Electronic control circuit for bi-directional motion

The disclosure provides an electronic motor control circuit which switches and amplifies to control a bi-directional electric motor, using a symmetrical circuit, each half of which consists of five transistors. The circuit has a logic level input and motor driving power output using, for control in each direction: an input transistor; a control transistor turned on and off by the input transistor; two matched, complementary motor drive transistors (at opposite terminals of the motor) turned on and off simultaneously by the control transistor; and a lockout transistor, also actuated by the control transistor, which is operatively connected to one of the motor drive transistors in the other half of the system to prevent short circuiting.

BACKGROUND OF THE INVENTION 
This invention relates to electronic control circuitry designed to control 
bi-directional, or back-and-forth, motion of a driven element. The driving 
energy is supplied by an electric motor, whose clockwise and 
counterclockwise motion is controlled. In many instances this rotary 
motion is converted into reciprocating motion of the driven member by 
suitable means, such as a lead screw. 
One of the important parameters of this invention is the capability of 
using logic level input voltage and current to develop sufficient power 
output to operate a motor which exerts substantial driving force. 
Another important parameter of this invention is the requirement that 
motion in one direction be accompanied by "lockout" of an 
oppositely-acting driving signal which could cause short circuiting. 
In some instances, the following truth table will be useful: 
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A = 0 A = 1 
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B = 0 Motor off - breaking 
Motor on - reverse 
B = 1 Motor on - forward 
Motor off - coasting 
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In other instances, the braking option may be unnecessary, or undesirable. 
There are numerous uses for such control systems. The assignee of the 
present invention has used such control systems in numerous 
driven-element-positioning machines, as disclosed in Sweeney U.S. Pat. No. 
4,353,019, issued Oct. 5, 1982; and in Sweeney application Ser. Nos. 
576,013, a continuation-in-part of 289,922, (now abandoned) filed Aug. 4, 
1981, and Ser. No. 439,299, filed Nov. 4, 1982. In FIG. 2 of application 
Ser. No. 576,013 and in FIG. 2 of application Ser. No. 439,299, 
motor-control circuitry is shown, which is substantially more complex than 
the present invention, and which is shown receiving signals from "axis 
interface" circuits interfaced between the motor-control circuitry and the 
CPU. 
There are various other significant uses for improved motor-control 
circuits of this type. For example, the assignee of the present 
application has incorporated the circuit of this invention in a medical 
device of the type which provides continuous passive motion "exercises", 
such as flexing of a patient's leg. 
This control circuitry is designed for use in a digitally-controlled 
system. The transistors involved in it function in the switch mode, 
operating essentially in the saturation and cutoff regions. Their 
amplification is very substantial, but no linearity of amplifier response 
is required. Variations in the speed, or driving energy, of the controlled 
motor are accomplished by pulse-width (or pulse-frequency) modulation at 
the input, i.e., by duty-cycle variations. 
SUMMARY OF THE INVENTION 
The purpose of the present invention is to provide bi-directional control 
of an electric motor by means of more economical and efficient electronic 
circuitry than that previously available, using a minimum number of 
transistor elements, while providing improved functional results. 
The control system is symmetrical, using identical component arrangements 
for each direction of motion. In each half of the control circuit, two 
motor-connected transistors, preferably a matched pair, one of which is 
NPN and the other of which is PNP, are used, respectively, to connect one 
terminal of the motor to the positive voltage level (the power source), 
and to connect the other terminal of the motor to the negative voltage 
level (ground). A third transistor simultaneously turns on and off the two 
motor-connected transistors. 
A positive "lockout" of a motor-connected transistor in the other half of 
the circuit is provided by a fourth transistor, which is also under 
control of the third transistor. 
In order to convert from the logic level voltage input of approximately 5 
volts to an operating voltage of, say, 30 volts, a fifth transistor in 
each half of the control circuit is used to control the third transistor. 
This fifth transistor switches the third transistor, and converts from low 
level input voltage to output level voltage. 
The entire function of using logic level input (e.g., 2.5 milliwatts) to 
provide motor drive output (e.g., 100 watts) is accomplished with a 
ten-transistor circuit, five associated with each direction of motion. 
Additionally, the power requirements, both in the off-state and in the 
on-state, are minimized.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT 
As shown in FIG. 1, a signal source, such as a CPU 12, supplies pulsed, or 
digital, signals on two output lines, A and B, which are operatively 
connected to a controlled electric motor 14 in such a way that a "high" 
logic signal on A accompanied by a "low" logic signal on B will cause the 
motor to move in one direction, whereas a "high" logic signal on B 
accompanied by a "low" logic signal on A will cause the motor to move in 
the opposite direction. By varying the duration of the "on" pulses as a 
percentage of the total on-off time of the motor 14, the power output of 
the motor can be varied. In the circuit illustrated, a low logic signal at 
both A and B will cause dynamic braking of the motor, whereas a high logic 
signal at both A and B will cause the motor to be turned off, but free to 
coast. The motor 14 is preferably a permanent magnet DC motor. 
Motor 14 is driven in one direction when: (a) transistor 16 is "on", 
providing emitter-collector current flow through transistor 16 between 
positive voltage reference 18 and motor terminal 20; and (b) transistor 22 
is "on", providing emitter-collector current flow through transistor 22 
between negative voltage reference 24 and motor terminal 26. Preferably, 
transistors 16 and 22 are a complementary pair, i.e., one is a PNP 
transistor (16 in the drawings) and the other is an NPN transistor (22 in 
the drawings), and they have substantially the same voltage and gain 
characteristics, and "turn on" and "turn off" times. 
Motor 14 is driven in the opposite direction when: (a) transistor 28 is 
"on", providing emitter-collector current flow through transistor 28 
between positive voltage reference 18 and motor terminal 26; and (b) 
transistor 30 is "on", providing emitter-collector current flow through 
transistor 30 between negative voltage reference 24 and motor terminal 20. 
Preferably, transistors 28 and 30 are also a complementary pair, 28 being 
shown as a PNP device and 30 as an NPN device. It is normally desirable 
that all four motor-connected transistors--16, 22, 28 and 30--have matched 
operating characteristics. 
One of the unique and advantageous features of the invention is that only 
five transistors are required to operate the motor in each direction, 
beginning with logic level input and ending with sufficient power to 
provide substantial driving energy. In addition to its switching 
functions, each branch comprising five transistors is designed to have a 
power amplification of approximately 18,000 to 1. The input power level is 
approximately 2.5 milliwatts, based on a logic level voltage of 5 volts 
and a current of 0.5 milliampere. The output transistors--16, 22, 28 and 
30--in order to have a substantial safety factor in their power handling 
capabilities, may have an available power capacity of 90 watts, based on a 
30 volt source and a maximum 3 ampere current. However, the circuit is 
designed to output about half that power--45 watts--at the power 
amplification ratio of 18,000. 
The A input signal is connected to the base of a transistor 32, and the B 
input signal is connected to the base of a transistor 34. The transistors 
32 and 34, which are shown as NPN devices, each have their collectors 
connected through resistances 36 to the positive reference voltage 18, and 
their emitters connected to the negative reference voltage 24. Thus 
transistors 32 and 34 convert the logic level input voltage swing of 
approximately 5 volts to the output voltage swing of the power source 
which may have a value of approximately 30 volts. The transistors 32 and 
34, in addition to their amplification function, constitute inverting 
switches, each of which turns on and off the next transistor in its branch 
of the control system. 
A transistor 38 is switched on and off by transistor 32; and a transistor 
40 is switched on and off by transistor 34. Transistors 38 and 40, which 
are shown as NPN devices, are important multi-purpose devices in this 
invention, since each of them simultaneously controls pair 16-22 or 28-30 
of the motor-connected transistors, and also operates one of two lockout 
transistors 42 or 44. Transistor 38 has its base connected through a 
resistance 46 to the collector of transistor 32; and transistor 40 has its 
base connected through a resistance 48 to the collector of transistor 34. 
The emitter of transistor 38 is connected to the base of transistor 22; 
and its collector is connected through a resistance 50 to the base of 
transistor 16, and also in parallel through a resistance 52 to the base of 
transistor 42, which is a PNP device. Transistor 40, which has the same 
functions as transistor 38, but in the opposite branch of the control 
circuit, has its emitter connected to the base of transistor 30; and its 
collector is connected through a resistance 54 to the base of transistor 
28, and also in parallel through a resistance 56 to the base of transistor 
44, which is a PNP device. 
Each of the lockout transistors 42 and 44 has its emitter connected to the 
positive reference voltage. The collector of transistor 42 is connected to 
the base of transistor 28; and the collector of transistor 44 is connected 
to the base of transistor 16. 
In analyzing operation of the circuit, first assume a 1 (logic high) signal 
input at A and a 0 (logic low) signal input at B. The high A input at the 
base of transistor 32 will cause that transistor to be turned on, 
providing emitter-collector current flow, and dropping its collector 
voltage signal to low. This low signal at the base of transistor 38 will 
cause that transistor to be turned off. With no current flowing in 
transistor 38, its collector voltage will be high, causing PNP transistors 
16 and 42 to be turned off, and its emitter voltage will be low, causing 
NPN transistor 22 to be turned off. 
The low B input at the base of transistor 34 will cause that transistor to 
be turned off, preventing emitter-collector current flow, and raising its 
collector voltage signal to high. This high signal at the base of 
transistor 40 will cause that transistor to be turned on. The flow of 
emitter-collector current in transistor 40 will close a series circuit 
between negative voltage reference 24 and positive voltage reference 18, 
in which emitter-base current flows in NPN transistor 30, causing that 
transistor to be turned on, and base-emitter current flows in PNP 
transistor 28, causing that transistor also to be turned on. 
Motor 14 will now be driven in one direction by the current flowing between 
its terminals 20 and 26, terminal 20 being connected to low voltage 
reference 24 through transistor 30, and terminal 26 being connected to 
high voltage reference 18 through transistor 28. 
The current flow in transistor 40, and the resulting voltage drop at its 
collector, puts a low level input signal on the base of PNP transistor 44, 
causing it to be turned on. Transistor 44, and its counterpart transistor 
42, are designed to be driven to saturation at a very low input level 
(less than the inherent transistor voltage drop), thus ensuring that the 
full voltage of the high reference voltage 18 will be connected to the 
base of transistor 16. This creates a lockout, providing positive 
assurance that transistor 16 will not be turned on while transistor 30 is 
turned on. The full voltage at the base of transistor 16 will hold it in 
the off state, regardless of the signal input to transistor 16 from 
transistor 38. Clearly, any simultaneous turn on of transistors 16 and 30, 
or of their counterparts 22 and 28, would be destructive, because a short 
circuit would be created between the positive reference voltage 18 and the 
negative reference voltage 24. 
Since the motor control system is symmetrical, the two branches operate in 
exactly the same way, but their operation is reversed when the motor is 
driven in the opposite direction. If the signal at A is 0 (logic low) and 
the signal at B is 1 (logic high), transistor 32 will be caused to turn 
off, and transistor 34 will be caused to turn on. The high voltage signal 
at the collector of transistor 32 will cause transistor 38 to turn on, 
thus providing a series current from the low voltage reference 24 through 
the emitter-base of NPN transistor 22 and through the base-emitter of PNP 
transistor 16. With both transistors 22 and 16 turned on, terminal 26 of 
motor 14 will be connected to low voltage reference 24, and its terminal 
20 will be connected to high voltage reference 18. The motor will be 
driven in the opposite direction from its motion when the A and B signals 
are reversed. The low voltage signal at the collector of transistor 38 
will turn on the NPN lockout transistor 42. This places full positive 
voltage at the base of transistor 28, preventing it from being turned on. 
In referring to the functions of the five transistors in each branch of the 
circuit, it may be useful to designate the transistors 32 and 34 as 
"input" devices, the transistors 38 and 40 as "control" devices (because 
each of them actuates three other transistors), the transistors 42 and 44 
as "lockout" devices, and the transistors 16, 22, 28 and 30 as 
"motor-drive", or "power", devices. 
A valuable aspect of the invention is the series connection of the gate 
circuits of the motor drive transistor pairs. Because the paired 
transistors 16-22 and 28-30 are matched PNP and NPN devices having their 
gate currents in series, they are ensured of being turned on to the same 
degree, thus avoiding any wasting of available motor driving power. 
While the control circuit of the present invention is useful whenever it is 
desired to control bi-directional motion with a minimum number of 
transistors, it is particularly impressive in its ability to provide 
bi-directional power output control directly from logic level input 
signals, using a total of only ten transistors. The use of each control 
device to operate the two power devices and also the lockout device is a 
significant contributor to the functional and economical advantages of 
this circuit. 
The motor 14 is turned off, but free to coast, if both A and B have input 
signals of 1 (logic high). This causes both control transistors 38 and 40 
to be turned off, and all of the motor drive transistors 16, 22, 28 and 30 
to be turned off. In this condition, the motor is able to coast without 
any braking effect resulting from the current generated by the motor. 
In the disclosed circuit, dynamic braking occurs if both A and B have input 
signals of 0 (logic low). There are some situations where dynamic braking 
would not be desirable; and the circuit should be altered for that reason. 
However, the circuit shown is designed to provide dynamic braking. With 
both A and B input signals low, transitors 32 and 34 are turned off, 
causing their collector voltage to go high, providing a high signal at the 
bases of transistors 38 and 40, thus causing them to turn on. The effect 
is to turn on drive transistors 22 and 30, and also to turn on lockout 
transistors 42 and 44, which prevent drive transistors 16 and 28 from 
turning on. Transistors 22 and 30 provide a shunt for the motor armature, 
i.e., a parallel path for the current generated by the BEMF of the moving 
motor (assuming that such motion has been occurring). Since these 
transistors are power transistors, the effect is to operate one as a 
transistor and the other 15 as a diode, their roles depending on the 
motor's direction of movement, and the polarity of its BEMF-generated 
current. Dynamic braking is caused by this low resistance shunting because 
the coasting of the permanent magnet motor acts as a generator with a low 
impedance load. 
FIG. 2 shows essentially the same motor actuating circuit as FIG. 1, except 
that "snubber" circuits are included for the four motor drive transistors 
16, 22, 28 and 30, and specific resistor values are shown as an example of 
a practical embodiment of the circuit. Obviously, these values are merely 
exemplary, and may be varied in accordance with the design parameters of a 
given system. In FIG. 2, the same identifying numerals are used where the 
elements correspond to those in FIG. 1. The resistor values are applicable 
to a circuit in which: (a) the NPN input transistors 32 and 34 are signal 
level transistors, which may be Motorola MPS series devices designated A 
05; (b) the NPN control and drive transistors 22, 30, 38 and 40 are power 
level transistors, which may be the type designated TIP 31; (c) the PNP 
drive transistors 16 and 28 are power level transistors, which may be the 
type designated TIP 32; and (d) the PNP lockout transistors 42 and 44 are 
power level transistors, which may be Motorola MPS series devices 
designated A 55. 
Between input line A and the base of transistor 32, there is a resistor 58; 
and between input line B and the base of transistor 34, there is a 
resistor 60. The resistors 58 and 60 are shown having a value of 11 
kilohms. A relatively high value of these resistors is needed to provide a 
high impedance input, thus minimizing the current demanded from the logic 
portion of the system. The other resistance values are generally dictated 
by the amplification requirements and the types of transistors chosen for 
the circuit. As previously stated, the devices and resistance values are 
chosen to cause switch mode operation, i.e., rapid change between cut-off 
and saturation at the leading, or trailing, edge of each input pulse. 
The load shifting circuitry (also termed "snubber", or "dump" circuitry) 
associated with each of the motor drive transistors (16, 22, 28, 30) 
comprises a diode 62 and a capacitor 64 in series, in a branch parallel to 
the respective transistor, and a resistor 66 shunting diode 62. The diodes 
may be the type identified as 1N 5402, and the resistors and capacitors 
may have the values shown in FIG. 2. The effect of the load shifting 
circuit is to charge the capacitor 64 through diode 62 during transistor 
turn-off, thus diverting current from the transistor. When the transistor 
is next turned on the capacitor will discharge through resistor 66. The 
effect of these load shifting circuits is to limit the peak transistor 
currents, thereby permitting each drive transistor to safely switch more 
power than it could without such protection. 
As shown in FIG. 2, the motor 14 is shunted through a capacitor 68 to 
bypass voltage transients, and is also shunted through a metal oxide 
varistor 70, which acts as a voltage-dependent resistance to limit peak 
voltages to a level below the 80 V peak reverse voltage rating of the 
motor drive transistors. 
It would be possible to substitute field effect transistors for the 
junction transistors shown, but such a change would result in greater 
power loss. As previously stated, low power loss is an important advantage 
of the present invention. It permits a maximum percentage of the available 
power to be used in driving the motor. rather than operating the 
transistors. In the off state, the only current flowing is that in the 
signal level input transistors 32 and 34. In the one state, power loss is 
reduced by operating the motor with a minimum number of transistors. 
From the foregoing description, it will be apparent that the apparatus 
disclosed in this application will provide the significant functional 
benefits summarized in the introductory portion of the specification. 
The following claims are intended not only to cover the specific 
embodiments disclosed, but also to cover the inventive concepts explained 
herein with the maximum breadth and comprehensiveness permitted by the 
prior art.