What is override control?
An “override” control strategy involves a selection between two or more controller output signals,
where only one controller at a time gets the opportunity to exert control over a process. All other
“de-selected” controllers are thus overridden by the selected controller.
If there are two or more conditions should be satisfied at the same time, override control is used. If one of the conditions satisfied for the controller output then output made to be true by override control system.
Consider a water distribution system which transports water from a tank to costumer, according to the costumer's need.
Water pump is driven by a variable-speed electric motor to draw water from a well and provide constant water pressure to a customer.Reducing pump speed in low-flow conditions will save a lot of energy over time compared to the energy that would be wasted by a constant-speed pump and control valve.
But this system has a defect, what happens when the level of the water in tank gone low below the suction pipe. The pump starts running dry, If the pump runs for too long with no water passing through it, the seals will become damaged. This will necessitate a complete shut-down and costly rebuild of the pump.
This problem can overcome by adding a level control system override to the system:
Connecting the level transmitter to a level controller, and using a low-select relay or function block to select the lowest-valued output between the pressure and level controllers. The level controller’s setpoint will be set at some low level above the acceptable limit for continuous pump operation.
If ever the well’s water level goes below this setpoint, the level controller will command the pump to slow down, even if the pressure controller is calling for a higher speed. The level controller will have overridden the pressure controller, prioritizing pump longevity over customer demand.