Automated Process Control of Water Pump


Thread Starter


How to design a control system to automatically maintain water level. Means it turns on pump when water in the tank is 20% and turns off the pump when the level is 80% percent of full scale value?

Which controller should i go for or the simple magnetic switch will serve the purpose.
Hi Depesh,

For such application, on the hardware side you will need a PLC with at least 1 channel AI (analog Input) with 4-20 mA signal and 1 channel DO (Discrete Output). A level transmitter with output of 4-20 mA and scaled to 0-100% will be needed to be connected to your AI.

On the logic software side, I suggest you to use combination of Set Reset (R-S) logic function block, Less than or Equal (LEQ) function block and Greater than or equal (GEQ) function block. When the measured level is equal or less than 20% the LEQ function block will be set RS function block and when the measured level is greater than or equal 80%, the GEQ function block will reset Rs function block. 

Your pump control can be as simple as the level switches wired into the motor control such that:

1-when in auto at 20% turn pump ON, needs a latching start circuit

2-when in auto at 80% turn pump OFF, opens to clear latch

you can add timers to give a minimum pump run time. You could do this logic in a PLC or DCS, depending how you do motor control. You should avoid logic latches and flip-flops.
The application, the size of the pump, the flow rate, and the criticality of the system will determine if a simple level switch and motor control logic is enough or if more complex with more redundancy is required.

Complete application information would improve the responses you get.
First of all thank you dwpatterson, i just wanted to know could i simply use single pole double throw switch to control my motor operation. If not then why? Will there be consequences if i do not use any of the traditional PID controllers in my process for control of level?
You will need a SPDT switch for each level setpoint.

Simply use a NO contact that closes when level is 20% to start your pump. In series with an Auto mode switch contact (auto=closed) the 20% level switch would energize the motor control start circuit and seal in the main contactor.

The 80% level switch will be wired as a NC stop contact with an Auto mode contact in parallel (auto=open). The 80% level switch will de-energize the seal in run circuit and stop the pump.

Since this is an ON-OFF application, simple level switches controlling the motor control will work.

You will have to decide if the controls need additional alarms or redundant level measurement.