Controls Glossary¶

reference: The desired state. This value is used as the reference point for a controller’s error calculation.
input: An input to the plant (hence the name) that can be used to change the plant’s state.
output: Measurements from sensors.
process variable: The term used to describe the output of a plant in the context of PID control.
setpoint: The term used to describe the reference of a PID controller.
error: Reference minus an output or state.
state: A characteristic of a system (e.g., velocity) that can be used to determine the system’s future behavior.
system: A term encompassing a plant and it’s interaction with a controller and observer, which is treated as a single entity. Mathematically speaking, a system maps inputs to outputs through a linear combination of states.
plant: The system or collection of actuators being controlled.
controller: Used in position or negative feedback with a plant to bring about a desired system state by driving the difference between a reference signal and the output to zero.
observer: In control theory, a system that provides an estimate of the internal state of a given real system from measurements of the input and output of the real system.
control input: The input of a plant used for the purpose of controlling it
control effort: A term describing how much force, pressure, etc. an actuator is exerting.
steady-state error: Error after system reaches equilibrium.
step response: The response of a system to a step input.
step input: A system input that is $0$ for $t < 0$ and a constant greater than $0$ for $t \geq 0$ . A step input that is $1$ for $t \geq 0$ is called a unit step input.
rise time: The time a system takes to initially reach the reference after applying a step input.
settling time: The time a system takes to settle at the reference after a step input is applied.
system response: The behavior of a system over time for a given input.
model: A set of mathematical equations that reflects some aspect of a physical system’s behavior.