PADILLA (Customer) asked a question.
I am writing to request assistance with the implementation of a closed-loop pressure control system for an extrusion machine. My name is Francisco, and I am working on this project at Parker Matamoros.
Project Objective:
The main goal is to automatically control the pressure of the extrusion machine by varying the speed of a motor, using a PID control loop programmed within the PLC.
Hardware Utilized:
- PLC: DL06 Series with F0-4AD2DA-1 analog I/O combination module (4 inputs / 2 outputs, 4-20mA).
- Variable Frequency Drive (VFD): GS4.
- HMI: C-more.
Current Configuration and Expected Operation:
- Pressure Measurement (PV): The actual pressure signal (Process Variable, PV) is captured via analog input CH1 of the F0-4AD2DA-1 module.
- Speed Control (CV): The motor speed (Control Variable, CV) is controlled using analog output CH1 of the F0-4AD2DA of the same module, which sends a 4-20mA signal to the GS4 VFD at the AI1 connections.
- PID Loop: The PID table has been configured in the PLC (starting at memory address V4000) to manage the control loop.
- Operating Mode Transition:
- Currently, the machine operates in a local/remote mode using a potentiometer (local in the VFD's) and analog control (remote mode in VFD's)
- Upon selecting "automatic control" on the HMI, the VFD is switched to remote mode (controlled by the PLC's analog output), and the PID loop should take over motor speed control to regulate the pressure to the desired Setpoint.
Problem Description:
When I initiate the automatic control sequence:
- I press the "Automatic" button on the HMI.
- The VFD is successfully switched to "Remote" mode.
- I input the Setpoint (SV) into the PLC.
- The Process Variable (PV) is being measured correctly from the analog input (CH1) of the F0-4AD2DA-1 module.
However, I am not observing any corresponding value or signal from the analog output (CH1) of the card, which is intended to control the motor speed via the VFD's analog input (AI1). The VFD is not receiving the 4-20mA speed reference signal.
Sincerely,
Francisco
Parker Matamoros
I have some suggestions. We have done many PID loop controls with the DL PLCs. In the C-more, make a PID screen showing all of the PID parameters with indicator lights and numeric displays, include a trend graph to show response to help in tuning. For output, monitor the actual raw output and scale it 0-100%. Also show status, like the PLC auto bit, etc. Showing the Bias value also helps in tuning, to monitor the Bias ramp up and ramp down over time. Also include a Hand-Off-Auto Radio Button. This allows manually testing output, invaluable test tool. All this can be tested before connecting actual hardware, using a 4-20ma simulator for input, and measuring ma output with a test meter (ma output going thru a 250-ohm resistor. Then vary the input and monitor HMI display and verify the 4-20 ma output. All this will prove out your PLC program and PID operation.
You can also monitor the PID parameters using DirectSOFT, but I found this is problematic (to say the least) in a plant setting during startup.
In your case, when you say "The VFD is not receiving the 4-20mA speed", you mean the PLC is outputting any 0ma? 4ma? as measured with a test meter? If 0ma, this could be wiring or the jumper setting inside the module (jumper should be set for 4-20ma). If 4ma, this could be your PID loop is not in auto, or programming issue. With the HMI screen (or DirectSOFT), you can verify PID Auto status, and verify actual raw output. Also, you can verify if you can run the VFD/motor in PLC PID manual mode. This can test your programming, wiring, and VFD config.
Thank you very much.
I understand there is one more step I am not taking; this is my first PID control that I am doing and also the first time I am using analog signals, but I consider myself very good and a quick learner.
I am taking the process variable directly from the analog card and sending the control output also directly from the analog card, but I feel that I need to perform what you mentioned about automatic or manual tuning for my equipment to function correctly. I will continue reviewing the documentation, or otherwise, I will use the VFD's PID instead, I feel that way it will be easier.
You can certainly get the PLC PID working. It does take some time. Yes, the VFD does have PID capability, but I still would use the PLC because you have the HMI to easily show and adjust all the PID variables and show a trend graph, so much easier to adjust, tune, monitor, and test. This is easier than going into VFD settings. Plant engineers do not want to go into VFD settings to adjust PID parameters.
(Yes, the PID in the DL PLCs is tricky, and I thought that's the way PID was supposed to be. But then some years ago, we went with the Do-more! Talk about making things easier!)
Anyway, here is a PID screen that may help. It allows PID monitoring, testing, tuning, all from one screen. Note "LOGIN STATUS". Some password protection planning is required because you want to limit the people tuning the loop. Note the "ALARM" light. You may want a number of alarms for the PID loop. Going to the "EVENT LOG" will display the actual alarms.
Can you send mi the screen shot for e mail please.
franciscojavier.padilla@parker.com
Wow thank you very much, your comments give me goosebumps, so I will continue trying with the PLC's PID and thanks for the screen