U.S. patent application number 09/821547 was filed with the patent office on 2001-09-06 for method and apparatus for a full-duplex electromagnetic transceiver.
Invention is credited to Shattil, Steven J..
Application Number | 20010019264 09/821547 |
Document ID | / |
Family ID | 23067450 |
Filed Date | 2001-09-06 |
United States Patent
Application |
20010019264 |
Kind Code |
A1 |
Shattil, Steven J. |
September 6, 2001 |
Method and apparatus for a full-duplex electromagnetic
transceiver
Abstract
A cancellation circuit removes interfering signals from desired
signals in electrical systems having antennas or other
electromagnetic pickup systems. The cancellation circuit provides
amplitude adjustment and phase adjustment to electrical signals
induced in an electrical system by received electromagnetic
signals. The amplitude-adjusted and phase-adjusted signals are
combined to cancel the effects of electromagnetic interference. In
an electromagnetic receiver, a plurality of receiver elements
provide the cancellation circuit with different proportions of
desired and interfering signals to enable removal of the
interfering signals. An electromagnetic-wave transmitter having
multiple transmitter elements is provided with a cancellation
circuit for canceling electromagnetic signals in at least one
predetermined region of space. A compensation circuit enables the
cancellation circuit to compensate for frequency-dependent phase
and amplitude differences in received signals and/or transmitted
electromagnetic waves having multiple frequencies and/or broadband
characteristics.
Inventors: |
Shattil, Steven J.;
(Boulder, CO) |
Correspondence
Address: |
Steve J. Shattil
4980 Meredith Way #201
Boulder
CO
80303
US
|
Family ID: |
23067450 |
Appl. No.: |
09/821547 |
Filed: |
March 29, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09821547 |
Mar 29, 2001 |
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08279050 |
Jul 22, 1994 |
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6208135 |
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Current U.S.
Class: |
324/225 ;
324/202; 324/207.17 |
Current CPC
Class: |
H05K 9/00 20130101 |
Class at
Publication: |
324/225 ;
324/202; 324/207.17 |
International
Class: |
G01N 027/72 |
Claims
1. An electromagnetic transceiver capable of simultaneously
transmitting and receiving electromagnetic signals, the transceiver
including: an antenna system capable of transmitting and receiving
the electromagnetic signals, a signal transmitter coupled to the
antenna system, the transmitter adapted to couple electromagnetic
signals to the antenna system for transmission, a receiver coupled
to the antenna system, the receiver adapted to be responsive to the
transmitted electromagnetic signals and electromagnetic signals
received by the antenna system, a cancellation circuit coupled to
the transmitter and to the receiver, the cancellation circuit
adapted to couple at least one cancellation signal to the receiver
that reduces the responsiveness of the receiver to the transmitted
signals, the cancellation circuit characterized by at least one of:
an amplitude-adjustment circuit adapted to compensate for amplitude
differences between the at least one cancellation signal and the
receiver response to the transmitted signals resulting from at
least one of: a) differences in propagation between the transmitted
signals and the at least one cancellation signal to the receiver,
and b) differences in the responsiveness of the receiver to the
transmitted signals and the at least one cancellation signal, and a
phase-adjustment circuit adapted to compensate for phase
differences between the at least one cancellation signal and the
receiver response to the transmitted signals resulting from at
least one of: a) differences in propagation between the transmitted
signals and the at least one cancellation signal to the receiver,
and b) differences in the responsiveness of the receiver to the
transmitted signals and the at least one cancellation signal
2. The electromagnetic transceiver recited in claim 1 wherein the
receiver is electrically coupled to the transmitter.
3. The electromagnetic transceiver recited in claim 1 wherein the
antenna system includes at least one receiving antenna and at least
one transmitting antenna and the receiver is electromagnetically
coupled to the transmitter.
4. The electromagnetic transceiver recited in claim 1 wherein the
cancellation circuit is electrically coupled to the transmitter and
electrically coupled to the receiver.
5. The electromagnetic transceiver recited in claim 1 wherein the
cancellation circuit is electrically coupled to the transmitter and
electromagnetically coupled to the receiver.
6. The electromagnetic transceiver recited in claim 1 wherein the
cancellation circuit is electromagnetically coupled to the
transmitter and electrically coupled to the receiver.
7. The electromagnetic transceiver recited in claim 1 wherein the
antenna system includes an array of antennas.
8. The electromagnetic transceiver recited in claim 1 wherein the
phase-adjustment circuit includes at least one delay device adapted
to delay at least one of the received signals or at least one of
the cancellation signals to provide relative phase adjustment
between the at least one receive signal and the at least one
cancellation signal.
9. The electromagnetic transceiver recited in claim 1 wherein the
cancellation circuit is adapted to provide at least one of
frequency-dependent amplitude adjustment and frequency-dependent
phase adjustment to cancel the receiver response to transmitted
signals having multiple narrowband frequencies.
10. A cancellation circuit coupled to an electromagnetic receiver,
the cancellation circuit adapted to couple at least one
cancellation signal to the receiver for reducing the responsiveness
of the receiver to transmitted signals generated by the
transmitter, the cancellation circuit including at least one of: an
amplitude-adjustment circuit adapted to compensate for amplitude
differences between the at least one cancellation signal and the
receiver response to the transmitted signals resulting from at
least one of: a. differences in propagation between the transmitted
signals and the at least one cancellation signal to the receiver,
and b. differences in the responsiveness of the receiver to the
transmitted signals and the at least one cancellation signal, and a
phase-adjustment circuit adapted to compensate for phase
differences between the at least one cancellation signal and the
receiver response to the transmitted signals resulting from at
least one of: a. differences in propagation between the transmitted
signals and the at least one cancellation signal to the receiver,
and b. differences in the responsiveness of the receiver to the
transmitted signals and the at least one cancellation signal
11. The cancellation circuit recited in claim 10 wherein the
phase-adjustment circuit includes at least one delay device adapted
to delay either or both at least one of the received signals and at
least one of the cancellation signals to provide relative phase
adjustment between the at least one receive signal and the at least
one cancellation signal.
12. The cancellation circuit recited in claim 10 wherein the
cancellation circuit is adapted to provide at least one of
frequency-dependent amplitude adjustment and frequency-dependent
phase adjustment to cancel the receiver response to transmitted
signals having multiple narrowband frequencies.
13. The cancellation circuit recited in claim 10 wherein the
transmitter and the receiver are electrically coupled to an antenna
system.
14. The cancellation circuit recited in claim 10 wherein the
cancellation circuit is adapted to be electrically coupled to the
transmitter.
15. The cancellation circuit recited in claim 10 wherein the
cancellation circuit is adapted to be electromagnetically coupled
to the transmitter.
16. The cancellation circuit recited in claim 10 wherein the
cancellation circuit includes a signal generator for generating the
at least one cancellation signal.
17. A method of simultaneously transmitting and receiving
electromagnetic signals including: providing for coupling of at
least one transmitted electromagnetic signal from at least one
transmitter to a communication channel and coupling at least one
received electromagnetic signal into at least one receiver that is
responsive to the at least one received signal and to the at least
one transmitted signal, providing for reducing the response of the
at least one receiver to at least one of the transmitted signals,
the step of reducing the receiver response including generating at
least one cancellation signal adapted to cancel the receiver
response to the at least one transmitted signal, the step of
generating the at least one cancellation signal including at least
one of: providing for amplitude adjustment of the at least one
cancellation signal to compensate for amplitude differences between
the at least one cancellation signal and the receiver response to
the transmitted signals resulting from at least one of: a)
differences in propagation between the transmitted signals and the
at least one cancellation signal to the receiver, and b)
differences in the responsiveness of the receiver to the
transmitted signals and the at least one cancellation signal, and
providing for phase-adjustment of the at least one cancellation
signal to compensate for phase differences between the at least one
cancellation signal and the receiver response to the transmitted
signals resulting from at least one of: a) differences in
propagation between the transmitted signals and the at least one
cancellation signal to the receiver, and b) differences in the
responsiveness of the receiver to the transmitted signals and the
at least one cancellation signal
18. The method recited in claim 17 wherein the step of providing
for phase-adjustment includes providing for delay of either or both
the at least one received signal and the at least one cancellation
signal.
19. The method recited in claim 17 wherein the step of providing
for phase-adjustment includes providing for at least one of
frequency-dependent amplitude adjustment and frequency-dependent
phase adjustment to cancel the receiver response to transmitted
signals having multiple narrowband frequencies.
20. The method recited in claim 17 wherein the step of generating
at least one cancellation signal includes providing for
electrically coupling a portion of the at least one transmitted
signal.
Description
[0001] This application is a division of Ser. No. 08/279,050, filed
Jul. 22, 1994, now U.S. Pat. No. 6,208,135, which is related to
application Ser. No. 08/097,272, filed Jul. 23, 1993, now U.S. Pat.
No. 5,523,526.
BACKGROUND OF THE INVENTION
[0002] The present invention relates to electromagnetic shielding
for shielding electromagnetic pickups, other types of electronic
equipment, and specific regions of space from electromagnetic
radiation, and more particularly to active electromagnetic
shielding for providing an electrical cancellation signal for
canceling electromagnetic radiation or canceling the response of an
electronic device to electromagnetic radiation.
[0003] It has long been known that voltages are induced in all
conductors exposed to changing magnetic fields regardless of the
configuration of such conductors. Electromagnetic radiation will
induce electrical signals in electronic devices according to the
laws of magnetic induction. Thus it has been desirable in some
applications of electronic instrumentation to reduce the inductive
noise caused by electromagnetic radiation.
[0004] A common method for providing electromagnetic shielding
involves the use of passive electromagnetic shielding. A passive
shield consisting of layers of high and low permeability material
may be used to attenuate electromagnetic radiation passing through
it. However, this passive electromagnetic shielding adds
substantial bulk and weight to the system that it shields.
[0005] Another method for providing electromagnetic shielding is to
utilize cancellation coils for generating a canceling
electromagnetic radiation in opposition to incident radiation
produced by external sources in order to cancel the effects of the
incident radiation. In U.S. Pat. No. 5,066,891, Harrold presents a
magnetic field sensing and canceling circuit for use with a cathode
ray tube (CRT). Magnetic flux gate sensors provide output signals
that are functions of detected fields. These signals are then used
to control the current in cancellation coils that produce a
cancellation magnetic field. Harold explains that it is of great
importance that the CRT in a color monitor be protected from the
effects of external magnetic fields, and, in particular,
time-varying magnetic fields. However, this method provides no
compensation to the frequency-dependent amplitude and phase
responses of the sensor that picks up incident electromagnetic
radiation and the device that generates the cancellation
radiation.
[0006] Likewise, in U.S. Pat. No. 5,132,618, Sugimoto shows a
magnetic resonance imaging system that includes active shield
gradient coils for magnetically canceling leakage fields that would
otherwise produce eddy currents in the heat-shield tube.
[0007] A common method for providing shielding to an
electromagnetic pickup is to utilize identical pickup coils
connected in series or in parallel so as to cancel the effects of
uniform electromagnetic radiation. Pizzarello shows such a system
in U.S. Pat. No. 5,045,784 for reducing inductive noise in a
tachometer coil. An electric tachometer is a coil of wire that may
be attached to a moving part of a motor that passes through a
stationary magnetic field. The motion of the wire through the
magnetic field induces a voltage that is indicative of the motor's
speed. However, if the motor is powered by electricity, changes in
the current powering the motor will cause a magnetic flux that also
produces a voltage in the coil. Pizzarello shows a stationary
pickup coil that is responsive to magnetic flux, and a means for
subtracting the pickup voltage from the tachometer voltage.
[0008] Likewise, in U.S. Pat. No. 4,901,015, Pospischil shows a
cancellation circuit for canceling the response of a magnetic
pickup to ambient electromagnetic fields. Pospischil describes
first and second pickups that are positioned in parallel with the
wavefronts of an interfering electromagnetic field. With such
placement, the electromagnetic field impinges simultaneously upon
the first and second pickups. The pickups are connected in
opposition. Thus, simultaneous impingement of the electromagnetic
field upon the pickups is expected to produce a 180-degree phase
displacement of the received signals.
[0009] If the electrical path lengths of the received signals in
Pospischil's cancellation system are different where they are
combined (summed), the relative phase difference between the
received signals will not have 180-degree phase displacement. Thus,
the signals will not cancel. Pospischil shows that differences in
the electrical path length occur when the propagation path lengths
of the signals received by the pickups are different (e.g., the
signals do not impinge upon the pickups simultaneously). These
differences in propagation path lengths can result from
reflections, multipath delay, superpositions of multiple received
signal components, or received electromagnetic signals having
non-perpendicular angles of arrival.
[0010] Pospischil does not identify nor compensate for electrical
path-length differences (e.g., differences in impedance) that occur
between different electromagnetic receivers (pickups). Such pickup
assemblies are also used with electric guitars and are known as
"hum-bucking" pickups. This technique is not effective for
providing a high degree of cancellation because slight differences
between the pickups, even pickups that are substantially identical,
cause the frequency-dependent amplitude and phase response of the
pickups to differ significantly from each other. Thus the pickup
signals will not be exactly out of phase and equal in amplitude
when they are combined.
[0011] A prior-art method for providing shielding to an
electromagnetic pickup from an electromagnetic source that produces
a non-uniform field is to "unbalance" either the pickup device or
the electromagnetic source. Such a method is described by Hoover in
U.S. Pat. No. 4,941,388. Hoover uses amplitude-adjustment
techniques to compensate for amplitude variations between the
responses of separate pickups to electromagnetic radiation
generated by an electromagnetic sustaining device that drives the
vibrations of a string on an electric guitar. However, Hoover does
not compensate for differences in the pickup coils which cause the
amplitude-variation of the responses of the pickups to be
frequency-dependent. Thus, Hoover's proposed solution results in
poor cancellation over a broad range of frequency. Furthermore,
Hoover does not compensate for phase-variations that occur between
different pickup coils. The resulting cancellation from the
unbalancing method is poor.
[0012] Hoover describes the operation of negative feedback in a
system where a magnetic pickup provides an electrical signal to a
magnetic driver that generates an electromagnetic field to which
the pickup responds. Hoover mentions that the system tends to drift
from the negative feedback condition at higher frequencies, and
identifies the cause of this drift as distortions in the
phase-response of the system resulting from the pickup, driver, and
amplifier in the system. Hoover does not present an effective
method for controlling the phase-response of the system, nor does
Hoover present the mathematical relationships between phase and
frequency resulting from the driver and pickup coils. Rather,
Hoover proposes the use of a low-pass filter to reduce the gain of
the system at which the negative feedback condition breaks
down.
[0013] Methods of active phase-compensation are described by Rose
in U.S. Pat. No. 4,907,483, U.S. Pat. No. 5,123,324, and U.S. Pat.
No. 5,233,123. Rose uses active circuits for determining the
frequency or frequency range of an electrical signal from an
electromagnetic pickup. Active phase-adjustment is applied to the
pickup signal, which is used to power an electromagnetic driver
that generates an electromagnetic driving force on a vibratory
ferromagnetic element of a musical instrument. The purpose of the
phase-adjustment of the pickup signal is to provide a driving force
to the vibratory element that is substantially in-phase with its
natural motion. Because the purpose of Rose's invention is to
improve the efficiency of the electromagnetic drive force on the
element, it is apparent that a passive phase-compensation circuit
would be preferable to Rose's active phase-compensation circuit.
However, Rose does not realize the mathematical relationships
between phase and frequency that provide the basis for constructing
a passive phase-compensation network. Furthermore, Rose's invention
does not provide simultaneous phase-compensation to more than one
harmonic.
[0014] Another method for providing electromagnetic shielding is to
orient the angle of a pickup coil to incident electromagnetic
radiation such that the electrical current induced in the coil by
the electromagnetic radiation will substantially cancel. One
application of this method is shown by Burke in the Handbook of
Magnetic Phenomena, published in 1986. Burke uses a transmitting
coil that produces electromagnetic radiation and a receive coil
that senses radiation. The two coils can be configured in such a
way that no energy is transferred between the transmitting and
receiving coils. Burke shows the receiving coil oriented with the
axis of its turns at right angles to the direction of the magnetic
field produced by the transmitting coil. Burke explains that the
instantaneous generated voltage of the receive coil is determined
by the instantaneous rate of change of the magnetic flux passing
through the coil. If the flux is directed at right angles to the
coil's axis, none of it is intercepted by the coil, and the
instantaneous rate of change through the coil is zero. This method
of cancellation was used in an electromagnetic sustain device for
electric guitars marketed by T Tauri Research of Wilmette Ill. in
November, 1988, and patented by Tumura, European Patent Application
No. 92307423.1 filed on Aug. 13, 1992, and U.S. Pat. No. 5,292,999.
The actual effectiveness of this technique is limited by several
factors, such as the uniformity of the pickup coil's windings, the
uniformity of the electromagnetic radiation near the pickup,
interference due to other nearby conducting materials, and the
difficulty of precisely positioning a pickup coil in a field whose
intensity varies as the inverse square of the distance from its
source.
[0015] Another method for providing active electromagnetic
shielding is the differential transformer also shown by Burke. The
differential transformer comprises a drive coil for generating a
magnetic flux, and two pickup coils wrapped around a ferromagnetic
core that includes a moveable armature that, when moved, varies the
reluctance of the magnetic path associated with each pickup coil.
If the two pickup coils are identical, and if the two magnetic
paths about which they are wound are identical, the voltages
induced in each pickup coil will be the same. However, Burke
explains that the two pickup coils nor the two magnetic paths can
be made exactly the same, therefore a differential transformer will
always have some output voltage under zero stimulus.
[0016] Coils of wire whose currents support magnetic fields in
space function as antennas radiating electromagnetic energy. There
are several cancellation methods used with antennas that act as
electromagnetic shielding. One of these methods is the basis of
operation for a sidelobe canceller that uses an auxiliary antenna
in addition to a main antenna. Combining the outputs from the two
antennas results in cancellation of the antenna beam pattern in the
direction of a noise source so that the effective gain of the
antenna in that direction is very small. Likewise, the multiple
sidelobe canceller addresses the problem of multiple noise
sources.
[0017] Delay-line cancellers are used in systems where multiple
radar pulses are transmitted. These cancellers are used to detect
moving objects. In a single-element delay-line canceller, a
received pulse is delayed and added to another pulse received later
so that the pulses reflected by stationary objects are out of phase
and thus cancel, whereas the pulses reflected by moving objects do
not cancel.
[0018] Several methods are used to allow an antenna to
simultaneously transmit and receive electromagnetic radiation. For
example, in a continuous wavelength radar system, a single antenna
may be employed since the necessary isolation between transmitted
and received signals is achieved via separation in frequency as a
result of the Doppler effect. The received signal enters the radar
via the antenna and is heterodyned in a mixer with a portion of the
transmitted signal to produce a Doppler beat frequency.
[0019] An intermediate-frequency receiver may use separate antennas
for transmission and reception. A portion of the transmitted signal
is mixed with an intermediate frequency, and then a narrow-band
filter selects one of the side bands as the reference signal, which
is mixed with the signal from the receiver antenna.
[0020] It is one object of the present invention to provide active
electromagnetic shielding for canceling the effects of
electromagnetic induction in electrical circuits. It is a related
object of the present invention to reduce interference between
transmitters and receivers of electromagnetic radiation that
operate simultaneously. It is another object of the present
invention to provide a cancellation circuit that allows a single
antenna element to simultaneously transmit and receive
electromagnetic radiation. It is still another object of the
present invention to compensate for frequency-dependent amplitude
and phase responses of electromagnetic receivers and
transmitters.
SUMMARY OF THE INVENTION
[0021] In accordance with the present invention, a cancellation
circuit is provided for canceling the inductive effects of
electromagnetic radiation. The cancellation circuit comprises a
means for acquiring or generating an electrical reference signal
that is similar in shape to the inductive electrical signal
produced by the electromagnetic radiation, an amplitude-adjustment
circuit that adjusts the amplitude of either or both the reference
signal and an electrical pickup signal containing an inductive
noise component, a phase-adjustment circuit that adjusts the
relative phase between the reference signal and the pickup signal
such that when these signals are combined, the inductive noise
component will be canceled, and a combining circuit that combines
the reference and pickup signals to produce a pickup signal that is
substantially free from inductive noise.
[0022] In one aspect of the present invention, the reference signal
is obtained from an electromagnetic pickup that is responsive to
external magnetic flux. In another aspect of the present invention,
the reference signal is obtained from part of the electrical signal
that is used to generate the external magnetic flux. In still
another aspect of the present invention, a signal generator
generates the reference signal.
[0023] The present invention provides substantial electromagnetic
shielding capabilities compared to prior-art shielding devices.
Because the present invention actively shields from electromagnetic
flux, it is non-intrusive compared to passive shielding
technologies, which use materials that are heavy and bulky and
require complete enclosure in order to provide optimum shielding.
Thus the present invention may be used in order to reduce or
eliminate the need for passive electromagnetic shielding in certain
applications. Furthermore, in addition to being superior for
shielding electromagnetic radiation compared to prior-art active
electromagnetic shielding technologies, the present invention may
be adapted to prior-art shielding devices to improve their
performance.
[0024] The cancellation effect of the present invention allows
electromagnetic pickups to operate in environments containing high
levels of electromagnetic noise. For example, the present invention
may be integrated into a sustaining device for a stringed musical
instrument (as described by Rose and Hoover) to provide a very
small sustain device that both picks up and drives the vibrations
of a string on the musical instrument. This sustain device would be
much smaller than the devices shown by either Rose or Hoover
because the improved shielding capability of the present invention
allows for the electromagnetic pickups (which pick up string
vibrations) and the driver (which generates an electromagnetic flux
to drive those vibrations) to be placed very close together (or
even share the same structure) without the effects of
electromagnetic interference. Other applications of the present
invention include electric tachometers that operate near devices
that generate large amounts of magnetic flux, and other
electromagnetic receivers such as radars that operate near sources
of electromagnetic radiation. This aspect of the present invention
allows an electromagnetic antenna to simultaneously operate as a
transmitter and receiver by decoupling the receiver-response to the
transmitted signal. The present invention may also be used to
cancel the response of a radar to ground clutter.
[0025] Another aspect of the present invention further includes a
compensation circuit for adjusting the pickup signal's amplitude
and/or phase in order to compensate for frequency-dependent
amplitude and phase responses of the pickup. The compensation
circuit may also compensate for frequency-dependent amplitude
and/or phase variations of electromagnetic flux generated by an
electromagnetic-flux generator, such as a drive coil. The present
invention may be integrated into a prior-art active magnetic
shielding circuit that generates a canceling magnetic of flux for
canceling external magnetic flux. The present invention provides a
more accurate response to external magnetic flux, and thereby
improves the cancellation effect of the circuit. Such a circuit may
be used to provide active electromagnetic shielding to instruments
that are sensitive to magnetic or electromagnetic fields. The
invention has applications as a shielding device for atomic clocks,
magnetic resonance imaging apparatus, tactical instrumentation,
cathode ray tubes, satellites, and spacecraft.
[0026] In another embodiment of the present invention, the
electromagnetic flux generated by the drive coil provides a
magnetic force upon a moving ferromagnetic element. The phase of
the electromagnetic flux generated by the system may be adjusted to
provide electromagnetic damping to the ferromagnetic element, and
thus act as a stabilizer for that element. The electromagnetic flux
may be adjusted in phase to drive the oscillations of the
ferromagnetic element (as discussed by Rose) The invention allows a
broad range of driving frequencies to be compensated, thus allowing
for the driving of the harmonics as well as the fundamental
frequency of the element.
[0027] These and other aspects of the present invention will become
apparent to those skilled in the art upon consideration of the
following detailed descriptions of the preferred embodiments.
BRIEF DESCRIPTION OF THE DRAWINGS
[0028] FIG. 1 is a schematic view of a prior-art cancellation
circuit.
[0029] FIG. 2 is a schematic view of a cancellation circuit of the
present invention.
[0030] FIG. 3A is a schematic view of a phase-adjustment circuit
that may be used in the cancellation circuit of the present
invention.
[0031] FIG. 3B is a schematic view of a phase-adjustment circuit
that may be used in the cancellation circuit of the present
invention.
[0032] FIG. 3C is a schematic view of a circuit of an embodiment of
the present invention.
[0033] FIG. 4A is a schematic view of a cancellation circuit of the
present invention that illustrates another method of
phase-adjustment.
[0034] FIG. 4B is a schematic view of a phase-adjustment circuit
that may be used in the cancellation circuit of the present
invention.
[0035] FIG. 4C is a schematic view of a phase-adjustment circuit
that may be used in the cancellation circuit of the present
invention.
[0036] FIG. 5 is a plot of cancellation relative to signal
frequency for three different cancellation circuits.
[0037] FIG. 6 is a schematic view of a cancellation circuit of the
present invention that generates an electromagnetic flux in
response to a pickup signal, and includes a compensation circuit
for compensating for frequency-dependent phase and/or amplitude
variations in electrical signals used to generate the
electromagnetic flux.
[0038] FIG. 7 is a schematic view of a cancellation circuit of the
present invention that includes a compensation circuit and provides
an electromagnetic drive force to a ferromagnetic element.
[0039] FIG. 8A is a schematic view of a compensation circuit of the
present invention.
[0040] FIG. 8B is a schematic view of a compensation circuit of the
present invention.
[0041] FIG. 8C is a schematic view of a compensation circuit of the
present invention.
[0042] FIG. 9 is a schematic view of a cancellation circuit of the
present invention wherein cancellation of incident electromagnetic
flux is achieved by generating an out-of-phase electromagnetic
flux.
[0043] FIG. 10 is a schematic view of a cancellation circuit of the
present invention wherein a pickup coil and a drive coil are
wrapped around the same core.
[0044] FIG. 11 is a schematic view of a cancellation circuit of the
present invention wherein pickup and drive coils are wrapped around
the same core.
[0045] FIG. 12 is a schematic view of a cancellation circuit of the
present invention wherein a reference signal is obtained from a
signal generator used to provide a drive signal that generates the
electromagnetic flux.
[0046] FIG. 13 is a schematic view of a cancellation circuit of the
present invention wherein a reference signal is obtained from
splitting the drive signal used to generate an electromagnetic
flux.
[0047] FIG. 14A is a schematic view of a cancellation circuit of
the present invention for a single-element transmit/receive system
that includes a harmonic-compensation circuit for canceling the
non-linear response of nearby magnetically permeable materials.
[0048] FIG. 14B is a schematic for a harmonic-compensation circuit
of the present invention.
[0049] FIG. 14C is a schematic for a harmonic-compensation circuit
of the present invention.
[0050] FIG. 14D is a schematic for a harmonic-compensation circuit
of the present invention.
[0051] FIG. 15 is a schematic view of a cancellation circuit of the
present invention for a single-element transmit/receive system.
[0052] FIG. 16 is a schematic view of a cancellation circuit of the
present invention used in a system that compensates for magnetic
fields.
[0053] FIG. 17 is a schematic view of a cancellation circuit of the
present invention used in a system that compensates for magnetic
fields.
[0054] FIG. 18 is a schematic view of a cancellation circuit of the
present invention used in a single-element transmit/receive
system.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0055] A prior-art balancing device for a pair of electromagnetic
pickups shown in FIG. 1 includes a two coil assemblies, 10 and 12,
two amplifiers, 14 and 16, and a combining means 18. The first
pickup coil 10 has values of resistance and inductance of R.sub.1
and L.sub.1, respectively. The second pickup coil 12 has values of
resistance and inductance of R.sub.2 and L.sub.2, respectively. The
pickup coil 10 is connected to the input of the amplifier 14 and
the pickup coil 12 is connected to the input of the other amplifier
16. The inputs to the amplifiers 14 and 16 each have a capacitor
C.sub.1 and C.sub.2, respectively, connected to electrical ground,
as is commonly done to filter out high-frequency noise and
interference from the pickup signals. The outputs of the amplifiers
14 and 16 are combined by a combining circuit 18, which may be a
voltage divider, a summing amplifier, or a differential
amplifier.
[0056] The pickup coils 10 and 12 are responsive to external
magnetic flux that induces a first electrical pickup signal in coil
10 and a second electrical pickup signal in coil 12. Due to
coil-positioning with respect to the external magnetic flux, coil
properties, and properties of materials (not shown) which the coils
10 and 12 may surround, the amplitude of the first electrical
pickup signal will most likely differ from the amplitude of the
second electrical pickup signal. Thus amplifiers 14 and 16 may be
used to change the amplitude of either or both of the first and
second electrical pickup signals. If the pickup signals are out of
phase, the combining circuit 18 is a voltage divider or a summing
amplifier. If the signals are in phase, then the combining circuit
18 is a differential amplifier. However, the relative phase between
the first and second electrical signals will tend to be
substantially different than 0 or 180 degrees, thus providing poor
cancellation of the signals at the output of the combining circuit
18.
[0057] The impedance Z.sub.1 of the first pickup coil 10 is related
to the coil's 10 resistance R.sub.1 and inductance
L.sub.1:Z.sub.1=R.sub.1+i.ome- ga.L.sub.1. Likewise, the impedance
Z.sub.2 of the second pickup coil 12 has the value:
Z.sub.2=R.sub.2+i.omega.L.sub.2, where .omega. represents the
frequency of the pickup signals multiplied by 2 Pi. The voltage
V.sub.1in at the input of the first amplifier 14 is
V.sub.1in=V.sub.1[(1-.omega..sup.2C.sub.1L.sub.1)-i.omega.C.sub.1R.sub.1]/-
[(1-.omega..sup.2C.sub.1L.sub.1).sup.2+.omega..sup.2C.sub.1.sup.2R.sub.1.s-
up.2],
[0058] where V.sub.1 is the voltage induced in the coil 10 by
external magnetic flux. The voltage V.sub.2in of the second pickup
signal is
V.sub.2in=V.sub.2[(1-.omega..sup.2C.sub.2L.sub.2)-i.omega.C.sub.2R.sub.2]/-
[(1-.omega..sup.2C.sub.2L.sub.2).sup.2+.omega..sup.2C.sub.2.sup.2R.sub.2.s-
up.2],
[0059] where V.sub.2 is the voltage induced in the coil 12 by
external magnetic flux. Incidentally, V.sub.1 and V.sub.2 are
proportional to the magnitude of external magnetic flux at the
locus of each pickup coil 10 and 12. The gain imparted to one or
both pickup signal voltages V.sub.1 and V.sub.2 by the amplifiers
14 and 16 can correct for differences in the amplitude between
V.sub.1 and V.sub.2 but can not correct for phase differences
between those signals. The phase of the first pickup signal voltage
V.sub.1in has the value
.O
slashed..sub.1=-ArcTan(.omega.C.sub.1R.sub.1/(1-.omega..sup.2C.sub.1L.s-
ub.1)),
[0060] and the phase of the second pickup signal voltage V.sub.2in
has the value
.O
slashed..sub.2=-ArcTan(.omega.C.sub.2R.sub.2/(1-.omega..sup.2C.sub.2L.s-
ub.2)).
[0061] It is typical for the values of L.sub.1 and R.sub.1 to
differ substantially from the values of L.sub.2 and R.sub.2,
respectively, even for pickup coils having identical numbers of
windings. For example, two coils of 34 gauge copper wire, each
wound 330 times around identical cores yielded values of resistance
of 16.5 and 16.7 ohms, and values of inductance of 205 uH and 194
uH, respectively. Thus when only the amplitudes of the two signals
are adjusted so that they are equivalent, the relative phase
between the signals prevents optimal cancellation of the
signals.
[0062] The expressions shown for the pickup voltages V.sub.1in and
V.sub.2in are very accurate, but not exact representations for
illustrating the differences in the phase variations between the
pickup coils 10 and 12. The exact impedance relations for the
pickup coils 10 and 12 should also include capacitive effects.
Other factors that may contribute to phase variations between the
signals produced by the pickup coils 10 and 12 include ground
oscillations, complexities resulting from the fact that each of the
pickup coils 10 and 12 acts as a source for the electrical pickup
signals, possible electrical loading between the two pickup coils
10 and 12, and variations in how the voltage leads the current in
the coils 10 and 12 resulting from inductance and capacitance in
each of the coils 10 and 12. Although the expressions shown for the
pickup voltages V.sub.1in and V.sub.2in do not provide exact
representations for the differences in the phase between the pickup
coils 10 and 12, these expressions are accurate to a great degree,
and represent the basis from which extremely effective cancellation
circuits can be designed. It will be appreciated that even more
precise representations of the electrical signals induced in
electromagnetic pickups can enable the design of cancellation
circuits that are even more effective.
[0063] An embodiment for a cancellation circuit of the present
invention is shown in FIG. 2 as including a pair of pickup coils 20
and 22, a pair of amplitude-adjustment circuits 24 and 26, a
phase-adjustment circuit 25, and a combining circuit 28. The pickup
coil 20 is connected to a phase-adjustment circuit 25. The output
of the phase-adjustment circuit 25 is connected to the input of an
amplitude-adjustment circuit 24. The pickup coil 22 is connected to
the input of an amplitude-adjustment circuit 26. The outputs of
both amplitude-adjustment circuits, 24 and 26, are connected to a
combining circuit 28. The output of the combining circuit 28
provides an electrical signal that is substantially free from the
effects of electrical noise caused by the response of the pickup
coils 20 and 22 to external magnetic flux.
[0064] The pickup coils 20 and 22 are responsive to external
magnetic flux that induces a first signal voltage V.sub.1 at the
output of the first pickup coil 20, and a second signal voltage
V.sub.2 at the output of the second pickup coil 22. The phase of
signal voltage V.sub.1 is .O slashed..sub.1 and the phase of signal
voltage V.sub.2 is .O slashed..sub.2. Because both .O
slashed..sub.1 and .O slashed..sub.2 are functions of signal
frequency .omega., we will write .O slashed..sub.1(.omega.) and .O
slashed..sub.2(.omega.). The pickup coil 20 is connected to the
input of a phase-adjustment circuit 25, which provides a
phase-shift F(.omega.)=(.O slashed..sub.2(.omega.)-.O
slashed..sub.1(.omega.)) to V.sub.1 that compensates for the
phase-difference between signals V.sub.1 and V.sub.2. The nature of
the phase-adjustment circuit 25 is determined by the frequency
range of signal cancellation required. The output of the
phase-adjustment circuit 25 is connected to the input of an
amplitude-adjustment circuit 24. The pickup coil 22 is connected to
an amplitude-adjustment circuit 26. Both amplitude-adjustment
circuits, 24 and 26, may provide amplitude-adjustment to the pickup
signals V.sub.1 and V.sub.2, respectively. In an alternative
embodiment, only one of the amplitude-adjustment circuits 24 or 26
may provide amplitude adjustment while the other circuit 26 or 24
acts only as a buffer. Because phase-adjustment circuits (such as
phase-adjustment circuit 25) typically change signal-amplitude as
well as phase, it is preferable that the amplitude-adjustment
circuits, 24 and 26, have little effect on signal phase. Thus the
amplitude-adjustment circuits, 24 and 26, may comprise
non-inverting amplifiers. The outputs of the amplitude-adjustment
circuits, 24 and 26, are combined in the combining circuit 28 in
order to cancel the effects of external magnetic flux picked up by
coils 20 and 22. Depending on whether the output signals of the
amplitude-adjustment circuits, 24 and 26, are in phase or out of
phase, the combining circuit 28 may comprise a voltage divider, a
summing amplifier, or a differential amplifier. It will be
appreciated that the coils 20 and 22 may be wrapped around a core
(not shown), such as a core comprising a ferromagnetic material. It
will also be appreciated that one or more additional
phase-adjustment circuits may be included in series with coil 20
and/or coil 22. Furthermore, it will be appreciated that
amplitude-adjustment circuits such as amplitude-adjustment circuit
24 may precede phase-adjustment circuits, such as phase-adjustment
circuit 25.
[0065] Several phase-adjustment circuits shown in FIG. 3 may be
used in the circuit shown in FIG. 2. The circuit in FIG. 3A is
commonly referred to as an "all-pass filter." The all-pass filter
provides a phase-shift of .O slashed.=180.degree.-2 ArcTan
(.omega.R.sub.6 C.sub.3) while producing little amplitude-variation
with respect to signal frequency. The circuit shown in FIG. 3B is
also an all-pass filter. It produces a phase-shift of .O slashed.=2
ArcTan (.omega.R.sub.8 C.sub.4). The all-pass filters in FIGS. 3A
and 3B may be preceded by a buffer amplifier (not shown).
[0066] It is sometimes desirable to have substantial
noise-cancellation over only a narrow frequency-range. This is
called "notch-cancellation" and may be used in a single-frequency
or band-limited system. One application for notch-cancellation is
when an external magnetic flux contains a weak signal having a
significantly different frequency than the noise that accompanies
it, then a cancellation circuit that cancels a narrow frequency
range including the noise, but not the desired signal, is
preferable. Ferromagnetic materials used for pickup cores have the
property of nonlinear responsiveness to magnetic flux. This
non-linear responsiveness is observed in a pickup signal as a
higher harmonic or intermodulation product of the frequency of the
magnetic flux. In order to observe the extent of the core
material's non-linearity, it is preferable to cancel only the
primary pickup signal, which has the same frequency .omega. as the
applied magnetic flux. Typically, the higher harmonic signatures
caused by a core's non-linearity is at least several orders of
magnitude less than the intensity of the primary signal induced in
the coil. Thus the method of notch-cancellation provides an
advantage over conventional electrical filtering techniques in both
simplicity and performance.
[0067] FIG. 3C shows another embodiment for a cancellation circuit
of the present invention. Two pickup coils 30 and 32 are each
connected to a phase-adjustment circuit 35 and 37, respectively.
The phase-adjustment circuits 35 and 37 are each connected to the
input of an amplitude-adjustment circuit 34 and 36, respectively.
The outputs of the amplitude-adjustment circuits 34 and 36 are
combined by a combining circuit 38. The output of the combining
circuit 38 is substantially free from inductive noise. The
phase-adjustment circuits shown in FIG. 3A and FIG. 3B may be used
as the phase-adjustment circuits 35 and 37 shown in FIG. 3C.
Furthermore, the phase-adjustment circuits shown in FIG. 3A and
FIG. 3B include a means for adjusting the amplitude of electrical
signals via adjustment of the resistors R.sub.5 and R.sub.7 in FIG.
3A, and resistors R.sub.9 and R.sub.10 in FIG. 3B.
[0068] For the case of notch-cancellation in which the relative
phase between the pickup signals from the first and second pickup
coils 30 and 32 is very small, such as when the coils 30 and 32 are
very close to being identical or when a phase-adjustment circuit
(not shown) has already created this condition, it is preferable to
select types of phase-adjustment circuits 35 and 37 that cause a
very narrow frequency-range in which the cancellation is
substantial. If the pickup signals from the two pickup coils 30 and
32 are in phase, this may be accomplished by selecting the all-pass
filter shown in FIG. 3A as one phase-adjustment circuit 35 and
selecting the all-pass filter shown in FIG. 3B as the other
phase-adjustment circuit 37. This selection is suggested because as
the signal-frequency ( changes, the phase of the pickup signal of
one of the phase-adjustment circuits 35 increases while the phase
of the pickup signal of the other phase-adjustment circuit 37
decreases, thus causing a rapid change in the relative phase with
respect to frequency .omega.. To maximize the change in the
relative phase near the "notch frequency" where the cancellation is
most substantial, one could select values of R.sub.6 and C.sub.3 in
FIG. 3A and values of R.sub.8 and C.sub.4 in FIG. 3B such that
.omega..sub.nR.sub.6C.sub.3 and .omega..sub.nR.sub.8C.sub.4 are
nearly equal to 1 for the notch frequency .omega..sub.n. To further
narrow the cancellation notch about the notch frequency
.omega..sub.n, the phase-adjustment circuits 35 and 37 may each
include multiple all-pass filters as shown in FIG. 3A and FIG.
3B.
[0069] For the case in which cancellation is desired over a broad
frequency range (such as when the pickups 30 and 32 are part of a
feedback circuit that is prone to oscillate, making it necessary to
cancel the higher harmonic terms that will accompany the primary
signal), phase-adjustment circuits 35 and 37 may be selected to
broaden the cancellation notch about the notch frequency
.omega..sub.n. For example, the choice of phase-adjustment circuits
35 and 37 for two pickup signals that are in phase may both be of
the type of all-pass filters shown in FIG. 3A or FIG. 3B where the
values of resistance R.sub.6 or R.sub.8 and capacitance C.sub.3 or
C.sub.4 are chosen to minimize the relative phase and amplitude
variations with respect to frequency between the two pickup
signals.
[0070] FIG. 4A shows how two phase circuits may be integrated into
a circuit comprising two pickup coils 40 and 42. Coil 40 is
connected to series element 45, which may include resistors and/or
inductors (not shown) connected in series with the coil 40.
Likewise, coil 42 is connected to series element 47, which may
include resistors and/or inductors (not shown) connected in series
with the coil 42. Series element 45 is connected to the input A of
an amplitude-adjustment circuit 44 and series element 47 is
connected to the input B of an amplitude-adjustment circuit 46.
Input A includes a resistor R.sub.3 connected to electrical ground,
and input B includes a resistor R.sub.4 connected to electrical
ground. Together, the series element 45 and resistor R.sub.3 form
one phase-adjustment circuit, and the series element 47 and the
resistor R.sub.4 form another phase-adjustment circuit. The outputs
of the amplitude-adjustment circuits 44 and 46 are connected to the
input of a combining circuit 48 that combines the output signals of
the amplitude-adjustment circuits such that the noise-signal caused
by external magnetic flux substantially cancels.
[0071] The effective impedance of the coil 40 at the input A of the
amplitude-adjustment circuit 44 includes the actual impedance of
the coil 40 added to the impedance of the series element 45, and is
represented by Z.sub.1=R.sub.1+i.omega.L.sub.1. The effective
impedance of the coil 42 at the input B of the amplitude-adjustment
circuit 46 includes the actual impedance of the coil 42 added to
the impedance of the series element 47, and is represented by
Z.sub.2=R.sub.2+i.omega.L.sub.2. The voltage of the pickup signal
induced in the coil 40 by external magnetic flux having frequency
.omega., measured at the input A is
V.sub.1A=R.sub.3V.sub.1/(R.sub.1+R.sub.3+i.omega.L.sub.1),
[0072] where V.sub.1 is the voltage-magnitude of the signal induced
in the pickup coil 40 by the external magnetic flux. The signal
voltage induced in the coil 40 by external magnetic flux having
frequency .omega., measured at the input B is
V.sub.2B=R.sub.4V.sub.2/(R.sub.2+R.sub.4+i.omega.L.sub.2),
[0073] where V.sub.2 is the voltage-magnitude of the signal induced
in the pickup coil 42 by the external magnetic flux. The phase of
the voltage of the pickup signal at the input A is
.O slashed..sub.1=ArcTan(-.omega.L.sub.1/(R.sub.3+R.sub.1)),
[0074] and the phase of the voltage of the pickup signal at the
input A is
.O slashed..sub.2=ArcTan(-.omega.L.sub.2/(R.sub.4+R.sub.2)).
[0075] In order that .O slashed..sub.1=.O slashed..sub.2 for a
broad range of signal-frequencies .omega., it is necessary that the
series element 45 and/or series element 47 be adjusted such that
L.sub.1/(R.sub.3+R.sub.1)=- L.sub.2/(R.sub.4+R.sub.2). This may
also be accomplished by adjusting resistors R.sub.3 and/or R.sub.4.
However, if we look at the equations for signal voltage at the
inputs A and B of the amplitude-adjustment circuits 44 and 46,
respectively, we note that equivalence of the ratios just discussed
does not, by itself, provide the condition whereby the magnitude of
the voltage difference V.sub.1A-V.sub.2B remains substantially
constant as .omega. changes. Thus, in order to assure optimal
cancellation over a broad range of signal frequencies, it is
necessary that the series elements 45 and 47 are adjusted such that
the effective resistances R.sub.1 and R.sub.2 are equivalent and
the effective inductances L.sub.1 and L.sub.2 are equivalent. It is
also necessary that resistance R.sub.3 equal resistance R.sub.4. It
is possible to replace resistors R.sub.3 and R.sub.4 with
capacitors (not shown) for filtering out high-frequency noise,
However, for optimal cancellation over a broad range of signal
frequencies, it is necessary that both capacitors (not shown) have
substantially equal values.
[0076] It will be appreciated from the equations representing the
voltages V.sub.1A and V.sub.2B at the amplifier inputs A and B,
respectively, that the series elements 45 and 47 may each include a
large value of series resistance so as to increase the effective
resistances R.sub.1 and R.sub.2 of the pickup coils 40 and 42,
respectively. This reduces the frequency-dependent amplitude and
phase variations of the pickup signals V.sub.1A and V.sub.2B.
However, it is preferable that the increase in the effective
resistances R.sub.1 and R.sub.2 of the pickup coils 40 and 42,
respectively, not be the only means of phase-adjustment used in the
circuit as other phase effects that are unrelated to the
signal-voltage equations for V.sub.1A and V.sub.2B tend to
occur.
[0077] Consider the circuit shown in FIG. 4A for a case in which it
is not optimized for canceling the effects of external magnetic
flux. An applied phase shift between voltages V.sub.1A and V.sub.2B
that matches their phases is
F(.omega.)=ArcTan(-.omega.L.sub.1/(R.sub.3+R.sub.1))-ArcTan(-.omega.L.sub.-
2/(R.sub.4+R.sub.2)).
[0078] A phase-adjustment circuit that provides the required
phase-shift may include a buffered input and precede either or both
amplitude-adjustment circuits 44 and 46, or may follow either or
both amplitude-adjustment circuits 44 and 46. The components of
this phase-adjustment circuit are shown in FIG. 4B and FIG. 4C.
[0079] The phase-adjustment circuit shown in FIG. 4B is a passive
lead network. An input voltage V.sub.in is applied across terminals
C and G. The output voltage V.sub.out of this circuit is measured
across terminals D and G. The output voltage is
V.sub.out=V.sub.inR.sub.12(i.omega.C.sub.10R.sub.11+1)/(R.sub.12(i.omega.C-
.sub.10R.sub.11+1)+R.sub.11).
[0080] If R.sub.11>>R.sub.12, then the phase-shift is .O
slashed.=ArcTan (.omega.R.sub.11C.sub.10).
[0081] The phase-adjustment circuit shown in FIG. 4C is a passive
lag network. An input voltage V.sub.in is applied across terminals
E and G. The output voltage V.sub.out of the circuit is measured
across terminals F and G. The output voltage is
V.sub.out=V.sub.inR.sub.14(((R.sub.14+R.sub.13)+i.omega.C.sub.15R.sub.14R.-
sub.13).
[0082] If R.sub.14>>R.sub.13, then the phase-shift is .O
slashed.=-ArcTan (.omega.R.sub.14C.sub.15).
[0083] The circuits in FIG. 4B and FIG. 4C provide the basis for
constructing F(.omega.)). The passive lead network shown in FIG. 4B
can be combined in series with the passive lead network shown in
FIG. 4C and use appropriate buffering between the lead and lag
networks (such as a buffered amplifier (not shown)). Values of
R.sub.14 and C.sub.15 are preferably selected such that
R.sub.14C.sub.14=L.sub.1/(R.sub.3+R.sub.1) and values of R.sub.11
and C.sub.10 are preferably selected such that
R.sub.11C.sub.10=L.sub.2/(R.sub.4+R.sub.2).
[0084] It should be appreciated that the phase-adjustment circuits
shown in FIG. 3 and FIG. 4 are only a few of the many
phase-adjustment circuits that can be used in a cancellation
circuit to substantially eliminate electrical noise in a pickup
signal caused by external magnetic flux. The phase-adjustment
circuits shown, as well as other phase-adjustment circuits, may be
used in combination for either broadening or narrowing the
frequency range where a high degree of signal cancellation occurs.
Phase-adjustment circuits may also be used for adjusting amplitudes
of electrical signals, and thus may be employed as combined phase
and amplitude-adjustment circuits.
[0085] A graphical analysis of different types of cancellation is
shown in FIG. 5. This graph shows three plots of cancellation (in
decibels) of a combined output of two pickups at an output of a
combining circuit For example, one or more plots in FIG. 5 may show
the voltage magnitude V.sub.out of combining circuit 48 shown in
FIG. 4A, divided by the voltage magnitude of the pickup signal at
one of the inputs to the combining circuit 48. The graph in FIG. 5
shows cancellation plotted relative to signal frequency (Hz).
[0086] Plot 1 of FIG. 5 represents cancellation obtained by a
circuit that does not compensate for phase-differences between two
pickup signals, such as the prior-art circuit shown in FIG. 1. Plot
2 of FIG. 5 illustrates "notch-cancellation" as explained above
with reference to the cancellation circuit shown in FIG. 3C. The
frequency at which the notch occurs can be changed by adjusting the
phase-adjustment circuits 35 and 37. Plot 3 of FIG. 5 represents
cancellation obtained by a cancellation circuit (such as the
cancellation circuit shown in FIG. 4A) that provides substantial
cancellation of external magnetic flux over a relatively broad
range of frequency. This curve illustrates a very broad notch
centered at a notch frequency .omega..sub.n. At frequencies below
and above the notch frequency .omega..sub.n, the degree of
cancellation begins to diminish. In the case where one uses a
cancellation circuit, such as the one shown in FIG. 4A, it is
possible to improve the cancellation at frequencies below the notch
frequency .omega..sub.n by adjusting the resistance in either or
both series elements 45 and 47 so that R.sub.1 better approximates
the value of R.sub.2. Thus, Z.sub.1 better approximates Z.sub.2 at
low frequencies. It is also possible to improve the cancellation at
frequencies above the notch frequency .omega..sub.n by adjusting
the inductance in either or both series elements 45 and 47 so that
L.sub.1 better approximates the value of L.sub.2. Thus, Z.sub.1
better approximates Z.sub.2 at high frequencies. Furthermore, the
overall level of cancellation over the entire frequency range may
be improved by adjusting the values of resistors R.sub.3 and
R.sub.4 such that the values of these resistances better
approximate each other. It will be appreciated that additional
phase circuits (not shown) may be used to provide compensating
phase-shifts at low and/or high frequencies in order to broaden the
notch centered at the notch frequency .omega..sub.n.
[0087] FIG. 6 shows a cancellation circuit of the present invention
that includes two pickup coils 60 and 62, phase-adjustment circuits
65 and 67, amplitude-adjustment circuits 64 and 66, a combining
circuit 68, a preamplifier 74, a power amplifier 76, and a drive
coil 70. The pickup coils 60 and 62 and/or the drive coil 70 may
also include one or more ferromagnetic cores (not shown).
[0088] Pickup signals are induced in the pickup coils 60 and 62 by
external magnetic flux. In this case, the external magnetic flux is
generated by the drive coil 70. The phase of the pickup signal from
the first pickup 60 is adjusted by the phase-adjustment circuit 65.
The phase of the pickup signal from the second pickup 62 is
adjusted by the phase-adjustment circuit 67 such that the phases of
the two pickup signals are substantially in phase (0 degrees) or
out of phase (180 degrees) for a broad range of signal frequencies.
The amplitude of the first pickup signal is adjusted by the
amplitude-adjustment circuit 64. The amplitude of the second pickup
signal may be adjusted by the amplitude-adjustment circuit 66.
However, the amplitude-adjustment circuit 66 may act only as a
buffer and provide no amplitude adjustment to the second pickup
signal. It will be appreciated that the amplitude-adjustment
circuits 64 and 66 may provide either gain or attenuation to the
pickup signals. It will also be appreciated that the
amplitude-adjustment circuits 64 and 66 may be replaced by a means
for adjusting the position and/or orientation of the pickup coils
in order to provide adjustment to the amplitude of the pickup
signals induced in either or both pickup coils 60 and 62. The
outputs of both amplitude-adjustment circuits 64 and 66 are
received by a combining circuit 68 that combines the pickup signals
such that the pickup signals induced by the external magnetic flux
generated by the driver 70 substantially cancel. The output of the
combining circuit 68 is amplified by a preamplifier 74. The output
of the preamplifier 74 is amplified into a drive signal by a power
amplifier 76. The drive signal is an amplified pickup response of
pickup coils 60 and 62 to an external magnetic flux other than the
external magnetic flux generated by the drive coil 70. This other
external magnetic flux may be from an electromagnetic source, such
as another drive coil (not shown), or may be caused by the motion
of a ferromagnetic element (not shown) in a magnetic field. The
drive signal flows through the drive coil 70 and generates an
external magnetic flux that cancels the magnetic flux generated by
the other magnetic source (not shown). The drive coil 70 may
generate a magnetic flux in response to a magnetic flux caused by
motion of a ferromagnetic element (not shown). The drive coil may
either drive or damp the motion of the ferromagnetic element (not
shown).
[0089] For the circuit shown in FIG. 6, it is necessary that a high
degree of cancellation is obtained for a broad range of
frequencies, else the circuit will undergo oscillation due to
direct magnetic feedback. In general, a circuit will oscillate at a
frequency at which the feedback gain is positive (i.e., when the
circuit gain exceeds the circuit losses). If the circuit in FIG. 6
achieves a cancellation profile similar to plot 3 in FIG. 5, the
circuit may oscillate at a relatively high frequency where the
cancellation is not as effective. However, a low-pass filter may be
included in the feedback circuit to reduce the feedback gain of the
circuit. For example, preamplifier 74 may comprise an active
low-pass filter. Likewise, one or more high-pass or bandpass
filters may be used to eliminate circuit oscillation. It is also
possible that the phase-adjustment circuits 65 and 67 and/or
amplitude-adjustment circuits 64 and 66 of the cancellation circuit
may be designed specifically to filter certain frequencies.
[0090] The phase-adjustment circuits 65 and 67 are designed
specifically for compensating for frequency-dependent
phase-variations between the pickup signals from the pickups 60 and
62. However, the phase-adjustment circuits 65 and 67 may provide an
overall phase-shift to the combined pickup signal at the output of
the combining circuit 68. This overall phase-shift may compensate
for the phase shift introduced to the drive signal as a result of
the frequency responses of the pickups 60 and 62 and the driver 70.
The phase-adjustment circuits 65 and 67 may also be used to
compensate for phase shifts in the circuit caused by other circuit
elements (not shown) that may precede the drive coil 70. The
phase-adjustment circuits 65 and 67, are preferably preceded by a
buffer (not shown), and may precede or follow the
amplitude-adjustment circuits 64 and 66.
[0091] An embodiment for a cancellation circuit of the present
invention is shown in FIG. 7. The circuit in FIG. 7 includes a
magnetic element 81, a ferromagnetic core 83, two pickup coils 80
and 82 wrapped around the core 83, and a phase-adjustment circuit
85. The input to the phase-adjustment circuit is shown connected to
electrical ground by a resistor R.sub.3. The output of the
phase-adjustment circuit is shown connected to the input of an
amplitude-adjustment circuit 84. The pickup coil 82 is shown
connected to the input of an amplitude-adjustment circuit 86. This
input also includes a resistor R.sub.4 connected to electrical
ground. The outputs of each amplitude-adjustment circuit 84 and 86
are connected to a combining circuit 88. The output of the
combining circuit 88 is connected to a compensation circuit 89. The
compensation circuit 89 is connected to an amplifier 96 that
amplifies an input signal into a drive signal that flows through a
drive coil 90 wrapped around a ferromagnetic core 93.
[0092] The pickup coils 80 and 82 are shown approximately
equidistant to the drive coil 90, which generates an external
magnetic flux F. The pickup coils 80 and 82 receive approximately
equal intensities of the external magnetic flux F generated by the
drive coil 90. Amplitude adjustment of the pickup signals induced
in the pickup coils 80 and 82 by the drive coil's 90 generated
external magnetic flux F compensates for differences that may occur
between the pickup signals, such as differences in the intensities
of the drive coil's 90 generated magnetic flux F at the location of
each pickup coil 80 and 82, distortions in the drive coil's 90
generated magnetic flux F resulting from nearby conducting or
magnetically permeable materials (such as ferromagnetic element 95)
and differences in the amplitude-responses (including
frequency-dependent amplitude responses) of the pickup coils 80 and
82 to magnetic flux. Likewise, the pickup signals induced in the
pickup coils 80 and 82 by uniform external magnetic flux and
magnetic flux generated by other magnetic sources (not shown)
disposed in the plane that is the perpendicular bisector of the
height dimension of the drive coil 90 are also substantially equal
in amplitude. Thus, after phase-shifting by the phase-adjustment
circuit 85 and combining by the combining circuit 88, the signals
induced in the pickup coils 80 and 82 by uniform external magnetic
flux, the magnetic flux F generated by the drive coil 90, and
magnetic flux generated by any other magnetic sources (not shown)
disposed in the plane that is the perpendicular bisector of the
height dimension of the drive coil 90 cancel.
[0093] The cancellation of the effects of the drive coil's 90
generated magnetic flux F on the combined pickup signal is altered
if either a permeable or conducting object enters the space shared
by the field patterns F of the drive coil 90 and the pickup coils
80 and 82. If the intruding object is permeable, the field pattern
F surrounding the pickup coils 80 and 82 is distorted, and energy
passes directly from the drive coil 90 to the pickup coils 80 and
82 through the distorted field. Thus if the ferromagnetic element
95 vibrates, the frequency of its motion is reproduced in the
combined pickup signal. Because of the cancellation of external
magnetic flux, the output of the combining circuit 88 comprises
only the pickup signals induced by the motion of the ferromagnetic
element 95.
[0094] The compensation circuit 89 provides a phase-shift to the
output signal of the combining circuit 88 in order to compensate
for the frequency-dependent phase variations between the magnetic
flux F applied to the ferromagnetic element 95 and the response of
the pickups 80 and 82 to the motion of the ferromagnetic element
95. The output signal of the phase-adjustment circuit 88 is
amplified by the amplifier 96 into a drive signal which flows
through the drive coil 90 and generates the magnetic flux F. This
magnetic flux F may be used to either drive or damp the motion of
the ferromagnetic element 95 depending on the phase of the drive
signal. It is preferable to provide drive forces to the
ferromagnetic element 95 such that the phase-relationship of the
drive force to the motion of the ferromagnetic element 95 is not
changed by the frequency of the element's 95 motion. This is
particularly important if the motion of the ferromagnetic element
95 comprises a plurality of different frequencies.
[0095] For the circuit shown in FIG. 7, the signals from the pickup
coils 80 and 82, after being combined, may have a total phase-shift
of
.O slashed..sub.p=ArcTan(-.omega.L.sub.2/(R.sub.4+R.sub.2)).
[0096] The phase-shift of the drive signal at the drive coil 90
is
.O slashed..sub.d=ArcTan(.omega.L.sub.d/R.sub.d),
[0097] where R.sub.d and L.sub.d are the resistance and inductance,
respectively, of the drive coil 90. Ignoring any phase-shifts
caused by other elements in the circuit, the total phase-shift
between the magnetic flux F generated by the drive coil 90 and the
response of the pickups 80 and 82 to the magnetic flux F generated
by the drive coil 90 is .O slashed..sub.t=.O slashed..sub.p+.O
slashed..sub.d. By adjusting the values of resistances R.sub.2,
R.sub.4, and R.sub.d, and/or the values of inductances L.sub.2 and
L.sub.p, or combinations thereof, it is possible to cause the
ratios L.sub.2/(R.sub.4+R.sub.2) and L.sub.d/R.sub.d be
substantially equal. Thus, the value of .O slashed..sub.t can be
made substantially zero for a broad range of signal frequencies,
.omega..
[0098] The circuits shown in FIG. 8A and FIG. 8B may be used in a
cancellation circuit as phase-adjustment circuits (such as
phase-adjustment circuit 85) for providing phase-shifts to pickup
signals before they are combined. The circuits shown in FIG. 8A
and/or FIG. 8B may be used in the feedback loop of FIG. 7 as a
phase-compensation circuit, such as phase-compensation circuit 89.
The circuit shown in FIG. 8A is a inverting amplifier. A buffer
amplifier 94 may precede the first impedance element Z.sub.11. The
second impedance element Z.sub.12 provides feedback between the
output and inverting input of amplifier 98. The gain resulting from
this inverting amplifier is G.sub.1=Z.sub.12/Z.sub.11. Thus the
ratio Z.sub.12/Z.sub.11 may be adjusted to compensate for
phase-shifts. The circuit shown in FIG. 8B is a non-inverting
amplifier comprising an amplifier 99 and gain-control impedance
values Z.sub.13 and Z.sub.14. The gain of the non-inverting
amplifier is G.sub.1=1+Z.sub.14/Z.sub.13. Likewise, the ratio
Z.sub.14/Z.sub.13 may be adjusted to compensate for
phase-shifts.
[0099] The circuits shown in FIG. 8A and FIG. 8B also provide a
means for compensating for frequency-dependent amplitude variations
in the feedback signal caused by the frequency response of the
pickup coils 80 and 82, the drive coil 90, and any other circuit
elements in the feedback loop. For example, the circuit shown in
FIG. 7 will produce a pickup signal voltage
V.sub.p=V.sub.BR/(R.sub.2+R.sub.4+i.omega.L.sub.2) at the input of
amplifier 86, where V.sub.B is the voltage induced in the pickup
coil 82 by external magnetic flux B. The voltage V.sub.B is
proportional to the magnitude of the magnetic flux B. The magnitude
of magnetic flux B is proportional to the amplitude of drive
current I.sub.d in the drive coil 90, where
I.sub.d=V.sub.d/(R.sub.d+i.omega.L.sub.d) and V.sub.D is the drive
voltage. Thus, the pickup signal voltage V.sub.p is proportional to
V.sub.BR/(R.sub.2+R.sub.4+i.omega.L.sub.2)
(R.sub.d+i.omega.L.sub.d). One can observe from the equation for
V.sub.p that as .omega. increases, the pickup signal voltage
V.sub.p decreases.
[0100] An example of a circuit design that can compensate for the
frequency-dependent amplitude variation of the pickup signal
voltage V.sub.p includes two inverting amplifiers (as shown in FIG.
8A) connected in series. The impedance element Z.sub.12 of the
first amplifier 98A comprises a resistor R.sub.12 (not shown) and
inductor L.sub.12 (not shown) connected in series. The value of the
resistor R.sub.12 is (R.sub.2+R.sub.4) and the value of the
inductor L.sub.12 is L.sub.2. The value of the impedance element
Z.sub.11 of the first amplifier 98A is .alpha.R.sub.4, where
.alpha. is a scalar constant. The impedance element Z.sub.12 of the
second amplifier 98B comprises a resistor R.sub.22 (not shown) and
inductor L.sub.22 (not shown) connected in series. The value of the
resistor R.sub.22 is R.sub.d and the value of the inductor L.sub.22
is L.sub.d. The value of the impedance element Z.sub.21 of the
second amplifier is .alpha.R.sub.4. The gain G.sub.1 of the first
amplifier is (R.sub.2+R.sub.4+i.omega.L.sub.2)/.alpha.R.sub.4. The
gain G.sub.2 of the second amplifier is
(R+i.omega.L.sub.d)/.alpha.R.sub.4. The total gain of this circuit
is G.sub.t=G.sub.1G.sub.2. The gain G.sub.t multiplies the pickup
voltage V.sub.p so that the frequency-dependent nature of the
pickup voltage amplitude is compensated. It should be noted that
the values of R.sub.2, R.sub.4, and R.sub.d may be increased to
reduce the frequency-dependent effects on the pickup signal voltage
V.sub.p. However increasing the value of R.sub.d substantially
reduces the magnitude of magnetic flux generated by the drive coil
90.
[0101] The frequency-dependent phase-shifts and amplitude
variations that typically occur between a pickup coil and drive
coil may be substantially compensated over a broad range of signal
frequencies .omega. via the selection of the values for electrical
components in the compensation circuits that adjust both amplitude
and phase-response. Typically, for a feedback system (such as shown
in FIG. 7) in which the motion of a ferromagnetic element is driven
by an external magnetic flux generated by the drive coil 90, the
frequency-dependence of the phase of the drive signal is of more
interest than the frequency-dependence of the amplitude of the
drive signal. However, for a feedback system in which a drive coil
generates a specific magnetic flux in response to an external
magnetic flux, such as in order to cancel an external magnetic flux
in a specific region of space, it is important to control both the
phase and amplitude of the drive signal.
[0102] Another embodiment for a cancellation circuit of the present
invention is shown in FIG. 9. The circuit in FIG. 9 includes a
pickup coil 104 wrapped around a pickup core 105. The pickup core
105 may be made of a ferromagnetic material, and the core 105 may
be magnetized. The pickup coil 104 is connected to a compensation
circuit 106 that is connected to a splitting circuit 108. The
splitting circuit 108 has a first output connected to a first
phase-adjustment circuit 109 and a second output connected to a
second phase-adjustment circuit 110. The first phase-adjustment
circuit 109 is connected to the input of a first amplifier 111 and
the second phase-adjustment circuit 110 is connected to the input
of a second amplifier 112. The output of the first amplifier 111 is
connected to a first drive coil 100, and the output of the second
amplifier 112 is connected to a second drive coil 102. Both the
first and second drive coils 100 and 102, respectively, are wrapped
around a drive core 101, and they generate a magnetic flux F. The
drive core 101 may be made of a ferromagnetic material, and it may
be magnetized. In this case, the drive core 101 is shaped so that
both of its endpoles are in close proximity to a ferromagnetic
element 115. The ferromagnetic element 115 induces a current in the
pickup coil 104 when its motion disturbs the distribution of
magnetic flux F that passes through the pickup coil 104. The shape
of the drive core 101 concentrates the magnetic flux lines F
generated by electrical current in the drive coils 100 and 102 so
as to provide a more efficient magnetic drive force to the
ferromagnetic element 115.
[0103] A first electrical pickup signal V.sub.D1 is induced in the
pickup coil 104 by magnetic flux generated by the first drive coil
100, and a second pickup signal V.sub.D2 is induced in the pickup
coil 104 by magnetic flux generated by the second drive coil 102.
An electrical pickup signal V.sub.Pickup is induced in the pickup
coil 104 by magnetic flux produced by other sources, such as the
ferromagnetic element 115 moving through a static magnetic field.
The electrical signals induced in the pickup coil 104 pass through
the compensation circuit 106 to the splitting circuit 108, which
splits the pickup signal into two drive signals. One of the drive
signals passes through the first phase-adjustment circuit 109 and
is amplified by the first amplifier 111. The other drive signal
passes through the second phase-adjustment circuit 110 and is
amplified by the second amplifier 112. The first drive coil 100 has
an effective resistance of R.sub.D1 and an effective inductance
L.sub.D1, which results in a total impedance of
Z.sub.1=R.sub.D1+i.omega.- L.sub.D1, where .omega. is the frequency
of the drive signal. The second drive coil 102 has an effective
resistance of R.sub.D2 and an effective inductance L.sub.D2
resulting in a total impedance of
Z.sub.2=R.sub.D2+i.omega.L.sub.D2. Because the impedance values
Z.sub.1 and Z.sub.2 of drive coils 100 and 102, respectively, tend
to differ from each other, a drive signal flowing through the first
drive coil 100 having the same frequency .omega. as a drive signal
flowing through the second drive coil 102 tends to differ in phase
and amplitude from the second drive signal.
[0104] The phase-adjustment circuits 109 and 110 compensate for
frequency-dependent phase differences between the first and second
drive signals. The amplifiers 111 and 112 may provide amplitude
adjustment to either or both of the drive signals to compensate for
amplitude differences between the signals. The phase adjustment and
amplitude adjustment is performed such that the signals V.sub.D1
and V.sub.D2 induced in the pickup coil 104 by the first and second
drive coils 100 and 102, respectively, are substantially equal in
magnitude and 180 degrees out of phase so they cancel. It will be
appreciated that the splitting circuit 108 may be used to adjust
the relative magnitudes of the drive signals in the drive coils 100
and 102. It will also be appreciated that the phase-adjustment
circuits 109 and 110 may be positioned so that they each follow the
amplifiers 111 and 112, respectively. Because phase adjustment and
amplitude adjustment need only be applied to one of the two drive
signals, this allows for removal of one of the phase-adjustment
circuits 109 or 110.
[0105] Another method of amplitude and phase adjustment involves
changing the effective resistance R.sub.D1 and R.sub.D2 and/or the
effective inductance L.sub.D1 and L.sub.D2 of either or both drive
coils 100 and 102. Therefore, the phase-adjustment circuits 109 and
110 follow the amplifiers 111 and 112, respectively. The
phase-adjustment circuits 109 and 110 may comprise resistors (not
shown) and/or inductors (not shown) connected in series with the
drive coils 100 and 102 so as to adjust their effective resistance
R.sub.D1 and R.sub.D2 and/or effective inductance L.sub.D1 and
L.sub.D2. In order for the relative phase between the magnetic flux
generated by each drive coil 100 and 102 to be substantially 180
degrees, the relationship ArcTan(.omega.L.sub.D1/R.sub.-
D1)=ArcTan(.omega.L.sub.D2/R.sub.D2) must hold for a wide range of
signal frequencies .omega.. Thus,
L.sub.D1/R.sub.D1=L.sub.D2/R.sub.D2. However, in order for the
relative amplitudes between the magnetic flux generated by each
drive coil 100 and 102 to be substantially equal for a broad range
of signal frequencies .omega., it is preferable that the effective
resistance R.sub.D1=R.sub.D2 and the effective inductance
L.sub.D1=L.sub.D2. It will be appreciated that because the
effective resistance R.sub.D1 and R.sub.D2 and the effective
inductance L.sub.D1 and L.sub.D2 can be adjusted to control both
the phase and amplitude of the drive signals in the drive coils 100
and 102, only a single amplifier (not shown) for amplifying a
pickup signal into a drive signal is necessary. An alternative
cancellation circuit may include a single amplifier circuit (not
shown) placed between the compensation circuit 106 and the
splitting circuit 108, and the amplifiers 111 and 112 may be
removed. The phase-adjustment circuits 109 and 110 can provide both
amplitude and phase-adjustment to the drive signals going to each
drive coil 100 and 102, as described above.
[0106] An embodiment for a cancellation circuit of the present
invention is shown in FIG. 10. The circuit in FIG. 10 includes a
first pickup coil 120 wrapped around a pickup core 121, a second
pickup coil 122 wrapped around a second core 131, and a drive coil
130 wrapped around the second core 131. The first pickup coil 120
is connected to the input of an amplitude-adjustment circuit 124.
The output of the amplitude-adjustment circuit 124 is connected to
the input of a phase-adjustment circuit 125. The second pickup coil
122 is connected to the input of an amplitude-adjustment circuit
126. The output of the amplitude-adjustment circuit 126 is
connected to a phase-adjustment circuit 127. The outputs of the
phase-adjustment circuits 125 and 127 are connected to a combining
circuit 128. The output of the combining circuit 128 is connected
to a compensation circuit 129. The compensation circuit 129 is
connected to an amplifier 136 that amplifies the input signal into
a drive signal that flows through the drive coil 130 and generates
a magnetic flux.
[0107] The pickup coils 120 and 122 are responsive to the magnetic
flux generated by the drive coil 130. However, due to the proximity
of the second pickup coil 122 to the drive coil 130, the second
pickup coil 122 receives a greater magnitude of magnetic flux
generated by the drive coil 130 than does the first pickup coil
120. It will be appreciated that the second pickup coil 122 may be
located inside of the drive coil 130, or the pickup coils 120 and
122 may be positioned, shielded or otherwise designed such that the
second pickup coil 122 receives greater magnetic flux generated by
the drive coil 130 than does the first pickup coil 120. The
amplitudes of the pickup responses of the first and second pickup
coils 120 and 122 induced by the magnetic flux generated by the
drive coil 130 are made equivalent by either or both of the
amplitude-adjustment circuits 124 and 126. The phases of the pickup
responses of the first and second pickup coils 120 and 122 induced
by the magnetic flux generated by the drive coil 130 are
compensated by either or both of the phase-adjustment circuits 125
and 127 so that when the pickup signals are combined in the
combining circuit 128, they substantially cancel. However, the
response of the pickup coils 120 and 122 to uniform external
magnetic flux results in a non-zero contribution to the combined
signal at the output of the combining circuit 128. The compensation
circuit 129 may comprise either or both phase-adjustment and
amplitude-adjustment circuits (not shown) for adjusting the phase
response and/or amplitude response of the drive signal. The drive
signal flows through the drive coil 130 and generates a uniform
magnetic flux inside the drive coil 130 that substantially cancels
the uniform magnetic flux inside the drive coil 130 generated by
other sources (not shown).
[0108] The core 121 may be a ferromagnetic core. However,
ferromagnetic materials tend to have a non-linear response to
magnetic flux, resulting in pickup signals comprising
higher-harmonic signals. The core 131 is preferably comprised of a
non-ferromagnetic material having a hollow center. If the core 121
is made of a ferromagnetic material, then it is preferable that the
core 131 be made of a similar ferromagnetic material so that the
non-linear responses of the cores 121 and 131 of the pickups 120
and 122 substantially cancel.
[0109] Because the drive coil 130 generates a very uniform magnetic
flux along its axis it is preferable that the region of space in
which cancellation of magnetic flux is desired be surrounded by the
drive coil 130. However, it will be appreciated that the region of
space in which cancellation is desired may be external to the drive
coil 130. It will also be appreciated that the second pickup coil
122 could be wrapped around the core 131 without being interwoven
with the drive coil 130, as shown. It will also be appreciated that
these methods for canceling magnetic flux may be used along with a
device that generates a static magnetic field to cancel external
static magnetic fields.
[0110] Another embodiment for a cancellation circuit of the present
invention is shown in FIG. 11. The circuit in FIG. 11 includes a
first pickup coil 140 wrapped around a core 141, a second pickup
coil 142 wrapped around the core 141, and a drive coil 150 wrapped
around the core 141. The first pickup coil 140 is connected to the
input of a phase-adjustment circuit 145. The output of the
phase-adjustment circuit 145 is connected to the input of an
amplitude-adjustment circuit 144. The second pickup coil 142 is
connected to the input of an amplitude-adjustment circuit 146. It
will be appreciated that either amplitude-adjustment circuit 144 or
146 may act only as a buffer, as amplitude-adjustment of only one
of the pickup coil 140 and 142 outputs may be necessary. The output
of the amplitude-adjustment circuits 144 and 146 are connected to a
combining circuit 148. The output of the combining circuit 148 is
connected to a compensation circuit 149. The compensation circuit
149 is connected to an amplifier 156. The amplifier 156 amplifies
its input signal into a drive signal that flows through the drive
coil 150, and generates a magnetic flux.
[0111] The pickup coils 140 and 142 are responsive to the external
magnetic flux generated by the drive coil 150. The pickup coils 140
and 142 maybe positioned relative to the drive coil 150 as shown in
FIG. 11 such that one of the pickup coils, such as pickup coil 140,
receives a greater amount of the magnetic flux generated by the
drive coil 150 than does the second pickup coil 142. Thus when the
amplitudes and phases of the pickup signals from each of the pickup
coils 140 and 142 are adjusted so that the contributions of
magnetic flux generated by the drive coil 150 cancel at the
combining circuit 148, the combined response of the pickup coils
140 and 142 to uniform external magnetic flux are substantially
non-zero. It will be appreciated that other methods may be used to
adjust the responses of the pickup coils 140 and 142 to external
magnetic flux, such as utilizing different numbers of coil windings
in the pickups 140 and 142, and/or changing the size, shape or
material of the core 141 which the pickup coils 140 and 142 are
wrapped around.
[0112] The compensation circuit 149 may comprise either or both
phase-adjustment circuits (not shown) and amplitude-adjustment
circuits (not shown) for adjusting the phase and/or amplitude
response of the drive signal so that the drive signal has a
specific amplitude and phase relationship to the external magnetic
flux impinging on the pickup coils 140 and 142. The drive signal
flows through the drive coil 150 and generates a uniform magnetic
flux inside the drive coil 150 that cancels the external magnetic
flux inside the drive coil 150.
[0113] It will be appreciated that the circuit shown in FIG. 11 may
be used to drive or damp the motion of a ferromagnetic element (not
shown) that generates a magnetic flux as it moves through a
magnetic field. In this case, the core 141 may be made of a
ferromagnetic material, and it may be shaped so that both endpoles
of the core 141 are in close proximity to the ferromagnetic element
(not shown) for providing a more powerful and concentrated driving
(or damping) force to the ferromagnetic element. To produce a
driving force, the drive coil 150 preferably generates a magnetic
flux that is in phase with the motion of the ferromagnetic element,
increasing in strength as the speed of the ferromagnetic element
toward the core increases. To produce a damping force, the drive
coil 150 preferably generates a magnetic flux that is out of phase,
hence opposing the motion of the ferromagnetic element.
[0114] An embodiment for a cancellation circuit of the present
invention is shown in FIG. 12. The circuit in FIG. 12 includes a
pickup coil 160 wrapped around a pickup core 161, a signal
generator 162, and a drive coil 170 wrapped around a second core
171. The pickup coil 160 is connected to the input of an
amplitude-adjustment circuit 164. The signal generator 162 provides
a signal to an amplifier 176 that amplifies the signal to produce a
drive signal. The drive signal flows through the drive coil 170 and
generates a magnetic flux. The signal generator 162 is connected to
the input of a phase-adjustment circuit 167. The output of the
phase-adjustment circuit 167 is connected to an
amplitude-adjustment circuit 166. The outputs of the
amplitude-adjustment circuits 164 and 166 are connected to a
combining circuit 168. The output of the combining circuit 168
provides a pickup signal that is substantially free from the
response of the pickup coil 160 to the magnetic flux generated by
the drive coil 170.
[0115] The cancellation circuit shown in FIG. 12 demonstrates that
in order to provide a pickup with a cancellation signal, it is not
necessary to have a second pickup device. In fact, any electrical
representation of a drive signal that has the proper phase and
amplitude characteristics may be used to cancel the response of the
pickup to external magnetic flux generated by that drive signal. In
this case, the waveform of the drive signal is generated by a
signal generator 162. The output of the signal generator 162 is
adjusted by a phase-adjustment circuit 167 and an
amplitude-adjustment circuit 166 before it is combined with the
output of the pickup coil 160. An amplitude-adjustment circuit 164
is shown connected to the output of the pickup coil 160 with a
resistor R connected to electrical ground. It will be appreciated
that either amplitude-adjustment circuit 164 or 166 may act only as
a buffer, as amplitude-adjustment of only one of the signals from
either the pickup coil 160 or the signal generator 162 is
necessary. It will also be appreciated that the output signal
V.sub.out of the combining circuit 168 may be supplied to an input
of the signal generator 162 to generate signals or control the
frequency and amplitude of the generated signals output to the
amplifier 176.
[0116] The magnetic flux generated by the drive coil 170 induces a
voltage V.sub.B in the pickup coil 160 that is proportional to the
magnitude of the magnetic flux. The magnitude of the magnetic flux
generated by the drive coil 170 is proportional to the drive
current I.sub.D in the drive coil 170. The drive current is
I.sub.D=V.sub.D/(R.sub.D+i.omega.L.sub.D), where R.sub.D and
L.sub.D are the effective resistance and inductance, respectively,
of the drive coil 170, and V.sub.D is the drive voltage. The drive
voltage is V.sub.D=G V.sub.O, where G is the gain of the amplifier
176 and V.sub.O is the signal voltage produced by the signal
generator 162. The voltage V.sub.P of the pickup coil 160 at the
input of the amplitude-adjustment circuit 164 is
V.sub.P=V.sub.BR/(R+R.sub.P+i.ome- ga.L.sub.P)=B
V.sub.OR/(R+R.sub.P+i.omega.L.sub.P)(R.sub.D+i.omega.L.sub.D- ),
where R.sub.P and L.sub.P are the effective resistance and
inductance, respectively, of the pickup coil 160, and B is a
proportionality constant that represents the contribution of gains
and losses in the circuit. The signal voltage at the input of the
combining circuit 168 connected to the output of the
amplitude-adjustment circuit 164 is
V.sub.CP=AV.sub.P=AB
V.sub.OR/(R+R.sub.P+i.omega.L.sub.P)(R.sub.D+i.omega.-
L.sub.D),
[0117] where A is the gain of the amplitude-adjustment circuit 164.
In the case where the combining circuit 168 is a differential
amplifier circuit, it is desirable that the input signal, V.sub.CO,
from the output of the amplitude-adjustment circuit 166 be
substantially identical to V.sub.CP in order for cancellation to
occur.
[0118] One possible design for a circuit that may be used as the
phase-adjustment and amplitude-adjustment circuits 167 and 166,
respectively, includes two inverting amplifier circuits connected
in series, as shown in FIG. 8C. The impedance element Z.sub.11 of
the first inverting amplifier circuit may comprise a resistor (not
shown) having the value (R+R.sub.P) and an inductor (not shown)
having the value L.sub.P, the resistor and inductor being connected
in series. The impedance element Z.sub.11 of the second inverting
amplifier circuit may comprise a resistor (not shown) having the
value R.sub.D and an inductor (not shown) having the value L.sub.D,
the resistor and inductor being connected in series. The impedance
element Z.sub.12 of the first inverting amplifier 98A, and the
impedance element Z.sub.11 of the second inverting amplifier 98B
may have values such that their product equals the value: AB R. Any
buffer amplifiers, such as amplifier 94, may provide unity gain.
Thus the input signal V.sub.CO into the combining circuit 168 is
substantially identical to the other input signal V.sub.CP, and
thus cancel. It will be appreciated that the example shown
illustrates only one of many designs for combined phase and
amplitude-adjustment circuits that may be used as amplitude and
phase-adjustment circuits 166 and 167, respectively. Furthermore,
it will also be appreciated that either or both phase and amplitude
adjustment may be performed on the signal from the pickup coil 160,
such as pickup signal V.sub.P, in addition to, or instead of the
phase adjustment and amplitude adjustment performed on the signal
V.sub.O generated by the signal generator 162. For example, the
amplitude-adjustment 164 circuit may be preceded or followed by, or
comprise a phase-adjustment circuit (not shown) that adjusts the
phase of the signal from the pickup coil 160.
[0119] Another embodiment for a cancellation circuit of the present
invention is shown in FIG. 13. The receiver and transmitter
elements in FIG. 13 include a pickup coil 180 wrapped around a
pickup core 181 and a drive coil 190 wrapped around a second core
191. The pickup coil 180 is connected to the input of an
amplitude-adjustment circuit 184 at Terminal A. An amplifier 196
generates a drive signal at Terminal B that flows through the drive
coil 190 and generates a magnetic flux. The output of the amplifier
196 is connected to the input of a phase-adjustment circuit 187.
Preferably, there is some sort of buffer (not shown) as part of the
phase-adjustment circuit 187. For example, the buffer (not shown)
may include a large value of resistance that forms a voltage
divider with the drive coil 190 and attenuates the input signal to
the phase-adjustment circuit 187. The output of the
phase-adjustment circuit 187 is connected to an
amplitude-adjustment circuit 186. The outputs of the
amplitude-adjustment circuits 184 and 186 are connected to a
combining circuit 188. The output of the combining circuit 188
provides a pickup signal at the input of the amplifier 196
(Terminal C) that is substantially free from the response of the
pickup coil 180 to the magnetic flux generated by the drive coil
190.
[0120] The cancellation circuit shown in FIG. 13 demonstrates that
a cancellation signal can be generated without requiring a second
pickup device. Part of a drive signal used to generate an external
magnetic flux may be combined with a pickup signal to cancel the
response of a pickup device to the external magnetic flux. The
signal V.sub.A at Terminal A represents a pickup signal
.beta.V.sub.D induced by the magnetic flux generated by the drive
coil 190, where V.sub.D is the voltage of the drive signal flowing
through the drive coil 190 and B is a scaling factor that
represents losses between the magnitude of the drive signal V.sub.D
in the drive coil 190 and the magnitude of the pickup coil's 180
response to that drive signal V.sub.D after being amplified by the
amplitude-adjustment circuit 184. The signal V.sub.A at Terminal A
also includes an additional pickup signal V.sub.Pickup induced by
magnetic flux generated by other sources other than the drive coil
190. Thus, V.sub.A=.beta.V.sub.D+V.sub.Pickup. The signal at
Terminal B is V.sub.B=V.sub.D+V.sub.DPickup, where V.sub.DPickup is
a signal that the drive coil 190 picks up due to external sources
of magnetic flux. If V.sub.D is much greater than V.sub.DPickup,
V.sub.DPickup can be ignored. Thus, V.sub.B.apprxeq.V.sub.D. The
signal V.sub.B is attenuated by a factor of .beta. and its phase is
adjusted in a feedback loop comprising amplitude-adjustment and
phase-adjustment circuits 186 and 187, respectively. Signal V.sub.B
is combined at the combining circuit 188 with the pickup signal
V.sub.A=.beta.V.sub.D+V.sub.Pickup from Terminal A. The resulting
output of the combining circuit 188 (Terminal C) is
V.sub.C=V.sub.Pickup. Thus, feedback resulting from the pickup
coil's 180 response to the magnetic flux generated by the drive
coil 190 is substantially eliminated.
[0121] A cancellation circuit of the present invention is shown in
FIG. 14A. A signal generator 202 generates an electrical signal
that is amplified by an amplifier 203 to produce a drive signal at
Terminal A. The drive signal flows through a drive coil 200 and
generates a magnetic flux. The drive coil 200 may be wrapped around
a core, such as core 201. Terminal A is connected to an
amplitude-adjustment circuit 204, which preferably draws only a
small portion of the drive signal. Thus, the input of the
amplitude-adjustment circuit 204 may include a buffer (not shown),
such as a high-value resistor. The output of the
amplitude-adjustment circuit 204 is connected to the input of a
phase-adjustment circuit 205. The signal generator 202 has a second
output (Terminal B) that produces a signal similar to the signal
input to the amplifier 203. However, the second output signal may
differ in phase and/or amplitude from the signal input to the
amplifier 203. The second output of the signal generator 202 may be
an output that is split from the input of the amplifier 203 by a
splitting circuit (not shown). Terminal B is connected to the input
of a harmonic-compensation circuit 210. The output of the
harmonic-compensation circuit 210 is connected to Terminal C, which
is connected to an amplitude-adjustment circuit 206. The output of
the amplitude-adjustment circuit 206 is connected to the input of a
phase-adjustment circuit 207. The outputs of both phase-adjustment
circuits 205 and 207 are connected to separate inputs of a
combining circuit 208. The combining circuit 208 produces an output
signal V.sub.out that results from cancellation of the drive
signals generated by the signal generator 202.
[0122] The drive coil 200 is responsive to external magnetic flux,
even while a drive signal is flowing through the coil 200. If the
drive coil 200 is not in close proximity to materials that have a
non-linear response to external magnetic flux, then higher harmonic
effects in the drive signal and/or pickup signal of the drive coil
200 are substantially negligible. Consequently, the harmonic
compensation circuit 210 may be replaced by a short circuit
connecting Terminal B to Terminal C. The signal voltage at Terminal
A is V.sub.A=V.sub.D+V.sub.Pickup, where V.sub.D is the voltage of
the drive signal and V.sub.Pickup is the voltage of the induced
pickup signal in the drive coil 200 resulting from external
magnetic flux. The signal voltage at Terminal B is: V.sub.B=Q
V.sub.D, where Q is a proportionality constant. Because the drive
coil 200 has a complex impedance Z.sub.D=R.sub.D+i.omega.L.sub.D,
and other circuit elements (not shown) associated with the drive
coil 200 may also contribute frequency-dependent terms to the
effective impedance of the coil 200, it is necessary that phase
adjustment be performed to compensate for phase variations between
the drive components V.sub.D of the signal voltages V.sub.A and
V.sub.B at Terminals A and B, respectively. After phase adjustment
and amplitude adjustment have been performed on either or both of
the signals V.sub.A and V.sub.B, these signals are combined in the
combining circuit 208 such that the signal components related to
the drive voltage V.sub.D cancel, leaving only a signal that is
related to the pickup signal V.sub.Pickup. This method of
cancellation allows a single element, such as the drive coil 200
shown in FIG. 14A, to simultaneously transmit and receive
electromagnetic signals. It will be appreciated that the output
signal V.sub.out may include an interface (not shown) to the signal
generator 202 to control the amplitude, frequency, and phase of the
signal generated by the signal generator 202.
[0123] It will be further appreciated that if the core 201 is made
of a ferromagnetic material, a voltage V.sub.H resulting from the
non-linear response of that material to the magnetic flux generated
by the drive coil 200 is included in the signal voltage V.sub.A at
Terminal A: V.sub.A=V.sub.D+V.sub.Pickup+V.sub.H. Such nonlinearity
creates even harmonics V.sub.H in the fundamental driving frequency
V.sub.D that are not reducible to zero by prior-art cancellation
techniques. Thus, it may be necessary to eliminate the signal
voltage V.sub.H resulting from the non-linear response of the core
201 material by providing the circuit with a filter (not shown) to
filter out harmonic effects. The circuit may include a
harmonic-compensation circuit, such as harmonic compensation
circuit 210.
[0124] FIG. 14B shows a circuit that provides harmonic
compensation. The circuit in FIG. 14B includes a coil 212 wrapped
around a core 211 that has a substantially identical non-linear
response to magnetic flux as does the core 201 shown in FIG. 14A.
The coil 212 may be oriented with respect to magnetic flux
generated by the coil 200 such that the electrical signals induced
in the coil 212 by the magnetic flux substantially cancel. The coil
212 may be provided with active magnetic shielding such as a
cancellation circuit (not shown), or magnetic shielding materials
(not shown) for substantially reducing the response of the coil 212
to magnetic flux generated by external magnetic sources (not
shown). The material of the core 211 responds to the magnetic flux
generated by the signal output V.sub.B of the signal generator 202
at terminal B and substantially reproduces the non-linear effect in
the signal V.sub.B that is used to cancel the signal V.sub.A.
[0125] FIG. 14C shows a circuit that may be used as the harmonic
compensation circuit 210. The circuit in FIG. 14C includes a
splitting circuit 213 for splitting the input signal from Terminal
B into two signals at output Terminals B1 and B2. The output
terminal (Terminal B2) is connected to an input terminal (Terminal
C2) of a combining circuit 223. Terminal B1 is connected to a
harmonic-generator circuit 214 that substantially reproduces the
shape of the harmonic signal V.sub.H in the signal voltage V.sub.A
at Terminal A. The harmonic-generator circuit 214 may include one
or more harmonic-generator circuits (not shown) known to persons
skilled in the art as "frequency-doublers" or "frequency-triplers."
The output of the harmonic-generator circuit 214 is connected to a
phase-adjustment circuit 218. The phase-adjustment circuit 218 is
connected to an input of an amplitude-adjustment circuit 219. The
output of the amplitude-adjustment circuit 219 is connected to an
input terminal (Terminal C1) of the combining circuit 223. A signal
that is proportional to the drive signal V.sub.D is input to the
combining circuit 223 at Terminal C2. A signal that is proportional
to the harmonic signal V.sub.H is adjusted in phase and/or
amplitude by the phase-adjustment circuit 218 and the
amplitude-adjustment circuit 219 before being input to the
combining circuit 223 at Terminal C1. The signal voltage V.sub.C at
the output (Terminal C) of the combining circuit 223 has amplitude
and phase relationships between signals V.sub.D and V.sub.H such
that when that signal V.sub.C is combined with the
amplitude-adjusted, phase-adjusted signal V.sub.A at the combining
circuit 218, the contributions of the harmonic terms V.sub.H and
the drive signal terms V.sub.D substantially cancel.
[0126] FIG. 14D shows a circuit that may be used as the harmonic
compensation circuit 210. The circuit in FIG. 14D includes a
splitting circuit 213 for splitting the input signal from Terminal
B into three signals at output terminals B1, B2, and B3. Terminal
B3 is connected to an input terminal (Terminal C3) of the combining
circuit 223. Terminal B2 is connected to the input of an
amplitude-adjustment circuit 216. Terminal B1 is connected to a
coil 212 that is wrapped around a core 211. The core 211 is made of
a material that has a non-linear response to magnetic flux that is
substantially identical to the non-linear response of the core 201
material. Thus, the output of the coil 212 comprises a signal
voltage having a component that is proportional to the drive signal
V.sub.D and a component that is proportional to the harmonic signal
V.sub.H. The coil 212 is connected to the input of a
phase-adjustment circuit 214. The output of the phase-adjustment
circuit 214 is connected to the input of a an amplitude-adjustment
circuit 215. The outputs of the amplitude-adjustment circuits 215
and 216 are connected to a combining circuit 217 that combines the
outputs such that the contributions that are proportional to the
drive signal voltage V.sub.D substantially cancel, providing a
signal that is proportional to the harmonic signal V.sub.H. The
output of the combining circuit 217 is connected to a
phase-adjustment circuit 218. The output of the phase-adjustment
circuit 218 is connected to the input of an amplitude-adjustment
circuit 219. The output of the amplitude-adjustment circuit 219 is
connected to the input Terminal C1 of the combining circuit 223.
The combining circuit 223 combines the harmonic signal V.sub.H and
the drive signal V.sub.D such that their relative proportion and
phase are substantially identical to the relative proportion and
phase of the harmonic signal V.sub.H and drive signal V.sub.D in
the signal V.sub.A.
[0127] It will be appreciated that the splitting circuit 213 and
the combining circuits 217 and 223 shown in FIG. 14C and FIG. 14D
may control relative amplitudes between the split and combined
electrical signals, thereby eliminating the need for
amplitude-adjustment circuits 215, 216, and 219. In FIG. 14C and
FIG. 14D, the output of the amplitude-adjustment circuit 219 is
shown connected to an input terminal of the combining circuit 223.
However, it will be appreciated that the output of the
amplitude-adjustment circuit 219 may be connected to the output
V.sub.out of the combining circuit 208.
[0128] The circuit shown in FIG. 15 is an embodiment of a
cancellation circuit of the present invention for a simultaneous
transmit/receive system. An electrical signal V.sub.B at a terminal
(Terminal B) is amplified into a drive signal V.sub.D at an output
(Terminal A) of an amplifier 233. Terminal A is connected to a
drive coil 230 that may be wrapped around a core 231. Terminal A is
connected to an amplitude-adjustment circuit 234 that may attenuate
the output of the amplitude-adjustment circuit 234 so that it is
substantially lower in amplitude than the drive signal V.sub.D. The
output of the amplitude-adjustment circuit 234 is connected to the
input of a phase-adjustment circuit 235. Terminal B is connected to
the input of an amplitude-adjustment circuit 236. The output of the
amplitude-adjustment circuit 236 and the output of the
phase-adjustment circuit 235 are connected to a combining circuit
238. The output of the combining circuit 238 is connected to a
preamplifier 232. The output of the preamplifier 232 is connected
to Terminal B.
[0129] The signal voltage V.sub.A at Terminal A comprises a drive
voltage V.sub.D, that flows through the drive coil 230 to generate
a magnetic flux, and a pickup voltage V.sub.Pickup induced in the
drive coil 230 by other sources (not shown) of magnetic flux. In
this case, it is preferable that the material comprising the core
231 has a substantially linear response to the magnetic flux so as
to minimize additive harmonic signatures V.sub.H caused by
non-linear responses of the core 231 material to magnetic flux. The
signal-voltage V.sub.B at Terminal B represents the drive signal
V.sub.D before it is amplified by the amplifier 233. The
phase-adjustment circuit 235 and the amplitude-adjustment circuits
234 and 236 adjust the relative phase and amplitude of the signals
V.sub.A and V.sub.B so that the components in the signals V.sub.A
and V.sub.B related to the drive signal V.sub.D substantially
cancel when they are combined at the combining circuit 238.
[0130] The total gain of a feedback loop is calculated by summing
the gains and losses of each component in the feedback loop. For
example, the gain of the first feedback loop in FIG. 15 is measured
starting at Terminal B and moving through the amplitude-adjustment
circuit 236 to the combining circuit 238, then through the
preamplifier 232 back to Terminal B. The amplitude-adjustment
circuit 236 and the preamplifier 232 may provide gain or
attenuation to the electrical signals. The combining circuit 238
provides an effective attenuation to the electrical signals by
canceling the electrical signals representing the drive signal
V.sub.D. Thus, if the total gain of the first feedback loop is less
than one, this part of the circuit will not cause oscillation.
[0131] The gain of the second feedback loop in the circuit shown in
FIG. 15 is measured starting at Terminal A and including the
amplitude-adjustment circuit 234, the phase-adjustment circuit 235,
the combining circuit 238, the preamplifier 232, and the amplifier
233. The amplitude-adjustment circuit 234 includes a means for
attenuating the signal V.sub.A at Terminal A. The preamplifier 232
and the amplifier 233 can provide substantial gain to the
electrical signal. The phase-adjustment circuit 235 generally has
little effect on the amplitude of the electrical signal. The
combining circuit 238 will provide a substantial effective
attenuation to the electrical signal flowing through it by
canceling the signal voltage that is related to the drive signal
V.sub.D. If this cancellation is large enough, it will cause the
total gain of the second feedback loop to be less than one. Thus,
the circuit will not oscillate.
[0132] The pickup signal V.sub.Pickup induced in the drive coil 230
is amplified and returned to the drive coil 230 to generate a
magnetic flux. However, the feedback effects of the drive signal
V.sub.D in the circuit are canceled to prevent oscillation. This
allows the drive coil 230 to simultaneously transmit and receive
electromagnetic signal. It will be appreciated that if the core 231
is made of a material that has a non-linear response to magnetic
flux, the cancellation circuit may be designed to cancel the
electrical signals resulting from the non-linear response of the
core 231 material. Different configurations of the cancellation
circuit are shown in FIG. 14B through FIG. 14D. It will also be
appreciated that either or both of the signals V.sub.A and V.sub.B
from Terminals A and B, respectively, may have amplitude adjustment
and/or phase adjustment applied to them such that the signal
components related to the drive signal V.sub.D will cancel at the
combining circuit 238. Furthermore, it will be appreciated that a
compensation circuit (not shown) may be included in the feedback
loop or may precede the drive coil 230 to provide a specific phase
and/or amplitude relationship between the pickup signal
V.sub.Pickup and the drive signal V.sub.D.
[0133] The circuit shown in FIG. 16 is an embodiment of a
cancellation circuit of the present invention that cancels both
static magnetic fields and magnetic flux in a specific region of
space. The circuit in FIG. 16 includes two magnetic-field sensors
240 and 242 that generate electrical signals that are proportional
to the scalar magnitude of the magnetic field strength in a
specific direction at the location of each sensor 240 and 242. The
sensors 240 and 242 may be flux gate sensors or the like. The phase
of the signal from the first sensor 240 is adjusted by a
phase-adjustment circuit 241. The amplitude of the signal from the
first sensor 240 is adjusted by an amplitude-adjustment circuit
244. The phase of the signal from the second sensor 242 is adjusted
by a phase-adjustment circuit 243. The amplitude of the signal from
sensor 242 is adjusted by an amplitude-adjustment circuit 246. The
outputs of the amplitude-adjustment circuits 244 and 246 are
connected to a combining circuit 247. The output of the combining
circuit 247 is connected to a compensation circuit 248, that is
connected to the input of an amplifier 249. The amplifier 249
amplifies an input signal to produce a drive signal V.sub.D that
flows through the drive coil 250 and produces a magnetic field that
is substantially parallel to, but opposite to the magnetic field
sensed by the sensors 240 and 242.
[0134] The first sensor 240 is positioned in close proximity to the
coil 250 or inside the coil 250 to sense both the magnetic field
generated by the coil 250 and the magnetic field generated by
external sources (not shown). The second sensor 242 is positioned
in such a manner so that it is more sensitive to magnetic fields
generated by external sources (not shown) than to the magnetic
field generated by the coil 250. Each of the sensors 240 and 242
has a specific ratio of response between the magnetic field
generated by the coil 250 and the magnetic field generated by the
external magnetic sources (not shown). It will be appreciated that
there are many ways to change the ratio of response of one of the
sensors 240 or 242, For example, the position of one of the sensors
may be adjusted. The important point is to provide one of the
sensors 240 or 242 with a different ratio of response than the
other sensor 242 or 240. The amplitude and/or phase of the signals
produced by the sensors 240 and 242 are adjusted by
amplitude-adjustment circuits 244 and 246, respectively, and
phase-adjustment circuits 241 and 243, respectively, such that the
components of the signals related to the drive signal V.sub.D
substantially cancel at the combining circuit 247. The output of
the combining circuit will comprise a voltage V.sub.Ext that is
proportional to the magnetic field intensity generated by the
external sources (not shown).
[0135] The amplitude of the signals produced by the sensors 240 and
242 may be adjusted in order to cancel a dc signal resulting from
the magnetic field generated by the drive coil 250. However, as the
magnitude of the drive signal V.sub.D changes in response to a
changing external magnetic field, there may be some response
anomalies between the two sensors 240 and 242 related to the
rate-of-change (flux) of the drive signal V.sub.D. Thus, it may be
necessary to compensate for flux-dependent amplitude differences
and rate-of-response (phase) differences between the two sensors
240 and 242.
[0136] The drive coil 250 generates a highly uniform magnetic field
within the region of space that it encloses. Thus, it is preferable
to utilize the interior of the coil 250 as the space in which
uniform magnetic fields will be canceled. However, due to the
inductive properties of the coil 250 and the possible
flux-dependent amplitude and phase characteristics of the sensors
240 and 242, it is necessary to provide phase and/or amplitude
compensation using a compensation circuit (such as compensation
circuit 248) in order to provide substantial cancellation of
magnetic flux.
[0137] The circuit shown in FIG. 17 is another embodiment of a
cancellation circuit of the present invention that cancels both
static magnetic fields and magnetic flux in a specific region of
space. The circuit in FIG. 17 includes a magnetic field sensor 262
that generates an electrical signal that is proportional to the
scalar magnitude of magnetic-field strength in a specific direction
at the location of the sensor 262. It will be appreciated that the
sensor 262 may be a flux gate sensor or the like. The signal
produced by the sensor 262 is sent to an automatic control unit
264. The automatic-control unit 264 controls the gain of an
amplifier 266 connected to a dc-level generator 265. The output of
the amplifier 266 is an amplified or attenuated dc-level drive
signal V.sub.D that is passed through a compensation circuit 267 to
a drive coil 260, which generates a magnetic field.
[0138] The magnetic-field sensor 262 is preferably positioned
inside the region of space where cancellation of magnetic fields is
desired. The magnetic-field sensor 262 produces a signal that is
proportional to the amplitude of the magnetic field it senses. The
magnetic field comprises the magnetic field generated by the drive
coil 260 and magnetic fields generated by other sources (not
shown). The automatic-control unit 264 determines if a magnetic
field is present at the sensor 262 and controls the amplifier 266
so that the drive signal V.sub.D in the drive coil 260 produces a
magnetic field that cancels the magnetic field at the sensor 262.
Because the drive coil 260 has inductive properties, there will be
an inductive lag in the drive signal V.sub.D flowing through the
drive coil 260 when the amplitude of that signal changes. Likewise,
the effective impedance Z.sub.D=R.sub.D+i.omega.L.sub.D of the coil
260 changes with signal frequency .omega.. Thus, a signal flux
results in an amplitude variation of the magnetic field generated
by the drive coil 260. The compensation circuit 267 provides phase
adjustment and amplitude adjustment to the drive signal V.sub.D so
that the drive coil 260 cancels both static and dynamic magnetic
fields.
[0139] It will be appreciated that the compensation circuit 267 may
also provide compensation for any flux-dependent amplitude and
phase variations in the response of the sensor 262. It will also be
appreciated that the automatic control unit 264 provide amplitude
and phase compensation to the drive signal V.sub.D. The circuits
shown in FIG. 16 and FIG. 17 show systems that cancel magnetic
fields along a single axis. However, a superposition of three such
circuits, each along an orthogonal axis, can provide complete
cancellation of magnetic fields in three dimensions.
[0140] Because coils of wire whose currents support magnetic fields
in space function as antennas radiating electromagnetic energy, it
is obvious that the cancellation and/or compensation circuits shown
above may be used in radar systems for providing interference
cancellation and simultaneous transmit/receive capability.
[0141] The circuit shown in FIG. 18 is a preferred embodiment of a
cancellation circuit of the present invention. A signal generator
276 generates an electrical generator signal V.sub.G that is
amplified by a power amplifier 274 and passed through a junction
272 to an antenna element 270. Antenna element 270 both emits and
receives electromagnetic radiation. The antenna element 270 is
responsive to other sources (not shown) of electromagnetic
radiation, thus producing an electrical pickup signal V.sub.P. The
junction 272 is connected to an input of a combining circuit 275
that receives the pickup signal V.sub.P along with a leakage signal
V.sub.L from the power amplifier 274, The leakage signal V.sub.L is
a portion of the generator signal V.sub.G. The signal generator 276
also produces a reference signal V.sub.R that is similar in shape
to the generator signal V.sub.G. The reference signal V.sub.R
passes through an amplitude-adjustment circuit 271 and a
phase-adjustment circuit 273 to an input of the combining circuit
275. An amplifier 278 amplifies the output of the combining circuit
275.
[0142] The junction 272, which the generator signal V.sub.G passes
through on its way to the antenna element 270, may be a circulator
(not shown) that directs most of the power from the power amplifier
274 to the antenna element 270. However, because the efficiency of
a circulator is frequency-dependent on the electrical signals
passing through it, the performance of the circulator (not shown)
is degraded by the use of large signal-bandwidths or multiple
frequencies. Therefore, some of the energy from the power amplifier
274 leaks into the combining circuit 275. The amplitude-adjustment
circuit 271 provides frequency-dependent amplitude adjustment to
the reference signal V.sub.R such that its amplitude is
substantially identical to the amplitude of the leakage signal
V.sub.L leaked from the power amplifier 274 into the combining
circuit 275. It will be appreciated that the amplitude-adjustment
circuit 271 may be a circulator that is similar to the circulator
used as the junction 272. Thus, the output of the
amplitude-adjustment circuit 271 is substantially proportional to
the amplitude of the leakage signal V.sub.L. The phase-adjustment
circuit 273 adjusts the phase of the reference signal V.sub.R such
that it cancels the leakage signal V.sub.L at the combining circuit
275. Preferably, the phase-adjustment circuit 273 produces a
substantially constant phase between the leakage signal V.sub.L and
the reference signal V.sub.R over the desired frequency range of
generated signals V.sub.G. Thus, the output of the combining
circuit 275 will comprise a pickup signal V.sub.P that is
substantially free from the effects of leakage signal V.sub.L
originating from the power amplifier 274.
[0143] It will be appreciated that many possible designs exist for
cancellation circuits that cancel the effects of interference
between the transmitting and receiving elements of a radiating
system. The circuit shown in FIG. 18 is only one of these designs.
Amplitude and/or phase-adjustment circuits (not shown) may be
interposed in the circuit between the junction 272 and the
combining circuit 275. Furthermore, in the case where
amplitude-modulated and/or frequency-modulated signals are
generated and received, the circuit may include filters (not shown)
for filtering out the carrier frequency before the cancellation
circuit removes the transmitted signal from the received
signal.
[0144] The phase-adjustment circuit 273 may include a delay
apparatus (not shown), such as delay lines, to delay part of the
reference signal V.sub.R so transmitted radiation reflected back
from nearby objects (such as ground clutter) is canceled from the
pickup signal V.sub.P. The reference signal V.sub.R may also
include electrical signals that are similar in shape to signals
induced by other noise sources (not shown) in the antenna element
270. Separate amplitude adjustment and phase adjustment may be
performed to cancel the response of the antenna element 270 to the
other noise sources (not shown). Furthermore, the antenna element
270 may be responsive to incident radiation for producing a drive
signal that allows an antenna, such as the antenna element 270, to
transmit electromagnetic radiation that cancels the reflection of
the incident radiation off of the antenna element 270 or some other
object (not shown). Thus, the reflected radiation may be canceled
at a distant receiver (not shown).
[0145] Magnetic pickups comprising pickup coils are shown in the
circuits of FIG. 2 through FIG. 15, however any type of
electromagnetic pickup may be used with these types of cancellation
circuits. Likewise, a compensation circuit may be used to
compensate for amplitude and phase variations arising from any
pickup device that produces an electrical pickup signal. A
cancellation circuit may be used to cancel the electrical signals
arising from the response of the pickup device to noise. For
example, a cancellation circuit may be used for canceling the
signals generated by an optical sensor's electrical response to
background electromagnetic radiation. In many of the figures, a
drive coil is illustrated as the element that generates an
electromagnetic field. However, compensation circuits may be used
to compensate for frequency-response characteristics exhibited by
any structures that generate electromagnetic fields.
[0146] The preferred methods of amplitude adjustment were shown to
be electrical gain and attenuation controls. However, it will be
appreciated that other methods of amplitude adjustment may be used,
such as adjusting the relative position of the pickup coils, the
drive coils, the cores for the pickup coils and drive coils, and/or
nearby permeable and/or conducting materials. It will also be
appreciated that the inductance of a coil may be changed by
changing the reluctance of the path seen by that coil's magnetic
field.
[0147] The magnitude of electrical current in the pickup coils was
considered to be very small. Thus, the formulation of the equations
representing the electrical pickup signals induced in the pickup
coils by magnetic flux have not included the inductive effects that
the pickup coils may have on each other. However, the scope and
spirit of the present invention would not be challenged by
considering the inductive effects between pickup coils when
designing the cancellation circuits. Furthermore, consideration of
the more subtle electromagnetic effects (such as how capacitance in
the pickup coils affects the induction of electrical signals in the
pickup coils) and how the cancellation and compensation circuits
that may be designed accordingly is anticipated by this
invention.
[0148] Although the invention has been described in detail with
reference to the illustrated preferred embodiments, variations and
modifications exist within the scope and spirit of the invention as
described and as defined in the following claims.
* * * * *