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BACKGROUND OF THE INVENTION
This invention relates to variable reluctance actuators of either the
linear or rotary type, and particularly to those whose mechanical force or
position may be controlled throughout a range of movement of their movable
element.
Variable reluctance electromagnetic actuators are well known in the art as
exemplified by the linear motion solenoid devices shown in U.S. Pat. Nos.
3,671,814, 4,434,450 and 4,450,427. Although such devices disclose the
possibility of controlling the force imposed by such actuators in a
constant, controlled manner independent of actuator position, in practice
they are unable to obtain this result. For example, in U.S. Pat. No.
3,671,814, a flux sensor is placed in the variable gap of the actuator's
magnetic circuit for controlling coil current such that the magnetic field
experienced by the flux sensor remains constant independent of position of
the actuator. Although holding the field in the variable gap constant
theoretically should produce constant force, in reality motion of the
actuator changes the boundary conditions of the magnetic field such that
the force produced varies significantly with motion. If the flux sensor is
not placed in the variable gap, as in U.S. Pat. Nos. 4,434,450 and
4,450,427, a further variable is introduced because, as the actuator
retracts, flux leakage circumventing the variable gap increases.
Accordingly, holding constant the magnetic field experienced by such a
fixed gap flux sensor likewise does not usually produce constant force
independent of motion. Moreover, permitting the variable gap to close
completely upon retraction, as taught by the latter two patents, further
varies the actuating force by increasing it abruptly as the actuator nears
full retraction.
None of the aforementioned variable reluctance actuators has a built-in
capability for position sensing or position control between two stop
positions. However, an integral means of position control for such
variable reluctance actuators is disclosed in the copending,
commonly-owned U.S. patent application of one of the inventors herein,
Ser. No. 639,187, filed Aug. 9, 1984. As disclosed in such patent
application, coil current which produces the actuator's magnetic field,
and the instantaneous magnetic flux density of such field, are sensed
concurrently and signals representative of each are fed to a divider which
divides the coil current magnitude by the flux density magnitude, yielding
a signal proportional to actuator position. Such a system, however,
requires both a flux sensor and a divider in the position-sensing circuit
which is costly. U.S. Pat. No. 3,413,457 discloses a general-purpose
analog computer circuit using a Hall effect sensor as a divider in a
constant-reluctance magnetic circuit. However, there is no suggestion of
how such principle could be applied to a variable reluctance magnetic
circuit to indicate position of a movable element.
SUMMARY OF THE PRESENT INVENTION
The present invention overcomes the foregoing disadvantages of force
control and position-sensing systems utilized previously in variable
reluctance actuators, and is applicable both to linear and rotary motion
types of actuators. The word "actuator" is used broadly herein to include
sensors as well as devices used principally to produce force or motion.
Substantially constant force control, independent of actuator position, is
achieved by variation, rather than stabilization, of the magnetic field
produced by the coil and sensed by the flux sensor. In essence, coil
current is controlled in response to a variably modified flux sensor
output signal, the modification being appropriate to compensate for such
variables as flux leakage and boundary conditions which change with
position. Also, the coil configuration is distributed nonuniformly
relative to the movable element of the actuator, and the variable gap is
prevented from closing completely upon full retraction. The result is that
the fIux density of the magnetic field produced by the coil, whether
measured in the variable gap or elsewhere, varies significantly during
motion, while the retracting force varies very little and, in any case, to
a much lesser degree than the flux density.
Simplified position sensing, without the need for a divider, is obtained by
automatic variation of the excitation current (or equivalent variation of
the excitation voltage) of the Hall sensor so that the output of the
sensor is always proportional to coil current. In a variable reluctance
magnetic circuit, such variation results in the sensor's excitation
current being representative of the position of the movable element
causing the variable reluctance.
The foregoing and other objectives, features, and advantages of the
invention will be more readily understood upon consideration of the
following detailed description of the invention, taken in conjunction with
the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a side, cross-sectional view of a simplified, exemplary variable
reluctance linear actuator constructed in accordance with the present
invention.
FIG. 2 is a view taken along line 2--2 of FIG. 1.
FIG. 3 is a diagram of an exemplary electrical circuit for producing
constant force control, usable with the actuator of FIG. 1.
FIG. 4 is a diagram of an exemplary electrical circuit for position sensing
and position control, usable with the actuator of FIG. 1.
DETAILED DESCRIPTION OF THE INVENTION
The mechanical structure of an exemplary, simplified variable reluctance
linear actuator constructed according to the present invention is shown in
FIGS. 1 and 2. The actuator employs a solenoid 10 wound on a spool 12
which may serve not only to provide the solenoid with shape but also as a
bearing for the movable element 14 of the actuator. Spool 12 would
typically be made of some type of nonmagnetic, nonconductive material,
such as nylon or polycarbonate. The element 14 is made of a suitable
magnetic material such as iron. As used herein, "magnetic material" is
defined as a material that exhibits enhanced magnetization when placed in
a magnetic field. The element 14, when placed within the solenoid 10 with
an electrical current therein, experiences a magnetic flux along its
longitudinal axis thereby producing a mechanical force tending to retract
it. Extension of the element 14 may be produced by an external or internal
opposing return force mechanism, such as a spring or fluid pressure
mechanism.
The actuator is provided with a first end cap 16, which also serves as a
stop for the movable element 14, a tubular case or core 18, and a second
end cap 20, all of which are preferably composed of magnetic material to
maximize the efficiency of the actuator. The end cap 20 is separated from
the casing 18 by a disc-shaped, nonmagnetic spacer 22 in order to provide
a location for a magnetic flux density sensor 26. The space 24 between the
inner surface 16a of the end cap 16 and the moving element 14 comprises a
variable reluctance air gap. This gap, whose reluctance varies with the
position of the element 14, accounts for the majority of the reluctance in
the primary magnetic circuit composed of the element 14, end caps 16 and
20, casing 18, the gap occupied by the spacer 22 and the variable gap 24.
The end cap 16 has a further spacer 16b of nonmagnetic material on the
inner surface thereof to prevent the variable gap 24 from closing
completely upon full retraction of the element 14, and the coil 10 is
shortened at its outer end 10a, i.e. its end most remote from the variable
gap 24, for reasons to be described hereafter.
An instantaneous magnetic flux density sensor 26 is disposed between the
end cap 20 and the casing 18 in the space created by the nonmagnetic
spacer 22. The spacer 22 extends completely through the primary magnetic
circuit of the actuator between the end cap 16 and casing 18 which,
although introducing some additional reluctance into the magnetic circuit,
serves to ensure symmetrical flux distribution and therefore an accurate
sample reading by the sensor 26. Preferably the sensor 26 comprises a Hall
effect transducer, although other flux sensors, such as magnetoresistive
devices which provide a signal representative of magnetic flux density,
might be used without departing from the principles of the invention.
Although a particular location of the sensor 26 is shown, it is to be
recognized that the sensor could be placed anywhere within the magnetic
circuit of the actuator. However, since the flux density of the field
produced by the coil does not vary identically everywhere in the magnetic
circuit, modifications to the control circuit may be appropriate for some
locations depending upon the characteristics of flux variation at those
locations.
It should be mentioned that the accuracy and effectiveness of the force
control, and position sensing and control, functions to be discussed
hereafter may depend on the quality of the magnetic material used in the
actuator structure. Preferably, such material should be as magnetically
soft as is feasible to minimize any unintended permanent magnetization
thereof and any resultant alteration of the actuator's magnetic circuit
characteristics.
Force Control System
The retracting force experienced by the moving element 14 as a result of
the current in the solenoid coil 10 is not, in reality, a simple function
of the total magnetic flux that the element experiences, nor of the flux
in the variable gap 24, nor of the flux in the gap defined by the spacer
22. A major variable to be taken into account is the fact that, as the
element 14 retracts, the area through which flux can leak from the element
14 to the casing 18 varies with the position of the element 14. Also, the
boundary conditions of the magnetic field in the variable gap 24 change
significantly as the position of element 14 changes. Moreover, if complete
closure of the variable gap 24 were permitted, the magnetic permeance of
the gap would increase abruptly as complete closure is approached. For all
of these reasons, controlling current in the solenoid coil 10, as the
element 14 is retracted, in such a way as to maintain constant the
magnetic field experienced by the flux sensor 26, regardless of where it
is placed, will usually not yield even an approximately constant
retracting force on the element 14. Instead, the sensed magnetic field,
whether in the variable gap 24 or elsewhere in the magnetic circuit, must
be controlled so as to vary with the position of the moving element 14 in
order to achieve substantially constant retracting force. Without such
control, retracting force is highly variable between full extension and
full retraction. For example, in the actuator of FIG. 1, retracting force
is relatively high at both full extension and full retraction, and lower
in the range of movement between these two extremes.
In the present invention, much of the substantial rise in retracting force
in the vicinity of complete retraction is eliminated by the provision of
the nonmagnetic spacer 16b which prevents complete closure of the variable
gap 24. The thickness of the spacer 16b will be different for each
different actuator design, but is easily determined for any design by
simply plotting retracting force against actuator position ("X" in FIG. 1)
while holding the sensed magnetic field constant, and thereby determining
the degree of retraction of element 14 which causes the force to begin to
rise rapidly near full retraction. The thickness of the nonmagnetic spacer
16b can then be selected so as to prevent closure of the gap 24 beyond
such point.
A significant, although more gradual, increase in retracting force in
proportion to greater extension of the moving element 14 would be produced
if the magnetic field experienced by the sensor 26 were held constant by
control of current in the coil. This phenomenon, caused by decreasing flux
leakage as the element 14 extends, and by changing boundary conditions of
the field in the variable gap, is corrected by the force control circuit
of FIG. 3, to be explained hereafter. The correction results in a
progressive decrease in sensed magnetic field during extension. This,
however, is accompanied by some relative elevation of the retracting force
in the vicinity of full retraction, despite the presence of the spacer
16b. It has been discovered that this latter elevation in force near full
retraction can be compensated for by distributing the turns of the
solenoid coil nonuniformly along the length of element 14. For example,
FIG. 1 shows shortening of the solenoid coil 10 at its end 10a remote from
the variable gap 24 such that a predetermined length "y" (FIG. 1) of
element 14, approximately equal to the length of the variable gap at the
point of retraction where such elevation in force begins without
shortening of the coil, is prevented from being coextensive with the coil
10 (although it is coextensive with the case 18) regardless of the
position of the element 14.
The final result of all of the foregoing adjustments is a retracting force
which is substantially constant throughout the range of motion of the
movable element 14, although the flux density of the magnetic field
produced by the coil varies significantly with such motion regardless of
whether such flux density is measured in the variable gap 24 or in the
fixed gap defined by the spacer 22. This is a somewhat incongruous result
from a simple theoretical point of view, because the retracting force of
element 14 would normally be thought to vary proportionally to the square
of the flux density, and therefore to a greater degree than the flux
density. Instead, the reverse is true, i.e. the flux density varies to a
greater degree than the force.
The circuit of FIG. 3 is the most significant part of the overall solution
to the constant force problem, because it effectively compensates both for
the variation in leakage flux between the moving element 14 and the casing
18, and the variation of the magnetic field boundary conditions, during
motion. Diode 30, connected to the power source, protects the circuit from
reverse voltage applications, and is connected to a voltage regulator 32.
Excitation current is supplied from current regulator 34 to the Hall
effect sensor 26 having excitation terminals 26a and 26b, and output
terminals 26c and 26d respectively.
An amplifier 38 controls the voltage on one of the excitation terminals 26b
so that one of the output terminals 26c is always kept at a common
reference potential. As a result, the flux sensor's amplifier 40
constitutes a simple amplifier, instead of a more complicated differential
amplifier having precision-matched resistors as is normally required. This
advantageous simplification of the circuit is applicable to virtually any
Hall sensor output circuit in a magnetic device.
The signal from output terminal 26d of the Hall effect sensor 26 is
presented to a summing junction 42 at the inverting input of amplifier 40
where it is compared to a force input reference signal which is adjustable
by means of adjustable potentiometer 43. The output of amplifier 40 is
presented to the inverting input of comparator 44, where it is combined
with a sawtooth signal at the noninverting input of comparator 44
generated by a sawtooth oscillator composed of amplifiers 46 and 48 and
their related circuitry. The output of comparator 44 controls the current
and/or voltage to the solenoid coil 10 by its control of a power
transistor 50 in a pulse width modulated switching mode dependent upon the
level of the output signal from amplifier 40. The result is such that when
the output signal from the Hall effect sensor 26 momentarily exceeds the
force input reference signal, transistor 50 decreases coil current, and
vice versa. Alternatively, the transistor 50 could be operated in an
analog mode, although power efficiency would be decreased. A flyback diode
52 is provided so that the current generated in the coil 10 by the
collapsing magnetic field, during periods when the transistor 50 is
switched off, recirculates through the coil causing the field to decay
exponentially rather than in an oscillatory manner.
Without the inclusion of resistor 54, the circuit of FIG. 3 would merely
control the transistor 50 so as to provide whatever coil current is
necessary to maintain the field sensed by the Hall effect sensor 26 at a
constant magnitude independent of position of the actuator, as in the
prior art. However, due to the feedback connection through resistor 54,
negative DC feedback is provided, and the DC gain of amplifier 40 is
thereby controlled. Such negative feedback requires that the output of the
Hall effect sensor change in order to effect a change in the coil current.
Accordingly, the current in the coil 10 does not increase with extension
to the extent that it otherwise would in the absence of the feedback
through resistor 54, with the result that the magnetic field measured by
sensor 26 is controllably decreased progressively with extension, to a
degree dependent upon the resistance of resistor 54. This compensates for
decreased flux leakage and varying boundary conditions caused by
extension, and thereby prevents the retracting force from increasing with
extension. Conversely, the sensed magnetic field is increased
correspondingly with retraction, thereby preventing the retracting force
from decreasing with retraction due to increased flux leakage and changing
boundary conditions. The necessary resistance of resistor 54, for any
particular actuator, is determined by sensing retracting force while
varying the position of element 14, and adjusting the value of resistance
54 to obtain the desired constant force characteristic.
The resistors 55, 56 and 57 in FIG. 3 are for the purpose of making the
force independent of any variations in supply voltage to the system.
Alternative methods of controlling the current or voltage to the coil in
response to the output of amplifier 40 could make these resistors
unnecessary.
If the flux sensor 26 were not located at the end of the actuator remote
from the variable gap 24, it would not be sensing the total of both flux
in the variable gap and leakage flux. For example, if located in the
variable gap 24, the flux sensor would sense only flux in such gap without
sensing any leakage flux. To obtain constant force, the circuit of FIG. 3
therefore would have to be modified to decrease the sensed field
progressively only during a first portion of extension and then increase
the sensed field thereafter to compensate for varying boundary conditions.
The advantage of placement of the Hall sensor 26, as shown in FIG. 1,
therefore, is that its sensitivity to the total flux, including leakage
flux, enables the force control circuit to compensate for all variables by
a progression in only one direction during extension or, alternatively,
during retraction.
Supplementary to the negative DC feedback of resistor 54, compensation for
leakage flux and the other foregoing variables to yield the desired
constant force characteristic could be aided in some configurations by
nonuniform shaping of the element 14 so that its cross-section and
reluctance vary with position, or by further nonuniform shaping of the
coil. Moreover, equivalent circuit alternatives or additions to the
negative DC feedback of resistor 54 could be employed to yield similar
results, such as modifying the shape of the sawtooth signal generated by
the aforementioned oscillator to vary the flux density sensed by the
sensor 26 in relation to changes in coil current.
Position Sensing and Control System
In the circuit of FIG. 3, where the flux is controlled so as to yield
substantially constant force, the position of the actuator can be
determined with reasonable accuracy by measuring coil current, for
example, by indicating the voltage difference across resistor 59 by means
of a voltmeter or other suitable readout device 59a. Alternatively, for
position sensing or control irrespective of force and coil current
variations, the circuit of FIG. 3 may be replaced by the circuit of FIG.
4. The position sensing feature of the circuit of FIG. 4 operates on the
principle that the position "X", i.e. the degree of extension of element
14, may be described at least approximately by the following equation:
X-k.about.Ic/B
where
X is position
k is a constant
Ic is the current in the coil
B is the flux density of the field produced by the coil.
Consequently, by dividing the output from the Hall effect sensor 26, which
is proportional to the flux density B, into the value of the current Ic in
the coil 10, a signal representative of position, irrespective of changes
in force and coil current, may be generated.
The foregoing principle applies with sufficient accuracy despite the
presence of flux leakage, and despite the placement of the sensor 26,
because the leakage reluctance increases with extension (due to decreasing
leakage area) as does the reluctance of the primary magnetic circuit (due
to increasing length of gap 24). Placement of the flux sensor 26 in the
variable gap 24, or in a fixed gap adjacent the variable gap 24, would
remove the effects of the leakage, in any case.
The operation of a Hall effect sensor is such that its output voltage Vh is
proportional to the product of its sensed flux density B and its
excitation current Ih. Therefore, if the excitation current Ih is
automatically variably controlled so that Vh and Ic are maintained
proportional to each other, the following relationships develop:
(Ih)(B).about.Vh.about.Ic
Ih.about.Ic/B
X-k.about.Ih
X.about.Ih+k
The circuit of FIG. 4, therefore, is designed to make it possible to sense
the position X of the actuator merely by measuring the excitation current
Ih of the Hall effect sensor 26.
In FIG. 4, as in FIG. 3, a diode 60 protects the circuit from reverse
voltage application at the supply, and supply voltage is controlled by a
voltage regulator 62. Amplifier 64 buffers the common supply voltage so
that some current can be drawn from the common bus without affecting its
voltage. Amplifier 66 controls one of the excitation terminals 26b of the
Hall effect sensor 26 to keep one of the output terminals 26c thereof at a
common reference potential equal to that at the output of amplifier 64,
for the reasons described previously. Amplifier 68, together with its
associated resistors, provides a voltage-controlled current source that
supplies the Hall effect sensor in a known manner independently of the
internal resistance of the sensor, which is variable with temperature.
The output of the Hall effect sensor 26 is combined at a summing junction
70, at the inverting input of amplifier 72, with a signal from amplifier
74 representative of the magnitude of current Ic in the coil 10. Amplifier
72 controls the excitation current Ih in the Hall effect sensor such that
the Hall sensor output Vh and the output of amplifier 74 are always equal.
Accordingly, the magnitude of the excitation current Ih of the Hall sensor
becomes proportional to the position of the actuator and is represented by
the signal at output 76. An adjustable potentiometer 78 is set so that the
position signal is accurate regardless of the amount of current in the
coil 10.
If position control, rather than merely position sensing, is desired, the
actual position of the actuator, as represented by the output of amplifier
72, is compared at a summing junction 84 with a position input reference
signal adjustable by means of potentiometer 82. The result of the
comparison is an error signal presented to the inverting input of
amplifier 80. Depending upon the direction of deviation of the movable
element 14 from the desired position, the output of amplifier 80 will
either increase or decrease to reduce the error signal. The output of
amplifier 80 is presented to a comparator 86 which combines it with the
output from a sawtooth oscillator composed of amplifiers 88 and 90,
respectively. Comparator 86 controls the duty cycle of a power transistor
92 in a pulse-width modulated manner to control coil current so as to
reduce the aforementioned error signal and thereby maintain the selected
position of the element 14. Diode 94 is a flyback diode used for the same
purpose as previously discussed. A position range adjuster 96 is used to
set the ratio between the motion of the actuator and the change in the
position feedback signal (the output of amplifier 72). Resistors 98, 100
and 102, and capacitors 104 and 106, are chosen and adjusted to achieve
stable and well-damped positioning performance. The shunt resistor 108
keeps the current in the coil 10 from decreasing to zero so that the
position feedback system will continue to operate when the transistor 92
is switched off.
As an adjunct to the general concept of position control of a variable
reluctance linear actuator, it is noteworthy that, just as force control
makes it possible to sense approximate position from the magnitude of the
coil current, position control makes it possible to sense approximate
actuating force from the magnitude of the coil current. Although the
relationships will normally not be linear, they will be predictable and
therefore appropriate calibration can yield useful readings. For example,
in FIG. 4, the output of amplifier 74 could be indicated at output 75 to
measure actuating force magnitude, at least approximately, because it is
representative of the magnitude of current in the coil 10.
Theoretically, the optimum setting of potentiometer 78 is such that the
independence of the position signal from coil current is optimized.
However, such setting would be dangerously close to a condition where
transient variables could render the position control system inoperative.
Accordingly, the practical optimum setting of potentiometer 78 preferably
permits a small dependence of the position signal on coil current. Such
small dependence can be at least partially compensated for by adjusting
the gain of amplifier 80 by adjustment of variable resistor 98, which
variably regulates the stiffness of the position control system, i.e., the
relationship between the magnitude of the correcting force and the
magnitude of deviation from a desired position.
The terms and expressions which have been employed in the foregoing
specification are used therein as terms of description and not of
limitation, and there is no intention in the use of such terms and
expressions of excluding equivalents of the features shown and described
or portions thereof, it being recognized that the scope of the invention
is defined and limited only by the claims which follow.
* * * * *
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