AiVibeCascade Control
Cascade control is a strategy in which the output of an outer, primary controller becomes the setpoint of an inner, secondary controller, so that disturbances affecting the inner variable are corrected before they upset the primary process variable.
A typical example is a reactor temperature loop whose output sets the setpoint of a coolant flow loop. If the coolant supply pressure changes, the fast inner flow loop corrects the flow before the reactor temperature moves noticeably. Each loop has its own measurement and PID controller, and only the inner controller drives the final control element, such as a valve or a drive.
Cascade control is common in temperature control of jacketed vessels, heat exchangers and furnaces, in level-to-flow arrangements and in drive systems, where a position loop sets a speed loop, which in turn sets a current or torque loop. It improves disturbance rejection and makes the outer loop less sensitive to nonlinearities in the actuator.
Cascade control works best when the inner loop responds considerably faster than the outer loop, commonly several times faster. The inner loop is tuned first, with the outer loop in manual, and the outer loop is then tuned with the inner loop in cascade mode. Mode handling, bumpless transfer and anti-windup across both controllers need care, so that saturation of the inner loop is visible to the outer one.
Key points
- Outer controller output becomes the inner controller setpoint
- Inner loop corrects local disturbances before they reach the main variable
- Inner loop should respond considerably faster than the outer loop
- The inner loop is tuned first, then the outer loop
- Drives commonly cascade position, speed and current loops
Where AiVibe comes in
AiVibe designs and manufactures the AiAmbA AI Factory, whose edge devices and AI agents let people talk to PLC, CNC and robot controllers in plain language. Agents only propose changes, a trained operator confirms each one, and the AI Factory never replaces a machine's control or safety systems.