Learn Automation with MINATA #65: Manual, Automatic and Handing Over Control Without a Bump

Manual, Automatic and Handing Over Control Without a Bump

Manual is not Auto switched off. It is a mode with its own control authority, its own permissive conditions and its own limits. Handling the transfer between Manual and Auto carelessly is where those sudden jolts on changeover come from.

Manual is a disciplined mode

In Manual, every jog and every command is still bounded by the mechanical interlocks, the safe zone and the equipment involved. "By hand" does not mean protection is set aside. The operator needs to see clearly which actuator is selected, which command is being held, and which condition is currently blocking the action.

A synchronisation point before Auto

Moving from Manual to Auto needs a synchronisation point, not merely a changed bit. Before Auto takes authority:

The actuators are in a valid position, not stuck part-way.
Every manual command has been released.
Setpoints and the sequence state agree with what is physically there.

If Auto takes over while a valve is open because of a hand command, the first step of the cycle can produce unexpected motion.

Bumpless transfer

For a continuous control loop such as a PID, switching between Manual and Auto needs a bumpless transfer: on entering Auto, the controller takes the present output as its starting point and initialises the integral term to match, so the output does not step. Going the other way, the manual value starts from the current output. This configuration has to be verified on the specific controller.

A worked engineering situation

A machine allows each actuator to be jogged in Manual, with full travel limits and interlocks. When the operator switches to Auto, the PLC checks that the actuators are in valid positions and that the hand commands have been released. If not, Auto does not take authority, and the HMI names the missing condition. The temperature loop uses a bumpless transfer, so the output does not jolt on changeover.

The synchronisation conditions and the transfer configuration follow the machine design and the controller documentation.

Common mistakes

Treating Manual as a mode where interlocks and limits no longer apply.
Switching to Auto by flipping one bit, with no synchronisation.
Letting Auto take authority while an actuator is stuck part-way.
Not releasing hand commands before entering Auto.
Skipping the bumpless transfer, so the loop output steps on changeover.

Manual and Auto checklist

[ ] Manual keeps its interlocks and limits, and displays them clearly.
[ ] Switching to Auto requires the actuators to be in valid positions.
[ ] Hand commands are released and setpoints agree with the plant.
[ ] If a condition is missing, Auto does not engage and the HMI says why.
[ ] Continuous loops use a bumpless transfer, verified on the controller.

A disciplined handover is what keeps a mode change from becoming a surprise. #66 goes into recipes and machine parameters — changing product without rewriting the program.

Read more automation knowledge at MINATA: https://minatavn.com/en/blog/industrial-automation

Previous — #64: Interlocks and permissives: https://minatavn.com/en/blog/automation-64-interlock-permissive

Next — #66: Recipes and machine parameters: https://minatavn.com/en/blog/automation-66-recipes-machine-parameters

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