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Old November 4th, 2013, 02:50 PM   #1
irondesk40
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Allen Bradley for temperature control again

Been working on a servo project but have had the project to control temperature using a allen bradley temperature controller to control temperature instead of Fuji temperature controller come up again. I have orderd the Allen bradley thermocouple input card and can read the temperature thanks to info posted on this site from some earlier questions i asked.
Another question i have is what would be the best approach to use to control a output that will be used to turn on a solid state relay? I had planned on using a analog solid state relay but due to cost that will not happen. Was told by boss to use the same solid state relays we have used which use a 3-32 volt input signal. I had planned on setting up a pid loop to control the temperature by a analog signal to the analog solid state relay. Once i read the teperature in with the thermocouple card in the allen bradley and if the temperature does not equal the desired set temperature would it work okay to do a compare statement to compare the actual temp to the desired temp and then start pulsing a regular output that would be used to control the solid state relay that is looking for a 3-32 volt signal? Would it be best to use a transistor output? if not, seems to me that the relay output on the allen bradley output card would not hold up very well in the long run.
Any advice would be greatly appreciated. Thanks
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Old November 4th, 2013, 02:57 PM   #2
Ken Roach
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In general you always use a solid-state DC output point on the PLC to run the SSR gate signal.

For ordinary resistive heaters, I use an SSR and a 1-second time period, which I divide up with a time-proportional output.

I just use a simple repeating timer with a millisecond timebase and a LEQ instruction on the Accumulated value.

The PID output is 0-100 (percent), so I multiply it by 10 and compare the result to the Accumulated value of the timer.

This is called "time proportional output" and is a very standard way to control resistive heaters.
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Old November 4th, 2013, 03:07 PM   #3
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would it be too much to ask if you could attach a sample program? there will be a total of 8 heaters so i would need 8 transistor outputs to use to turn on the 8 Solid State Relays, but still confused. i had a pretty good grasp when i had planned on using a analog output but now just brain dead. Thanks a lot
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Old November 4th, 2013, 03:18 PM   #4
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Do you need PID control or would a gap control work.
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Old November 4th, 2013, 04:03 PM   #5
Ken Roach
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I agree that you need to take a step back and think about what you're controlling. Most heat control systems use a PID equation, but some just act like a programmable thermostat.

Please post details on the PLC model you are using, as well; there are a handful of different A-B programming platforms.

Attached is a screenshot of what I was talking about with a time-proportional output.
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File Type: jpg Time_Proportional_Output.jpg (62.9 KB, 213 views)
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Old November 4th, 2013, 08:02 PM   #6
irondesk40
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thanks. i am using a micrologix 1400 plc. Machine will have 8 heat sections and the way i see it i would have to have 2 of the thermocouple cards and 8 point DC output card. I was pretty sure how to do this once it came back up using a analog Solid State Relay and a PID loop but with the on/off type output was not quite sure the best approach and be able to accurately control the temperature. Thanks
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Old November 4th, 2013, 08:10 PM   #7
irondesk40
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Quote:
Originally Posted by Ken Roach View Post
I agree that you need to take a step back and think about what you're controlling. Most heat control systems use a PID equation, but some just act like a programmable thermostat.

Please post details on the PLC model you are using, as well; there are a handful of different A-B programming platforms.

Attached is a screenshot of what I was talking about with a time-proportional output.
Ken, i would have never thought of that. Just not enough time to think through how to properly do anything. I was very confident using the analog Solid State Relay and a PID loop but that looks like a great way and cheaper. The analog relay is very expensive. In your experience how accurate was you able to control the temperature. I really like the little fuji units, basically because i can concentrate on the AC drives and Servo controllers and operation of the machine and not have to deal with the temperature. Just does not make sense to me in my mind to dedicate plc processing time etc. to control the temperature when the little fuji units do a excellent job. Thanks
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Old November 4th, 2013, 08:12 PM   #8
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forgot to mention the temperature is usually around 410 degrees.
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Old November 4th, 2013, 08:17 PM   #9
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Quote:
Originally Posted by cornbread View Post
Do you need PID control or would a gap control work.
PID control was how i was leaning toward doing it when i was going to use a analog solid state relay but due to cost that is now not a option. Could you explain gap control. I am open to all suggestion. Analog temperature control is the only way i have ever done it.
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Old November 14th, 2013, 04:09 PM   #10
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ken still confused

ken, i have recieved the thermocouple card and can read the temp in such as 70 degrees F. I can set a setpoint for the desired temp such as 400 degrees F.
having difficult time setting up a micrologix 1400 to use the ouput of a pid to pulse a output to a solid state relay. I read and downloaded a tech note about using a timed pulse width output to control a output. But having difficult time getting it to work. I know i am doing something stupid but spent all week getting a unidrive and a redlion communicating and just plain brain dead. Could you possibly post a example that would show this. thanks
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Old November 14th, 2013, 04:33 PM   #11
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forgot to attach my file. not sure what i am doing wrong. I read the temp in and if i manually put in a setpoint to equal the actual temp the output stays on. what am i possibly doing stupid. At this point, any suggestions would be greatly appreciated.
thanks
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File Type: zip THERMOCOUPLETESTINGONBENCH_Ken_test.zip (13.8 KB, 68 views)
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Old November 14th, 2013, 04:56 PM   #12
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here is some more testing. i was able to get the output to come on if the setpoint is higher than the actual temp. Am i doing this the proper way or am i looking at this wrong.
any advice would be appreciated. I have never done temp control with the plc and they expect it to work as good as the fuji controllers and having a difficut time.
thanks.
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File Type: zip THERMOCOUPLETESTINGONBENCH_KEN_TEST5.zip (15.0 KB, 17 views)
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Old November 15th, 2013, 03:01 PM   #13
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i was able to do some reading today, Boss not in, and i think i have a working example of how to control the temp using a allen bradley micro 1400.
A big learning experience. Hate i asked so many questions, but tried for hours every day to get through to ab tech support and not much luck and my local ab programming rep not that great. This site is a life saver.
If anyone feels like taking a look, please feel free to comment and tell me anything stupid or different i should look at.
Thanks
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File Type: zip Micrologix1400_TempControl_Bench_Gain200.zip (15.1 KB, 81 views)
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