PID Control Accuracy

[Bit_Bucket_07]

You put a PID in place to control the flowrate. The "root problem", as you call it, is nearly always there, which is why you spend a lot of time and effort configuring and tuning control loops to regulate flow, pressure, level, current, speed etc., etc.

The purpose of a PID controller is to allow closed loop control without the need for constant human intervention. The PID controller is not capable of overcoming the laws of physics, and is not intended to provide a software patch for a hardware problem. If a process is so dysfunctional that it cannot be controlled by a highly skilled operator, then resolving the mechanical dysfunction within the process is the appropriate solution.

On the other hand, it is entirely possible that the application in question does not actually require that the PV never deviate more that +/- 1% of SP in order to be functional. All I know is that the top poster has stated that this is the desired goal.

 

[daba]

The purpose of a PID controller is to allow closed loop control without the need for constant human intervention.

Exactly so, and it can do a better job than a human

The PID controller is not capable of overcoming the laws of physics

It is not "a law of physics" that a flow-rate, left uncontrolled, will fluctuate. It happens all the time, and for many reasons, source tank changeovers, particulate debris entering the pipework, gas blowout, sluice gate opening/closing, weir starvation, sudden floods, etc. I have worked with PIDs that have had the toughest of challenges, and yet have made them work successfully...

... and is not intended to provide a software patch for a hardware problem.....

.....If a process is so dysfunctional that it cannot be controlled by a highly skilled operator, then resolving the mechanical dysfunction within the process is the appropriate solution.....

A PID controller is in no way a "software patch". It is a tried and tested algorithm that works, it's been around far longer than PLCs. I was cleaning slidewires of the drums of Eurotherm controllers back in the 70's, re-calibrating them, and sending them back for another 2-3 months service....

Most times it isn't a hardware problem at all, nor is it "dysfunctional" - it's just the way it is.... Look at it the other way - "here's what the flow does guys - can you tame it ?"

Show me an operator that can cope with the forced discharge of a mash filter which has been running for 2 hours at 120 brls/hr, and the initial discharge peaks at over 1000 brls/hr, it's all over by the time you've got up off your seat.

On the other hand, it is entirely possible that the application in question does not actually require that the PV never deviate more that +/- 1% of SP in order to be functional. All I know is that the top poster has stated that this is the desired goal.

Which is why I enquired about the actual process under discussion....

 

[Michael Faulknor]

I would like to update my tune experience in the past two day. I totally tune 3 loop, two flow control, one level control. I use AB PIDE instruction which has auto tune function built in. It can generate PID parameter and result is pretty good: +/- 1% tolerance probably 50% of the time and +/- 2% tolerance probably 90% of the time. The requirement is to be within +/- 1% tolerance, look like I need to do some fine tune to the loop to get ride of > +/- 1% toleranc and autotune function can not help me. Could anyone give me some instruction on how to fine tune the loop?
Thanks firstCamcanpro,

Eliminate the suspicions with the system controls: I/P - calibrate it to the desired scale for your control valve (3-15 or other, refer to the name plate or rating on the valve itself). Then refer to your Control Valve manual and set the 'positioner' accordingly, may have a simple 'zero adjust' screw. Then test the valve from a manually controlled 4-20mA signal for operation.

If this is an established system that has operated in the past and the PID was already tuned, chances are your devices involved in the process are where you will find your trouble.


You can also Refer to RS Knowledge Base for answers or examples:
20410 - RSLogix 5000: PIDE - Function Block Enhanced PID
49243 - Example program for PIDE in Single loop,Cascade and Ratio Control
53929 - Comparing PID and PIDE Instructions

Concerning your E+H Flow Meter,you should refer to your E+H Rep for help from their device, I've seen where they have tech support that can provide set up assistance if not a visit.

I would eliminate the suspicions of your process control in the system with regards to 'Is the I/P the problem?' 'Is the Valve the issue?' Establish a standard of expectation...then tune, set up, and run the 'software'.

Simple things cause more people to trip then Dog poo or banana peels.

 

[Bit_Bucket_07]

Simple things cause more people to trip then Dog poo or banana peels.

Heh!

That's a good one!

 

[Peter Nachtwey]

There is a way to compute the accuracy of a PID.

 

[g.mccormick]

What is the time of the +/-5% deviations? The I term is mainly incharge of long term stability. If the deviations are fairly long, the properly tuned PID should be able to control. One thing that may hinder this is the resolution and time constant of the controlled device. For instance, if you are using a valve that is very slow you will probably never get it working correctly.