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Claims  |
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What we claim is:
1. An incremental motion motor controller for a linear, variable reluctance
stepper motor comprising:
a first cylindrical motor member having a plurality of regular helical
teeth with grooves therebetween, said teeth having a pitch P and a width
equal to P/2;
a second cylindrical motor member surrounding at least a portion of said
first member, said second member comprising two annular poles separated by
an annular magnet, each pole having two annular elements separated by an
annular winding, each element having a set of regular helical teeth with
grooves therebetween, said teeth having a pitch P and a width equal to
P/4, the sets of teeth in each pole being offset longitudinally from each
other by an amount equal to (n.+-.1/2)P, n being an integer, said poles
being offset from each other in the longitudinal direction by an amount
equal to (m.+-.1/4)P, m being an integer, said windings characterized by
no-motion signals having unperturbed waveforms and by motion-dependent
signals having perturbed waveforms;
waveform generator means synthesizing waveforms modeled after said
no-motion signals;
comparator means for comparing said synthesized waveforms with said
motion-dependent waveforms; and
means for controlling the energization of said windings in response to said
comparison.
2. A motor controller according to claim 1 wherein said second motor member
includes annular flux "regulators" disposed on each side of said magnet
and between said magnet and each pole, the cross section of said flux
"regulators" narrowing from said magnet toward each of said poles.
3. A motor controller according to claim 1 wherein said magnet is a
permanent magnet.
4. A motor controller according to claim 3 wherein said permanent magnet is
a rare earth permanent magnet.
5. A motor controller according to claim 1 wherein said synthesized
waveform is offset in the direction of perturbation.
6. A motor controller according to claim 1 or 5 wherein said no-motion
signal has an exponential waveform.
7. A motor controller according to claim 6 wherein said no-motion signal
has an exponential waveform of the general type V=a(1-e.sup.- bt), where a
and b are constants.
8. A motor controller according to claim 7 wherein said motion-dependent
signal has a perturbed exponential waveform of the general type
V=a(1-e.sup.- bt), where for some t>0, b=b(t), first decreasing in value
and then increasing in value.
9. An incremental motion motor controller for a disk, rotary, variable
reluctance stepper motor comprising:
a disk-shaped rotor mounted on a shaft and having a plurality of
wedge-shaped, radially extending equally spaced teeth having an angular
pitch P and an angular width equal to P/2; and
a stator comprising two poles, one disposed on each side of said rotor,
each stator pole having two sets of wedge-shaped, radially extending,
equally spaced teeth having an angular pitch P and an angular width equal
to P/4, the locii of said sets of stator teeth describing circles having
two different radii, the sets of teeth in each stator pole being offset
from each other by an angular amount equal to (n.+-.1/2)P, n being an
integer, the two stator poles being offset from each other by an angular
amount equal to (m.+-.1/4)P, m being an integer, each stator pole also
including an annular magnet and an annular winding, said windings
characterized by no-motin signals having unperturbed waveforms and
motion-dependent signals having perturbed waveforms;
waveform generator means for synthesizing waveforms modeled after said
no-motion signals;
comparator means for comparing said synthesized waveforms with said
motion-dependent waveforms; and
means for controlling the energization of said windings in response to said
comparison.
10. A motor controller according to claim 9 wherein said rotor further
includes an annular-shaped iron ring disposed between said shaft and said
rotor teeth to conduct flux between said stator poles.
11. A motor controller according to claim 9 wherein said magnet is a
permanent magnet.
12. A motor controller according to claim 11 wherein said permanent magnet
is a rare earth permanent magnet.
13. A motor controller according to claim 9 wherein said synthesized
waveform is offset in the direction of perturbation.
14. An incremental motion motor controller for a cylindrical, rotary
variable reluctance stepper motor comprising:
a cylindrical rotor having a plurality of longitudinally extending teeth
with grooves therebetween, said rotor teeth having an angular pitch P and
an angular width equal to P/2; and
a cylindrical stator surrounding said rotor, said stator having two annular
poles separated by an annular magnet, each stator pole having two sets of
equally spaced, longitudinally extending teeth having an angular pitch P
and an angular width equal to P/4, the sets of teeth in each stator pole
being offset from each other by an angular amount equal to (n.+-.1/2)P, n
being an integer, the two stator poles being offset from each other by an
angular amount equal to (m.+-.1/4)P, m being an integer, the sets of teeth
in each stator pole being separated by an annular windings, said windings
characterized by no-motion signals having unperturbed waveforms and
motion-dependent signals having perturbed waveforms;
waveform generator means for synthesizing waveforms modeled after said
no-motion signals;
comparator means for comparing said synthesized waveforms with said
motion-dependent waveforms; and
means for controlling the energization of said windings in response to said
comparison.
15. A motor controller according to claim 14 wherein said magnet is a
permanent magnet.
16. A motor controller according to claim 15 wherein said permanent magnet
is a rare earth permanent magnet.
17. An incremental motion motor controller for a variable reluctance
stepper motor having at least two windings characterized by no-motion
signals having unperturbed waveforms and motion-dependent signals having
perturbed waveforms; said controller comprising:
waveform generator means for synthesizing waveforms modeled after said
no-motion signals;
comparator means for comparing said synthesized waveforms with said
motion-dependent waveforms; and
means for controlling the energization of said windings in response to said
comparison.
18. A motor controller according to claim 17 wherein said means for
controlling the energization of said windings comprises:
means for determining the difference between a desired motor position and
the present motor position;
means for determining the direction for stepping; and
means for stepping said motor so as to reduce said difference to zero.
19. A motor controller according to claim 17 or 18 wherein said synthesized
waveform is offset in the direction of perturbation.
20. A motor controller according to claim 17 or 18 wherein said no-motion
signal has an exponential waveform.
21. A motor controller according to claim 17 or 18 wherein said no-motion
signal has an exponential waveform of the general type V=a(1-e.sup.- bt),
where a and b are constants.
22. A motor controller according to claim 21 wherein said motion-dependent
signal has a perturbed exponential waveform of the general type
V=a(1-e.sup.bt), where for some t> 0, b=b(t), first decreasing in value
and then increasing in value.
23. An incremental motion motor controller for a DC variable reluctance
stepper motor having first and second windings characterized by no-motion
signals having unperturbed waveforms and motion-dependent signals having
perturbed waveforms, said controller comprising:
first and second motor drive circuits connected, respectively, to said
first and second windings for energizing same;
first waveform generator means for synthesizing waveforms modeled after
said no-motion signals produced by said first winding;
second waveform generator means for synthesizing waveforms modeled after
said no-motion signals produced by said second winding;
first comparator means connected to said first winding and to the output of
said first waveform generator means for comparing the synthesized
waveforms modeled after said no motion signals produced by said first
winding with said motion-dependent waveforms produced by said first
winding;
second comparator means connected to said second winding and to the output
of said second waveform generator means for comparing the synthesized
waveforms modeled after said no-motion signals produced by said second
winding with said motion-dependent waveforms produced by said second
winding; and
processor means connected to said first and second motor drive circuits,
said first and second waveform generator means and to said first and
second comparator means for controlling the energization of said first and
second windings in response to said comparisons.
24. A motor controller according to claim 23 wherein said processor means
comprises:
means for determining the difference between a desired motor position and
the present motor position;
means for determining the direction of stepping; and
means for stepping said motor so as to reduce said difference to zero.
25. A motor controller according to claim 24 further including means for
decelerating said motor when said difference has been reduced to a
predetermined value.
26. A motor controller according to claim 23 or 24 wherein said first and
second motor drive circuits each comprises a four transistor H-bridge
connected across the winding, said four transistors being selectively
energized in pairs under control of said processor means to effect current
reversal through the winding and stepping of said motor.
27. A motor controller according to claim 23 or 24 wherein said first and
second waveform generator means each comprises two waveform generators and
wherein said first and second comparator means each comprises two
comparators.
28. A motor controller according to claim 23 or 24 wherein said no-motion
signals have exponential waveforms.
29. A motor controller according to claim 28 wherein said synthesized
waveforms are offset in the direction of perturbation.
30. A motor controller according to claim 28 wherein said exponential
waveforms are of the general type V=a(1-e.sup.- bt), where a and b are
constants.
31. A method for controlling a variable reluctance stepper motor having
first and second windings characterized by no-motion signals having
unperturbed waveforms and motion-dependent waveforms having perturbed
waveforms, said method comprising the steps of:
(a) synthesizing waveforms modeled after said no-motion signals;
(b) comparing the synthesized waveforms with the motion-dependent
waveforms; and
(c) controlling the energization of said windings in response to said
comparison..
32. The method of claim 31 or 30 further including the step of offsetting
the synthesized waveforms in the direction of perturbation.
33. The method of claim 31 or 32 wherein said synthesized waveforms are
exponential waveforms.
34. The method of claim 33 wherein said exponential waveforms are of the
general type V=a(1-e.sup.-bt), where a and b are constants.
35. A method for controlling a variable reluctance stepper motor having
first and second windings characterized by no-motion signals having
unperturbed waveforms and motion-dependent signals having perturbed
waveforms, said method comprising the steps of:
(a) determining the difference between a desired motor position and the
present position;
(b) determining the direction of stepping;
(c) synthesizing waveforms modeled after said no-motion signals;
(d) comparing the synthesized waveforms with the motion-dependent
waveforms;
(e) controlling the energization of said windings in response to said
comparisons; and
(f) stepping said motor so as to reduce said difference to zero.
36. The method of claim 35 further including the step of decelerating said
motor when said difference has been reduced to a predetermined value.
37. The method of claim 36 wherein said synthesized waveforms are
exponential waveforms. |
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Claims |
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Description |
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BACKGROUND OF THE INVENTION
It is well known that stepping motors of all kinds exhibit instabilities at
certain combinations of drive, load inertia and operating frequency. These
instabilities result from the fact that the force/displacement
characteristic at each cardinal position of the stepper is like a spring
constant which, acting on the inertia mass of the moving part, results in
a highly undamped mechanical resonance. Operation of the stepper at the
resonant frequency or at harmonics of this frequency will often result in
erratic performance. While there are timing methods which will uniformly
accelerate and decelerate a stepping motor through its resonance
frequencies, these methods are all subject to the requirement that the
load must be nearly constant. A well-compensated stepping motor drive
system which smoothly accelerates, slews and decelerates a given load will
usually perform very badly if the load is doubled or halved.
Manufacturers and users of stepping motors have developed various
techniques for controlling stepper motors in a closed-loop manner.
Feedback stepper controls may be classified into two groups: (1) velocity
feedback systems, in which a signal indicative of mechanical stepping rate
is developed and used to modify the drive frequency; and (2) pulse
position or timing feedback systems, in which an output is derived from
either the motor itself or a separate transducer and used directly to
control stepper drive switching. Velocity feedback is implicit in the
latter systems and the response of the stepper to time varying loads is
much more rapid. The most successfully known method of feedback control
involves the addition of an external position measuring device such as an
electro-optical transducer. Signals from the transducer are used to
confirm and count steps and in some systems the transducer output is used
directly to time driving pulses to the motor.
Investigators have tried to derive feedback signals directly from the
windings of a stepper motor. The principal advantage of this approach
would be lower costs. A secondary advantage in high performance systems
would be the elimination of the inertia of a separate transducer, which
can be a significant part of the total load. The problems most frequently
encountered are that the windings of most stepper motors have a high
degree of cross-coupling and the feedback signals are small compared with
drive voltages. In conventional steppers, as the motor speed changes the
relative magnitudes of these voltages vary significantly. The most
successful known system is one which measures average motor current which
is indicative of average motor speed because the motor back EMF reduces
motor current. At least one such closed loop motor control system is
commercially available. The response of such a system to load transients
is extremely poor, however, because the averaging process involves a long
time constant.
BRIEF DESCRIPTION OF THE INVENTION
It has been discovered that stepping motors of the type disclosed in
application Ser. No. 926,311 filed July 20, 1978 and entitled "Variable
Reluctance Stepper Motor" are characterized by the production of a motion
dependent electrical signal that can be employed in the incremental motion
motor controller of the present invention to time the drive pulses in an
optimum fashion so as to achieve reliable stepper operation during
acceleration, slewing and deceleration under widely varying load
conditions.
Variable reluctance stepper motors characterized by such a motion dependent
signal generally comprise cooperating fixed and moving structures having
tooth-like patterns of equal pitch but with the fixed and moving teeth
having different lengths, measured in the direction of movement. The
smaller teeth are divided into groups which are offset from one another by
fractions of a tooth pitch to provide a bi-directional motor capable of
being stepped from one position to the next with the teeth tending to
produce force in the desired direction carrying essentially all of the
flux and the teeth tending to produce force in the opposite direction
carrying essentially no flux.
One embodiment of a linear variable reluctance stepper motor characterized
by such a motion dependent signal comprises a cylindrical slider and a
rod-shaped stator. The slider comprises two poles separated by a permanent
magnet. Each pole comprises two sets of one or more spiral teeth separated
by a winding. Each winding is continuously energized by a direct current,
the direction of which is controlled. The stator comprises a toothed
member having uniformly spaced spiral teeth having a pitch P and a width
equal to P/2. The spiral slider teeth have a pitch P and a width equal to
P/4. The slider tooth sets on each pole are offset from each other by an
amount equal to (n.+-.1/2)P, n being an integer. The poles of the slider
are offset from each other by an amount equal to (m.+-.1/4)P, m being an
integer.
One embodiment of a disk, rotary variable reluctance stepper motor
characterized by such a motion dependent signal comprises a rotor and a
stator having two poles. Between the stator poles is a disk-shaped rotor
having uniformly spaced radial teeth having an angular pitch P and an
angular width equal to P/2. Each stator pole comprises two sets of radial
teeth, the locus of each set describing a circle with a different radius.
The stator teeth have an angular pitch P and an angular width equal to
P/4. Associated with each stator pole is a permanent ring magnet and a
winding. The sets of teeth in each stator pole are offset from each other
by an angular amount equal to (n.+-.1/2)P, n being an integer. The stator
poles are offset from each other by an angular amount equal to (m+1/4)P, m
being an integer. Each winding is continuously energized by a direct
current, the direction of which is controlled.
One embodiment of a cylindrical, rotary variable reluctance stepper motor
characterized by such a motion dependent signal comprises a rotor and a
stator having two poles. The rotor is cylindrical and has longitudinally
extending teeth with grooves therebetween, the rotor teeth having an
angular pitch P and an angular width equal to P/2. The stator is
cylindrical and surrounds the rotor. The stator has two annular poles
separated by an annular magnet. Each stator pole has two sets of equally
spaced, longitudinally extending teeth having an angular pitch P and an
angular width equal to P/4. The sets of teeth in each stator pole are
offset from each other by an angular amount equal to (n.+-.1/2)P, n being
an an integer. The two stator poles are offset from each other by an
angular amount equal to (m.+-.1/4)P, m being an integer. The sets of teeth
in each stator pole are separated by an annular winding which is
continuously energized by a direct current, the direction of which is
controlled.
Thus, the variable reluctance stepper motors suitable for use with the
incremental motion motor controller of the present invention comprise a
pair of windings continuously energized by direct currents the direction
of which is controlled. The motion dependent signal employed in the
incremental motion controller of the present invention is derived from
that end of the winding which has most recently switched to the lower
(e.g., ground) voltage. In the absence of any motion, the voltage will
exhibit an unperturbed waveform which, in the case of a linear variable
reluctance stepper motor, may be of the type V=a(1-e.sup.-bt), where a and
b are constants. When the stepper motor is allowed to move, however, the
change in reluctance induces a transient voltage in the winding which is
superimposed on the unperturbed waveform and results in a perturbed
waveform. In the case of a linear variable reluctance stepper motor the
perturbation may be such as might be caused if "b" in the expression
V=a(1-e.sup.-bt) were not constant but, for some time t>0, b=b(t), first
decreasing and then increasing in value. See, e.g., the lower curves in
FIGS. 8A and 8B. As described in greater detail hereinafter, the
incremental motion motor controller of the present invention compares the
motion induced (perturbed) signal with a synthesized signal which is
modeled after the no-motion (non-perturbed) waveform and which is offset
from the no motion waveform in the direction of perturbation so as to
control the timing of the drive pulses and achieve reliable stepper
operation during acceleration, slewing and deceleration under widely
varying load conditions.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a simplified view in section of a linear, variable reluctance
stepper motor for use with the motor controller of the present invention.
FIG. 2 is a simplified plan view, partially sectioned, of a cylindrical,
rotary, variable reluctance stepper motor for use with the motor
controller of the present invention;
FIG. 3A is a simplified view in section of a disk, rotary, variable
reluctance stepper motor for use with the motor controller of the present
invention;
FIG. 3B is a simplified view of the tooth arrangement of the motor of FIG.
3A;
FIG. 4A is a simplified view in section of a slider element for the motor
of FIG. 1;
FIG. 4B is a simplified partial plan view of a stator for use with the
spiral slider element of FIG. 4A;
FIG. 5 is a functional block diagram of one embodiment of the incremental
motion motor controller of the present invention;
FIG. 6 is a schematic diagram of one embodiment of the motor drive circuit
of FIG. 5;
FIG. 7 is a schematic diagram of one embodiment of the function generator
and comparator circuits of FIG. 5;
FIGS. 8A, 8B and 8C are graphical representations of typical waveforms
associated with one embodiment of the incremental motion controller of
FIG. 5; and
FIGS. 9A, 9B and 9C are flow diagrams of one embodiment of the programs for
the processor of FIG. 5.
DETAILED DESCRIPTION
One embodiment of a linear variable reluctance motor useful with the
incremental motion motor controller of the present invention is
illustrated in FIG. 1. Linear variable reluctance motor 10 comprises a
cylindrical stator 11 and an annular slider 14. Stator 11 has teeth 12 and
interspersed grooves 13. The teeth have pitch P and a width P/2. Stator 11
is preferably formed from 2.5% silicon iron. Annular slider 14 slides
along stator 11 on support bearings 15. Slider 14 comprises poles 16 and
17 separated by ring permanent magnet 18, preferably a samarium cobalt
magnet. Pole 16 comprises two annular slider elements 19 and 20 while pole
17 comprises two annular slider elements 21 and 22. Slider elements 19 and
20 are separated by winding 23 while slider elements 21 and 22 are
separated by winding 24. Between ring permanent magnet 18 and poles 16 and
17 are flux "regulators" 25 and 26. The slider elements and flux
"regulators" are also formed of 2.5% silicon iron. Annular rings 27 and
28, also of 2.5% silicon iron, provide flux paths between slider elements
19-20 and 21-22, respectively.
Whereas stator teeth 12 have a pitch P and a width equal to P/2, slider
teeth 29 have a pitch P and a width equal to P/4. In addition, the teeth
in slider elements 19 and 20 (as well as the teeth in slider elements 21
and 22) are offset from each other by an amount equal to (n.+-.1/2)P, n
being an integer. The teeth of poles 16 and 17 are offset from each other
by an amount equal to (m.+-.1/4)P, m being an integer. Motor 10 is
"stepped" from one linear position to the next by reversing the direction
of current in one of the two control windings 23 and 24, both of which are
continuously energized.
One embodiment of a cylindrical rotary variable reluctance motor useful
with the incremental motion motor controller of the present invention is
illustrated in FIG. 2. Motor 40 comprises a cylindrical rotor 41 having
longitudinally extending teeth 42 with grooves 43 therebetween. Teeth 42
have an angular pitch P and an angular width equal to P/2. Grooves 43 may
be filled with a non-magnetic material so that rotor 41 presents a smooth
outer periphery. Stator 44 is provided with two poles, 44a and 44b.
Associated with each of stator poles 44a and 44b are two sets of
longitudinally extending stator teeth 45-46 and 47-48. Positioned between
stator teeth 45-46 and 47-48 are control windings 49a and 49b,
respectively. Stator teeth 45, 46, 47 and 48 have an angular pitch P and
an angular width equal to P/4. Stator poles 44a and 44b are separated from
each other by ring permanent magnet 49. Stator teeth 45 and 46 (as well as
stator teeth 47 and 48) are displaced from each other by an angular amount
equal to (n.+-.1/2)P, n being an integer. Stator poles 44a and 44b are
offset from each other by an angular amount equal to (m.+-.1/4)P, m being
an integer.
Control windings 49a and 49b are continuously energized by direct currents
of a magnitude sufficient to create an MMF equal to that developed by
permanent magnet 49, the direction of current being selected to cause the
magnetic flux developed by the control winding either to aid or oppose the
magnetic flux of permanent magnet 49. The four possible combinations of
current direction establish flux paths through the rotor and stator teeth
which are analogous to those created in the linear stepper motor of FIG.
1. The rotary stepper motor is stepped by changing the direction of
current of one of control windings 49a or 49b.
One embodiment of a disk, rotary variable reluctance motor useful with the
incremental motion motor controller of the present invention is
illustrated in FIGS. 3A and 3B. Motor 50 comprises a rotor 51 mounted on a
non-magnetic, preferably stainless steel, shaft 51a by means of a central
collar 51b, which may be made from an epoxy resin, upon which is mounted
an integral soft iron ring 51c. Two stator poles 52 and 53 are disposed on
opposite sides of rotor 51. Stator pole 52 comprises two sets of radial,
wedge-shaped teeth 54, 55, the locus of each set describing a circle with
a different radius. Stator pole 53 comprises two sets of similar stator
teeth 56, 57. Associated with stator poles 52 and 53 respectively are
permanent ring magnets 57 and 58 preferably of samarium cobalt.
Surrounding ring magnets 57 and 58 respectively are coils 59 and 60 having
leads 59a-59b and 60a-60b adapted for connection to sources of current
whose direction is controllable.
Rotor 51 comprises equally spaced, wedge-shaped radial teeth 61. Rotor
teeth 61 and stator teeth 54-56 are preferably made from vanadium
premendur. Rotor teeth 61 have an angular pitch P and an angular width
P/4. Stator teeth 54, 55 (as well as stator teeth 56, 57) are offset from
each other by an angular amount equal to (n.+-.1/2)P, n being an integer.
Stator poles 52 and 53 are offset by an angular amount equal to
(m.+-.1/4)P, m being an integer. FIG. 3B shows the spatial relation
between rotor teeth 61 and stator teeth 54, 55. Both the rotor and stator
teeth are preferably embedded in epoxy rings.
The direct current continuously applied to control windings 59, 60 is of a
magnitude substantially equal to the MMF of a ring-shaped permanent
magnets 57 and 58, either aiding or opposing. Incremental stepping of the
disk-type stepping motor is controlled by switching current direction as
described earlier. Although not shown for purposes of simplicity, it
should be understood that stator halves 52, 53 are typically enclosed
within a non-magnetic housing.
FIGS. 4A and 4B show a spiral slider element and stator for use with the
linear variable reluctnce stepper motor of FIG. 1. Spiral slider element
19 is comprised of a hollow cylindrical shell 19a having an outwardly
extending circular flange 19b provided for mounting purposes. The hollow
interior is provided with a tooth pattern comprised of teeth 29 arranged
in a regular helix, each tooth having a pitch P and a width equal to P/4.
Grooves 30 are three times as wide as teeth 29. Four spiral slider
elements are employed in each motor 10 (elements 19, 20, 21 and 22 of FIG.
1). FIG. 4B shows the stator 11 having interspersed teeth 12 and grooves
13. Teeth 12 and grooves 13 have a pitch P and a width equal to P/2. Teeth
12 of the stator form a continuous helix and have square threads.
In a preferred method for fabricating stator 11 an iron bar, preferably
2.5% silicon iron, is machined on a latch and helical threads of the
desired depth are cut therein. The threaded bar is then tin flashed with a
very thin electro-deposit of elemental tin so as to make soldering
material to the threaded bar possible. Next a tinned trip of electrically
conducting, non-magnetic material is fitted into the groove 13 extending
the length of the bar. This material may, for example, be copper or
aluminum. The strip may be secured to the bar by a screw 11a. After the
strip has been placed in groove 13 the bar is soldered, for example by
dipping, so as to fill up the space around the tinned strip with solder.
Next the bar is ground to a diameter slightly smaller than the desired
finished diameter. It may, for example, be ground to a diameter 0.002
inches less than the desired finished diameter. The ground rod is now
preferably copper flashed with a very thin copper coating a fraction of a
thousandth of an inch thick. The ros is now plated with, for example, a
thin layer of a non-magnetic material such as chromium to produce a very
hard finish. When chromium is used for plating it is necessary to go
through the step of copper flashing. When other plating materials such as
nickel are used, the copper flashing step may be omitted.
In the finished structure support bearings 15 ride on stator 11 (See FIG.
1). Where the plating on stator 11 is hard, support bearings 15 are of a
softer material. Conversely, where the surface of stator 11 is soft, the
support bearing material is harder. For example, where the surface of
stator 11 is very hard, e.g., chromium, support bearings 15 are preferably
of an oilite sintered bronze material. Alternatively, support bearings 15
may be eliminated and the grooves between teeth 23 of the slider elements
may be filled with a Teflon loaded material called "Delrin" or with a
nylon loaded material called "Rulon" and employed directly in lieu of
bearings 15. Where stator 11 is plated with a material which is not quite
as hard as chromium, such as nickel, then support bearing 15 may be made
from aluminum treated with the "Tufram" process of General Magnaplate
which has the effect of oxidizing the aluminum to make polycrystalline
alumina after which the voids are impregnated with Teflon. This is a very
hard bearing material which has a low coefficient of friction.
It should be understood that the tooth arrangements heretofore described
may be reversed in that the wide teeth or the narrow teeth may be provided
on the fixed or on the moving part, the opposite tooth configuration being
placed on the moving and fixed parts respectively. In addition, the
permanent magnets, although preferably formed of samarium cobalt, can be
formed of any suitable material. Alternatively, they may be
electromagnets. Magnetic paths may be either solid or laminated and the
coils may be located as shown or wound directly around the teeth to
provide different coupling for their MMF's. In the linear embodiment, the
cross section of the inner member need not be round but may be square,
hexagonal or any other desired shape. An inner member having a round cross
section is preferred because it is easier to manufacture.
The number of teeth employed and, therefore, the size of motion increments,
is not limited by any ratio or formula involving pole and slot counts as
is the case with vernier steppers. If the desired number of rotary steps
is divisible by four two but not by four, a motor car be designed to
provide directly this capability. If the desired number of steps is
divisible by then the motor must have two electrical steps per design
step. To provide an odd number of steps per revolution of the motor, the
motor must be designed with four electrical steps per design step. In most
cases, however, one to three steps can be added to the design value to
simplify the design. On the other hand, linear motors can be designed to
have any pitch within the practical limits of physical size and gap
tolerances. Although gap tolerances should be close, they fall well within
practical ranges.
FIG. 5 is a functional block diagram of one embodiment of the incremental
motion controller of the present invention. It comprises a processor 70,
motor drive circuits 71, 72, waveform generators 73, 74, 75, 76 and
comparators 77, 78, 79, 80. The motor drive circuits continuously supply
direct current to the motor windings which may, for example, be windings
23 and 24 of the linear variable reluctance motor of FIG. 1. Processor 70
may, for example, be a Rockwell 6502 microprocessor. Motor drive circuit
71 may, for example, be of the type shown in FIG. 6. Waveform generator 74
and comparator 80 may, for example, be of the type shown in FIG. 7.
Each motor drive circuit comprises an H-bridge which drives its winding
with essentially constant current in one of two directions. The feedback
signal used to control stepper timing is derived from the end of the
winding which has been most recently switched to the lower in this case
(ground) voltage. At the moment of switching the winding produces an EMF
that causes the diode in parallel with the lower switching transistor to
conduct, and the voltage reaches approximately -1.0 volts. As the energy
in the winding inductance is dissipated, this voltage rises above ground
and approaches a positive voltage equal to the IR drop of the winding
current through the forward resistance of the driving transistor and the
resistance of the 1.5 ohm current limiting resistor. In the absence of any
motion produced EMF, the voltage will exhibit an unperturbed waveform
which, in the case of a linear variable reluctance stepper motor of the
type shown in FIG. 1, may be of the form V=a(1-e.sup.-bt), where a and b
are constants.Such waveforms are shown, for example, in the upper curves
in FIGS. 8A and 8B.
If, however, the stepper is allowed to move in response to the condition
described above, then the reluctance change at the end of the motor
controlled by this winding will produce a momentary EMF, superimposed on
the unperturbed waveform. In the case of a linear variable reluctance
stepper motor of the type shown in FIG. 1, the perturbation may be such as
might be caused if "b" in the expression V=1(1-e.sup.-bt) were not
constant but, for some time t>0,b=b(t), first decreasing and then
increasing in value. Such perturbed waveforms are shown, for example, in
the lower curves in FIGS. 8A and 8B. The lower curve in FIG. 8A, which is
typical of the motion voltage produced with light load. If the stepper is
more heavily loaded, however, the motion voltage will resemble that shown
in the lower curve in FIG. 8B. The motion voltage occurs later, indicating
a slower mechanical response to the reversal of winding current.
In the incremental motion motor controller of the present invention a
signal is synthesized in waveform generators 73-76 which is modeled after
the no motion (non-perturbed) waveform and is offset from the no motion
waveform in the direction of the perturbation. The motion induced
(perturbed) signal is compared with the synthesized signal using
comparators 77-80. In the case of linear variable reluctance stepper motor
of the type shown in FIG. 1, the synthesized and perturbed waveforms,
together with the comparator output are shown graphically in FIG. 8C. The
first crossing, indicated by the comparator output going to its low output
voltage, occurs at the time which is nearly optimum for acceleration
switching. The second crossing, indicated by comparator output going to
its higher output voltage, occurs at a nearly optimum time for
deceleration switching. Depending on the desired program of motion, one of
these transitions can be used to cause reversal of the winding current at
the other motor pole so that the next mechanical state is selected. That
winding, in turn, will produce a motion control voltage which can be used
to time the next reversal of the first winding current, and so on.
As will be readily appreciated by those skilled in the art, the waveform of
the no motion signal and the motion induced signal is heavily dependent on
the physical construction of the motor. In the case of other linear
embodiments or in the case of rotary embodiments of the variable
reluctance stepper motors disclosed herein, those waveforms may or may not
be exponential in character. They might, for example, be parabolic or
hyperbolic. They may even take a form which does not readily lend itself
to mathematical expression. Nevertheless, such waveforms can still be
synthesized in waveform generators using, for example, piecewise linear
approximations, read only memories (ROM's), microprocessors or a
combination thereof.
It will also be understood that those skilled in the art that if the
stepper motor encounters a hard stop or an excessive load, then the motion
voltage and resulting comparator outputs will not be produced and the next
state of the windings will not be selected. This condition can be used to
signal an overload or to stop counting and indicate the actual position
reached by the stepper. A correct response to a hard stop or an excessive
load is possible only with a step-by-step feedback technique. It is not
possible with a velocity averaging feedback technique.
With a large but not excessive load the timing of the first crossover at
the comparator input will be delayed, as shown in FIGS. 8A and 8B, and
switching to the next state will be automatically delayed. The stepper
will therefore automatically slow down in response to increasing load and
speed up when the load is decreased or removed, and this response will
occur on a step-by-step basis.
While an incremental motion motor controller could be used to control
stepper motion directly, without the counting of steps, by a system which
connects the comparator outputs directly to winding controls, most actual
systems will require a means of step counting and a method for commanding
the stepping motor to move to various positions according to a fixed or
adjustable sequence. A flexible control device which can produce this type
of response is a microprocessor, incorporating control programs which
select the winding states and feedback signals, providing full positioning
performance. The flow charts for one suitable program are shown in FIGS.
9A, 9B and 9C, with FIGS. 9B and 9C showing the "STEPS" and "DELCSN"
subroutines.
The value POSNOW is the present position of the stepper motor, counted up
and down as the stepper actually moves. POSCOM is the commanded new
position, set at a new value before entering the program from the Monitor.
The program starts by computing the difference between POSNOW and POSCOM.
The flag indicating the sign of this difference is DIRFLG. The absolute
difference is OFFSET, a quantity which will be counted down to zero as
positioning proceeds. If OFFSET is initially zero the routine returns to
Monitor. If not, the first step is taken by subroutine STEPS. After this,
the program checks whether 2 steps remain and, if so, switches to a
deceleration routine. If more than 2 steps remain, the DELSCN subroutine
examines the selected feedback comparator for a motion signal. When it
occurs, the next step is taken.
The deceleration routine uses a subroutine DELSCN also. But when the
comparator has made a transition to zero, the routine continues by
examining the same comparator for a transition to one. When that occurs,
the next step is taken and this routine repeats until OFFSET=.phi..
The STEPS subroutine corrects the value of POSNOW by adding DIRFLG, which
is either +1 or -1. The lowest order 2 bits of POSNOW are now masked and
used to select the state of output windings to cause motion to the
position POSNOW. This is done by using the low order bits of POSNOW as
part of an index. After a delay the same bits are used to construct
another index which selects the generator for the comparison transient.
Finally, the OFFSET is decremented, and the subroutine exits.
The DELSCN subroutine initially delays action to allow the winding current
transient to die down. Then an index is computed to select the proper
input for feedback. The input is repeatedly scanned by a loop whose
iterations are counted. If no feedback has occurred by the time initial
value LOOPCT .phi. has been decremented to zero, the program exits to
Monitor because the stepper has taken too long to move. In so doing, it
must correct the stack pointer (SP) because it is jumping out of a
subroutine. When the input from the comparator is found to be zero, the
subroutine exits normally.
The program described is, and will usually be, part of an overall control
program which communicates with a source of commanded positions and may
feed back condition reports to this source. The routines described are
sufficient to provide positioning response to a digital command, however,
and comprise only 180 bytes.
Tests with a linear variable reluctance stepper motor of the type shown in
FIG. 1 have demonstrated that under closed loop control reliable stepping
rates of about 400 steps/second are readily achieved even though in an
open loop mode the motor will not reliably step at over about 150
steps/second. Under closed loop control the motor has been shown to be
quite insensitive to load, running from 400 steps/second down to less than
50 steps/second with increasing load, and always indicating actual steps
completed after a hard stop
While the incremental motion motor controller of the present invention has
been described in detail in connection with linear variable reluctance
stepper motor it will be appreciated by those skilled in the art that it
is equally applicable to rotary disk and cylindrical rotary embodiments as
well. In addition, while the program disclosed in FIGS. 9A-9C and in the
Appendix adjusts the timing of current switching on the basis of position,
it will be appreciated by those skilled in the art that the timing of
current switching may also be adjusted on the basis of a velocity signal,
derived from the time between steps. Even higher speeds may be attained by
a control system which commands a position more than one step away from
the dynamic position. This technique may be particularly effective where
the delay in energizing windings becomes a significant limiting factor in
stepper performance. Under these circumstances, the feedback signal will
have to be measured on the other winding and a delay adjustment may be
necessary to avoid producing reverse forces or torques.
Although two waveform generators are shown in FIG. 5 for each winding, it
will be appreciated by those skilled in the art that where the no-motion
waveform produced when one end of a winding is switched to the lower
potential is substantially the same as when the other end of the winding
is switched to the lower potential, then only a single waveform generator
is needed for each winding. Similarly, only one comparator would be
required for each winding if it were time shared. When employing
integrated circuits, however, it is often simpler to avoid time sharing
and to use multiple components, the additional size and cost being
nominal.
Although unnecessary to enable persons skilled in the art to make and use
the present invention, a complete program for both open loop and closed
loop positioning is annexed hereto as an Appendix since that constitutes
the best mode contemplated by the applicant of carrying out the invention.
Although this invention has been described with respect to its preferred
embodiments, it should be understood that many variations and
modifications will now be obvious to those skilled in the art.
Accordingly, the scope of the invention is limited, not by the specific
disclosure herein, but only by the appended claims.
APPENDIX
______________________________________
Instruc-
tion
Memory Mne-
Location
monic Mode Opcode Comments
______________________________________
0400 00 OPSELL
0401 04 OPSELH
0402 CF STDEL
0403 00 POSCOM
0404 00 POSNOW
0405 00 DIRFLG
0406 00 OFFSET
0407 00 DELIND
0408 7F INDLD
0409 1F SENDEL
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