PLC based water level controller

by barkkathulla 2012-09-22 11:30:53

The water is drawn from the tank by another system, as needed and our example system must manage the water level in the tank.Using only digital signals, the PLC has two digital inputs from float switches (tank empty and tank full). The PLC uses a digital output to open and close the inlet valve into the tank.
If both float switches are off (down) or only the 'tank empty' switch is on, the PLC will open the valve to let more water in. If only the 'tank full' switch is on, the valve turns off. Both switches being on would signal that something is wrong with one of the switches, as the tank cannot be both full and empty at the same time. Two float switches are used to prevent a 'flutter' condition where any water usage activates the pump for a very short time causing the system to wear out faster.
An analog system might use a load cell (scale) that weighs the tank, and a rate valve. The PLC could use a PID feedback loop (see section below) to control the rate valve. The load cell is connected to one of the PLC's analog inputs and the rate valve is connected to another of the PLC's analog outputs. This system fills the tank faster when there is less water in the tank. If the water level drops rapidly, the rate valve can be opened wide. If water is only dripping out of the tank, the rate valve adjusts to slowly drip water back into the tank.
In this system, to avoid 'flutter' adjustments that can wear out the valve, many PLCs have a "deadband". A technician adjusts this deadband so the valve moves only for a significant change in rate. This will in turn minimize the motion of the valve, and reduce its wear.A real system might combine both approaches, using float switches and simple valves to prevent spills, and a rate sensor and rate valve to optimize refill rates. Backup and maintenance methods can make a real system very complicated.

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