Requirements Capture

Requirements have already appeared throughout the previous sections. We first identified the controlled variable and the requirements that define the desired behaviour. We then showed how the desired value becomes the reference, which is compared with the measured output to generate an error. Finally, the complete feedback-control system showed how the controller, actuator, process, measurement device and feedback path work together to achieve the required behaviour.

Requirements capture steps back to an earlier point in the engineering lifecycle: where does the requirement come from, and how is it defined before the control system is designed?

In industry, requirements capture is commonly undertaken as part of systems engineering or requirements engineering. Stakeholder needs are clarified, operating conditions are identified and the resulting requirements are reviewed before being used to guide design and verification. The process is normally iterative rather than a single one-off activity.

Requirements capture often begins with a stakeholder need. These statements describe the outcome that matters to the user or customer, but they may initially be too broad to verify directly.

The engineering requirement is more useful because it identifies the controlled variable, the target and an allowable tolerance. The operating conditions must also be defined so that the requirement can be tested consistently.

A requirement should normally describe what the system must achieve, rather than immediately prescribing how it must be implemented. For example, maintaining 25°C ± 1°C is a performance requirement. Specifying a PID controller would normally be a design decision unless the solution is explicitly constrained to use PID.

The temperature-control system can now be viewed from a requirements perspective. The requirement defines the desired behaviour; the feedback-control architecture provides the mechanism used to achieve and assess that behaviour.

In this example, the reference comes from the temperature requirement, the measurement device provides evidence of the actual temperature, and the error shows the difference between required and measured behaviour. The controller and actuator are design elements used to influence the process, while the open door represents a disturbance that should be considered when defining the operating conditions.

Once requirements have been agreed, engineers maintain traceability between the original need, the requirement, the design used to implement it and the evidence used to verify it.

This becomes particularly important when a requirement changes. A tighter temperature tolerance, for example, may affect the sensor, controller, actuator, model and verification test.