Control Systems
Control Systems tests architecture and logic rather than single-loop tuning. Expect PLC, DCS, SCADA, feedback, cascade, feedforward, ratio, split-range, alarms, interlocks, and safety instrumented systems.
Core rules
- A feedback loop measures the controlled variable, compares it with setpoint, and adjusts the output. Cascade uses a primary controller output as the secondary setpoint; the secondary must be faster than the primary.
- Feedforward acts on a measured disturbance before the error develops; it usually needs a feedback trim to correct model error.
- Split-range uses one controller output to sequence two final elements. Define the signal breakpoints precisely and check for overlap or deadband.
- PLC scan order matters: read inputs, execute logic, write outputs. Interlocks and trips should fail safe and be independent of normal control where required.
- A SIS is separate from basic process control; do not use it for routine control or bypass it without formal management of change.
Worked example
A split-range scheme uses 4–12 mA for valve A (0–100%) and 12–20 mA for valve B (0–100%). At 8 mA, the controller is 25% through its full 16 mA span, but valve A is at (8−4)/8 = 50%, and valve B is at 0%. At 16 mA, valve A is at 100% and valve B is at (16−12)/8 = 50%. At exactly 12 mA, define whether A is 100% and B is 0%, or whether a small deadband exists. The reasoning is to convert the signal into each valve's own 8 mA segment, not the full 16 mA span.
Traps
Tuning the primary faster than the secondary in cascade, using the SIS as a normal controller, assuming split-range valves share the same 4–20 mA span, and forgetting alarm deadband are common. Also check whether an alarm is high-high, low-low, or deviation.
Key points
- Cascade secondary must be faster.
- Feedforward needs feedback trim.
- Convert split-range signals per valve segment.
- Respect PLC scan and fail-safe logic.
- Keep SIS independent and controlled.