Britman
New Member
I have an application where I want to test a PLC programme. In the past, working with DCS systems, I have used process simulators that had been built by the companies I worked for. The principle is that an analogue output which would normally go to say a control valve, is filtered to simulate a process, (i.e. time delay etc.)
This signal is then feedback to an input to complete the control loop. As far as I can tell these simulators are basically RC circuits. I have no problem with that. However, I have a situation where the PLC output is not the usual 4 – 20mA. We are what we call ‘split ranging’ an output, (actually using 2 outputs), to drive 2 control valves. One valve responds to 4 – 12mA, and the other 12 – 20mA. I wish to create a simulation of the process using these two outputs, but I need to sum them such that my return signal is 4 – 20mA. If I can subtract 12 mA from the first signal and then add the remaining 0 – 8mA to the 4 – 12mA signal, I shall have my 4 – 20mA. I came up with the attached drawing.
Would this work? I am assuming that the optimum setting of the variable pot would be 2KΩ? Is it felt that the attached circuit would subtract 12mA in a stable manner? If not, can anyone suggest something that will? Thanks, Chris.
This signal is then feedback to an input to complete the control loop. As far as I can tell these simulators are basically RC circuits. I have no problem with that. However, I have a situation where the PLC output is not the usual 4 – 20mA. We are what we call ‘split ranging’ an output, (actually using 2 outputs), to drive 2 control valves. One valve responds to 4 – 12mA, and the other 12 – 20mA. I wish to create a simulation of the process using these two outputs, but I need to sum them such that my return signal is 4 – 20mA. If I can subtract 12 mA from the first signal and then add the remaining 0 – 8mA to the 4 – 12mA signal, I shall have my 4 – 20mA. I came up with the attached drawing.
Would this work? I am assuming that the optimum setting of the variable pot would be 2KΩ? Is it felt that the attached circuit would subtract 12mA in a stable manner? If not, can anyone suggest something that will? Thanks, Chris.