D

It is widely accepted that setpoint should be removed from D-term. However, when I mentioned setpoint should also be removed from P-term, I got quite a few objections. Attached below is my reply to these objections. I would like to hear comments from this email group, especially PID controller designers from various PLC, DCS, and microcontroller manufactures.

After its half-a-century development, I think it is high time that we, the so-called control people or control engineers, made it clear what is the TRUE PID. I believe this discussion will benefit the whole industry.

Danyang Liu, PhD of Process Control.

---------------------------------------------

Kari and others,

I am very pleased that my BESTune posting has attracted so much attention.

Thank you all.

Since 1998 while I developed my first PID tuning software BESTune using Matlab/Simulink, I have been using the TRUE PID equation:

CV=Kp*(-PV)+Ki*int(SP-PV)+Kd*(d/dt)(-PV)

The reason why SP should be removed from the P-term is the same with reason why SP should be removed from D-term, which is:

"A step change in SP sometimes can cause an unwanted big change in CV"

The only difference between the two is: When SP changes, the bad effect (sudden large change in CV) of using Kd*(d/dt)(SP-PV) will last longer than using Kp*(SP-PV). You can see this clearer if you look at the discrete form of TRUE PID.

Most industrial PID designers noticed the bad effect of using Kd*(d/dt)(SP-PV) and they removed SP from the D-term. Unfortunately, they still keep SP in the P-term, such as PIDs from A.-B., which I would call semi-PIDs. Until 1998 I proposed the TRUE PID, I did not find any PIDs using the TRUE PID equation.

I noticed that some people believe in some applications such as motion control, textbook PID should still be used in order that PV tracks the

changing SP more quickly. Well, I don't think so. The reason is very funny here: when textbook PID and semi-PID are tuned in a conservative way, the

bad influence of step change in SP helps PV to track SP more quickly.

This is why in my BESTune version 3, I asked users to reduce the tuning related controller tightness when we have to tune textbook PIDs or even semi-PIDs.

In TURE PID, the response speed of PV can be raised by increasing Ki and/or reducing Kp and/or Kd. This is easy to understand if you know that the I-term acts like an accelerator and the P-term and D-term act like two brakes.

Finally, may I ask you to do me a favor:

(1) If you are PID controller designers, please at least include the TRUE PID as an option in your PID controllers. How to implement TRUE PID? Here it is:

CV(k)=CV(k-1)+Ki*(LUT)*[SP(k)-PV(k)] /* I-term, accelerator

-Kp*[PV(k)-PV(k-1)] /* P-term,

brake 1

-(Kd/LUT)*[PV(k)-2*PV(k-1)+PV(k-2)] /* D-term,

brake 2

This is the form when CV POSITIVELY affects PV, i.e., increase in CV will cause PV to increase.

If you only allow positive values for setting Kp, Ki, and Kd, then if CV NEGTIVELY affect PV, you should change the sign of SP and PV before using

the above TRUE PID equation.

(2) Please forward this email to any PID designers or PLC, DCS, Microcontroller vendors (such as Allen-Bradley, Bailey, Honeywell, Modicon, Siemens, Foxboro, etc, etc.)

Kari called me Mr./Ms. BESTune.com. Well, "BESTune.Com" is the name of my own business (Until now I am not making money at all). My name is Danyang Liu.

Thank you all for your attention.

Danyang Liu, PhD of Process Control

BESTune.Isclever.Com

P.S.: I have update BESTune version 3, which includes both PI and PID tuning.

"Kari Olson" <[email protected]> wrote in message

news:8amunm$eto$[email protected]...

> Thanks for the reason; I forgot the limitations of actuator response for a

> moment.

>

> I guess my real point was to ask why a PID which acts on -PV for P and D

but

> SP-PV for I should be considered the "TRUE PID" by Mr./Ms. BESTune.com. I

> can concede that it may _work_ the best in practice (ignoring solutions

> which filter the SP or rate limit changes to it, which are not under

> discussion here). I don't see how it can be considered the canonical PID

> controller when it is a PID controller on -PV in parallel with a SP

> integrator with the integral gains coordinated.

>

> My two cents,

> -Kari

>

> BESTune (The BEST PID Loop doctor) wrote in message

> >I believe the type of PIDs with P-term and D-term acting on (-PV) is

really

> >a correct implementation of PID control algorithm. A.-B.'s PID (with only

> >D-term acting on (-PV)) is NOT a completely correct implementation of

TRUE

> >PID algorithm. It can only be called a semi-PID algorithm. And PIDs with

> all terms acting on (SP-PV) are too far away from the TRUE PID.

> >

> >Could someone tell me which PID controller implemented the TRUE PID

> >algorithm (i.e. the third type PID in BESTune version 3)?

> >

> >BESTune.Isclever.Com