Posted in: About stepper motors in general

Stepper Motor FAQ Part 2

The post The most frequently asked questions about stepper motors, part 2, is a continuation of part 1, so if you have not read part 1 and want to familiarize yourself with the basics of how stepper motors work, we recommend that you read Stepper Motor FAQ, Part 1.

1. What is stepper motor resonance?

Stepper motors are unique among electric motors in that they move in a sequence of discrete steps (hence their name) rather than in continuous motion. At each step, small vibrations occur due to the inertia of the moving rotor, which slightly overshoots (or in some cases does not reach) the step position at each step and then oscillates until it settles at the correct step position. Resonance only occurs if the frequency of these oscillations matches the natural frequency of the motor. Then there is an audible noise, vibrations, lost steps, stopping or even rotation of the engine in the opposite direction. In order to mitigate or completely eliminate the resonance, we help each other in various ways:

  • Acceleration through the resonant region
    Resonance is only problematic at the natural frequency of the stepper motor. This means that it only occurs in a certain speed band. This usually occurs at 100-200 pulses per second in full step mode and also at higher speeds. If your motor is rotating at a speed below or above the resonant speed, you may not even notice the resonance itself, even if you go through the resonant speed when accelerating the motor. The reason is that it takes time for resonance to occur. The problem arises if you stay in the resonance area for a few seconds. Since the resonance problem only occurs at certain speeds, you can avoid it by accelerating through the resonant region.

  • Using microsteps
    Reducing the step size of the stepper motor is the most common way of mitigating the resonance effect. Using the microstepping method, we divide each step into 2, 4, 8, 16 and more microsteps. With smaller step sizes, the rise and fall of current in each winding is more gradual, leading to smaller differences in torque between individual steps. This means that overshooting the position of each step is less extreme, the oscillation time is shorter, and vibration and noise are greatly reduced.

  • Change in rotor inertia
    The resonance frequency of a stepper motor is proportional to the torque stiffness of the motor and inversely proportional to its inertia. The resonant frequency can be changed by changing at least one of these parameters. 

  • Installation of a mechanical shock absorber
    A mechanical damper on the motor shaft can add additional inertia to the shaft and help absorb vibration and provide a stable damping effect.

  • System inertia change
    If the motor is under load, similar to a mechanical damper, the inertia of the rotor will be much greater and the oscillations will be significantly reduced.

  • Using a reducer
    While reducers are typically used to increase the torque of a motor, the addition of a reducer also means that the motor will have to run at a higher speed - often outside of its resonant region.

  • Current reduction
    The motor will produce less torque at a lower input current. As a result, less energy will be produced to move the rotor (ie lower dτ/dθ, torque stiffness). Many applications where low speed is required will run more smoothly. Make sure you have enough torque reserve when using this technique.

  • Increasing the inductance of the coils
    The resonance of a stepper motor induces an alternating current in the motor winding, and the alternating current disturbs the direct current flowing through the winding. By increasing the inductance of the motor winding, it can prevent resonance or shift it to lower frequencies.

  • Increasing the number of phases
    A multi-phase motor will have a smaller step angle, similar to a microstep function. A higher step resolution requires less energy to spin the rotor, so there is less step position overshoot as discussed above.

2. What is the recommended ratio of motor inertia to load?

This depends on the acceleration you expect from the system. For stepper motors, it is best to choose a ratio between the inertia of the load and the motor of around 1:1 for good acceleration. Using a gear reducer is a good option to reduce the inertia ratio mismatch, as the head gear reduces the load inertia by the square of the gear ratio.

As the ratio between load and motor inertia increases, especially when it is above 10, unwanted ringing and vibrations occur when decelerating at the end of rotation. The reason is that the greater the inertia of the load, the greater the overshoot of the ideal position of each step (more on this above), leading to greater oscillations back and forth around the final position of each step.

One more thing needs to be mentioned at this point. It is usually necessary to connect the motor shaft to the load shaft with some kind of coupling. A clutch that has a high torsional yield, or acts like a spring, can cause ringing problems even if the inertia ratio is 1:1, as the energy stored in the torsion is released during deceleration and causes more overshoot of the ideal position of each step. Therefore, in addition to the appropriate ratio between the inertia of the load and the motor, the selection of the appropriate clutch is also very important.

3. How to reduce ringing at the end of rotation?

Chiming is the unwanted back and forth movement of the rotor during engine stoppage. Friction in the system, faster decelerations/accelerations are methods to help mitigate ringing. Also, don't forget the proper motor-to-load inertia ratio as described above.

4. Why does the stepper motor stop during the no-load test with strong vibration and rattling?

In order to accelerate properly, the motor needs a load with an inertia approximately equal to its inertia (see answer above). Any resonance that develops in the motor is greatest in the no-load condition.



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