July-August 2021

HYDRAULICS Eliminating resonance in stepper motor applications Stepper motors are a popular option in a host of positioning applications, as they can be simply driven, step-by-step, without the need for an encoder or additional device to provide position feedback information. Commutation from an external electronic driver moves the rotor from one stable position to the next, and as long as one phase is energised without switching to the next phase, the rotor holds a stable position. If we consider the operating mechanism more closely, however, we can see the potential for problems. If the rotor moves slightly ahead of the target position the motor will develop a negative torque to pull the rotor back to the target position. On the other hand, when the rotor has not yet reached the target position, a positive torque will draw the rotor towards it. In these conditions, oscillation can easily occur. What this means in practice is that if the rotor speed and/or the inertia from the load causes overshoot from the target position, the rotor will then start oscillating around the target position, with a natural frequency that is a factor of the holding torque, the number of poles and the total moment of inertia. The magnitude of the oscillation will reduce over time with the inherent damping of the system, but if the oscillation is not over before the next step is commanded there is a risk of resonance, with the mechanical system responding with oscillations of greater amplitude. The problems that can be caused by resonance should not be underestimated, potentially leading to a loss of steps, possible changes in direction of rotation and general erratic motion. Measures to prevent resonance Resonance typically occurs when the commutation frequency is close to the mechanical system’s natural frequency of vibration. It is not always possible to keep the commutation frequency away from the resonance frequency, and thus avoid it. It may be an option instead to shift the system’s natural frequency either up or down. This can be done by working on two of the parameters that Engineers are increasingly turning to stepper motors for a simple and cost-effective positioning solution. However, due to the design of these motors and the way they are driven, there is an inherent risk of resonance under certain operating conditions. This can manifest as vibrations, which impacts on the synchronisation between the command and the actual rotor position – ultimately affecting accuracy. Portescap application engineer, Daniel Muller, discusses the importance of preventative measures to avoid the resonance that can lead to these problems. Stepper motors are a popular option in a host of positioning applications, as they can be simply driven, step-by-step, without the need for an encoder or additional device to provide position feedback information. 38 HYDRAULICS & PNEUMATICS July/August 2021 www.hpmag.co.uk