Abstract:
A control system for a stepping motor comprises flowing an electric current to a plurality of phases in succession and driving the stepping motor, and flowing a loop current to a coil of the phase which is not concerned in the driving of the stepping motor when the deceleration area of the stepping motor is reached, thereby obtaining a braking force.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a control system for a stepping motor. 
     2. Description of the Prior Art 
     When a stepping motor shifts from its moving state to its stopped state, the mover of the stepping motor usually effects attenuation vibration near the objective stable point and becomes stationary. In order to shorten the time until the mover stops via this attenuation vibration, there has heretofore been proposed a system in which the kinetic energy of the mover when it has reached the stable point is brought close to 0 by manipulating the excitation phase or the excitation time and thereby the vibration is quickly brought to a close. 
     Such a conventional system, however, can hardly obtain the desired result when the load applied to the motor fluctuates and in addition, the mode in which the vibration comes to an end differs greatly each time depending on the irregularity of the torque of the motor itself, and this has led to a problem that where the motor is actually used, the time until the vibration comes to an end (the damping time) must unavoidably be set with a sufficient allowance. 
     So, where high-speed operation of the motor is required, it has been unavoidable in the conventional system to provide a position detector or the like and construct a close loop control system using the detection output thereof, and this in turn has led to the bulkiness and expensiveness of the control system including the stepping motor. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a control system for a stepping motor whereby in the deceleration time duration interval of the stepping motor, a loop current is flowed to a coil of a phase which is not concerned in driving, thereby obtaining a braking force. 
     It is another object of the present invention to provide a stepping motor driving apparatus in which in the deceleration time duration interval of the stepping motor, a closed loop is formed by a coil of a phase to which no electric current is flowed, whereby a braking force can be obtained. 
     It is still another object of the present invention to provide a stepping motor driving apparatus in which in the deceleration time duration interval of the stepping motor, a closed loop current path including a coil is formed by a switch, whereby a braking force can be obtained. 
     It is yet still another object of the present invention to provide a stepping motor driving apparatus in which in the deceleration time duration interval of the stepping motor, a closed loop is formed by a current controlling switch, whereby a braking force can be obtained. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a circuit diagram showing an embodiment of the present invention; 
     FIG. 2 is a timing chart showing an example of the operation timing of the signal of each portion shown in FIG. 1; 
     FIGS. 3A and 3B illustrate the driving states of stepping motors by the control system according to the present invention and the system according to the prior art; 
     FIG. 4 is a circuit diagram showing another embodiment of the present invention; and 
     FIG. 5 is a timing chart showing an example of the operation timing of the signal of each portion shown in FIG. 4. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The invention will hereinafter be described in detail with reference to the drawings. 
     Referring to FIG. 1 which shows an embodiment of the present invention, this embodiment exemplarily shows a case where a 4-phase stepping motor is bipolarly driven In FIG. 1, L1 and L2 within a broken line designate the windings (driving coils) of the stepping motor. S1-S4 denote phase signals. Correspondingly to the states of these signals, transistors Tr1-Tr8 are rendered conductive and phases I-IV are driven. D1-D12 designate diodes, and Tr9 and Tr10 denote transistors. Correspondingly to switch signals SW1 and SW2, the transistors Tr9 and Tr10 are respectively rendered conductive. 
     If, for example, the transistor Tr9 is rendered conductive by the switch signal SW1 being energized when only the phase II is driven by the phase signal S2, no electric current will flow when the mover is stopped, but when the mover is in motion, an electric current will flow through the route of Tr9→ ground→D6→L1→D9→Tr9 or the route of Tr9→ground→D5→L1→D10→Tr9 by a counter electromotive force produced in the coil L1. This electric current flows always in a direction to apply a brake to the mover. 
     A control circuit is comprised of a CPU and puts out signals S1-S4 for rendering transistors Tr5-Tr10 conductive or non-conductive and signals SW1 and SW2. 
     The specific operation of such a construction will now be described. 
     FIG. 2 is a timing chart showing an example of the operation timing of the signal of each portion shown in FIG. 1. Here, let it be assumed that up to time t o  is an acceleration time duration interval and the stepping motor is driven by 2-phase excitation and that the time after time t 0  is a deceleration interval. In the deceleration interval from after time t 0  to time t 1  close to time t 2  whereat the objective position is arrived at, 1-phase excitation is effected and the switch associated with the coil which is not driven then is closed. That is, for example, during phase I driving, the switch signal SW2 is switched on, and during phase II driving, the switch signal SW1 is switched on. These switch signals SW1 and SW2 may be produced during the execution of the control procedure of the control device of an instrument using the stepping motor or may be produced by a simple logic circuit using the stop signal of the motor and signals S1-S4. For example, provision may be made of a first OR gate for receiving the inputs of the signals S1 and S3, a second OR gate for receiving the inputs of the signals S2 and S3, a first AND gate for directing the output of the first OR gate and the stop signal, and a second AND gate for directing the output of the second OR gate and the stop signal, and the outputs of the first and second AND gates may be used as the switch signals SW1 and SW2, respectively. The above-described control is performed by a control circuit in accordance with the control program in a ROM. 
     According to such a construction, in the deceleration time duration interval (from t 0  to t 1 ), a loop current I 0  flows to either of the coil L1 concerned in phase I and phase III or the coil L2 concerned in phase II and phase IV, and a brake can continue to be applied to the mover (rotor) and therefore, the pulse of the deceleration time duration interval can be made shorter than in the prior art. Also, the loop current I 0  is great if the speed of rotation of the rotor is high, and is small if the speed of rotation of the rotor is low and therefore, the braking force by the loop current I 0  becomes substantially proportional to the speed of rotation. Thus, even in a case where the load applied to the motor fluctuates, the operation of the rotor is very stable, and the damping time (from t 2  to t 3 ) can also be made shorter than in the prior art system, and the damping of the system can be improved. 
     In the present embodiment, the state of stoppage is 2-phase excitation and therefore, the final pulse from time t 1  till time t 2  is switched off for both of switch signals SW1 and SW2 and drivers two phases, i.e., phase III and phase IV. 
     FIGS. 3A and 3B respectively show the relations between the time t and the drive frequency f of the stepping motors according to the present embodiment and the prior art. As is apparent from the comparison between these graphs, according to the present embodiment, the time required until the stoppage at the objective position is made shorter than in the prior art. 
     In the above-described embodiment, the transistors Tr9 and Tr10 are rendered conductive by the use of the switch signals SW1 and SW2, whereby an electric current flows to the idle coil which is not concerned in the phase drive, but this may be accomplished also by using the phase drive signal in the following manner. 
     FIG. 4 shows another embodiment of the present invention. In FIG. 4, only the construction of the coil L1 side is shown. G1 designates a NAND gate receiving as inputs the inverted signal of a phase drive signal S1 by an inverter G3 and a phase drive signal S3, and G2 denotes a NAND gate receiving as inputs the inverted signal of the phase drive signal S3 by an inverter G4 and the phase drive signal S1. The outputs of these gates are used for the switching of transistors Tr1 and Tr2, respectively. 
     Thus, when the coil L1 is not driven even if the signal S1 or S3 is energized, the transistors Tr1 and Tr2 are not rendered conductive by gates G1-G4, but an electric current generated by a counter electromotive force flows through the route of Tr5→ground→D6→coil L1→Tr5 or the route of Tr6→ground→D5→coil L1→Tr6. Although not shown in FIG. 4, the coil L2 side may also be of a similar construction. The operation timing of the signal of each portion in the present embodiment is such as shown in FIG. 5. 
     In the present embodiment, design is made such that from the point of time at which acceleration changed over to deceleration, a loop current begins to flow to the winding which is not concerned in driving, but alternatively, design may of course be made such that the loop current begins to flow during acceleration or after the lapse of a predetermined time after the shift to the deceleration time duration interval and also, the present invention is of course applicable to a case where the stepping motor is trapezoidally driven such as driven in the form of acceleration-equal speed-deceleration. 
     Further, the above-described two embodiments have been shown with respect to a case where during stoppage, the 4-phase stepping motor is driven in a mode in which it is 2-phase or 3-phase excited, but of course, the present invention can also be very effectively applied to a case where, for example, during stoppage, the 4-phase stepping motor is driven in a mode in which it is 1-phase excited or in other mode, or to other motors than the 4-phase stepping motor. 
     As is apparent from the foregoing description, according to the present invention, the deceleration time duration interval and damping time can be easily shortened with the stepping motor controlled by the open loop control and therefore, complicated closed loop control in which a position detector or the like is added becomes unnecessary and accordingly, compactness and inexpensiveness of a control system which can operate the stepping motor at a high speed can be realized.