On/Off Control with a Deadband

The Core Idea

On/off control uses two output states rather than a continuously adjustable command. A deadband separates its switching thresholds, allowing the previous state to hold while the measurement stays between them.

What you will learn

  • Control a heater using only 0% and 100% output.

  • Explain why two switching limits prevent rapid switching at one threshold.

  • Choose a deadband that balances command switching and temperature swing.

Why it matters

Some processes can meet their requirements with simple switching control. The useful choice depends on the allowed process variation and switching frequency.

Try a thermostat before tuning PID

This heater starts at 20 °C and targets 50 °C. Its output can be fully on at 100% or fully off at 0%. The controller samples every 100 ms, with no measurement noise in this comparison.

The deadband width is the full distance between the two limits, centered on setpoint (SP). A width of 0.4 °C means on at or below 49.8 °C and off at or above 50.2 °C. Between those limits, keep the previous output state.

Predict the temperature and output traces. Run for 60 simulated seconds, record the switching count and final temperature swing, and save the trace. Then widen the band and restart from the same 20 °C start.

Choose a useful compromise

The brief allows at most 60 output transitions in 60 s, excluding the initial switch-on, and at most 6 °C peak-to-peak process variable (PV) swing over the final 30 s. Meet both limits. A transition is one change from on to off or off to on; a full cycle includes two transitions.

Compare widths of 0.4 °C, 4 °C, and 8 °C in separate full runs. Keep the target fixed. The measured swing can slightly exceed the threshold spacing because the process is sampled, rather than checked continuously.

Why the output keeps changing

While the heater is on, temperature rises; while it is off, the process loses heat toward ambient. Between the two switching limits, the controller remembers its last state. This is the same hysteresis idea used in the alarm lesson.

On/off control can suit equipment designed for switching. A proportional controller can instead request intermediate outputs, such as 30%, when the actuator supports them. That gives another way to hold temperature without this deliberate full-on/full-off cycle.

Hint — Name both limits

For a width of 4 °C around 50 °C, the lower limit is 48 °C and the upper limit is 52 °C. Heating turns on at the lower limit and off at the upper one; it retains its state between them.

Worked comparison after your runs

The 0.4 °C band causes about 339 command transitions and about 0.9 °C of ending swing. A 4 °C band gives about 56 transitions and 4.2 °C of swing, meeting both limits. A 5 °C band also passes.

An 8 °C band reduces switching to about 28 transitions, but the ending swing grows to about 8.4 °C, exceeding the temperature limit. The widest band is therefore not automatically the best choice.

Check your understanding

Record your chosen width, its lower and upper limits, switching count, and temperature swing. Explain what an intermediate output could change, then continue to proportional action. Manual completion is also available.

Enable JavaScript to run the embedded simulator and complete the interactive exercises.