Sign in to follow this  
Followers 0
Rojay

Micrologix PID

6 posts in this topic

Can anyone help me with my PID issues, I can't seem to get them to work. The 3276 - 16383 represents the 4 - 20 mA scale. The 3153 - 15365 represents the actual values of a 4 - 20mA signal injected into the input. The 8761 represents the 9GPM required flow. Any help would be appreciated. Thanks in advance, Rojay PID.pdf PID.doc

Share this post


Link to post
Share on other sites
you can used your 3153 and 15365 as your Input min and Input Max, then for your scaled Min and Scaled max. used the min and max of your raw data. (ex. flow meter . at the name plate you can see the minumum and maximum flow, let see " 0 to 10000Kg/Hr.) so at your scale min = 0 and Scale max= 10000 Edited by Gnex

Share this post


Link to post
Share on other sites
What do you mean by doesn't work? What is or isn't working? We need more information to help you out.

Share this post


Link to post
Share on other sites
Everyone thanks for replying. I do have all of my JSR commands in Main2, I just deleted all of the other logic for file size considerations. At first I tried running the straight PID, with my needed 9GPM setpoint level, and had no output from my PID. So I added to the value that represents my 9GPM, thinking that my PID would only output the needed correction factor. I was told to have my input represent my flowmeter range, but it being only a 0-20GPM transmitter, I wanted more resolution than that. I tried using a float, but the PID didn't like that. If you have any other questions that mighy help you figure out what I am doing wrong, please ask.

Share this post


Link to post
Share on other sites
You don't need to offset the output by the setpoint. The output of a PID is essentially 0-100%, representing the amount of output that the PID controller calculates that you need to move the process variable towards the setpoint. The output is actually scaled from 0-16383, which is intended to be moved directly to an analog output without additional scaling. There is also a sub-element of the PID datatype that returns the control variable as a percentage (PD9:0.CVP). It shows up on the setup screen labeled "Control Output CV (%)". Looks to me like your control variable is pegged at 16383 (100%), which is the max output value. That means the PID controller is doing something, otherwise you'd get a zero. Make sure the setpoint and process variable are scaled to the same range so that the error calculation comes out right, but most likely you just need to tune the PID. Here's a quick test: try changing the setpoint to be less than the process variable. The control variable should drop to zero.

Share this post


Link to post
Share on other sites
Thanks everyone. I am going to the panel tomorrow, and will change the program to the values to 0-16383, SPS to 7372 (9GPM on a 0-20GPM scale 45%)and get rid of the add block. After that it will just be adjusting my gains to get the results I need. I will read up on some techniques to do this before I go. I will keep everyone posted.

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!


Register a new account

Sign in

Already have an account? Sign in here.


Sign In Now
Sign in to follow this  
Followers 0