creating a deadband with RSLogix5000

Hello again! so the PLC I programmed was installed and tested and ready to run the chiller. The rep from Johnson Controls came up and discovered that our engineer made a mistake on the valve and control specifications. What Johnson wanted was a pneumatic modulating valve and a PID controller set up with the process variable as water inlet temperature and the setpoint at 18 degrees ( which I was told originally was. What was installed was a 3 way solenoid valve that actuates open when the temperature was 13 degrees or lower and a bypass pump was then started to bypass the chiller, ( water is already cold enough, does not need chilling ) and above 13 degrees the valve actuates closed and water runs straight in the chiller and the bypass pump is not running. So its too late to change out the valve and there is a way to make this valve work properly without constantly actuating around 18 ( i.e. water temp floating around 17-19 degrees ). I was told I could put in a deadband so that when the water temp reaches 18 degrees and climbing the valve will actuate closed, not just right at 18 degrees, and vice versa when the water temp reaches 18 degrees and dropping the valve will actuate open not just at 18 degrees. How do I go about adding this "deadband" into my ladder logic. I will load up my existing program on the site under downloads. please help as the rep is on site today and would like to see this thing run!! thanks

If you are using the ladder logic PID instruction then PidTagname.DB is your deadband parameter. If you are using the function block PIDE then PidTagName.ZCDeadband and PidTagName.ZCOFF are the deadband parameters and deadband on/off bits.

for some reason the DEDT "deadtime" is not available, the instruction is under "process control" which isn't even a part of my instruction set on my rslogix5000

You'll need to create a few new tags: high_sp, low_sp, and temp_db. I also recommend creating a tag for the setpoint so that you can change the setpoint without having to do an edit to the program, so temp_sp. See the changes below.

There are two PID instructions. There is a ladder logic version which you should have in your RSLogix5000. Then there is a function block version called PIDE - you need the language extensions or L5K-pro to use that version.

Are you creating the program or are you working with a program that was created by someone else?

If the program was created by someone else who had the language extensions and who used PIDE then you won't be able to access the function block routine... HOWEVER, you can still access the PIDE tag and change the deadband parameters using the tag monitor. You can also programatically set numeric values in the PIDE tag using the ladder logic MOV instruction, and change PIDE tag bits using OTE, OTL, and OTU instructions.

TConnolly - he's not using a PID function at all so none of those options are going to be available. It's a simple on/off control based on temperature.

cbuysse said:

You'll need to create a few new tags: high_sp, low_sp, and temp_db. I also recommend creating a tag for the setpoint so that you can change the setpoint without having to do an edit to the program, so temp_sp. See the changes below.

View attachment 28248

Click to expand...

wow, this is awesome!! wish I could have thought of that, that will work perfect. thanks so much!

p.s. I also added a less than instruction so that when the temp is below 10 degrees the bypass motor cannot start at all. this also helps for testing as well because the temperature transmitter was not yet wired up, so the analog signal was 0, so the pump would have have energized as soon as I downloaded the program.

Cbuysse, I just start to learn PLC. Could you please explain how to come up with the expression used in the program? Thanks.

thanks

If you have a fixed deadband, then just use Latch and Unlatch. When at 17 latch bypass bit. When at 19 unlatch bypass bit.