An Apparatus for Interactively Accelerating an Electric Drive Vehicle

Technical Field

This invention relates generally to an apparatus for controlling the operation of a bypass contactor of an electric drive vehicle and more particularly to an apparatus for interactively delaying energization of the bypass contactor in response to the electric drive motor being current limited.

Background Art

In the field of electric drive vehicles, motor speed control circuits are typically of the chopper or pulse control variety. However, during periods of high speed operation it is advantageous to employ a bypass contactor to shunt the chopper circuit and provide full battery voltage to the electric motor. This control scheme is well known in the art and serves the useful purpose of protecting the chopper circuit from overheating and overcurrent conditions while providing maximum acceleration and velocity.

Typically, these controls monitor accelerator pedal position and energize the bypass contactor after the pedal has been held at its maximum position for a preselected duration of time (e.g. 2 seconds) . The delay in operation of the contactor allows the vehicle to accelerate smoothly to a speed relatively near the maximum velocity before directly connecting battery voltage to the motor. Thus, pre¬ venting sparking and degradation of the contactor tips and large inrush currents to the drive motor causing jerky operation.

The acceleration rate of electric drive vehicles is dependant upon a number of conditions, including weight of the vehicle, load, current limitations of the chopper circuit, and electric drive motor characteristics. Accordingly, control schemes which force the bypass contactor to be energized after a set period of time cannot optimize acceleration, reduce contactor tip degradation, or prevent large inrush currents to the drive motor. For example, if the delay period is selected to optimize acceleration of an unloaded vehicle, then the delay period will be too short for a loaded vehicle. The contactor will be energized at a relatively low speed resulting in "jerky" operation, sparking, and a large inrush current. Conversely, a delay period that is selected to optimize acceleration of a loaded vehicle will be too long when the vehicle is operated unloaded. The vehicle will quickly accelerate to near maximum velocity and remain at that speed until the time period elapses and the bypass contactor is energized. While the previously mentioned problems are avoided, vehicle performance is less than optimal and the chopper circuit is susceptible to overheating and overcurrent during this extended operation. The present invention is directed to overcoming one or more of the problems as set forth above.

Disclosure of the Invention According to the present invention, apparatus is provided for use on an electric drive vehicle having an electric motor controllably ener- gizable via a power switching element to selectively bypass the power switching element during a pre- selected mode of operation, the apparatus comprising:

a bypass contactor connected in parallel with the power switching element; means for delivering a signal indicative of a desired duty cycle of the power switching element; means for detecting motor current; means for delivering a current limit signal in response to the detected current being greater than a preselected reference value; means for receiving the desired duty cycle signal, comparing the desired duty cycle signal to a maximum duty cycle signal, and closing the bypass contactor a variably selected duration of time after receiving the desired duty cycle signal having a value equal to the maximum duty cycle signal, the selected duration of time being adjustable in response to receiving the current limit signal.

Brief Description of the Drawings

For a better understanding of the present invention, reference may be had to the accompanying drawings, in which:

Fig. 1 is a block diagram of one embodiment of the present invention;

Fig. 2 is a schematic illustration of an overall motor control system incorporating the present invention; and

Fig. 3 illustrates a flowchart representation of one embodiment of the software of the present invention.

Best Mode For Carrying Out The Invention

Fig. 1 illustrates an apparatus 10 for controlling the supply of power to a motor 12 and functioning to control motor speed and limit motor current. The apparatus 10 includes a power source 32,

and means 14 for generating a plurality of different motor speed command signals on an output line 16, each in response to a respective desired motor speed, a controllable supplying means 18 delivering motor speed control signals to the motor 12 via a line 20, and re- sponsively coupling power to the motor 12. A microprocessor means 22 receives the command signals, generates respective motor speed control signals in response to the command signals, and delivers the control signals to the supplying means 18 via a line 24 to couple and decouple power to the motor 12. A bypass contactor 21 shunts the supplying means 18 and connects the power source 32 directly to the motor 12 during high speed, high power operation. The first means 22 also acts to control the bypass contactor 21. Operation of the bypass contactor 21 normally occurs at a selected time after the operator requests full speed operation via the command signal generator 14. A first transducer means 40 supplies a current signal in response to the magnitude of current flowing through the motor 12 and a second means 26 receives the current signal, generates a blocking signal on a line 28 in response to the current signal exceeding a preselected value, and delivers the blocking signal to the controllable supplying means 18 to selectively decouple power to the motor 12 in response to the control signal. A line 28 bypasses the means 22 and, in addition to the line 24, acts as a control input to the supplying means 18. Therefore, the supplying means 18 is controlled by the second means 26 independently of the first means 22, as will be further described, and continues to operate should the first means 22 malfunction or otherwise fail, for example, by calling for unwanted maximum power and corresponding motor speed. Similarly, operation of

the bypass contactor 21 is altered in response to receiving a signal from the first transducer means 40 via a line 25.

FIG. 2 shows a motor control system 30 which incorporates the apparatus 10 and is shown controlling a pair of motors 12, 12' . The second motor 12 ' is similar to the first motor 12 and items relating thereto operate in similar fashion and are shown with identical numerals followed by a prime mark ( * ) . The duplicated circuitry will not generally be discussed hereafter.

The control system 30 includes the power source 32, such as a vehicle battery, that supplies power to the motors 12 and 12' through a switch 34. The motor 12 includes an armature 36 and a field wind¬ ing 38 that are connected in series with a shunt 40.

The direction of energization of the motors 12, 12' is controlled by the system 30 which includes normally open/closed forward contacts respectively 44, 46, 44', and 46', together with normally closed/open reverse contacts respectively 48 ,50, 48', and 50'. Directional contactor coils 52, 52 ' which control the state of the contacts and an on-off switch 54, such as a transistor which is controlled by the output of the microprocessor means 22 on a line 56, are connected in series with the power source 32. When the switch 54 is closed, the coil 52 is energized to change the state of the contacts from that shown. Deenergizing the coil 52 returns the contacts to the normal state shown.

A directional control switch 58 and associated circuitry, having a forward position F, a neutral position N and a reverse position R, provide signals to microprocessor means 22, which in this example is a programmable microprocessor 74 that

performs a number of control functions. Thus, under software control, the microprocessor 74 responds to S the switch 58 by closing/opening respectively the switches 54, 54' energizing the coil 52, 52' changing the state of the contacts.

The motor speed command signal generating means 14 includes a means 76 for generating digital

10. numbers on the output line 16 representing the command speed data. For example, the numbers are 4-bit digital numbers ranging from 0000 to 1111. Each digital number in the range 0000-1111 is a different preset motor speed command signal for the motors 12

15 and 12*. For example, the number 0000 commands zero speed, the number 1011 commands half speed and the number 1111 commands 90% of full or maximum speed. The 90% pulse train is not immediately delivered, but is delayed to allow smooth acceleration of the

20 vehicle. Under software control, the microprocessor 74 responds to the digital numbers on the line 16 by outputting preselected pulse trains on the lines 24 and 24*. The pulse trains are different in duty factor or proportion of pulse "on-time" to pulse

25 "off-time".

The generating means 76 is coupled to an accelerator pedal 78 via a linkage 80 and includes a transducer and signal conditioning circuit 82 which converts positional information of the accelerator aσ pedal 78 to any one of the digital numbers 0000-1111 on the line 16.

Table 1 shows one example of a suitable relationship between the accelerator pedal 78 position, the corresponding motor speed command

35 signals, and the resulting pulse train duty factors.

TABLE 1

Accelerator Pedal Pulse Train Position (% of Motor Speed Duty Factors Maximum) Command Signals 1 %.

0.00 0000 0.00 6.67 0001 4.50 13.33 0010 9.00 20.00 0011 13.50

10. 26.67 0100 18.00 33.33 0101 23.00 40.00 0110 27.00 46.67 0111 32.00 53.33 1000 37.00

15 60.00 1001 41.00 66.67 1010 46.00 73.33 1011 50.00 80.00 1101 60.00 86.67 1101 70.00

20 93.33 1110 80.00 100.00 1111 90.00

The circuitry includes two-input NOR gates 84, 84' having first inputs connected to lines 24, 24' 5 and outputs coupled via lines 86, 86' to on-off power switching elements 88, 88' which have power transistors 90, 90' connected intermediate the power source 32 and the motors 12, 12*. The gates 84 and 84' pass or do not pass the pulse trains on the 0 respective lines 24 and 24' to the lines 86 and 86' in response to the state (logic 0 or logic 1) of the respective second inputs, as described below.

Assume the gates 84 and 84 ' are enabled to pass the respective pulse trains on the lines 24 and 5 24'. In response to the pulses on the lines 24 and

24' being high or logic 1, the outputs of the gates 84 and 84* on respective lines 86 and 86' are low or logic 0. As can be seen, the means 18 and 18* respond by turning off the transistors 90 and 90' and 5 deenergizing the motors 12 and 12' . In response to the pulses on the lines 24 and 24' being low or logic 0, the outputs of the gates 84 and 84' on respective lines- 86 and 86* are high or logic 1. The means 18 and 18* respond by turning on the transistors 90 and

10. 90* to energize the motors 12 and 12*.

As previously mentioned, the means 26 decouples power from the motors 12 and 12' in order to limit motor current to a maximum value. The means 26 includes a differential amplifier 92 having inputs 94

15 coupled across the shunt 40 to monitor or sense current flow through the armature 36 of the motor 12. The output voltage signal of the amplifier 92 on a line 96 is directly proportional to the armature current. An operational amplifier or comparator 98

20 has one input (+) coupled to the line 96 for receiving the voltage signal. Another input (-) of the comparator 98 receives a reference voltage signal on a line 100 via the output of a potentiometer 102 to provide a maximum alternate current signal. The

25 reference voltage signal on the line 100 is at one of two levels for reasons described below.

A latch 103 has a set input S connected to the output of the comparator 98 via a line 104. A reset input R of the latch 103 is coupled to the line

₃₀ 24 carrying the pulse train output of the microprocessor 74. The latch 103 has an output coupled to a line 106, which is one input to a comparator 110. The other input to the comparator 110 receives a reference voltage signal on a line 112 from the output 35 of a voltage divider 114. The output of the

comparator 110 is coupled to a line 116 which is the other input to the gate 84.

The means 26' has corresponding components for limiting current through the motor 12 ' . The components are indicated by identical numerals followed by a prime mark ( ' ) .

In the operation of the means 26, if the current through the armature 36 does not exceed the maximum allowable current, then the output of the gate 84 on the line 86 follows the waveform of the pulse train on the line 24 and biases the transistor 90 on and off. A low pulse or logic 0 on the line 24 ultimately turns on the transistor 90 and a high pulse or logic 1 on the line 24 ultimately turns off the transistor 90.

If the current through the armature 36 exceeds the preselected maximum value, the output of the gate 84 on the line 86 is continually a logic 0, irrespective of the pulse train on the line 24, and turns off the transistor 90. In response to the current in the motor armature 36 exceeding the maximum value, as selected by the reference voltage signal on the line 100, the transistor 90 is turned off to stop the supply of current to the motor 12 until the current falls below this maximum value. Thus, the output of the comparator 110 on the line 116 is the blocking signal supplied to the gate 84 for controllably modifying the pulse trains delivered by the first means 22. An undesirable thermal condition is another condition that can occur and make it desirable to limit current through the motors 12 and 12'. That is, motor current flow should be limited in the event of overheating the power switching element 88 including the transistor 90 or the switching element 88'

including the transistor 90'. Consequently, a means is provided for protecting the elements 88 or 88' against overheating.

This means includes a thermal switch 122, for supplying a temperature signal in response to the temperature of the supplying means 18 exceeding a preselected value, and means 26 for generating the blocking signal in response to receiving the temper¬ ature signal. In operation, if the power switching elements 88 and 88' have not overheated, the reference voltage signal on the lines 100 and 100' is set to a first selected value for current limit control of current through the armatures 36 and 36', as previously described.

However, should the switch 88 or 88' overheat, a lower reference voltage signal on the lines 100 and 100' results. This lower reference voltage signal lowers the setting for the current limit and results in a lowered allowable maximum current flow through the motors 12 and 12' . The reduced current flow will allow the overheated switch 88 or 88* to cool.

During high speed, high load operation it has been found advantageous to bypass the power transistor 90 via a bypass contactor 21 connected in parallel with the transistor 90. A contactor coil 136, which controls the state of the contacts 140 of the bypass contactor 21, and an on-off switch 138, such as a transistor, which are connected in series with the power source 32. Switch 138 is controlled by the output of the first means 22 on a line 139. When switch 138 is closed, the coil 136 is energized to change the state of the contacts 140 from that shown. When switch 138 is opened, the coil 52 is deenergized

to return the contacts 140 to the normal state shown. Operation of the normally open bypass contactor 140 is effected by the microprocessor 74 under software control. Means 141, 141' detect the state of energization of the power switching element 88, 88* to determine if current limit conditions exist. Such an arrangement acts as a feedback for the control signals delivered on the line 24. Delivery of a logic "low" signal on the line 24 to the microprocessor 74 causes the power transistor to be biased on. Accordingly, by monitoring the voltage on line 148 during the periods when the line 24 has been pulled "low" the microprocessor 74 detects a current limit condition in response to line 148 being low. Operation of the software to alter performance of the bypass contactor 21 is explained in conjunction with Fig 3.

FIG. 3 is a flowchart of a computer program suitable for controlling the apparatus 10. Upon beginning the flowchart of FIG. 3 at the block 200 labeled START, program control progresses through the following sequence and to increase the duty cycle of the power switching element 88 at a preselected rate. In decision block 202, the microprocessor 74 inputs the information that is received on the line 16 relating to the accelerator pedal 78 position or speed demand. This signal is compared to the present pulse rate to determine if the operator is requesting acceleration. If the vehicle is in steady state operation, control passes to block 204 where the pulse rate is maintained at its exisiting value. Thereafter control continues to loop through the blocks 202,204 until the operator attempts to accelerate by depressing the accelerator pedal 78. Once acceleration has been requested, control passes to

block 206 where an acceleration rate timer is decremented on each pass through the control loop. The acceleration rate timer is a counter used to determine the rate at which the pulse rate is incremented. The counter starts at initial reset value determined as a function of the desired pulse train duty factor and is incremented or decremented in various decision blocks. Decision block 208 routes control to decision block 210 only if the acceleration rate timer has not reached zero. If the acceleration rate timer has reached zero, decision block 208 routes control to decision block 216 wherein the pulse rate is increased.

Referring to Table 2, it can be seen that the acceleration rate timer is varied according to the present pulse train duty factor. For example, at 60% - 80% duty factors the acceleration rate timer is set to produce a 200 msec, delay for each factor and at 90% the timer delay is limited to 100 msec. Below 50% the entire time delay is limited to a total of 250 msec. Therefore, the system will change normally from 0% to 100% duty factor in less than 1 second.

However, during current limiting the acceleration rate timer is overridden by a time out timer which is normally significantly longer than the acceleration rate timer. When current limiting is occurring and the timeout timer is not equal to zero, the acceleration rate timer remains effectively unchanged as the program loops through blocks 210, 212, 214, 202, 206 and 208. This provides a time interval during which the pulse rate is not incremented regardless of the value of the acceleration rate timer. The longer period of time allows the vehicle to accelerate further without increasing the pulse rate, thereby delaying ultimate

energization of the bypass contactor 21,21'. Means 211 alters the rate of increase of the power switching element 88 duty cycle in response to receiving the current limit signal. The means 211 includes a means 213 for decreasing the rate of change of the duty cycle in response to receiving the current limit sig¬ nal. In decision block 210, the microprocessor 74 inputs the information that is received on the lines 148,148' relating to the voltage level of the emitters of the power transistors 90,90'. If the voltage level the lines 148,148' is "low" while the output signal on the lines 24,24' is "low", the control routine assumes that current limiting has biased the power transistors 90,90* off. In the event of current limiting, control passes to block 212 where the time out timer is decremented and the acceleration rate timer is incremented. Incrementing the acceleration rate timer offsets decrementing the timer in the block 206. In this manner, the acceleration rate timer is maintained at its maximum value for the entire period of the time out timer. Accordingly, if the vehicle accelerates sufficiently to overcome current limiting during the time out timer period, control reverts back to the acceleration rate timer allowing the vehicle a short period of time to continue accelerating before energizing the bypass contactor 21. For example, a vehicle may become current limited while ascending a ramp, but will revert back to normal operation after reaching level ground.

In decision block 214 the time out timer is compared to zero and, if not equal to zero, control returns to the start. Conversely, if current limiting does not exist, the decision block 210 bypasses the blocks 212,214 and returns control to the start.

Ref rring again to Table 2, the relationship between the time out timer setting and the pulse train duty factor is shown. Below the 50% duty factor the time out timer remains unchanged from the acceleration rate timer. This is generally because current limiting ordinarily does not occur in this range of duty factors. However, at 60% - 80% the time allowed for acceleration increases to 900 msec for each factor and at 90% the time out timer is set to 100 msec. Accordingly, the time allowed prior to energizing the bypass contactor 21 approximately triples to about 3 seconds. The delay periods shown herein are for illustrative purposes only and actual delay periods will vary according to the vehicle and application. Control will continue to pass through the control loop until either the acceleration rate or time out timer is decremented to zero. At that time control passes from the decision blocks 208,214 to block 216 where the pulse rate is incremented to the next higher pulse rate and both the time out and acceleration rate timers are set to their desired values. Each time one of the timers times out, the process will repeat and control will return to the start until the maximum pulse rate has been reached. Means 219 closes the contacts 140 of the bypass contactor 21 in response to the duty cycle of the power switching element 88 reaching a preselected maximum rate. Once the maximum pulse rate has been attained, decision block 218 passes control to block 220 where the bypass contactor 21,21' is closed. At this time the process is complete, but will be repeated each time the operator attempts to accelerate to maximum velocity.

TABLE 2

Pulse Train Acceleration Time Out

Duty factors Rate Timer Timer

(msec) (msec)

0 - 50 250 (total) 250(total)

60.00 200 900

70.00 200 900

80.00 200 900

1.0 90.00 100 100

In block 216, the microprocessor 74 inputs the information relating to the direction of desired motor rotation. A means 217 detects the direction of

15 energization and reduces the rate of increase of the duty cycle in response to the motor 12 being energized for reverse rotation. The direction of vehicle travel can be important in determining the optimum time delay prior to bypass contactor energization. Electric lift

20 trucks typically employ front wheel drive with forward mounted forks and rear wheel steering. Accordingly, during forward operation, a sudden acceleration results in weight transfer to the rear.wheels which only enhances steering. Alternatively, during reverse 5 operation, a sudden acceleration transfers weight to the front drive wheels thereby reducing weight on the steering wheels. It is possible that an overloaded vehicle could have sufficient weight transfer to reduce steering responsiveness. To mitigate this

30 possibility the acceleration rate timer and time out timer are substantially longer during reverse operation.

35

Industrial Applicability

In the overall operation of the vehicle and the vehicle control system, assume, for example, that the directional control switch 58 is in the forward position F and that the accelerator pedal 78 is at a half-speed position. The microprocessor 74 receives a direction control signal on the line 64 and the digital number 1011 on the line 16. Consequently, the microprocessor 74 closes the switch 54 to energize the coil 52 and produces the pulse train on the lines 24 and 24* corresponding to half-speed. As a result, the motors 12 and 12' are energized to move the vehicle in the forward direction at half speed.

Then, for example, should the accelerator pedal 78 be released to a quarter speed position or depressed to a full speed position, the microprocessor 74 will respond to the respective digital numbers on the line 16 to generate the corresponding pulse trains on the lines 24 and 24*, thereby energizing the motors 12 and 12' to drive the vehicle at quarter speed or full speed.

In response to the current through the motors 12 or 12' exceeding a preselected maximum value, the second means 26 and 26* will limit this current, irrespective of the magnitude of current or power that would otherwise be coupled to the motor 12 or 12 ^r due to the pulse train on respective lines 24 and 24*. The maximum value can be a relatively high or low value depending on whether the power switching ^" elements 88 or 88' overheat, as determined by the means 118.

Current limiting can have significant impact on the smooth operation of the vehicle, in that, if current limiting occurs and the control continues

incrementing through the pulse rates until the bypass contactor 21 is closed, the differential between the applied voltage and battery voltage may be large. Accordingly, closing the contacts 140,140' of the bypass contactors 21,21' when the effective pulse rate is at, for example, 60% will produce a marked increase in torque resulting in jerky operation. Conversely, not energizing the bypass contactor in response to a current limiting condition can have undesirable impact on the operation of the vehicle. For example, a vehicle carrying a large load up an inclined ramp may become stalled and closing the contacts 140,140* of the bypass contactors 21,21' is the only means for accelerating the vehicle. Thus, jerky operation may be permissible in certain operating conditions.

During normal operation without current limiting the microprocessor 74 controllably increases the duty cycle of the chopper circuit at a preselected rate. However, when the microprocessor detects current limiting by monitoring the lines 148,148', the rate of increase of the chopper circuit duty cycle is altered. The rate of change is decreased during current limiting to allow the vehicle to further accelerate; however, the contacts 140,140' of the bypass contactors 21,21' will still close, but at a time delayed from that of normal operation to allow the vehicle more time to accelerate.