GE Series 90-30 PIDs

Bit_Bucket_07

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I don't have a lot of experience with the 90-30 PID, but I'm working on some for an existing system. My question concerns manual operation. It is my understanding that the CV upper and lower clamps as well as slew control are applied to the output when operating in manual mode. I don't want to live with this restriction. Instead, I (and my client) would like for the operator to have complete control over the output value when operating in manual mode.

A solution that I've come up with is to never use manual mode with the PID enabled, and to simply disable the PID and use a separate operator entry for the manual mode CV reference. I've tested the idea, and it works, but I don't know how to accomplish bumpless transfer with this configuration. I was hoping that some of you who are more familiar with the 90-30 PID could suggest a means for accomplishing this.
 
One possibility would be change the upper and lower clamps and the slew rate when you put the loop into manual. Upper Clamp is located at the starting address plus 9, lower clamp is starting address plus 10 and minimum slew time is at starting address plus 11.
 
One possibility would be change the upper and lower clamps and the slew rate when you put the loop into manual. Upper Clamp is located at the starting address plus 9, lower clamp is starting address plus 10 and minimum slew time is at starting address plus 11.


That would be an option. I am already allowing many of the PID parameters to be adjusted via the HMI by those with an adequate access level (Operator may only adjust Setpoint), so I know what you're talking about here. Is there no other way to bring the PID in line with a CV reference that is outside of the PID?

Also, is there a way to adjust the PID's manual output value aside from using the Up and Down discrete inputs to the PID block? Can I write values directly to Ref+13? I haven't tried that. If I did copy the external manual output value to Ref+13 while the PID is disabled, would that then give me a bumpless transfer upon enabling the PID?
 
Yes, you can write directly to Ref + 13 in manual. When you switch back to auto Ref + 13 will track the CV output from the PID function block. The Up and Down inputs will bump the manual CV by one count per PID solution. To change the size of the increment you would have to add ladder logic outside the PID block.
 
Yes, you can write directly to Ref + 13 in manual. When you switch back to auto Ref + 13 will track the CV output from the PID function block. The Up and Down inputs will bump the manual CV by one count per PID solution. To change the size of the increment you would have to add ladder logic outside the PID block.


Sounds like my best bet is to leave the PIDs enabled unconditionally and switch from manual to auto mode instead, then write the external manual entry into Ref+13 and zero out the clamps and slew rate when in manual mode. I'm afraid that simply writing the manual output value into Ref+13 while the PID is disabled would not give me a bumpless transfer. Sounds like I need a transition from manual to auto mode to get bumpless transfer.

Thanks for sharing your knowledge.
 
Don't zero the clamps. Set the upper to +32000 and the lower to -32000. If you set them to zero you'll get zero output. Do zero the slew time.


Yes, I understand about the clamp values, since I had to allow for the HMI to access those parameters. I was just speaking in general of the concept of eliminating the upper and lower clamps while in manual mode. If I understand correctly, I could actually use a value of zero for the lower clamp, since none of my outputs will ever need to go negative.
 
Here is some info that may help.
A doc explaining the PID configuration word.
Also an Excel spreadsheet, I did many moons ago to calculate values.
 
Here is some info that may help.
A doc explaining the PID configuration word.
Also an Excel spreadsheet, I did many moons ago to calculate values.


Thanks Nathan. Do you know if setting the value of Ref+13 while the PID loop is disabled would result in the PID making a bumpless transfer to that value for the CV when it is subsequently enabled?

I'll soon get a chance to find out for myself when I go back on-site. I'm having to write these loop controls in online mode with a running process underway, so I've had to be very careful.
 
Thanks Nathan. Do you know if setting the value of Ref+13 while the PID loop is disabled would result in the PID making a bumpless transfer to that value for the CV when it is subsequently enabled?

I'll soon get a chance to find out for myself when I go back on-site. I'm having to write these loop controls in online mode with a running process underway, so I've had to be very careful.

NO...I do not remember. I would suspect that power down of PID block will clear all calculations....and I would think that on power up of the PID it would calculate from 0, and if it starts up with a large difference between PV and SP, CV would come up based on PID.
Are you using the old LM90 Software or have you converted the logic to Machine Edition?
 
NO...I do not remember. I would suspect that power down of PID block will clear all calculations....and I would think that on power up of the PID it would calculate from 0, and if it starts up with a large difference between PV and SP, CV would come up based on PID.
Are you using the old LM90 Software or have you converted the logic to Machine Edition?


It's Machine Edition. I'm not concerned about power down. What I'm wondering is what happens when the PID enable input transitions on. I know that a transition from manual to auto provides bumpless transfer, but I don't know if that would hold true for a transition from disabled to enabled.
 
Disabling the PID function will freeze the internal registers, particularly the integral storage. If there is a significant change in the PV while the function is disabled, upon re-enabling the proportional component of the CV will immediately be recalculated, but it will take time to correct integral component of the CV.
"Bumpless" transfer from manual back to automatic is accomplished by the function manipulating the internal stored integral component to account for the changes to the CV while in manual operation. That's why you need to keep it enabled.
 
Disabling the PID function will freeze the internal registers, particularly the integral storage. If there is a significant change in the PV while the function is disabled, upon re-enabling the proportional component of the CV will immediately be recalculated, but it will take time to correct integral component of the CV.
"Bumpless" transfer from manual back to automatic is accomplished by the function manipulating the internal stored integral component to account for the changes to the CV while in manual operation. That's why you need to keep it enabled.


That's what I thought. I'm just going to do what we first discussed. That will work.
 

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