Motor control apparatus and method

Motor control systems are commonly associated with motors found on electric industrial vehicles, such as lift trucks. Advantageously, the control of such motors should be optimized such that current is timely supplied to the motors in conjunction with selection of the appropriate motor direction. The instant apparatus 10 includes a logic device 24 for receiving speed and direction demand signals from respective speed and direction selection devices 20,22. The logic device 24 responsively produces respective motor speed and direction command signals. A motor control device 26 receives the command signals and directs the electrical current from a power source 18 through the motor 12. A transducer device 38 senses the actual direction of electrical current flow through the motor 12 and produces responsive feedback signals. The logic device 24 produces at least one motor direction interrogation pulse in response to receiving the speed and direction demand signals and prior to delivering the motor command signals. The interrogation pulse directs a small amount of electrical current from the power source 18 through the motor 12, insufficient to actually cause rotation of the motor 12. The logic device 24 then compares the direction demand signals and the direction feedback signals and determines if and when command signals can properly be delivered to the motor control device 26.

DESCRIPTION 
1. Technical Field 
This invention relates generally to an apparatus and method for controlling 
an electric motor associated with a vehicle and, more particularly, to an 
apparatus and method for controllably determining the status of the 
direction contactors associated with an electric motor and optimally 
operating the motor only when the contactors are in the proper logical 
state. 
2. Background Art 
Bidirectional electric motors are associated with various industrial 
equipment in common use today. For example, industrial work vehicles, such 
as lift trucks, commonly have several associated electric motors. In 
particular, electric lift trucks include traction or drive motors having 
bidirectional capability. The direction of operation of a drive motor is 
commonly controlled by the orientation of switching contacts surrounding 
either the armature or field of the motor. In order to control motor 
rotation direction, one or the other set of contacts is closed while a 
related set is opened, causing electrical current to flow in a particular 
direction through the motor winding. 
Such systems suffer from various problems and limitations. One particular 
problem is the time required for the direction contactor to change contact 
states. Since the contactor is magnetically actuated by electrical current 
flowing through an associated coil, a finite period of time is required 
for the contacts to switch from one state to another. In order to allow 
for unavoidable variations in switching time from one contactor to 
another, time delays are commonly designed into the control circuitry to 
deal with the worst case expected switching time. If the time delay is too 
short, arcing will occur between the contact tips, greatly reducing 
contactor life. On the other hand, if the delay period is too long, the 
system becomes less responsive than is optimum. This is especially 
critical in cases where plugging or electrical breaking is desired. In 
these situations, it is important that switching times be minimized in 
order to best utilize the advantages offered by electrical breaking. 
However, in prior systems the delays can never be less than the 
anticipated worst case contactor switching time associated with particular 
contactors in the system. 
In addition to the problems associated with providing for switching delays, 
it is common in high current applications for contact tips to 
inadvertently become stuck or welded together. In such cases, when a 
signal is sent to the contactor coil requiring a change in contact status, 
switching does not occur because of the welded contacts. This is a 
particular problem when the contactor is designed to change direction of a 
traction motor on a work vehicle. For example, if the contactor associated 
with the work vehicle is positioned to drive the vehicle forward, and 
reverse direction is desired, selection of reverse direction followed by 
application of current to the motor windings could result in the vehicle 
moving in the wrong direction in the event of welded contacts. Some 
mechanism is desirable for dealing with this situation. 
One example of an apparatus designed to deal with welding of contact tips 
is found in the United Kingdom Pat. No. GB 2 118 381 B published on Dec. 
4, 1985, and issued to Wolfgang Schuckert. This patent teaches use of 
complex logic circuitry for sensing the direction of motor rotation 
following application of current to the motor, and for determining if the 
sensed direction is appropriate in view of the signals supplied to the 
system indicating the desired direction. However, while this system will 
detect the presence of a welded contact, it is only effective after actual 
motor current is applied to the system. Therefore, the vehicle will begin 
to move in the contactor-selected direction, regardless of the validity of 
the contactor status. In other words, the failed contactor will only be 
sensed after the vehicle is energized for actual operation. No 
optimization of contactor utilization can occur, because any delays 
required for engaging the contactors are still required by the disclosed 
system. In fact, the description is only that of a failed contactor 
sensor, and does not solve the other problems described above. 
The present invention is directed to overcoming one or more of the problems 
as set forth above. 
DISCLOSURE OF THE INVENTION 
In one aspect of the present invention a motor control apparatus for 
controlling a motor associated with a vehicle is provided. The vehicle 
includes an electric motor, an electric power source, a speed selection 
element for controllably producing motor speed demand signals, and a 
direction selection element for controllably producing motor direction 
demand signals. A logic device receives the speed and direction demand 
signals and responsively produces respective motor speed command and motor 
direction command signals. A motor control device receives the speed and 
direction command signals and responsively directs electrical current from 
the power source through the motor. A transducer senses the actual 
direction of electrical current flowing through the motor and produces 
responsive motor direction feedback signals. The logic device produces at 
least one motor direction interrogation pulse in response to receiving the 
speed and direction demand signals. The motor control device receives the 
direction interrogation pulse and responsively directs a predetermined 
amount of current from the power source through the motor. The 
predetermined amount of current is insufficient to cause the motor to 
rotate. The logic device compares the direction demand signals and the 
direction feedback signals and produces the speed command signals only in 
response to the comparison results indicating that the actual motor 
direction corresponds to the desired motor direction. 
In a second aspect of the present invention, a method for controlling a 
motor associated with a vehicle is provided. The vehicle includes an 
electric motor, an electric power source, a speed selection device for 
controllably producing motor speed demand signals, and a direction 
selection device for controllably producing motor direction demand 
signals. The method includes the steps of controllably directing a 
predetermined amount of electrical current from the power source through 
the motor. The predetermined amount of current is insufficient to cause 
the motor to rotate. The actual direction of electrical current flowing 
through the motor is then sensed and respective motor direction feedback 
signals are produced in response to the sensed direction. Speed and 
direction demand signals and the direction feedback signals are received, 
and the direction demand signals and direction feedback signals are 
compared with one another. Electrical current responsive to the speed 
demand signals is directed from the power source through the motor only in 
response to the comparison of signals indicating that the actual motor 
direction corresponds to the desired motor direction. 
The instant invention provides a motor control system which advantageously 
responds directly to the actual status of the direction contactor 
associated with an electric motor. Timing of the application of current to 
the electric motor is precisely controlled in response to the direction 
contactor achieving the desired status. Failure of the contacts to achieve 
the desired status prohibits electrical current from being delivered to 
the motor and, in a preferred embodiment, causes an error signal to be 
generated.

BEST MODE FOR CARRYING OUT THE INVENTION 
Referring first to FIG. 1, an apparatus embodying certain of the principles 
of the present invention is generally indicated by the reference numeral 
10. It should be understood that the following detailed description 
relates to the best presently known embodiment of the apparatus 10. 
However, the apparatus 10 can assume numerous other embodiments, as will 
become apparent to those skilled in the art, without departing from the 
appended claims. 
The apparatus 10 is associated with a vehicle, for example, an industrial 
work vehicle such as a lift truck. The particular vehicle associated with 
the apparatus 10 forms no part of the instant invention and is not shown 
in the drawings. The apparatus 10 includes an electric motor 12 having 
armature and field windings 14,16, an electric power source 18, a speed 
selection device 20, and a direction selection device 22. In the preferred 
embodiment, the electric power source 18 is a storage battery of the type 
generally associated with electric traction vehicles. 
The speed selection device 20 is adapted to controllably produce motor 
speed demand signals, and can be, for example, a potentiometer type analog 
device or a digital encoder. In any event, output signals delivered from 
the speed selection device 20 are suitable for electronic processing and 
for controlling the actual motor speed of the electric motor 12. The 
direction selection device 22 controllably produces motor direction demand 
signals. In the typical embodiment, the direction selection device 22 is a 
simple spring biased, center "off" switch capable of selecting either 
forward or reverse motor direction. Operating in conjunction with one 
another, the direction selection device 22 and the speed selection device 
20 are the operator control elements utilized to operate the motor 12 at 
the desired speed and in the desired direction. 
A logic device 24 is adapted to receive the speed and direction demand 
signals from the speed selection device 20 and direction selection device 
22. Responsively, the logic device 24 produces respective motor speed 
command and motor direction command signals. In a preferred embodiment, 
the logic device 24 is a properly programmed microprocessor. 
A motor control device 26 is adapted to receive the speed and direction 
command signals and responsively controllably direct electrical current 
from the power source 18 to at least one of the motor armature and field 
windings 14,16. In a typical embodiment, the motor control device 26 
includes a solid state chopper circuit 28. In response to receiving the 
speed command signals, the chopper circuit 28 delivers predetermined 
current pulses from the power source 18 to the motor 12. 
The motor control device 26 also includes a direction control circuit 30 
which receives the direction command signals and responsively supplies 
electrical current from the power source 18 to a direction contactor 32. 
The direction contactor 32 includes a coil 34 which magnetically operates 
associated contacts 36a-d. A transducer device 38 is adapted to sense the 
actual direction of electrical current flowing through the motor 12, and 
to produce respective motor direction feedback signals in response to the 
sensed current direction. 
The transducer device 38 includes first and second resistors 40,42 
connected to respective ends of the one of the motor armature and field 
windings. In the embodiment shown in FIG. 1, the first and second 
resistors 40,42 are connected to respective ends of the armature 14 
intermediate the respective pairs of direction contacts 36a-d. The 
opposite ends of the resistors 40,42 are connected to respective buffers 
44,46. Each of the input terminals to the buffers 44,46 is also connected 
to the positive logic supply voltage through a respective diode 48,50. 
Output signals from each of the buffers 44,46 are delivered to the logic 
device 24. 
Industrial Applicability 
Operation of the apparatus 10 is best described in relation to its use on a 
vehicle, for example, an industrial vehicle such as an electric lift 
truck. Assume first that the vehicle is stationary and that power has not 
been applied to either the direction contactor 32 or to the motor -2. In 
the embodiment shown in FIG. 1, one set of direction contacts 36a,b are 
normally closed and the other set of direction contacts 36c,d are normally 
open. Consequently, a current path is established from the power source 18 
through the chopper 28, the field 16, the direction contact 36a, the 
armature 14, and the direction contact 36b, back to the power source 18. 
However, because the chopper 28 is turned "off", no current flows at this 
time. 
Assuming now that the vehicle is to be operated, the logic device 24 first 
produces at least one motor direction interrogation pulse in response to 
receiving speed and direction demand signals from the speed selection 
device 20 and the direction selection device 22. The motor control device 
26 receives the direction interrogation pulse and responsively 
controllably directs a predetermined amount of electrical current from the 
power source 18 through the motor 12. In other words, the chopper 28 is 
turned "on" very briefly by the logic device 24 and allows a small amount 
of current to flow through the motor 12. The predetermined amount of 
current is insufficient to overcome the inertia of the motor 12 and 
associated work vehicle elements. Therefore, the motor 12 does not rotate. 
The transducer device 38 senses the actual direction of current flow 
through the motor 12 and responsively provides motor direction feedback 
signals to the logic device 24. The logic device 24 then compares the 
direction demand signals and the direction feedback signals and produces 
the speed command signals only in response to the comparison of signals 
indicating that the actual motor direction corresponds to the desired 
motor direction. 
Once the logic device 24 senses that the direction contactor 32 has 
attained the demanded control status, vehicle control progresses in 
accordance with commonly known methods of such control. However, the logic 
device 24 continues to repeatedly compare the produced direction demand 
signals with the actual direction feedback signals. The motor speed 
command signals continue to be produced only in response to each 
comparison of the demand and feedback signals indicating that the actual 
motor direction continues to correspond to the desired motor direction 
established by the direction demand signals. Therefore, even during 
on-going operation of the motor 12, failure of the direction contactor 
will immediately cause the motor speed command signals to stop being 
produced. 
The speed command signals are initially produced by the logic device 24 
only after correspondence between the direction demand signals and the 
direction feedback signals occurs. This eliminates any effect from contact 
bounce, which is known to cause contact arcing and pitting. 
At any time during vehicle operation, in response to the signals delivered 
to the logic device 24 indicating a lack of correspondence between the 
direction demand signals and the actual motor direction feedback signals, 
an error signal is produced by the logic device 24. However, this error 
signal is produced only after a predetermined time following production of 
the direction interrogation pulse. Therefore, a failed contactor 32 will 
cause the error signal to be produced. The error signal can be utilized by 
the vehicle control logics to disable the vehicle, to sound an alarm, or 
to take other appropriate vehicle action. 
As noted above, in the preferred embodiment of the instant invention the 
logic device 24 is a properly programmed microprocessor. In FIG. 2, a 
functional flowchart defining the internal programming for such a 
microprocessor is demonstrated. From this flowchart, a programmer of 
ordinary skill can develop a specific set of program instructions that 
performs the steps necessary to implement the instant invention. A 
description of the flowchart follows: 
Assume first that no motor direction has been selected and the vehicle is 
stationary, and that the normal or deenergized status of the direction 
contacts 36a-d is such that a forward motor direction is the default 
orientation. Therefore, the direction selection device 22 has not been 
operated, and the contacts 36a-d of the contactor 32 are in their normal, 
de-energized positions. Beginning at the block 100 labeled START, the 
status of the direction selection device 22 is continuously monitored at 
the block 102. Assuming that the direction selection device 22 has not 
changed states, the program continuously loops back to the block 100. Once 
a desired change in direction is detected at the block 102, the actual 
direction selected is determined at the block 104. Assuming that the 
forward direction has been selected, control passes to the block 106. 
At the block 106 a timer register of the logic device 24 is reset and 
begins counting. Once the timer has begun counting, an interrogation pulse 
is delivered at the block 107. The transducer device 38 is then monitored 
at the block 108 to determine whether forward status of the contactor 32 
(corresponding to forward motor rotation) has been attained. If not, the 
timer is examined at the block 109 to determine whether it has timed out. 
If time remains, control loops back to the block 107 where another 
interrogation pulse is delivered, and the cycle repeats. 
The duration of the timer interval determines when the system will indicate 
that a failure of the contactor 32 has occurred. The timer is, therefore, 
established at some duration longer than the maximum switching time that 
the contactor 32 should ever require. If this maximum time is exceeded, it 
can be assumed with a fair degree of certainty that the contact tips have 
failed or that something in the switching circuitry for the contactor 32 
is defective. In any case, it is undesirable to energize the motor 12 in 
this situation because the actual motor direction may be the opposite of 
that desired. Therefore, an error signal is generated at the block 110. 
However, in the normal course, the contactor 32 will switch in far less 
than the maximum timer duration, and efficiency is optimized. This is in 
opposition to conventional contactor control design in which a 
predetermined delay must be established at the maximum or worst switching 
time anticipated from the contactor 32. Therefore, the instant system 
response is optimized in accordance with actual system dynamics. 
Assuming that forward direction is successfully detected by the transducer 
device 38, the program then progresses to the block 112 in which the motor 
12 is pulsed at the speed commanded in response to the speed demand device 
20. The direction selection is continuously monitored at the block 113 for 
any change and the actual direction of motor rotation is continuously 
monitored at the block 114. Assuming that the direction selection device 
22 and the contactor orientation has not been modified, control loops back 
to the block 112 where motor command pulses are repetitively produced at 
the required rate. 
In response to detecting an unexpected change in the orientation of the 
direction contacts at the block 114, motor pulsing is stopped at the block 
115 and an error signal is generated at the block 110. In response to 
detecting a change in the direction selection device 22 at the block 113, 
motor pulsing is stopped at the block 116 and the direction of the 
direction selection device 22 is then determined at the block 118. If the 
direction selection device 22 has not been switched to the reverse mode, 
control passes back to the start block 100. This will be the case in the 
event that the direction selection device 22 is moved to the neutral 
position and no further motor pulsing is required at that time. 
In the event that the reverse direction has been selected at the block 118, 
control then passes to the right side of the flowchart of FIG. 2, where 
the coil 34 of the contactor 32 is energized at the block 120. As 
described previously with respect to the left side of the flowchart of 
FIG. 2, a timer register of the logic device 24 is reset at the block 124 
and begins counting. The interrogation pulse is delivered at the block 
126, and the transducer device 38 is monitored at the block 128 to 
determine if the reverse status of the contactor 32 has been attained. If 
not, the timer is examined at the block 130 to determine whether it has 
timed out. If it has not done so, control loops back to the block 126 
where another interrogation pulse is delivered and the cycle repeats. If 
the timer has timed out at the block 130, the error signal is generated at 
the block 110. 
Assuming that the contactor 32 attains the reverse status prior to timeout 
of the timer register, control passes to the block 132 in which the motor 
12 is pulsed at the commanded speed. This is exactly the same as the motor 
pulsing found on the left side of the flowchart. Again, at the blocks 134 
and 135 the direction selection device 22 and the contactor 32 orientation 
are continuously monitored for a change, and if no change is sensed the 
program loops to produce the desired motor speed command pulses. 
A change in the orientation of the direction contacts detected at the block 
135 results in control passing to the block 115 where motor pulsing is 
stopped, followed by generation of the error signal at the block 110. Once 
a change in the status of the direction selection device 22 is detected at 
the block 134, program control passes to the block 136 where motor pulsing 
is stopped, and then to the block 138 where the direction selection device 
22 is inspected to determine whether the forward direction has been 
selected. If not, it is assumed that the direction selection device 22 has 
been placed in the neutral position, and the program returns to the start 
block 100. Assuming that the forward direction has again been selected, 
the coil 34 of the contactor 32 is de-energized at the block 140, and 
control then passes to the block 105, where the process of determining 
whether the contacts have switched back to the normally de-energized 
position begins. 
It will be appreciated by those skilled in the art that it is not essential 
to incorporate all of the steps represented in the flowchart of FIG. 2 in 
a given system, nor is it necessary to implement the steps of FIG. 2 in a 
microprocessor as described. However, such implementation is deemed to be 
the best mode of practicing the invention owing to the broad and 
widespread availability of suitable microprocessor circuits, the 
widespread use of such circuits in industrial vehicle logic controls, the 
cost reduction normally obtained by utilizing microprocessors, and the 
flexibility afforded by such programmed devices. 
The described embodiment of the present invention advantageously optimizes 
the application of drive current pulses to the traction motor 12 of an 
industrial vehicle, while avoiding premature application of current to the 
motor 12. In addition, the instant invention is capable of detecting 
failure of the contactor 32 or welding of the contact tips 36a-d. The 
apparatus 10 is capable of determining operability of the direction 
control system prior to application of drive current to the motor 12, and 
continues to monitor the direction selection contactor 32 status 
throughout the operation of the motor 12. 
Other aspects, objects, advantages and uses of this invention can be 
discerned from a study of the drawings, the disclosure and the appended 
claims.