Abstract:
An improved control system for energizing a stepping motor varies the width of the stepping pulse to the stepping motor as the motor transfers from one balanced state to another in order to provide additional starting torque and disconnects the energizing source from the motor when the motor is in its balanced state in order to conserve energy.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to an improved control system for energizing a stepping motor; and, more particularly, to a system which is efficient in terms of the overall power consumed by the motor. 
     2. Description of the Prior Art 
     Stepping motors of the type to which this invention is applicable are well known in the art. Such stepping motors have a number of angular rotor positions at which the rotor is balanced. An electrical energy pulse causes the rotor of the motor to step from one balanced position to the next. The direction of rotation of the motor is determined by the sequence in which the field coil windings are energized as the total angle through which the rotor travels is a function of the number of pulses supplied to the field coil windings. 
     As will be appreciated by those skilled in the art, such stepping motors are advantageous in that the angular position of their rotor can be simply and accurately determined by counting the number of pulses supplied to the field coil windings. Such motors are used widely in numerical control machine tool systems and also in other process control systems. In the case of a process control system, for instance, the position of the stepping motor rotor can be varied as a function of the difference a feedback signal from a rebalancing element between a process variable signal and in order to rebalance the system. 
     In order to operate satisfactorily in most control systems, a stepping motor must usually develop a large torque as it initially steps from one balance state to a new balance state. the inertia or friction of the motor&#39;s rotor and the inertia or friction of the connected load call for this large torque. In addition, in control system applications, it is often desirable that the delivered torque vary as the motor steps through a series of positions. 
     It will further be appreciated that in a control system application the motor may sit at its balance point for considerable periods of time. If the field coils are energized during these periods, the system will be inefficient in terms of power consumption and also the motor will tend to overheat. 
     SUMMARY OF THE PRESENT INVENTION 
     Accordingly, one object of the present invention is to provide a control system for energizing a stepping motor which can cause the stepping motor to follow an input signal for instance, a process variable signal, so that the stepping motor provides sufficent torque when it transfers from one balanced state to the other. 
     Another object of this invention is to provide the system for energizing the stepping motor which is operated by a single power supplying source and is useful for saving the electric power consumption by the stepping motor. 
     According to this invention, these other objects of the invention are achieved by a control system in which the width of the stepping pulse to the stepping motor can be varied when the stepping motor transfers from one balanced state to the other and wherein the power supply to energize the stepping motor is interrupted when the stepping motor is in a balanced state. Advantageously, the power supply to the system is a single power supply, or a power supply having an equivalent ability to that of two power supplies. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Another object and advantages of this invention will be more fully understood from the description taken in conjunction with the accompanying drawings, in which: 
     FIG. 1 is a schematic diagram (partially in block form) of one embodiment of the stepping motor control system of this invention; 
     FIG. 2 is a schematic diagram of the stepping motor control circuit shown in block form in FIG. 1; 
     FIG. 3 is a schematic diagram of an alternate embodiment of the invention; and 
     FIG. 4 is another schematic diagram of still another embodiment of the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to the drawings, FIG. 1 shows a schematic diagram of the system for energizing a stepping motor, which is designated by the reference letter M in accordance with the teachings of the present invention. In this figure, the emitter of a transistor Q 1  is coupled to the positive terminal of a direct current power supply, the collector of which is coupled to operational common which is a constant voltage. A zener diode D 1  provides a constant potential at the base of the transistor Q 1 , and transistor Q 1  supplies from its collector a constant current. A second zener diode D 2  applies a constant potential to the collector of transistor Q 1 . The collector of transistor Q 1  supplies a constant current to one terminal of a potentiometer VR 1  whose other terminal is connected to ground. The wiper arm of potentiometer V R , which is mechanically coupled to, and driven by stepping motor M, is electrically coupled to one input of an operational amplifier OA 1 . 
     The other input to operational amplifier OA 1  is the process variable signal PV. It will be appreciated that the output of amplifier OA 1  is a function of the difference between the signal coupled to the process variable terminal PV and the feedback rebalancing potential developed by the potentiometer VR 1 . 
     The output of amplifier OA 1  is coupled to the light emitting elements of a pair of photocoupler switches PC 1  and PC 2 . An emitter follower transistor Q 2 , the base of which is supplied with a constant voltage from the zener diode D 1 , is used as a constant voltage source for the photocoupler switches. 
     Zener diode D 3  provides a positive bias, with respect to the constant voltage from the emitter of Q 2 , to one end of the light emitting element of photocoupler PC 1  . Similarly, zener diode D 4  provides a bias or level shift from this constant voltage source to the other terminal of photocoupler PC 2 . 
     The power for the light receiving elements of photocouplers PC 1  and PC 2  is supplied with a constant voltage from transistor Q 2 . The outputs of these photocouplers develop, respectively, a voltage signal which is transmitted to a stepping motor energization control circuit CON as a control signal for controlling the forward or backward rotation of the stepping motor M. In addition, the outputs of photocouplers PC 1  and PC 2  are coupled as inputs to a NOR gate G 1 . 
     In operation, photocoupler PC 1  turns on when the output of operational amplifier OA 1  exceeds the bias established on the light emitting element by transistor Q 2  and zener diode D 3 . Similarly, photocoupler PC 2  turns on when the output of operational amplifier OA 1  falls below the bias established by transistor Q 2  and zener diode D 4  on the light emitting element of photocoupler PC 2 . When the output signal of operational amplifier OA 1  is between these levels, both photocouplers are off. 
     NOR gate G 1  produces an output signal which is transmitted to the gate of an MOS (metal oxide semiconductor) switch whenever PC 1  or PC 2  is turned on. An RC network, comprising condenser C 1  and resistor R 1 , couples the output from NOR gate G 1  to the base of the metal oxide switch MOS. 
     The resistance between the source and drain of the MOS forms a component part of an oscillator circuit OSC 1  which also comprises resistor R 2 , condenser C 2  and a programmable uni-junction transistor PUT which serves as the active element of the oscillator. When either PC 1  or PC 2  comes on, NOR gate G 1  provides an output signal which temporarily increases the impedance between the source and drain of MOS and thereby decreases the oscillating frequency of OSC 1  for a period of time determined by the time constant of the RC network comprising resistor R 1  and a capacitor C 1 . 
     Thus, it wil be appreciated that the output of oscillator OSC 1 , whose output is coupled as a clock pulse input to the stepping motor energization control circuit CON, decreases initially from its normal steady state value and returns gradually to this frequency over a period of time determined by the time constant of the differential circuit R 1 , C 1 . 
     Referring now to FIG. 2 in addition to FIG. 1, FIG. 2 shows an embodiment of the energization control circuit CON. In this figure, a pair of D flip-flops FF 1  and FF 2  are connected in cascade with one another. The Q output of flip-flop FF 2  is the D input to flip-flop FF 1  and the Q output of FF 1  is the D input of FF 2 , establishing a loop so that the outputs of FF 1  and FF 2  are reversed by two clock pulses CP with the phase shifted by one clock pulse cycle with respect to one another. 
     A data selector DS comprising AND gates and OR gates receives inputs from the flip-flop circuits FF 1  and FF 2  and also signals from PC 1  and PC 2  for forward rotation K A  and backward rotation K B . 
     For forward rotation, for example, pulses delayed one after the other by one clock pulse and each having a pulse width equal to two clock pulses appear sequentially on the output terminals DS 1  thru DS 4 . In the case of backward rotation, pulses gaining one after the other by one clock pulse each of which is two pulse widths wide are coupled to the output terminals. In either case, the transistors Q 3  thru Q 6  are switched over from on to off or vice versa by each output pulse of the data selector DS to energize the stepping motor in the proper sequence or order. 
     In operation, the system for driving the stepping motor shown in FIGS. 1 and 2 is in its quiescent state and the motor M is not energized when the feedback voltage determined by the position of the wiper arm of the potentiometer VR 1  is almost equal to the input signal (the process variable signal PV in this case). In this situation, operational amplifier OA 1  has a positive output voltage proportionate to the difference between both input voltages. The magnitude of this output is within the range of the threshold levels established by the zener diodes D 3  and D 4  so that the electric current does not flow to the light emitting elements in the photocouplers PC 1  and PC 2 . The output of photocouplers PC 1  and PC 2  is, in this case, logic 0 and all the outputs from the data selector of energization control circuit CON are logic 0. In this state, transistors Q 3  thru Q 6  are in their cutoff state. Accordingly, no energization current is supplied to the stepping motor M although the oscillating circuit OSC.sub. 1 continues to supply clock pulses having a steady state, constant period to the energization control circuit CON. 
     Summarizing the above operation, the circuit is in the balanced state when the difference between the input signal voltage and the feedback voltage is smaller than a predetermined small value; and, in this condition, the current to energize the stepping motor M is interrupted. 
     Next, assuming there is a difference between the input signal and the feedback voltage so that the output voltage of the operational amplifier OA 1  exceeds the voltage range of the threshold level, one of the outputs of photocouplers PC 1  or PC 2  becomes logic 1; which of the photocouplers becomes logic 1 depends upon whether the potential at the terminal PV exceeds or is less than the potential established by the potentiometer. Logic 1 from PC 1  or PC 2  provides a control signal for the energization of control circuit CON to enable the appropriate gates forward or background rotation of motor M. 
     Simultaneously upon the input of either logic 1, the output of NOR gate G 1  drops to low level and the oscillating frequency of the oscillating circuit OSC 1  is lowered initially, and then increased gradually towards its steady state frequency over a period determined by time constant of resistor R 1  and the condenser C 1 . As a result of this, the stepping motor is supplied initially with the energization current having relatively lower frequency (wide pulse widths) and then is supplied gradually with the energization current having higher frequency. The motor is thereby controlled to the rotated forwardly or backwardly. The rotation of the stepping motor ceases when the feedback voltage coincides with the input voltage, and the motor energization current ceases. In other words, the energization current is not supplied to the stepping motor when it is in the balanced state but is supplied temporarily, with the pulse initially having a wide pulse width, when the stepping motor rotor transfers from the balanced state. Thereby, the consumption of electric power in the balanced state is saved while at the same time a large torque is obtained when the stepping motor starts rotating. 
     FIG. 3 is a circuit diagram of another circuit for driving a stepping motor embodying the present invention for indicating and/or recording the standard predetermined quantity or set point SP in this case wherein the circuit makes the rotation of angle of the stepping motor follow the preceding signal. The same component for the same function as that shown in FIG. 1 is indicated with like reference numerals or symbols. In FIG. 3, an operational amplifier OA 2  has the feedback voltage from VR 2  which varies in accordance with the rotation angle of the stepping motor M. This feedback voltage is compared with another input voltage SP, the standard set point. A second operational amplifier OA 3  is used as an inverted amplifier with unity gain, providing a logically inverted output of the output of the amplifier OA 2 . The outputs from the operational amplifiers OA 2   and OA 3  are connected respectively to the cathodes of the diodes D 5  and D 6 . The outputs of the amplifiers OA 2  and OA 3  are also coupled to the energization control circuit CON as a forward rotation signal (the output from OA 2 ) or a backward rotation signal (the output from OA 3 ), for example. 
     The anode voltage of diodes D 5  and D 6  is the forward voltage of the diode D 7  ; diodes D 5  and D 6  are used to clamp the negative output from OA 2  or OA 3 , respectively, for protection of the NOR gate G 1  and the AND gates of the data selector DS. 
     The output of the NOR gate G 1  is applied to the gate of an MOS through the condenser C 1  and the differential circuit comprising the resistor R 1  as in the embodiment of FIG. 1, and the impedance variation of MOS changes the oscillating frequency of the voltage control oscillator VCO, which includes inverting amplifiers INV 1  and INV 2  as its active elements. The output of the voltage control oscillator VCO is transmitted as a series of clock pulses to the energization control circuit CON. 
     The negative power supplied to the operational amplifier OA 2  and OA 3  can be obtained by full-wave rectification of an oscillating output of the oscillator OSC 2  operated with the positive power supply. The full-wave rectification can be performed with transistors Q 7  and Q 8 , and diodes D 8  thru D 11 , which is then stabilized to a constant voltage by zener diode D 12 . 
     In accordance with the circuit constructed as in FIG. 3, when the difference between a standard set point SP and the voltage corresponding to the rotation angle of the stepping motor is out of the range of allowable deviation, amplifiers OA 2  and OA 3  become saturated and have outputs which are logically inverted with respect to one another, thereby supplying the forward or backward rotation signal to the energization control circuit CON. Simultaneously, these outputs change the frequency of the voltage control oscillator VCO to an initially relatively low frequency then gradually back to its normal steady frequency. As a result of this operation, the stepping motor starts revolving to make the feedback input voltage of the operational amplifier OA 2  approach to the standard set point SP. When both voltages coincide, the forward and backward rotation signals both become logic 0, and the revolution of the stepping motor stops. In this balanced state, the energization current to motor M is interrupted. 
     FIG. 4 shows another embodiment of the circuit for driving the stepping motor in accordance with the present invention. This circuit is similar to that of FIG. 3 but is used advantageously where there is no need to provide the stepping motor with as large torque as the circuit in FIG. 3 does. As no large torque is required, the voltage control oscillator VCO becomes unnecessary and the oscillating output of the oscillator OSC 2  has a constant frequency. The negative power source is conveniently derived from clock pulse for the energization control circuit CON. 
     As described above, in conjunction with FIG. 2 and 3, the difference between the input signal SP and the feedback voltage established by the variable resistor VR 4  is compared by the operational amplifier OA 2 . When said difference is within the range of the predetermined allowable deviation, transmission of the forward or backward rotation signal to the energization control circuit for the stepping motor is ceased, and energization current supply to the stepping motor is interrupted. When said difference is out of said range, a rotation direction signal is transmitted to the energization control circuit.