Greetings all,
I'm new to PLC and I've been reading through many of the posts concerning PID process control in RSLogix 500. I have programmed the control incorporating all that I've gathered through all these great posts and replies, however I am not getting the PID to fully control my process and wanted to ask what I am missing.
I've got a pump that puts out a 0-100% output signal to set flow that is measured on a flowmeter from 0 to 1500 milliliters. The idea is to set the flowrate and have the pump dial in the frequency to maintain the flow set point. What I have done is taken the 0-1500 milliliter flowrate and scaled it to 0-16383 to use at N17:14 for the process variable of the PID. I've then taken the 0-16383 control variable N7:85 and scaled that back to 0-100 to send to the pump.
I do get the frequency to start the pump and give me flow, however it doesn't seem to be responding to the error or trying to close the gap between the flow set point and measured flow. The frequency on the pump seems to be a function of the Gain from the following set up screen and no actual controlling occurs.
The above pic is with the process off, so in actual operation the setpoint SP would have a number: 1092 corresponding to SP of 100 ml/min. There is a corresponding control output CV(%) say 12.1% which seems to correspond only to whatever I set the Gain to. Let's say at a gain of 1.0, I get a flow at the meter of 89 ml/min and NO ACTION takes place in order to close the gap, i.e. the frequency does not increase. At a Gain of 1.1, I get an output of 12.6% and a flow close to 100 ml/min, but it isn't actually being controlled by the PID.
What I thought might be the problem is the scaled error SE box had 0 even with the process running (I thought since it doesn't register an error, it doesn't see the need to take action) so I have added another rung (not displayed) to mov the calculated error to this bit, again this did nothing for the PID to actually control the pump. I know I am missing something here, can someone more experienced please help me out? Thanks for all your patience and excuse my ignorance.
Thanks in advance,
pd
I'm new to PLC and I've been reading through many of the posts concerning PID process control in RSLogix 500. I have programmed the control incorporating all that I've gathered through all these great posts and replies, however I am not getting the PID to fully control my process and wanted to ask what I am missing.
I've got a pump that puts out a 0-100% output signal to set flow that is measured on a flowmeter from 0 to 1500 milliliters. The idea is to set the flowrate and have the pump dial in the frequency to maintain the flow set point. What I have done is taken the 0-1500 milliliter flowrate and scaled it to 0-16383 to use at N17:14 for the process variable of the PID. I've then taken the 0-16383 control variable N7:85 and scaled that back to 0-100 to send to the pump.
I do get the frequency to start the pump and give me flow, however it doesn't seem to be responding to the error or trying to close the gap between the flow set point and measured flow. The frequency on the pump seems to be a function of the Gain from the following set up screen and no actual controlling occurs.
The above pic is with the process off, so in actual operation the setpoint SP would have a number: 1092 corresponding to SP of 100 ml/min. There is a corresponding control output CV(%) say 12.1% which seems to correspond only to whatever I set the Gain to. Let's say at a gain of 1.0, I get a flow at the meter of 89 ml/min and NO ACTION takes place in order to close the gap, i.e. the frequency does not increase. At a Gain of 1.1, I get an output of 12.6% and a flow close to 100 ml/min, but it isn't actually being controlled by the PID.
What I thought might be the problem is the scaled error SE box had 0 even with the process running (I thought since it doesn't register an error, it doesn't see the need to take action) so I have added another rung (not displayed) to mov the calculated error to this bit, again this did nothing for the PID to actually control the pump. I know I am missing something here, can someone more experienced please help me out? Thanks for all your patience and excuse my ignorance.
Thanks in advance,
pd