Stepper motors are known to generate audible noise as they operate, which is undesirable for many applications. What are the root causes of the audible noise?
The root causes of the stepper motor audible noise could come from mechanical factor, magnetic factor or electrical factor.
Mechanical noise is caused by the physical component’s vibration in the structure of the stepper motor. Common examples contributing to noise include unsecured mounting structures, bent shafts, and loose bearings. Other mechanical noise factors include motor housing, balance of rotor, and bearing choice. All of these examples cause unnecessary vibrations and make audible noise.
Magnetic noise is caused by the magnetic materials expanded or contracted in response to a magnetic field. In a stepper motor, magnetostriction deforms the iron and pulls the rotor and stator teeth toward each other in the air gap, causing audible noise.
Electrical noise is caused by the winding current frequency components in 20Hz to 20kHz audible range. There are three reasons could affect the winding current frequency components: micro-stepping setting, step frequency setting, PWM frequency setting and current deviation from the current reference setting.
Mechanical and magnetic noise is related to the motor design. Electrical noise could be related to the motor driver device. This [FAQ} is mainly focus on the electrical noise.
When the driver commands a new step, similar to a pulse response, the rotor overshoots and oscillates around the next position, leading to mechanical vibrations and noise. The oscillation frequency is the motor system resonant frequency. To reduce these oscillations, modern stepper drivers employ micro-stepping to divide one full step into smaller micro-stepping. As a result, the rotor is now stepped in much smaller angles; Less over- and under-shoots which can reduce the audible noise.
Step Frequency Setting
If the step frequency is between 20Hz and 20 kHz, then the step frequency and all the higher harmonics will appear in the audible range. By increasing the step frequency beyond the audible range, the audio nose is reduced and at the same time overall smoother rotation is achieved. A higher levels of micro-stepping setting allows a higher step frequency and reduce noise for a same motor speed.
PWM frequency and Current Ripple
If the PWM frequency falls within the audible band, the noise associated with PWM frequency will be audible. Increasing the switching frequency is the first method to reduce audible noise. To reduce the ripple and therefore audible noise, a stepper motor should operate with a minimized ripple current.
Comparing with Mixed decay or other decay modes, the slow decay can have a smaller ripple current control which are shown in the top of this slide. The smart turn ripple control algorithm uses slow decay for current regulation, but keeps fixed ripple current instead of a fixed PWM off time as other decay modes’.
Decay Mode Setting
The smoothest and quietest operation of a stepper motor happens when perfect sinusoidal current waveforms are applied to the windings. Any ripple in the current waveform is a deviation from the desired shape and causes an uneven torque in the motor, which manifests as vibration and noise during operation.
However, due to back-emf, especially at high speeds and on decreasing steps, a fixed PWM off time slow decay cannot apply enough voltage to the winding terminal to bring the output current back to ideal value. The smart tune dynamic decay or ripple control decay attempts to operate mostly in slow decay with inserting periods of fast decay or extending the PWM off time to maintain the ideal waveform if current runs away. Therefore, a smart tune decay mode which can follow the ideal waveform at all conditions is the ideal candidate for reducing audible noise from stepper motors.
Zero-cross Current Error
To avoid the high side FET turn-on switching noise coupling to the current sense circuit, stepper motor drivers start to sense the output current after a blanking time. A high current-sense blanking time could cause the average current for the step to be significantly higher than the intended value near the zero-cross section. Or, a too high PWM off time could cause the average current to be significantly lower than the intended value near the zero-cross section. This distortion is to create unsmooth current waveform - which leads to wobbling, vibrations and audible noise.
TI devices can support down to 1usec current sense blanking time and different PWM off time option to give respectively smoothness current around zero-cross.