U.S. patent number 3,671,814 [Application Number 05/136,397] was granted by the patent office on 1972-06-20 for electromagnet with a field-responsive control system.
This patent grant is currently assigned to Voith Getrieb KG, Heidenheim (Brenz), Federal Republic of. Invention is credited to Heinrich Dick.
United States Patent |
3,671,814 |
|
June 20, 1972 |
ELECTROMAGNET WITH A FIELD-RESPONSIVE CONTROL SYSTEM
Abstract
In an electromagnet, to generate a force which is independent
from the armature position, an on-off control system for the
excitation current is provided which operates as a function of the
magnetic field intensity. The control system causes the excitation
current to oscillate between two values and thus have a constant
mean value according to a preset desired value. The timelag of the
inductivity or its change between two close values is utilized for
measuring the magnetic flux density and for a comparison with a
desired value.
Inventors: |
Heinrich Dick (Heidenheim,
DE) |
Assignee: |
Voith Getrieb KG, Heidenheim
(Brenz), Federal Republic of (N/A)
|
Family
ID: |
5768784 |
Appl.
No.: |
05/136,397 |
Filed: |
April 22, 1971 |
Foreign Application Priority Data
|
|
|
|
|
Apr 22, 1970 [DE] |
|
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20 19 345.7 |
|
Current U.S.
Class: |
361/154; 361/170;
361/188 |
Current CPC
Class: |
G05F
7/00 (20130101) |
Current International
Class: |
G05F
7/00 (20060101); H01h 047/32 () |
Field of
Search: |
;317/123,DIG.6 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: L. T. Hix
Attorney, Agent or Firm: Edwin E. Greigg
Claims
1. In an electromagnet of the type that includes (a) a magnet coil,
(b) an iron cladding surrounding said magnet coil, (c) an armature
movable within said coil and defining an air gap with a part of
said cladding, (d) means for supplying an excitation current to
said coil to generate a magnetic field passing through said air gap
and exerting an inwardly directed attracting force to said armature
and (e) means exerting an outwardly directed force on said
armature; the intensity of said magnetic field being dependent upon
the position of said armature from said cladding, the improvement
comprising a circuit means for regulating said excitation current;
and circuit means including A. a magnetic field
intensity-responsive means disposed within said cladding and
responding at least indirectly to the intensity of said magnetic
field, B. setting means for obtaining signals corresponding to a
desired value of magnetic force, C. comparator means for comparing
the output signals of said magnetic field intensity-responsive
means with those of said setting means and D. switching means for
regulating the admission of said excitation current to said magnet
coil in response to the output signals of said comparator means for
providing in said air gap a magnetic field being of constant
magnitude at least as to a mean value with respect to time and
being
2. An improvement as defined in claim 1, wherein said means defined
in (A) responds directly to the intensity of said magnetic field
and is disposed
3. An improvement as defined in claim 2, including A. a field
resistor constituting said magnetic field intensity-responsive
means, B. means for applying the voltage drop across said field
resistor to said comparator means for comparing said voltage drop
with a desired potential difference prevailing at said setting
means and C. an amplifier having input means for receiving the
output signals of said comparator means, said amplifier having
output means connected to said switching means for supplying the
latter with an amplifier output current which is at least decreased
when said voltage drop exceeds said desired potential difference
and which is increased when said desired potential
4. An improvement as defined in claim 3, including A. a resistance
bridge circuit containing 1. said field resistor, 2. a variable
resistor constituting said setting means, B. means for connecting
two diagonal measuring points of said resistance bridge circuit to
two inputs of said amplifier, C. a first transistor having a base
to which the output signals of said amplifier are applied; said
first transistor having a collector-emitter leg, D. a second, or
power transistor having a base to which the signals of the
collector-emitter leg of said first transistor are applied; said
second transistor having a collector-emitter leg; said first and
second transistors forming part of said switching means and E. a
direct voltage source connected to diagonal feed points of said
resistance bridge circuit and, through the collector-emitter leg of
said
5. An improvement as defined in claim 1, wherein said means defined
in (A)
6. An improvement as defined in claim 5, including A. an auxiliary
winding disposed inside said magnet coil and constituting said
magnetic field intensity-responsive means; said auxiliary winding
generates output signals induced therein by the excitation current
flowing in said magnet coil and B. an integrating circuit connected
to said auxiliary winding; said integrating circuit is connected to
said comparator means for applying
7. An improvement as defined in claim 5, wherein said magnetic
field intensity-responsive means is constituted by said magnet coil
itself; and improvement further includes A. a differentiating
circuit connected to said magnet coil to receive therefrom output
signals that include a voltage component due to the self-induction
in response to the excitation current; said differentiating circuit
is adapted to suppress a direct voltage component of the coil
output signal due to the ohmic resistance of said coil, B. a first
integrating circuit connected to said differentiating circuit for
delivering a voltage proportional to the change of the magnetic
flux in said air gap and C. a second integrating circuit connected
to said first integrating circuit for delivering a voltage
proportional to the product of the momentary value of the
excitation current and the momentary value of the inductance of
said magnet coil; said last named voltage is applied to said
comparator means including said setting means.
Description
This invention relates to an electromagnet with a stationary,
ironclad coil and a movable armature projecting through an open
location of the iron cladding; said armature is drawn to the iron
cladding by the magnetic field generated by virtue of current
flowing through the coil. Assuming a constant excitation current,
upon movement of the armature towards the iron cladding, the flux
density increases. The electromagnet is further of the type that
includes a force accumulator (gravitational force, spring, pressure
cushion) urging the armature to move away from the iron
cladding.
It is known to generate a distance-independent linear force by
means of a plunger coil device which is characterized by a circular
cylindrical magnetic field generated by a permanent magnet or by a
direct current and having radially extending short magnetic field
lines into which a thin-layer coil is axially immersed. Depending
on the magnitude of the current flowing through the plunger coil,
the latter is exposed to a greater or lesser axially orientated
force which is independent form the position of the coil provided
that all turns of the plunger coil are disposed in the undisturbed
magnetic field. A plunger coil device of this kind, however, is
capable of generating only comparatively small forces. Plunger coil
devices designated for larger forces are unproportionately large
and heavy. The best plunger coil devices are able to produce a
force corresponding to approximately 0.4 times their own dead
weight. It is also a disadvantage that the required control power
is very high and that the coil constitutes the moving part. Apart
from their large weight, plunger coil devices are very expensive
due to their complex structure and the requirements for high
precision in the manufacture of the coil.
Although relatively large forces may be generated by a small magnet
of the kind mentioned heretofore, the attracting force on the
armature depends to a large extent on its position. Thus, the
attracting force increases hyperbolically as the armature
approaches the coil core. Although, by a suitable design of the
magnetic field (partial field line shunt) an approximately linear
force/distance curve may be obtained in zones and thus the effect
of distance within each zone is substantially eliminated, such
design restricts the magnet to the zone of minimum power. To permit
the generation of larger forces independently of the distance would
necessitate the provision of a magnet of very large dimensions. The
disadvantageous results are extensive space requirements, large
weight and a high current consumption. Moreover, a
stroke-independent attracting force can be achieved only along
short distances of displacement caused by the attracting force.
It is an object of the invention to provide an improved
electromagnetic device of simple and light-weight structure which
is adapted to generate a relatively large, distance-independent
linear force and with which the magnitude and the direction of said
force may be altered in a rapid manner.
Briefly stated, according to the invention, there is provided an
electromagnetic device of the aforeoutlined type which includes a
means for regulating the excitation current. Said means comprises a
transducer element which is responsive to the magnetic field
intensity and which is disposed in the air gap between the armature
and the iron cladding. The transducer element, which may be a
Hall-generator or a field resistor, upon command by a desired value
setter, regulates the excitation current to obtain a magnetic field
excitation which is constant at least as far as its average value
with respect to time is concerned thus resulting in a constant,
distance-independent magnetic force.
The invention will be better understood as well as further objects
and advantages of the invention will become more apparent from the
ensuing detailed specification of several exemplary embodiments
taken in conjunction with the drawing.
BRIEF DESCRIPTION OF THE DRAWING
FIG. 1 is a circuit diagram of an embodiment of the invention,
including an electromagnet in longitudinal section;
FIG. 2 is a circuit diagram of a further embodiment of the
invention, including, in longitudinal section, an electromagnet
designed as a solenoid valve;
FIG. 3 is a circuit diagram of still another embodiment of the
invention, including an associated electromagnet in longitudinal
section;
FIG. 4 is a circuit diagram of still a further embodiment of the
invention, including an associated electromagnet in longitudinal
section and
FIG. 5 is a circuit diagram of still another embodiment.
DESCRIPTION OF THE EMBODIMENTS
Turning now to FIG. 1, there is shown an electromagnet generally
indicated at 1, having a coil 2, and iron cladding, 3, 3'
surrounding the coil 2 and an armature 4 axially movable therein.
The radial face 4a of armature 4, together with a projection 5
integral with the iron cladding 3 in the coil core defines an air
gap 6. A spring 7 is disposed between the projection 5 and the
radial face 4a of the armature 4 to urge the latter outwardly thus
tending to increase the air gap 6.
A field resistor 8, responsive to the magnetic field strength, is
affixed (e.g. glued to the end face of the projection 5. The field
resistor 8 may be constituted by a semiconductor element which
alters its resistance in the same sense as the change of a
traversing magnetic flux. Thus, the voltage drop across the field
resistor is a direct measure of the attracting force of the
armature. The two terminals of the resistor 8 are brought out
through a bore provided in cladding 3.
The electronic circuit associated with the magnet 1 comprises a
regulator part 9 and a switch part 10, which are connected through
conductors 12 and 13 to a voltage source such as a battery 11. The
regulator part incorporates a resistance bridge circuit formed of
the field resistor 8, as well as a fixed resistor 14 and a variable
resistor 15, 16. Between the resistors 14 and 8 there is disposed a
measuring point 148, whereas another measuring point 165 is located
between the two resistor parts 16 and 15 of the variable resistor
15, 16. From the battery 11 a constant voltage is applied to a feed
point 168 between the resistors 16 and 8 and to a feed point 145
between resistors 14 and 15. The two potentials, of which that at
165 may be arbitrarily set, are compared with each other in the
resistance bridge circuit. This tapped voltage is applied through a
series resistor 23 to one input -E of an amplifier V which has two
inputs +E and - E and an output A. The measuring point 148 is
connected through the series resistor 24 to the input + E. When the
resistance of the field resistor is reduced assuming -- initial
potentials at both measuring points 148 and 165 -- the potential at
point 148 will increase relative to that at point 165. As a result,
the potential increases at output A relative to the terminal .+-.0
of the amplifier, assuming an initial state of identical
potentials. The amplifier output A is connected to the base of a
transistor T.sub.1. The increase of potential at the amplifier
output A and thus at the base of the transistor T.sub.1 caused by a
drop of the resistance of the field resistor 8, allows current to
flow through the collector-emitter leg of the transistor T.sub.1
thus driving a power transistor I.sub.2. As a result, the
collector-emitter leg of the power transistor T.sub.2 -- connected
in series with the coil 2 of the magnet 1 between the feed
conductors 12 and 13 -- is rendered conductive and thus the coil
feed circuit is closed.
The aforedescribed response to a change in the magnetic field
strength in air gap 6 takes place practically without a time lag.
Stated in different terms, the coil 2 is energized as soon as the
potential at the point 148 increases with respect to the adjustable
potential at point 165 due to a decrease in the resistance of the
resistor 8.
Starting from the preceding low magnetic flux, a stronger magnetic
field will thus be progressively built up in the air gap 6 so that,
among other effects, the armature 4 is slightly displaced towards
the projection 5 against the force exerted by the spring 7. The
increase of the magnetic field intensity causes an increase of the
resistance of the field resistor 8. Such increase, in turn results
in a dropping of the potential at the measuring point 148 relative
to the potential at the measuring point 165. The dropping of the
potential continues as long as the potential at 148 is smaller than
the potential at 165. A negative input voltage will thus be applied
to the amplifier V and accordingly, the potential at the output A
will become increasingly negative relative to the zero point .+-.
0. As the output potential passes the .+-. 0 point value in the
direction of negative values at the base of transistor T.sub.1, the
collector-emitter leg of the latter becomes non-conductive, whereby
the power transistor T.sub.2 is cut off. This results in a
de-energization of the coil 2.
By virtue of a bypass diode D connected parallel with the coil 2 in
the direction of the preceding current flow, the magnetic field in
the air gap 6 decays exponentially and relatively slowly. The
simultaneous increase of the air gap due to the outward movement of
the armature 4 as urged by spring 7, reduces the flux passing
through the field resistor 8 and thus, the resistance of the latter
drops. This reduction in resistance once again causes the coil 2 to
be energized. This results in an increase of the resistance of the
field resistor 8 which, in turn, causes the coil 2 to be
de-energized, etc.
The aforedescribed regulation of the magnetic field intensity may
be regarded as a two-point control which oscillates with a systemic
frequency. This frequency comprises square-wave pulses of identical
amplitude and represents the on-off switching frequency for the
coil current. A magnetic field excitation of greater or lesser
intensity will be needed dependent upon the position of the
armature 4 and the pulling force to be exerted by the magnet.
Accordingly, a lower or higher frequency will be set by the system.
This is governed by the voltage drop which is determined by the
resistance of the field intensity-responsive resistor 8 and which
is compared with a set (desired) potential difference. By virtue of
the latter, it is possible in practice to compare and regulate the
magnetic flux with another desired value.
A practical application of the magnet according to the invention is
illustrated in FIG. 2. The control circuit shown therein is fully
equivalent to that illustrated in FIG. 1. In the resistor bridge
circuit of FIG. 2, instead of the variable resistor 15, 16 of FIG.
1, two fixed resistors 15a and 16a are provided between the two
feed points 145 and 168. The amplifier input -E is connected to the
variable output of a function generator 14" which may be, for
example, a sinusoidal generator adjustable with respect to
frequency and amplitude or a generator adapted to supply from a
given moment, upon receipt of a command signal, a defined ramp
function with adjustable parameters. Or, a tacho generator may be
used which produces an rpm-analogous potential difference at the
feed point 145 with respect to the other feed point 168.
The magnet 1' of FIG. 2 is an electrohydraulic transducer wherein
the armature is formed of a piston 4' of a pressure limiting valve
generally indicated at 17. The displacement of the piston 4' in
response to the magnetic field results in a greater or lesser
restriction of the volumetric flow delivered by the pump 20 through
the throttle formed by the control lands 18 and 19. Depending on
the attracting force of the magnet 1', a greater or lesser pressure
is built up upstream of the throttle (i.e. in the delivery side of
the pump 20). The pressure which is indicated by the pressure gauge
21 may be directed through the connecting conduit 22 to any desired
loads and may be limited as to its maximum value by means of the
pressure limiting resistor 24. The generated pressure is also
transmitted to the radial end face of piston 4' in the air gap 6
through a radial and an axial bore provided in the piston 4'. In
this manner, in the air gap 6 a pressure cushion is generated which
acts against the attracting force of the magnet. The magnitude of
said pressure cushion is immaterial, provided a counter-force is
produced which will counteract the attracting force of the magnet.
In order to ensure that the forces urging the piston 4' outwardly,
and thus the attracting forces generated by the magnet, do not
become excessive and that the generated pressure does not affect
the entire cross section of the piston, there is provided a
reducing pin 7' which is slidably disposed in the axial bore of
piston 4' in a fluid tight manner and which, exposed to the
generated pressure, abuts the projection 5. The pressures which may
be controlled by the electrohydraulic transducer 1', 17 are very
large. Pressures of up to 50 kg/cm.sup.2 or more may be controlled
with ease by means of an electromagnet having a weight of
approximately 200 g. The oscillation superimposed on the entire
system, enables the transducer to respond very rapidly and permits
a corresponding output signal to follow with great rapidity the
changes in the input values.
Turning now to FIG. 3, in the control system for regulating the
magnetic force, a so-called Hall generator 8' is used which,
similarly to the field resistor 8 of FIGS. 1 and 2, is also
disposed in the air gap 6. The generator 8' requires a constant
feed current which is supplied by a voltage source 25. At the two
output terminals of the generator 8' there appears a voltage which,
assuming a constant feed current, is proportional to the magnetic
flux traversing the generator. If the magnet coil fed directly by
the power amplifier through a diode D.sub.1 is energized, the Hall
generator will supply a voltage which increases with the inward
movement of the armature and the corresponding increase of flux
density. The amplifier inputs +E, -E are connected to two circuits
in which current flows in opposite directions. One circuit, formed
by the lower resistor part 15' of a variable resistor 15', 16', a
series resistor 23' and the amplifier input, is adjustable at will
to set the driving potential difference by varying the location of
the tapping point 165'. The outer circuit is formed by the Hall
generator 8' and a series resistor 24'. The polarity of the Hall
generator in the circuit must be such that the Hall voltage opposes
the driving potential difference across the resistor part 15'. When
the Hall voltage exceeds the potential difference across the
resistor 15', the potential of the point 168' shifts towards the
negative range so that an input signal of a polarity in accordance
with the terminal designation appears at the amplifier input -E,
+E. The input signal causes a corresponding amplified potential
increase with respect to .+-.0 at the amplifier output A. Because
of the blocking effect of the diode D.sub.1, the shift of the
amplifier output into the positive range causes the magnet coil to
be de-energized. In response to the now decreasing magnetic field
and the outward movement of the magnet armature 4 as urged by the
spring 7, the Hall voltage will drop. At one moment during this
process the Hall voltage will become smaller than the voltage
increase across the resistor 15, and the point 168' will become
positive relative to the other measuring point 148'. An input
signal with a polarity opposite to that of the terminal designation
will then appear at the amplifier input -E, +E resulting in the
appearance at the amplifier output A of a correspondingly amplified
powerful potential drop relative to .+-.0 so that the magnet coil 2
is energized through the diode D.sub.1. The aforedescribed
energization and de-energization is repetitive similarly to the
embodiment described in connection with FIG. 1. Here too, a
systemic switching frequency will appear.
The desired value of coil excitation for the magnet according to
FIG. 3 (i.e. the force to be exerted by the magnet) may be adjusted
on the variable resistor 15', 16' or may be preset by a function
generator provided instead of the variable resistor similarly to
FIG. 2. Or, the auxiliary voltage source 25 may be replaced by a
function generator of the kind heretofore described for setting the
desired value for coil excitation. The Hall voltage generated by
the Hall generator is proportional to the product of its feed
current and magnetic flux so that the magnetic intensity can also
be affected by the control current which flows through the Hall
generator.
By means of the embodiment illustrated in FIG. 4 a voltage
responsive to the magnetic field intensity is generated in a
different manner. The magnet system is provided with an auxiliary
winding 2" disposed within the coil 2'. This auxiliary winding may
be regarded as the secondary winding of a transformer, the
secondary voltage of which depends on the change, with respect to
time, of the field line density of the surrounding magnetic field.
As already described, during the control of the excitation current
a systemic oscillation takes place. The exciter coil 2' is supplied
practically only with the positive half waves of a "square-wave"
voltage whose mean value with respect to time is equal to the
excitation current required for the specified armature pull. This
means that the magnetic field is continuously increased and then
decreased through the bypass diode D. The said magnetic field is
detected by the auxiliary coil 2" on the terminals of which a
voltage appears which is proportional to the change of magnetic
flux with respect to time. Since it is desired, however, to obtain
a voltage which is proportional to the flux itself, the voltage
delivered by the coil has to be integrated with respect to time.
For this purpose there is provided an amplifier V.sub.1, the inputs
of which are connected with the output terminals of the auxiliary
winding 2" and which is associated with a feedback capacitor C. The
capacitive feedback of the amplifier output to one of the amplifier
inputs gives the amplifier its integrating characteristics. Thus,
between measuring points 148" and 168" of the resistance bridge
circuit a generator is provided which delivers a voltage
proportional to the magnetic flux in the magnet 1". The effect of
this generator and the mode of operation of this embodiment is
equivalent to that of the precedingly described embodiment.
FIG. 5 shows a practical application of the invention wherein the
magnet is void of any separate magnetic field-sensitive transducer.
The role of the transducer necessary for the regulation of the
excitation current is taken over by the magnet coil itself which is
shown as an inductance L and as an ohmic resistance R.sub.L. L is
the momentary inductance of the magnet system depending on the
position of the armature of the magnet and the coil size, while
R.sub.L is the ohmic resistance of the copper windings. The circuit
system is based on the principle that the excitation current in the
magnet system can be measured as a voltage drop across a measuring
resistor R.sub.M which is serially connected to the coil L,
R.sub.L. This current or the measuring voltage taken from the
terminals of the measuring resistor R.sub.M contains a constant
direct voltage component resulting from the voltage drop across the
two ohmic resistances R.sub.L and R.sub.M in addition to a voltage
component which is proportional to the product of the induction and
the change of the excitation current and which varies in accordance
with the buildup and decay of the magnetic field. The aforenoted
constant direct voltage component of the measuring signal initially
obtained is first suppressed by means of a differentiating circuit
formed of a capacitor 19 and a resistor 30 and is then integrated
in a first integrating stage V.sub. 1, C.sub.1. The output voltage
of the latter is proportional to the change of flux in the magnet
system L, R.sub.L. This signal is again integrated in a second
integrating stage V.sub.2, C.sub.2 to provide a voltage which is
proportional to the flux density prevailing in the magnet. Since
the circuit is based on a voltage which is proportional to the
product of the momentary induction and the momentary excitation
current, it follows that the signal obtained from the output of the
second integrating stage is, too, dependent upon the position of
the armature. Thus, similarly to the previously described
embodiments, the armature position too is taken into consideration
in the measurement. The signal finally obtained is compared with a
desired potential adjustable at a variable resistor 31, and,
depending on whether the signal or the desired potential
predominates, the coil L, R.sub.L is connected to or disconnected
from the current supply through the switching amplifier V' and the
two transistors T.sub.1 and T.sub.2.
Although the circuit according to FIG. 5 is more complex from a
technological point of view, it is advantageous in that the
inventive principle can be practiced with conventional
electromagnets without any modification of the magnet system
itself.
In the different embodiments described hereinabove, the momentary
magnetic field strength in the magnet is measured in four different
ways and the work coil is energized or deenergized depending on
whether the signal characterizing the magnetic field strength is
greater or smaller than an adjustable desired value. Since the
buildup of a magnetic field or the diminishing of an existing
magnetic field are phenomena which have a timely course and since
the magnetic field excitation, with preset and closely adjacent
values, requires a certain period of time, this period, as proposed
by the invention, may be utilized for signal measurement and for
comparison with desired values. Depending on the results of the
comparison, corrective measures are taken. The inertia inherent in
the inductance provides sufficient time for a two-point regulation
which is a feature utilized in accordance with the invention.
It is thus seen that the transistor circuit connected to the output
of the amplifier in the embodiments is, in fact, a solid-state
on-off power switch which converts the regulating system for the
excitation current into a two-point control system. This feature
involves two advantages. In the first place, the on-off control
limits the turn-on period of the power transistor in the power
dissipation range to the required minimum (due to the steep voltage
increase at the transistor the range of power dissipation is very
rapidly traversed), so that the losses at the power transistor are
maintained at a very small value. In the second place, the magnet
armature is subjected to small oscillations which eliminate static
friction and hysteresis effects.
* * * * *