What do P, I and D do?
This is how an engineer tests a control loop: move the setpoint by a step and watch how the temperature follows. Here that happens again and again, between 50 and 60 °C. Move the controller sliders while it runs and watch what happens to the line: on the right you can already see the next step coming.
- Temperature
- Setpoint
What you see
- Overshoot
- At temperature after
- Remaining offset
The process is a first-order process with dead time (K = 1 °C per %, τ = 15 s, θ = 2 s, 55 °C at 50 % valve position). The controller has the ISA standard form, D action on the measurement and a scan time of 0.5 s. The slider for I runs from Ti = 60 s (weak) to Ti = 2 s (strong). Optimal here means: at the setpoint as fast as possible, with at most 0.3 °C overshoot and a calm valve. A different process needs different settings. The behaviour of P, I and D stays the same.
Further reading
- Article
PID controller explained: how it works, settings and process dynamics
How does a PID controller work? Learn what P, I and D do, how process dynamics determine the right settings and how to recognise control behaviour in trend data.
- Article
Process control: the basics of industrial control engineering
How does process control work in the process industry? Learn how feedback, feedforward and process dynamics come together in stable, well-performing control.
P, I and D work exactly the same way in your plant.
This is a simulated process. In practice the setpoint usually stays put and the controller mainly absorbs disturbances; a step like this one shows how well it does that. In a real control loop we take the time constant, dead time and gain from your own trend data and calculate the settings from them. Would you like to know whether your control can be better? Take the quickscan, or read exactly how a PID controller works.