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8 DRIVING MOTORS - DC & STEPPER 203

+Volts

A1

N

A1

S

+ Volts

Figure 8-5 Simplified 2-phase stepper motor – full-stepping sequence.

Table 8-1 Full-stepping sequence for a 2-phase motor.

Coil Contact Voltage

Smaller step angles can be achieved when a half-stepping drive sequence is used. This involves energising only one of the coil windings for every second step angle as shown in Figure 8-6 and Table 8-2.

204 8 DRIVING MOTORS - DC & STEPPER

1 2 3 4

Figure 8-6 Half-stepping sequence for a 2-phase motor.

Table 8-2 Half-stepping sequence for 2-phase motor.

Coil Contact Voltage

Stepper motors are categorised depending upon the way current is driven through their windings as will now be explained.

8 DRIVING MOTORS - DC & STEPPER 205

Bipolar Motor

This type of motor uses coil currents that reverse in direction throughout the stepping sequence, as shown in the preceding text. To achieve this reversal of current, bipolar voltages are applied to the coil windings. A bipolar power source with +ve, –ve and ground potentials can be used with four switching transistors to drive this type of motor. However, the usual means of implementing this form of drive is to use a single polarity power supply with eight switching transistors, configured using two separate H-bridge circuits as shown in Figure 8-7. Bipolar type motors are easily recognized since they have only four connection leads.

Figure 8-7 Bipolar drive using two H-bridge circuits.

Unipolar Motor

This type of motor uses coil winding currents which flow in one direction only. In order to obtain a reversal of magnetic field at each stator tooth, the coil is wound in two halves as shown in Figure 8-8. One half of the coil is wound clockwise around the stator tooth and the other half of the coil is wound anticlockwise around the stator tooth – known as a bifilar winding.

+ Volts

N

Figure 8-8 Unipolar motor – bifilar coil.

206 8 DRIVING MOTORS - DC & STEPPER

The wire connection halfway through the coil is connected to the positive power supply. Only one half of the coil is energised at a time. A north pole is produced when one half of the coil is energised by connecting its end to ground potential; a south pole is produced by energising just the other half of the coil by grounding its end. This is shown in Figure 8-9, Figure 8-10 and Table 8-3. Note that a north pole is produced when coil winding A1 or B1 are grounded; conversely grounding A2 or B2 results in a south pole.

+V +V

Figure 8-9 Unipolar coil drive.

Figure 8-10 Full-stepping sequence – unipolar motor.

Table 8-3 Full-stepping sequence for unipolar motor.

Coil Contact Voltage

1

2

3

4