Torque controller system having a torque processor with improved tractive effort distribution

A torque controller system provides improved torque distribution as a function of wheel diameter size in a vehicle propelled by electric traction motors. The system includes one or more torque processors each having a multiplier coupled to receive an input signal and configured to produce an output signal which is the product of the input signal and a variable multiplier value generated by a respective torque distribution generator based on a predetermined torque distribution control function.

BACKGROUND OF THE INVENTION 
The present invention is related to torque control of a vehicle, such as a 
locomotive or a transit vehicle, propelled by traction motors, and, more 
particularly, to a torque controller system for providing improved torque 
distribution in that vehicle. 
Locomotives and transit vehicles as well as other large traction vehicles 
are commonly powered by electric traction motors which are coupled to 
drive one or more axles of the vehicle. Locomotives and transit vehicles 
generally have at least four axle wheel sets per vehicle with each 
axle-wheel set being connected via suitable gearing to the shaft of an 
electric motor commonly referred as a traction motor. In a motoring mode 
of operation, the traction motors are supplied with electric current from 
a controllable source of electric power, such as an inverter. The traction 
motors apply torque to the axles which, in turn, apply torque to the 
wheels of the vehicle. The wheels exert tangential force or tractive 
effort on the surface on which the vehicle is traveling (e.g., the 
substantially parallel steel rails of a railroad track). Alternatively, in 
an electrical braking mode of operation, the motors operate as axle-driven 
electrical generators; that is, torque is applied to the motor shafts by 
their respectively coupled axle-wheel sets which then exert braking effort 
on the surface, thereby retarding or slowing the motion of the vehicle. 
For efficient operation, either in the motoring or in the braking mode of 
operation, the vehicle is required to provide a substantial level of 
adhesion between its wheels and the surface on which the vehicle is 
traveling. In view of that requirement, the vehicle is generally required 
to achieve the maximum reachable adhesion on every axle-wheel set while, 
due to cost considerations, the respective power ratings of the 
controllable power source, the traction motor, wiring, and other equipment 
coupled to drive each axle-wheel should be as low as feasible for a given 
application. Due to various factors, such as wear and tear, or improper 
maintenance, the size of the diameter of the vehicle wheels may change 
relative to one another. Such wheel diameter differences can produce 
unequal vertical force or weight on each axle. The unequal vertical force 
could also be due to dimensional variations on the platform or the trucks 
where respective ones of the axle-wheel sets are mounted. If, for example, 
the vertical force on a given axle-wheel increases due to one or more of 
the above-listed factors, then the available tractive effort on that 
axle-wheel would increase and this situation would require a higher rated 
power equipment to make use of the increased tractive effort. It will be 
appreciated that when the vertical force increases on a given axle-wheel, 
there is a corresponding vertical force reduction to the other axle-wheels 
since the total vertical force in the vehicle remains constant. The 
reduced vertical force in turn produces a reduction in available tractive 
effort on the other axle-wheels and thus the rating of the power equipment 
coupled to drive these other axle-wheels would be less relative to the 
power equipment coupled to the axle-wheel with increased vertical force. 
Thus, it is desirable to operate each axle-wheel set such that each 
requires substantially the same power rating relative to one another, and 
it is desirable that each axle-wheel produce substantially the same level 
of tractive effort under worst case operating conditions, that is, when 
maximum tractive effort is truly required. 
Presently available torque controllers are generally configured so that the 
torque supplied to each axle-wheel set under normal steady state operating 
conditions is substantially the same relative to one another, regardless 
of the vertical force any given axle-wheel set actually receives. This 
substantially even torque distribution would make tractive effort on a 
smaller diameter wheel greater than on a larger diameter wheel and would 
cause the smaller wheel to wear out faster. Thus, it would be advantageous 
to have a torque controller system which would allow for distributing the 
tractive effort such that it would be greater on axles with larger 
diameter wheels than on axles with smaller diameter wheels, at least under 
normal operation, that is, not during operational conditions which require 
worst-case tractive efforts. Any shift or distribution of tractive effort 
to the larger axle-wheel would make a corresponding reduction in tractive 
effort to the smaller axle-wheels; consequently, there would be a 
reduction of the wear rate of those axle-wheels compared to the wear rate 
of the larger wheel. Tractive effort distribution which takes into account 
the actual tractive effort requirements of each axle-wheel set in the 
vehicle would advantageously result in eventually all of the wheels having 
a substantially similar diameter with respect to one another since during 
most operations the locomotive will be running at high speeds. 
BRIEF SUMMARY OF THE INVENTION 
A torque controller system provides improved torque distribution in a 
vehicle propelled by electric traction motors. The torque controller 
system includes a subtractor for receiving a total torque command signal 
as a minuend input signal and for receiving a total torque feedback signal 
as a subtrahend input signal. The subtractor is configured to produce a 
total torque difference output signal. A torque regulator is coupled to 
receive the total torque difference output signal to generate a total 
torque regulated output signal. A divider is coupled to receive the total 
torque regulated signal and configured to divide the regulated signal into 
a plurality of split torque command output signals corresponding to the 
number of respective loads, (e.g., axle-wheel sets) individually 
controlled by the torque controller. Each of a plurality of multipliers is 
coupled to receive a respective one of the split torque command output 
signals and is configured to produce a respective split torque command 
product output signal. Each of the multipliers has a torque distribution 
generator coupled thereto for generating a variable multiplier value based 
on a predetermined torque distribution control function. Each of a 
plurality of subtractors is coupled to receive a respective one of the 
split torque product output signals as a minuend input signal and for 
receiving a split torque feedback signal corresponding to a respective one 
of the loads being controlled by the torque controller. Each of the 
subtractors is configured to produce a split torque difference output 
signal. Each of a plurality of regulators is coupled to receive a 
respective one of the split torque difference signals and to generate a 
split torque regulated signal; and a summer is configured for receiving 
the split torque feedback signals from each of the loads to generate the 
total torque feedback signal. Each combination of a respective one of the 
multipliers and a respective one of the torque distribution generators 
comprises a torque processor which conveniently provides individual 
processing to a respective one of the split torque command signals based 
on wheel diameter variation in the wheels of the vehicle.

DETAILED DESCRIPTION OF THE INVENTION 
A torque controller system according to preferred embodiments of the 
present invention may be utilized in various types of alternating current 
(AC) induction motor powered vehicles such as, for example, transit cars 
and locomotives. For purpose of illustration, the invention is described 
herein as it may be applied to a locomotive. For example, a propulsion 
system 10 of FIG. 1 includes a variable speed prime mover 11 mechanically 
coupled to a rotor of a dynamo electric machine 12 comprising a 3-phase 
alternating current (AC) synchronous generator or alternator. The 3-phase 
voltages developed by alternator 12 are applied to AC input terminals of a 
conventional power rectifier bridge 13. The direct current (DC) output of 
bridge 13 is coupled via DC link 14 to a pair of controlled inverters 15A 
and 15B which invert the DC power to AC power at a selectable variable 
frequency. The AC power is electrically coupled in energizing relationship 
to each of a plurality of adjustable speed AC traction motors M1 through 
M4. Prime mover 11, alternator 12, rectifier bridge 13 and inverters 15A, 
15B are mounted on a platform of the traction vehicle 10, such as a 
four-axle diesel-electric locomotive. The platform is in turn supported on 
two trucks 20 and 30, the first truck 20 having two axle-wheel sets 21 and 
22 and the second truck 30 having two axle-wheel sets 31 and 32. 
Each of the traction motors M1-M4 is hung on a separate axle and its rotor 
is mechanically coupled, via conventional gearing, in driving relationship 
to the associated axle-wheel set. In the illustrative embodiment, the two 
motors M1 and M2 are electrically coupled in parallel with one another and 
receive power from inverter 15A while motors M3 and M4 are coupled to 
inverter 15B. However, in some instances, it may be desirable to provide 
an inverter for each motor or to couple additional motors to a single 
inverter. Suitable current transducers 27 and voltage transducers 29 are 
used to provide a family of current and voltage feedback signals 
respectively representative of the magnitudes of current and voltage in 
the motor stators. Speed sensors 28 are used to provide speed signals 
representative of the rotational speeds W1-W4 in revolutions per minute 
(RPM) of the motor shafts. These speed signals are readily converted to 
wheel speed in a well known manner. For simplicity, only single lines have 
been indicated for power flow although it will be apparent that the motors 
M1-M4 are typically three phase motors so that each power line represents 
three lines in such applications. 
The magnitude of output voltage and current supplied to rectifier bridge 13 
is determined by the magnitude of excitation current supplied to the field 
windings of the alternator 12. The excitation current is set in response 
to an operator demand (throttle 36) for vehicle speed by the controller 26 
which is in turn responsive to actual speed as represented by signals 
W1-W4. The controller 26 converts the speed command to a corresponding 
torque command for use in controlling the motors M1-M4. Since AC motor 
torque is proportional to rotor current and air gap flux, these quantities 
may be monitored; or, alternatively, other quantities such as applied 
voltage, stator current and motor RPM may be used to reconstruct motor 
torque in controller 26. A more detailed analysis of such techniques is 
given in U.S. Pat. No. 4,243,927 and in a paper published in IEEE 
Transactions on Industry Applications, Vol. IA-13, No. 1, January 1977, 
entitled "Inverter-Induction Motor Drive For Transit Cars," by Plunkett 
and Plette. 
FIG. 2 shows an exemplary prior art torque controller system 100 configured 
to command substantially the same level of torque to each of the vehicle 
axle-wheel sets, independently of the vertical force which is actually 
received by any of such axle-wheels. As shown in FIG. 2, a full torque 
command signal (TT*) such as may be generated in controller 26 (FIG. 1), 
is received by a subtractor 102 as a minuend input signal, and a total 
torque feedback signal is received by subtractor 102 as a subtrahend input 
signal so that subtractor 102 generates a total torque difference output 
signal. A torque regulator 104, using conventional control techniques 
well-known to those skilled in the art, such as proportional plus integral 
(P-I) control techniques, is coupled to receive the total torque 
difference signal to produce a total torque regulated output signal. A 
signal divider 106 splits or divides the total torque regulated signal 
into a plurality of n split torque command output signals which correspond 
to the number of loads, i.e., axle-wheel sets, which are individually 
controlled by the torque controller. It will be appreciated by those 
skilled in the art, that each respective load may include either a single 
axle-wheel set or multiple axle-wheel sets, such as the multiple 
axle-wheel sets which may be mounted in a single truck. For example, if 
the total torque regulated signal corresponds to a torque value of 12,000 
ft-lb and the number of axle-wheel sets is six, then each split torque 
command signal would have a torque value which corresponds to about 2,000 
ft-lb. It will be further appreciated that if one of the axle-wheel sets 
is disabled due to any reason, e.g., thermal overload, regulator 104 would 
continue to keep the overall tractive effort substantially constant. 
For the sake of simplicity of illustration and explanation, FIG. 2 shows 
only the circuitry associated with the ith split torque command signal 
wherein i is any positive integer from 1 to n. It will be appreciated, 
however, that in general there will be a plurality n of the circuits 
having reference numerals with the i subscript. A subtractor 110.sub.i is 
coupled to receive a respective one of the split torque command signals 
(e.g., T.sub.i *) as a minuend input signal, and a split torque feedback 
signal (T.sub.i) as a subtrahend input signal. As explained above, the 
torque feedback signal may be computed using well-known techniques using 
applied voltage, motor RPM, and rotor current in a feedback torque 
calculator 118.sub.i. The split torque difference signal from subtractor 
110.sub.i is received by a torque regulator 112.sub.i which, as discussed 
above, may use well-known P-I control techniques so as to produce a 
regulated split torque signal. A power converter 114.sub.i, such as either 
of inverters 15A or 15B (FIG. 1), receives the regulated split torque 
signal to generate a suitable set of power signals for driving a motor 
116.sub.i for driving the ith axle-wheel set (not shown). A summer 120 is 
coupled to receive each split torque feedback signal so to generate the 
total torque feedback signal which is applied to subtractor 102. It will 
be appreciated that, controller 100 does not have the capability to 
distribute the split torque command signals in a manner that compensates 
for axle-wheel sets having different tractive efforts needs relative to 
one another such as may be the case if one or more of the wheels in a 
given axle has a diameter which deviates from the average diameter of the 
vehicle's wheels. 
FIG. 3 shows a block diagram of a torque controller system 200 configured 
in accordance with preferred embodiments of the present invention that 
allows for improved torque distribution in the vehicle which is 
advantageously implemented relatively easily in existing torque 
controllers. Although the block diagram of FIG. 3 shows hardware 
components, it will be further appreciated that the invention is 
preferably implemented in a computer, such as the microprocessor-based 
controller 26 of FIG. 1. For the sake of simplicity, circuitry which may 
be used in either of torque controllers 100 or 200 is identified with the 
same reference numeral and the operational description of such circuits 
will not be repeated. A plurality of processors such as processor 
201.sub.i made up of a multiplier 202.sub.i and a torque distribution 
generator 204.sub.i allows for individually processing the split torque 
command signals based on wheel diameter size. As shown in FIG. 3, prior to 
subtractor 110.sub.i, the multiplier 202.sub.i is coupled to receive a 
respective one of the split torque command signals (e.g., T.sub.i *) to 
produce a respective split torque command product output signal. The 
multiplier 202.sub.i is coupled to the torque distribution generator 
204.sub.i which generates a respective variable multiplier value based on 
a predetermined torque distribution function which compensates for wheel 
diameter size variation. By way of comparison, in controller 100, although 
no multiplier is explicitly shown, it will be appreciated that in essence 
each split torque command signal is equally weighed as if it were 
multiplied by a unity factor. On the other hand, as best seen in FIGS. 4 
and 5, in controller 200, each respective split torque command signal 
(e.g., T.sub.i *) can be multiplied by a respective multiplier value which 
varies as a function of wheel diameter size. This would allow for 
matching, based on wheel diameter variation, the actual tractive effort 
needs of the axle-wheel set coupled to motor 116.sub.i, for example. 
Patent application Ser. No. 09/118,875 (docket 20-LC-1901), titled "Wheel 
Diameter Calibration System For Vehicle Slip/Slide Control", filed on Jul. 
20, 1998 and commonly assigned to the same assignee of the present 
invention, describes a technique and apparatus which may be conveniently 
used to measure wheel diameter size while the locomotive is in motion. 
However, it should be understood that any other technique which allows for 
measuring wheel diameter size in operation could be readily employed in 
the implementation of the present invention. 
FIGS. 4 and 5 show one exemplary torque distribution function which may be 
used by a respective torque distribution function generator, such as 
generator 204.sub.i, to generate the variable multiplier value used by the 
multiplier coupled to generator 204.sub.i, in this case multiplier 
202.sub.i. More specifically, FIG. 4 shows a torque distribution function 
which may be used in the case where the wheel diameter size (Wi.sub.d) of 
the ith wheel is larger than the average wheel diameter size in the 
vehicle. For vehicle speeds equal to or below a predetermined vehicle 
speed (S1), the value of the multiplier factor may have a value 
substantially equal to unity, i.e., one. For vehicle speeds above the S1 
speed, the multiplier value may vary as a function of vehicle speed up to 
a maximum multiplier value. The multiplier value variation up to that 
maximum value may be substantially linear. Once the maximum value is 
reached, regardless of increased vehicle speed, the multiplier value would 
remain substantially constant. In this case, the rate of change of the 
multiplier value or slope in the linear range of operation can be computed 
from the following equation: 
##EQU1## 
wherein m is the slope in the linear range of operation, K is a suitable 
scale factor, W.sub.Di is the ith wheel diameter and WA is the average 
wheel diameter size in the vehicle. 
FIG. 5 shows the torque distribution function which may be used in the case 
where the wheel diameter (W.sub.Di) of the ith wheel is smaller than the 
average wheel diameter size in the vehicle. As before, for vehicle speeds 
up to or below the predetermined wheel speed (S1), the multiplier may have 
a value of one. However, for vehicle speeds above the S1 vehicle speed, 
the multiplier may vary as a function of vehicle speed down to a minimum 
multiplier value. The multiplier value variation down to its minimum value 
may be substantially linear. Once the minimum value is reached, regardless 
of increased vehicle speed, the multiplier value would remain 
substantially constant at the minimum multiplier value. 
FIGS. 6 and 7 illustrate another example of the torque distribution 
function which may be used by the torque distribution function generator. 
As shown in FIG. 6, when the ith wheel has diameter size which is larger 
than the average wheel diameter size, then from a first predetermined 
vehicle speed (S1) to a second predetermined vehicle speed (S2), the 
multiplier value can be chosen to vary substantially linearly so that the 
value increases with increasing vehicle speed up to the second vehicle 
speed value. For vehicle speeds above the second vehicle speed, the 
multiplier value remains substantially constant at a value which is higher 
than unity. For vehicle speeds below the first speed value, the multiplier 
value is substantially equal to one. As shown in FIG. 7, when the ith 
wheel has a diameter size which is smaller than the average wheel diameter 
size, then from the first predetermined vehicle speed to the second 
vehicle speed, the multiplier value can be chosen to vary substantially 
linearly so that the value decreases with increasing vehicle speed up to 
the second vehicle speed value. For vehicle speeds beyond the second 
vehicle speed, the multiplier value remains substantially constant at a 
value which is below one. Once again, for vehicle speeds below the first 
speed value, the multiplier value is substantially equal to one. 
It will be understood that the specific embodiments of the invention shown 
and described herein are exemplary only. Numerous variations, changes, 
substitutions and equivalents will occur to those skilled in the art 
without departing from the spirit and scope of the present invention. 
Accordingly, it is intended that all subject matter described herein and 
shown in the accompanying drawings be regarded as illustrative only and 
not in a limiting sense and that the scope of the invention be solely 
determined by the appended claims.