RSLogix500 PID help

[realolman]

I have been troubleshooting a system containing PID's that doesn't perform correctly (it's nothing dangerous)

the setup screen shows a setpoint. In some setup screens I can change the setpoint right in the setup screen and others I can't.

Seems to me you should have to be able to change the setpoint from outside the setup screen.

How do you set them up to change the setpoint from outside the setup screen?

The help doesn't seem to.
thank you

 

[Ron Beaufort]

In some setup screens I can change the setpoint right in the setup screen and others I can't.

in the setup screens that you can NOT change, the number/value is probably coming from some other source ...

for examples:

from an operator's HMI screen ...

or from some scaling operation in the ladder logic ...

How do you set them up to change the setpoint from outside the setup screen?

you told us that you're using RSLogix500 – but you didn't tell us what hardware platform ... so there are TWO possible answers to your question ...

FIRST ... with an INTEGER control block ...

let's say that your PID's control block address (23 words long) starts at N7:100 ... then "WORD 2" of the control block will be the Setpoint ... so in this example, N7:102 would contain the setpoint value ... (sanity tip: start counting at "word ZERO" for N7:100) ...

so if you wanted to change the Setpoint from within your ladder logic, you could use something like an MOV (Move) command to write some particular value into the Setpoint location ... specifically, N7:102 would be the Destination of the Move ...

SECOND ... with a "PD" type control block ...

let's say that your PID's control block address is PD9:0 ... then in this example, PD9:0.SPS would contain the setpoint value ...

so if you wanted to change the Setpoint from within your ladder logic, you could use something like an MOV (Move) command to write some particular value into the Setpoint location ... specifically, PD9:0.SPS would be the Destination of the Move ...

if this doesn't help enough - then post again with more details - and consider posting your entire program file ... the more we know about your system, the more detailed we can make the help that we offer ...

 

[realolman]

Thanks to your help I found where the setpoint was being changed.

I figured it had to be changed somewhere else... seems to me a setpoint that you have to access the ladder logic wouldn't be widely useful .

What threw me is that on some of them you CAN change it in the setup screen. I'll have to see if I can determine why.

Thank you

 

[Ron Beaufort]

What threw me is that on some of them you CAN change it in the setup screen. I'll have to see if I can determine why.

in many (most?) cases like this, you'll find that the MOV command which changes the Setpoint has been conditioned with a ONE SHOT action ... for example: you'll often find an OSR or an ONS instruction on the MOV command's rung ...

this is usually done so that more than one source can be used to control the Setpoint ... basic idea: whichever rung "votes last" gets to override any previous settings ...

again, seeing your program would allow us to help you nail this down ...