System for controlling a separately excited constant load DC electric motor

A separately excited constant load DC electric motor is controlled in resse to a source of current for the desired motor speed. Current from the source applied to the motor armature is modified by a motor loss simulator circuit responsive to the source. The circuit simulates the losses the motor is expected to have in response to the current derived from the source, whereby the actual motor speed corresponds with the desired motor speed despite the loss. The losses are simulated during acceleration and deceleration as well as steady state intervals of the current source, which can be actuated to drive the motor in both directions.

FIELD OF THE INVENTION 
The present invention relates to a system for controlling a separately 
excited, constant load DC electric motor, and more particularly to such a 
system in which compensation is made for the losses occurring in the motor 
as a function of a signal controlling the desired motor speed. 
BACKGROUND OF THE INVENTION 
Separately excited DC electric motors are often used because the speed 
thereof can be adjusted over a wide range by varying the supply voltage 
for the motor. In theory, for a given supply voltage and flux, the load 
characteristic of these motors is independent of the direct current which 
flows through them. However, it is observed in practice that the motor 
speed decreases linearly in response to energizing current increases. The 
speed decrease is due to resistive losses which occur as a result of the 
motor internal resistance, which is the sum of the resistances of the 
motor armature and the commutator windings. A formula which represents 
this phenomenon is: 
EQU E=U-RI, 
where: 
E=the electromotive force of the motor, 
U=the motor supply voltage, 
R=the motor internal resistance, and 
I=the motor armature current. 
The product RI represents the motor internal resistive or voltage drop. In 
cases where the motor operates at a constant supply voltage, increases in 
the shaft speed are regulated by varying the control current I, which must 
increase to reduce the time required for the shaft speed to increase and 
decrease. Consequently, if it is desired to adjust the shaft speed 
quickly, a considerable resistive drop develops to greatly upset the 
adjustment process. This is why separately excited DC electric motors 
which are used under these conditions have control systems including a 
loss compensating arrangement which enables the required speed to be 
reached in the required time. 
A common prior art system for controlling a separately excited DC electric 
motor comprises a generator for deriving a control signal for the motor 
speed, a power amplifier connected to drive the motor in response to the 
control signal to adjust the motor speed, and a feedback loop for 
compensating, as a function of the control signal, for motor losses. The 
feedback loop feeds back a signal proportional to the motor current to the 
input of the amplifier. To derive the signal proportional to the motor 
current, a small feedback resistor is placed in series with the motor 
armature to derive a voltage proportional to the amplitude of the motor 
armature current. 
A compensating feedback loop of this nature has been found to be 
ineffective when it is desired to adjust the motor speed very rapidly, 
equivalent, for example, to the time required for the motor commutator to 
turn past one or a few bars or strips. In such a case, slots between the 
bars produce a very irregular signal in the feedback resistor. The 
irregular signal includes one or a few current surges which, even when 
effectively filtered, cannot be used as a feedback signal. Thus, there is 
only an advantage in using this kind of compensating feedback loop when 
the required speed changes are equivalent to the commutator turning past 
many bars, so that a suitable feedback signal can be obtained. If there is 
a small angle through which the commutator turns while the speed is 
changing, the compensating arrangement is ineffective. 
The invention proposes a control system which employs a compensating 
circuit having the advantage of being effective regardless of the 
commutator rotation angle. The improved compensating circuit is also of a 
very simple construction, and dispenses with a feedback resistor which 
necessarily adds to the internal resistance of the motor in conventional 
control systems. 
BRIEF DESCRIPTION OF THE INVENTION 
According to the invention, a separately excited, constant load DC electric 
motor is controlled by an improved motor loss compensating circuit 
including a motor loss characteristic simulator circuit responsive to a 
motor control signal derived from a generator. The circuit derives a motor 
loss representing signal which is combined with the control signal in such 
a way that the speed of the motor corresponds to the required speed. 
It is, accordingly, an object of the invention to provide a new and 
improved compensating circuit for a separately excited DC motor. 
Another object of the invention is to provide a new and improved 
compensating circuit which is capable of very rapidly adjusting the speed 
of a separately excited DC motor. 
A further object of the invention is to provide a circuit capable of 
adjusting the speed of a separately excited DC motor during the interval 
required for the motor commutator to pass one or a few bars. 
An additional object of the invention is to provide a new and improved 
compensating circuit for a separately excited DC motor wherein there are 
no armature losses introduced as a result of the compensating circuit. 
The advantages and features of the invention will be clearly apparent from 
the following description, which is given with reference to the 
accompanying drawings.

DETAILED DESCRIPTION OF THE INVENTION 
In FIG. 1, a system 10 for controlling a separately excited DC electric 
motor 12 includes an input signal source 14, such as a push button 
keyboard. Generator 16 responds to the signal derived by source 14 to 
generate a control signal for the motor by determining the speed 
corresponding to the signal received from source 14 and the time required 
to reach this speed. Power amplifier 18 drives the armature of motor 12 in 
response to the control signal derived by the generator 16. Circuit 20 
responds to a signal indicative of the current supplied to the motor to 
develop a compensating signal for the motor internal resistance drop. 
A compensating arrangement of the conventional kind servo-controls motor 12 
by means of a feedback loop including differential amplifier 22 and 
feedback unit 24. In accordance with the familiar principles of 
servo-control, differential amplifier 22 has first and second inputs 
respectively responsive to the control signal derived from generator 16 
and the signal produced at the output of feedback unit 24. 
Unit 24 includes a very low impedance resistor having one grounded terminal 
and a second terminal 24a connected in series with the armature of motor 
12, as illustrated in FIG. 2. Connected in parallel with resistor 26 is a 
fixed, high impedance resistive potentiometer 28 having a tap forming 
output 24b of feedback unit 24. In certain cases feedback unit 24 includes 
a filtering network (not shown) for the reasons given above. 
The operation of control system 10 is now explained with reference to 
waveforms of FIG. 6. Assume that waveforms A and B represent the signals 
derived from source 14. Waveform A orders motor 12 to run in the forward 
direction during periods t1-t2 and t4-t5, while signal B orders the motor 
to run in reverse during interval t3-t4. 
Waveform C represents a control signal which generator 16 derives in 
response to waveforms A and B to control the required speed change 
pattern. Thus, starting in the forward direction occurs during the period 
t1-t'1 and stopping in the forward direction during the period t2-t'2. It 
is assumed that normal circumstances occur so the starting and stopping 
periods differ. Starting and stopping in the reverse direction 
respectively occur between times t3-t'3 and t4-t'4. Since time t'4 occurs 
during the interval t4-t5, while the signal commanding running in the 
forward direction is derived, generator 16 determines the predetermined 
period of forward starting, which extends between times t'4 and t"4. At 
time t5 which corresponds to the derivation of the stop forward running 
signal from source 14, generator 16 orders a stop in the predetermined 
stop period t5-t'5. 
Since the predetermined starting periods are shorter than the predetermined 
stopping periods, the current which flows through the motor 12 must be 
greater during starting than during stopping, as shown by the waveform of 
FIG. 6E. These differing currents cause different resistive drops in the 
armature of motor 12 at the terminals of the resistor 26 in the feedback 
unit 24. Differential inputs of differential amplifiers 22 respectively 
respond to control signal C, derived by generator 16, and the output 
signal from terminal 24b of feedback unit 24. 
As mentioned above, the resistance of feedback unit 24 adds to the internal 
resistance of motor 12 and the unit is disturbed by interference, pulse 
type signals which are introduced into the feedback signal by edges of the 
commutator bars; the interference signals cannot effectively be filtered 
if there are only a few interfering pulses during the acceleration 
(starting) and deceleration (stopping) intervals. 
FIG. 3 is a block diagram of a system 30, according to the invention, for 
controlling a separately excited, constant load DC electric motor 32. As 
in prior art system 10, control system 30 comprises input signal source 
34, a control signal generator 36, similar to generator 16, and a power 
amplifier 38 which energizes motor 32 so the motor speed corresponds to 
the desired speed indicated by the control signal produced by generator 
36. Control system 30 according to the invention differs from the prior 
art control system 10 by employing a compensating circuit 40 which is 
connected in series between generator 36 and the input to power amplifier 
38, while motor 32 is connected directly to ground. 
In FIG. 6 is shown the principle employed by the invention for compensating 
losses by the resistive armature drop. As mentioned supra, the resistive 
drop corresponds to the product of the internal motor resistance and the 
value of the current flowing through the motor. Because the internal motor 
resistance is a relatively fixed characteristic and the motor current is a 
known function of the motor inertia and acceleration, the resistive drop 
causes a linear reduction in the motor electromotive force, in conformity 
with the previously stated equation E=U-RI. To cancel the resistive drop 
effects, the supply current and motor internal resistance are simulated by 
combining a loss signal representing the internal motor drop, as a 
function of the control signal supplied by the generator 36, with the 
control signal in such a way that the supply voltage to the motor produces 
the required speed. In other words, the product RI is simulated to form a 
loss signal S.sub.p, which is added to the control signal U so that the 
electromotive force E=U+S.sub.p RI=U. Because source 34 is operated in the 
same way as source 14 in FIG. 1 and generator 36 therefore derives the 
same control signal C, FIG. 6, as is derived by generator 16, the 
compensating circuit 40 supplies signal D to amplifier 38. Signal D is the 
sum of signal C and a component proportional to current I, as represented 
by waveform E in FIG. 6; wavefore E occurs during the motor starting and 
stopping periods. 
In other words, compensating circuit 40 responds to the control signal 
derived from generator 36 to generate a signal representing the losses 
occurring in motor 32, and combines the loss representing signal with the 
control signal from generator 36, so the motor speed corresponds to the 
desired speed. FIG. 4, a general diagram of the combination formed by 
signal source 34, generator 36 and compensating circuit 40, shows how 
easily the invention is put into practice. 
Signal source 34, FIG. 4, causes motor 32 to run in the forward and reverse 
directions by including two switches 42 and 42', respectively connected in 
series between opposite polarity DC voltage sources B+ and B- by resistors 
44 and 44'. The embodiment of generator 36 and compensating circuit 40 
illustrated in FIG. 4 has the advantage of combining the generator and 
compensating circuit, as seen infra. 
A feed forward circuit including resistor 48 connects common terminal 45 
for switches 42 and 42' to terminal 46 so the voltage at the terminal 
represents the desired steady state speed of motor 32. The signal at 
terminal 46 controls circuits 36,40, as well as generator 34, as indicated 
infra with regard to FIG. 5. In FIG. 4, terminal 45 is also connected to 
non-inverting input (+) of differential amplifier 50, having a grounded 
inverting input (-). Differential amplifier 50 has an output connected by 
resistor 52 to an inverting input (-) of differential amplifier 54, having 
a grounded non-inverting feedback path including series capacitor 56 and 
resistor 58, connected between output terminal 60 and the inverting input 
terminal of amplifier 54. Terminal 60 of differential amplifier 54, is 
connected to terminal 64, the output of compensating circuit 40 from which 
is extracted input signal D, FIG. 6, for power amplifier 38. Common 
terminal 57 for capacitor 56 and resistor 58 is connected to noninverting 
input (+) of buffer differential amplifier 62, having a negative feedback 
loop whereby the amplifier output terminal 46 and inverting input (-) are 
directly connected. 
The amplitude of a constant voltage supplied to motor 32 after the motor 
has started is determined by the values of each of supply voltages B+, B-, 
resistors 44, 44' and 48. The time required for motor 32 to start and stop 
is set by the circuit including amplifiers 50, 54 and 62, as well as 
resistors 52 and 58, and capacitor 56; this circuit is in parallel with 
resistor 48. Differential ampifier 50 is a buffer, while differential 
amplifier 54 is an integrator by virtue of capacitor 56 and resistor 52. 
Consequently, the slope of signals C and D in the starting and stopping 
periods is determined by the values of resistor 52 and capacitor 56. In 
the general case where the starting, i.e., accelerating, and stopping, 
i.e., deceleration, periods differ, the integration constant for the 
integrating amplifier must be altered so they differ during these periods. 
In the illustrated example, where the starting period is shorter than the 
stopping period, resistor 52 and capacitor 56 form a time constant for the 
stopping period. A shorter starting time constant is established by 
connecting resistor 52' in parallel with resistor 52 to lower the 
resistance in the signal path at the inverting input of amplifier 54. If 
terminal 57 were connected directly to terminal 60, differential amplifier 
62 would supply terminal 46 with the required control signal C for motor 
32, assuming that the control signal voltage after starting is determined 
by the voltage supplied by feed forward resistor 48 to terminal 46 as seen 
above. 
Compensating circuit 40, FIG. 3, is formed by adding resistor 58 between 
terminals 57 and 60. The voltage between terminals 57 and 60 is 
proportional to the current flowing through motor 32 because: (1) the 
motor internal voltage drop is proportional to the current flowing through 
the motor, (2) the motor current is inversely proportional to the required 
starting and stopping times and thus to the integration constants of 
differential amplifier 54, and (3) the current in resistor 52 (which 
determines the stopping time-constant) or in resistors 52 and 52' (which 
determine the starting time-constant) is the same as that flowing through 
resistor 58. The internal drop is simulated by adjusting the value of 
resistor 58 to ensure that the voltage between terminals 57 and 60 equals 
the motor internal drop IR. The voltage between terminals 57 and 60, 
representing the motor internal drop, is added to the integrated voltage, 
at output terminal 60 of amplifier 54, which represents the slope of 
signal C during the starting and stopping periods, so that the voltage at 
terminal 64 is illustrated by waveform D, FIG. 6. 
FIG. 5 is a circuit diagram of an actual circuit based on the general 
circuit diagram shown in FIG. 4. Thus, components which are the same as 
those in FIG. 4 are shown in FIG. 5 with the same references. The two 
chief differences are (1) a decoupling capacitor 56' in parallel with 
resistor 58 to simulate the motor internal drop, and (2) a switching 
network 66 to connect or disconnect resistor 52' depending upon whether 
the period corresponds to the starting or stopping of the motor. 
Switching network 66 is relatively simple and responds to the output signal 
from terminal 46, as applied to a non-inverting input of differential 
amplifier 68, having an inverting input (-) connected to ground via 
resistor 70 and to negative feedback resistor 72, connected to the 
amplifier output. The output of amplifier 68 is connected to a pair of 
biasing circuits respectively provided for opposite conductivity type 
field-effect transistors 74 and 76. The biasing circuits respectively 
comprise diodes 78,80 respectively connected in series with grounded 
resistors 82,84. Transistors 74 and 76 include gates respectively 
connected to the common terminals for diodes 78,80 and resistors 82,84. 
Source electrodes of transistors 74 and 76 are connected in parallel to 
the inverting input (-) of differential amplifier 54, while drain 
electrodes of these transistors are connected to one terminal of resistor 
52' via diodes 86 and 88 which are biased to conduct current in the same 
direction as transistors associated with them. 
The circuit illustrated in FIG. 5 includes input terminals 90 and 92 
respectively responsive to input signals (such as signals A and B, FIG. 6) 
commanding the motor to run forward and in reverse. The signals at 
terminals 90 and 92 are processed by an input circuit 94 containing like 
conductivity type field effect transistors 96 and 98, having gate 
electrodes respectively connected to input terminals 90 and 92. Resistors 
100 and 102 are respectively connected between drain electrodes of 
transistors 90 and 92 and B+ and B- supply voltage source terminals, while 
source electrodes of these transistors have a common terminal 104, in turn 
connected to resistor 48. Oppositely polarized diodes 106 and 108 shunt 
terminal 104 to ground. Terminal 104 is also connected to a non-inverting 
input of a differential amplifier 110, having an output connected to 
resistor 112. Amplifier 110 includes a negative feedback resistor 114 
between its output and inverting input, also connected to ground by 
resistor 116. Oppositely polarized diodes 118 and 120 respectively shunt 
the drains of transistors 96 and 98 to ground, while oppositely polarized, 
series diodes 122 and 124 limit the maximum voltage applied by sources B+ 
and B- to resistors 52 and 52'. 
In response to positive and zero voltages being respectively applied to 
terminals 92 and 90, whereby transistors 98 and 96 are respectively 
conducting and cut off to order motor 32 to run forward, the voltage at 
terminal 64 changes as a function of the values of supply voltage B- and 
resistors 102 and 48. With transistor 98 conducting, a negative DC voltage 
is coupled from source B- to terminal 104; the voltage at terminal 104 is 
limited by the junction voltage of diode 106. In response to the negative 
voltage at terminal 104, amplifier 68 develops a negative voltage that is 
coupled through diode 80 to forward bias transistor 76, whereby resistor 
52' is connected in parallel with resistor 52 during starting interval 
t1-t'1 so the time constant of integrating amplifier 54 is relatively low. 
Amplifier 54 responds to the output of amplifier 110, which, under the 
stated conditions, is a negative voltage proportional to the voltage at 
terminal 104. The negative voltage at terminal 104 is converted by 
amplifier 54 into a voltage having three superimposed components. One of 
the components is a positive going linear ramp having a slope inversely 
proportional to the combination of parallel resistors 52 and 52' 
multiplied by capacitor 56. The second component is a constant positive DC 
level having a value determined by the value of resistor 58 divided by the 
parallel value of resistors 52 and 52'. The third component is a negative 
going exponential ramp having a time constant determined by the product of 
the values of resistor 58 and capacitor 56. By selecting the value of 
capacitor 56' to be sufficiently large, the third component can be 
effectively ignored so the signal at terminal 64 is a ramp superimposed on 
a constant value during interval t1-t'1 as indicated by waveform D. 
Because resistor 58 and capacitor 56 are connected between terminals 57 and 
60, the voltage coupled from terminal 57 to amplifier 62 is essentially a 
positive going ramp. The positive going ramp 62 is combined by amplifiers 
62 and 68 with the negative voltage fed through resistor 48. In response 
to the combined voltage at the output of amplifier 68 reaching a 
predetermined level at time t'1, diode 80 becomes back biased, causing 
transistor 76 to open circuit, so resistor 52' is no longer effectively 
connected in parallel with resistor 52 to the input of amplifier 54, to 
increase the input impedance to amplifier 54. Simultaneously with 
transistor 76 being back biased, a threshold sensing circuit (not shown) 
responds to the output of amplifier 68 to remove the forward bias on 
transistor 98, so zero input voltage is applied to integrating amplifier 
54. The output of amplifier 54 therefore remains constant at the same 
level it has at instant t'1. 
At time t2 generator 36 applies a forward bias to terminal 90 so transistor 
96 is forward biased and a positive voltage is applied to terminal 104. 
The positive voltage at terminal 104 causes a negative going step change 
and a negative going ramp at terminal 64. The ram is coupled to terminal 
57 and has a slope determined by resistor 52 and capacitor 56. The 
positive voltage supplied from terminal 104 to resistor 48 is subtracted 
in amplifiers 62 and 68 from the voltage at terminal 57, causing both 
diodes 78 and 80 to be back biased so field effect transistors 74 and 76 
are cut off and resistor 52' is not effectively connected in parallel with 
resistor 52. When the negative going ramp reaches a predetermined value at 
t'2, the combined voltage at the output of amplifier 68 again actuates the 
threshold sensing circuit to remove the forward bias applied to transistor 
96. 
For reverse running, the operation is similar to that stated for the 
forward running condition, but transistor 96 is initially forward biased 
and a positive voltage is applied by amplifier 68 to forward bias diode 78 
and transistor 74 during the interval t3-t'3, whereby resistors 52 and 52' 
are in parallel. During the interval t4-t'4, neither of transistors 74 nor 
76 is forward biased, whereby only resistor 52 is effectively in circuit. 
During the interval t'4-t"4, transistor 76 is again forward biased to 
reduce the integration time constant. 
While there have been described and illustrated one specific embodiment of 
the invention, it will be clear that variations in the details of the 
embodiment specifically illustrated and described may be made without 
departing from the true spirit and scope of the invention as defined in 
the appended claims.