Disturbances

Definition

A disturbance ( Trigger) is an event that results in disruption.

Disturbances are expected when the system is in a routine state, and is prepared to handle them.

Run-time (expected) disturbances

Common sources of disturbances include

• Functional disturbance is an external trigger that motivates the system development and justifies the development costs. (for example, various alarm systems: medical monitors and public alarm generators about fire, environmental hazards, intruders, etc.)
• External hazards, such as obstacles (for example, in transportation),
• Hostile activity
• Exceptional situations, such as slippery road, exceptional slope, sharp curve etc.
• Fault (hardware fault, power failure, communication interference, etc.)
• Operational errors: mistakes, slips, omission of required actions
• Exceptional change of an operational parameter.

Design-time (unexpected) disturbances

• Software bug, which under certain circumstances in normal operation results in changing the machine state
• Design mistakes - such as incomplete specifications, usability mistakes, inter-unit coordination mistakes

Classification

Disturbances may be classified by the following qualifiers:

• Trigger
• Predicted time
• Ease of recovery