1. Field of Use
The present invention relates to stepping motors and more particularly to drive circuits for stepping motors.
2. Prior Art
As known, a step motor comprises a rotor which consists of a permanent magnet having toothed pole pieces and a toothed stator having phase windings which, when energized, induce magnetic fields in the stator. In general, each phase winding is connected at one end to a D.C. voltage source and at the other end to a switching device (generally a power switch transistor. Such device when switched on allows an energizing current to flow through the phase winding. When the phase windings of a stepping motor are energized according to a suitable sequence, the magnetic fields induced in the stator interact with the rotor causing its rotation. In particular, the rotor is fed with an angle corresponding to a step for each current switching occurring in the phase windings.
At present, in order to improve the stepping motor performances, so-called current chopping control systems are used. Such "chopping" systems maintain the energization current in the phase windings to pre-established values during the entire time that the phase windings are energized. The "chopping" systems intermittently feed the phase windings. In other words, they switch off the feeding current when it reaches a prefixed value. This allows the established current to flow through a recirculating path and to decay with a time constant determined by the electrical characteristics of the recirculating path. The feeding current is again switched on after a predetermined time interval or when the circulating current is decayed to a second prefixed value, and so on for the entire energization time of the phases. Examples of such systems are given by U.S. Pat. Nos. 4,107,593 and 3,812,413.
A limit to the chopping frequency establishing when the energized windings are connected and disconnected from the voltage source, is determined by operational limitations of the power transistors used as switch devices. In fact, it has been found that such transistors which are controlled to work in the extreme conditions of cutoff and saturation, do not switch immediately from a working state to another but rather respond to the applied control signals after some delay time. In particular, the most critical situation occurs when a transistor switches from the saturation condition (ON) to the cutoff condition (OFF).
In such case, the saturation current nearly falls to zero after a relatively long delay time which for the most part is determined by so-called storage time. This is the time required to remove the minority carriers from the base of the transistor. Thus, during the time interval proportional to the storage time, the transistor operates in a condition of high collector-emitter voltage drop at the same time a high current is flowing therethrough. During the period of switching from ON to OFF, a considerable peak of power is required to be absorbed by a switching transistor. At high switching frequencies, for example, those above the acoustic frequencies, a current chopping control system, besides requiring the use of switching transistors with high power dissipation (therefore very expensive), also produces a degradation in stepping motor efficiency.
One solution to the problem is to remove the main cause of the switching losses, that is, remove or at least reduce the storage time of a power switch transistor during its ON to OFF switching.
A method known in the art for reducing the storage time of a transistor consists of applying a reverse voltage to the base of the switch transistor so as to speed up the removal of the minority carriers from its base. Besides presupposing the use of both positive and negative voltages in the stepping motor drive circuit with a consequent increase in the complexity of the power supply, the method also requires that high level control signals be applied to the transistors. This results in more expensive drive logic circuits.