### Glossary of Control Engineering Terms - G

Gain: this is defined as the change in input divided by the change in output. A system with high gain will react more to the input changing. Gain can be specified as a unit-less ratio, or in terms of decibels (dB) = 20*log10(unit-less gain ratio). Also called magnitude.

Gain Margin: For a linear system (usually the controller*plant transfer function, i.e. the open loop system) this is the amount of gain that can be added before it would be unstable when a feedback loop (unity gain) is closed around it. This is determined from the system gain at the frequency when the phase shift is -180 degrees. If the gain is greater than 1 (or > 0dB) at this frequency, then closing the loop will result in an unstable system. If the gain is less than 1 (or < 0dB) then the closed loop system will be stable. The gain margin is therefore a measure of how far the system is away from being closed loop unstable.

Gain Scheduling: a class of adaptive control, where the gains are adjusted depending upon some measurement of current operating condition. For example, the flight dynamics of aircraft change with altitude, and a gain scheduled flight controller would have different sets of controller gains for different altitudes.