Proportional Feedback 4-20mA Process Control

Digital type chemical dosing pumps are normally featured with the basic constant manually flow rate as well as the advance proportional analog 4-20mA & proportional pulse feedback control!

In analog proportional control, the signal value is calculated from a linear relationship based on the variance from set-point with a maximum output set at a fixed maximum variance. Say for example, in a typical pH control where the output to an acid metering pump will vary from 4-20 mA as the pH increases from 8-10pH. At pH8 or less, 4 mA is sent to achieve 0% output of the pump. As the pH increases to 10 or greater, the output will increase linearly of which 20mA will be sent to the pump to achieve 100% output. In more advance control scenarios, the algorithm to determine the output value may incorporate Integral and Derivative calculations (PID Control) to enable a more predictive output based on the speed of the process response to the output. Even with PID control, the actual output signal remains a 4-20mA resulting in a proportional 0-100% response of the control device.

Proportional Pulse control is receiving from a controller, varying pulse output based on deviation from setpoint, similar to proportional analog control. Using the same acid feed control example as above, the controller will send 0 pulses per minute to the metering pump at pH8 or less; increasing the pulse/minute output proportional for when pH10 or greater and the maximum pulse output will be generated. The metering pump will stroke one time for each pulse it receives, the increasing of pH will enabling an increase in the volume output of the metering pump.

Metering Pump, also known as Chemical Dosing Pump – have whether analogue, digital, solenoid driven. mechanical, hydraulic or plunger piston actuated, risen as a technology of first choice for the injection of chemicals in processing and agriculture applications.