AdvancedTopic 72 of 101Signals, Control & Comms
Feedforward and cascade control
Add anticipatory action or nested loops to improve rejection.
In depth
Feedforward cancels measured disturbances before they affect output (u_ff=-G_d/G_p d). Cascade control nests a fast inner loop (e.g. valve flow, motor current) inside a slower outer loop (temperature, speed), rejecting inner disturbances early. Ratio, override and split-range are other strategies.
Key points
- Feedforward needs a model
- Inner loop 3–5× faster
- Combine with feedback trim
ExampleA furnace temperature loop uses fuel-flow cascade to handle supply pressure changes.
What is Feedforward and cascade control?
Add anticipatory action or nested loops to improve rejection.
Why is it important to understand feedforward and cascade control?
Feedforward and cascade control add anticipatory action and inner loops, which reject disturbances faster than feedback alone.
What is feedforward control?
Measuring a disturbance and acting on it before its effect appears in the output, which reduces the error it causes.
Why is cascade control useful?
An inner fast loop removes disturbances early, so the outer slow loop has an easier job and performs better.
How is Feedforward and cascade control used in practice?
Typical applications include adding a feedforward flow measurement to a heat exchanger to counter inlet changes, using a cascade loop to hold a valve position inside a temperature loop and improving the disturbance rejection of a boiler feedwater system.
What should I learn before Feedforward and cascade control?
Study "Adaptive control" first (topic 71 of 101 in Signals, Control & Comms). It covers the groundwork this topic builds on.
What are the key points of Feedforward and cascade control?
Feedforward needs a model. Inner loop 3–5× faster. Combine with feedback trim.
Can you give an example of Feedforward and cascade control?
A furnace temperature loop uses fuel-flow cascade to handle supply pressure changes.
What comes after Feedforward and cascade control?
The next topic is "PLC and industrial control", which builds directly on this one.
How difficult is Feedforward and cascade control?
It is classed as Advanced in Signals, Control & Comms (topic 72 of 101). It is an advanced topic. It assumes you are comfortable with the intermediate topics in Signals, Control & Comms.