【正文】 . In other words, alternate steps will be strong and weak. This does not represent a major deterrent to motor performance—the available torque is obviously limited by the weaker step, but there will be a significant improvement in lowspeed smoothness over the fullstep mode.Clearly, we would like to produce approximately equal torque on every step, and this torque should be at the level of the stronger step. We can achieve this by using a higher current level when there is only one winding energized. This does not over dissipate the motor because the manufacturer’s current rating assumes two phases to be energized the current rating is based on the allowable case temperature). With only one phase energized, the same total power will be dissipated if the current is increased by 40%. Using this higher current in the onephaseon state produces approximately equal torque on alternate steps (see Fig. 5).Fig. 3 Full step currentFig. 4 Half step current Half step current, profiledWe have seen that energizing both phases with equal currents produces an intermediate step position halfway between the onephaseone positions. If the two phase currents are unequal, the rotor position will be shifted towards the stronger pole. This effect is utilized in the micro stepping drive, which subdivides the basic motor step by proportioning the current in the two windings. In this way, the step size is reduced and the lowspeed smoothness is dramatically improved. Highresolution micro step drives divide the full motor step into as many as 500 micro steps, giving 100,000 steps per revolution. In this situation, the current pattern in the windings closely resembles two sine waves with a 90176。 phase shift between them (see Fig. 6). The motor is now being driven very much as though it is a conventional AC synchronous motor. In fact, the stepper motor can be driven in this way from a 60 HzUS (50HzEurope) sine wave source by including a capacitor in series with one phase. It will rotate at 72 rpm.Fig. 6 Phase currents in micro step modeStandard 200Step Hybrid MotorThe standard stepper motor operates in the same way as our simple model, but has a greater number of teeth on the rotor and stator, giving a smaller basic step size. The rotor is in two sections as before, but has 50 teeth on each section. The halftooth displacement between the two sections is retained. The stator has 8 poles each with 5 teeth, making a total of 40 teeth (see Fig. 7). 200step hybrid motorIf we imagine that a tooth is placed in each of the gaps between the stator poles, there would be a total of 48 teeth, two less than the number of rotor teeth. So if rotor and stator teeth are aligned at 12 o’clock, they will also be aligned at 6 o’clock. At 3 o’clock and 9 o’clock the teeth will be misaligned. However, due to the displacement between the sets of rotor teeth, alignment will occur at 3 o’clock and 9 o’clock at the other end of the rotor.The windings are arranged in sets of four, and wound such that diametricallyopposite poles are the same. So referring to Fig. 7, the north poles at 12 and 6 o’clock attract the southpole teeth at the front of the rotor。 the south poles at 3 and 9 o’clock attract the northpole teeth at the back. By switching current to the second set of coils, the stator field pattern rotates through 45176。. However, to align with this new field, the rotor only has to turn through 176。. This is equivalent to one quarter of a tooth pitch on the rotor, giving 200 full steps per revolution.Note that there are as many detent positions as there are full steps per rev, normally 200. The detent positions correspond with rotor teeth being fully aligned with stator teeth. When power is applied to a stepper drive, it is usual for it to energize in the “zero phase” state in which there is current in both sets of windings. The resulting rotor position does not correspond with a natural detent position, so an unloaded motor will always move by at least one half steps at poweron. Of course, if the system was turned off other than in the zero phase state, or the motor is moved in the meantime, a greater movement may be seen at powerup.Another point to remember is that for a given current pattern in the windings, there are as many stable positions as there are rotor teeth (50 for a 200step motor). If a motor is desynchronized, the resulting positional error will always be a whole number of rotor teeth or a multiple of 176。. A motor cannot “miss” individual steps – position errors of one or two steps must be due to noise, spurious step pulses or a controller fault.Fig. 8 Digital servo driveDigital Servo Drive Operation shows the ponents of a digital drive for a servo motor. All the main control functions are carried out by the microprocessor, which drives a DtoA converter to produce an analog torque demand signal. From this point on, the drive is very much like an analog servo amplifier.Feedback information is derived from an encoder attached to the motor shaft. The encoder generates a pulse stream from which the processor can determine the distance traveled, and by calculating the pulse frequency it is possible to measure velocity.The digital drive performs the same operations as its analog counterpart, but does so by solving a series of equations. The microprocessor is programmed with a mathematical model (or “algorithm”) of the equivalent analog system. This model predicts the behavior of the system. It also takes into account additional information like the output velocity, the rate of change of the input and the various tuning settings.To solve all the equations takes a finite amount of time, even with a fast processor – this time is typically between 100ms and 2ms. During this time, the torque demand must remain constant at its previouslycalculated value and there will be no response to a change at the input or output. This “update time” therefore bees a critical factor in the