Dear All,

I have a situation where i need to control the speed and the stroke of a metering pump using only one Analog output signal. The speed will be calculated automatically by the controller using PID control loop. The setpoint will calculated from the required dosing rate and the process variable will the flow of the pump. The stroke will be a direct setpoint from the operator. My question is how to include the effect of the stroke setpoint in the control loop? i mean what will be the effect of the stroke in the analog output from the controller?

best regards,
Ibrahim

 

Ibrahim,

1. You cant do it. If you want to control 2 variables (Speed & Stroke length) you need 2 outputs.

2. It is normal practice to use the PID controller to adjust a dosage rate. If required the PID output is added to a throughput rate feed forward to give a desired flow target.

For example, if you are adding a chemical to a liquid stock flow, the PID calculates a % change in chemical required and the feed forward compensates for changes in stock flow.

The PID output does NOT control the metering pump directly.

Next you calculate the desired Speed and Stroke for the metering pump. This means if the operator adjusts the stroke for example, then the Speed calculation immediately adjusts to keep a constant chemical flow without the need to adjust the PID output.


Rob

www[.]lymac.co.nz