Micrologix To Compact Logix PID temp cntrol

irondesk40

Member
Join Date
Jan 2008
Location
nc
Posts
630
Have some machines that i used a micrologix 1400 plc and using thermocouple inputs and outputs to control solid state relays to maintain the desired temperature of some press molds on some machines. With this setup the temperature can be maintained with 1 degree of accuracy from 0-420F.
Now looking at swapping out the micrologix with a compact logic 1769-L24er and working at the bench to duplicat the program.
Running into a issue with the temperature control. IN the micrologix i use the control variable output to determine the preset times for timers used to turn on the outputs to solid state relays. If the desired temperature requested is higher than the actual output then the control variable of the PID is a larger number. As the desired temperature gets closer to the actual temperature the value of the control variable gets smaller which causes shorter output cycles to the solid state relays.
Trying to simulate this with the compact logix and the control variable is always either 0 or 100%, it never varies from 0-100%.
Confused as to what i am doing wrong. Could someone that has a lot more experience with the compact logix take a look and see what i am doing wrong. I will attempt to attach some screen shots. Due to the 300k size limit had to break them up into mulitple screen shots.
Thanks
 
Also, you need to move your PID block to a periodic task (you have to create one), instead of being in a subroutine of the main task. Make sure the time period of the periodic task equals the PID block's update time.

Also, an unrelated issue, you probably need to change your scaling. Your process variable is scaled like a MicroLogix, you will want to scale it to something like 4000 to 20000 for your analog input (and also configure your AI module for 0-20mA or 4-20mA or whatever). Engineering units are typically something useful to an operator like 0 - 500.0 (*F). And your control variable may need to be something like 0-1000 for your PWM control.
 

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