The state machine pattern: a batch sequence that cannot get stuck

Why a state machine and not a pile of flags

Any process with an order to it (fill, hold, drain) belongs in a state machine. The alternative is a heap of BOOL flags that interlock over time, and six months later nobody knows which combinations are reachable. A state machine makes the state one number: the controller is in exactly one state, always, and that number goes straight to the HMI so an operator can see where the sequence stopped.

The skeleton

CASE Step OF
  0:   (* idle *)
  1:   (* fill *)
  2:   (* soak *)
  3:   (* drain *)
  9:   (* fault *)
END_CASE;

Leave gaps in the numbering (0, 10, 20) if you expect steps to be inserted later. A transition is one line, Step := 2;, and nothing else.

Entry and exit actions

An entry action runs once when a step begins: reset a timer, clear a counter, open a valve. You detect entry by comparing the step with the previous step:

NewStep  := Step <> PrevStep;
PrevStep := Step;

NewStep is true for exactly one scan after every change. Use it to reset the step timers. The exit action is what you guarantee before leaving: close what you opened.

Every step needs a timeout

This is the line between a demo and a plant program. Any step that waits for a field event (a level to arrive, a limit switch to close) needs a timeout. Valves seize, pumps lose suction, transmitters die. Without a timeout the sequence waits forever and the operator sees "running" while nothing moves.

The fault state

The fault state does two things: it puts the outputs in a safe position, and it stays there. It never self-clears. Getting out needs a person: an explicit reset, and then you start from idle, not from the step that failed, because the state of the process is no longer known.

How you restart

Three rules that hold up in the field:

• Stop aborts to idle, not to pause, unless the process genuinely allows a pause.
• Reset clears the fault and does not start. A reset that also starts the machine surprises whoever is standing next to it.
• On power-up, start at state 0. Never resume from retained memory unless you designed for it on purpose.