Stepmotors convert electrical pulses into incremental mechanical movements. Each “step” is a set angle, providing precise linear positioning downstream. Sometimes, stepmotors are unable to step when pulsed; they may be overloaded or the pulse frequency may be too high. Commanded steps not taken are “lost.”
Lost steps are often thought to be caused by a motor that's too small or a load that's too great. But back-driving and external commutation errors may also be at fault.
Back-driving, an unwanted reversal of the shaft, alters the alignment of stator and rotor teeth, taking the motor out of its optimal torque position. Upon starting, the first step in the pulse sequence merely re-aligns the stator and rotor and is effectively lost because there is no advance from what was thought to be the starting position. A way to avoid this is pulsing the motor into the last executed step position before the commanded motion (commutation) begins.
Q: How can test results about lost steps be analyzed?
A: Analysis depends on the type of motion profile generated. For a start-stop test, the controller applies a fixed frequency to the driver while the motor is connected to the load. A motor that doesn't start may be carrying a heavier-than-normal load and must be replaced; otherwise, the frequency may be set too high and should be reduced. In general, designers should size stepmotors for the highest expected torque/speed point with an additional 30% safety factor from the published torque-speed curve.
For a trapezoidal acceleration profile, three failure types exist.
A motor that doesn't start is diagnosed as previously discussed.
If the motor doesn't finish its acceleration ramp, the cause may be resonance, supply voltage, or speed. To avoid resonance, increase acceleration, choose a start-stop frequency above the resonance point, or employ half or microsteps. Half and microsteps increase commutation frequency and achieve the same top speed as a full step. For a low supply voltage, increase voltage, test a lower impedance motor, or use a current-mode driver when implementing a voltage driver. The solution for excessive speed is simple: reduce it.
A motor that completes the acceleration ramp but stalls directly after running at constant speed may be suffering from excessive torque, causing vibrations that it cannot dampen. One solution is to reduce jerk, either by selecting a slower acceleration rate or using two different acceleration levels: higher at the start and lower during top speed. Another option is to increase torque and system stiffness upon reaching peak speed. Increasing current for two steps before and four or five steps after the switch accomplishes this and limits the vibration angle.
Q: What is back-driving?
A: Even though stepmotors include gears to combat back-driving, they tend to wind up while moving and conversely return energy to the motor when it begins to coast. If back-driving exceeds one step, the motor lacks sufficient torque to complete its first step and either doesn't start or starts only after missing four full steps (one commutation cycle). One solution is programming the commutation to apply the same motor current and polarity every time the motor currents switch off.
Q: How are external commutation errors addressed?
A: When fewer than four steps per revolution are lost, users must check the commutation. If power is lost, the four control bits (Phase 1, Phase 2, Enable 1, Enable 2) lose the counter status required for an uninterrupted stepping sequence. If the counter can't find its position at power-up, the motor executes uncontrolled steps due to the difference between actual (rotor) and perceived (counter) positions. To solve this, users can record the four-bit word (greater for more sophisticated drives) before power-off to reload from memory upon initialization.