PID Control Action Mode in RSLOGIX 5000

plcnovel

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Hi All
In RsLogix 5000 PID instruction Help menu, it says that .CA member of the PID structure is responsible for Control Action:

IF .CA = 0 then E = SP - PV
IF .CA = 1 then E = PV - SP

Now from RSlogix 500, it says specifically that E = SP - PV is Reverse Acting Mode where as E = PV - SP mode is a Direct acting mode.

My question is, Is it true for RSLOGIX 5000 PID Instruction as well. Means can we say the following:

If .CA = 0 then mode is REVERSE (E = SP - PV)
If .CA = 1 then mode is DIRECT (E = PV - SP)

Because in RSLOGIX 5000 help menu for PID it doesn't say that E = PV - SP is DIRECT ACting and E = SP - PV is a REVERSE ACting

Thnaks in advance
 
The terminology is pretty standard. Reverse and Direct acting. E=PV-SP is just another way of saying Direct acting. It means Error = Process Variable - Setpoint. Basically, if your actual process value is greater than the setpoint and the appropriate controller action is to increase the output, that is direct acting. The output moves in the same direction as the error. In a reverse acting loop, the output moves opposite of the error.

For example, a temperature control loop using a heating (more output = more heat) would require a reverse acting loop.

If it uses cooling instead, (more output = more cooling) then a direct acting loop is required.
 
mellis said:
....[with] Direct acting... The output moves in the same direction as the error. In a reverse acting loop, the output moves opposite of the error.

For example, a temperature control loop using a heating (more output = more heat) would require a reverse acting loop.

If it uses cooling instead, (more output = more cooling) then a direct acting loop is required.



Definition, definition, definition.

It's all in how you define your terms. From the AutomationDirect.com uer manual chapter on PID Loop Operation

ADC D2-250 manual said:
...which we call “direct-acting”. This means that when the control output increases, the process variable also eventually increases. Of course, a true process is usually a complex transfer function that includes time delays. Here, we are only interested in the direction of change of the process variable in response to a control output change.
Most process loops will be direct-acting, such as a temperature loop. An increase in the heat applied increases the PV (temperature). Accordingly, direct-acting loops are sometimes called heating loops.

The manual goes on to say that reverse-acting loops are often called cooling loops.

SO...
Allen-Bradley (500) output direct/reverse with respect to error. (EDIT: The PID equation always direct with respect to error. Direct vs Reverse refers to how error is calculated.)

Koyo (ADC) output direct/reverse with respect to (expected) effect on PV

(EDIT: should have said
RSLogix500
Reverse-acting = Heating
Direct-acting = Cooling

Koyo
Reverse-acting = Cooling
Direct-acting = Heating)


Worry more about how the system will act, than whether or not it is called a direct-acting or reverse-acting loop.
 
Last edited:
Bad terminology

plcnovel said:
Hi All
In RsLogix 5000 PID instruction Help menu, it says that .CA member of the PID structure is responsible for Control Action:

IF .CA = 0 then E = SP - PV
IF .CA = 1 then E = PV - SP
.CA = 0 ---> positive gain
.CA = 1 ---> negative gain

plcnovel said:
Now from RSlogix 500, it says specifically that E = SP - PV is Reverse Acting Mode where as E = PV - SP mode is a Direct acting mode.
Yes, but what they are really saying is that when .CA = 1 they multiply the gain Kc by -1 for you. There controller has a negative gain.
 
Hi Peter
Thanks for your calrification. So it means , lets say, we have a discharge pressure loop in a compressor, would it be a negative gain or positive gain loop controller or it depends upon the process
 
plcnovel said:
Hi Peter
Thanks for your calrification. So it means , lets say, we have a discharge pressure loop in a compressor, would it be a negative gain or positive gain loop controller or it depends upon the process

It depends on the process.

If you need to increase the output to increase the PV, then it is positive gain.

If you need to increase the output to decrease the PV then it is a negative gain.

"Discharge pressure loop"
Does that means you open a vent to decrease pressure? I.e. the PID loop output would be a % open of a vent valve. The more open it is, then the faster the pressure goes down? Then it would be a negative gain loop.
 
Fromt he instruction help in RSLOgix 5000

ControlAction BOOL Control action request. Set to calculate error as E = PV - SP; clear to calculate error as E = SP - PV.
Default is cleared.

Edited...
I see what your asking now, rockwell never states which is direct or reverse.
IF .CA = 0 then E = SP - PV
Then when your PV goes above SP your output will decrease to bring PV back.
and when you PV goes below SP your output will increase to bring PV back.
This is Direct acting

IF .CA = 1 then E = PV - SP
Then when your PV goes above SP your output will Increase to bring PV back.
and when you PV goes below SP your output will Decrease to bring PV back.
This is Reverse acting

Since default is cleared I would also assume that .CA = 0 then E = SP - PV represents Direct acting as I don't think Reverse acting would be the default

Edit....
Lol I am tired and just realized this thread is 4.5 years old. :(
 
Last edited:
...
Worry more about how the system will act, than whether or not it is called a direct-acting or reverse-acting loop.


^ this.


TL;DR




Wow.

It appears as if, to avoid confusion, Rockwell intentionally chose to not use these terms at all in the RSLogix 5000 documentation.

Because, as is plain both in this thread and a few other places I looked, there is neither a universal nor a de facto nor even a prevalent convention for the meaning of the terms "Direct Action/Acting" and "Reverse Action/Acting" in PID controls.

If one is competent enough to attempt using a PID, then RA's mathematical formulae

  • Δ.OUT = Kc x Δ.ERR + ...
  • If .CA = 0 => .ERR = .SP - .PV
  • If .CA = 1 => .ERR = .PV - .SP
will unambiguously provide what one needs to know, without risking any confusion by picking one convention for those terms.

In some PLCs, there is no .CA-equivalent, and .ERR = .PV - .SP without exception; to invert the controller response one simply sends the measurement to .SP and the setpoint to .PV (assuming Kc must be positive).

I don't remember any convention from controls class four decades ago; if I had a preference it would be based on grammar and semantics: Direct/Reverse Acting Controller seems to refer to the controller output response direction to a change in the measurement controlled; Direct/Reverse Acting System seems to refer to the measurement response direction to a change in the output. However the suffix Controller or System is rarely added after Acting. Also, the issue is not at all further helped by the muddled use of the terms Acting and Action. So it not worth dying on either hill.




P.S. Yes, I see it's an old thread
 

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