When a step motor makes a move from one step to the next, the rotor doesn’t immediately stop. The rotor actually passes up its final position (overshoots), then goes past it in the opposite direction (undershoots), then moves back and forth until it finally comes to rest.

We call this “ringing,” and it occurs every time the motor takes a step. In most cases, the motor is commanded to move to the next step before it comes to a rest.

Unloaded, the motor exhibits a fair amount of ringing. This ringing translates into motor vibration. The motor will often stall if it is unloaded or under-loaded, because the vibration is high enough to cause the motor to lose synchronism. Loading the motor properly will dampen these vibrations. The load should require somewhere between 30% to 70% of the torque that the motor can produce, and the ratio of load inertia to rotor inertia should be between 1:1 and 10:1. For shorter, quicker moves, the ratio should be closer to 1:1 to 3:1.

A step motor will exhibit much stronger vibrations when the input pulse frequency matches the natural frequency of the motor. This phenomenon is called resonance. In resonance, the overshooting and undershooting become much greater, and the chance of missing steps is much higher. The resonance range may change slightly due to the damping effect of the load’s inertia.

Troubleshooting Vibration and Resonance

A two-phase step motor can only miss steps in multiples of four full steps (equivalent to one tooth pitch or pole pitch). If the number of missing steps is a multiple of four, vibration or overloading may be causing a loss of synchronism. If the number of missing steps is not a multiple of four, an electronics problem is most likely the issue. There are a number of ways to get around resonance. The easiest way is to avoid the resonant speed range altogether. The resonant frequency for a two-phase motor is around 200pps; motors can be started at speeds above the resonant range. Accelerating quickly through the range is recommended if the motor must be started at a speed below the resonance range.

Half stepping and microstepping are also effective means of reducing vibration. Both methods reduce the size of each motor step. When the motor step angle is made smaller, the motor will vibrate less. The motor does not have to travel as far for each step, and less energy will be wasted in overshooting and undershooting. Step motors react differently to different loads. Make sure that the motor is sized properly to the load.