(1) Field of the Invention
The present invention relates to a driving circuit for a stepping motor being constituted to perform bipolar driving.
(2) Description of the Related Art
A driving circuit of a bipolar system has been used as a driving circuit for a stepping motor.
A driving circuit of a bipolar system is shown in FIG. 9. As shown in the figure the driving circuit is constituted with a first and a second bridge circuits, 5 and 6, connected between a DC power source 1 and coils, 3 and 4, of a stepping motor 2. The first bridge circuit 5 is formed with a first, a second, a third and a fourth switching elements, Q1, Q2, Q3 and Q4, being composed of transistors, and the second bridge circuit 6 is formed with a fifth, a sixth, a seventh and an eighth switching elements, Q5, Q6, Q7 and Q8 being composed of transistors. In order to feedback the counter electromotive force (accumulated energy) induced in the coils, 3 and 4, to the power source, a first to an eighth diodes, D1 to D8 are connected in antiparallel to the first to the eighth switching elements, Q1 to Q8, respectively.
An end of the first coil 3 is connected to the middle point between the first and the second switching elements, Q1 and Q2, and the other end of the first coil 3 is connected to the middle point between the third and the fourth switching elements, Q3 and Q4. An end of the second coil 4 is connected to the middle point between the fifth and the sixth switching elements, Q5 and Q6, and the other end of the second coil 4 is connected to the middle point between the seventh and the eighth switching elements, Q7 and Q8.
An end of the DC power source 1 is connected to the junction point 7 (a terminal on the power source side) of the first and the third switching elements, Q1 and Q3, of the first bridge circuit 5, and to the junction point 8 (a terminal on the power source side) of the fifth and the seventh switching elements, Q5 and Q7, of the second bridge circuit 6, and the other end of the DC power source 1 is connected to the junction (ground) of the second and the fourth switching elements, Q2 and Q4, of the first bridge circuit 5, and to the junction point (ground) of the sixth and the eighth switching elements, Q6 and Q8, of the second bridge circuit 6.
An exciting signal generator 9 is a well-known circuit which transmits exciting signals shown in FIGS. 11(A), 11(B), 11(C) and 11(D), to lines, 10, 11, 12 and 13, to drive a stepping motor 2 with a predetermined exciting system (for example, a bipolar exciting system). The exciting signal line 10 is connected to the control terminals (bases) of the first and the fourth switching elements, Q1 and Q4, the exciting signal line 11 is connected to the control terminals of the second and the third switching elements, Q2 and Q3, the exciting line 12 is connected to the control terminals of the fifth and the eighth switching elements, Q5 and Q8, and the exciting signal line 13 is connected to the control terminals of the sixth and the seventh switching elements, Q6 and Q7. A resistor is connected to each of the base lines of switching elements, Q1 to Q8.
The principle of the constitution of the stepping motor 2 is as shown in FIG. 10; for example, it comprises a stator core 18 having a first, a second, a third and a fourth magnetic poles, 14, 15, 16 and 17 being disposed at intervals of 90 degrees and a rotor 19 being composed of a permanent magnet having an N pole and an S pole at intervals of 180 degrees. The first coil 3 is wound on the first and the third magnetic poles, 14 and 16, and the second coil 4 is wound on the second and the fourth magnetic poles, 15 and 17.
When the stepping motor 2 is driven by a bipolar two-phase exciting system, the first to the eighth switching elements, Q1 to Q8, are ON/OFF controlled by the exciting signal shown in FIGS. 11(A), 11(B), 11(C) and 11(D). In the period of time, t1 to t3, shown in FIG. 11(A), the first and the fourth switching elements, Q1 and Q4, are made ON simultaneously. Thereby, a current in a first direction is made to flow through the first coil 3 in a circuit constituted with the DC power source 1, the first switching element Q1, the first coil 3 and the fourth switching element Q4.
In the period of time, t3 to t5, shown in FIG. 11(B), the second and the third switching elements, Q2 and Q3, are made ON, and a current in a second direction is made to flow through the first coil 3 in a circuit constituted with the DC power source 1, the third switching element Q3, the first coil 3 and the second switching element Q2.
In the period of time, t2 to t4, shown in FIG. 11(C), the fifth and the eighth switching elements, Q5 and Q8, are made ON, and a current in the first direction is made to flow through the second coil 4 in a circuit constituted with the DC power source 1, the fifth switching element Q5, the second coil 4 and the eighth switching element Q6.
In the period of time, t4 to t6, shown in FIG. 11(D), the sixth and the seventh switching elements, Q6 and Q7, are made ON, and a current in the second direction is made to flow in the second coil 4 in a circuit constituted with the DC power source 1, the seventh switching element Q7, the second coil 4 and the sixth switching element Q6.
When any one of the switching elements, Q1 to Q8, is changed from ON to OFF, a counter electromotive force is induced in the first coil 3 or the second coil 4. When the second and the third switching elements, Q2 and Q3, are changed from ON to OFF, and the first and the fourth switching elements, Q1 and Q4, are changed from OFF to ON, based on the counter electromotive force induced in the first coil 3, a current is made to flow in a circuit constituted with the first coil 3, the first diode D1, the DC power source 1 and the fourth diode D4, and the accumulated energy in the first coil 3 is fed back to the DC power source 1. At this time, since the first and the fourth diodes, D1 and D4, are made ON, the application of a high voltage based on the counter electromotive force to the first and the fourth switching elements, Q1 and Q4, is prevented; thus the first and the fourth switching elements, Q1 and Q4, can be protected.
When the switching elements, Q1, and Q4 to Q8, other than the above-mentioned second and the third switching elements, Q2 and Q3, are changed from ON to OFF, a similar action to the above is performed.
When a current based on the counter electromotive force is flowing through the first and the fourth diodes, D1 and D4, for example, even if exciting signals are given to the first and the fourth switching elements Q1 and Q4, these switching elements, Q1 and Q4, are not made ON. Therefore, when the period of time in which a current based on the counter electromotive force flows (feedback current period) is long, since an actual exciting period is disposed after the feedback current period, it becomes necessary to lengthen the period of the exciting signal; thereby, there occurs a problem that the stepping motor 2 cannot be operated at a high rotational speed.
A method described in the following is well-known to the public as a method for solving the problem: in the method, in order to make the feedback current period short and to increase the pullout torque of the stepping motor 2, as shown in FIG. 12, a first and a second capacitors, 20 and 21, are connected in parallel to respective bridge circuits, 5 and 6.
In FIG. 12, the first capacitor 20 is connected between the junction point 7 of the first bridge circuit 5 and the ground, and a diode 22 for preventing feedback is connected between the junction point 7 and the DC power source 1. The second capacitor 21 is connected between the junction point 8 of the second bridge circuit 6 and the ground, and a diode 23 for preventing feedback is connected between the junction point 8 and the DC power source 1. Except the above-mentioned points, the circuit is constituted in the same manner as that shown in FIG. 9.
In the circuit shown in FIG. 12, owing to the capacitors, 20 and 21, for example, as shown in FIGS. 13(A) and 13(B), at the time of t1, the exciting signal is given to the first and the fourth switching elements, Q1 and Q4, and when the second and the third switching elements, Q2 and Q3, are made OFF, a current based on the counter electromotive force induced in the first coil 3 is made to flow through a circuit constituted with the first coil 3, the first diode D1, the first capacitor 20 and the fourth diode D4. Thereby, the accumulated energy in the first coil 3 is absorbed by the capacitor 20. The period of time in which a current flows based on the counter electromotive force can be adjusted by the capacity of the capacitor 20, so that it is made possible to operate the stepping motor 2 at a high rotational speed by making the feedback period shorter than that in the circuit described in FIG. 9. The energy accumulated in the capacitor 20 functions as a power source to make a current flow in the first direction in the first coil 3 when the first and the fourth switching elements, Q1 and Q4, are made ON; therefore, it is made possible to increase the pullout torque of the stepping motor 2.
In the circuit shown in FIG. 12, however, when the charged voltage of the capacitors, 20 and 21, is made higher than the breakdown voltage of the switching elements, Q1 to Q8, by the dispersion of circuit constants such as the first and the second coils, 3 and 4, or the capacitors, 20 and 21, the breakdown of the switching elements may occur in the OFF period and in some case they may be destroyed. In FIG. 13(C), the charged voltage Vc of the capacitor 20 is shown, and at a time t1, when the switching elements, Q2 and Q3, are turned OFF (change from ON to OFF), it is charged up to a voltage Vsp which is higher than the source voltage Vs, and if the Vsp becomes higher than the resisting voltage of the second switching element Q2 or of the third switching element Q3, there used to be a fear that the breakdown of a switching element can occur.