Analog flow control

I've programmed discrete I/O control in ladder logic but not analog in or outputs. I have two 4-20ma flow signal inputs that are presently run manually. Flow 1 is a continous flow rate and I want to mix Flow 2 at 2% of Flow 1. Flow 2 has a VFD drive on the pump. Can this be done automatically in ladder logic? We use an Automation Direct DL06 for our discrete I/O.

billbone said:

I've programmed discrete I/O control in ladder logic but not analog in or outputs. I have two 4-20ma flow signal inputs that are presently run manually. Flow 1 is a continous flow rate and I want to mix Flow 2 at 2% of Flow 1. Flow 2 has a VFD drive on the pump. Can this be done automatically in ladder logic? We use an Automation Direct DL06 for our discrete I/O.

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I am not familiar with the DL06, but assuming you have an Analog Output to either control a Valve or the VFD, then absolutely, it is done all the time. Normally a PID Instruction would be used for this, but I have also used a simple Ratio in some cases. It will depend on your exact requirements. There are a number of members who have experience with Automation Direct PLCs that I am sure will help you if you give them enough information to work with.

Stu....

This is not a problem. Depending on the type of pump for flow two you will need one or control two loops.

The first step is to calculate flow 2 as % of flow 1. If the flow 2 pump is peristaltic or other positive displacement type then all you need to do is calculate the speed that corresponds to that flow and send the signal to the VFD.

If flow 2 pump is centrifugal then the flow 2 becomes a setpoint for a 2nd loop, the flow control loop. In this case you would use PID or another algorithm to adjust the speed of the pump until flow 2 equals the calculated setpoint.

PS avoid duplicate posts!