Regulation system for decoupled efficiency optimized operation of DC traction motors

A regulation system for controlling a dc motor having a separately excited armature and field includes microprocessor based pulse width (duty cycle) and frequency modulation control of an armature amplifier and a field amplifier. Motor operating efficiency is optimized through microprocessor control of the field amplifier as a function of actual armature current, actual field current and desired armature speed. Sensed armature current is multiplied by a fixed optimal field constant to generate an optimal field current control signal. The optimal field current control signal is summed with a signal representative of field current required for desired armature speed less a signal representative of actual field current to generate a field current error signal. A controller receives the field current error signal and generates a pulse width modulated field current control effort signal which is applied to the field amplifier. An armature voltage controller receives an input signal which is the difference between desired armature speed and actual armature speed. The input signal is generated only when actual field current exceeds a predetermined field saturation current value. The controller generates a pulse width modulated armature control effort signal which is applied to the armature amplifier only if actual armature current is below the maximum allowable armature current.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates generally to control systems for dc traction motors 
and more particularly to systems for separately driving the armature and 
field circuits of a dc traction motor while optimizing energy efficiency. 
2. Related History 
DC electric motors have functioned as a motive source in various vehicles 
ranging from trains, to boats, automobiles, delivery vans, golf carts, 
trucks and fork lift vehicles. In many applications including automobiles, 
material handling trucks and fork lifts, the on-board stored energy supply 
comprised batteries which accounted for a significant proportion of the 
total vehicle weight. Depending upon the vehicle application, motor torque 
loads significantly varied and were affected by vehicle load variations, 
the incline of the travel path, (such as a loading ramp) and vehicle 
acceleration. It was common for vehicle pay-load variations to represent a 
change in the order of 33% of the total motor torque requirements. 
In vehicles wherein the motive energy was furnished by an on-board battery 
system, the main motive element, commonly known as a traction system, 
included a series-wound dc motor coupled to one or more drive wheels 
through a reduction gear. Control of the direction of rotation of the 
motor was effected by controlling the polarity across the motor 
field-armature. Series-wound dc motors were limited to operation along 
their characteristic commutation curve and, as a result, variations in 
torque load resulted in variations of motor speed, hence vehicle speed. 
Separately excited motor control systems have been devised for 
independently and variably exciting the armature and field windings and 
thus remove the constraint of operation only along the motor's commutation 
curve limit. It has been found impractical, however, to implement systems 
with separately excited and variable armature and field control in 
conjunction with series-wound dc motors because the field current levels 
of such motors required large and relatively expensive control systems, 
specifically the field control portion of the control systems. 
The substitution of shunt-wound dc motors for reducing the size and cost of 
the control systems as opposed to series-wound dc motors has been 
attempted. Under conventional control, wherein the field excitation of a 
shunt wound dc motor was constantly applied, the shunt-wound dc motor was 
incapable of providing a high starting torque and could not serve as a 
replacement for a series-wound dc motor. With the use of a motor control 
system wherein the armature and field of a shunt-wound dc motor were 
separately excited and variably controlled, shunt-wound dc motors were 
capable of operating with the high starting torque characteristics of a 
series-wound dc motor. 
Systems for controlling a separately excited shunt-wound dc motor though 
microprocessor-based independent pulse width modulated frequency control 
of an armature chopper and a field H-bridge were disclosed in U.S. Pat. 
No. 5,070,283 and U.S. Pat. No. 5,039,924, both of which were issued to 
the present applicant. 
The inventions disclosed in the aforementioned patents generally comprised 
an armature voltage amplifier connected to the shunt-wound dc motor 
armature for varying the applied armature voltage and a field current 
amplifier for controlling the direction of armature rotation and for 
varying the voltage applied to the field winding. Rather than allowing the 
field current to follow armature status, a processor was employed to 
effect complete decoupling of torque and speed characteristics (patent 
5,070,283). The processor received signals representative of a reference 
armature speed value and a reference armature current value and employed 
matrix algebra to transform such signals into filtered input references. A 
controller received signals representative of sensed armature current and 
sensed armature speed and generated conditioned motor output signals, 
utilizing matrix algebra. The difference between one of the filtered input 
reference signals and one of the conditioned motor output signals 
comprised a decoupling armature effort signal applied to the armature 
amplifier and the difference between the other filtered input reference 
and the other conditional motor output comprised a decoupling field effort 
signal applied to the field circuit. Both decoupling effort signals were 
generated for maintaining constant armature speed with varying torque 
loads. 
In patent 5,039,924, motor efficiency was optimized with a processor which 
controlled the field current amplifier responsive to desired armature 
speed, sensed armature speed, sensed armature current and sensed field 
current. The processor functioned with a feedback controller which 
generated an optimal ratio as a function of the sensed armature speed and 
the sensed field current. 
While the systems disclosed in these prior patents functioned to provide 
motor operation capable of producing variable torque while maintaining 
constant speed or functioning to optimize motor efficiency, such systems 
were subject to certain disadvantages in terms of cost and practical 
efficiency. 
The systems disclosed in both of the aforementioned patents functioned 
independently of each other and required separate algorithm sets for 
different conditions and could not be incorporated for simultaneous 
operation in a single system. 
Additionally, the prior optimization system was premised upon the 
unnecessary constraint that armature control must operate totally 
independent of field control and field control must operate totally 
independent of the armature control whereas practical implementation must 
recognize the status of the armature circuit for optimized field control 
and the status of the field circuit for armature control. 
The operational constraints for totally independent control found in 
patents 5,039,924 and 5,070,283 required complex and unnecessary 
programing from a practical system operation standpoint and significant 
manufacturing costs in terms of initial programming expense and hardware 
configuration. 
Further, the applicant realized that by driving only the armature circuit 
to maintain constant speed under varying torque loads and allowing the 
field current to follow the driven armature current, adequate decoupling 
of torque and speed characteristics could be attained which would suffice 
for practical implementations, without the added cost and complexity of 
separately driving the field circuit to achieve decoupling control. 
The applicant herein has appreciated that interrelationships between 
armature and field conditions, such as, mutual inductance, mandate a 
control system wherein the field and armature are not driven totally 
independently of one another. 
Further, the applicant herein recognized the need for a single control 
system capable of simultaneously providing practical levels of 
torque/speed decoupling and simultaneous optimization of motor efficiency. 
SUMMARY OF THE INVENTION 
A regulating system for a dc traction motor provides through armature 
circuit regulation, speed commutation curve characteristics of a motor and 
through field circuit control, simultaneous optimization of motor 
efficiency. The system incorporates microprocessor-based pulse width (duty 
cycle) and frequency modulation control of separately excited field and 
armature circuits of a shunt-wound dc motor. 
Optimization of motor efficiency is achieved through processor control of a 
field current (H-bridge) amplifier and control of an armature voltage 
amplifier. 
A value representative of sensed armature current is divided by an optimal 
ratio comprising the square root of field resistance over armature 
resistance to generate an optimal field current control signal which is 
summed with a signal representative of field current required for the 
desired armature speed less a signal representative of actual field 
current to generate field current error signal. 
The field current error signal is received at a field current controller 
which generates a pulse-width (duty cycle) modulated field current control 
effort signal. The field current demand signal is applied to the drivers 
of an H-bridge circuit together with a predetermined field frequency 
signal for frequency and pulse width modulation control of field current 
for optimized decoupled control of the field circuit. 
An armature voltage controller receives a control signal which is a 
function of desired armature speed and actual armature speed. The control 
signal is generated only when the actual field current exceeds a 
predetermined field saturation current value. The controller generates a 
pulse width (duty cycle) modulated armature control effort signal 
indicative of the control effort required to equate the actual armature 
speed with the desired armature speed only after the field is fully 
saturated. When the armature current level is below the maximum allowable 
armature current, the pulse width modulated control effort signal is 
applied to the driver of a chopper circuit (armature amplifier). 
An armature frequency controller also receives the pulse width modulated 
control effort signal and generates a desired armature frequency signal as 
a function of the pulse width modulated control effort signal. The desired 
armature frequency signal is applied to the base driver of the chopper 
circuit along with the pulse width modulated control effort signal for 
both variable frequency and pulse width, i.e., duty cycle, modulation 
control of the chopper circuit. 
From the foregoing compendium, it will be appreciated that it is an aspect 
of the present invention to provide a regulation system of the general 
character described for simultaneous field optimized and armature 
decoupled torque/speed operation of dc traction motors which is not 
subject to the disadvantages of the background history aforementioned. 
A consideration of the present invention is to provide a regulation system 
of the general character described for both field optimized and armature 
decoupled torque/speed operation of dc traction motors, wherein armature 
and field control are interrelated. 
A feature of the present invention is to provide a regulation system of the 
general character described for decoupled torque/speed operation through 
armature circuit control of dc traction motors which simultaneously 
functions to optimize operating efficiency through field circuit control. 
Another consideration of the present invention is to provide a regulation 
system of the general character described for both field optimized and 
armature decoupled torque/speed operation of dc traction motors which 
implements microprocessor based pulse width modulation and frequency 
control of a chopper and an H-bridge circuit. 
Another feature of the invention is to provide a regulation system of the 
general character described for both field optimized and armature 
decoupled torque/speed operation of dc traction motors wherein the 
detrimental effects of mutual inductance which would be encountered upon 
simultaneous energization of the armature and field circuits are avoided. 
Yet a further aspect of the present invention is to provide a regulation 
system of the general character described which achieves optimized 
efficiency and practical levels of decoupled torque/speed operation in dc 
traction motors with reduced programming requirements. 
To provide a regulation system of the general character described which is 
relatively low in cost and is suitable for economical mass production 
fabrication is yet another feature of the present invention. 
With these ends in view, the invention finds embodiment in certain 
combinations of elements, arrangements of parts and series of steps by 
which the said aspects, features and considerations and certain other 
aspects, features and considerations are hereinafter attained, all as more 
fully described with reference to the accompanying drawings and the scope 
of which is more particularly pointed our and indicated in the appended 
claims.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring now in detail to the drawings, the reference numeral 10 denotes 
generally a regulation system constructed in accordance with and embodying 
the invention for combined field circuit optimized and armature circuit 
decoupled torque/speed operation in a dc traction motor. The regulation 
system 10 controls the operation of a dc traction motor 12 which comprises 
an armature 14 and a field coil 16, depicted in schematized format in FIG. 
1 as separated from one another and joined together by dot and dash lines. 
The armature 14 is separately excited from the field coil 16 and current 
flow from a battery source B+ through the armature 14 is under the control 
of an armature amplifier or chopper circuit 18. Similarly, the motor field 
coil 16 is energized from the battery source B+ through an H-bridge field 
control circuit 20. 
Pursuant to the invention, optimization of efficiency and simultaneous 
decoupling of torque/speed characteristics of the motor 12 is achieved 
through the employment of a microprocessor 22. 
It should be noted, from an observation of FIG. 1, that the armature 14 is 
depicted in an operating state rotating at a speed W.sub.m and generating 
a torque T.sub.g while driving a load 24 having a torque load T.sub.1 with 
a viscous damping torque loss T.sub.d. Associated with the armature 14 is 
an encoder 26 which generates a signal W.sub.e, the frequency of which is 
a function of actual, i.e. instantaneous, armature rotation speed. 
An operator throttle 28 generates a signal V.sub.r which is representative 
of the desired armature rotational speed. The throttle signal V.sub.r is 
received at a profile filter 30 which, in turn, generates a desired 
rotational direction/speed signal W.sub.m. 
The desired rotational direction/speed signal W.sub.m applied to a field 
strength filter 32 which, in turn, generates a reference signal I.sub.f, 
representative of the desired field current necessary to support the 
desired rotational direction/speed. The field current reference signal 
I.sub.f is generated by the field strength filter 32, premised upon the 
phenomena of field weakening and field strengthening. 
Armature response is a function of field air gap flux which affects back. 
EMF and generated torque torque T.sub.g. When the field current is 
reduced, the air gap flux is reduced, which reduces back EMF which 
increases armature current, resulting in increased armature speed W.sub.m. 
The desired field current signal I.sub.f is selected by the field strength 
filter 32 based upon desired rotational direction/speed W.sub.m. At low 
values of W.sub.m it is desirable to strengthen the field and vice versa. 
A typical range of desired field current signals may be in the order of 4 
to 30 amps. The signal I.sub.f is applied to a summer 34 to be summed with 
further signals representative of the instantaneous conditions of motor 
operation. 
A signal 36, indicative of actual field current, I.sub.f is read at a field 
current tap 36 and applied to the summer 34. Additionally applied to the 
summer 34 is a signal I.sub.f, representative of optimal field current 
value, which is a function of actual armature current. 
Actual armature current I.sub.a, is read at an armature current tap 38. A 
line 40 extends from the armature current tap 38 to an optimal field 
constant multiplier 42. At the optimal field constant multiplier 42, the 
actual armature current I.sub.a is divided by an optimal field constant 
ratio K.sub.IaIf. The optimal field constant ratio K.sub.IaIf comprises 
the square root of the field resistance divided by the armature resistance 
as indicated in U.S. Pat. No. 4,247,807 issued Jan. 27, 1981 to Wilson and 
incorporated herein by reference. 
Generation of the optimal field ratio is defined, in Microprocessor-Based 
High Efficiency Drive of a DC Motor, IEEE Volume 1 E-34, No. 4 by Hong and 
Park pp. 433-440 equation 5 (p. 434) however, such equation may be further 
reduced to the square root of field resistance over armature resistance. 
The optimal field constant multiplier 42 generates an optimal field 
current control signal I.sub.f which is applied to the summer 34 together 
with the signal I.sub.f. 
The summer 34 generates an error signal E.sub.f, representative of field 
current error which comprises the sum of the desired field current signal 
I.sub.f and the optimal field current control signal I.sub.f less the 
actual field current I.sub.f. The field current error signal, E.sub.f is 
applied to a field current controller 44. 
The field current controller 44 may comprise, for example, a 
proportional/integral/derivative controller which sums proportional, 
derivative and integral gain portions to generate a pulse width (duty 
cycle) modulated field current control effort signal, PWM.sub.f. 
Alternatively, other controllers such as fuzzy logic controllers may be 
employed. 
The pulse width (duty cycle) modulated field current control effort signal 
PWM.sub.f is applied to base drives of four transistors 46, 48, 50 and 52 
of the H-bridge circuit 20. For frequency modulation control of the 
H-bridge circuit 20, a field frequency constant generator 54 generates a 
field frequency signal f.sub.f which is also applied to the base drives of 
the transistors 46, 48, 50 and 52. The fixed frequency is optimally 
selected at approximately 400.+-.50 hz, which represents a compromise 
between switching loss characteristics and field current ripple. 
To provide decoupled torque/speed armature control, the regulation system 
10 also applies the desired rotational direction/speed reference signal 
W.sub.m to a summer 56 through a conventional switching device 58 which 
may function as a transistor under the control of a saturation controller 
60. 
The saturation controller 60 operates as a comparator which generates a 
signal for closing the switching device 58 only if actual field current 
I.sub.f equals or exceeds a predetermined magnetic saturation field 
current limit I.sub.k. The saturation controller 60 and the switching 
device 58 thus serve to assure that the chopper circuit will not be 
activated unless sufficient field current exists to fully saturate the 
field. As such, the mutual inductance drop in field current which is in 
response to the application of a step-input to the armature will be 
accommodated. 
Once the field is saturated, the switching device 58 closes and the desired 
rotational direction/speed signal W.sub.m applied to the summer 56. 
Additionally applied to the summer 56 is a signal W.sub.m representative 
of actual armature speed. Such signal is derived from the armature encoder 
26 which generates the signal W.sub.e which is indicative of instantaneous 
armature speed. The signal W.sub.e is received at a velocity constant 
multiplier 62 and multiplied by a velocity constant to generate the actual 
armature rotational speed signal W.sub.m. 
The summer 56 generates an output signal E.sub.a representative of armature 
error or demand. The armature error signal E.sub.a is applied directly to 
an armature voltage controller 64 which may comprise a 
proportional/integral/derivative controller or a fuzzy logic controller 
and generates a pulse width (duty cycle) modulated output signal 
PWM.sub.a, representative of armature control effort signal. 
The PWM.sub.a signal is transmitted to a multiplier 66. Also transmitted to 
the multiplier is a maximum armature current constant K.sub.Ia. 
The maximum armature current constant K.sub.Ia has a value of either 0 or 1 
depending upon whether or not actual armature current is greater than a 
predetermined maximum allowable current. The multiplier 66 thus functions 
as a gate. 
In order to generate the maximum armature current constant signal K.sub.Ia, 
a maximum armature current constant generator 68 is provided. The 
generator 68 receives the actual armature current signal I.sub.a appearing 
on the line 40 and generates a low armature current constant signal, i.e. 
0, if the actual armature current is equal to or greater than the maximum 
permitted armature current. In the event the actual armature current is 
less than the maximum permitted armature current, the generator generates 
a high armature current constant signal, i.e. 1. 
In the event the armature current constant signal is high, the multiplier 
66 generates a pulse width (duty cycle) modulated armature control effort 
signal PWM.sub.a, which is the same as the signal PWM.sub.a generated by 
the armature voltage controller 64. 
The pulse width (duty cycle) modulated signal PWM.sub.a is applied to the 
base drive circuitry of the armature amplifier or chopper circuit 18. 
Simultaneously applied to the chopper circuit base drive is an armature 
frequency signal f.sub.a which is generated by an armature frequency 
controller 70. The armature frequency controller 70 receives the pulse 
width modulated armature control effort signal PWM.sub.a and in turn, 
generates the corresponding armature frequency signal f.sub.a. 
A relatively high frequency signal e.g. 5,000 hz, is desired, at low duty 
cycle and relatively low frequency signal f.sub.a, e.g. tending towards 
zero, is desired when the duty cycle is relatively high. Since the 
armature winding represents a sixth order system with little inductance, 
armature frequency selection is primarily based upon current ripple. 
Referring now to FIG. 2 wherein a flow chart of the operation of the 
regulation system in conjunction with control of the H-bridge circuit is 
shown, it will be seen that the microprocessor 22 first reads the throttle 
signal V.sub.r as illustrated in a block 72. Thereafter, the desired 
rotational direction/speed signal W.sub.m which is a function of the 
throttle signal V.sub.r and time is calculated as illustrated in a block 
74. The steps of reading the throttle signal and computing the desired 
rotational direction/speed signal is a continuing one as indicated by a 
return loop in FIG. 2. 
After the desired rotational direction/speed signal W.sub.m is calculated, 
the desired field current signal I.sub.f is computed as indicated at a 
block 76. 
The microprocessor is programmed to then read the actual field current 
I.sub.f as indicated in a step 78, read the actual armature current as 
indicated in a block 80 and then calculate the optimal field current 
control signal I.sub.f by dividing the actual armature current by the 
optimal field constant as illustrated in a block 82. 
The microprocessor is then programmed to calculate the field current error 
signal E.sub.f by adding the optimal field current control signal and the 
desired field current and subtracting the actual field current as 
indicated in a block 84. 
Thereafter, the microprocessor proceeds to calculate the pulse width (duty 
cycle) modulated field current control effort signal PMW.sub.f as 
indicated in a block 86. The microprocessor is programmed to additionally 
read the field frequency signal f.sub.f, generated by the field frequency 
constant generator 54, as indicated in a block 88. 
Finally, the microprocessor 22 applies the pulse width (duty cycle) 
modulated field current control effort signal PMW.sub.f together with the 
field frequency signal f.sub.f to the H-bridge circuit 20, as indicated at 
a step 90. 
Turning now to FIG. 3 wherein a flow chart of the regulation system 
operation for decoupled torque/speed control of the armature 14 is 
illustrated, it should be noted that, initially, the microprocessor 22 
reads the throttle signal V.sub.r as indicated in a block 92. Thereafter, 
the desired rotational direction/speed signal W.sub.m is calculated as a 
function of the throttle signal and time as indicated in a block 94. The 
steps of reading the throttle signal and calculating the desired 
rotational direction/speed signal is continuous as indicated by a return 
loop. 
The program then proceeds with reading the actual field current as 
indicated by a step 96 and then determines whether or not the field 
current level is sufficient to saturate the field as indicated in a step 
denoted 98. If the field current is insufficient, the program exits; if 
the field current is sufficient, hence the field is saturated, the program 
then proceeds to read the armature speed signal W.sub.e, generated by the 
encoder 14, as indicated in a block 100 and then calculates the armature 
rotational speed signal W.sub.m as a function of the encoder signal 
W.sub.e, as indicated in a block denoted 102. 
The armature error or demand signal E.sub.a is calculated at a step 104 by 
subtracting the actual armature speed signal W.sub.m from the rotational 
direction/speed reference signal W.sub.m. 
Thereafter, the armature control effort signal PWM.sub.a is computed as 
indicated in a block 106. 
The program then proceeds to read the actual armature current as indicated 
in a block 108. A determination is thereafter made as to whether or not 
the actual armature current is less than the maximum permissible armature 
current as indicated in a block 110. 
If the actual armature current is equal to or greater than the maximum 
permissible, a zero output armature current constant signal is generated 
as indicated in block 112, while if the actual armature current is less 
than the maximum permissible, a unitary armature current constant output 
signal is generated as indicated in a block 114. 
The microprocessor then proceeds to multiply the armature control effort 
signal PWM.sub.a by the armature constant, either zero or one, and 
generate the pulse width (duty cycle) modulated armature control effort 
signal PWM.sub.a as indicated in a block 116. 
Additionally, the program proceeds to calculate an armature frequency value 
as a function of the signal PWM.sub.a as indicated in a block 118 and 
apply the armature frequency signal f.sub.a and the pulse width (duty 
cycle) modulated armature control effort signal PWM.sub.a to the chopper 
circuit 18 as indicated in a block 120. 
Thus, it will be seen that there is provided a regulation system for 
armature circuit decoupled torque/speed operation with simultaneous field 
circuit optimization of dc traction motors which achieves the various 
aspects, features and considerations of the present invention and which is 
well adapted to meet the conditions of practical usage. 
Since various possible embodiments might be made of the present invention 
without departing from the spirit of the invention, and since various 
changes might be made in the exemplary embodiment shown herein, it is to 
be understood that all matter herein described or shown in the 
accompanying drawings is to be interpreted as illustrative and not in a 
limiting sense.