leitmotif
Member
Have situation that has both me and the instructor at school stymied.
PROJECT OVERVIEW Simultating using water pumped thru pipe embedded in concrete to melt snow or ice on sidewalk.
EQUIPMENT Allen Bradley SLC (forgot to bring home model number) with NT 4 thermocouple mV module. Using only two of the 4 thermocouples. Thermocouples are type K.
Each thermocouples is immersed in its own bath (warm water and ice bath). Over the short time frame temp in each bath may vary by a degree or two. Thermocouples are not reversed (checked by heating with match - temp goes up).
The LES (less than) instruction is used to compare thermocouple readout to a desired temp - in case of ice bath 40 F and in case of warm water bath 100. One LES instruction for each case.
PROBLEM
1. The temp readout displayed by the LES instruction fluctuates wildly ie from -454 to + 100 over a matter of seconds.
2. The LEDs on the thermocouple go out occasionally indicating a fault.
3. When the LEDs on the thermocouple module go out the outputs also fail stopping the system.
Instructor thinks there is a mismatch between the PLC processing speed and the processing speed fo the thermocouple module ie the PLC is outrunning the thermocouple module.
SOLUTIONS attempted
1. Adjusted the filter rate - did not help.
2. Ran thermocouple leads (they are continuous no splices) as far away from any power conductors as possible
3. He added a timer so that each LES instruction is activated every five seconds.
4. This eliminated the fluctuations
5. BUT then caused the outputs to short cycle every five seconds.
6. The short cycling can be overcome using another timer configured to TOF
7. We then used the Move (MOV) to put a copy of the input into another file (N11:10 and 11) in hopes that this would yield a stable reading - no help
This methodology strikes me like putting bandaid on gangrene. I want to understand what is causing the fluctuations (gangrene) and eliminate the problem.
THINGS I NEED to do (after rereading the manual)
1. Make dang double sure the NT 4 is compatible with the PLC. There is a table in teh manual for this.
2. Trouble shooting section in manual says I can check bits in the inputs for trouble codes.
3. Heck if I know
QUESTION
1. What is causing the fluctuations?
2. How do I overcome the problem?
Thank you for the help
Dan Bentler
PROJECT OVERVIEW Simultating using water pumped thru pipe embedded in concrete to melt snow or ice on sidewalk.
EQUIPMENT Allen Bradley SLC (forgot to bring home model number) with NT 4 thermocouple mV module. Using only two of the 4 thermocouples. Thermocouples are type K.
Each thermocouples is immersed in its own bath (warm water and ice bath). Over the short time frame temp in each bath may vary by a degree or two. Thermocouples are not reversed (checked by heating with match - temp goes up).
The LES (less than) instruction is used to compare thermocouple readout to a desired temp - in case of ice bath 40 F and in case of warm water bath 100. One LES instruction for each case.
PROBLEM
1. The temp readout displayed by the LES instruction fluctuates wildly ie from -454 to + 100 over a matter of seconds.
2. The LEDs on the thermocouple go out occasionally indicating a fault.
3. When the LEDs on the thermocouple module go out the outputs also fail stopping the system.
Instructor thinks there is a mismatch between the PLC processing speed and the processing speed fo the thermocouple module ie the PLC is outrunning the thermocouple module.
SOLUTIONS attempted
1. Adjusted the filter rate - did not help.
2. Ran thermocouple leads (they are continuous no splices) as far away from any power conductors as possible
3. He added a timer so that each LES instruction is activated every five seconds.
4. This eliminated the fluctuations
5. BUT then caused the outputs to short cycle every five seconds.
6. The short cycling can be overcome using another timer configured to TOF
7. We then used the Move (MOV) to put a copy of the input into another file (N11:10 and 11) in hopes that this would yield a stable reading - no help
This methodology strikes me like putting bandaid on gangrene. I want to understand what is causing the fluctuations (gangrene) and eliminate the problem.
THINGS I NEED to do (after rereading the manual)
1. Make dang double sure the NT 4 is compatible with the PLC. There is a table in teh manual for this.
2. Trouble shooting section in manual says I can check bits in the inputs for trouble codes.
3. Heck if I know
QUESTION
1. What is causing the fluctuations?
2. How do I overcome the problem?
Thank you for the help
Dan Bentler