Apparatus for controlling an electrical vehicle drive system

A torque command proportional to accelerator position is provided at low speeds with a transition to a torque command dependent on accelerator pedal position that varies to provide constant horsepower at high speeds. The transition occurs between constant torque command and torque command providing constant power at decreasingly lower speeds with decreasing accelerator pedal position. The torque command provides the control input to a traction motor for an electric vehicle.

BACKGROUND OF THE INVENTION 
This invention relates to an apparatus for supplying a flux and torque 
command to an electric vehicle drive system having as an input vehicle 
accelerator pedal position. 
One of the many criteria in the design of electric vehicle drive systems is 
that the electric vehicle drive system approximate the driving 
chacteristics of the conventional internal combustion automobile. 
Specifically, the electric vehicle drive system must be capable providing 
smooth vehicle acceleration and deceleration in accordance with driver 
commands supplied to the vehicle drive system through the accelerator 
pedal. 
Another criteria in the design of electric vehicle drive systems is that 
such a drive system be rugged so as not to require frequent maintenance. 
Further, such vehicle drive systems must be efficient to maximize vehicle 
range. 
Configuring the electric vehicle drive system from an inverter-induction 
machine drive allows a rugged and efficient electric vehicle drive system 
to be realized. However, the inverter-induction machine drive, when 
controlled by conventional techniques such as torque regulation and using 
an optimum size inverter, exhibits a dead zone at high speeds because of 
the inability of the inverter-induction machine drive to supply constant 
torque. 
It is an object of the present invention to provide an electric vehicle 
drive system which achieves smooth control of vehicle acceleration and 
deceleration at all speeds. 
It is another object of the present invention to provide a balancing speed 
characteristic as a function of accelerator pedal position for driving at 
a constant speed. 
It is still another object of the present invention to provide an electric 
vehicle drive system, responsive to operator commands as transmitted 
through the vehicle accelerator pedal, which approximates the driving 
characteristics of an internal combustion engine automobile. 
BRIEF SUMMARY OF THE INVENTION 
Briefly, in accordance with one of the embodiments of the present 
invention, an electric vehicle drive system for providing smooth control 
of vehicle acceleration and deceleration in accordance with operator 
commands transmitted through the vehicle accelerator pedal has a torque 
command generator that provides a torque signal proportional to 
accelerator pedal position and a torque signal varying to provide constant 
horsepower. The constant torque signal is selected by the torque command 
generator at low speeds and the torque signal varying to provide constant 
horsepower selected at higher speeds with a smooth transition from 
constant torque to constant horsepower occurring at lower speeds for 
decreasing accelerator pedal position. The torque command is coupled to a 
control system for a traction motor.

DETAILED DESCRIPTION OF THE DRAWING 
Referring now to the drawing, wherein like reference numerals designate 
like elements and more particularly FIG. 1 thereof an operator command A* 
which can be derived from accelerator pedal position in block 2 is coupled 
to a torque command generator 1. The torque command generator develops two 
torque signals which are coupled to a minimum selector 3 within the torque 
command generator to develop a torque command. The torque command 
generator provides an operator command A* to a block 4 to develop a torque 
signal proportional to A* and couples to the torque signal from block 4 to 
one input of the minimum selector. The torque command generator also 
squares the operator command A* in a multiplier 5, multiplys the output of 
multiplier 5 by a constant S.sub.1 /T.sub.o in gain block 7 and divides 
the output of block 7 by a signal representative of vehicle speed S in 
divider 9. The output of divider 9 is connected to the other input of 
minimum selector 3. The minimum selector 3 has an operational amplifier 11 
with the resistor 13 in series of the input of the operational amplifier 
and a feedback resistor 15 between the input and output of the operational 
amplifier. One input of the minimum selector is connected to resistor 13. 
The two resistors 13 and 15 are of equal value providing unity gain. A 
diode 17 is connected between the output of the operational amplifier 11 
and feedback resistor 15 with the anode of the diode connected to the 
operational amplifier output. Similarly, an operational amplifier 19 has a 
resistor 21 in series with the operational amplifier input and a feedback 
resistor 23 connected between the operational amplifiers input and output. 
Resistors 21 and 23 are of equal value providing unity gain. A diode 25 is 
connected between the output of operational amplifier 19 and feedback 
resistor 23 with the anode of diode 25 connected to the operational 
amplifier output. The other input to minimum selector 3 is connected to 
resistor 21. The outputs of the unity gain amplifiers were coupled 
together and through resistor 27 to a negative power supply to provide a 
current sink. The outputs of the unity gain amplifiers are also connected 
to a signal inverter 29 to change the sign of the minimum torque signals 
selected as the torque command. The inversion is necessary due to the 
inverting action of the operational amplifiers in the minimum selector. 
The torque command is connected to a flux command generator 31. The flux 
command generator provides a minimum flux command at all times and a 
maximum flux command at maximum torque command. The torque command from 
the minimum selector 3 and the flux command from flux generator are 
connected to the appropriate inputs of a control system for an A.C. 
machine 63. 
In the embodiment of FIG. 1, a current controlled PWM control is shown. The 
torque command is compared to a derived torque to generate an error signal 
at summer 33. The output of summer 33 passes through a gain block 34 to 
generate an angle command signal. The angle command is compared to a motor 
angle signal sin .theta. and the difference is obtained at summer 35. The 
error from summer 35 passes through gain block 37 and provides a frequency 
command f*. The flux from generator 31 is compared to the derived machine 
flux in summer 39 and the error signal is sent through gain block 41 
resulting in a current amplitude command .vertline.I.vertline.. Waveform 
generator 43 supplies each of three current regulators 45, 47 and 49 with 
one of the three sinusoidal reference signals, each of the reference 
signals being in a three-phase relationship with one another. The 
amplitude and frequency of each of the three sinusoidal signals generated 
by the waveform generator, varying in accordance with the frequency 
command and amplitude command signal input to the waveform generator. The 
current regulators 45, 47 and 49 in addition to having inputs from the 
waveform generator each have an input from current sensors 51, 53 and 55 
which are connected to the output of a power inverter 57. The current 
regulators provide pulse width modulated signals to the inverter 57. Power 
is supplied to the inverter by a D.C. supply through a filter consisting 
of a series inductor 59 and a parallel capacitor 61. The output of the 
inverter which consists of three lines A, B, and C is connected to the 
stator winding of motor 63. Block 65 derives motor angle, torque and flux 
for use as feedback signals from motor voltages and currents. 
Referring to FIG. 2, a torque command generator and a flux generator are 
shown connected to a control system for a D.C. machine 64. The flux 
command generator and torque command generator are configured as 
previously described. The output of the torque command generator passes 
through again block 66 and field current command I.sub.F * in divider 67 
to develop an armature current command I.sub.A *. The output of the flux 
command generator after passing through gain block 76 is used as a field 
current command I.sub.F * to the D.C. machine control system. The armature 
current command is compared actual armature current in summer 69. The 
error is sent to gain block 71 and then to the armature control. The 
armature control is supplied with power from a D.C. source through a 
filter consisting of series inductor 73 and a parallel capacitor 74. The 
output of the armature controller is connected across the armature of the 
D.C. motor 75. The field current command I.sub.F * is compared to motor 
field current and the difference obtained in summer 77 is sent through 
gain block 79 to a field controller 81 which also receives power from the 
D.C. supply through a filter. The output of the field controller is 
conected to the field winding 83 of the motor. 
The operation of FIG. 1 will now be described. The torque command generator 
1 develops a torque command as a function of accelerator pedal position 
for different speeds. Referring to FIG. 3, the relationship between torque 
command and speed for different values of accelerator pedal position is 
shown. If a simple constant torque command was supplied to the motor 
control system at all speeds, the command would be ineffective at higher 
speeds due to the power limited nature of the inverter and the motor. 
Also, a natural balancing point between motor torque output and vehicle 
resistance at high speeds would not be clearly defined because of the 
shallow angle of intersection between the constant torque line and the 
vehicle friction and windage line. By using a constant torque command 
whose value varies proportionally to accelerator position at low speeds 
and a torque command varying to maintain constant power at high speeds 
dead zones at high speeds due to commanding excessive torque are 
eliminated. In addition, a natural balancing speed for a given accelerator 
pedal position is achieved giving a driving control characteristic similar 
to a gasoline engine powered automobile. The vehicle speed at which the 
transition between constant torque and torque varying to maintain constant 
power changes with operator command to obtain a good range of control at 
high speeds. The transition between constant torque command mode and the 
torque command varying to produce constant power mode can occur, for 
example, along a line passing through the origin and a point having a 
maximum speed at which a maximum torque at nominal battery voltage is 
desired. The torque T.sub.o is the maximum torque commanded at maximum 
accelerator pedal position. The point along the transition line where the 
torque command crosses is a function of accelerator pedal position. The 
horsepower resulting from the constant torque magnitude times the speed at 
the transition line crossing is the horsepower maintained as vehicle speed 
is increased beyond the transition line into the constant horsepower mode 
assuming fixed accelerator position. 
The transition of voltage to frequency control in the motor control system 
does not have to correspond to the transition called for by the torque 
command from constant torque to constant power. The same torque command 
generator can be used with different motor controllers having different 
characteristics as long as the torque called for at any given speed by the 
torque command generator does not exceed the motor control system 
capabilities. 
An example of the drive system maximum torque and maximum power capability 
is shown in FIG. 3 as dashed lines. A desirable way of matching the torque 
command generator to the drive system maximum capability is to make the 
point (Si,T.sub.o) correspond to the equivalent intersection of the drive 
system maximum capability lines. 
Assuming the vehicle is standing still and the accelerator pedal is 
depressed to one-half its travel distance, the constant torque signal 
proportional to the accelerator pedal position will be produced in block 4 
of FIG. 1 and connected to one input of the minimum selector 3. The torque 
signal providing constant power at the current speed of the vehicle is 
provided at the other input of the minimum selector 3. At low speeds the 
constant torque signal will be selected since it is the lesser of the two 
torque signals. As the vehicle speed increases the constant power curve 
and the constant torque curve crosses at the transition line. By using the 
minimum selector, discontinuities in torque command are avoided. As the 
vehicle speed increases the constant power torque signal is selected as 
the torque command. If the accelerator pedal is kept at its half way 
position, the windage and friction curve of the vehicle intersect with the 
constant power curve providing a constant balancing speed. 
The flux generator always provides a minimum amount of flux to avoid delay 
in an initial motor response. At maximum torque command, the flux command 
is at a maximum. At light loads the flux commanded is reduced to increase 
motor efficiency. The torque command generator and the flux command 
generator can be used with any A.C. motor control requiring a torque and 
flux command. The current controlled PWM control system shown is explained 
in greater detail in my U.S. Pat. No. 4,320,331 issued Mar. 16, 1982 (Ser. 
No. 80,479) entitled "Transistorized Current Controlled Pulse Width 
Modulated Inverter Machine Drive system", filed Oct. 1, 1979 and assigned 
to the General Electric Company. 
The operation of the torque and flux generator of FIG. 2 is the same as 
that described in FIG. 1. The D.C. motor control used in FIG. 2 needs an 
armature current command and the field current command. Torque is 
proportional to field current times armature current. Therefore, the 
torque command after passing through gain block 66 is divided by the motor 
field current in divider 67 to obtain an armature current command. Field 
current is proportional to motor flux neglecting saturation and therefore 
the flux is used as a field current command after passing through gain 
block 76. 
The torque command generator and the flux generator and the flux generator 
of the instant invention provide command signals in response to 
accelerator pedal position that achieve smooth accelerator control for an 
A.C. or D.C. electric vehicle, without a dead zone at high speeds. In 
addition, the present invention provides a balancing speed characteristic 
as a function of accelerator position for driving at constant speeds. 
It is understood that the foregoing detailed description is given merely by 
way of illustration and many modifications can be made without departing 
from the spirit or scope of the present invention.