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BACKGROUND OF THE INVENTION
This invention relates to stepper motors of the rotary or linear type.
A stepper motor is an electromagnetic incremental actuator. A stepper motor
of the variable reluctance or permanent magnet type includes a magnetic
stator structure and a magnetic moving structure which comprises a rotor
in the rotary motor case and a slider in the linear motor case. Either the
moving structure or the stator structure has windings associated with
various pole positions which are sequentially and selectively energized to
produce incremental motion of the moving structure.
The stationary positions of linear and rotary stepper motors after each
step, hereinafter referred to as the "step positions," are inherent in the
magnet structure. In other words, the moving structure will move to a
predetermined, stable stepped position is response to the energization of
one or more windings.
In many applications, this incremental motion as provided by stepper motors
is particularly desirable. One such application is found in printers of
the type disclosed in copending application Ser. No. 809,646 filed June
24, 1977 wherein a linear stepper motor is utilized to advance a movable
print point in a serial impact printer. The magnetically inherent step
positions of a rotary stepper motor may be utilized to position a daisy
character element in a serial impact printer as disclosed in copending
application Ser. No. 809,923 filed June 24, 1977.
However, the magnetically inherent step positions of a linear or rotary
stepper motor may be insufficient or inadequate in many applications
including serial impact printers. For example, the magnetically inherent
step positions may not provide a sufficient number of steps in a printer
where very small steps are required as necessitated by certain print
characters or certain spacing between print characters. In addition, the
magnetically inherent step positions may be improperly located.
In this connection, it will be understood that a very high degree of
precision is required of a printer linear stepper motor associated with a
carriage as well as the rotary stepper motor associated with the print
element. However, such precision may be difficult to achieve in the
magnetic structure although the discrete steps of the stepper motor still
provide distinct positioning control advantages in approaching the step
position. In other words, the stepper motor affords control advantages in
coarse positioning but may be inadequate for fine positioning.
Heretofore, feedback or closed-loop control of stepper motors has been
utilized to control the selective energization of the motor winding. See
Theory and Applications of Step Motors, Kuo, West Publishing Company 1974,
pp. 252-272 and 279. However, the closed loop or feedback control has not
been utilized to supplement or modify the inherent step positions but
merely to control the motor in reaching those inherent step positions.
In U.S. Pat. No. 3,906,326, a DC motor is stopped at predetermined
positions using optical feedback to position the motor However, the motor
is not of the stepper type and there is therefore no effort to supplement
or change the inherent step positions of such a motor since the DC motor
disclosed has no such step positions.
SUMMARY OF THE INVENTION
It is an object of this invention to provide stepper motors with one or
more stop positions which may differ from the magnetically inherent step
positions.
It is a further object of this invention to provide a stepper motor with
accurate stop positions independent of the accuracy in the magnetic
structure of the stepper motor.
It is also an object of this invention to have the movable magnetic
structure of the stepper motor stop quickly without oscillation.
It is also an object of this invention to provide a motor which is of
relatively low cost due to simplicity of design and the elimination of
brushes.
It is also an object of this invention to provide a motor which is reliable
due to the simplicity of design and the elimination of brushes.
It is a further object of this invention to achieve stable positions with
no steady-state power dissipation unlike normal variable reluctance motors
which may require detent current to effect stable positions.
In accordance with these and other objects, a means and method are provided
for operating a stepper motor comprising a movable magnetic structure, a
stationary magnetic structure and a plurality of windings associated with
one of the structures at a plurality of pole positions where the movable
magnetic structure is inherently capable of moving between discrete
magnetically determined step positions.
In accordance with this invention, the location of the movable magnetic
structure is sensed and the distance between the sensed location and a
predetermined stop position which may differ from the magnetically
inherent step position is determined. The windings of the stepper motor
are then energized so as to reduce this distance with the movable magnetic
structure stopping substantially at the predetermined stop position.
In a particularly preferred embodiment of the invention, sensor means which
may optionally sense the location of the movable magnetic structure
relative to the predetermined position is coupled to position indicating
means which generate a position signal representing the distance between
the location of the movable magnetic structure and the predetermined
position. A motor reference signal is then generated by a reference means
coupled to the position indicating means. Comparison means coupled to the
reference means compares the motor reference signal to a signal
representing the state of the motor and motor control means coupled to the
comparison means selectively energizes the windings of the motor in
response to the comparison so as to reduce the distance between the
location of the movable structure and the predetermined position.
In the preferred embodiment of the invention, the signal representing the
state of the motor represents current flow through the energized windings.
The reference signal represents the required current flow to reduce the
distance towards zero. The motor control means then adjusts the current
flow through the motor so as to reduce the distance. The adjustment of the
motor control means is accomplished by periodically interrupting the
current flow through the energized windings so as to vary the average
current therethrough.
In accordance with one very important aspect of the invention, the error
signal generated by the error means represents the distance as well as the
velocity of the movable magnetic structure in approaching the
predetermined position. More particularly, the error signal includes a
position component less a velocity component.
In accordance with this invention, the sensor means may sense any of a
plurality of locations of the moving magnetic structure relative to a
predetermined position. In the alternative, the sensor means may sense a
single location relative to a plurality of stop positions. Moreover, a
plurality of sensor means may be utilized with each of the sensor means
sensing the locations(s) of the movable magnetic structure relative to a
different predetermined stop position(s).
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a partially schematic, block diagram illustrating one embodiment
of the invention;
FIG. 2 is a diagram of waveforms which are utilized to explain the
operation of the embodiments shown in FIG. 1;
FIG. 3 is a schematic diagram of a portion of the motor control circuitry
shown in FIG. 1;
FIG. 4 is a top plan view of a linear motor utilized in another embodiment
of the invention;
FIG. 5 is a view of the motor of FIG. 4 taken along line 5--5;
FIG. 6 is a block diagram of an embodiment of the invention utilizing the
linear motor of FIGS. 4 and 5;
FIG. 7 is a diagram of waveforms utilized to explain the embodiment of FIG.
6; and
FIG. 8 is a schematic diagram of a portion of the motor control circuitry
shown in FIG. 6.
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT
Referring to the embodiment of the invention shown in FIG. 1, a rotary
stepper motor 10 comprising a stator structure 12 including a plurality of
pole positions 1-4 having windings 14 associated therewith is magnetically
coupled to a rotor 16 having a plurality of pole faces 18. As the windings
14 are selectively energized at the various pole positions 1-4 in
different phases, the pole faces 18 and the rotor 16 will be advanced in
accordance with well-known stepper motor techniques. For example,
energization of the winding 14 at the pole position 1 of the stator 12
will produce an incremental rotation of the rotor 16 so as to
substantially align the adjacent pole face 18 with the pole position 1.
Similarly, subsequent energization of the winding 14 associated with the
pole position 2 will produce alignment between that pole position and the
adjacent pole face 18. In this manner, the rotor 16 is rotated to
inherently stable step positions which are determined by the magnetic
structure of the stator 12 and the rotor 16. The actual energization of
the windings 14 is sequenced and controlled by circuitry well-known in the
art and designated as winding sequence and control 20.
In accordance with this invention; the stepper motor is not limited to the
magnetically inherent step positions of the motor. Rather, different or
additional step or stop positions are achieved.
Before proceeding with a description as to how the modified. step positions
are achieved, the following definitions will be helpful. As used herein,
the word "position" or "positions" describes one or more distinct points
which are fixed or stationary with respect to the stationary magnetic
structure. The word "location" or "locations" describes one or more
distinct points which are fixed or stationary with respect to the moving
magnetic structure and move therewith. The description will now proceed
relying on the definitions.
The rotor 16 is provided with a circular array of darkened areas or hash
marks 22. Predetermined locations between the darkened areas 22 relative
to a sensing position 24 represented by the intersection of dotted lines
26 and 28 are determined by optical sensing system 30. The optical sensing
system 30 includes a light source in the form of a light emitting diode 32
which is in optical communication with the location 24 along the path 26
and a light detecting means in the form of a phototransistor 34 which is
in optical communication with the position 24 along the path 28.
As the darkened area 22 and the locations therebetween pass the position
24, a sinusoidal light pattern is generated which in turn generates
sinusoidal current flow through the transistor 34 and a resistor 36
connected to the emitter thereof. The sinusoidal current flow is depicted
to waveform a in FIG. 2 which illustrates that current flow increases when
the space between the darkened area 22 is at position 24 and current flow
decreases when the darkened area 22 is at the position 24.
The resultant sinusoidal voltage waveform which is produced across the
resistor 36 is applied to a differential amplifier 38 along with a
reference voltage supplied by the tap of a potentiometer 40 connected
between a reference voltage V.sub.ref and ground. Where the tap on the
potentiometer 40 is appropriately set so as to correspond with the DC
level represented by the abcissa of the waveform a in FIG. 2, the output
from differential amplifier 38 represents the distance between the
locations x of the rotor 16 and the predetermined position 24. By
providing a plurality of darkened areas 22 and locations x therebetween as
shown in FIGS. 1 and 2, the distance between the plurality of locations
and the predetermined position 24 is represented at the output of the
differential amplifier 38. As shown in waveform a of FIG. 2, the modified
step positions are represented by axis crossings S.sub.1, S.sub.2,
S.sub.3, S.sub.4 and S.sub.5 where the locations x are aligned with the
position 24.
In accordance with another important aspect of the invention, the output
signal from the differential amplifier 38 is applied to the circuitry for
generating a signal having a distance component as well as velocity
component. In this connection, the output signal from the differential
amplifier 38 is applied to circuit means 24 which multiplies the signal by
an appropriate constant A and applied to circuit means 44 which
differentiates and multiplies the output signal by an appropriate constant
B so as to produce a velocity component. The two components are then
summed at a differential amplifier 46 and multiplied by an appropriate
constant C by circuit means 48 so as to produce a position signal
representing the distance of a predetermined location on the rotor 16 from
the stop position 24 and the velocity at which that location is
approaching the predetermined stop position.
In accordance with this invention, this distance and velocity signal is now
utilized to control the motor so as to assure that the rotor 16 will stop
at one of the modified, desired stop positions S.sub.1, S.sub.2, S.sub.3,
S.sub.4 and S.sub.5. In this connection, the output signal from the
circuit 48 is applied to a circuit 50 which determines the absolute value
of the position and velocity signal. The output from the circuit 50 is
then applied through a switch 52 when the switch is in the position shown
in phantom, to a comparator 54 where it is compared with the actual
current flowing through the energized windings 14 of the motor 10. The
particular winding which is energized is determined by the winding
sequence and control circuit 20 in response to the polarity of the
position and velocity signal as determined by a polarity determining
circuit 56 and a stop control circuit 58 schematically depicted as a
switch 59 associated with a reference voltage +V which coacts with the
switch 52. When a stop is initiated, the switch 59 is placed in the state
shown in phantom.
When the motor 10 is running, the switch 59 is in the position shown in
full. Similarly, the switch 52 is in the position shown in full so that a
current reference source 61 is connected to the comparator 54 to limit the
current through the windings 14 of the motor as determined by the source
61.
In order to control the current to the windings 14 of the motor in an
analog manner during stopping so as to reduce the position and velocity
signal to zero, the comparator 54 is responsive to the absolute value of
the position and velocity signal as well as a signal representing the
current through the energized windings 14 of the motor as determined by a
motor current resistor 60. As long as the signal representing the motor
current is less than the absolute value of the position and velocity
signal, the output from the comparator 54 remains high. That high signal
is applied to the data input of a D-type flip-flop 62 so as to produce a
high going output applied to winding sequence and control circuitry 20 in
response to clock pulses from a clock 64 which are applied to the clock
input of a D-type flip-flop 62. When the absolute value of the position
and velocity signal falls below the motor current signal, the output of
the flip-flop 62 will go low so as to interrupt the current flow through
the windings. When the current flow through the windings falls below the
position and velocity signal, the output from the comparator 54 will again
go high causing the output from the flip-flop 62 to go high so as to again
supply current to the windings as will now be described in more detail
with reference to FIG. 3.
As shown in FIG. 3, the windings 14 are connected between the voltage
supply +V and ground through a plurality of power transistors 66 and the
motor current sensing resistor 60 which is connected to the comparator 54.
The winding sequence and control 20 as shown in FIG. 3 comprises a counter
68, a decoder 70 and a plurality of AND gates 72 associated respectively
with the transistors 66. As the output from the flip-flop 62 changes state
reflecting that the motor sensor current exceeds the current reference
provided by the position and velocity signal, the AND gates 72 are
inhibited so as to turn any conducting transistor 66 off. The particular
transistor 66 which is conducting is under the control of the counter 68
of the decoder 70 as will now be described.
During normal running operation, the input to the counter 68 from the
sensor 30 advances the counter 68 and the decoder 70 decodes the count so
as to pass a high going signal to the appropriate gate 72 which is enabled
by the high output from the flip-flop 62. At the time of stopping, an
output from the polarity circuitry 56 will go high or low so as to
appropriately modify the decoded output from the decoder 70 which result
in intermittent enabling of the appropriate AND gate 72 depending on the
state of the flip-flop 62.
Referring now to waveform b in FIG. 2, torque curves for the rotary motor
are illustrated for energization of the various windings 14.sub.1-4 as a
function of rotor position. Magnetically inherent step positions are
depicted by axis crossing I.sub.1-5 for the various windings. For example,
energization of the winding 14 represented by waveform 14.sub.3 will
result in an inherently stable step position at axis crossing I.sub.3
since a position to the left of I.sub.3 will produce a positive torque
causing the motor to advance to I.sub.3 and a position to the right of
I.sub.3 will produce negative torque causing the motor to return to
I.sub.3. In accordance with this invention, the magnetically inherent
stepping positions I.sub.1-5 differ from the stop positions S.sub.1
-S.sub.5 by a slight displacement along the position axis. The manner in
which the rotor is stopped at a selected modified step position, e.g.,
stop position S.sub.2, will now be described.
Assume that the switch 52 is closed at point x on the position axis. At
that moment in time, the winding 14 represented by the torque curve
14.sub.3 is energized. In order for a predetermined location on the rotor
16 to step at position S.sub.2, it it necessary to apply a braking torque
to the rotor 16. This braking torque may be provided by energizing the
winding 14 represented by the torque curve 14.sub.1 and this is
accomplished automatically at the decoder 70 in response to the positive
polarity output from the polarity sensing circuit 56 and the stop control
circuit 58. As the distance to stop position S.sub.2 is reduced, the motor
current will exceed the current reference applied to the comparator 54 and
the AND gate 72 associated with that particular winding 14 will be
inhibited so as to interrupt current flow. When the current flow again
falls below the current reference supplied to the comparator 54, the AND
gate 72 will again be enabled. If the polarity of the error changes and
the location on the rotor 16 overshoots stop position S.sub.2, the
polarity output from the polarity circuit 56 will again change the decoded
output from the decoder 70 so as to apply a positive torque resulting from
energization of the winding 14 represented by the torque curve 14.sub.3
with the flip-flop 62 changing state so as to interrupt the current flow
until such time as the current reference representing the position and
velocity signal reaches the axis crossing corresponding to rotor position
S.sub.2.
Reference will now be made to a linear motor embodiment of the invention.
FIGS. 4 and 5 disclose a linear motor which forms the subject matter of
copending application Ser. No. 809,646 filed June 24, 1977.
The motor comprises a stator 100 including an active portion 112 and an
inactive portion 114. Pole positions 116 extend along the length of the
stator 112 with the pole positions of the active portion 114 being
energized by windings. 118. A slider 120 which is located in the air gap
between the active portion 114 and the inactive portion 112 moves
longitudinally along the motor in the direction depicted by the arrows.
In order to provide a position and velocity feedback, a timing band 122 as
shown in FIG. 5 extends along the length of the motor. The timing band
comprises a plurality of openings 124 which are sensed by optical sensing
means comprising phototransistors 126 which are exposed to a light source
(not shown) on the opposite side of the band 122 through a mask comprising
openings 128. In this embodiment of the invention, the distance between a
single location on the slider 120 and a plurality of stop positions must
be determined. Further details concerning the optical sensing of position
in a printer are disclosed in copending application Ser. No. 833,271 filed
Sept. 14, 1977 which is incorporated herein by reference.
The openings 128 associated with each of the transistors 126 are
appropriately spaced so as to generate two separate signals as shown in
waveform a of FIG. 7. By providing the two separate signals from each of
the transistors 126, more stop positions may be provided for the slider
120. In some applications such as movable print point printers as
disclosed in the aforementioned application Ser. No. 833,271 (Attorney's
Docket RM-828/EX-L-4) where the slider 120 would carry the print head,
closely spaced stop positions as provided by the two signals may be
required.
It will be understood that waveforms may be modified as shown in dotted
lines if a different sensing arrangement such as that shown in copending
application Ser. No. 833,351 filed Sept. 14, 1977, which is incorporated
herein by reference, and wherein the petals of a daisy-type printing
element are sensed directly.
In this embodiment of the invention, the windings 118 are energized in
pairs by circuitry shown in FIG. 8. The windings 118.sub.1 -118.sub.4 are
connected to power supplies +V and -V through switching transistors
130.sub.1-4. By saturating the transistors in pairs, i.e., 130.sub.1 and
130.sub.2, 130.sub.2 and 130.sub.3, 130.sub.3 and 130.sub.4 and 130.sub.4
and 130.sub.1, the windings 118.sub.1-4 are energized in pairs. Diodes 132
provide current circulating paths when switching from one pair to another.
For example, a current circulating path 134 through a diode 132 is
utilized when changing energization from the winding pair 118.sub.1 and
118.sub.2 to the winding pair 118.sub.2 and 118.sub.3 while the transistor
130.sub.2 is turned off. Resistors 136 and 138 are utilized to sense the
current flow through the windings 118.sub.1-4.
In order to achieve the stop position S.sub.1-12 as shown in waveform a of
FIG. 7, the circuit shown in FIG. 6 is utilized. As shown therein, a motor
control microprocessor 140 such as an F-8 manufactured by Fairchild Camera
and Instrument Corporation provides control of the energization sequence
and a regulator current 142 controls the current to the motor windings. In
addition, the microprocessor 140 operates in conjunction with a stop
circuit 144 including the components described in FIG. 1 to control the
current to the windings 118.sub.1-4 so as to achieve the modified step
positions which differ in part from the magnetically inherent stepping
positions I.sub.1-6. More particularly, the stop positions S.sub.1,
S.sub.3, S.sub.5, S.sub.7, S.sub.9, and S.sub.11 correspond with step
positions I.sub.1, I.sub.2, I.sub.3, I.sub.4, I.sub.5 and I.sub.6 whereas
the stop positions S.sub.2, S.sub.4, S.sub.6, S.sub.8, S.sub.10 and
S.sub.12 are additional. An output 145 of the stop circuit 144 is
connected to the current regulator 142 which switches the transistors
130.sub.1-4 so as to properly control the current in accordance with the
position and velocity signal generated by the stop circuit 144. Another
output 146 from the stop circuit 144 provides a polarity input to the
microprocessor 140.
In order to properly locate the stop positions S.sub.1-12 shown in waveform
a of FIG. 7, it is necessary to calibrate the output from the
phototransistors 128 shown in FIG. 5. For this purpose, a D/A converter
148 in combination with a comparator 150 is provided and the calibrated
output from the D/A converter 148 is applied to the stop circuit 144. A
switch 152 selectively connects the phototransistors 128 to the stop
circuit 144.
Referring now to the torque curves of waveform b in FIG. 7, the manner in
which the slider 120 is stopped will be described. Assume that a decision
to stop is made at location x when the windings 118.sub.3 and 118.sub.4
are energized. At that moment, a negative braking torque is required and
the windings 118.sub.1 and 118.sub.2 are energized. The current through
the windings 118.sub.1 and 118.sub.2 is controlled in response to the
magnitude of the waveform 128.sub.1 approaching stop position S.sub.2. The
braking current applied to the windings 118.sub.1 and 118.sub.2 is reduced
as the distance and velocity of the location on the slider relative to the
stop position S.sub.2 is reduced. The same technique is utilized to stop
at all of stop positions S.sub.1 -S.sub.12 even though some of the stop
positions S correspond with inherent step positions I.sub.1 -I.sub.6. In
this connection, it will be noted that the difficulty in holding
tolerances on the magnetic structure may produce step positions I.sub.1
-I.sub.6 which are slightly displaced from the desired stop positions.
However, for some applications, the step positions I.sub.1 -I.sub.6 may be
satisfactory so as to permit the use of a mixture of magnetically inherent
and modified step positions. Where such a mixture is desirable, the
microprocessor 140 is programmed to control the current regulator 142 at
the inherent step positions so as to override the stop circuit 144.
In the embodiment described with respect to waveform b of FIG. 7, some of
the stop positions correspond to the inherent step positions I-I.sub.6.
However, this need not be the case as shown in waveform c of FIG. 7
wherein the stop positions are equally spaced on either side of the
inherent step positions, i.e., the stop positions correspond exactly to
positions S.sub.1 -S.sub.12 (only positions S.sub.7, S.sub.8, S.sub.9 and
S.sub.10 are shown) which are equally spaced on either side of the
inherent step positions.
It will be understood that the linear stepper motor of the embodiment shown
in FIGS. 4-8 might be replaced by a rotary stepper motor. Moreover, where
the rotary motor controls the printing element, a rotary stepper may also
be used in a printer. In this connection, reference is made to the printer
of copending applications Ser. No. 809,923 filed June 24, 1977 which is
incorporated by reference herein along with copending application Ser. No.
833,351 filed Sept. 14, 1977 which describes sensing the location of a
rotor having print elements integral therein.
It will also be understood that the phrase magnetic stepper motor as used
herein refers to variable reluctance as well as permanent magnet stepper
motors which are characterized by inherently stable step positions which
occur in response to sequential energization of individual windings or
winding combinations.
Although a particular embodiment of the invention has been shown and
described and various modifications suggested, other modifications and
embodiments will occur to those of ordinary skill in the art which will
fall within the true spirit and scope of the invention as set forth in the
appended claims.
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