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Claims  |
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We claim:
1. An apparatus for the electromagnetic control of the suspension of an
object including:
an electromagnet including an inductive coil,
a source of electric power for the electromagnet which source includes a
controllable electric supply device capable of delivering a controlled
electric supply to the coil of the electromagnet,
a control signal producer for producing an error control signal in response
to an incremental change in a parameter related to the position of the
object relative to the electromagnet, and
a negative feedback control loop for feeding said error control signal
generated by the control signal producer to said source of electric power
to adjust the electric supply to the coil so as to stabilize the
suspension of the object relative to the electromagnet, wherein said
control signal producer includes:
means for detecting a component V.sub.L of voltage across the coil which is
a function of pure inductance L of the coil,
means for detecting a current i flowing through the coil,
means for deriving, from the detected value of V.sub.L and the detected
value of i, opposing signals, and
means for balancing said opposing signals whereby said error control signal
generated by the control signal producer represents a deviation from
perfect balance of said opposing signals and comprises a correction signal
applied to the electrical supply device for restoring said balance.
2. An apparatus as in claim 1 wherein the control signal producer is such
that in operation balancing of said opposing signals provides at least one
of proportional control and derivative control of the suspension of the
object, where control, required to balance a perturbation caused by a
random disturbance to the suspension of the object, is proportional
control when proportional to the relative position of the object and is
derivative control when proportional to the rate of change of the relative
position of the object.
3. Apparatus as in claim 1 wherein said means for deriving said opposing
signals includes derivation means by one of integration and
differentiation with respect to time of one of the signals representing
V.sub.L and i.
4. Apparatus as in claim 3 wherein said derivation means comprises a means
for integrating a signal proportional to or representing the measured
value of V.sub.L with respect to time and balancing said integrated
proportional signal against a signal proportional to or representing the
measured value of i for producing said error control signal giving
proportional control.
5. Apparatus as in claim 1 wherein said deriving means comprises a means
for balancing a signal proportional to or representing the measured value
of V.sub.L against a time derivative of a signal proportional to or
representing the measured value of i for producing said error control
signal giving derivative control.
6. Apparatus as in claim 1 wherein the control signal producer includes at
least two types of control provided together, the apparatus according to
the present invention together comprising a plurality of parallel control
signal producers and feedback loops for generating the respective pairs of
opposing signals derived from V.sub.L and i and for providing a
combination of error control signals to give the required forms of
feedback control.
7. Apparatus as claimed in claim 6 wherein the control signal producers and
loops include means which is such as to enable in operation the inductance
of the coil of the electromagnet to be measured by the control signal
producer to allow the electric supply to the coil to give integral control
of the position of the controlled object.
8. Apparatus as in claim 1 wherein said electromagnet comprises a magnetic
core carrying an inductive coil.
9. Apparatus as in claim 1 wherein said object is an object influenced by
the magnetic field generated by said electromagnet.
10. Apparatus as claimed in claim 1 wherein the electric supply device
comprises a power amplifier circuit connected to a fixed voltage source
capable of providing a controlled direct current of a controlled level to
the electromagnet.
11. Apparatus as claimed in claim 1 wherein the control signal producer
includes means for measuring the voltage V across the coil of the
electromagnet, and means for deriving from the measured values of V and i
parameters V' and i' respectively which are the phase lagged versions of V
and i.
12. Apparatus as claimed in claim 10 further including a second coil
connected in series with said inductive coil so that the same current will
flow through the inductive coil and the second coil, the second coil
connected in a resonant circuit, and the apparatus further includes means
for injecting through the inductive coil an a.c., signal having a
frequency within the resonance peak of the resonant circuit.
13. Apparatus as claim 12 further including:
a means for measuring the amplitude of the alternating voltage across a
component in the resonant circuit,
an error detector for comparing the amplitude measured by the amplitude
detector with a reference signal and
an integrator for producing the time integral of error signals provided at
the output terminal of the error detector, the output of the integrator
being applied to an input of the controllable electric supply device.
14. Apparatus as claimed in claim 1 wherein the control signal producer is
connected to the respective terminals of the electromagnet.
15. Apparatus as claimed in claim 14 wherein the control signal producer is
for providing proportional control and includes a processing circuit which
includes a first operational amplifier having its inputs provided by
resistive connections to the terminals of the coil of the electromagnet,
an integrator connected to the first operational amplifier so as to
integrate its output, and a second operational amplifier having a first
input terminal connected to the output terminal of the integrator and a
second input terminal having a resistive connection to one of the
terminals of the coil.
16. Apparatus as claimed in claim 15 wherein the processing circuit
includes means for providing a positive current component in the
processing circuit.
17. Apparatus as claimed in claim 1 in which the suspension of an object is
controlled by a plurality of inductive coils.
18. Apparatus as claimed in claim 17 wherein the apparatus comprises an
active electromagnetic bearing and said object comprises
19. Apparatus for the electromagnetic control of the suspension of an
object including:
an electromagnet including at least one inductive coil,
a source of electric power for the electromagnet which source includes a
controllable electric supply device capable of delivering a controlled
electric supply to the coil of the electromagnet,
a control signal producer for producing an error control signal in response
to an incremental change in a parameter related to the position of the
object relative to the electromagnet and
a negative feedback control loop for feeding said error control signal
generated by the control signal producer to said source of electric power
to adjust the electric supply to the coil so as to stabilize the
suspension of the object relative to the electromagnet, wherein said
control signal producer includes:
means for detecting a component V.sub.L of voltage across the coil which is
a function of pure inductance L of the coil,
means for detecting a current i flowing through the coil,
means for deriving, from the detected value of V.sub.L and the detected
value of i, opposing signals, and
means for balancing said opposing signals whereby said error control signal
generated by the control signal producer represents a deviation from
perfect balance of said opposing signals and comprises a correction signal
applied to the electrical supply device for restoring said balance wherein
said at least one coil comprises two coils, each coil being connected to
said electric supply device, and to said control signal producer, a
feedback loop being provided from the control signal producer to said
electric supply device, wherein said electric supply device for the two
coils comprises:
two voltage sources; and
a common power amplifier driving two transistor devices, each transistor
device connected to one of the coils and one of the two voltage sources,
the two voltage sources being of opposite polarity whereby the incremental
current changes applied to the two coils are equal and opposite.
20. An apparatus for the electromagnetic control of the suspension of an
object said apparatus including:
a first electromagnet including a first inductive control coil;
a second electromagnet including a second inductive control coil;
a source of electric power for each of the electromagnets, said source
including:
a first controllable electric supply device for delivering a controlled
electric supply to the first, coil; and
a second controllable electric supply device for delivering a controlled
electric supply to the second coil,
a first control signal for producer for monitoring voltage across and
current through the first, control coil and for producing, from the
monitored voltage and current values, a first error control signal in
response to an incremental change in a parameter related to the position,
relative to the first electromagnet, of the object being suspended,
a second control signal producer for monitoring voltage across and current
through the second control coil and for producing, from the monitored
voltage and current values, a second error control signal in response to
an incremental change in a parameter related to the position, relative to
the second electromagnet, of the object being suspended, and
first, second third and fourth feedback control loops, the first loop being
connected from the first control signal producer to the first controllable
electric supply device, the second loop being connected from the second
control signal producer to the second controllable electric supply device,
the third loop being connected from the first control signal producer to
the second controllable electric supply device and the fourth loop being
connected from the second control signal producer to the first
controllable electric supply device, wherein the first error control
signal provided by the first control signal producer is connected in an
equal but opposite manner to the first, and third loops, and the second
error control signal provided by the second control signal producer is
connected in an equal but opposite manner to the second and fourth loops.
21. Apparatus as in claim 20 and wherein the first and second control
signal producers produce first and second error control signals,
respectively, said first and second error control signals comprising a
means for controlling the rate of change of position of the suspended
object. |
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Claims |
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Description |
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RELATED APPLICATIONS
An Application of even date by John Frederick Curtis et al assigned to the
present Assignee and claiming priority from UK Patent Applications
9200087.6 and 9222017.7 and a corresponding U.S. application Ser. No.
07/986,733, based thereon, was filed Dec. 8, 1992 and abandoned in favor
of U.S. Continuation application Ser. No. 08/347,066.
RIGHTS ARISING FROM FEDERALLY SPONSORED RESEARCH
Nil.
The present invention relates to apparatus for the electromagnetic control
of the suspension of an object.
Electromagnetic control of the position of an object by suspension or
levitation has been employed in a number of commercial applications in the
field of industrial engineering. Such applications have included
passenger-carrying vehicles, conveyor systems, flow meters, frictionless
bearings, tool spindles, centrifuges, alternators, pumps, compressors and
balances. The present invention is concerned with systems for the
electromagnetic control of the suspension of an object which are suitable
for use in such applications and which systems are of the kind including
an electromagnet, a source of electric power for the electromagnet which
source includes a controllable electric supply device capable of
delivering a controlled electric supply to the electromagnet, a control
signal producer for generating an error control signal in response to an
incremental change in a parameter related to the position of the object
relative to the electromagnet and a negative feedback control loop for
feeding an error control signal generated by the control signal producer
to the electric supply device to adjust the electrical supply to the
electromagnet so as to stabilise the suspension of the object relative to
the electromagnet.
In systems known in the prior art the position of the object has been
stabilised by monitoring the position of the object relative to the
electromagnet and producing an error control signal in the manner
described to stabilise the position of the object following random
disturbances to the suspension of the object caused by changes in the
overall force acting upon the object. In some applications the rate of
change of relative position of the object has been stabilised either
together with the position, or as a separate control, by monitoring the
rate of change of position of the object relative to the electromagnet and
producing an error control signal in the manner described to stabilise the
rate of change of position or, in other words, to damp oscillations of the
object, caused as a result of random disturbances to the suspension of the
object. In the latter case the error control signal may be generated by
measuring relative position and differentiating the signal produced or by
measuring the rate of change of relative position directly. An example of
an application where rate of change of position is controlled without
relative position per se is a shaft damper employed on long shafts, eg a
propeller shaft on a ship.
The relative position of the object is the separation or gap between the
control electromagnet and the object being controlled and in the prior art
systems is monitored by a transducer forming part of the control signal
generator for the feedback loop. Such transducers have included devices
which are photocells (detecting the interruption of a light beam by
movement of the object); magnetic (comprising a gap flux density
measurement device, eg Hall plate); inductive (eg employing two coils in a
Maxwell bridge which is in balance when the inductance of the coils is
equal); I/B detectors (in which the ratio of the electromagnet coil
current and magnetic flux produced is determined to provide a measure of
the gap between electromagnet and object; for small disturbances the
division may be replaced by a subtraction); and capacitive (employing an
oscillator circuit whose output frequency varies with suspension gap).
Direct measurement of the rate of change of position has been carried out
in the prior art by "derivative transducers" such as a coil wound around a
permanent magnet.
The use of the gap or derivative measurement transducers in prior systems
has not been entirely satisfactory. Usually, the transducer has an upper
temperature limit of operation. The transducers have a discrete physical
size and the space occupied by the transducer reduces the space available
for the electromagnet and therefore the force which can be exerted on the
object.
This problem is significant where the object is a shaft which is able to
bend, eg at a resonant rotation frequency in its so-called "free-free"
mode, confining the positions in which the transducer may be located.
SUMMARY OF THE INVENTION
According to the present invention there is provided an apparatus for the
electromagnetic control of the suspension of an object including an
electromagnet including an inductive coil, a source of electric power for
the electromagnet which source includes a controllable electric supply
device capable of delivering a controlled electric supply to the coil of
the electromagnet, a control signal producer for producing an error
control signal in response to an incremental change in a parameter related
to the position of the object relative to the electromagnet and a negative
feedback control loop for feeding an error control signal generated by the
control signal producer to the electric supply device to adjust the
electrical supply to the electromagnet so as to stabilise the suspension
of the object relative to the electromagnet, wherein the said control
signal producer includes means for detecting the component V.sub.L of
voltage across the coil of the electromagnet which is due to pure
inductance L of the electromagnet, means for detecting the current i
flowing through the coil of the electromagnet, means for deriving from the
detected value of V.sub.L and the detected value of i opposing signals and
means for balancing the opposing signals whereby an error control signal
generated by the control signal producer represents a deviation from the
perfect balance of the said opposing signals and causes a correction
signal to be applied by the electrical supply to restore the balance.
Balancing of the said opposing signals may provide one or more of
proportional and derivative control of the suspension of the object. In
such forms of control the restoring force required to balance a
perturbation caused by a random disturbance to the suspension of the
object is proportional respectively to the relative position of the object
or to the rate of change of the relative position of the object.
The said opposing signals may comprise a signal proportional to or
representing the measured value of V.sub.L which has been integrated with
respect to time and balanced against a signal proportional to or
representing the measured value of i to give proportional control.
Alternatively, or in addition, the said opposing signals may comprise a
signal proportional to or representing the measured value of V.sub.L
balanced against a signal which is the time derivative of a signal
proportional to or representing the measured value of i to give derivative
control.
Thus, the required form of control is obtained by deriving opposing signals
by suitable integration or differentiation with respect to time of one of
the signals representing V.sub.L and i and comparing it with the other.
Where two or more types of control, eg proportional and derivative control,
are required to be provided together the apparatus according to the
present invention may comprise a plurality of parallel circuits or loops
for generating the respective pairs of opposing signals (derived from
V.sub.L and i) employed to provide an error control signal to give the
required form of feedback control.
The said electromagnet may comprise in its most usual form a magnetic core
on which an inductive coil is provided (through which current is supplied
to energise the electromagnet). However, the electromagnet may
alternatively be an inductive coil without a magnetic core.
The said object may be any of the objects of types known to be influenced
by the magnetic field generated by an electromagnet, eg a magnetic
material, a permanent magnet, an electromagnet or a current-carrying
conductor.
The electric supply device may comprise a controlled power amplifier
circuit connected to a fixed voltage source, thereby delivering a
controlled direct current of a controlled level to the coil of the
electromagnet.
The amplifier of the said amplifier circuit is preferably a high gain
amplifier and may be an amplifier of any one of classes A, B, C, D and E.
Preferably, the gain of the amplifier is greater than 200, desirably
greater than 500.
The voltage V across the terminals of an electrical coil of an
electromagnet is given by the equation:
##EQU1##
where R is the resistance of the coil, i is the current passing through
the coil and L is the inductance of the coil. The said means for detecting
the parameters i and V.sub.L measure respectively the resistive component
equal to the term iR in the voltage V expressed in Equation 1 and also the
inductive component equal to the sum of the terms L.sup.di /.sub.dt and
i.sup.dL /.sub.dt. These last two terms may alternatively be expressed as
N.sup.dO /.sub.dt where N is the number of turns of the coil and O is the
magnetic flux developed due to inductance of the coil when carrying
current. For proportional control the said control signal producer
integrates N.sup.dO /.sub.dt and sets this equal to the term iR (by the
said means for balancing the opposing signals derived from V.sub.L and i).
The effect of the operations performed by the said control signal producer
for proportional control may be expressed as follows.
The voltage component across the coil due to pure inductance L of the coil
V.sub.L is given by
V.sub.L =N.sup.dO /.sub.dt Equation 2
the intergral of V.sub.L is given by
V.sub.L dt=NO+K.sub.1 Equation 3
where K.sub.1 is a first constant and N is a second constant (as stated
above representing the number of turns in the coil). The balancing of the
integral of the voltage component across the coil due to pure inductance L
and the voltage component due to pure resistance R of the coil may be
represented by:
V.sub.L dt-K.sub.3 i=0 Equation 4
where K.sub.3 is a third constant and i is the current through the coil.
By combining with Equation 3, Equation 4 becomes:
NO+K.sub.1 -K.sub.3 i=0 Equation 5
The known relationship between the flux O linking to an object a distance x
from an electromagnet comprising a coil carrying a current i is:
O=.sup.i /K.sub.4 x Equation 6
where K.sub.4 is a fourth constant.
By combining with Equation 5, Equation 6 for proportional control by the
apparatus of the present invention becomes:
O(N-K.sub.3 K.sub.4 x)=-K.sub.1 Equation 7
Thus, by rearranging Equation 7 the expression for becomes:
##EQU2##
The known relationship between the force F on the object and the flux O
(where K.sub.5 is a constant) is:
F=K.sub.5 O.sup.2 Equation 9
Finally, by combining Equation 9 with Equation 8:
##EQU3##
where K.sub.A, K.sub.B and K.sub.C are alternative constants.
The relationship between total force F acting upon the object and distance
x from the electromagnet to the object is therefore given by Equation 10
and, as illustrated below (FIG. 2), in the working region of the curve
graphically representing this relationship, the curve approximates locally
to a straight line of positive slope similar to that representing the
simple analogous relationship of force v extension for a spring obeying
Hooke's law. The effect of this relationship is that the control of the
total force
The parameters V.sub.L dt and K.sub.3 i as in Equation 4 may acting upon
the object is proportional control be monitored by measuring V.sub.L and
iR which are respectively the inductive and resistive components of the
voltage across the coil.
It can be shown by analysis that by deriving parameters V' and i' which are
respectively phase lagged versions of V and i defined above a system may
be built which also operates in the manner described by Equation 10. The
control signal producer of the apparatus according to the present
invention may therefore include sensors capable of sensing the parameters
V and i, means for deriving the parameters V' and i' therefrom and means
for processing the measured values of these parameters to provide an error
control signal for the electric supply device which provides proportional
control of the suspended object.
The means for sensing the voltage V across the coil and the means for
sensing the current i may comprise known means. For example, the component
of voltage V may be measured directly by a separate search coil wound on a
magnetic core which searches for changes in magnetic flux. The current i
may be measured by a separate ferromagnetic, eg ferrite, ring plus Hall
plate.
The parameters V.sub.L and i may however be measured by a processing
circuit connected directly to the respective terminals of the coil and
these parameters are preferably measured in this way.
The present invention therefore allows proportional control of the
electromagnetic suspension of an object by an electromagnet to be achieved
without the use of separate transducers, ie without transducers not
physically connected to the coil as in the prior art. Such proportional
control provides stiffness to oscillations of the object about its mean
position caused by fluctuations in the power supply. High stiffness
implies that little movement will occur when an oscillating force is
applied (assuming the frequency is not close to the resonant frequency of
the system).
For example, a processing circuit for proportional control may comprise an
operational amplifier having its inputs provided by resistive connections
to the coil terminals; the operational amplifier may be connected to
provide an output to an integrator whose output in turn forms one input to
a further operational amplifier having another input provided by a
resistive connection to the electromagnet coil, the output of the further
operational amplifier providing an error control signal for the electric
supply device, eg power amplifier circuit, providing current supply to the
coil. An example of the construction and operation of such a circuit is
described in further detail below.
As illustrated in more detail below, where such a circuit is connected to
the terminals of the coil to measure the values of voltage across and
current through the coil, so-called derivative control by ensuring that a
positive component of current exists at the output of the circuit, another
desirable known kind of control, is additionally achieved in which there
is an approximately linear relationship of positive slope between the
total force acting upon the suspended object and the time derivative of
the relative position of the object.
Such derivative control provides desirable damping to oscillations of the
object about its mean position caused by random disturbances to the
suspension of the object. High damping implies that little velocity will
occur when an oscillating force is applied.
In a number of systems it is desirable to provide both stiffness and
damping of random oscillations. A lower mechanical impedance results from
higher stiffness and/or damping.
A preferred alternative way of achieving derivative control in addition to
the aforementioned proportional control in apparatus according to the
present invention is as follows. A signal representing the measured value
of the parameter V.sub.L defined above is balanced against a signal
representing the measured value of the parameter di/dt where i is the
current through the coil and any difference between the balanced signals
is employed as an error control signal in a feedback loop and applied,
together with the aforementioned error control signal, to the electrical
supply to the coil. Thus, it is preferred to have in parallel with the
aforementioned negative feedback control loop providing proportional
control an additional negative feedback loop providing derivative control
in the manner described. Such derivative control together with the
aforementioned proportional control may be achieved without the use of
separate transducers.
Thus, as well as providing a novel means of providing proportional and
derivative control of the electromagnetic suspension of an object the
present invention allows the problems associated with separate transducers
as described hereinbefore to be avoided. Electromagnetic control of the
suspension of an object may be achieved at higher temperatures which could
otherwise harm the separate transducers as used in the prior art and
greater electromagnetic control force per unit area may be obtained. As
the measurement system can be formed by using less discrete parts,
suspension control may be achieved more cheaply and more reliably than
with prior art systems.
It can be shown in the following way that both proportional and derivative
control achieved in the manner described above can give both damping and
stiffness to oscillations caused by a random disturbance to the suspension
of the object.
For small perturbations the force F acting upon the controlled object is
proportional to flux O produced by the electromagnet:
F=c.sub.1 O Equation 11
Also current i through the coil of the electromagnet and position x of the
object relative to the electromagnet are related:
i=c.sub.2 x+c.sub.3 O Equation 12
To control stiffness and damping force is required to be a function of x
and dx/dt:
##EQU4##
The `pure inductance` voltage V.sub.L is given in Equations 2 and 3 above.
From Equation 3:
V.sub.L dt=NO+c.sub.7 Equation 14
From Equation 12:
##EQU5##
From Equation 15:
##EQU6##
Substituting Equations 15 and 16 into Equation 13:
##EQU7##
From Equation 2:
##EQU8##
From Equation 14:
##EQU9##
Substituting Equations 18 and 19 into Equation 17:
##EQU10##
Which may be re-written as:
##EQU11##
So the requirement to control stiffness and damping stated as Equation 13
is met if Equation 22 is satisfied.
Equation 22 is satisfied by combining:
(a) positive feedback of i;
(b) positive feedback of di/dt;
(c) Negative feedback of V.sub.L dt;
(d) Negative feedback of V.sub.L.
This combination is achieved by having proportional control obtained by
balancing a signal representing V.sub.L dt against one representing i and
by having derivative control obtained by balancing a signal representing
V.sub.L against one representing di/dt as described above. The correct
choice of c.sub.A, c.sub.B, c.sub.C, c.sub.D and c.sub.E is required to
satisfy Equation 22. There is an effect on stiffness and damping if any
controls are varied but predominantly the stiffness is affected by c.sub.A
and c.sub.C whilst the damping is controlled by c.sub.B and c.sub.D. The
parameters may be combined together in a way that allows independent
control of stiffness and damping. If c.sub.B and c.sub.D are set to zero
then Equation 22 simplifies to:
##EQU12##
This is of identical form to Equation 4 above.
In any application where changes in the resistance of circuit resistors in
apparatus embodying the invention may be caused by changes in temperature
of the operating environment such resistors may be variable resistors each
controlled to have a fixed resistance value over a range of operating
temperatures.
The said control signal producer(s) and feedback loop(s) may be used in
conjunction with one or more other sensors and feedback loops to provide
control of other parameters related to the suspension of the object by the
electromagnet. For example, the deviation between an instantaneous
position and a desired mean position of the object may be measured and
controlled in one of the ways known in the prior art, eg using one of the
known gap measuring methods described above. The inductance of the
electromagnet coil may alternatively be measured to provide position
control. The inductance is a measure of the distance separating the object
from the electromagnet. Control of the inductance therefore allows setting
of the distance. The inductance may be measured by one of the prior art
methods which are known to those skilled in the art for the measurement of
inductance but is desirably measured in the following manner which is a
novel technique per se and is the subject of copending UK Patent
Application Nos. 9200087.6 and 9222017.7 by the present Assignee.
Connected in series with the first mentioned coil of the electromagnet
which for the purpose of reference is herein called the "control coil" is
a second coil so that the same current flows through the control coil and
the second coil. A resonant circuit is formed by connecting a capacitor to
the second coil or to a third coil which is in a mutually inductive
relationship with the second coil so that the second and third coils act
as the primary and secondary coils of a transformer. An a.c. signal of
constant peak voltage amplitude and having a frequency within the
resonance peak of the resonant circuit, preferably at the resonant
frequency, is injected into the circuit containing the control coil and
the second coil. The impedance or inductance of the control coil varies
with any incremental change in the distance separating the electromagnet
and the suspended object resulting in a varying current component at the
a.c. frequency. The tuned circuit provides means for detecting only the
a.c. component of the current through the control coil, the amplitude of
the a.c. component of current being sensed by measuring, by an amplitude
detector, the amplitude of the alternating voltage across a component, eg
the capacitor, of the resonant circuit.
Where the said second coil is itself in the resonant circuit the said
amplitude detector requires isolation from the d.c. components in the
second coil and this may be provided by connecting the respective leads to
the said amplitude detector through capacitors.
Where the said second coil is employed together with a third coil as a
transformer the a.c. signal injected through the second coil (or primary
coil of the transformer) induces a voltage alternating at the same
frequency to appear across the third coil (or secondary coil of the
transformer). The induced voltage is a measure of the varying a.c.
component of the current flowing through the control coil at the applied
frequency.
The amplitude of the alternating voltage detected by the said amplitude
detector is compared in an error detector with a reference signal
comprising a desired mean level, and the error signal comprising
variations between the measured amplitude and the desired mean level is
integrated in an integrator and thereafter employed as a control signal in
a closed negative feedback loop (in parallel with aforementioned loop)
connected to the aforementioned controllable electric supply device, eg
power amplifier circuit, to adjust the electrical supply to the control
coil to maintain the inductance at a desired mean level.
The said injected a.c. signal may be applied as an input to the circuit of
the controllable electric supply device and thereby superimposed upon the
nominally d.c. output of that device supplied to the control coil.
Desirably, although not essentially, the said resonant frequency is
greater than the normal audio range and is preferably in the range 15 to
25 kHz, eg 20 kHz.
It can be shown that the control of position of the object by measurement
of the inductance of the control coil in the manner described provides
so-called integral control of the suspension of the object which is
another known desirable kind of control in which the relationship between
the total force acting upon the object and the time integral of the
incremental distance moved by the object is a linear relationship of
positive slope.
Since the measurement of inductance in the manner described may be carried
out essentially by connecting a second coil to the control coil the use of
a separate transducer may again be avoided. Thus, so-called PID control,
which is a combination of proportional, integral and derivative controls,
of the object suspension may be achieved without the need for separate
transducers, ie without transducers not connected to the control coil.
The apparatus according to the present invention may include a plurality of
electromagnets each controlled in the manner of the present invention. For
example, a pair of electromagnets working together may be employed to
control the suspension of an object in one dimension. Control of such a
pair of electromagnets may be achieved by providing a controllable
electric supply device connected to each electromagnet, a control signal
producer and a feedback loop from the control signal producer to the
electric supply device all as described hereinbefore.
An active electromagnetic bearing for a rotating shaft constituting the
object being suspended may comprise two pairs of electromagnets controlled
in the manner described, the members of each pair working together to
control one dimensional suspension, the overall bearing providing two
dimensional suspension.
A complete suspension system for a shaft may comprise two or more such
active bearings acting radially upon the shaft and one active thrust
bearing acting upon an end of the shaft.
Use of apparatus according to the present invention to suspend a rotating
shaft advantageously allows the problem of the restriction on the choice
of location of separate transducers caused by bending of the shaft in its
free-free mode to be avoided.
Where a plurality of electromagnets are employed these may be formed by the
provisions of the electromagnet coils on different portions of a common
core in a known manner. The core may be shaped and laminated in known
manner. For example, it may be a solid ring shape with the object at the
axis of the ring. The ring shape may have projections facing toward the
axis some or all of the projections carrying the control coils. The core
may be one of the other shapes well known in this field.
In apparatus according to the present invention comprising two
electromagnets each having a feedback loop controlling a parameter
relating to the relative position of the object, the control signal
producer for producing an error control signal for each loop may comprise
a single circuit common to the feedback loops associated with both
electromagnets. The control signal producer may have an output which is
applied as a negative signal to the electric supply device of one
electromagnet and as a positive signal to the electric supply device of
the other electromagnet and vice versa (as appropriate). The output from
the common circuit for each feedback loop may be applied in conjunction
with an error control signal from a control signal producer dedicated to
the control coil of the electromagnet with which it is associated.
The electric supply device for delivering current to the two electromagnets
may comprise a common power amplifier driving two transistor devices each
connected to one of the electromagnets and to a separate voltage source,
the two voltage sources being of opposite polarity whereby the current
changes applied to the two coils are equal and opposite. In this case only
one feedback loop and error control signal is required as an input to the
power amplifier.
Active bearings embodying the present invention may be employed in any
applications requiring contactless, maintenance-free, non-lubricated
bearings. Such applications include bearings for moving parts in machines
handling dangerous materials, eg radioactive, explosive toxic or
biologically active materials, high speed bearings for vacuum pumps, food
processing (where lubricants would cause contamination), bearings for high
temperature environments and bearings for low temperature environments
(where oil and other lubricants would freeze).
Embodiments of the present invention will now be described by way of
example with reference to the accompanying drawings, in which:
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a circuit diagram of apparatus embodying the invention;
FIG. 2 is a graph of force versus distance for an object whose suspension
is controlled by apparatus as shown in FIG. 1;
FIG. 3 is a circuit diagram of an additional feedback loop circuit which
may be employed in conjunction with the feedback loop in the apparatus
shown in FIG. 1;
FIGS. 4 and 5 are circuit diagrams of alternative arrangements for the
feedback loop shown in FIG. 3;
FIG. 6 is a block circuit diagram of apparatus embodying the present
invention including two active electromagnets for controlling the
suspension of an object;
FIG. 7 is a circuit diagram of alternative apparatus embodying the present
invention including two active electromagnets for controlling the
suspension of an object;
FIG. 8 is a circuit diagram of further apparatus embodying the present
invention;
FIG. 9 is a side view of an experimental arrangement to compare the
properties of a control system embodying the invention with those of a
conventional system; and
FIG. 10a-10d are a series of figures representing oscilloscope display
traces obtained by using the arrangement shown in FIG. 9.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The control circuit embodying the invention shown in FIG. 1 measures by tw | | |
