Constraining options

The effects of rules

The effect of rules may be either absolute, leaving no other choice, or conditional, enabling the operators to bypass them.

Absolute constraint: disabling the event that might violate the rule. This option is applicable operator's activity, which the machine can reject before execution. It is not applicable to many machine generated events, resulting from hardware faults.
Conditional constraint: enabling the risky event, but acknowledging the operators about the exceptional situation .

The dilemma

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 exceptional situations.

Example

The 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 fault and on the situation ( explanation ).