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BACKGROUND OF THE INVENTION
This invention relates to variable reluctance actuators, particularly
variable reluctance actuators whose mechanical force may be controlled
throughout the range of movement of their movable actuation element.
Variable reluctance actuators operate on the principle that a magnetic
material, when placed in a magnetic field, will experience a mechanical
force tending to move the material in a direction parallel to the field,
the mechanical force at any point on the surface of the material being
proportional to the square of the flux density of the magnetic field
experienced at that point. A magnetic material is a material that exhibits
enhanced magnetization when placed in a magnetic field.
In a practical variable reluctance actuator a movable element made of
magnetic material, typically in the form of a ferromagnetic plunger, is
subjected to a magnetic field generated by an electrical current in a coil
so that it transmits the resultant force to some other device for
actuation. Such an actuator is referred to as a "variable reluctance"
actuator because as the movable element, which makes up part of a magnetic
circuit, moves in response to mechanical force, it varies the reluctance
within the magnetic circuit, ordinarily by changing the dimension of an
air gap.
A typical example of a variable reluctance actuator is a linear actuator
comprising a plunger mounted for sliding inside the core of a solenoid.
(Although the term "solenoid" is loosely used commonly to refer to such a
device as a whole, it is used herein in its technical sense to refer to a
coil comprising one or more layers of windings of an electrical conductor
ordinarily wound as a helix with a small pitch.) Such linear actuators are
used, for example, in vehicles, household appliances, and a variety of
industrial applications, such as for controlling valves.
Variable reluctance actuators are to be distinguished from actuators in
which mechanical force is created as a result of current passing through a
conductor oriented perpendicular to a magnetic field, thereby creating
lateral force on the conductor, the conductor typically being wound in the
form of a movable solenoid. In general, the latter are more difficult to
construct and provide less actuation force per unit volume.
A principal problem with variable reluctance actuators which limits the
applications to which they may be put is that the mechanical force
experienced by the moving element in the actuator changes as a function of
the position of the moving element. Ordinarily the change is non-linear,
the force increasing more rapidly as the effective air gap in the device
decreases, since the decrease in air gap produces a decrease in circuit
reluctance and a concomitant increase in circuit flux. This generally
causes the moving element to release energy in the form of undesirable
vibration and noise when it collides with a stop for limiting its
excursion, and makes controlled positioning of the element difficult.
While the force can theoretically be controlled by controlling the current
in the solenoid this has heretofore been difficult to accomplish
effectively. Consequently, such devices are ordinarily used in simple
on-off applications where the vibration and noise resulting from collision
of the moving element with a stop is of little or no significance, and are
often relatively crude devices.
Previous approaches to controlling the mechanical force created by variable
reluctance actuators so as to employ them in more sophisticated
applications have employed transducers to measure the mechanical force or
the position of the moving element to provide feedback for controlling the
current in a solenoid. One example of such an approach is shown by Keller
U.S. Pat. No. 3,584,496 wherein a force-sensitive transducer is employed
to measure the mechanical force applied by the moving element of a
variable reluctance actuator and the output of the transducer is employed
to control the current in the solenoid of the actuator. In Umbaugh U.S.
Pat. No. 3,697,837, the position of the moving element is also detected by
a displacement-sensitive transducer to control the current in a solenoid
and, hence, the position of the moving element.
Some drawbacks of measuring the actual mechanical force experienced by the
moving element, which requires a device sensitive to change in physical
dimensions, such as a strain gauge, are that such devices are typically
sensitive to orientation, inertia, and shock, have slow response times,
and require complex circuits to control the current in the magnetic field
generating coil. While devices for measurement of the position of the
moving element can be more readily employed to adjust the position of the
moving element, they are subject to some of the same problems. Moreover,
they cannot be used to adjust the mechanical force without knowledge of,
and compensation for, the force-position characteristic of the actuator.
Since the force experienced by the moving element of a variable reluctance
actuator is proportional to the square of the magnetic flux density
experienced by the element, it would be desirable to measure that magnetic
flux density directly. Although coils have been used to detect a change in
magnetic field strength in bi-stable variable reluctance actuators, as in
Massie U.S. Pat. No. 3,932,792, a coil cannot be used to measure the
instantaneous magnitude of magnetic field strength, or flux density. An
alternative would be to use a Hall effect device, whose output is a
function of the magnetic flux density that it experiences. While Hall
effect devices have been used in connection with permanent magnets as
position detectors, as in Brace et al., U.S. Pat. No. 4,319,236, it is
believed that they have not been used to measure the flux density
experienced by a moving element in a variable reluctance actuator.
It would also be desirable to control the flux density in the moving
element of a variable reluctance actuator by controlling the duty cycle of
the solenoid in order to maximize energy efficiency. Electronic circuits
for switching the current in a solenoid on and off in a varible reluctance
actuator, including a flyback diode for protecting the circuitry from
unacceptable voltage excursions during the collapse of the magnetic field
in the solenoid, have been used, for example, in electronically driven
pumps, as shown in Maier et al., U.S. Pat. No. 3,293,516; however, such
devices are bistable, and do not control the current in the solenoid to
maintain substantially constant flux density in the moving element.
In addition, it would be desirable to control the position of the moving
element of a variable reluctance actuator based upon the magnetic and
electrical characteristics of the actuator itself, rather than an external
transducer subject to difficult-to-control variables.
SUMMARY OF THE INVENTION
The present invention provides a variable reluctance actuator whose force
and position can be effectively and simply controlled. It avoids the
problems of external transducers subject to uncontrollable variables by
directly measuring the ultimate quantity that determines the mechanical
force experienced by the moving element, that is, the flux density in the
magnetic circuit, and controls the current in a solenoid based thereon. It
provides a simple and efficient circuit for maintaining substantially
constant flux density. It also provides a servo mechanism for controlling
the position of the moving element based upon the electrical and magnetic
characteristics of the actuator itself, with reference to a position input
signal.
The magnetic flux density experienced by the moving element of the actuator
is measured by the placement of a flux density sensor in the magnetic
circuit of the actuator. An example of such a sensor is a Hall effect
device. The output of the flux density sensor is fed to a control circuit
for controlling the current in the solenoid to maintain substantially
constant flux density and, hence, substantially constant force.
A "chopping" circuit is used to maintain the flux density substantially
constant by controlling the duty cycle of the solenoid. In this manner
external current is either connected or disconnected to the solenoid and
energy losses in the control circuit components are minimized. A flyback
diode connected in parallel with the solenoid permits current in the
solenoid to recirculate when external current is turned off, thereby
producing an exponential, rather than oscillatory, decay of the magnetic
field in the solenoid, which tends to reduce energy losses and protects
the control circuitry. Alternatively, analog control of the current in the
coil may be provided in response to a flux density sensor.
The force exerted by the actuator may be adjusted by providing a magnetic
field that biases the flux density sensor, or by amplifying the sensor
signal. A biasing field may also be employed to achieve a desired
force-displacement characteristic for the actuator.
The output of the flux density sensor may be divided into a signal
representative of the measured current in the solenoid to produce a signal
representative of the position of the moving element of the actuator. The
position-representative signal may then be compared to an input control
signal to adjust the force experienced by the moving element until it has
travelled to a desired position.
Although the preferred embodiment of the invention, a variable reluctance
linear actuator, employs a moving element experiencing essentially
constant flux distribution, the invention can be adapted to devices whose
moving element experiences changing flux distribution of a predictable, or
empirically measureable, character. Such devices may be used, for example,
to create rotational motion.
Therefore, it is a principal object of the present invention to provide a
novel controlled force variable reluctance actuator.
It is another object of the present invention to provide a variable
reluctance actuator wherein current in a magnetic field-generating coil of
the actuator is controlled in response to measurement of magnetic circuit
flux density.
It is another object of the present invention to provide a variable
reluctance actuator wherein the force experienced by the moving element
thereof may be selectively controlled.
It is yet another object to provide a variable reluctance actuator which
employs a high energy efficiency control circuit.
It is a further object of the invention to provide a variable reluctance
actuator in which the relationship between the force experienced by the
moving element and the position of the moving element may be selectively
controlled.
It is yet a further object of the present invention to provide a variable
reluctance actuator wherein the position of the actuator may be controlled
without the use of external position transducers.
It is an object of the present invention to provide a simple, constant
force variable reluctance linear actuator.
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 shows an exemplary curve representing the force-displacement
relationship of the moving element of an open loop variable reluctance
linear actuator operated at constant current.
FIG. 2a shows a side, cross-sectional representation of a preferred
embodiment of a variable reluctance linear actuator according to the
present invention.
FIG. 2b shows a cross-sectional view of the actuator of FIG. 2a, taken
along line 2b--2b thereof.
FIG. 3 shows a schematic diagram of a control circuit for the actuator of
FIG. 2a.
FIG. 4 shows force-displacement curves for various embodiments of variable
reluctance actuators according to the present invention.
FIG. 5a shows a schematic diagram of an alternative embodiment of the
actuator of FIG. 2a wherein the level of constant force may be adjusted.
FIG. 5b shows an alternative embodiment of the actuator of FIG. 2a wherein
the force-displacement curve is modified to provide a predetermined linear
relationship between force and displacement.
FIG. 5c shows a schematic diagram of an alternative embodiment of the
actuator of FIG. 2a wherein mechanical force is controlled by an analog
signal.
FIG. 6 shows a schematic diagram of another alternative adjustable force
control circuit for an actuator according to the present invention.
FIG. 7 shows a block diagram of an alternative variable reluctance actuator
servo control circuit according the present invention, including a
position adjustment feature.
FIG. 8 shows an alternative embodiment of a variable reluctance actuator
according to the present invention wherein the actuator produces
rotational motion and the moving element experiences variable flux
distribution.
DETAILED DESCRIPTION OF THE INVENTION
As shown in FIG. 1, the mechanical force f.sub.m experienced by the moving
element of a variable reluctance actuator as a result of the magnetic
field generated by a coil to which a constant current is supplied
ordinarily changes in a non-linear manner as a function of displacement x
of that element, the force decreasing with increasing displacement in the
direction of increasing reluctance. In that case, it necessarily follows
that the magnetic flux density B experienced by that element varies as
well, since the force is proportional to the square of the flux density.
However, since the flux density can be controlled by controlling the
current applied to the actuator, the force can likewise be controlled by
controlling that current.
The mechanical structure and magnetic circuit of a preferred embodiment of
a controlled force variable reluctance actuator can be understood with
reference to FIGS. 2a and 2b. As shown in FIG. 2a, the actuator employs a
solenoid 10 wound on a form 12, preferably a spool, which may serve not
only to provide the solenoid with shape but as a bearing for the moving
element 14 of the actuator. The moving element, or actuation means, 14 is
made of a material classified as "ferromagnetic", for example, iron. In an
actuator such as that shown in FIG. 2a, commonly known as a linear
actuator, the moving element is commonly referred to as a plunger.
The form 12 would typically be made of some type of plastic material, such
as nylon or polycarbonate material. The moving element 14, when placed
within the solenoid as shown, will experience a magnetic flux density
generally along its longitudinal axis thereby producing a mechanical force
tending to pull the moving element into the core of the solenoid.
In order to increase the efficiency of the actuator, it is provided with a
hat-shaped first end cap 16, which also serves as a stop for the plunger,
a casing 18, and a disc-shaped second end cap 20, all of which preferably
comprise ferromagnetic materials. The first end cap 16 is slightly
separated from the casing 18 by a disc-shaped spacer 22 in order to
provide a location for a magnetic flux density sensor. The space between
the first end cap 16 and the moving element 14 comprises a variable
reluctance air gap 24 and accounts for the majority of the reluctance in
the magnetic circuit. The two end caps 16 and 20, the casing 18, the
moving element 14, the spacer 22, and the air gap 24 provide a magnetic
circuit to which the magnetic flux created by the solenoid is essentially
confined.
It is to be recognized that the end caps, casing, and plunger might be made
of other than ferromagnetic materials without departing from the
principles of the invention. The end caps and casing might not even be
made of magnetic material, though the actuator would consequently be less
efficient. The spacer 22 is preferably made of a non-magnetic material;
although this introduces some additional reluctance into the magnetic
circuit, it serves to ensure symmetrical flux distribution.
A magnetic flux density sensor, or measurement means, 26 is disposed
between the first end cap 16 and the casing 18. Preferably the sensor
comprises a Hall effect device, such as a Hall effect switch or analog
semiconductor. Hall effect switches provide an "on" or "off" binary output
based upon a threshhold level of magnetic field density. Analog Hall
effect devices provide a variable analog output signal that is a function
of the magnetic flux density. The nature and operation of such devices is
commonly known in the art. Although a particular placement of the sensor
26 is shown, it is to be recognized that the device could be placed
anywhere within the magnetic circuit of the actuator without departing
from the principles of this invention. Moreover, other sensor devices,
such as magnetoresistive devices (devices whose resistance varies with
experienced flux density), which provide a signal representative of
magnetic flux density might also be used without departing from the
principles of this invention.
Since the force experienced by the moving element 14 as a result of the
current in the solenoid is a function only of the magnetic flux density
that it experiences, the magnetic flux density experienced by the sensor
26 provides a direct measurement of the mechanical force experienced by
the moving element. Moreover, in the actuator shown, since the
distribution of magnetic flux density experienced by the moving element 14
is constant, the magnetic flux density experienced by the sensor 26 is
directly proportional to the flux density experienced by the moving
element.
Turning to FIG. 3, a control circuit is provided for controlling the
current in the solenoid based upon the output of the sensor 26. In this
preferred embodiment of a control circuit the sensor 26 comprises a Hall
effect switch 28 having a positive power supply input 30, a common, or
negative, supply connection 32, and a binary output 34. When the magnetic
flux density to which the switch 28 is exposed exceeds an operating point,
defined by the characteristic of the switch, the binary output goes "low";
when the flux density decreases below a release point, the output goes
"high." Since the operating point and release point differ from one
another, the resultant hysteresis provides for unambiguous or
non-oscillatory switching. An example of a suitable device is the
UGN-3030T/U bipolar Hall effect digital switch manufactured by Sprague
Electric Company, 70 Pembroke Rd, Concord, N. Hamp.
A control transistor 36 has its collector connected to the solenoid 10 and
its emitter connected to the common, or negative, power connection 38, so
as to be in series with the solenoid. The base of the transistor is biased
on by a resistor 40 so that when the output of the Hall switch 28 is high,
the transistor is switched on and current flows from the positive power
connection 42 through the solenoid 10 to the negative supply 38; yet, when
the output from the Hall switch goes low, it pulls the base voltage low
and, hence, shuts the transistor 36 off so as to disconnect external
current from the solenoid 10.
When external current to the solenoid 10 is cut off by the control
transistor 36, the magnetic field in the solenoid will begin to collapse.
A flyback diode 44 is provided so that the current generated in the
solenoid by the collapsing magnetic field will recirculate through the
coil causing the field to decay exponentially, at a rate determined
essentially by the inductance and resistance of the solenoid. In the
absence of the diode the field would decay in an oscillatory manner, due
to the distributed capacitance of the solenoid, which would create eddy
current losses in the magnetic circuit, as well as produce voltage spikes
that could damage the transistor. Thus, as a result of the flyback diode
44 the magnetic field tends to remain more nearly constant. To achieve
this result the flyback diode 44 must be connected in opposite polarity to
the external power applied to the solenoid.
As the magnetic field in the solenoid begins to collapse, the magnetic flux
density experienced by the Hall effect switch drops. As soon as it drops
below the release point, the transistor is turned on again, thereby
supplying current to the solenoid and reestablishing the magnetic field.
The circuit thus turns on and off so as to maintain the magnetic flux
density in the magnetic circuit essentially constant; hence, the force
experienced by the moving element 14 is also maintained essentially
constant. In actuality, the flux varies slightly with a periodicity
dependent upon the characteristic hysteresis of the Hall switch 28 and the
time constants in the control circuit, which establish the duty cycle of
the solenoid. A change in position of the moving element causes the
transistor to turn on or off for different periods of time, that is, it
changes the duty cycle. The result of this control circuit is that the
force remains essentially constant regardless of displacement of the
moving element, as shown by curve 46 in FIG. 4. Also, since the transistor
is operating in a switching mode, it dissipates very little energy and the
circuit operates very efficiently.
Turning to FIG. 5a, an alternative embodiment employs a modification of the
control circuit of FIG. 3 wherein a second coil 48 is magnetically coupled
to the Hall switch 28 so as to bias the level of magnetic flux that the
Hall switch experiences. By varying the current in the second coil 48
using, for example, a potentiometer 50, the force experienced by the
moving element 14, though constant, can be adjusted selectively.
In FIG. 5b another modification of the control circuit of FIG. 3 employs a
third coil 52 connected in series with the solenoid 10 and magnetically
coupled to the Hall switch 28. This coil can be used to provide the
actuator With a characteristic whereby the mechanical force experienced by
the moving element has a substantially linear relationship to
displacement. Where the third coil 52 is coupled to the Hall switch 28 so
as to add to the magnetic flux the mechanical force will be inversely
proportional to the displacement, as shown by curve 54 in FlG. 4; whereas,
if the third coil 52 is coupled so as to substract from the magnetic flux,
the mechanical force will be directly proportional to displacement as
shown by curve 56 of FIG. 4.
Turning to FIG. 5c, an analog version of a control circuit employs a Hall
device 58 having an analog, rather than a binary, output 60 which drives a
control transistor 62 biased by a resistor 64 and connected in series with
the solenoid 10. (It is assumed that the Hall device 58 is actually an
analog circuit incorporating a Hall effect sensor and that the output
provides negative feedback to transistor 62.) The amount of current
allowed to flow through the solenoid 10 is thus proportional to the output
of the Hall device. Since the solenoid 10 is not simply turned on and off
a flyback diode is unnecessary. Such an embodiment would exhibit less
energy efficiency, but can be used in applications where the slight
oscillation associated with the control circuit of FIG. 3 is undesirable.
FIG. 6 shows yet another embodiment of an actual control circuit employed
to selectively provide constant force in a variable reluctance actuator
without the addition of another coil. ln the circuit the actuator solenoid
66 is controlled by a Darlington pair transistor device 67. A flyback
diode 68 is provided, since the solenoid is controlled in a switching
mode. An analog Hall device 69 is employed in this circuit. A suitable
device would be, for example, the THS 102A Hall effect sensor manufactured
by Toshiba America, Inc., 2441 Michelle Dr., Tustin, Calif. Constant
current input is provided to the Hall device by zener diode 70, resistors
71 and 72, capacitor 73, and amplifier 74. The output from the Hall device
69 is amplified by amplifier 75, whose output is connected to the input of
the Darlington device 67. The gain of the amplifier's output is controlled
by fixed resistor 76 and variable resistor 77, thereby setting the median
level to which the Darlington device 67 responds. A hysteresis function is
employed so as to switch the transistor on or off in an unambiguous
manner. The hysteresis function is provided by resistors 78, 79 and 80,
and capacitor 81. Resistor 82 provides biasing for the Darlington device
67. This circuit is simply exemplary, and the manner of design and
construction of this, or a similar, circuit would be commonly known to a
person skilled in the art.
A control circuit for an embodiment of the invention that includes position
control is shown in FlG. 7. Although contemplated for use with a linear
actuator such a that shown in FlG. 2a, the control circuit could also be
used with actuators having other geometric characteristics. This control
circuit is similar to the previously discussed control circuits in that it
includes a Hall effect sensor 83 magnetically coupled to the magnetic
circuit of the actuator solenoid 84, which is operated in an on-off mode
by a switching transistor 85. A flyback diode 86 is included to
recirculate current generated by collapse of the field in the solenoid.
The transistor 85 is turned on and off by a switch controller circuit 87,
which includes a summing junction 88, such as a differential amplifier,
for adjusting the output of the Hall effect sensor 83 up or down, based
upon an error signal input 89, a hysteresis circuit 90 for ensuring that
the output to the transistor 85 either turns the transistor on or off, for
maximum efficiency, and an amplifier 91, associated with the hysteresis
circuit 90 for providing any needed gain for operating the transistor. It
is to be recognized that this is a functional description and that a
variety of different specific circuits for providing the features of the
switch controller 87 could be designed by a person skilled in the art.
The error signal 89 is employed to vary the force on the moving element so
as to move it to and maintain it at a selected position. In a linear
actuator of the type shown by FIG. 2a the circuit permeance p is a
substantially linear inverse function of position x. The position may be
described by the following equation:
x=k(i/B)
where
x=position
k=a constant, and
i=the current in the solenoid.
Consequently, by dividing the output from the Hall effect sensor, which is
proportional to the flux density B, into the value of the current in the
solenoid 84, a signal representative of position may be generated.
The control circuit is provided with a current sensor 92 whose output 94 is
a signal representative of current in the solenoid and a divider 96 that
divides the output 98 from the Hall effect sensor into the output 94 from
the current sensor to produce a position signal output 100. Although the
position signal output 100 is very nearly directly proportional to the
position of the moving member in a linear actuator, slightly non-linear
characteristics may exist due to the geometry of the device. Accordingly,
a practical control circuit may include a circuit for compensating for
non-linearity, such as linearizer filter 102. Its output signal 104 is a
linearized representation of moving element position. The signal 104 is
compared to a position input signal 106 by a summing junction 108, such as
differential amplifier, to produce as a result the error signal 89. When
the linearized position signal 104 differs from the position input signal
106 the error signal 89 takes on a non-zero value causing the force
experienced by the moving member to change until the moving member has
relocated to the desired position, at which point the error signal would
take on a zero, or equivalent, value.
The mechanical portion of the control force actuator may take on other than
a linear configuration. Moreover, the moving element need not necessarily
move within the core of | | |
