The Control Hierarchy and the Control Engineer's Role
The layers of process control from the valve to economic optimisation, what each layer depends on, and the judgements a control engineer is responsible for.
13 min
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Layers that stand on each other
Process control on a modern plant is built in layers. At the bottom are the final elements and measurements: valves, drives and transmitters. Above them is the regulatory layer: single PID (proportional, integral, derivative) loops, cascades, ratios and selectors in the distributed control system (DCS). Above that sits advanced process control (APC), usually a multivariable model predictive controller. At the top, optimisation chooses the targets that make the most money within the plant's limits.
Beside all of these, and independent of them, is the safety instrumented system (SIS), which takes the plant to a safe state when limits are exceeded. It is not a control layer; it is a protection layer.
Why the order matters
Each layer can only be as good as the layer under it. An optimiser that asks for a higher throughput cannot get it if the multivariable controller is switched off. The multivariable controller cannot hold constraints if the flow loops it writes to are oscillating. And no flow loop can perform if its valve sticks. The existing course material puts it plainly: advanced control multiplies whatever is underneath it, including the problems.
This gives the control engineer a working rule. When a higher layer performs badly, look down before you look up. Most "advanced control problems" are regulatory problems in disguise, and most regulatory problems are valve or measurement problems.
What each layer is for
Turn controller output into a real change, and report the process truthfully and quickly. Speed: milliseconds to seconds.
Judgements that belong to the control engineer
Diagnose from the bottom up
Arrange the checks in the order a control engineer should make them when an advanced controller performs badly.
- 1Check the advanced controller model and constraints
- 2Check the optimisation targets
- 3Check the regulatory loops for oscillation, manual mode and saturation
- 4Check the final elements and measurements for faults
A multivariable controller keeps being switched off by operators because it "makes the unit unstable". Where should the investigation start?