Specifications

Appendix A
Resonance, Ringing & Damping—
Discussion & Theory
In this appendix we will discuss resonance and ringing in step motors. This information will
help you configure the ZETA6104’s damping features—anti-resonance, active damping, and
electronic viscosity.
All step motors have natural resonant frequencies, due to the nature of their mechanical
construction. Internally, the rotor acts very similarly to a mass suspended on a spring—it can
oscillate about its commanded position. Externally, the machine, mounting structure, and
drive electronics can also be resonant, and interact with the motor. During a move, two types
of problems can arise from these causes: resonance and ringing transients.
Resonance (Steady
State Response)
Resonance is a steady state phenomenon—it occurs when the motor’s natural resonant
frequencies are excited at particular velocities. It is not caused by transient commands that we
give the motor. If you slowly increase your motor’s speed from zero to 20 rps, for example,
you may notice “rough” spots at certain speeds. The roughness is resonance; it is depicted in
the next drawing.
Time
Velocity
Commanded
Resonances
Actual
Instead of moving at the commanded velocity, the motor is oscillating between speeds faster
and slower than commanded. This causes error in rotor position.
Resonance points can differ in intensity. The drawing shows a typical case—as motor speed
increases, resonances of varying levels occur. Usually, the motor can accelerate through the
resonance point, and run smoothly at a higher speed. However, if the resonance is extreme,
the rotor can be so far out of position that it causes the motor to stall.
Resonance is affected by the load. Some loads are resonant, and can make motor resonance
worse. Other loads can damp motor resonance. To solve resonance problems, system
designers will sometimes attach a damping load, such as an inertial damper, to the back of the
motor. However, such a load has the unwanted effect of decreasing overall performance, and
increasing system cost.
The ZETA6104 has internal electronics that can damp resonance, and increase system
performance. No external devices are necessary.