CompactLogix - tieback vs software manual etc.

We have a couple of instances in our control systems where we want to, for example, force a PID controlled motor to a minimum speed or force a 4-20mA control valve fully open. In the past our guys just fiddled the setpoint in the PLC and hidden from the HMI to make the PID do the right thing but this tends to mess things up when reverting to auto and has reliability issues, plus a few customers have complained that this is just bad practice.

I've tried a couple of other options under the pressures of site conditions and with customers breathing down my neck - 1) leaving in auto but limiting output so that the motor/valve goes where we want it, 2) switching to manual and then writing an output in. Both worked but seem ungainly and over complex. Now I have 5 mins it looks like the tieback function is what I should have been using all along. Any other ideas?

On the HMI the Valve should have a HAND-OFF-AUTO "switch" (pushbutton) and a Numeric Entry field.
If you want to force the 4-20mA valve fully open, put it in HAND using the HMI and enter 100% in the Numeric Entry Field which the PLC logic you wrote should scale and convert to 20mA. The valve will then open 100%.

When you are done, put the valve back in AUTO and the process will regain valve control.

You should not be doing ANY bandaid changes to the code or fiddling with anything.
and yes that Is very bad practice.

GTUnit said:

On the HMI the Valve should have a HAND-OFF-AUTO "switch" (pushbutton) and a Numeric Entry field.
If you want to force the 4-20mA valve fully open, put it in HAND using the HMI and enter 100% in the Numeric Entry Field which the PLC logic you wrote should scale and convert to 20mA. The valve will then open 100%.

When you are done, put the valve back in AUTO and the process will regain valve control.

You should not be doing ANY bandaid changes to the code or fiddling with anything.
and yes that Is very bad practice.

Click to expand...

The forcing to 100% etc. I'm interested in is part of the auto sequence. We already have a manual function available to the operator via the HMI but it's when we need to override the normal PID control in the auto sequence that the problem arises.

Fiddling with the setpoint is bad practice - switching into software manual when you don't want the PID to control the devices is not fundamentally wrong but just messy. Have you used tieback?

I have but its been a while. I was doing bumpless transfer between 3 blower motors.
Below is a good thread with some more info:
http://www.plctalk.net/qanda/showthread.php?t=36877

switching into software manual when you don't want the PID to control the devices is not fundamentally wrong but just messy.

Click to expand...

can you give us some details on what you mean by "messy" ? ...

TIP: most people use the Software Manual Mode for what you're trying to do (if I understand you correctly) – and find that it works quite well (not messy at all) in most situations ...

if your situation won't work – maybe there's some underlying problem that we might be able to help you with – if we can see your existing code ...

note that my earlier "tieback" description which has been linked above was written for the PLC-5 platform ... it's a good start – BUT - there are some differences in the ControlLogix/CompactLogix systems ...

it would be helpful if you could post your existing code ... you'd have to zip it first – forum rule ...