Cascade control
The cascade control is a cascading of several controllers , the associated control loops are nested. The controller output variable of one controller (master controller) serves as a reference variable for another (slave controller). The overall control route is thus divided into smaller, more controllable sections. Compared to a direct acting controller, the control accuracy increases.
The cascade control is used, for example, for the position control of CNC machines :
s: position (position), v: speed (speed), a: acceleration, U: voltage, I: current in the motor
red - inner control loop, here current control loop with PI controller :
- Controller 1: PI (red);
- Controlled system 1: PT 1 (red); Inductance of the electric motor,
green - middle control loop, here speed control loop with PID controller :
- Controller 2: PID (green);
- Controlled system 2: PT 1 (green); Mass of engine and machine part
blue - outer control loop, here position control loop with P controller :
- Controller 3: P (blue);
- Controlled system 3: I (blue); Speed changes position
gray - pre-control of speed and acceleration
The inner control loops (speed and current control loops ) must be faster than the outer ones. This means that their time constants must be smaller for the cascade control to work. The current controller (PI) is the fastest of all.
The pre-control of the speed and acceleration is not part of the cascade control. It improves leadership behavior. Due to the speed pre-control, the position error is approximately zero at constant speed. When all disturbance variables are corrected, the following applies: Actual position = target position. The acceleration pre-control reduces the speed error during acceleration.
Further examples
- Control of ventilation and air conditioning systems
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- Room temperature / supply air temperature cascade control or
- Room humidity / supply air humidity cascade control
- The effects of energy storage and dead times due to the system expansion are thereby reduced.
- Positioner for control valves
- The process controller (PI or PID) specifies a target position that the positioner (P or PD) moves to and largely compensates for the effects of friction ( hysteresis ) and inlet pressure (non-linearity) in the valve.