Auto/ Manual Mode of PID Controller- why two modes?

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I recently came across concept of auto /manual mode for PID controllers. I was wondering why two modes are required. PID is intended to change output according to set point and auto regulate during disturbances. Can just one mode not be used for both changing set point and regulation? why two separate circuits?
 

Hi,

I recently came across concept of auto /manual mode for PID controllers.
Click to expand...
I think it's a good idea to give us the (link to the) document...

I assume it tells you "what" is auto and manual....

Klaus
 

The linked article discusses specific details of controllers manual to automatic switching and is obviously not intended to teach basic terms of control engineering. Nevertheless it gives you an idea about the purpose of mode switching, if you just read it thoroughly.

Consider that a controller is not necessarily always active. It may be activated and deactivated at times. Manual mode means that a controller is deactivated, in other words not reacting on set point and process value variations. Instead the controller output ("manipulated value") is either hold at a defined initial value or controlled by a different source. "Bumpless" is a keyword in mode switching, start controller operation without any unwanted excursions of the manipulated value.
 

Hi FvM
My question is why two different modes of operation. I wonder where such a scheme can be used? conventional process loops have only only one mode of PID settings, which allows user to change the set point and handle disturbances as well
 

They're not talking about different types of controllers or different controller settings.

They're talking about the 'bump' that might occur when control is turned completely off or back on.



I.E. if this is a car when you turn the cruise control on to 60mph when you're going 40mph a 'bumpless' controller won't floor the gas.
 

Hi

Further googling gave me the idea that in manual mode output is set according to operator and in auto mode, PID behaves as regulator to maintain the set value. I could understand that this would facilitate different ramp rates for manual and auto. I suppose in manual mode we could go faster and in auto mode we could be some what slower.
 

Basically there's no "ramp rate" in manual mode. Automatic control operation is paused and the manipulated valued overridden by a different source.
 

If SP can be varied through controller, then why bother having two circuits and worry about bumpless transfer?
 

If you don’t want a manual mode then don’t have a manual mode.

It’s a system level choice.
 

When the application demands it. I presume, you'll recognize when it's the case. For the time being, don't worry about manual mode.
 

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