【文章內(nèi)容簡介】 s just vectors 1,3, 5 and 7 in sequence. Clearly, the direction of rotation can be reversed by making the energisation sequence 7, 5, 3, 1 etc. A superior excitation scheme makes use of positions 2, 4, 6 and 8, where both phases are always excited. This provides improved torque stiffness at each holding position as the stiffness (given by the gradient of the torque/position characteristic) is least in the direction of the reference axes and greater elsewhere. Additionally, since both windings are now involved in torque production, the total available torque for any given winding current is increased by a factor of .Both of the previously described schemes generate four discrete positions per electrical cycle, known as ‘full step’ mode. Given that, with 50 rotor teeth, there are 50 cycles per revolution, either of these simple schemes can position the machine rotor in any one of 200 unique positions. The discrete positioning characteristic of the hybrid stepping motor makes it ideal for use in digitally controlled motion applications. Higher positional resolution can be achieved by using all eight positions, known as the ‘half step’ mode of operation. An extension of this, known as ‘microstepping’ creates many more unique flux directions by using a greater range of different phase current binations.Power converters for hybrid stepping drivesSince hybrid stepping motors generate torque using a magnetically polarised rotor, the polarity flux in the machine is important, which results in the requirement for bipolar stator flux excitation.One method of providing bidirectional flux is to have bifilar wound phase coils: a technique frequently applied to small stepping motors where drive simplicity is required. Such motors are frequently known as ‘unipolar’, since the current in any particular winding only needs to flow in one direction. Fig. 10 shows the typical power circuitry used in a small drive employing a bifilar wound motor.The circuit of Fig. 10 achieves full motor control with just four power switches, all of which have a mon connection at OV Several semiconductor manufacturers produce integrated circuits containing four switches and the necessary sequencing logic to directly drive a small unipolar stepping motor under the control of digital inputs for direction and step increment.The circuit of Fig. 10 has a major drawback in that it relies on the resistance of the phase windings to dissipate energy and hence to allow the current in a winding to fall to zero when it is switched off. The circuit of Fig. 10 is therefore limited to motors with high resistance windings. A simple improvement to the circuit is to add a power Zener diode, as shown in Fig. 11.The addition of the Zener diode allows a negative voltage to appear across the phase coil at turn off, which rapidly reduces the current to zero. Energy is dissipated in the Zener diode as well as the phase resistance while this happens. The choice of Zener voltage is a promise, as a large voltage is required to maximise the rate of change in current, but the peak voltage appearing at the switches is the supply voltage plus the Zener voltage. The Zener voltage must therefore be chosen so as not to require excessive voltage ratings of the switching devices.The primary disadvantage of the bifilarwound unipolar excitation scheme is that, in each phase, only half the total winding area can be used to carry current at any one time. This results in higher resistive losses in the machine which is particularly serious in larger machines with high power outputs. In the larger drives, an increase in the plexity of the power electronics can be justified by improved performance of the drive. In these cases, each phase has just one winding which then requires bipolar excitation. The circuit of Fig. 12 is monly used to drive such motors.The circuit of Fig. 12, known as an ‘Hbridge’ due to the appearance of the schematic layout, requires eight power switching devices to control the bipolar energisation of two motor phases. With this arrangement, care must be taken in the power switch control to avoid ‘shootthrough where two switches in series across the DC power supply conduct simultaneously with destructive results.It was shown in the preceding analysis of motor operation that torque production in a stepping motor is due to a flow of current in the stator windings. The action of the power converters presented here is to impose a particular voltage .across the phase winding rather than a particular current through it.In a small stepping motor, the relatively high resistance of the phase windings naturally limits the current when voltage is applied. Sometimes each phase winding has an added series resistor, known as a ‘forcing resistor’, which makes the constant voltage output of the power converter more like a constant current source.Whilst resistance is a practical and costeffective way to limit current in a small drive, the inefficiencies of this are intolerable in a large drive. In these cases, a current sensor can provide feedback to a control system which regulates current to a preset level. This is achieved by using the power converter repetitively to switch. the voltage supply to the motor phase on and off. The ratio of on time to off time is varied according to the feedback level such that the average current level in the winding is equal to the control system’s preset level. This method of current control, known as switching regulation or pulse width modulation, can achieve very high efficiencies and is ubiquitous in highpower stepping drives.Further developmentsThe production of rotation by incremental steps is well established in mercially available stepping drives. This approach in either full or half step forms can be achieved with simple control circuitry, but has some undesirable features. Operating the motor in discrete steps involves accelerating the rotor from one position of rest, then allowing