Frequen-cy asked questions

Many modern servo filters provide some facility for frequencydependant filtering. This is useful for compensating for mechanical systems that have a resonance at a certain frequency or speed, or to reduce high frequency noise.

The most common implementation of such a filter is known as a bi-quad filter, shown in figure 6. By choosing the right values for A1, A2, B0, B1, and B2 this filter can function as a notch filter, band-pass filter, high or low-pass filter. If you are not familiar with use of a bi-quad filter, there are a number of facilities that provide information including the website www.octave.org. This website includes a tool that lets you calculate values for A1, A2, etc. based on the frequency filtering characteristics you want for your system.

Conclusion

Servo tuning need not be more difficult than other typical motion tasks such as sizing a motor. There are a number of standard manual methods available, two of which, step-response tuning and zone-based tuning, are discussed in this article. Autotuning holds out the promise of eliminating human involvement in the process of servo tuning, but at present, most auto-tuning packages are designed to provide workable initial values, which are then further hand-optimized for a specific application.

Once your basic PID parameters have been determined, techniques such as feed-forward, and bi-quad filtering can be used to further improve performance, or increase smoothness.

Regardless of the process by which you arrive at your tuning parameters, make sure that you exercise your system over the complete load range expected for your application, and if possible, on both new and older hardware to insure that your system will run correctly under real-world conditions.