Glossary of Control Engineering Terms - C
Cascade: A system with two (or more) controllers configured such that the output of the "Master" controller is the setpoint for the "Slave" controller. This is often employed to accommodate different process dynamics with multiple controllers, where each controller is tuned for its particular process dynamics (rather than have a single controller that is compromised in its operation). Cascade control is commonly used to control level, with the slow "Master" loop controlling the level by demanding more or less flow from the faster "Slave" loop which regulates flow into the vessel. This allows the "Slave" loop to quickly counteract any changes in flow due to pressure variations etc., rather than having to wait until they affect the level.
Closed Loop: A controller in automatic mode. Controller is driven by an error generated from reference minus the plant output or feedback signal. The controller then calculates the corrective action to apply to the plant to drive the plant output to match the reference, i.e. make the error zero.
Controller: A device (mechanical, electrical or computerised) that minimises the difference between a desired value (often called the setpoint or reference) and a measurement (often called the process variable) by manipulation of an actuator. More sophisticated controllers can use several measurement and/or several actuators.
Corner Frequency: For first order systems, the corner frequency is the frequency where the magnitude starts to roll-off (3dB below the steady-state gain) and the phase shift is -45 degrees. Also: corner frequency = 1/(time constant) rads/s.
Critically-damped: A linear system that has the fastest response without any overshoot is said to be critically damped. This corresponds to the case where all of the system poles have the same (negative) real value. This applies to 2nd-order (or higher-order) systems.
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