SIMATIC 545 PID

[Maintpro 0]

I am having problems with the PID operations of my SIMATIC 545 PLC. We use the controller in a wax melting operation. The wax kettles are heated by steam and the PID is adjusting the valve position. The valve position range is -25% to 100%. The problem is that when the PV is the same or greater than the SP, the the valves are occasionally still open 50% or sometimes greater (I say occasionally because it is not every time). This leads to overheating of the kettles and burning of the wax. If we turn the steam off (this takes the loop out of auto) and then back on again, the output will correct itself. We have had this system in place for a number of years now and according to the operators, it has never worked properly to begin with. My loop perameters are as follows: POS/VEL PID ALGORITHM: POS LOOP VFLAG ADDRESS: C702 SAMPLE RATE: 2.00 SEC PROCESS VARIABLE ADDRESS: WX177 PV RANGE LOW: 0 HIGH: 300 PV IS BIPOLAR: NO SQUARE ROOT OF PV: NO 20% OFFSET ON PV: YES RAMP/SOAK PROGRAMMED: NO RAMP/SOAK FOR SP: NO ALARM DEADBAND: 0 MONITOR LOW-LOW/HI-HI: NO MONITOR LOW/HI: NO PV ALARMS LOW-LOW: 0 LOW: 0 HIGH: 100 HIGH-HIGH: 100 REMOTE SETPOINT: NONE CLAMP SP LIMITS LOW: O HIGH: 300 LOOP GAIN: 50 RESET: 35 RATE: 0 FREEZE BIAS: NO DERIVATIVE GAIN LIMITING: NO LIMITING COEFFICIENT: 10 SPECIAL CALCULATION ON: NONE SPECIAL FUNCTION: NONE LOCK SETPOINT: NO LOCK AUTO/MANUAL: NO LOCK CASCADE: NO ERROR OPERATION: NONE REVERSE ACTING: NO MONIOR DEVIATION: NO DEVIATION ALARM YELLOW: 100 ORANGE: 100 MONITOR RATE OF CHANGE: NO RATE OF CHANGE ALARM: 0 MONITOR BROKEN TRANSMITTER: NO This is the first time that I have had to deal with a PID system and am uncertain if these perameters, especially the gain, are correct. I am also wondering if the problems are due to fluctuations in the process variable and if I should freeze this value for loop use by moving it into a V-register. I have monitored the program and found that temperature values received by the plc are correct. Any help you can provide would be greatly apprecieated.

[Maintpro 0]

30 views and no replys. What gives? Doesn't anyone have an opinion on what could be causing my temp issues? If this was viewed by all in the other forum then I appologize for being upset. I am not very familiar with PID operation and am just looking for answers.

[Crossbow 296]

Perhaps nobody has read it that know the Siemens 545 PLCs and PID control. Taking an angry tone is certainly not a way to get people to help. Keep in mind none of us is paid for this, we do it merely because we can. If it's a serious problem and you need a quick solution, perhaps it's best to try to contact the people that wrote the program/built the machine, or contact Siemens directly.

[Guest Jim Rozewicz]

If you are using 505 WorkShop for your software, FasTrak's support people will try to help as much as possible. The only problem there, is that they aren't experts on PIDs either. But, they may be able to direct you to further help. You may want to consider logging or trending your LOOP values to be sure they are performing as expected. FasTrak has the Data Logger and Trender built into 505 WorkShop, and can be gotten seperately for unlimited data points, at a very low cost. This may help you monitor your values better. Feel free to contact me, and at the least I may be able to direct you to further help. jim@fast-soft.com 262.238.8088 x1062

[Ken Moore 23]

This guy posted the same question on another board, got lots of answers, but never responded back. I know quite a bit about the 505 pid loops. I asked a few questions to narrow down his problem, but he didn't want to put any effort into his problem. Ken LINK