Principle 43

In order to control a process four conditions are required:

1. Goal Condition: The controller must have a goal or goals (e.g. to maintain the setpoint).

2. Action Condition: The controller must be able to affect the state of the system. In engineering, control actions are implemented by actuators.

3. Model Condition: The controller must be (or contain) a model of the system.

4. Observability Condition: The controller must be able to ascertain the state of the system. In engineering terminology, observation of the state of the system is provided by sensors.

(Leveson, Engineering a Safer World, pp.54-55).