The servo system will resonate due to the flexibility or gap of the mechanical components. Resonance can degrade system performance, cause audible noise, and in extreme cases can even damage hardware.
Servo adjustment is the process of setting the controller gain to optimize the servo performance, but as the gain increases, the number and severity of resonance usually increase.
When the natural frequency of the system is excited, mechanical resonance occurs. When the servo feedback includes the component of the natural frequency of the system, the natural frequency amplification factor will be added to the controller gain, which will cause severe vibration and instability of the servo loop.
Options to reduce resonance include mechanical solutions, such as using harder couplings, shafts and drive components (screws, belts, gearboxes), but this is difficult to achieve while maintaining performance and cost. Another option is to reduce the inertia ratio of the load to the motor by changing the load parameters, using a higher inertia motor or using a higher reduction gearbox. But load parameters are difficult to change, motors with higher inertia (usually larger) require additional torque, and other factors play a role in the selection of gear ratios-making these choices undesirable or not feasible at all.
From the perspective of servo adjustment, resonance can be reduced or eliminated by reducing the control gain. However, this can compromise the performance of the servo system by reducing its bandwidth and therefore its responsiveness. While maintaining the highest possible control gain, the easiest way to reduce resonance is to add a filter in the control loop.
Resonance can be classified as high-frequency, intermediate-frequency, or low-frequency, depending on how close they are to the control loop bandwidth (above, close, or lower, respectively). Most servo systems experience high-frequency resonance, which manifests as a high-pitched scream. Two types of filters commonly used for high-frequency resonance are notch filters and low-frequency filters.
The notch filter attenuates or reduces a specific narrow frequency range near the center frequency (notch). Frequencies above or below the specified range are passed unchanged-therefore, another term for notch filters is "band stop filter". Signals near the notch (center) frequency are severely attenuated, but the attenuation drops at either end of the specified range.
One disadvantage of notch filters is that they will be ineffective if the resonant frequency changes significantly. Changes in resonance frequency may be caused by changes in load inertia. For example, in distribution applications, the load varies with the distribution of products. As mechanical parts "wear" and connections become loose or gaps increase, they may also change as compliance and gaps increase.
Low frequency filter
The low-frequency filter attenuates high-frequency signals that exceed its bandwidth, while keeping low-frequency signals unchanged. However, because they act on the high frequency response of the system, they will degrade the performance of the servo system. Also, if the problem frequency is low, using a low-frequency filter with a bandwidth close to the servo response will cause the servo system to become unstable.
Most solutions to the resonance problem use one or more notch filters and low frequency filters.
Some servo drives will configure filter parameters in their "auto-tuning" function. If the filter has little effect on resonance (usually at frequencies below 500 Hz), other drive functions (for example, vibration suppression) can reduce its tedious parameter adjustment settings.
The types of resonances described here are called "motor-side resonances" because they are reflected back to the motor and captured by the encoder. This means that they can be reduced or eliminated by adjusting the control loop gain and applying filters.
Another type of resonance is called "load side resonance". These resonances can be caused when the load or end effector vibrates due to the compliance of the connection with the transmission system and the motor. Load-side resonance is usually not captured by the encoder, so it will not be affected by servo loop adjustment or filtering. Load-side resonance is particularly common in robots, cranes, and other designs with high cantilever loads.