Constraints may be designed as absolute, leaving no other choice, or conditional, enabling the operator to bypass them.
Who should be in charge of constraining the operation according to the rules? who should be at the helm?
A common practice of controlling the machine behavior is by relying on the operator's capability to track the machine state and to prevent exceptional states. For example, operator's control is commonly employed in many plants in the process industry, based on charts of various process parameters. The problem with this approach is that the operators are not very reliable: they often fail to track the machine state and they make mistakes.
The design decision should depends on the trigger and on the situation.
Updated on 28 Mar 2016.