One of the plant’s more profitable lines produces what is basically a plastic film. Lately, the thickness variance has been exceeding the allowable limit so often that scrap rates are unacceptable and delivery times are unreliable. Initially, the process engineer focused on potential problems with the production of the slurry that’s fed to this machine. There were some minor issues with the feed valves for each raw material, the vat temperature control, and the mixing unit. These didn’t totally explain the variances, but were corrected anyhow.
Now the process engineer suspects drive motor speed variations, but he had an electrician check the variable frequency drive (VFD) and all of its settings and diagnostics are fine. How would you solve this?
Begin by ensuring the drive is getting good instructions from the programmable logic controller (PLC), or other type of controller. With the line running, measure each sensor input to see if it’s within the expected range. If not, you must methodically locate and correct the cause(s).
Take these steps:
- Check the sensor wiring and connections, where practical. Any millivolt sensing cabling will be very sensitive to induced signals; reroute as needed.
- Calibrate each sensor. Adjust or replace as needed.
- Calibrate each sensor loop; simulate a signal back to the PLC and read it there to see if the PLC is reading what is sent to it. If not, the problem is in the signal cabling.
