Note: Descriptions are shown in the official language in which they were submitted.
~.5~B9
The present invention relates to controllers for
variable reluctance stepper-motors. This application is a
divisional application of Canadian application Serial No.
326,855, filed May 3, 1979.
One known variable reluctance stepper motor employs
a toothed rotor/stator combination in which rotation of the
rotor causes a cyclic variation in the reluctance of the
magnetic circuit, which includes the rotor and stator teeth
and t-he gap therebetween. The gap changes dimension as
the rotor and stator move relative to one another. A motor
capable of providing continuous stepping motion is comprised
of two or more such sets of rotor/stator teeth, as well as
separate magnetic circuits extending therethrough, which
circuits are capable of being selectively energized. The
different rotor/stator teeth are staggered in relative
angular positions. By selective energization of the magnetic
circuits, the rotor is caused to assume successive positions
of least reluctance for the respective magnetic circuits,
depending on the magnetic and mechanical variables designed
into the motor and the method of controlO In conventional
variable reluctance stepper motors, the rotor and stator
teeth are of the same or nearly the same width. This
configuration produces the greatest possible difference
between maximum and minimum reluctanceO
It is also 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,
mb/ - 2 - ~
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acting on the inertia mass of the moving part, results in a
~ highly undamped mechanical resonance. Operation of the stepper
3 at the resonant frequency or at harmonics of this requency will
4 often result in erratic performance, While there are timing
5 methods which will uniformly accelerate and decelerate a stepping
6 motor through its resonance frequencies, these methods are all
7 subject to the requirement that the load must be nearly constant.
8 A well-compensated stepping motor drive system which smoothly
9 accelerates, slews and decelerates a given Load will usually
l0 perform very badly if the load is doubled or halved. '
l2¦ Manufacturers and users of s~epping motors have
l3,1developed various techniques for controlling stepper motors in
i411a closed-loop manner, Feedback stepper controls may be classified
l511into two groups: (l) velocity feedback systems, in which a
16~ signal indica~ive of mechanical stepping ra~e is developed and
17 used to modify the drive frequency; and (2) pulse position or
18 timing feedback systems, in which an output is derived from
l9 either the motor itself or a separate transducer and used
20 directly to control stepper drive switching. Velocity feed~ack
21 is implicit in the latter systems and the response of the
22 stepper to time varying loads is much more rapid, The most
23 ¦success~ul known method of feedback control involves the addition ,
24 of an external position measuring device such as an electro-optica
transducer, Signals from the transducer are used to confirm and
26, count steps and in some systems the transducer output is used
271 directly to t~'me driving pulses to the motor,
28, ,
291l Investigators have tried to derive feedback signals '
q0' directly from the windings of a stepper motor, The principal
, - 3 -
s38g l~
- advantage of this approach would be lower costs. A secondary
2 ;vantage in high performance systems would be the elimina~ion
3 of the inertia of a separate transducer, which can be a
4' significant part of the total load, The problems most frequently ,
si~encountered are that the windings of most stepper motors have a
6 high degree of cross-coupling and the feedback signals are small
7 compared with drive voltages, In conventional steppers, as the
g,motor speed changes the relative magnitudes of these voltages
glflvary 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
12llcurrent- A~ least one such closed loop motor control system is I ~
,commercially available, The response of such a system to load
transients is extremely poor, however, because the averaging
¦¦process involves a long time constant.
18
19 l
21 ,,,
23 ,
261 .,
27 11i
28 ji
29~
~0'
,
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38~
The invention relates to a 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 wave-
forms. The controller comprises: waveform generator means
for synthesizing waveforms modeled after the no-motion signals;
comparator means for comparing the synthesized waveforms with
the motion-dependent waveforms; and means for controlling
the energization of the windings in response to the comparison.
BRIEF DESCRIPTION OF THE INVENTION
Thus, the present invention involves a novel variable
controller for a reluctance stepper motor. The variable
reluctance stepper motor minimizes undesirable restoring
forces and produces a more efficient stepper motor of higher
sensitivity. As will be described more fully below, the
teeth on one of the cooperating elements of the motor are
wider than the teeth on the other cooperating element, while
the pitch of the teeth on both elements is the same. In
addition, the narrower teeth are divided into groups and
are offset with respect to each other by fractions of a tooth
pitch.
In a linear embodiment the motor comprises a
cylindrical slider and a rod-shaped stator. l'he 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
current whose direction is controlled. Thus, there are four
possible combinations of current directions.
mb/ ; 5
The magnitude of the current is such that the
magneto-motive force (MMF) across the teeth on each side
of the winding is equal to that produced by the permanent
magnet. Current in a given direction will produce a
minimum MMF across the teeth on one side of the winding
and a maximum MMF across the teeth on the other side of
the winding because the direction of the winding MMF aids
the permanent magnet MMF on one side and opposes it on
the other.
The stator of the linear variable reluctance stepper
motor 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 + l/4)P, m being an integer.
In a disk, rotary embodiment the motor 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 + l/2)P, n being an
integer. The stator poles are offset from each other by an
mb/ - 6 -
3~S3B~
angular amount equal to (m + l/4)P, m being an integer.
Each winding is continuously energized by a current whose
direction is controlled.
In a cylindrical, rotary embodiment the motor
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 integerO 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.
The above-described variable reluctance stepper
motors are characterized by the production of a motion
dependent electrical signal that can be employed in the
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.
mb/', 7
~ hus, the variable reluctance stepper motors suitable
for use with the incremental mGtion 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(l - e-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 unper-
turbed 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 h in the
expression V = a(l - e-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.
mb/ - 8 -
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BRIl~F DESCRIPTION OF THE DRAWINGS
3 Figure i is a simplified view in section of a linear
4 variable reluc~ance stepper motor of ~he present invention;
Figure 2 is a simplified plan view, partially
6 sectioned, of a cylindrical, rotary, variable reluctance stepper
7 motor of the present invention;
8 Figure 3A is a simplified view in section of a disk,
g rotary, variable reluctance stepper motor of the present
lO,iinvention;
Figure 3B is a simpli~ied view of the tooth arrangement i
12ljf the motor of Fig. 3A:
13~1 Figure 4A is a simplified view in section of a slider
llelement for the motor of Fig. l;
15 ~ Figure 4B is a simplified partial plan view of a
16 stator for use with the spiral slider element of Figure 4A; 2
17 Figure 5 is a functional block diagram of one
18 embodiment of the motor controller of the present invention;
19 Figure 6 is a schematic diagram of one embodiment of
20 the motor drive circuit of Fig. 5;
21 Figure 7 is a schematic diagram of one embodiment of
22 the function generator and comparator circuits of Fig. 5;
23 Fîgures 8A, 8B and 8G are graphic represen~ations of
24 typical waveforms associated with one embodiment of the motor
25 ! controller of Fig. 5; and
26 Figures 9A, 9B and 9C are flow diagrams of one
27!l,embodiment of the programs for the processor of Fig. 5.
28
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~ DETAILED ~ESCRIPTION
2 - .
3 Fig. 1 is a view in section of a linear, variable
4 reluctance stepper motor 10 in accordance with the present in-
5- vention. Stator 11 is an elongated cylindrical rod having
6 teeth 12 and interspersed grooves 13. The ~eeth have a pitch P
7 and a width P/2. Cylindrical slider 14 slides along stator 11
8 on support bearingsl5. Slider 14 comprises poles 16 and 17
9 separated by ring permanent magnet 18, preferably a rare earth
10~ magnet. Pole 16 comprises two annular slider elements 19 and 20
llli!while pole 17 comprises two annular slider elements 21 and 22.
12¦;Slider elements 19 and 20 are separa~ed by winding 23 while ~i
1311slider elements 21 and 22 are separated by winding 24. Between
14!!ring permanent magnet 18 and poles 16 and 17 are flux "regulators7'
15¦ 25 and 26. Annular rings 27 and 28 provide flux paths between
16 slider elements 19-20 and 21-22, respectively.
17 .
18 Whereas the stator teeth 12 have a pitch P and a
19 width equal to P/2, the slider teeth 29 have a pitch P and a
20 width equal to P/4. In addition, the teeth in slider element 19
21 and 20 (as well as the teeth in slider elements 21 and 22) are
22 ofset from each other by an amount equal to (n ~ 1/2)P, n being
23 an integer. The teeth o poles 16 and 17 are offset from each
24 other by an amount equal to (m ~ 1/4)P, m being an integer.
26 Motor 10 is "stepped" rom one linear position to the
2711 next by rever~ing the direction of curren~ in one of the two
28 control windings 23 and 24. There is no tendency for the slider
29 ! to move in the wrong direction. The direction of motion is
~,o dependent solely upon which winding current is reversed. Also,
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selectively inactive teeth do not se~ up any fvrces whic'n conClict
2 rth the direction of the desired forces created by the magnetic
3 energization of the active teeth. Thus, the design of Fig. 1
4 enables the motor to be stepped in either direction and with
s`good stability.
7 Fig. 2 depicts a cylindrical, rotary v~riable reluctance
8 stepper motor 40 in accordance with the present invention. Motor
gj 40 comprises a cylindrical rotor 41 having longitudinally extending
, teeth 42 with grooves 43 therebetween. Teeth 42 have a pitch P
and a width equal to P/2. Grooves 43 may be filled with a
121lnon-magnetic material so that rotor 41 presents a smooth outer
13i periphery~ I
141 . , '.
15~ Stator 44 is provided with two poles9 44a and 44b.
16l Associated with each of stator poles 44a and 44b are two sets of
17 longitudinally extending stator teeth 45-46 and 47-48. Positioned
18 between stator teeth 45-46 and 47-48 are control windings 49a and
19 49b, respectively. Stator teeth 45, 46, 47 and 48 have a pitch P
and a width equal to P/4. Sta~or poles 44a and 44b are separated
21 ~rom each other by ring permanent magnet 49. Stator teeth 45 and
22 46 (as.well as stator teeth 47 and 48) are displaced from each
23 ~ other by an angular amount equal to ~n + 1/2)P, n being an
24 I integer. Stator poles 44a and 44b are offset from each other by
25 ! an angular amount equal to (m + 1/4jP, m being an integer.
26 '~ I
271~ Control windings 49a and 49b receive control currents
281 of a magnitude suf~icient to creat an MMF equal to that developed
29' by permanent magnet 49, the direc~ion of current being selected
30' to cause the magnetic flux develo~ed by the control winding
' '
~ ~ ~5~ ~
1 either to aid or oppose the magnetic 1ux o~ perrnanen~ ma~ne~ 49.
2 ~he four possible combinations of current direction es~ablish
8 flux paths through the rotor and stator teeth which are analogou~
4 to those created in the linear stepper motor of Fig. 1. The
5 rotary stepper motor is stepped by changing the direction of
6 current o~ one of control windings 49a or 49b.
, !
8, Figs. 3A and 3B depict a disk, rotary variable
gilreluctance stepper motor 50 in accordance with the present in- j
OI,vention The motor comprises a rotor 51 mounted on a non-magnetir
lljishaft 51a by means of a central collar 51b upon which is mounted
12llan integral soft iron ring 51c. Two stator poles 52 and 53 are
13lldisposed on opposite sides o~ rotor 51. Stator pole 52 comprises
411two sets of radial, ~edge-shaped teeth 54, 55, the locus of each
15 ¦set describing a circle with a different radius. Stator pole 53
16 ¦comprises two sets of similar stator teeth 56, 57. Associated
17¦ with stator poles 52 and 53 respectively are permanent ring
181 magnets 57 and 58 Surrounding ring magnets 57 and 58 respec-
19 tively are eoils 59 and 60 having leads 59a-59b and 60a-~Ob
adapted for connection ~o sources of current whose direction is
21 controllable.
22
Z3 Rotor 51 comprises equally spaced, wedge-shaped radial
24 teeth 61 having an angular pitch P and an angular width P/2.
Stator téeth 54-57 have an angular pitch P and an angular width
26¦ Pl4 Stator teeth 54, 55 (as well as stator teeth 56, 57) are
27¦ioffset from each other by an angular amount equal to (n + 1/2)P,
28lin being an integer. Stator poles 52 and 53 are offset by an
29' angular amount equal to (m + 1/4)P, m being an integer. Fig. 3B
shows the spatial relation between rotor teeth 61 and stator
- 12 -
1 teeth 54, 55. ~ 53 ~9
3 The current applied to control winding 59, 60 is of
4 a magnitude substantially equal to the MMF of ring-shaped
5 permanent magnets 57, 58, either aiding or opposing. Incremental
6 stepping of the disk-type stepping motor is controlled by switch-
7 ing current direction as described earlier. Although not sho~m
8, for purposes of simplicity, it should be understood that stator
9I halves 52, 53 are typically enclosed within a non-magnetic
lO, housing.
l2~1 Figs. 4A and 4B show a spiral slider element and
13,,stator for use with a linear variable reluctance stepper motor
l4i1of Fig. l. Spiral slider element l9 is comprised of a hollow
lSIl cylindrical shell 19a having an outwardly extending circular
16 flange l9b provided for mounting purposes. The hollow interior
17 is provided with a tooth pattern comprised of teeth 29 arranged
18 in a regular helix, each tooth having a pitch P and a width
l9 equal to P/4. Grooves 30 are three times as wide as teeth 29.
Four spiral slider elements are employed in each motor lO
21 (elements l9, 20, 21 and 22 of Fig. l). Fig. 4B shows the
22 stator .ll having interspersed teeth 12 and grooves 13. Teeth 12
23 and grooves 13 have a pitch P and a width equal to P/2. Teeth 12
241 of the stator form a continuous helix and have square threads.
251
261 It should be understood that the tooth arrangements
27l heretofore described may be reversed in that the wide teeth or
28' the narrow teeth may be provided on the fixed or on the moving
29' part, the opposite tooth configuration being placed on the moving
and ~ixed parts respectively.
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53B~
In addition, although the permanent magnets are
2 referably formed o~ samarium cobalt, they can be formed of
3 any suitable material. Alternatively, they may be electromagnet~.
4 Magnetic paths may be either solid or laminated and the coils
j
5 may be located as shown or wound directly around the teeth to
6 provide different coupling for their MM~'s. In the linear
7 embodiment, the cross section of the inner member n~ed not be
8 round but may be square, hexagonal or any other desired shape.
9lAn inner member having a round cross section is preferred because
lOI it is easier to manufacture.
111i. '
12¦l It can be seen from the foregoing description that the
13~present invention provides novel and highly useful variable
14'reluctance stepper motors. Undesirable opposing forces generated
15 Iby those teeth which do not ~orm a portion of the active flux
16 path are minimized, so long as the currents in the control i
17 windings develop an MMF which rreates a balanced condition with -:
18 the MMF of the permanent magnet, thereby resulting in a highly
19 useful orce per unit weight of the reluctance force motor.
21 The number of teeth employed and, therefore, the size
22 of motion increments, is not limited by any ratio or formula
23 involving pole and slot counts as is the case with vernier
24 steppers. If the desired number o~ rotary steps is divisible
by four, a motor can be designed to provide directly this capabil-
26l ity. If the desired number of steps is divisible by two but not
27li~ by four, then the motor must have two electrical steps per design
, step. To provide an odd number of steps per revolution of the
29 motor, the motor must be designed with four electrical steps per
~o design step. In most cases, however, one to three steps can be
, . . .
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3 ~
~ added to the design value to simplify the design. On the other
? ~and, linear motors can be designed ~o have any pitch within the
3 practical limits of physical size and gap tolerances. Although
4 gap toler~nces should be close, they fall well within practical
5 ranges.
7l In addition to the above capabilities and variations,
8 other variations are possible. For example, the motors may be
g rotary or linear as described above. Alternatively, a rotary
stepper may be mounted upon movable elements of a linear stepper,
11, or vice versa. As a further alternative, in a linear embodiment
121' either the slider or stator or both m2y be driven so as to impart ¦
13l!rotational movement thereto.
14
15j Fig. 5 is a functional block diagram of one embodiment
161 of the motor controller of the present invention. It comprises
17 a processor 70, motor drive circuits 71, 72, waveform generators
18 73, 74, 75, 76 and comparators 77, 78 79, 80 The motor drive
19 circuits continuously supply direct current to the motor windings
20 which may, for example, be windings 23 and 24 of the linear
21 variable reluctance motor of Fig. 1. Processor 70 may, for
22 example, be a Rockwell 6502 microprocessor. Motor drive circuit
23 71 may; for example, be of the type shown in Fig. 6. Waveform
24 generator 74 and comparator 80 may, for example, be of the type
25 ~shown in Fig. 7.
261,
2711 Each motor drive circuit comprises an H-bridge which
2glldrives its winding with essentially constant current in one of
29! two directions. The feedback signal used to control stepper
,
30 timing is derived from the end of ~he winding which has been
~ 15 -
. .
r~ f
~ mos~ recently switched to the lower (in this case ground) voltage.
2 ~t the moment of s~7itching the winding produces an EMF tha~ causes
3 the diode in parallel with the lower switching transistor to
4 conduct J and the voltage reaches approximately -1.0 volts. As
5 the energy in the winding inductance is dissipated, this voltage
6 rises above ground and approaches a positive voltage equal to the
7 IR drop of the winding current through the forward resistance of
8 the driving transistor and the resistance of the 1.5 ohm current
9l limiting resistor. In the absence of any motion-produced EMF, the
lO,voltage will exhibit an unperturbed waveform which, in the case
~ of a linear variable reluctance stepper motor of the type sho~n
12l~in Fig. 1, may be of the form V = a(l - e-bt), where a and b
13¦,are constants. Such waveforms are shown7 for example, in the
141!upper curves in Figs. 8A and 8B.
1511
16¦ If, however, the stepper is allowed to move in I -~
17 response to the condition described above, then ~he reluctance
18 change at the end of the motor controlled by this winding will
19 produce a momentary EMF, superimposed on the unperturbed waveform.
ZO In the case of a linear variable reluctance stepper motor of the
21 type shown in Fig. 1, the perturbation may be such as might be
22 caused.i~ "b" in the expression V = a(l - e-bt) were not
23 constant but, for some time t~O, b = b(t)~ first decreasing and
24 ¦then increasing in value. Such perturbed waveforms are shown,
25¦¦ for example, in the lower curves in Figs. 8A and 8B. The lower
26' curve in Fig. 8A is typical of the motion voltage produced with
27,~ light load. I the stepper is more heavily loaded, however, the
28, motion voltage will resemble that shown in the lower curve in
29 Fig. 8B. The motion voltage occurs later in Fig. 8B, indicating
a slower mechanical response to the reversal of winding current.
,
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In the motor controller of ~he present inven~ion a
2 . ignal is synthesized in waveform generators 73-76 ~,lhich is
3 modeled after the no-motion (non-perturbed) waveform and is
4 offset from the no-motion waveform in the direction of the
5 perturbation. The motion-induced (perturbed) signal is compared
! '
6 with the syn~hesized signal using comparators 77-80. In the case
7,of a linear variable reluctance stepper motor of the type shown
8. in Fig. 1, the synthesized and perturbed waveforms, together wi~
g,ithe 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
3j'the comparator output going to its higher output voltage, occurs
l~at a nearly op~imum time for deceleration switching. Depending
5i¦on the desired program of motion, one of these transitions can be
16¦1used to cause reversal of the winding current at the other motor
171 pole so that the next mechanical state is selected. That winding,
18 in turn, will produce a motion control voltage which can be used
19 to time the next reversal of the first winding current, and so on.
.
21 As will be readily appreciated by those skilled in the
22 art, the waveforms o the no-motion signal and the motion-induced ¦
23 signal are heavily dependent on the physical construction of the
24 motor. In the case o other linear embodiments or in the case of
rotary embodiments of the variable reluctance stepper motors
26l! disclosed herein, those waveforms may or may not be exponential
27' in character. They might, for example, be parabolic or
28j hyperbolic. They may even take a orm which does not readily
29 lend itself to mathematical expression. Nevertheless, such
waveorms can still be synthesized in waveform generators
using, for example, piecewise linear approximations) read onl~J
2 ~mories (ROM's), microprocessors or a combination thereof.
4 It will also be understo_d by those skilled in the art
5 that if the stepper motor encounters a hard stop or an excessive
6 load, then the motion voltage and resulting comparator outputs
7 will not be produced and the next state of the windings will no~ !
8 be selected. This condition can be used to signal an overload
g'or to stop counting and indicate ~he actual position reached by
l0 the stepper. A correct response to a hard stop or an excessive
load is possible only with a step by-step feedback technique.
12l,¦It is not possible with a velocity averaging feedback technique.
13i ~,
14l~l With a large but not excessive load the timing of the
15l~!first crossover at the comparator input will be delayed, as shown i
16 in Figs. 8A and 8B, and switching to the next state will be
17 automatically delayed. The stepper will ~herefore automatically
1~ slow down in response to increasing load and speed up when the
19 load is decreased or removed, and this response will occur on
a step-by-step basis.
21
22 . While an incremental motion motor controller could be
23 used to control stepper motion directly, without the counting
24 of steps, by a system which connects the comparator outputs
25 ¦ directly to winding controls, most actual systems will require
26 ' a means of step counting and a method for commanding the stepping
271,' motor to move to various positions according to a fixed or ad-
~8, justable sequence. A flexible control device which can produce
29 this type of response is a microprocessor incorporating control
programs which select the winding states and feedback signals,
, .
- 18 -
1 -~roviding full positioning performance. The flow charts for one
2 suitable program are shown in Figs. 9A, 9B and 9C, with Figs
3 and 9C showing the "STÉPS" and "DELCSN" subroutines respectively.
5.! The value POSNOW is the present position of the stepper
6 motor, counted up and down as the stepper actually moves. POSCOM i
7 is the commanded new position, set at a new value before entering !
8 the program from the Monitor The program starts by computing
g the difference between POSNOW and POSCOM The flag indicating
the sign of this diference is DIRFLG. The absolute difference
11 is OFFSET, a quantity which will be counted down to zero as
12 positioning proceeds If OFFSET is initially zero, the routine
13; returns to Monitor If not, the first step is taken by sub-
14 routine STEPS. After this, the program checks whether 2 steps
ls,~remain and, if so, switches to a deceleration routine I~ more
16!jthan 2 steps remain, the DELSCN subroutine examines the selected
17 ¦ feedback comparator for a motion signal When it occurs, the
18 !next step is taken.
19 .
The deceleration routine uses subroutine DELSCN also.
21 But when the comparator has made a transition to æero, the
22 i routine continues by examining the same comparator for a
23 I transition to one. When that occurs, the next step is taken
24~1 and this routine repeats until OFFSET = 00
2s!
26 The STEPS subroutine corrects the value of POSNOW by
27 adding DIRFL~, which is either +1 or -1. The lowestorder 2 bits
28; Of POSNOW are now masked and used to select the state of output
29 Of 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
!
- 19 - '
~ 5~
a delay the same bi~s are used to construct another index whic'n
2 ~elects the generator for the comparison transient. Finally, the
3 OFFSET is decremented and ~he subroutine exits.
The DELSCN subroutine initially delays action to allow
6 the winding current transient to die down. Then an index is
7 computed to select the proper input for feedback. The input is
8 repeatedly scanned by a loop whose iterations are counted. If
g no feedback has occurred by the time initial value LOOPCT 0 has
10 been decremented to zero, the program exits to Monitor because
11 the stepper has taken too long to move. In so doing, it must
12, correct the stack pointer (SP) because it is jumping out of a
13 subroutine. When the input from the comparator is found to be
14 zero, the subroutine exits normally.
16, The program described is, and will usually be, part of
17¦ an overall control program which communicates with a source o~
18¦1 commanded positions and may feed back condition reports to this
lgli source. The routines described are sufficient to provide
20 ~ positioning response to a digital command, however, and comprise
21 I only 180 bytes.
22 1
23¦1 Tests with a linear variable reluctance stepper motor
~4il o the type shown in Fig. 1 have demonstrated that under closed
25¦! loop control reliable stepping rates of abou~ 400 steps/second
26 are readily achieved even though in an open loop mode the motor
27 will not reliably step at over about 150 steps/second. Under
28 closed loop con~rol the motor has been shown to be quite
29 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.
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1 l~hile the incremental motion motor controller of the
2 present invention has been described in detail in connection with -
3 a linear variable reluctance stepper motor it will be appreciated
4 by those skilled in the art that it is equally applicable to
5 disk, rotary and cylindrical, rotary embodiments as well. In
6 addition, while the program disclosed in Figs. 9A-9C adjusts
7 the timing of current switching on the basis of position, it
8 will be appreciated by those skilled in the art that the timing
g of current switching may also be adjusted on the basis of a
10 velocity signal, derived from the time between steps. Even higher
11 speeds may be attained by a control system which commands a
12 position more than one step away from the dynamic position. This
13! technique may be particularly efective where the delay in
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4-energizing windings becomes a significant limiting factor in
15~' stepper performance. Under these circumstances, the feedback
16l, signal will have to be measured on the other winding and a delay
1711 adjustment may be necessary to avoid producing reverse forces
18¦l or torques.
19
Although two waveorm generators are shown in Fig. 5
21 for each winding, it will be appreciated by those skilled in the
22 art that where the no-motion waveform produced when one end of
231 a winding is switched to the lower potential is substantially
24¦l the same as when the other end of the winding is switched to the
251¦ lower potential, then only a single waveform generator is needed
26. for each winding. Similarly, only one comparator would be
27 required for each winding i it were time shared. When employing
28 integrated drcuits, however, it is often simpler to avoid time
29 sharing and to use multiple components, the additional size and
cost being nominal.
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~ 53
Although this invention has been described with respect
2 ~o its preferred embodiments, it should be understc,od that many
3 variations and modifications will now be obvious to those skilled
4 in the art. Accordingly, the scope of the invention is limited,
5 not by the speci~ic disclosure herein, but only by the appended
6 claims.
8 What I claim is:
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