Manual/Auto and a Smooth Handover

The Core Idea

Manual and automatic modes choose the output in different ways. A smooth handover prepares the automatic controller so taking control does not suddenly change the command sent to the process.

What you will learn

  • Distinguish the operator's manual command from automatic controller output (CV).

  • Transfer control without an avoidable output jump.

  • Check the temperature response after the transfer, not just the first CV sample.

Why it matters

An otherwise useful tune can still cause a large output change during a mode transfer. Operating changes need to be checked as carefully as steady automatic control.

Warm the heater in manual

This familiar heater starts at 20 °C, targets 50 °C, and accepts 0–100% output. Manual mode holds 30% output. Automatic settings are P = 2% output per °C, I = 0.2% output per (°C·s), D = 0.

Run in manual. The simulation pauses at 30 simulated seconds so you can inspect process variable (PV) and the steady manual command. Predict the first automatic output if the controller uses its present error with an unprepared integral contribution.

Leave Track manual output on transfer unchecked, select Automatic, and resume to 90 s. Record the first output change and the later temperature response. Save this trace for comparison.

Prepare the second transfer

Use Restart to return to the same manual start; your selected gains and tracking option are retained. Enable Track manual output on transfer, run to the 30 s pause, select Automatic, and resume. Compare the output around the transfer with your saved run.

The check allows a manual-to-auto jump no greater than 1 percentage point of output, a peak no higher than 53 °C, and average absolute error no greater than 0.6 °C over the final 18 s. Finish the full 90 s in auto, with at least 20 s before and 40 s after the transfer.

What tracking prepares

In manual, the operator's command drives the process. Automatic control must take over from that operating point. Here the transfer prepares the integral contribution so P plus I begins near the current manual output.

This avoids an unnecessary step in CV. It does not guarantee perfect temperature control: the next scans still respond to error and process behavior. A small change on the first automatic scan is possible because integration resumes.

Explained simply

Imagine handing a steering wheel to another driver. Matching its current position before taking over prevents a sudden turn. The new driver must still make sensible corrections afterward.

Optional calculation

For this controller's heating direction, the starting integral contribution is prepared as manual CV − Kp × error, with D = 0. The stored integral is this contribution divided by Ki. This transfer therefore needs a nonzero I gain to supply the required bias.

The output limit and anti-windup still apply. This lesson uses the existing transfer behavior; a real controller's tracking and manual-mode settings must be checked in its documentation.

Worked interpretation after the trials

After 30 s at 30% heating, PV is near 48.5 °C. Without tracking, the first automatic command falls from 30% to roughly 3%, and the process then has to recover. With tracking, the first automatic command stays near 30%, with only a small integration change.

Both runs may eventually approach the target. Their ending temperature alone would miss the avoidable transfer jump, which is why the check inspects the actual handover event as well.

Check your understanding

Record CV immediately before and after each transfer, then describe the later PV response. Explain what tracking prepared and why a smooth first sample still needs a complete recovery check. Manual completion is also available.

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