Discrete, step-by-step movement enables stepper motors to have precise positioning capabilities, which may also lead to some undesirable performance characteristics, namely vibration and audible noise due to resonance.
Due to the inertia of the rotor, the stepping motor will naturally show a small vibration in each step, which will cause the motor to slightly overshoot (or undershoot in some cases) stepping position and oscillating until the correct step is reached. The timing "stable". If the frequency of these oscillations matches the natural frequency of the motor, resonance will occur, resulting in audible noise, vibration, and in extreme cases, loss of step or stall.
Although manufacturers use a variety of design techniques to reduce stepper motor resonance (such as special winding configurations or low inertia rotor design), users or OEMs can still take some measures to reduce stepper motor system resonance.
The resonant frequency (ω) of a stepping motor is proportional to the square root of the torque stiffness (K) of the motor divided by the inertia (J). Changing any parameter will change the resonant frequency of the motor.
Reduce the step size
The most widely recommended method to reduce the resonance of a stepper motor is to reduce the step size of the motor. This can be done by microstepping or using a 5-phase stepper motor.
Microstepping is a control method of a stepper motor that electronically divides each step into smaller increments. When the step size is small, the build-up and attenuation of current in each winding is more gentle, and the torque change between steps is smaller. This means that the position overshoot is less extreme, the settling time is shorter, and vibration and noise are greatly reduced.
Similarly, a five-phase stepper motor reduces the step pitch of the motor by reducing the offset between the stator and rotor teeth to 1/10 of the tooth pitch, resulting in a step angle of 0.72 degrees (in contrast, The step angle of the stepper motor is 1.8 degrees (standard 2-phase stepper motor). Similarly, a smaller step size means that the torque required to move the rotor to the next position is smaller, so the accumulation and attenuation of current is no longer so extreme, and overshoot can be reduced.
Stepper motor resonance
Adding a mechanical damper is a simple way to reduce the resonance of a stepper motor.
Mechanical dampers (whether internal viscous dampers or external damping devices) can eliminate or reduce the resonance of the stepper motor by changing the inertia of the system, thereby changing the resonance frequency of the motor. If the motor does resonate, the damper will absorb some vibration energy, which is generated when the motor oscillates and is fixed in the step position.
Although a gearbox is usually used to increase the torque of the motor, adding a gearbox will also increase the inertia on the motor side of the transmission system, thereby reducing the resonance frequency of the motor (see the equation above). Adding a gearbox to a stepper motor also means that the motor will need to run at a higher speed-usually beyond its resonance frequency range.
Use chopper driver
Another way to reduce resonance in a stepper motor is to precisely control the current flowing to the motor so that it only receives the amount of current required for each step (and therefore torque). This is usually done by a stepper driver called a chopper driver, which uses pulse width modulation to change the voltage and current output to the motor and precisely control the motor torque.