Siemens LOGO PI regulating

[drbitboy]

interesting. where do the two missing close-parentheses go? I would assume they are at the end of the formula, because if one was earlier (i.e. just before the -Kp), then it would be an normal PI control.

 

[Peter Nachtwey]

CO=min(max(CO(n-1)+Ki*T*(SP-PV)-Kp*(PV-PV(n-1)),100),0)

 

[drbitboy]

huh? I need to double-check, but that is no different than the regular PI equation.


[Update: I did the double-check; mathematically that is no different from the regular PID equation (with Kd=0)]


Are you sure it is not this (originally-missing parens highlighted):

CO=min(max(CO(n-1)+Ki*T*(SP-PV-Kp*(PV-PV(n-1))),100),0)

 

[drbitboy]

Where is the pressure being measured i.e. how far downstream from the pump (OP mentioned that the system there could be 100m or even 1km of pipe)?


And where is the valve?

 

[drbitboy]

@rQx: can you post your code e.g. a screen shot of the blocks, at least of the section that handles the PI controller's PV, SP and CV.

 

[Peter Nachtwey]

There is a small difference. The proportional gain is only multiplied by changes in the PV not the error. The difference is that the I-P doesn't add a zero to the closed loop transfer function. Zeros extend the bandwidth of the response ( make it fast ) but it also will cause over shoot if it isn't in the right place. There other feature is that the proportional term will subtract from the control output as the PV approaches the SP and can offset what the integrator adds to the control output. It will reduce over shoot.


The accumulator gives the I-P term a chance to respond to the pressure changes due to the valve changing.

 

[drbitboy]

There is a small difference. The proportional gain is only multiplied by changes in the PV not the error. The difference is that the I-P doesn't add a zero to the closed loop transfer function. Zeros extend the bandwidth of the response ( make it fast ) but it also will cause over shoot if it isn't in the right place. There other feature is that the proportional term will subtract from the control output as the PV approaches the SP and can offset what the integrator adds to the control output. It will reduce over shoot.

The accumulator gives the I-P term a chance to respond to the pressure changes due to the valve changing.

I understand the expressed form is different, but if SP is constant, which is usually the case, then it can be no different, mathematically (and can therefore be no different functionally), from the regular PID equation.

Since it is no different functionally, I don't see how we can get an added zero.

This is straight algebra; what am I missing?
_

 

[Peter Nachtwey]

You are right if the SP doesn't EVER change. But..

In a normal PI controller the integrator winds up and doesn't unwind until it over shoots the SP.


If the SP doesn't change then it is the velocity or incremental mode that make all the difference because reduction of the error subtracts from the accumulated output.


The time integrator time constant or ratio of Kp/Ki needs to be fast enough to respond to the pressure changes.

 

[drbitboy]

While @PeterN composes the post that will straighten me out, that got me to thinking about constant SP, which took me back to the externally-added Derivative action (D-action) idea ...

One way to do ad D-action would be to calculate the rate of change of PV externally, and if it goes above a certain positive value (e.g. let's call it the WHDL - water hammer detection limit), the external logic could subtract a delta, scaled from that rate, from the setpoint SP. Another piece of the external logic could gradually reduce the accumulated deltas down to zero, so it has no effect in normal operation i.e. when the PV is not rising "quickly."

I know this is a Rube-Goldberg/Heath-Robinson approach, but if you've ever messed with LOGO! you'll understand why i.e. it's needs must.

Sidebar: This approach relative to built-in Derivative-action what operator "Reset" used to be relative to Integral action, in the P-only, fixed bias, pre-PID days of yore. I learned from @OldChemEng the other day that that is why Integral action is traditionally called Reset: on P-only controllers, if the process changes (load, cooling medium temperature, etc.), the controller would respond but always end up with an offset of PV relative to SP; this is well understood and often covered in PID tutorials. The operator's solution was to "reset" the SP to a faux value to get the PV back to the target SP.

 

[drbitboy]

You are right if the SP doesn't EVER change. But..

In a normal PI controller the integrator winds up and doesn't unwind until it over shoots the SP.

If the SP doesn't change then it is the velocity or incremental mode that make all the difference because reduction of the error subtracts from the accumulated output.

The time integrator time constant or ratio of Kp/Ki needs to be fast enough to respond to the pressure changes.

Otay, but you have still not shown that the equation you presented, with one paren on the I term and one on the P term, is any different, or will respond any differently, from a normal PID equation (with Kd=0).