U.S. patent application number 10/955507 was filed with the patent office on 2005-03-17 for snr improvement by selective modulation.
Invention is credited to Swain, William Hall.
Application Number | 20050057242 10/955507 |
Document ID | / |
Family ID | 34273127 |
Filed Date | 2005-03-17 |
United States Patent
Application |
20050057242 |
Kind Code |
A1 |
Swain, William Hall |
March 17, 2005 |
SNR improvement by selective modulation
Abstract
The accuracy of certain sensors is improved by improving their
signal to noise ratio (SNR) in the presence of an interfering
noise. Sensors were discovered which have a SNR which substantially
changes when an operating parameter is selectively modulated to
different magnitudes. In the simplest form, the sensor is operated
where it is both stable and close to its best SNR. This is usually
faster and less costly, but the noise is never completely
eliminated. This invention has first been applied to Swain
Meter.RTM. type clamp-on DC ammeters. Some results are good--the
benefit in SNR is between 2 and 4. .RTM. Swain Meter is a
registered Trademark of the William H. Swain Co.
Inventors: |
Swain, William Hall;
(Sarasota, FL) |
Correspondence
Address: |
William H. Swain
4662 Gleason Ave.
Sarasota
FL
34242
US
|
Family ID: |
34273127 |
Appl. No.: |
10/955507 |
Filed: |
October 1, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10955507 |
Oct 1, 2004 |
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08579395 |
Dec 27, 1995 |
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Current U.S.
Class: |
324/127 |
Current CPC
Class: |
Y10T 29/4902 20150115;
G01R 15/186 20130101 |
Class at
Publication: |
324/127 |
International
Class: |
H04B 007/10 |
Claims
1. A method and process for constructing and using a sensor with
reduced error for measurement or control including means: a core of
low magnetic reluctance material, here called SQ, a coupling sense
winding on said core having a number of turns, here called N.sub.s,
an inverter having an output current, here called i.sub.s, and an
average said output current here called I.sub.s, and also
constructed such that said inverter has an operating parameter
which is the peak value in either direction of said current, here
called I.sub.sm, a low input impedance means converting the said
average value I.sub.s of said inverter current to an output voltage
here called V.sub.c, and said method includes: positioning said
core so that it is influenced by a conductor carrying a signal
current I to be measured, said position being within the effective
range of a magnetic field noise, here called N, causing at least
part of an error in the form of a change in zero offset of said
output voltage V.sub.c, wherein the sensitivity of said V.sub.c to
said noise N is here called .PSI., and defined as the change in
said V.sub.c due to a unit change in said noise N divided by a gain
g, i.e., 12 V c / N g ,where said g is defined as the change in
said output V.sub.c due to a unit change in said signal current I;
i.e., 13 g V c I ,and said method also includes series connecting
said N.sub.s, said inverter, and said low input impedance means
converting; and adjusting said means, including said N.sub.s and
said I.sub.sm, so that the change in said gain g is considerably
less than the change in said noise sensitivity .PSI., as said noise
sensitivity .PSI. is reduced from a maximum to a value considerably
less than said maximum, said reduced being accomplished by altering
the value of said means, especially the number of turns on said
winding N.sub.s and the said peak inverter current I.sub.sm, said
altering being preferably in the direction of a greater value of
the product of said N.sub.s and said I.sub.sm, and operating said
sensor with said product of said N.sub.s and said I.sub.sm set so
that said noise sensitivity .PSI. is considerably reduced below
said maximum, thereby constructing and operating said sensor with
said reduced error in zero offset due to said noise N.
2. A Swain Meter type non-contact direct current ammeter with
improved accuracy for measurement or control, which comprises: a
core, here called SQ, of low magnetic reluctance material, a
coupling sense winding, here called N.sub.s, on said core SQ, an
inverter with power supply, here called X, with output terminals
with a current i.sub.s flowing which has an average value I.sub.s,
and also a peak value I.sub.sm which is an operating parameter, all
of said currents flowing in either direction in said output
terminals, a low input impedance means converting said average
current I.sub.s to an average output voltage V, a current carrying
conductor carrying a signal input current I, which is to be
measured or controlled, positioned so that said current I
influences said core SQ, and said core SQ is within the effective
range of an interfering magnetic field noise, here called N, and
said coupling sense winding N.sub.s series connected with said
output terminals of said inverter X and said low input impedance
means converting, said operating parameter I.sub.sm set to a
substantially greater magnitude than the magnitude corresponding to
the minimum signal to noise ratio, here called SNR, so that thereby
the said SNR is considerably increased over said minimum, so that
said non-contact ammeter has considerably greater accuracy in the
presence of said interfering magnetic field noise N.
3. I claim a method for making a more accurate implement for at
least one of measurement or control including the steps: Construct
a port for desired input signal I, which of necessity makes a port
for undesired error producing interference N, construct a port for
said implement's output V.sub.c, acquire an Essential
Characteristic type sensor having an output V responsive to said
desired input signal I, and also responsive to said undesired error
producing interference N, and further having an operating parameter
of magnitude Q; show that said Essential Characteristic type sensor
has a useful said Essential Characteristic evidenced by a signal to
noise ratio SNR of said sensor observed to change a lot when the
said magnitude Q of said operating parameter is modulated over a
practical range; provide said implement equipped to: support said
sensor so as to: considerably reduce said undesired interference N
relative to said desired signal I at said output V.sub.c by holding
said magnitude Q in a higher said SNR state and coupling said
sensor output V to said implement output V.sub.c.
4. I claim a more accurate sensor with implement for at least one
of measurement or control, including said sensor having a strong
Essential Characteristic, and also an output V responsive to a
physical quantity input I, the gain g given by 14 g V I ,and said
output V also responsive to an undesired error producing
interference N, the sensitivity .PSI. being 15 V N ,and said sensor
also having an operating parameter of magnitude Q which modulates
said .PSI., and to a lesser extent said g; said sensor having been
shown by at least one of calibration, proven manufacturing process,
or other demonstration to have said strong said Essential
Characteristic, i.e., the said sensitivity .PSI. changes a lot more
than said gain g when said magnitude Q is driven over a practical
range of values; and also including: an error reduction form of
said implement, fitted to support said sensor, and fitted to drive
said magnitude Q and hold it at a constant value, which is
predetermined to cause said sensor to operate with said
interference sensitivity .PSI. a lot less than was heretofore
customary, while said gain g is still good, thereby making said
sensor with said implement substantially more accurate than
comparable transducers for said physical quantity I in the presence
of said interference N.
5. I claim a method for making a more accurate sensor with
implement for at least one of measurement or control, made in
steps: obtain a said sensor having an output V responsive to a
physical quantity input I, the gain g given by 16 g V I ,and said
output V is also responsive to an undesired error producing
interference N, the sensitivity .PSI. being 17 V N ,and in
addition, said sensor has an operating parameter of magnitude Q
which modulates said .PSI., and to a lesser extent said gain g; at
least one of calibrate, or make by a proven process, or otherwise
assure that said sensor has a strong Essential Characteristic
evidenced by observing that said Sensitivity .PSI. changes a lot
more than said gain g when said magnitude Q is driven over a
practical range of values; and: provide an error reducing form of
said implement, fitted to support said sensor, and also fitted to
drive said magnitude Q and hold it at a constant value, and by at
least one of measurement or a proven process, set said magnitude Q
at a value corresponding to a said sensitivity .PSI. which is a lot
less than heretofore while said gain g is still good, thus making
said sensor with implement substantially more accurate than
comparable transducers for said input I in the presence of said
interference N.
Description
[0001] This is a Divisional Application on copending application
Ser. No. 08,579,395 filed 27 Dec. 1995. This Divisional Application
is filed under 37 CFR 1.53(b)(1). No new matter is introduced.
REFERENCES
[0002] 1) Copending application Ser. No. 08,579,395 of William H.
Swain, filed 27 Dec. 1995. This contains a description for the
herein claims.
[0003] 2) "Special" Status was granted for my 1995 Application on
26 Nov. 2002.
[0004] 3) U.S. Pat. No. 3,768,011 granted to William H. Swain
[0005] 4) U.S. Pat. No. 6,278,952 of William H. Swain on the
Rgage.
[0006] 5) U.S. Pat. No. 6,323,635 of William H. Swain on the
MER2.
FEDERAL SPONSORSHIP
[0007] None
MICROFICHE
[0008] Not applicable
BACKGROUND OF THE INVENTION
[0009] This invention relates to sensors and/or implements for
measurement or control.
[0010] The object of the invention is to improve accuracy by
reducing error in the sensors output when in the presence of an
interfering noise source. An example is a magnet near the
sensor.
[0011] This Divisional Application is for method and means to get
SNR improvement by selective modulation. In connection with my 1995
Application I have called this the "Better SNR" species. This work
has primarily been directed to improving the accuracy of clamp-on
direct current ammeters by reducing the zero offset error due to an
interfering magnetic field; especially a non-uniform field caused
by a nearby magnet.
[0012] The Examiners action of 18 Nov. 2003 on my referenced 1995
Application, in effect, restricted the claims to the "Better SNR
Species" or the "Combiner Species". On page 4, line 13 to page 5,
line 2 he wrote:
[0013] "With the election of any one of the above inventions
further election of species is required as follows:
[0014] This application contains claims directed to the following
patentably distinct species of the claimed invention:
[0015] 1. The simpler form species where SNR is substantially
improved by operating at a more favorable operating parameter,
wherein noise is not cancelled as set forth at the bottom of page 1
of Applicant's specification and identified by Applicant on page 3
of his Appeal Brief as the "Better SNR species".
[0016] 2. The "Combiner species" as illustrated in FIGS. 9, 11, and
12 and those portions of the specification respectively related
thereto and by the description of combining the outputs of two
separate sensors as described on page 1 of the specification, the
combiner species again being identified by Applicant on page 9 of
his Appeal Brief as the "combiner species"."
[0017] I elected the "Combiner Species". This Divisional
Application is for the simpler "Better SNR Species". The invention
is here called SNR Improvement by Selective Modulation.
[0018] This invention solves the problems of complexity and need
for adjustment to suit the magnetic environment encountered with
the "Combiner Species". I presented it on page 1 of my 1995
Application.
[0019] "In a simpler form, SNR is substantially improved by
operating at a more favorable operating parameter magnitude. Noise
is not canceled, but this form can be faster and cost less".
[0020] I also presented this SNR improvement by Selective
Modulation invention in the first paragraph of the Abstract of my
1995 Application. Page 52 includes:
[0021] "In the simplest form, the sensor is operated where it is
both stable and close to its best SNR. This is usually faster and
less costly, but the noise is never completely eliminated."
[0022] Original claims 12 and 13 of my 1995 Application on pages
49-51 are for this invention.
[0023] This Application is based entirely on my 1995 application
Ser. No. 08,579,395. It contains no new matter. Basis for the
illustrations and examples will be found in the 1995 description,
drawings and claims. Original 1995 claims 12 and 13 kappear herein
as claims 1 and 2.
SUMMARY OF THE INVENTION
[0024] This SNR improvement invention is inherently simpler,
faster, and more adaptable to a wide range of magnetic interference
(noise) environments.
[0025] The "Discovery", illustrated in 1995 FIGS. 4, and 5 was that
some Swain Meters and other sensors changed their Signal to Noise
Ratio (SNR) a lot when the magnitude of an operating parameter was
changed. This is not a result of filters, data manipulation, etc.
It is an intrinsic change. For example, I can do math faster with
fewer errors after a good nights sleep.
[0026] The method is given at the bottom of 1995 page 1:
[0027] " . . . SNR is substantially improved by operating at a more
favorable operating parameter magnitude . . . "
[0028] This can be done by stopping the counter 24 in 1995
"Combiner Species" FIG. 9 and operating full time in the high SNR
state {circle over (B)}, where the switches are in the {circle over
(4)} position, with operating parameter I.sub.sm at 0.4 Amp.
Definitions are given on 1995 pages 32-34.
[0029] Herein FIG. 6 illustrates high SNR state {circle over (B)}.
Herein FIG. 7 shows an implementation. It is 1995 FIG. 9 with the
unneeded state switching parts removed.
[0030] My original 1995 claim 12, states much the same. It is
herein claim 1.
DESCRIPTION OF THE DRAWINGS
[0031] There are many 1995 antecedents and considerable 1995 basis
for these herein drawings. These include:
[0032] Herein drawing FIG. 1 is the same as 1995 FIG. 1, but with
the Hall devices (5) removed. Hall devices are not considered
herein.
[0033] Herein FIGS. 2, 3, 4, and 5 are the same as 1995 FIGS. 2, 3,
4, and 5.
[0034] Herein FIG. 6 illustrates the above descriptions on 1995
page 1 and in 1995 Abstract.
[0035] Herein FIG. 7 is an implementation for my 1995 original
claim 12. It is 1995 FIG. 9 with unneeded switching removed.
[0036] In the drawings:
[0037] FIG. 1 is a functional diagram of a sensor with a split
magnetic core SQ surrounding a conductor carrying a current I to be
measured. The core of a Swain Meter will have a coupling sense
winding N.sub.s.
[0038] FIG. 2 illustrates interference from the uniform magnetic
field H.sub.u due to a very remote and large field such as that of
the earth, H.sub.e.
[0039] FIG. 3 illustrates interference from the non-uniform
magnetic field H.sub.n due to a magnet near the sensor.
[0040] FIG. 4 is a graph illustrating the essential characteristic
discovered in a type of clamp used in some Swain Meters. As the
operating parameter I.sub.sm increases, the signal gain increases
only slightly, but the normalized output zero offset due to noise,
here called , first increases and then decreases to half and
less.
[0041] FIG. 5 is a graph illustrating the essential characteristic
in terms of signal to noise ratio SNR for 5" diameter aperture clip
#88.
[0042] FIG. 6 illustrates the method of this invention. It is a
partial copy of FIG. 5 with added annotation showing the preferred
setting of the Operating Parameter, and the SNR result, as
indicated on page 1 of my 1995 Application. It includes:
[0043] "In a simpler form, SNR is substantially improved by
operating at a more favorable operating parameter magnitude. Noise
is not canceled, but this form can be faster and cost less."
[0044] This is also the basis for FIG. 6 which shows that operation
is continuous at the operating parameter magnitude of 0.4 Amp. Then
the SNR is 29.
[0045] FIG. 7 illustrates the preferred way of implementing this
method. It is a copy of my 1995 FIG. 9 with parts deleted which are
not needed for this simpler form. The switch 18 is replaced by a
wire which causes the inverter to operate continuously at operating
parameter magnitude 4, i.e., 0.4 Amp. There is no operation at
operating parameter magnitude 2, i.e., 0.2 Amp.
[0046] FIGS. 6 and 7 also illustrate the method taught in paragraph
1 of the Abstract of my 1995 Application. Page 52 includes:
[0047] "In the simplest form, the sensor is operated where it is
both stable and close to its best SNR. This is usually faster and
less costly, but the noise is never completely eliminated."
[0048] FIG. 8 is a block diagram type representation of a sensor of
this invention. It has a signal input I, an unavoidable interfering
noise input N and an output V. This comes from 1995 FIG. 13 on page
66. FIG. 13 is discussed on 1995 page 8, 14, and eq. a) through eq.
j) to 1995 page 22.
[0049] The operating parameter port Q is not an input. Instead, it
acts more like a modulator access, which primarily governs the
sensitivity of the noise input N, but only slightly affects the
sensitivity of the signal input I.
DESCRIPTION OF THE INVENTION
SNR Improvement by Selective Modulation
[0050] General
[0051] This invention can be applied to improve the accuracy of
sensors for measurement and control. It has been applied to reduce
the zero offset error of clamp-on DC ammeters of the Swain
Meter.RTM. type. .RTM. Swain Meter is a Registered Trademark of the
William H. Swain Co.
[0052] Purpose
[0053] Interference type noise causes an error in the output of
some sensors. The purpose of the present invention is to improve
the accuracy by improving the signal to noise ratio (SNR) of
sensors and associated implements for measurement or control. A
sensor and/or implement may also be called a transducer or signal
translator. A particular purpose is to improve the accuracy of
sensors for clamp-on or non-contact DC ammeters of the Swain
Meter.RTM. type, by reducing error due to zero offset caused by
interference from non-uniform magnetic fields due to local magnets,
and also by uniform fields due to more remote magnets such as the
earth.
[0054] Method and Means
[0055] It was discovered that certain sensors have the Essential
Characteristic, i.e., they have a sensitivity to an interfering
noise which changes a great deal more than the sensitivity to a
signal input when the magnitude of an operating parameter is
changed. We call this selective modulation.
[0056] A method for improving accuracy is to reduce sensitivity to
noise while keeping good sensitivity to signal, i.e., increasing
Signal to Noise Ratio (SNR).
[0057] Having got a sensor having the Essential Characteristic,
adjust the magnitude of the Operating Parameter to get the SNR
needed. Noise is not canceled, but this "Better SNR" method can be
faster and cost less than the "Combiner" method.
[0058] Outline of Contents
[0059] The remainder of this specification includes the following
sections:
[0060] Introduction begins with the Swain Meter.RTM. Patent of
William H. Swain, U.S. Pat. No. 3,768,011. FIGS. 1, 2, and 3 show a
basic clamp for a non-contact DC ammeter of the Swain type with
coil N.sub.s (2), and they show the effects of interfering magnetic
noise H.sub.n (8) and H.sub.u (9). Hall Devices (5) are not
considered here.
[0061] The herein Introduction is the same as that in my 1995 pages
8-11 except that reference to Hall devices is deleted.
[0062] Discovery that many Swain sensors had a zero offset error Z
heavily dependent on the magnitude of operating parameter I.sub.sm,
but stable gain g for the input signal I, is shown in FIG. 4.
Normalized output error and noise sensitivity .PSI. are introduced,
along with signal to noise ratio SNR. This is plotted in FIG. 5.
Both FIG. 4 and FIG. 5 illustrate the Essential Characteristic
needed in a sensor for successful SNR improvement by selective
modulation.
[0063] The herein Discovery is the same as that in my 1995 pages
11-13 except that:
[0064] Under Essential Characteristic the error ""Magnitude" Field
H.sub.n (8)" is corrected to ""Magnetic" Field H.sub.n (8) . . . ";
and
[0065] Reference to Hall device is deleted; and
[0066] "Corrected" has been changed to "reduced."
[0067] General Method and Mathematical Relationship
[0068] This section provides a theory for use in evaluating the
Essential Characteristic of a sensor and its suitability for
improving SNR.
[0069] This herein section is the same as that in the corresponding
section in 1995 pages 13-16 except that:
[0070] "Canceling" is changed to "reducing".
[0071] Eq. i) and eq. j) are omitted because they relate primarily
to error canceling in the Combiner Species.
[0072] Reference to 1995 FIG. 6 for a hypothetical sensor is
deleted. Herein I discuss FIG. 4 which shows the result of actual
measurements on 5" clip #88.
[0073] Likewise FIG. 8 is replaced by FIG. 5.
[0074] Reference to Hall devices is deleted.
[0075] Non-Contact Ammeter Implementation for Swain Meter
[0076] This section shows a method and a practical design embodying
the invention as shown in FIGS. 6 and 7. This worked using clip #88
(characterized in FIG. 4 and FIG. 5). Details are discussed.
[0077] This section begins as 1995 page 31 but then changes to
discuss SNR improvement instead of eror cancelation.
[0078] Antecedent and basis for this section is 1995 statements
on:
[0079] Page 1,
[0080] Abstract,
[0081] Original claim 12 which is herein claim 1, and
[0082] Original claim 13 which is herein claim 2.
[0083] The Construction and Results section gives some detail on
the construction of 5" clip #88 and its operation in FIG. 7. A
benefit of 2 to 4 to one was measured.
[0084] Antecedent and basis for construction is in:
[0085] 1995 page 12 which describes 5" clip #88, and
[0086] 1995 page 36 which describes 5" clip #88, C16, and R17.
[0087] Antecedent and basis for results is 1995 FIGS. 4 and 5 which
show the measured benefit of this SNR Improvement invention.
[0088] Conclusion is that the method can be widely applied to
considerably improve accuracy.
[0089] Antecedent and basis is:
[0090] on the bottom of my 1995 page 1, and
[0091] the first paragraph of my 1995 Abstract, page 52.
[0092] Introduction
[0093] Swain Meter type clamp-on DC ammeters have gained wide
acceptance because they are generally sensitive and accurate and
available in a variety of forms for measuring 10 mA to 500 Amp.
direct current with sensors from 1/4" to 5 feet in diameter. A
clamp-on type sensor is shown in FIG. 1 herein.
[0094] A sensor plus implement combination can be constructed using
the concepts of U.S. Pat. No. 3,768,011 to serve as a non-contact
ammeter. In FIG. 2 therein, resistor R.sub.s can be made quite
small--100 ohms or less, and capacitor C quite large--1000 micro
farad or more.* The output voltage V.sub.c across capacitor C and
resistor R.sub.s will henceforth be written simply as V, and in
some places, assumes a more general meaning. More gain is assumed
to be available if needed.** * In some designs we have replaced
R.sub.s and C with the low impedance input of a high current
capability operational amplifier. This can be a lot faster, and it
also converts the average current I.sub.s in the sense winding
N.sub.s to an output voltage. ** Here we assume that where gain is
needed, it is available. The voltage across resistor R.sub.s in
FIG. 2 of the Patent may be only a few millivolts. The means for
boosting this to a volt, essentially free of added error, are
widely known.
[0095] The output voltage V is sensitive to an input signal current
I, and also to an interfering noise N which causes an output zero
offset Z. FIG. 8 represents a sensor with functional symbols. An
equation can be written to relate these:
V=gI+Z
[0096] Accuracy is dependent on g--this may be 1.000 V per Amp on a
particular range*--and on Z. The values of g and Z should be
constant over all values of input signal I, and also over all
values of noise interference N. * In some designs we have replaced
R.sub.s and C with the low impedance input of a high current
capability operational amplifier. This can be a lot faster, and it
also converts the average current I.sub.s in the sense winding
N.sub.s to an output voltage.
[0097] We have got 1% type control over the gain g, and also good
control over zero offset Z due to the magnetic field of the earth
H.sub.e. On a 1/4" clip this can be as low as 0.+-.1 ma. peak
equivalent input current in response to a full vertical north-south
spin in the earth's field H.sub.e. We call the earth field uniform,
H.sub.u as shown in FIG. 2 herein.
[0098] The most difficult type of interference noise N to control
has been that due to a strong non-uniform magnetic field H.sub.n
such as that shown in FIG. 3. A stray magnet, perhaps in a weld in
a pipe, a sector of magnetized sheet metal in an automobile near
the battery cable, or a magnetized fastener near the sensor can
produce a considerable zero offset error Z. When the clamp-on
sensor is moved from nearby to really around the conductor carrying
the current to be measured, the intensity and direction of the
effective non-uniform field H.sub.n changes, and this changes the
zero offset Z, and so reduces the accuracy of output V.
[0099] The method and means shown herein have improved accuracy by
reducing noise, not only from H.sub.n, but also, to a lesser
degree, from H.sub.u.
[0100] FIG. 1 represents a clamp-on type of non-contact sensor
having a low magnetic reluctance core 1 which is split at the lips
61. These have a large cross section area to provide low magnetic
reluctance all around the magnetic core path.* If it is for a Swain
Meter, it will have a coupling sense winding 2. It may be called a
signal translator or transducer because the input current 7 sets up
an input field 3 which influences, i.e., upsets the magnetic state
of the core 1 and thus causes an average current 4 to flow in
coupling sense winding 2 when connected to a suitable inverter. An
output voltage is available when this current 4 flows through a
resistor 17 called R.sub.s. * This is not essential. We have made,
for special applications, non-contact ammeters wherein the core is
an open ended shape, or even a flat bar. The coupling between the
input current and the core is not as good as when there is a low
reluctance path all around the input current, but signal input
current positioned near the core still influences the core, i.e.,
alters the magnetic state of the core enough so that some
measurements are practical. It is expected that the method of this
invention will also reduce error in these.
[0101] Stray magnetic fields such as those shown in FIG. 2
(H.sub.u) and FIG. 3 (H.sub.n) produce a zero offset error because
all non-contact DC Ammeters measure the current 7 by measuring the
magnetic field 3 or flux density 6 set up in the magnetic core
material of the sensor by the input current 7. Some H.sub.u or
H.sub.n gets into the core in FIG. 1 and produces a zero offset
error Z.
[0102] The zero offset error Z tends to be less if the core is
continuous, with no split. When the core is split at the lips 61,
it is preferred that these have low magnetic reluctance, often by
virtue of large surface area.
[0103] The input current 7 sets up an input field 3. It is largely
uniform and constant and circular about the current carrying
conductor 7. In FIG. 1, input field 3 and input flux path 6 go
evenly all around the core of the clamp.
[0104] This is not true of a non-uniform field (H.sub.n) 8 such as
that due to a magnet 10 near the clamp, as shown in FIG. 3. This is
also not true of a uniform field H.sub.u 9, which may be produced
by the Earth's magnetic field (H.sub.e). This is shown in FIG.
2.
[0105] It may be that selective modulation of the signal and noise
is feasible because the signal I.sub.i acts circumferentially, but
the noise H.sub.n and H.sub.u act partially in the core and
partially outside.
[0106] In Swain Meters the zero offset (Z) produced by the Earth
(H.sub.e) or another uniform field (H.sub.u) has been reasonably
well controlled and reduced to a magnitude low enough to measure
direct current to within .+-.1 ma. when using a 3/4" clip. U.S.
Pat. No. 3,768,011 shows the concept of peak magnetizing current
(I.sub.sm) and uniform coupling sense winding (N.sub.s) used to get
such zero stability when the field is uniform (FIG. 2), and the
core is small. But these techniques still allow a substantial zero
offset (Z) when the core is large (over 4"), or when the field is
strong and non-uniform (FIG. 3). We especially want to correct this
error. We also want to further reduce the error due to H.sub.u.
Discovery
[0107] The inventor discovered that the output V of many Swain
Meter clamps was a lot less sensitive (1/2 to 1/3 in some sensors)
to a change in the intensity of a non-uniform magnetic field
H.sub.n when the magnitude of an operating parameter I.sub.sm was
doubled or tripled. And the sensitivity (gain) to a change in
signal input current I stayed constant to within a few percent.
[0108] Essential Characteristic
[0109] FIG. 4 shows the approximate sensitivities for a five inch
diameter aperture clip #88. This is an illustration of a sensor
having the essential characteristic:
[0110] Firstly, the signal gain g (13) sensitivity to signal input
I (7) is constant within a few percent as an operating parameter
I.sub.sm (12) changes from 0.18 A to 0.5 Amp peak; and
[0111] Secondly, the zero offset (11) sensitivity to a unit change
in intensity of a non-linear magnetic field H.sub.n (8) is reduced
to well under half over the same range of I.sub.sm (12).
[0112] The equation relating these quantities is V=gI+Z.
[0113] Zero offset is given in terms of =Z/g, where the input
current I equivalent to the zero offset Z is obtained by dividing
the zero offset Z by the signal gain g. The result (14) is plotted
in FIG. 4.
[0114] The data in FIG. 4 shows the approximate behavior of 5" dia.
aperture clip #88. It uses concepts shown in U.S. Pat. No.
3,768,011, especially in connection with FIG. 2 and FIG. 4 therein.
Clip #88 is outlined in FIG. 1 herein. The primary parts are:
[0115] A core SQ (1) having five layers of 0.725" wide-4D low
reluctance steel from Magnetics of Butler, Pa.,
[0116] The core is mounted on a support and arranged so that the
magnetic reluctance around the full magnetic path is minimized.
Care should be used to avoid forcing or bending the steel because
stresses and strain may produce a poorer core.
[0117] A uniform coupling sense winding N.sub.s (2) of about 1000
turns of #22 magnet wire. A symmetrical and balanced form is
preferred. The winding resistance should be less than 5 ohms.
[0118] Half inch lips (61) which are constructed to mate well so
that the magnetic reluctance all around the core is minimized.
[0119] The essential characteristic for successful SNR improvement
by selective modulation shown in FIG. 4 for clip #88 plots--in
effect--noise sensitivity .PSI. times gain g against the operating
parameter I.sub.sm. This is from 1 O ' N ,
[0120] where is still the equivalent input current of a zero offset
Z and N is a unit of noise, in this case, magnetic field
H.sub.n.
[0121] Signal to noise ratio SNR is the reciprocal of noise
sensitivity .PSI., i.e., 2 SNR = 1
[0122] SNR is, in a way, easier to understand, and it can help in
writing claims, partly because it is basic. FIG. 5 is an SNR plot
of the same #88 clip over the same operating parameter I.sub.sm
range of magnitudes as in FIG. 4. It shows SNR, which is the signal
sensitivity (gain g) divided by the noise sensitivity (g.PSI.)
changing from a minimum of about 13 at about 0.07 Amp I.sub.sm to
over 50 as I.sub.sm approaches 0.5 Amp peak.
[0123] The essential characteristic necessary for good SNR
improvement by selective modulation can be measured and presented
in several ways, but that shown in FIG. 5--the plot of SNR vs.
Operating Parameter is now considered the most basic. A good
characteristic such as that in FIG. 5 has a substantial change in
SNR--two to one or more--over a practical range of the condition of
the operating parameter. It is not necessary that the gain g be
nearly constant. Useful improvement can be had when the gain g
changes 40% as the operating parameter Q is driven from one
condition to another.
[0124] General Method and Mathematical Relationship
[0125] Since it appears likely that someone will find sensors
and/or implements for measurement or control of diverse physical
quantities such as position or chemical concentration we need a
general method and/or procedure for determining if the sensor has
the essential characteristic, and if so, how to use selective
modulation to improve accuracy by reducing error. Statements of the
general method follow.
[0126] A general method for correcting error in the output of a
sensor caused by interference from a noise is presented with
reference to FIGS. 4, 5, and 6. These represent 5" sensor #88. They
are presented to illustrate the analysis.
[0127] A sensor is represented as having an output V which changes
in response to a signal input I, and the output also has an error Z
due to interference from a noise N. FIG. 8 presents this with
functional symbols. Restated:
V.ident.gI+Z, where Eq. a)
[0128] the gain g of the sensor is 3 g V I . Eq . b )
[0129] This is the sensitivity or gain of the sensor's output V to
a signal input I.
[0130] A partial derivative symbol .delta. is used to indicate that
the gain g is the change in sensor output V divided by the change
in sensor signal input I.
[0131] In Eq. a), if the input I is zero, the output V equals the
error Z due to noise. Or if there is an input but it is held
constant, then the change in output V in the presence of an
interfering noise N is the same as the change in error Z due to
this same noise N. Therefore, the gain g times the sensitivity of
the sensor's output V to a noise N, is: 4 V N = Z N Eq . c )
[0132] The importance of an error Z in the output is better shown
in terms of an equivalent noise input which will have the same
effect on the output V as an input signal I. Since both and I are
to be thought of as inputs, the signal input sensitivity, i.e., the
gain g applies to both. Therefore, we define by: 5 O ' Z g ; so g =
Z O ' . Since Eq . b ) gives g = V I , and Eq . c ) gives V = Z ,
then Z O ' = V I , Z O ' = Z I , so Eq . d ) O ' = I . Eq . e )
[0133] Thus has the effect of an input, i.e., is the noise
equivalent input of error Z, which is the result of interfering
noise N.
[0134] The ratio of the noise equivalent input to the interfering
noise N which caused it is the noise sensitivity .PSI.. This is
defined: 6 O ' N . Eq . f )
[0135] We get a little more direct meaning of .PSI. by noting that:
7 g = Z O ' , so O ' = Z g . Also Z = V , so = V / N g . Eq . g
)
[0136] Thus we see that the sensor noise sensitivity .PSI. is the
change in sensor output V divided by the change in the interfering
noise N, all divided by the sensor gain g whereby the change in
sensor input I changes the sensor output V.
[0137] Put another way, .PSI. is the sensitivity of the sensor's
output V to an interfering noise N, all divided by the sensitivity
of the sensor's output V to signal input I, i.e., .PSI. is the
inverse of SNR. Restated: 8 = V / N V / I
[0138] Since gain g is defined in Eq. b) as 9 V I ,
[0139] the above is just another way of writing Eq. g).
[0140] FIG. 4 is a graph showing the essential characteristic of
sensor #88, presented here to help illustrate the method. The
signal to noise ratio (SNR) changes a lot when an operating
parameter Q changes its condition.* By this I mean that the signal
gain g (13) changes only a few percent when the operating parameter
I.sub.sm (12) changes enough to cause the noise sensitivity .PSI.
(14) to change by a factor of two or more, or vice versa. *
Operating parameter Q can be any of a variety of physical
quantities able to change condition. It can be a chemical mixture
proportion, electric current, fluid pressure, etc. The change in
the condition of Q can be a magnitude, as in peak current I.sub.sm
changing condition from 0.2 to 0.4 Amp. Or it can be a change in
power supply voltage or source impedance, a change in frequency
used in a modulator, a change in direction of an applied force,
etc.
[0141] By SNR I mean the sensitivity of the sensor's output V to
the signal I divided by that to noise interference .PSI.. 10 In Eq
. b ) V I = g , and In Eq . g ) V N = g , so SNR = g g , or SNR = 1
Eq . 1 )
[0142] FIG. 5 is a SNR graph of the essential characteristic of
this 5 inch diameter sensor #88.
[0143] Operating parameter Q can be thought of as an input to a
modulator, or as the modulator itself. Functionally, a change in Q
causes a change in the SNR of the sensor.
[0144] Non-Contact Ammeter Implementation for Swain Meter
[0145] This section begins as 1995 page 31.
[0146] To build a non-contact DC ammeter according to this SNR
improvement invention you need at least two things:
[0147] 1) A clip or clamp sensor which has the essential
characteristic of the discovery shown in FIG. 4 between points (A)
& (B); namely, the signal gain g remains relatively constant
while the response =Z/g to a field H.sub.n changes substantially*
and repeatably (it can be calibrated) with some operating parameter
(a bias, local saturation, mechanical modulation, or as in FIG. 4,
the peak magnetization current I.sub.sm). It is not required that
the teachings of U.S. Pat. No. 3,768,011 be used.
[0148] 2) Support means, which can be electronic +/or mechanical,
which implement the method, i.e., the mathematical relation, to
produce a sensor output (V) which is a linear function of the input
current I to be measured. The sensor performs the correction by
making use of the essential characteristic (FIG. 4) or equivalent
to reduce the noise (error due to a magnet). This can be
implemented by herein FIG. 7. This is 1995 FIG. 9 with parts
removed so that operation is always in state {circle over (B)}
where all switches are set in position {circle over (4)}, and where
the Operating Parameter is I.sub.sm set at 0.4 Amp.
[0149] The method is illustrated by FIG. 6. Data describing the
noise sensitivity of the sensor are plotted on FIGS. 4 and 5. FIG.
6 is a simplified version of FIG. 5 which shows the operating point
selected for 5" sensor #88. When the Operating Parameter I.sub.sm
is set at 0.4 Amp the SNR is 29. This is a useful improvement over
the SNR of 13 typically resulting from use of our earlier
methods.
[0150] FIG. 7 shows the interconnections of parts used to build an
SNR Improvement by Selective Modulation implement previously called
a "Better SNR" sensor and implement. This starts where the cover
drawing (FIG. 2) of U.S. Pat. No. 3,768,011 filed in 1970 left
off.
[0151] FIG. 7 illustrates interconnections for my original 1995
claim 12 and also claim 13 which is herein claim 2. This apparatus
claim on 1995 page 51 includes:
[0152] " . . . said operating parameter I.sub.sm set to a
substantially greater magnitude than the magnitude corresponding to
the minimum signal to noise ratio, here called SNR, so that thereby
the said SNR is considerably increased over said minimum, so that
said non-contact ammeter has considerably greater accuracy in the
presence of said interfering magnetic field noise N."
[0153] The effect of 1995 claim 13 is to set all 1995 FIG. 9
switches to the {circle over (4)} position. This is shown in the
herein SNR improvement drawing FIG. 7.
[0154] For example:
[0155] The above requirement of 1995 claim 13 on page 51:
[0156] " . . . said operating parameter I.sub.sm set to a
substantially greater magnitude . . . "
[0157] is met when I.sub.sm means (12) is set by switch (18) to 0.4
Amp. These interconnections are defined on the bottom of 1995 page
32 and the top of page 33. Page 33 continues:
[0158] " . . . Polarity switch 19 goes to the {circle over (4)}
position, . . . and
[0159] " . . . gain switch 20 is in the high position . . . "
[0160] 1995 page 33 continues at paragraph 2:
[0161] " . . . During the {circle over (B)} state, the voltage
V.sub.c across resistor 17 and capacitor 16 are applied to polarity
switch 19 through low pass filter 21 which attenuates potentials,
both common and differential mode, above f.sub.o/3 . . . "
[0162] Herein FIG. 7 includes no switches because original 1995
claim 13 requires none. It states:
[0163] "A Swain Meter type non-contact direct current ammeter with
improved accuracy for measurement or control, which comprises:
[0164] a core, here called SQ, of low magnetic reluctance
material,
[0165] a coupling sense winding, here called N.sub.s, on said core
SQ,
[0166] an inverter with power supply, here called X, with output
terminals with a current i.sub.s flowing which has an average value
I.sub.s, and also a peak value I.sub.sm which is an operating
parameter, all of said currents flowing in either direction in said
output terminals,
[0167] a low input impedance means converting said average current
I.sub.s to an average output voltage V,
[0168] a current carrying conductor carrying a signal input current
I, which is to be measured or controlled, positioned so that said
current I influences said core SQ, and
[0169] said core SQ is within the effective range of an interfering
magnetic field noise, here called N, and
[0170] said coupling sense winding N.sub.s series connected with
said output terminals of said inverter X and said low input
impedance means converting,
[0171] said operating parameter I.sub.sm set to a substantially
greater magnitude than the magnitude corresponding to the minimum
signal to noise ratio, here called SNR, so that thereby the said
SNR is considerably increased over said minimum, so that said
non-contact ammeter has considerably greater accuracy in the
presence of said interfering magnetic field noise N."
[0172] Herein FIGS. 7 and 1 meet this requirement. For example:
[0173] " . . . a core, SQ . . . " is item 1 in FIG. 7 and herein
FIG. 1.
[0174] " . . . N.sub.s . . . " is item 2 in FIG. 7 and herein FIG.
1.
[0175] " . . . Inverter . . . X . . . " is item 15 in FIG. 7.
[0176] " . . . I.sub.s . . . " is item 4 in FIG. 7 and herein FIG.
1.
[0177] " . . . Signal . . . I . . . " is item 7 in FIG. 7, and FIG.
1.
[0178] " . . . Noise . . . N . . . " is caused by field 8 and
magnet 10 in FIG. 7 and in FIG. 1.
[0179] " . . . Operating Parameter I.sub.sm is set . . . " is item
(12) tied in position {circle over (4)} by wire to 18 in FIG.
7.
[0180] " . . . Low impedance means . . . " is items 16 and 17 in
FIG. 7.
[0181] In herein FIG. 7 the filter 21 is defined on page 33, par. 2
above, with its connection.
[0182] " . . . Ammeter . . . " is supported with means in FIG. 7.
The filter 21, the amplifier 26, and the output 28 condition the
voltage V.sub.c.
[0183] Herein FIG. 7 had no need of switch related parts 18, 23,
24, 25, 19, 20, and 22. These are discussed on 1995 pages
32-34.
[0184] The operating parameter I.sub.sm is " . . . set to a
substantially greater magnitude . . . " by the wire in herein FIG.
7 which continuously joins the I.sub.sm 4 terminal to the
connection for switch 18 to cause operation at 0.4 Amp
continually.
[0185] This implements the method illustrated in FIG. 6. As claimed
in 1995, " . . . said operating parameter I.sub.sm (is) set to a
substantially greater magnitude . . . ", i.e., to 0.4 Amp, not 0.2
Amp, so that " . . . thereby the said SNR is considerably increased
. . . ", i.e., from 13 to 29.
[0186] FIG. 7 herein is my 1995 FIG. 9 with parts used for the
"Combiner Species" removed because they are not needed for this SNR
Improvement by Selective Modulation invention. FIG. 7 is preferred
over a cut down version of 1995 FIG. 11 because FIG. 7 is
simpler.
[0187] FIGS. 6 and 7 herein also serve to illustrate the method
claimed in my 1995 claim 12. My 1995 page 50 includes:
[0188] "and adjusting said means, including said N.sub.s and said
I.sub.sm, so that the change in said gain g is considerably less
than the change in said noise sensitivity .PSI., as said noise
sensitivity .PSI. is reduced from a maximum to a value considerably
less than said maximum, said reduced being accomplished by altering
the value of said means, especially the number of turns on said
winding N.sub.s and the said peak inverter current I.sub.sm, said
altering being preferably in the direction of a greater value of
the product of said N.sub.s and said I.sub.sm,
[0189] and operating said sensor with said product of said N.sub.s
and said I.sub.sm set so that said noise sensitivity .PSI. is
considerably reduced below said maximum,
[0190] thereby constructing and operating said sensor with said
reduced error in zero offset due to said noise N."
[0191] FIG. 6 herein shows I.sub.sm set to 0.4 Amp. FIG. 7 herein
shows the inverter operated continuously at operating parameter
level 4.* * Operating Parameter level 4 in my 1995 Application
corresponds to 0.4 Amp. This is apparent in 1995 Table I on page
24. It is also seen in the last sentence of 1995 page 32 and the
first sentence of page 33.
[0192] In FIG. 7 a special inverter is connected in series with the
winding on the core of the non-contact sensor. This core may be
solid, or split to form a clamp or clip. Capacitor C shunted by
resistor R.sub.s are also in series. All are constructed so that
the average current I.sub.s flowing in the loop is proportional to
the input current I.sub.i. Then the average voltage V.sub.c across
C and R.sub.s is also proportional to I.sub.i. Voltage V.sub.c is
the input signal to the filter and amplifier combination (21) and
(26).
[0193] In FIG. 7 if the capacitor C (16) is large, and also if
resistor R.sub.s (17) is large, the time required for V.sub.c to
reach a final value in one state can be excessive. This and other
reasons led us to build a filter and an operational amplifier which
remove the inverter frequency f.sub.o components of v.sub.c and
provide gain. Then the output V.sub.o can be 1 Volt per Ampere
input current (7).
[0194] It is well known in the art how to build suitable filter
(21) and amplifier (26) combinations. Then R.sub.s (17) can be
small, i.e., less than 100 ohms; and C (16) can be moderate, say
470 microfarad.
[0195] In FIG. 7 the special inverter 15 operating at frequency
f.sub.o is series connected with the sensor's coupling sense
winding 2 and the parallel combination of capacitor 16 and resistor
17. Input current 7 influences the magnetic material in the core 1,
and so also does the magnet 10. So the average current 4 in the
loop produces a voltage V.sub.c across capacitor 16 and resistor 17
which is proportional to the input current 7, and also proportional
to the effect of noise magnet 10 and its non-uniform field 8. In
this implementation, the means driving the operating parameter
I.sub.sm (12) at 0.4 Amp. is a fixed lead connecting to a resistor
selected to operate I.sub.sm at 0.4 Amp continuously. No switch is
needed.
[0196] Construction and Results
[0197] Several preliminary forms of this invention have been built
and tested with mixed results. The best so far uses the
implementation shown in FIG. 7 and a 5" diameter aperture clip #88,
constructed using structures and processes outlined in U.S. Pat.
No. 3,768,011 and in the same general form (see FIG. 1) as clips
sold December 1995. The top part of 1995 page 36 shows that the
steel core* 1 has 5 layers of 0.725" wide, 4 mil thick type D steel
tape from Magnetics in Butler, Pa. The clip's coupling sense coil 2
has about 1000 turns of #22 magnet wire with a resistance of 3 or 4
ohms. At point (A) on FIG. 4 the peak magnetization current 12 is
about 0.2 A, and at point (B) it is about 0.4 A. Operation is
continuous at point (B), where I.sub.sm=0.4 A. Point (A) at 0.2 Amp
is closer to that used in the prior art. * A low reluctance ferrite
or low reluctance steel laminations may be used for the core 1. So
far we have gotten better results with the 4 mil steel tape.
[0198] FIG. 4 plots the equivalent input current of the zero offset
Z due to a standard magnet as a function of I.sub.sm, the peak
current in the coupling sense winding N.sub.s. This 0.2 to 0.4 Amp.
peak current is flowing in N.sub.s.congruent.1000 turns on a 5"
diameter core, SQ. What really counts is the peak magnetic field
intensity H.sub.sm acting on the steel of the core. Since 11 H sm =
N s I sm 1 ,
[0199] where l is the mean flux path length, we can reduce I.sub.sm
if we increase N.sub.s, or reduce l, etc.
[0200] 1995 Page 36 also shows that capacitor 16 is 470 .mu.F.
Resistor 17 is 200 ohms. The filter 21 has a cutoff frequency of
about 100 Hz.
[0201] The implementation, outlined in FIG. 7, runs on 12 volts
with f.sub.o roughly equal to 400 Hz.
[0202] When tested with a non-uniform magnetic field H.sub.n from a
nearby speaker magnet, the zero offset error was one ampere
equivalent input under the previous conditions not using this
invention. The noise or zero offset error in the SNR improved
output was generally less than .+-.0.5 Amp. equivalent input
current. This is a two to one benefit. The benefit is usually 2 to
4.
[0203] 1995 FIGS. 4 and 5 show that 5 inch diameter aperture clip
#88 had a benefit of at least two, but it could be run up to four
to one.
[0204] The usual zero offset error rating for Swain Meter 5" clips
is less than .+-.40 mA equivalent input current due to the uniform
(H.sub.u) field of the earth (H.sub.e).
[0205] In FIG. 7 the output V.sub.o (28) of amplifier (26) was
proportional to the input current (7). It was substantially free of
components of the inverter (15) at frequency f.sub.o. The zero
offset sensitivity to magnetic interference noise (10) was reduced
to less than half that with conditions prior to the SNR Improvement
invention, so the output V.sub.o had improved SNR by over a factor
of two. This acts to improve accuracy of measurement of direct
current because the zero offset error due to a non-uniform
(H.sub.n) field is reduced.
[0206] Conclusion
[0207] This SNR Improvement method can be widely used to improve
the accuracy of sensors and implements for measuring and/or
controlling physical quantities. Our experience to date is
primarily with reducing zero offset error noise from interfering
magnetic fields acting on non-contact DC ammeters.
* * * * *