This flowchart describes an autotuning procedure used in conjunction with PID controllers and uses three phases. The first derives a value for D by applying a method like zone-based tuning. The second phase derives values for P and I using the relay-test approach. And the third lets users apply system information to hand-optimize results.

Many modern servo filters provide some facility for frequency dependant filtering. That is, filtering that affects only certain domains of the overall operational frequency spectrum. This is useful for compensating for mechanical systems that have a resonance at a certain frequency or speed, or to reduce high-frequency noise.

To calculate a biquad filter's output, one must combine user-programmed coefficients A1, A2, B0, B1, B2, and K with current and previous input and output values.

The most common implementation of such a filter is known as a biquad. Biquads are common in digital control systems because they are flexible and easy to implement. They can be programmed to operate as notch, bandpass, band reject, or high or low-pass filters. Combining user-programmed coefficients A1, A2, B0, B1, B2, and K with current and previous inputs and outputs generates a biquad's output.

Selecting filter coefficients that meet desired characteristics is not exactly straightforward. For help, designers can visit informative websites such as www.octave.org, which provide tools to calculate these values. A biquad filter is represented as:

Y_n = K(B0X_n + B1X_n-1 + B2X_n-2 + A1Y_n-1 + A2Y_n-2)

where: Y_n is the filter's output at time n
X_n is the filter's input at time n
K is a positive scalar
B0, B1, B2, A0, A1 are programmable biquad coefficients.