Failure protection circuit for a two-motor lift truck

An electrically operated dual motor drive system having first and second reversible traction motors (14, 16) driven by a power source (18). Motor control circuitry (10, 12) for controlling the direction of current flow through the motors (14, 16) is designed to be actuated by only two coils (36, 42). Preferably, control circuits (10, 12) employ an arrangement of normally open (32a, 32b, 38a, 38b) and normally closed (34a, 34b, 40a, 40b) contacts such that the two motors (14, 16) operate in reverse directions in the event of a complete system malfunction. A line contactor (20) is advantageously utilized to simultaneously serve as a main power disconnect for a variety of motors within the system.

DESCRIPTION 
1. Technical Field 
This invention relates to electrically operated dual motor drive systems 
for industrial vehicles and the like and more particularly, to electronic 
circuitry for direction control. 
2. Background Art 
Multiple-motor electric vehicles such as industrial lift trucks are known 
in the art. At least one such vehicle employs reversible traction motors 
individually coupled to the left and right drive wheels. Each motor 
includes control circuitry for selecting the direction of motor operation. 
In the prior art, such control circuitry includes two sets of contacts for 
each motor, one set causing current to flow from a power supply through 
the motor in one direction and the other set causing current to flow 
through the motor in the opposite direction. A two-motor vehicle requires 
a total of eight contacts and four coils. 
To move in a given direction, the operator selects the appropriate position 
on a direction selector device. The associated contacts are energized to 
direct current through each of the motors in the same direction thereby 
creating vehicle movement in the selected direction. 
When the user places the direction selector in the reverse direction, the 
remaining coil of each motor is selected and the previously energized 
coils are de-energized. Under these conditions, the motor control circuit 
contacts which are selected cause the current to flow through the motors 
in the reverse direction and thereby imparting vehicle motion in the 
opposite direction. Placing the direction selector in a neutral position 
de-energizes all four of the coils. This, in turn, returns all of the 
motor control contacts to their normally open conditions such that no 
current is capable of flowing through the motors. 
While the above-described arrangement is satisfactory, a number of 
advantages could be realized if it were possible to safely reduce the 
number of solenoid coils. One advantage is the elimination of certain 
packaging constraints, thus reducing vehicle size and weight. Another is 
the reduction in the portion of current from the battery system which goes 
into non-propulsion activities. Still another is the elimination of 
control functions and electrical transient sources. The present invention 
is directed to overcoming one or more of the problems as set forth above. 
DISCLOSURE OF THE INVENTION 
The present invention overcomes the disadvantages of the prior art 
techniques through the provision of uniquely designed motor control 
circuitry which is selectively activated by only two coils instead of the 
previously used four, thereby resulting in substantial cost and space 
savings. In general, this is accomplished in an apparatus for controlling 
the direction of rotation of two DC traction motors as may be found, for 
example, in an electrically powered vehicle, said apparatus including 
first circuit means for establishing complemental or oppositely directed 
current paths from a DC source through one of the traction motors, second 
circuit means for establishing similar complemental current paths from the 
DC source to the second traction motor, each of the first and second 
circuit means being of the control-signal-responsive type and being 
further arranged so as to normally close one of the two complemental 
circuits in the uncontrolled or signal-free condition. Whenever the 
normally closed circuit is the selected circuit, no control signal need be 
sent to at least one of the first or second circuit means; only when a 
change of condition is desired must a control signal be generated and 
sent. In an illustrative embodiment, a first motor control circuit for one 
of the motors includes a first set of normally open contacts for directing 
current through the first motor in one direction when actuated. A second 
set of contacts for the first motor control circuit is normally closed and 
serves to direct current through the first motor in a second direction. A 
second motor control circuit for the other motor similarly includes a set 
of normally closed contacts and a set of normally open contacts. Only two 
coils are utilized, each coil actuating both sets of contacts in its 
associated motor control circuit. Logic control circuitry is employed for 
selectively energizing either the first or second coil.

BEST MODE FOR CARRYING OUT THE INVENTION 
Referring to FIG. 1, the preferred embodiment of the invention is shown in 
connection with an electrically operated dual motor system including left 
and right traction motor control circuits 10 and 12, respectively. Left 
motor control circuit 10 includes a reversible electric traction motor 14, 
with the right circuit 12 similarly including a reversible electric 
traction motor 16. Motors 14, 16 include armatures 15, 17 and field 
windings 19, 119, respectively, as known in the art. Both traction motor 
circuits 10 and 12 receive DC current from a DC power source such as a 
large heavy duty battery 18. A line contactor 20 couples the electrical 
power to motor control circuits 10 and 12 when energized by its associated 
coil 21. The mean current value through traction motor circuit 10 is 
controlled by a power switching element 22. Similarly, traction motor 16 
is connected in series with a power switching element 23 which is 
substantially identical to element 22. As is known in the art, power 
switching elements 22 and 23 are pulse and/or frequency modulated to vary 
the mean voltage applied to their respective motor control circuits 10 and 
12 according to the desired motor speeds. The modulation of power 
switching elements 22 and 23 is controlled by a central controller 24 
which receives various status information from the system as input signals 
and provides selected output signals depending upon the status of the 
input signals. Preferably, controller 24 is a micro-processor such as a 
number 3870 component manufactured by Mostek Corporation. Controller 24 
supplies output signals on lines 26 and 28 to control the modulation of 
switching elements 22 and 23, respectively. Generally, the modulation 
control signals are determined by the state of an accelerator position 
sensor circuit 30 which detects the position of a speed select device. 
Power switching elements 22 and 23 may be independently or differentially 
varied according to the various operating parameters of the vehicle in 
which traction motors 14 and 16 are mounted. 
Special attention is now drawn to the direction control contacts of the 
motor control circuits 10 and 12. Left circuit 10 includes a first set of 
normally open contacts 32a and 32b. When actuated or closed, contacts 32a 
and 32b direct current through armature 15 in a first direction which is 
from right to left with reference to FIG. 1. A second set of contacts 34a 
and 34b are also provided. Unlike contacts 32a and 32b, contacts 34a and 
34b are normally closed such that they direct current in an opposite 
direction through armature 15 in their steady state or de-actuated mode. 
In such state, contacts 34a and 34b direct current from battery 18 from 
left to right through armature 15. Both sets of contacts 32a, 32b, and 
34a, 34b are actuated by a single coil 36. When energized, coil 36 serves 
to close normally open contacts 32a and 32b and simultaneously open the 
normally closed contacts 34a and 34 b. The energization of coil 36 is 
controlled by appropriate signals from controller 24. 
Focusing attention on the right traction motor control circuit 12, it also 
includes two sets of contacts. Normally open contacts 38a and 38b direct 
current flow through armature 17 in the direction characterized by a left 
to right current flow when actuated or closed. It should be noted that the 
closure of normally open contacts 38a and 38b causes current flow through 
the right traction motor armature 17 in an opposite direction from the 
current flow through left traction motor armature 15 when normally open 
contacts 32a and 32b are actuated. Normally closed contacts 40a and 40b 
when de-actuated cause current flow in a right to left direction through 
right traction motor armature 17. Again, it should be noted that this 
direction is opposite from the current flow through left traction motor 
armature 15 when the normally closed contacts 34a and 34b are in a closed 
position. Both sets of contacts 38a, 38b, and 40a, 40b are actuated by a 
single coil 42. Coil 42 is energized by an appropriate signal from 
controller 24. When energized, coil 42 serves to close the normally open 
contacts 38a, 38b and to open the normally closed contacts 40a, 40b. 
Left motor control circuit 10 further includes a plugging diode 44 and a 
flyback diode 46 to provide a re-circulating path for current which is 
generated by motor 14 when coating; i.e. when driven by the load or its 
own inertia. Circuit 10 is completed by the provision of a bypass 
contactor 50 which operates, when actuated, to provide an alternate 
current path for motor 14 during high load or maximum speed conditions 
thereby bypassing power switching element 22. Similarly, right motor 
control circuit 12 also includes a plugging diode 52, flyback diode 54, 
and a bypass contactor 58, all of which operate in a similar manner to 
their counterparts in the left motor control circuit 10. Both bypass 
contactors 50 and 58 are controlled by coil 59 coupled to controller 24. 
The system of the present invention further includes a power steering motor 
60 which is also powered by battery 18 when line contactor 20 is actuated. 
The energization of a lift motor 62 is controlled by the position of a 
lift contactor 64. Lift contactor 64 is actuated upon closure of a lift 
switch 66 which energizes lift contactor coil 68. 
Controller 24 includes a plurality of inputs which receive various status 
condition signals within the system as detected by a variety of sensing 
devices. In addition to the accelerator circuit 30 noted above, switch 74 
provides a control signal indicative of the position of a key switch in 
the vehicle and switch 76 detects the mounting of the operator onto a seat 
for driving the vehicle. A three positioned direction selector switch 78 
selectively provides control input signals on line 80 when in a forward 
position, on line 82 when in a reverse position, with neither signal lines 
being selected when in a neutral position. Other input signals to 
controller 24 are derived from a wheel position sensor 84 which provides 
control signals indicative of the position of a rotating steering wheel as 
will be more fully described under the heading "Industrial Applicability". 
Signal lines 86, 87 serve to provide an indication of the condition of 
power switching elements 22, 23 and bypass contactors 50, 58 during system 
operation as will also be more fully described under the following 
heading. Fuses 88, 90, and 92 complete the system circuitry to provide 
overload protection at various points within the system. 
Industrial Applicability 
Referring now to FIG. 2, the operation of the system of FIG. 1 will now be 
described with reference to the application of said system to an 
electrically powered three wheel industrial vehicle 94 such as a fork-lift 
truck. In such an application the armature 15 of left traction motor 14 is 
mechanically connected in driving relationship with the left front wheel 
96 and the armature 17 of right traction motor 16 is similarly associated 
with the right wheel 98 of the vehicle. Wheel position sensor 84 detects 
the angular position of the rear dirigible wheel 100 of the vehicle. The 
output of sensor 84 may be utilized to differentially vary the switching 
rates of power switching elements 22 and 23 at extreme angles of wheel 
100. Key switch 74 is located near the operator station. Seat switch 76 is 
arranged in a known manner to close when an operator assumes a driving 
position on the seat. Accelerator circuit 30 detects the position of the 
accelerator and provides output signals indicative of its displacement. 
Lift switch 66 provides the operator with a means for selectively 
activating a lift mechanism (not shown) which is generally associated with 
such vehicles. A manually operable direction selector 78 controls the 
selection of forward or reverse movement of the vehicle 94. 
In normal operation, the operator first turns on key switch 74. With 
reference to FIG. 1, this supplies power over line 102 from battery 18 to 
controller 24. Controller 24 then determines the status of seat switch 76. 
If the operator has mounted the seat thereby closing switch 76 the 
controller 26 will provide an output signal to energize line contactor 
coil 21. This actuates line contactor 20 to couple battery 18 power to the 
motor control circuits 10 and 12. Immediately upon closure of line 
contactor 20, controller 24 checks the status of input lines 86 and 87. 
Lines 86, 87 will be substantially at the positive battery voltage if 
power switching elements 22 and 23 and bypass contactors 50, 58 are 
properly operating at this stage of the operation. If power switching 
elements 22, 23 are short circuited or if bypass contactors 50, 58 are 
closed, then signal lines 86 and/or 87 will be substantially at the 
negative battery voltage or ground level. If such a condition exists, 
controller 24 will remove power from the line contactor coil 21 thereby 
de-energizing line contactor 20 and removing power from the system. If the 
start-up check succeeds, the operator will place direction selector 78 in 
either the forward or reverse position. The coupling of the wiper of the 
direction selector 78 to one of lines 80 or 82 will cause controller 24 to 
selectively energize coil 36 or 42. Assume that the user selects the 
forward position in which line 80 is contacted. Controller 34 responds by 
energizing coil 36 which, in turn, closes normally open contacts 32a and 
32b and simultaneously opens normally closed contacts 34a, 34b of the left 
motor control circuit 10. Note that the contacts in the right motor 
control circuit 12 are not affected. With the contacts in this state, the 
current flow through both motors 14 and 16 is in a uniform direction 
causing them to rotate in the forward direction. The current path through 
left motor 14 is established by contacts 32a and 32b, with the current 
flowing from right to left through armature 15. The current path through 
right motor armature 17 is from right to left through contacts 40a and 
40b. Controller 24 responds to control signals from accelerator circuit 30 
by modulating the switching elements 22 and 23 in a manner known in the 
art to control vehicle speed. 
If the user desires the vehicle 94 to move in a reverse direction, selector 
78 is moved such that the wiper contacts line 84. In response thereto, 
controller 24 energizes coil 42 instead of coil 36. Under these conditions 
the left motor control circuitry 10 contacts remain in their steady state 
condition but the contacts of the right motor circuitry 12 are actuated. 
Such actuation causes normally open contacts 38a, 38b to close and the 
normally closed contacts 40a and 40b to open. Hence, current flow through 
the motors 14 and 16 is directed therethrough in the same left to right 
direction causing uniform motor rotation in the reverse direction. Note 
again that the energization of coil 42 does not effect the contacts in the 
left motor control circuit 10. 
Thus, it can be seen that the present invention provides uniquely designed 
motor control circuitry which requires only two directional coils in 
comparison with the four coils previously used in the industry. In the 
prior art four coil scheme, the control logic would of necessity energize 
two coils at a time thereby causing an appreciable amount of current flow 
drain in the circuit. In addition to the savings of costs and space, the 
present invention minimizes these troublesome circuit conditions since 
only one coil is energized at a time. 
When the selector 78 is in the neutral position in which neither line 80 or 
82 is contacted by the wiper, controller 24 does not energize either of 
coils 36 or 42. Consequently, the contacts of the left and right motor 
control circuits 10 and 12 assume their steady state condition. 
If the system is functioning normally, controller 24 will de-energize coils 
21 and 59 to open line contactor 20 and bypass contactors 50, 58, 
respectively, to remove power to the motor control circuitry in the event 
of a detected system malfunction, such as a short circuit condition in the 
power switching elements 22, 23. However, in the event of total failure of 
the controller 24 in which neither of the coils 36 or 42 are energized, 
the steady state condition of the motor control circuit contacts will 
condition the current flow through their respective motors such that they 
operate in opposite directions or counter-rotate. By referring to the 
state of the contacts shown in FIG. 1, it can be seen that current will 
flow through the left motor 14 in a left to right direction whereas 
current flow through motor 16 will be in a right to left direction. 
Accordingly, a run away condition of the vehicle is prevented. Instead of 
rotating in unison, the motors now operate in opposite directions to 
substantially counteract each other. Depending upon the position of the 
steering wheel 100, vehicle 94 will either remain stationary or will move 
in a generally circular path. 
In view of the foregoing it can now be realized that the present invention 
not only provides economies of cost and space, but it also provides 
distinctive fail-safe measures in the event of complete system 
malfunction. It is also important to note that the same line contactor 20 
which is used to apply power to the power steering motor 60 is used as the 
main system power disconnector. This line contactor in the four coil prior 
art approach was used merely to supply power to the power steering motor. 
In the present invention, however, it provides a dual function without an 
increase in manufacturing costs and facilitates the implementation of the 
unique two coil approach of the present invention. In fact, the disclosed 
motor control circuit contact arrangement need not be always employed if 
other fail-safe provisions are instead utilized. 
Therefore, while this invention was described in connection with a 
particular example thereof, various modifications will become apparent to 
one skilled in the art. For example, although the invention has been 
described with reference to a three wheel industrial vehicle having two 
traction motors and a single dirigible wheel, it is equally applicable to 
four wheeled vehicles having two dirigible wheels as well as to track 
laying vehicles having no dirigible wheels. In addition, the invention may 
be applied to other dual motor devices. Similarly, the controller 24 may 
be replaced with equivalent hard wired circuitry including solenoid 
operated relays and other conventional logic circuitry. Other aspects, 
objects and advantages of this invention can be obtained from a study of 
the drawings, the disclosure and the appended claims.