1. Field of the Invention
The present invention relates to a method of controlling an overdrive type stepping motor and a drive circuit employing the same method.
2. Description of the Related Art
In the case of driving a stepping motor requiring high holding torque, a high voltage is applied to the phase windings of a stepping motor to reduce the rise time of an exciting or driving current supplied to the phase windings. In this case, the high voltage needs to be changed to a low voltage or cut off when the motor reaches a given position to prevent overheating of the stepping motor caused by the supply of overcurrent to the phase windings. Therefore, the driving current supplied to the phase windings must be suppressed.
The conventional method of controlling an overdrive type stepping motor will be described with reference to FIGS. 9 and 10. FIG. 9 is the drive circuit and FIG. 10 is a timing chart showing the operation of that circuit.
The drive circuit of FIG. 9 comprises a pulse generator 1 for generating a chain of pulses, an overdrive signal generator 2 for generating overdrive signals upon reception of the pulses, a distributing circuit 3 for producing exciting signals, transistors 4a to 4d for receiving phase exciting signals, a transistor 4e for receiving a power source voltage +E1 and phase windings 5a to 5d of a stepping motor.
The drive circuit starts operation when the pulse generator 1 produces the pulse signal (a) as illustrated in FIG. 10. An overdrive signal (b) is produced by the overdrive signal generator 2 upon reception of the pulse signal (a). The overdrive signal (b) goes high, thereby turning on the transistor, by way of a transistor 4f for a given time so that the power supply voltage +E1 is supplied to the phase windings 5a to 5d.
The distributing circuit 3 selectively produces exciting signals (c) to (f) upon reception of the pulse signal (a), thereby turning on or off the transistors 4a to 4d in a given sequence so that the driving current is supplied to the phase windings 5a to 5d a discrete combination of two phases.
If the stepping motor is the one-two alternating phase exciting type, in which the exciting or driving current is supplied alternately to the one phase winding or the two phase windings of the stepping motor, the exciting signals (c) to (f) as illustrated in FIG. 10 are supplied from the distributing circuit 3 to the transistors 4a to 4d so that the driving currents are supplied to the phase windings 5a to 5d in the directions of the arrows denoted at i1 to i4 in FIG. 9, thereby rotating the stepping motor.
A cyclic change of the pulse signal (a) produced by the pulse generator 1 influences the speed of revolution of the stepping motor. A change of the pulse width of the overdrive signal (b) produced by the overdrive signal generator 2 influences the magnitude of the driving current.
However, when the stepping motor is used to its performance limit in accordance with the above-mentioned stepping motor controlling method, the phase winding current rises up sharply at the trailing edge of the excitation due to a magnetic saturation, as shown in the broken lines in FIG. 10, thus resulting in large amount of heat from the stepping motor. If, in order to overcome this problem, the width of the overdrive signal (b), is shortened enough current cannot be supplied at the rising edge of the phase excitation. Thus, the motor torque is decreased.
If the source voltage +E1 is high and unstable, the driving current is excessively supplied to the phase windings, thereby heating the motor. If the power source voltage +E1 is low, the driving current does not sufficiently to supply the phase windings, thereby reducing the torque. To cope with this problem, a chopper type constant current drive circuit may be employed, but it will cause high manufacturing cost.