Abstract:
A direct current, variable duty cycle control circuit for the manual control of the supply of power from a direct current source to a motor is disclosed including a transistor arrangement operable when enabled to couple the direct current source to the motor along with means for enabling that transistor arrangement during an operator determinable portion of each of a series of time intervals and a sensing circuit receptive to an overload condition of the transistor arrangement for disabling that transistor arrangement. The disclosed system includes an operator control member effective in a first position to prevent the energization of the motor by preventing the supply of electrical power to the control circuit, effective in a second series of positions to vary the portion of each time interval during which the motor is supplied power and effective in a third position to deliver power continuously to the motor by bypassing the transistor arrangement.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates generally to direct current motor control circuitry and more particularly to a solid state motor speed control circuit which varies the motor speed by varying the duty cycle of the voltage applied to the motor. 
     Basic motor speed control circuits have in the past employed a fixed direct current voltage source with a variable resistance in series with, for example, the motor armature, so that an operator might vary the series resistance thereby also varying the voltage applied to the motor armature and therefore also the motor speed. This series variable resistor arrangement provides adequate speed control, however, particularly at reduced speeds, substantial I 2  R losses occur in the series variable resistance, resulting in substantially reduced efficiency of the system. In installations employing a rechargeable storage battery, such as an electrically powered golf cart or forklift truck, a large fraction of the energy consumed by the system is dissipated as heat from the series control resistor and the elimination of this heat loss would provide for either a substantial increase in the operating time between recharging sessions or a substantial reduction in the size of the storage battery required while maintaining the operating time between battery chargings substantially the same. 
     In addition to the inefficiency and heat dissipating problems, such series resistance controls are plagued with electro-mechanical contact problems including the service and maintenance problems associated therewith. Motor overload protection circuitry is difficult or expensive to incorporate and the frequent recharging of the energy source shortens the life of that source. Despite these problems, the series control resistor is still commercially employed because of its relatively low initial cost. 
     Various attempts have been made to avoid the foregoing disadvantages of series resistance control arrangements by, for example, electronically controlling the duty cycle of energy supplied to a motor. Such electronic controls may, for example, employ a silicon controlled rectifier in series with the motor armature and selectively gate that silicon controlled rectifier to conduct during selectable portions only of repeated time intervals. These prior art duty cycle motor controllers have not met with great commercial success, due in part to their relatively high initial cost and lack of overload protection for the control circuit elements. 
     SUMMARY OF THE INVENTION 
     Among the several objects of the present invention may be noted the provision of a solid state direct current motor speed control circuit characterized by its reduced cost, improved operating efficiency, and high reliability; the provision of a motor control circuit employing solid state switching of the motor armature current where the drive for the solid state switching arrangement is derived from the voltage across the field of the motor; the provision of circuitry for manually controlling the power supplied from a direct current source to a motor including circuitry for sensing an overload condition and for interrupting the power supply in response thereto; the provision of a solid state motor speed control circuit including a single operator control member which, in a first position, prevents motor operation by precluding energization of the control circuit which, in a second series of continuous or discrete positions, functions to vary the proportion of successive time intervals during which power is supplied to the motor, and which, in a third position, bypasses the control circuitry to continuously directly energize the motor; and the provision of a solid state motor control circuit of generally improved operating characteristics. 
     In general, and in one form of the invention, circuitry for the manual control of the power supplied from a source to a motor includes a transistor switching arrangement which is operable when enabled to couple the source to the motor along with means for enabling that transistor switching arrangement during selectively determinable portions of each of a series of time intervals and includes circuitry for disabling the transistor arrangement in response to an overload condition of that transistor arrangement. Manual control is achieved by a single operator control member which is effective in a middle range of positions to determine the portion of each time interval while, in one extreme position, the transistor enabling circuitry is disabled and in the other extreme position the transistor switching arrangement is short-circuited or by-passed to supply power directly from the source to the motor. 
     Also in general, and in one form of the invention, a motor speed control circuit includes a direct current source, the motor armature, the motor field winding, and a power transistor switching arrangement connected in series along with a driver transistor arrangement for enabling the power transistor arrangement during controllable portions of successive time intervals and circuitry for limiting the collector-emitter voltage of the driver transistor arrangement to less than the voltage of the source by at least the voltage across the motor armature. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block schematic diagram of a direct current motor speed control arrangement in one form of the invention; 
     FIG. 2 is a wave form illustrating the voltage output from the variable duty cycle timer during two different illustrative conditions; and 
     FIG. 3 is a detailed schematic diagram of one implementation of the speed control circuit of FIG. 1. 
    
    
     Corresponding reference characters indicate corresponding parts throughout the several views of the drawing. 
     The exemplifications set out herein illustrate a preferred embodiment of the invention in one form thereof and such exemplifications are not to be construed as limiting the scope of the disclosure or the scope of the invention in any manner. 
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring first to FIG. 1, the direct current motor speed control arrangement of the present invention is seen to include in general a direct current source such as a rechargeable storage battery 11 and a power switch 13 which may, for example, be a transistor arrangement which is operable when enabled to couple the source 11 to a direct current motor which, as illustrated, comprises the series combination of the motor armature 15 and the motor field winding 17. The switching arrangement 13 is enabled during a selectively determinable portion of each of a series of time intervals by a switch driver circuit 19 which has its voltage between lines 21 and 23 limited to less than the voltage of the source 11 by at least the voltage drop across the motor armature 15. The switch driver in turn is enabled for the selectively determinable portions of the successive time intervals by a variable duty cycle timer 25. Any overload condition, as indicated by an excessive voltage drop across the power switching arrangement 13 combined with the switch driver being in an activated state is indicated by a signal on line 27, which signal is effective to interrupt operation of the timer 25. The combination of excessive voltage across the power switch and driver in the conductive state is sensed by logic circuit 39. Closure of the switch 29 provides a voltage source for timer 25, while closure of switch 31 effectively short-circuits or bypasses the solid state power switching arrangement 13, connecting armature 15 and field winding 17 directly in series with the battery 11. 
     An operator control member 33, which might in an exemplary environment, such as a battery powered golf cart, take the form of an operator accessible foot-actuated accelerator pedal, functions upon initial depression by a slight amount to close switch 29 and functions when fully depressed to close also switch 31. Such a pedal may be spring loaded to return to the position where both switches 29 and 31 are open, if the operator, for example, removes his foot from the pedal. In intermediate positions of the operator control member, a representation of the degree of depression is supplied by way of line 35 to the variable duty cycle timer 25, with various positions of that operator control member varying the duty cycle of the timer. Thus, the operator control member might be coupled to a potentiometer with depression of the control member varying the resistance of that potentiometer. 
     FIG. 2 depicts two exemplary output wave forms supplied from the variable duty cycle timer 25 to the switch driver 19 by way of line 37. During the first (left) portion of the timer output wave form, a time interval or cycle is represented by T 2 , and during this interval, driver 19 is enabled for the portion of the time interval represented by T 1  with the duty cycle being by definition T 1  divided by T 2 . So long as the potentiometer setting remains constant, this output on line 37 continues with the total cycle time of T 2  and with the switch driver periodically enabled during the time interval T 1 . If the potentiometer setting is changed to a new value, the wave form on line 37 may become as depicted further toward the right in FIG. 2, with a period T 3 , during which an &#34;on time&#34; of T 4  occurs. The duty cycle is now T 4  divided by T 3 . The repetition rates associated with T 2  and T 3  may be substantially the same or may vary somewhat with variations in the potentiometer setting depending on the particular circuitry employed and minor variations in this period or repetition rate have no substantial effect on circuit operation since the important feature is the ratio of driver &#34;on time&#34; to the total time interval. Also while the enabling signal in FIG. 2 has been depicted as a negative going pulse, either positive or negative logic may be employed in any particular implementation. Further, while the wave form of FIG. 2 depicts only two different duty cycles, the timer may be arranged to provide duty cycle signals varying from zero or nearly zero to one or nearly one, depending upon the particular setting in which the time proportioning control circuit is used. 
     Turning now to the more detailed time proportioning control circuit schematic diagram of FIG. 3, switch 29 is initially closed upon operator actuation of the operator control member to supply voltage from battery 11 to the variable duty cycle timing circuitry. This operator control member varies the position of the wiper on potentiometer 97 through its range and at the other extreme of the operator control member movement, switch 31 is closed to directly connect the battery 11, armature 15 and field winding 17 in series. The closure of switch 31 of course occurs to obtain a duty cycle equal to one and represents full motor speed. A separate operator control may be provided for changing the double pole double throw switch 45 from the position shown so as to connect to contact 49 on the one hand and contact 53 on the other hand to provide for reversing the direction of motor rotation. 
     For intermediate positions of the operator control member, that is, positions through which the potentiometer 43 is varied after the closure of switch 29 and prior to the closing of switch 31, current flows from the battery 11 through armature 15 and field winding 17, through the parallel connected transistor array, of which only units 113 and 115 are shown, to complete the circuit back to battery 11. 
     The transistor array represented by 113 and 115 is enabled to conduct only during portions of successive time intervals with the duty cycle for that portion determined by the particular setting of potentiometer 97. 
     The timing circuitry of FIG. 3 is built around a commercially available integrated circuit 59 which operates as an astable mutivibrator providing an output (as illustrated in FIG. 2) by way of resistor 105 to the base of the transistor 107. The duty cycle of the enabling signal supplied through resistor 105 is numerically the ratio of the resistance between pins 6 and 7 on integrated circuit 59 to the sum of the resistance between pins 7 and 8 plus twice the resistance between pins 6 and 7. This integrated circuit 59 is a NE555. 
     The timer output as supplied to the base of transistor 107 goes &#34;low&#34; during the enabling portion of the duty cycle causing transistor 107 to conduct. Conduction by transistor 107 supplies a base enabling current to Darlington transistors of the driver array represented in FIG. 3 by Darlingtons 109, 111, rendering them conductive. The source of current for the high base current drive required by the output transistors in the parallel array represented by 113, 115 in FIG. 3 is provided by the low voltage dropped across the motor field 17. This method of supplying base drive is advantageous in that it supplies base current drive in proportion to load current, and from a low voltage source consisting of the drop across field 17, thus minimizing the power consumed by the base drive circuit. Aso, at high motor current, the magnetic circuit 16 of the motor may saturate if all of the armature current is allowed to pass through the field, so that shunting a fraction of the current into the driver circuit as in FIG. 3 will not result in a significant decrease in the flux density of field 17. Rendering of the driver array 109- 111 conductive allows current through resistor array 136-137 and to the base terminals of output array 113-115 via the emitters of driver array 109-111. Current into the base terminals of output transistors 109-111 is established as equal values from one unit to the next by resistor array 136-137. Thus, with substantially equal base currents, the output transistors will share the load current in proportion to their gain. Since the gain of the output transistors does not vary substantially at high currents, the load current division within array 113-115 is good. Conduction by the output transistors in turn causes a rise in field and armature current. When the output from timer circuit 59 goes &#34;high&#34;, transistor 107 is turned off, interrupting current to the bases of driver array 109-111 and thereby also turning off output array 113-115. The sudden cessation of current in the output transistors causes a stepped conduction in diode 75 so as to maintain a continuity of current in field 17 and armature 15. 
     Suppose now that the circuit of FIG. 3 is operating at a relatively high duty cycle with transistor array 109-111 conducting most of the time but that the operator control member has not been moved quite far enough to close switch 31. Suppose further that the collector-emitter voltage drop across transistors 113-115 becomes larger than some specified value indicating excessive current flow through those transistors and an overload condition. With the hypothesized overload, the voltage on line 87 exceeds some specified value, reverse biasing diode 89 so that that diode no longer conducts when transistor 107 conducts. This in turn causes an increased current flow through the variable resistor 91, in turn increasing the voltage level at the base of transistor 93, rendering that transistor conductive. When transistor 93 conducts, it essentially prevents the accumulation of any charge on capacitor 95, which is the timing capacitor for the integrated circuit 59. Hence, the short-circuiting of capacitor 95 renders the timing circuit 59 inoperative, precluding the further enablement of the power transistors 109-111 and obviating the overload condition. The level on line 87 at which transistor 93 conducts is of course set by adjustment of potentiometer or variable resistor 91, and the total voltage drop across that resistor increases as the potential on line 87 increases. Thus, for some choices of circuit parameters and certain settings of the potentiometer 91, diode 89 may still be conducting when the potential at the base of transistor 93 becomes sufficient to cause that transistor to conduct. In either event, overload protection is effectively provided. 
     Component values and a brief comment as to the component function for the circuit are as follows: 
     
         __________________________________________________________________________Component Value        Function__________________________________________________________________________122   2000 μf   Voltage suppression and load-line              improvement for transistors 109, --, 111.117   220 ohm 4w   Forms power supply for drive oscillator 59.119   120 ohm 2w   Acts as dummy load on power supply during              the &#34;off&#34; period to maintain near constant              voltage on capacitor 121 without expense              of a zener diode.99    1000 ohm     Timing function; resistor 97 is connected97    25000 ohm         Pot. to vehicle accelerator pedal.101   1000 ohm124   1000 ohm     Limits discharge current from timing              capacitor 95.103   2200 ohm     Bypass of emitter-base of transistor 107              for leakage control.105   4700 ohm     Base drive for transistor 107.126   510 ohm      Base drive for Darlington transistor 111.128   4700 ohm     Drives transistor 93 when diode 89 is              reverse biased.91    5000 ohm         Pot. Controls trip level of protection circuit.130   2200 ohm     Leakage bypass.132   10 ohm       Leakage bypass.134   lead resistance              Aid in dividing collector currents. about .002 ohm136   1.0 ohm      Dropping resistor - also assures equal              base currents for output transistors.95    0.33 μf   Timing capacitor.140   .05 μf    Noise suppression for the NE555.93    2N5128       Part of feedback, protection loop.107   2N4354       Intermediate stage.109   Darlington   Driver for output transistor 113.113   2N5575       Output transistor, 80 amp continuous,              100 amp peak rating.142   6 amp diode, Freewheel diode for portion of 100 volt     armature current which goes through              base-drive Darlington 109.75    High current Freewheel diode for field and diode        armature current.__________________________________________________________________________ 
    
     From the foregoing it is now apparent that a novel arrangement for controlling the energy supplied from a direct current source to a motor has been disclosed meeting the objects and advantageous features set out hereinbefore as well as others and that modifications as to the precise arrangements and details may be made by those having ordinary skill in the art without departing from the spirit of the invention or the scope thereof as set out by the claims which follow.