Control Basics: On-off, Modulating and PID
Every automatic control loop measures something, compares it with a set point, and moves something to close the gap. On-off control switches between two states; modulating control moves smoothly; a PID controller decides how far and how fast to move.
You'll learn
- Describe a control loop
- Compare on-off and modulating control
- Explain proportional, integral and derivative action
- Recognize a badly tuned loop
Before this: Pressure and temperature measurement
Measure, compare, correct
A control loop has a measurement (steam pressure), a set point (the pressure wanted), a controller that compares the two, and a final element it moves (the burner firing rate). The difference between measurement and set point is the error; the controller acts to reduce it.
- On-off control: fully on below the set point, fully off above it, with a small dead band between. Simple; the measured value always swings.
- Modulating control: the final element moves continuously to any position, holding the measured value close to set point.
P, I and D
| Symbol | Meaning | Unit |
|---|---|---|
| Error: set point minus measurement | — | |
| Proportional gain | — | |
| Integral gain | per second | |
| Derivative gain | seconds |
- Proportional: output in proportion to the error now. Fast, but on its own it leaves a steady offset.
- Integral: output grows as long as any error remains, which removes the offset. Too much and the loop overshoots and oscillates.
- Derivative: responds to how fast the error is changing, damping overshoot. Sensitive to noise, so often left out on noisy loops such as flow.
Tuning and its symptoms
| Symptom | Likely cause |
|---|---|
| Slow, steady offset from set point | Too little integral |
| Continuous oscillation | Too much gain or integral |
| Overshoot then slow settle | Integral too strong; derivative may help |
| Valve chattering, noisy output | Derivative on a noisy signal, or gain too high |
| Good on a test, poor at low load | Process gain changes with load; valve oversized |
Tuning should be done with the process at a representative load, one change at a time, recording the response. Many poor loops are not tuning problems at all: a sticking valve, an oversized valve working near closed, or a sensor in the wrong place will defeat any tuning.
Key takeaways
- A loop measures, compares with a set point and moves a final element.
- On-off swings around the set point; modulating holds it.
- P acts on the error now, I removes offset, D damps rapid change.
- Sticking or oversized valves and badly placed sensors defeat any tuning.