U.S. patent number 3,860,860 [Application Number 05/347,498] was granted by the patent office on 1975-01-14 for motion control system for direct current motors, particularly in sewing machine uses.
This patent grant is currently assigned to Photocircuits Division of Kollmorgen Corporation. Invention is credited to Larry M. LeBlanc, Robert J. Moran, Constantine J. Pateuk.
United States Patent |
3,860,860 |
Moran , et al. |
January 14, 1975 |
MOTION CONTROL SYSTEM FOR DIRECT CURRENT MOTORS, PARTICULARLY IN
SEWING MACHINE USES
Abstract
A motion control system for the direct current motor of a sewing
machine wherein an external brake applies a retarding force to the
motor and the sewing machine needle mechanism once the motor has
reached a predetermined essentially stable level. Braking is
controlled by monitoring the emf or current of the motor when it is
in a dynamic braking mode and by initiating stabilized speed
control of the motor when the emf or current obtains a
predetermined level. The external brake thereafter is energized
upon occurrence of a signal representing the arrival of the needle
mechanism at a predetermined location in the needle cycle.
Inventors: |
Moran; Robert J. (Littleton,
MA), Pateuk; Constantine J. (Billerica, MA), LeBlanc;
Larry M. (Natick, MA) |
Assignee: |
Photocircuits Division of
Kollmorgen Corporation (Glen Cove, NY)
|
Family
ID: |
23363940 |
Appl.
No.: |
05/347,498 |
Filed: |
April 3, 1973 |
Current U.S.
Class: |
318/375; 112/275;
318/269 |
Current CPC
Class: |
H02P
3/12 (20130101); D05B 69/26 (20130101) |
Current International
Class: |
D05B
69/26 (20060101); D05B 69/22 (20060101); H02P
3/12 (20060101); H02P 3/06 (20060101); H02r
003/16 () |
Field of
Search: |
;318/265,269,366,367,369,370,371,375,379,380,368 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Schaefer; Robert K.
Assistant Examiner: Duncanson, Jr.; W. E.
Attorney, Agent or Firm: Morgan, Finnegan, Durham &
Pine
Claims
We claim:
1. A braking control system for a dc motor adapted for energization
by an external power source, comprising:
external braking means for applying a retarding force to the
motor;
means responsive to the back emf of the motor when the motor is
unenergized by the external power source to develop a speed
representative signal; and
braking control means responsive to the speed representative signal
for energizing the external braking means at a time when the motor
attains a given speed level.
2. The motor braking control system of claim 1, further
comprising:
means responsive to deenergization of the motor by the external
power source for dynamically braking the motor to slow it to said
given speed level; and
means for driving the motor at said speed level for at least a
limited period of time prior to energization of the external
braking means.
3. A motion control system for a sewing machine having a needle
mechanism driven by a dc motor comprising:
means for applying a dc signal to the motor to drive the needle
mechanism during normal operation of the machine;
a braking circuit including means for connecting the motor to a
series current path during braking operation thereof;
means responsive to the position of the needle mechanism for
developing a position signal;
braking means for applying a retarding force to the needle
mechanism; and
a brake control circuit jointly responsive to the position signal
and to the dynamic braking motor current in said series current
path for energizing the braking means so as to stop the needle
mechanism at a predetermined position during braking operation of
the system.
4. The motion control system of claim 3, wherein:
the series current path includes a low dc impedance and the brake
control circuit responds in part to the voltage signal developed
across said impedance.
5. The motion control system of claim 3, wherein:
the braking circuit includes a switch operable to apply the dc
drive signal during normal operation and to disconnect the motor
from the dc signal while connecting said motor to the series
current path during braking operation.
6. The motion control system of claim 3, wherein:
the braking circuit includes switch means connected serially in the
series current path and responsive to the brake control circuit for
obtaining dynamic braking of the motor during selected portions of
the braking operation.
7. A motor control system for a sewing machine having a needle
mechanism driven by a d.c. motor comprising:
means for applying a d.c. signal to the motor to drive the needle
mechanism during normal operation of the machine;
a braking circuit including switch means for connecting the motor
to a series current path during braking operation thereof for
obtaining dynamic braking of the motor during selected portions of
the braking operation;
means responsive to the position of the needle mechanism for
developing a position signal;
braking means for applying a retarding force to the needle
mechanism;
a speed control drive circuit for applying a d.c. drive signal to
the motor for a selected period of time during braking operation;
and
a brake control circuit jointly responsive to the position signal
and to the motor current in said series current path during braking
operation for energizing the braking means so as to stop the needle
mechanism at a predetermined position.
8. The motion control system of claim 7, wherein:
the speed control drive circuit is controlled by the brake control
circuit so as to be operable when the motor speed obtains a
predetermined level.
9. The motion control system of claim 8, wherein:
the speed control drive circuit is operable to drive the motor with
a preselected torque.
10. The motion control system of claim 9, wherein the speed control
drive circuit comprises:
a regulator for applying a variable dc current signal to the motor
in response to a variable control signal;
means for generating the variable control signal, said signal
having a first signal component variable in accordance with motor
speed variations and a second signal component which is essentially
constant in value.
11. The motion control system of claim 10, wherein said generating
means comprises:
means responsive to current in the motor for producing the first
signal component;
reference potential means connected intermediate the speed
responsive means and said regulator for developing the second
signal component; and
means for applying a signal from the brake control circuit to the
regulator, said signal being greater in potential than the
reference potential.
12. The motion control system of claim 8, wherein:
the speed control drive circuit is responsive to the position
signal so as to be rendered inoperable when the motor has obtained
said predetermined level and the needle mechanism, while in motion,
reaches a preselected position.
13. The motion control system of claim 12, wherein:
the brake control circuit energizes the braking means upon
cessation of operation of the speed control drive circuit.
14. The motion control system of claim 13, wherein:
the braking means is responsive to an electrical signal; and
the brake control circuit generates an electrical signal having a
predetermined duration so as to operate the braking means for a
corresponding period of time.
15. The motion control system of claim 8, wherein:
the brake control circuit comprises a pulse generator operable to
produce an electrical control pulse when the motor exceeds a
predetermined speed level,
the braking circuit switch means being responsive to the control
pulse so as to effect dynamic braking for the duration thereof.
16. The motion control system of claim 15, wherein:
the position signal comprises at least one signal generated each
time the needle mechanism arrives at a predetermined position in a
needle motion cycle; and
the brake control circuit pulse generator generates an enabling
signal for the speed control drive circuit, the generator being
operative to initiate the enabling pulse upon cessation of the
electrical control pulse and to extinguish the enabling pulse upon
the occurrence of a position pulse.
17. The motion control system of claim 16, further comprising:
means responsive to a needle position command for overriding the
response of the pulse generator to motor speed so as to initiate
the enabling signal for the speed control drive circuit upon such
command.
18. The motion control system of claim 17, wherein:
the position signal comprises at least one additional signal
separated in time from the first position signal, the pulse
generator responding to such other position signal so as to
extinguish the enabling signal initiated by the needle command,
thereby to stop the needle mechanism in a second predetermined
position.
19. The d.c. motor control system of claim 7, wherein:
the impedance altering means causes said low impedance to become a
high impedance thereby to diminish the dynamic braking current.
20. A motor control system for a sewing machine comprising:
a direct current low inertia motor for driving the mechanism;
a source of variable direct current for energizing said motor
during normal operation;
a direct current impedance controllable between a high impedance
value and a low impedance value sufficient to dynamically brake the
motor;
a controllable motor drive circuit for providing a source of direct
current to run said motor at a reduced speed during selected
portions during braking operation of the sewing machine;
a switch connecting the motor to said variable direct current
source during normal operation and, during braking operation, for
disconnecting the motor from said variable direct current source
while connecting the motor to a first series current path including
said controllable impedance and to a second series current path
including said controllable motor drive circuit; and
a control circuit responsive to the dynamic braking current in said
motor during braking operation for altering said impedance from a
low value to a high value when the motor speed is reduced to a
desired level, and for bringing said controllable motor drive
circuit into operation once said impedance is of high value.
21. The motor control system of claim 20, further comprising;
means responsive to the needle mechanism for developing a position
signal representing the location of the needle relative to a
desired position;
said control circuit being also responsive to said position signal
so as to terminate operation of said drive circuit upon occurrence
thereof, whereby said motor, during braking operation, is first
dynamically braked and then is controllably run at reduced speed
until the needle attains a predetermined location.
22. The motor control system of claim 21, further comprising;
electromechanical braking means responsive to said position signal
for applying an external retarding force to the motor so as to stop
the needle at a desired predetermined position.
23. A low inertia d.c. motor control system comprising:
means responsive to a braking command for short circuiting the
motor through a current path having a low impedance;
means for detecting the dynamic braking current in said path to
develop a control signal when the magnitude of the current obtains
a given value;
means responsive to the control signal for controllably altering
the impedance in said current path so as to thereby control the
dynamic braking current for the motor in accordance with the speed
of said motor; and
speed regulating means including a current source,
a current control device in series with the motor to provide a
variable current to the motor from a direct current source in
response to a variable speed regulation signal during a period of
time when said dynamic braking is not effective, and
means jointly responsive to the control signal and to the current
through said motor for developing the speed regulation signal.
Description
BACKGROUND OF THE INVENTION
Our invention relates to motion control of direct current motors of
the type used in sewing machines and the like. It specifically
concerns an improved motion control system wherein the parameter of
motor current or motor emf is used to control the motor, such as by
using auxiliary braking means to bring the motor to a predictable
stop from a controlled intermediate speed level.
Control of industrial type sewing machines places stringent demands
upon the machine's electrical control system. First, the sewing
machine drive is subjected to continuously varying loads, and the
speed of the sewing machine must be capable of continuous control
by the operator, preferably being infinitely variable over the
operating range. Second, it should be possible to stop the sewing
machine in a relatively short period of time and in a minimum
distance of needle travel under operator control. Third, it must be
possible to stop the sewing machine with the needle either in the
up position or in the down position. Thus, a precise positional
control, and not merely random stopping, is both desirable and
necessary.
Another requirement, particularly in sewing machine work, is the
capability of rapid acceleration and deceleration. This requirement
makes the direct current low inertia motor, directly connected to
the needle mechanism, an ideal drive source, as it may be
accelerated and decelerated rapidly owing to its relatively high
torque and its relatively low inertia. By the same token, less
braking torque is required in order to bring the motor, and the
mechanism it drives, to a complete stop.
It has been the usual practice to relay upon dynamic braking of the
drive motor in order to bring the needle mechanism to a halt. If
dynamic braking alone is the only source of braking, it is apparent
that a specific needle position cannot be assured inasmuch as the
motor could be at any of an infinite number of intermediate speeds
when braking is initiated, even should it be possible to start
braking in a particular part of the mechanical cycle. In order to
meet this problem, it has been the very recent practice in
commerical sewing machine drives to implement a complex motor
control sequencing operation wherein the motor first is slowed to a
predetermined speed level and then stabilized there at least
momentarily. Upon reaching this speed level, the motor subsequently
is dynamically braked to a stop in response to a pulse signal that
is generated when the needle mechanism, in its cycle, passes some
known location.
Prior art commercial systems embodying this technique yield good
performance when the sequencing control system for the motor is
functioning properly. It has been the experience of operators,
however, that these systems are much too complex, require frequent
serving, are very expensive and are difficult to maintain in
operation.
The complexity of these systems is attributable to several factors.
First, they implement photo-responsive devices and circuits for
generating pulses whose repetition rate must be reliably detected
to control system operation and motor speed. While pulse train
techniques do eliminate the need for tachometers and other types of
speed transducers, they unnecessarily complicate the circuitry.
Another complicating factor is attributable to the motor drive
system itself, which operates on unidirectional current pulses of
varying duty cycles obtained from the regulated triggering of
several silicon controlled rectifiers (SCR's) fed by a multiphase
source. In order to obtain dynamic braking in this type of circuit,
special circuitry is used to disable the SCR triggering system
during dynamic braking and to reactivate the drive system to
provide a low torque drive of the dc motor during the
speed-stabilized mode of operation prior to the final braking
step.
A further complicating factor may reside in the use of silicon
controlled rectifiers for short circuiting the direct current motor
during dynamic braking. A special purpose circuit must be
incorporated in this case just to release dynamic braking at the
end of the first braking period. This is because the SCR tends to
continue conducting until the current through it is nearly
extinguished.
It is an important object of the invention to overcome the problems
inherent in the present day commercial systems while retaining the
advantages of these systems in dc motor motion control. One
principal and important advantage of our invention in that regard
is the elimination of much complexity previously thought to be
indispensable.
Another object of the invention is to provide a simple, reliable
and low cost sewing machine control system which gives positive and
accurate control of the motor and the sewing machine mechanism.
Another object of the invention is to join compatibly the dc motor
drive signal developed from rectifying a continuously variable
alternating current signal with a sewing machine motion control
system using an intermediate speed regulated drive.
SUMMARY OF THE INVENTION
Briefly, these and other objects of the invention are achieved
through a control system in which the direct current motor is
dynamically braked in response to a braking command, during which
time the dynamic energy of the motor, e.g., the current or voltage
generated by the motor in a braking mode, is used to develop a
control signal for the system.
In a preferred embodiment, the dynamic braking current is used to
detect a predetermined desired motor speed, and the motor speed
thereafter is regulated by a current regulating device in series
with the motor. The motor is put into the dynamic braking mode by
disconnecting the normal variable direct current source from the
motor terminals and connecting the motor terminals to the braking
control system. Series current regulation of the motor continues
until the needle mechanism reaches a predetermined position, at
which time external braking means is brought into play to stop the
motor from a known velocity in a prescribed time and travel
distance.
In accordance with an additional feature of the invention, the
dynamic braking current is used to develop a variable signal which,
when added to a fixed voltage reference, provides a control signal
for regulating the motor during the speed stabilized mode.
DETAILED DESCRIPTION OF THE DRAWINGS
The following detailed description of a preferred embodiment should
be referred to for a complete understanding of the invention. In
the drawings:
FIG. 1 is a schematic diagram of a motor control system in
accordance with the invention; and
FIG. 2 is a series of graphs, A-I, of representative waveforms and
functions at various points in the system of FIG. 1.
DESCRIPTION OF THE PREFERRED EMBODIMENT
NORMAL MOTOR DRIVE
Referring to the left-hand portion of FIG. 1, a high torque, low
inertia direct current motor 10 is excited, during normal
operation, through a switch 12 from a conventional full wave bridge
14. The bridge, in turn, is excited by a continuously variable
alternating current source provided by the variable transformer 15.
This variable transformer includes a movable slider 16 mechanically
coupled to the operator's treadle 18 of the sewing machine, as is
indicated by the dashed line. The autotransformer 15 is supplied by
an alternating current source from the line, e.g., 220 volts/60 Hz.
applied across the transformer terminals 17a, 17b. As the operator
depresses the treadle, a continuously increasing alternating
current voltage appears at the movable contact 16 and therefore at
the input to the full wave bridge 14. A full wave rectified signal
thus drives the motor 10 in a forward direction.
A motor drive circuit of this type is disclosed in the co-pending
application of Moran et al. for "Sewing Machine Drive Control",
Ser. No. 259,171, filed June 2, 1972 and assigned to the assignee
of this invention.
The waveform applied to the motor terminals by the normal drive
circuit can be seen from FIG. 2D, illustrating the qualitative
nature of the motor drive signal as the operator completely
releases the treadle pressure. Essentially, the motor terminal
voltage includes the back emf (the dashed line) upon which is
superimposed the full wave rectified voltage from the bridge
14.
BRAKING
The system is put into the braking mode when the operator releases
pressure on the sewing machine treadle. When the treadle returns to
its normal position, the contact 16 of the variable transformer is
shorted to the terminal 17b and thus essentially reduces to zero
the applied ac signal, as already observed in FIG. 2D.
Simultaneously, the movable contact 13 of the switch 12 moves from
the normally closed position to make contact with the terminal 20,
thereby connecting the motor to the motion control system for the
sewing machine needle mechanism (the mechanism itself not being
shown). With the switch 12 in this position, the motor is
conditioned for both (a) dynamic braking; and (b) receiving
regulated current to drive the motor at a controlled predetermined
intermediate speed. Motor velocity during all operating modes is
depicted in FIG. 2A, region (0) in that curve being the
decelerating motor speed contour during normal drive as the treadle
returns to its rest position.
Braking operation is automatically initiated in order to bring the
sewing machine needle mechanism to a halt with the needle in the
full down position, when the treadle is released. Braking occurs in
three steps: (1) dynamic deceleration from existing speed to a
predetermined plateau velocity (see region (1) of FIG. 2A); (2)
constant speed operation at the plateau velocity until the needle
mechanism arrives at a preselected location in its cycle (region
(2)); and (3) combined dynamic and external braking to bring the
motor to a complete stop (region (3) of FIG. 2). In the event that
the operator commands the machine to stop with the needle in the
full up position, a command is given and the machine drives from
the bottom stop to the top (needle up) stop automatically by
accelerating to the plateau velocity and, thereafter, being braked
to a complete stop by dynamic and external braking. The latter mode
takes place in regions (4) and (5) of FIG. 2A.
Dynamic braking is accomplished by connecting the motor in a low
impedance series current path comprised of the low impedance
resistance 22 and the nominal impedance of the emitter-collector
path of the switching transistor 24. During dynamic braking, the
transistor 24 is turned on by an appropriate signal supplied to the
transistor base via the amplifier 25. This signal is developed by
the braking circuit comprised in part of the flip-flop pulse
generator 27, which is totally responsive to speed-representative
and position-representative signals.
When motor 10 is in the dynamic braking mode, the short circuit
current, i.e., the dynamic braking current, develops a voltage
across the resistance 22 which is proportional to motor speed. This
voltage is compared in the comparator 30 with a reference voltage
applied to the amplifier 31. As the voltage across the resistance
22 drops below a predetermined value set by this reference voltage,
the output of the differential comparator 30 goes low to induce a
pulse (FIG. 2E) at the output of the negative edge detector 33.
This latter pulse represents the motor's obtaining the preselected
speed level and is fed through an OR gate 35 to the set (S) input
of the braking control flip-flop 27, thereby driving the set (Q)
output high and the reset (Q) output low.
The current in the dynamic braking resistor 22 is illustrated in
FIG. 2C, with the corresponding position of the mode switch 12
being shown in FIG. 2B. Rapid deceleration of the motor due to
dynamic braking is clearly seen to occur in region (1) of the graph
of FIG. 2A.
Since the braking control flip-flop 27 is normally in the reset
condition, its Q output is normally high. This condition causes an
enabling signal to be applied to the amplifier 25 to turn on the
transistor 24. Thus, when the treadle is released by the sewing
machine operator, dynamic braking results from the short circuiting
of the motor through the resistive impedance 22.
Dynamic braking continues until the voltage across the resistor 22
is equal to the set reference voltage 31, causing the pulse in FIG.
2E to fall. This negative falling edge is sensed by the detector 33
and results in a pulse (34 in FIG. 1) which sets the flip-flop 27.
This sequence drives the Q output of the unit 27 high so as to
enable a speed control regulating device while at the same time
causing the transistor switch 24 to turn off. The Q output, it is
seen, is applied via the amplifier 38 to the base of a current
regulating transistor 40, the collector of the latter being
connected to an appropriate direct current source +V. At this point
in the braking cycle, the circuit transfers to operation in the
region (2) of FIG. 2A, wherein the motor is held at a constant, or
plateau, velocity until such time as a position pulse is sensed.
Speed regulation is accomplished as follows.
SPEED REGULATION
Speed regulation occurs in regions 2 and 4 of FIG. 2A by supplying
a regulated current to the motor through the transistor 40. In
those regions the amplifier 42 is active and amplifies the voltage
across the resistor 22. The gain of amplifier 42 is chosen such
that the voltage applied to the anode of the zener diode 44 equals
the i.sub.a R.sub.a (armature) voltage drop in the motor. The
signal presented to the motor via the transistor 40 and the diode
45 is thus set by the zener diode and the i.sub.a Ra drop in the
motor.
In operation, local torque variations causing the motor speed to
change are senses by the amplifier 42 and presented to the zener
diode 44 as the variable component of a speed control signal. Any
voltage changes in this signal component are reflected at the base
of the transistor 40, which is excited with the sum of the zener
diode 44 voltage and the variable i.sub.a R.sub.a drop voltage
whenever the amplifier 38 receives an enabling signal from the Q
output of the flip-flop 27. In response to signal variations at its
base, the transistor 40 modifies the voltage at the diode 45 to
readjust the i.sub.a R.sub.a drop in the motor circuit, thus
keeping the motor speed constant. The motor terminal voltage is
equal essentially to the sum of the motor back emf and the i.sub.a
R.sub.a drop. Thus, motor speed is always corrected toward a
constant value by varying the terminal voltage to maintain the same
back emf, determined by the zener voltage, anytime i.sub.a R.sub.a
tends to vary as a result of back emf changes.
The absolute value of the motor speed is set by the zener voltage.
A small forward bias is set on the transistor 40 by the resistors
47, 48.
The transistor 40 continues to conduct until the flip-flop 27 is
reset. Throughout this period, therefore, the motor 10 drives under
such torque as is necessary to maintain constant speed until the
needle mechanism arrives at a known position.
POSITIONAL STOP CONTROL
Position pulses are generated in order to give a positional
reference for the initiation of the final braking step region (3)
in FIG. 2A. To this end an irregularly shaped ferromagnetic cam 50
rotating with the needle mechanism and cooperating with the
reluctance pick off coil 52, generates pulses each time the lobes
of the cam pass the pick off point. Those pulses are of the
opposite polarities (FIG. 2F) and are applied to respective
amplifiers 54, 55. The outputs of the amplifiers are gated in the
AND gates 57, 58 with gating signals from the position selector
flip-flop 60. The AND gate outputs are applied via the OR gate 62
to the reset (R) input of the braking control flip-flop 27.
Normally, the bottom stop (needle down) position is desired as the
final needle position. That is, the Q output of the flip-flop 60 is
normally high, allowing only the pulses passing to the AND gate 57
to be passed on to the braking control circuit 27. Accordingly,
unless overridden by the operator, the needle will be brought to
stop in the down position as a result of the flip-flop 60 normally
being in the reset condition.
In summary, the braking control pulse generating unit 27, normally
in the reset condition, becomes set (for normal operation) whenever
motor speed drops below a pre-selected level. As long as motor
speed remains above this level, the output of the voltage
comparator 30 is high (FIG. 2E), and a pulse is applied to the set
(S) input of the flip-flop 27 only upon extinction of the high
level output of the comparator 30. The flip-flop 27 output (FIG. 2G
illustrating Q output thereof) then commands regulated speed
operation, region (2), until such time as the needle mechanism
arrives at a predetermined location. This results in the generation
of a position pulse applied to the reset (R) input of the flip-flop
27, extinguishing the enabling pulse to the speed regulator and
again instituting dynamic braking in region (3) of FIG. 2.
At this time, braking is supplemented by an external brake, such as
the electromechanical brake represented by the amplifier 70, brake
energizing solenoid 72 and the braking armature 73. Yet other types
of brakes may be used satisfactorily with the invention, such as of
the eddy current, magnetic particle positive mechanical detent
types, etc. The braking unit 70-73 is brought into operation
through detection of the trailing edge of the set (Q) output of the
flip-flop 27. A trailing edge detector 75, which may be a
differentiator, for example, accepts the Q output of the circuit
27; the detector output in turn triggers a one-shot pulse generator
76 to produce the waveform of FIG. 2H. During the fixed period of
this pulse, the external brake is energized.
In the event that the operator wishes to select the top needle
position, a command is given by moving one of the controls on the
sewing machine, usually a knee-operated lever 77. This applies a
signal (FIG. 2I) to the set (S) input of the needle position
flip-flop 60, the top stop selector switch being shown
schematically by the switch 78 in FIG. 1. This same command is
applied through the amplifier 80 directly to one of the inputs of
the OR gate 35 and, thence, to the set input of the flip-flop 27.
This immediately sets the flip-flop and places the system in the
driven mode illustrated as region (4) in FIG. 2. The motor is
operated for a period of time sufficient to bring the needle
mechanism to a position where it can be stopped in the full up
location.
When the top stop command is applied to the flip-flop 60, the
condition of the Q and Q outputs changes to remove the gating
signal from the AND gate 57 and to enable the AND gate 58, allowing
the top stop position pulses (FIG. 2H negative pulse) to pass to
the OR gate 62 and thence to the R input of the flip-flop 27. The
motor thus continues to drive until such time as the needle reaches
the proper top location, thereupon extinguishing the pulse at the Q
output the flip-flop 27 and generating a braking pulse at the
output of the one-shot 76. The system then enters the external
braking region (5) illustrated in FIG. 2.
From the foregoing it should be immediately appreciated that
dynamic braking occurs only as long as the transistor 24 conducts
as a result of an enabling signal from the braking control
flip-flop. Since this signal directly controls braking, additional
circuits found in systems using silicon rectifiers are not
necessary. In addition, since the signal developed across the
resistance 22 represents both motor speed and i.sub.a R.sub.a drop
of the motor, it may be used both to detect plateau motor velocity
and to correct speed variations from plateau velocity making
unnecessary complex circuits for pulse counting and the like. All
of this is an immense simplification over known commercial devices
with a concommitant saving in initial investment and maintenance
time.
A further advantage of the invention is obtained as a result of the
function switch 12, which removes the normal drive source from the
motor during the position control sequence. This makes possible to
use of the separate current regulating circuit, including the
transistor 40, without having to disable in some manner the normal
drive circuitry. Thus, although conceptually this approach appears
to result in a duplication of circuits, it in fact results in a
great simplification.
Although our development has been described with reference to a
preferred embodiment, modifications and variations are possible
without departing from the spirit and scope of the fundamental
principles involved. For example, both positive and negative pulse
logic can be used and numerous types of devices for developing
position pulses and for applying external braking forces are
available to those in the art and are well known. Also, the
invention is not limited to any particular type of direct current
motor, and any high response low inertia dc motor will provide
satisfactory performance. The U 12 M4H motor available from the PMI
Division of Kollmorgen Corporation, Glen Cove, New York is one such
motor in this category which is satisfactory for use in the
invention.
* * * * *