PID with RSLogix500 question

[busarider29]

Setting up a PID instruction to control the temperature of oil in a small tank. First time using PID instruction. The heaters are 480VAC energized from a solid state relay with 110 VAC coil. Following the instructions in the Logix500 instruction set manual, it has a small program for time proportional control on page 9-31 which I plan to implement in my program. Is the latching output in that program the direct output to the coil to my SSR? Should I be using an analog output address to energize the coil??

 

[Lancie1]

Is the latching output in that program the direct output to the coil to my SSR?

If you mean the Latch for Output O:1.0, then yes.

Should I be using an analog output address to energize the coil??

No, not if you use the time-proportioning method. It only uses digital ON/OFF outputs.

Where you need to use analog outputs is when you have some type of controller (say an SCR temperature controller). Then you can feed the scaled PID output directly to the temperature controller. Without the temperature controller, you can use the PID with the time-proportioning method. Basically you are setting the % time (of the 10-second cycling timer T4:0) that the output stays ON. You definitely need solid-state relays for this, as they have to switch on and off a bunch of times.

 

[busarider29]

If you mean the Latch for Output O:1.0, then yes.

No, not if you use the time-proportioning method. It only uses digital ON/OFF outputs.

Where you need to use analog outputs is when you have some type of controller (say an SCR temperature controller). Then you can feed the scaled PID output directly to the temperature controller. Without the temperature controller, you can use the PID with the time-proportioning method. Basically you are setting the % time (of the 10-second cycling timer T4:0) that the output stays ON. You definitely need solid-state relays for this, as they have to switch on and off a bunch of times.

Thanks for clear feedback. I was about 90% sure those were my direct digital outputs but wanted to be completely sure before proceeding. I'm sure I'll have more questions with the PID thing before I have it working so stand by....LOL .

 

[Mickey]

What is your temperature tolerances? You may not need a PID.

A simple on-off with a deadband would be a whole lot easier.

 

[busarider29]

What is your temperature tolerances? You may not need a PID.

A simple on-off with a deadband would be a whole lot easier.

Temperature tolerances are somewhat tight. I could certainly do it without the PID like we have it on some other machine setups. However, I thought this would be a good opportunity to learn PID as it certainly would be useful in some of our other applications such as oil flow control, motor speed ramp up control, etc. This is a new machine that I'm attempting to implement this on. I may have to temporarily set it up like our other machines until I get the PID working and dialed in.