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PID Auto Mode/Manual Mode Settings

12/24/2018 6:57 AM

Hi,

I came across concept of bumpless transfer with reference to PID controllers. I was wondering why two modes are required in the first place. Conventional temperature PID controllers, directly responds to user inputs and also perform regulator function simultaneously. Why then is it required to bypass a controller?

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#1

Re: PID Auto Mode/Manual Mode Settings

12/24/2018 8:20 AM

Why then is it required to bypass a controller?

It depends on what the controller is used for. You may want to drive the system away from the setpoint temporarily and then return to automatic control with the original set point. Here is one example for manual override:

https://www.homebrewtalk.com/forum/threads/pid-manual-mode.509659/

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#2
In reply to #1

Re: PID Auto Mode/Manual Mode Settings

12/24/2018 9:32 AM

HI

My question is, can controller not be used for varying SP as well as for regulation? I mean let the large step got through the controller ( instead of bypassing) and when the desired set point is reached, let is regulate

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#3
In reply to #2

Re: PID Auto Mode/Manual Mode Settings

12/24/2018 10:50 AM

No reason why not. If you want the new set point, that would be the way to do it. Here is an example:

Your car has cruise control. If you want a new speed, just bump it up or down. You changed the set point.

If you want to pass a slow driver, you step on the gas and speed up (manual control). When you release the accelerator, the car returns to the old set point. This would be an example of manual override.

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#4
In reply to #3

Re: PID Auto Mode/Manual Mode Settings

12/24/2018 10:59 AM

Hi

This is my point as well. Then no need to switch between auto or manual mode. Only one mode could do. Change the set point at will and then regulate when done.

But why PID controllers come with two options?

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#5
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Re: PID Auto Mode/Manual Mode Settings

12/24/2018 11:51 AM

Because PID units without both modes do not sell as well as those that do have both modes.

Also manual mode can be very useful when troubleshooting a failing system.

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#6
In reply to #5

Re: PID Auto Mode/Manual Mode Settings

12/24/2018 11:56 AM

"Because PID units without both modes do not sell as well as those that do have both modes."

This is commercial concern and does not address the need for two modes. Finally can SP be changed in auto mode? is yes then manual mode is unnecessary

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#7
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Re: PID Auto Mode/Manual Mode Settings

12/24/2018 12:19 PM

"Also manual mode can be very useful when troubleshooting a failing system."

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#8
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Re: PID Auto Mode/Manual Mode Settings

12/24/2018 6:29 PM

Thanks SE. But it is obvious that vishnu's modesty and experience is vastly superior to all on this subject. I am amazed they even bother with us mortals.

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#10
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Re: PID Auto Mode/Manual Mode Settings

12/25/2018 3:34 AM

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#12
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Re: PID Auto Mode/Manual Mode Settings

12/27/2018 12:06 AM

If the controller tuning is slow due to process characteristic, or due to an upstream or downstream failure, an alert operator can override the tuning to correct for an upset before the tank overflows, for example.

For instance, the fill valve for a tank is wide open, trying to keep up with a rapidly falling tank level. The downstream process stops taking the liquid out of the tank suddenly, the pump trips off or the valve closes. The process gain of the tank level is slow, lots of gallons of fluid for a very small change in level, for example.

The operator sees the high level alarm on the tank, sees the fill valve is at 100%, slowly, slowly, slowly closing, and puts the level control PI controller in manual and sets 0% for the valve, stopping the flow into the high level tank.

While you could do the same thing perhaps almost as fast response by setting 0% for the level set point, you then have to reset the level, and hope you remember where it should be, among all your other duties.

Also, some processes are complex feedback with long delays, they are easier to start up in Manual, until close to your final quality product, then once all processes stabilized, put the process in Auto control. Bumpless transfer is best for this, the operator just pops it in Auto and can then attend to other things, and not have to compensate for control induced upsets due to controller integration windup, for instance.

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#9

Re: PID Auto Mode/Manual Mode Settings

12/24/2018 10:49 PM

Reasons PID controllers have a manual mode option.

1. temporary manual mode to deal with upsets
The process has an upset and the operators action is to go to X % output to recover or idle or do whatever necessary for that process in that situation.

2. Manual station
There are points in processes where the loop is always controlled manually and a PID controller is used for that purpose
- because it can be located local to where the manual control observation is most convenient
- because it has its own HMI, both a display showing the PV and the % output and the functionality (buttons) to move the output to wherever the operator wants to move it.

3. Loop commissioning.
When a new loop is commissioned, one of the tasks is to force the output to zero and see if the valve responds to whever 0% is supposed to be, force to 100%, force to 50% and confirm valve response.

4. Trouble shooting
I can't tell you how much I cursed when commissioning a pair of mass flow meters on a atmosphere furnace with PLC control where only a corporate engineer with PLC development software could force an output to modulating valve on the gas lines. Guess who wasn't around that day? The HMI had no manual mode for those loops. The shenanigans I had to go through to do a basic loop start-up.
Another use: Controller is not tuned properly and the PV is oscillating all over, chasing its tail. The user doesn't believe the controller can even hold a steady temperature.

Put the controller in manual and force the output to 25%. Wait 10, 15 minutes. The temperature won't be at setpoint, but the temperature will be stable, which proves that it can happen, even if not in modulating mode.

Comments on manual mode.

Generally ovens/furnaces are better off without manual mode. Once upon a time, when I answered the support line for a PID controller used primarily in thermal markets, about 70% of the complaints were "Your controller is broken. It's not controlling. It's stuck at XXX (PV value higher or lower than setpoint)."

I'd say, look in the upper right corner. Is there a letter M highlighted in red that you can see? Usually the answer was yes.

I'd say, "touch the button labelled M A N, just once, so the lit-up letter M goes out. Then go get a cup of coffee and when you finish your coffee, go check the controller. If it's not controlling, give me a call back at extension 123.
Popping the controller from manual mode back into automatic mode solved about 95% of the reported problems for loops that had been working but were now "broken".

That general stuck-in-manual problem disappeared over time as the manufacturer got smart and provided the means to enable or disable the auto/manual mode in the setup. Then, commissioning and troubleshooting techs can enable the auto/man key when needed and disable it general use.
But, in the process industries, it is quite common for an operator to throw a loop into manual mode to deal with an upset in the process.

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#11

Re: PID Auto Mode/Manual Mode Settings

12/26/2018 1:12 PM

The more appropriate question is "When", not <...Why...>.

The answer to this question can be found in the plant's Control Philosophy and Functional Design Specification documents. As every application is different, the forum cannot answer the question better than the individual user.

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#13

Re: PID Auto Mode/Manual Mode Settings

12/27/2018 6:45 PM

Automatic bumpless transfer refers to a centralized computer system that is controlling a process via a field located PID controller.

If the computer fails,the controller reverts to the manual mode,and uses the same setpoint and parameters as was used in the auto mode.Thus,the "bumpless" description.

It also allows the operator to change setpoints in the field if required when the need arises without having to change a program plan in the main computer.

The manual mode is also used for calibration check of the controller by injection of inputs and outputs that do not interfere with the centralized computer.

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#14
In reply to #13

Re: PID Auto Mode/Manual Mode Settings

12/27/2018 7:09 PM

bumpless also goes in the other direction, manual to auto, or from one regulator mode to another, such as between speed and torque control for rotating machinery, as an example. Setpoint tracking, PV tracking, MV tracking are enabled to facilitate the various transfer modes.

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