control//feedback control//control loop//setpoint

The setpoint is the value a control loop is told to hold, the 21 degrees on a thermostat or the 65 % level in a tank, and it is the one input of the loop that a person or a higher-level program sets on purpose. In process plants operators change setpoints from their screens; in a machine a sequence or a trajectory planner changes them many times a second, and then the setpoint is called the **reference** \(r(t)\) and the job becomes tracking rather than holding.

The setpoint is the value a control loop is told to hold, the 21 degrees on a thermostat or the 65 % level in a tank, and it is the one input of the loop that a person or a higher-level program sets on purpose. In process plants operators change setpoints from their screens; in a machine a sequence or a trajectory planner changes them many times a second, and then the setpoint is called the reference r(t)r(t)r(t) and the job becomes tracking rather than holding.

How the setpoint changes matters as much as its value. A step from 50 to 80 degrees asks the controller for everything at once: a large error, a proportional kick, a derivative spike if the derivative acts on the error, and possibly an actuator pinned at its limit while the integral winds up. Industrial controllers soften this in three standard ways.

A setpoint ramp limits how fast the setpoint may move, so the loop follows a slope it can manage instead of a cliff. Furnaces and anything sensitive to thermal stress use it.

Taking the derivative of the measurement instead of the error removes the spike, since the measurement moves smoothly even when the setpoint jumps (derivative action).

Setpoint weighting passes only a fraction of a setpoint change to the proportional term, so a loop can be tuned hard against disturbances and still answer setpoint changes gently. This is the two-degree-of-freedom PID, offered for instance as the PID Controller (2DOF) block in Simulink.

Holding and tracking ask different things. A constant setpoint against slow disturbances is the regulator problem of process plants; a moving reference, as in a robot arm, usually needs feedforward from the reference itself, because feedback alone always trails it.