Strategies To Avoid Stepper Motor Resonance Problems
When an application requires a reliable, low-cost motor with simple operation, stepper motors can be hard to beat. However, the same straightforward design and driving method that make stepper motors popular can lead to resonance problems in certain conditions.
When an application requires a reliable, low-cost motor with simple operation, stepper motors can be hard to beat. They can be driven step by step without needing an encoder or Hall sensor for rotor position feedback.However, the same straightforward design and driving method that make stepper motors popular can lead to resonance problems in certain conditions. This article will review different ways to avoid resonance issues and ensure trouble-free motion.
Stepper motor phases are commutated sequentially by an external electronic driver that subsequently moves the rotor—which often carries a permanent magnet—from one stable position to the next. An appropriate motor will provide a sufficient torque to move the rotor and load to the next step after each commutation. If the torque is not sufficient or if the speed is too high, the synchronization between the driver and the actual rotor position can be lost.
At each step, the rotor tends to align its poles with the stator's poles. As long as one phase is energized continuously—without switching to the next phase—the rotor holds a stable position.
Avoid natural frequencyResonance typically occurs when the commutation frequency is close to the mechanical system's natural vibration frequency. Accordingly, the most basic way to prevent the occurrence of resonance is to keep the commutation frequency away from the system's natural frequency. Working with a different commutation frequency can require other changes in the application in order to keep the same speed, but such changes are not always possible. Shift natural frequencyInstead of changing the commutation frequency, you can shift the natural frequency higher or lower, based on your constraints or challenges, in order to prevent the commutation frequency from matching it. This can be done by working on the two factors that influence the natural frequency: the holding torque and the total inertia in the system.
Prevent resonance with microsteppingThe higher the energy brought into the mechanical system, the higher the risk to trigger a resonance phenomenon. To prevent this, microstepping can be a good alternative to driving a stepper motor with full steps. Each microstep has a smaller step angle and requires less energy to move from one stable position to the next. Because the target position overshoot is smaller along with the magnitude of the oscillation, microstepping is often an effective way to avoid resonance. In addition, microsteps generally offer lower noise, less vibration and smoother operation. Damping via frictionFriction provides a braking torque that is opposed to the instant direction of rotation. This torque is constant, regardless of motor speed. While it helps dampen the oscillation and prevents resonance, keep in mind that friction also adds to the load applied on the motor at any speed. Therefore, it is important to ensure that the motor provides sufficient performance when adding friction to prevent resonance. Viscous friction also provides a braking torque, but its magnitude depends on the motor speed. The higher the speed, the stronger the viscous damping. For this reason, viscous damping provides strong braking at the beginning while the speed and oscillation amplitude are greater and only very light braking once the oscillation is smaller—unlike dry friction which provides the same braking magnitude even at very low speed. That makes viscous friction desirable for oscillation damping within a very short time and without adding too much load onto the motor. Different phenomena can bring viscous friction into a system:
While stepper motors offer easy and cost-effective positioning, their step-by-step, sequential operation can lead to resonance issues under certain conditions. Sometimes the resonance can be solved by acting on a single cause. But, depending on the motor technology and design, there may be additional frequency ranges such as mid-frequency resonance that can trigger resonance, apart from natural oscillation frequency. Motor providers can help you determine the frequency ranges that are likely to trigger resonance and offer solutions to prevent the issues from occurring. For more information about Portescap stepper motors, visit our product page.
Daniel Muller is an application engineer at Portescap.
Check out our free e-newsletters to read more great articles..Avoid natural frequency Shift natural frequency Holding torque: Inertia. Prevent resonance with microstepping Damping via friction Eddy currents. Back-EMF. Electronic damping. An external mechanical damper.