Most process loops have one job: hold a measurement at setpoint. But real plants have multiple constraints that any single loop can violate. A compressor's throughput controller wants to push hard; the discharge pressure controller wants to keep things safe; the motor amp limit doesn't care about either. Override control — also called selector control — lets multiple PID controllers compete, with a high or low selector picking the winning output to send to the valve.

The architecture: Two or more PIDs each compute an output based on their own PV and setpoint. A low selector (LSS) or high selector (HSS) block passes the smallest (or largest) signal to the final control element. The "losing" controllers are bypassed but kept tracking so they don't wind up.

Classic example — gas compressor antisurge:

• Primary controller: discharge pressure PID, output drives suction throttle valve open to push more gas.
• Override controller: motor current PID with setpoint at 95% of nameplate amps.
• A low selector picks the smaller output. Normally pressure wins. If the motor hits its amp limit, that controller's output drops below the pressure controller's, the selector switches, and the valve throttles back — protecting the motor without tripping the plant.

The anti-windup problem: The losing PID keeps seeing error but its output is ignored. Without intervention, integral action ramps the output to saturation. When conditions shift and that loop should take over, it's already 100% wound up — bumpless transfer becomes a violent bump. The fix is external reset (also called integral tracking): every PID's integrator is fed the actual selector output, not its own computed output. The losers track the winner, ready to take over smoothly the instant their constraint becomes binding.

Rule of thumb for selector direction:

• If "more output = more aggressive/risky," use a low selector — the most conservative controller wins.
• If "more output = safer/backed off," use a high selector — again, the controller demanding the safest position wins.

Tuning gotcha: The override loop is usually tuned tighter than the primary because it's protecting against a hard limit (equipment damage, trip). You want it to grab control quickly and decisively. Conversely, the primary can be detuned slightly so it doesn't fight the override during transitions.

Other real applications: boiler drum level vs. steam flow vs. firing rate; pump discharge pressure vs. NPSH margin; reactor temperature vs. cooling jacket flow; turbine speed vs. exhaust temperature. Anywhere a single actuator must respect multiple competing limits, a selector beats interlocks — interlocks trip the plant; overrides keep it running at the constraint.