Hi everybody,
I need some help on PID loops in a project. I have done this project for a pasteurizer and I control the temperatures in two compartments with PID loops controlling analogue pressure valves which in turn control two pneumatic steam valves. The project has been working perfectly well for more than a year. About 3 weeks ago I recieved a call from the customer and I visited the plant. Sometimes one, other times both PID loops behave in a very strange way, then the problem goes away for a couple of days and then back again.
The problem is that lets say I have a SP of 60 Celsius and the actual temperature is 59.9, the PID will start from 6400(closed valve) to 7000 then somewhere close to 8000 then suddenly 32000 then again back to 8000 and continues like this until temperature goes close to 60.8 where it goes gradually below 60 and starts again. The customer didnt change anything on the PID settings neither on his process.
The steam and air pressures are ok.
I am using Siemens S7-200 and the PID subroutines are locked by SIEMENS so I cannot see anything in this Subroutines.
Any ideas would be appreciated
I need some help on PID loops in a project. I have done this project for a pasteurizer and I control the temperatures in two compartments with PID loops controlling analogue pressure valves which in turn control two pneumatic steam valves. The project has been working perfectly well for more than a year. About 3 weeks ago I recieved a call from the customer and I visited the plant. Sometimes one, other times both PID loops behave in a very strange way, then the problem goes away for a couple of days and then back again.
The problem is that lets say I have a SP of 60 Celsius and the actual temperature is 59.9, the PID will start from 6400(closed valve) to 7000 then somewhere close to 8000 then suddenly 32000 then again back to 8000 and continues like this until temperature goes close to 60.8 where it goes gradually below 60 and starts again. The customer didnt change anything on the PID settings neither on his process.
The steam and air pressures are ok.
I am using Siemens S7-200 and the PID subroutines are locked by SIEMENS so I cannot see anything in this Subroutines.
Any ideas would be appreciated