Editor's Choice


Case history 186: Don’t always trust valve position feedback signals

March 2023 Editor's Choice PLCs, DCSs & Controllers

I recently encountered an interesting problem in a minerals recovery processing plant. The loop in question was a gas flow control to a burner and was considered very important for the process temperature control. The operators reported that the loop cycled badly in automatic, and was very difficult to control manually. The C&I technicians had tried all sorts of tunings without any improvement.

They included this loop in a series of optimisation tests we were doing as part of the practical we held after they had finished the classroom part of my basic control course. They said they knew that valve problems often cause problems for control but that this loop had a valve position feedback signal on it and the feedback signal closely followed the controller output signal so they were pretty sure it wasn’t the cause of the problem.

The first test we normally carry out is a Closed Loop As Found test and we recorded how the loop responded to setpoint (SP) changes in automatic with the original existing tuning parameters. However, as reported, the loop cycled badly.

The second test was an Open Loop test where the controller is placed in manual, and various steps are then made on the PD (controller output). Part of this test is shown in Figure 1. Unfortunately we did not record the valve position feedback signal. It can be immediately seen that there was in fact a huge valve problem as the valve was not only apparently sticking in places but also had huge ‘almost hysteresis’ of over 10%. I say ‘almost hysteresis’ because it did move a little on most reversals and then stuck whilst the PD had to move a further 10% before the valve moved again. This was the major reason for the cycling.

The question is why the valve feedback signal followed the PD so closely. The answer is, and this is something every control practitioner should be aware of, valve position feedback signals generally do not track the actual valve position, but track the position of the actuator. Now depending on the type of valve and actuator, there are often various linkages joining the two. In this case the valve was a butterfly valve, which is a rotary valve. The actuator was a spring and diaphragm type which has a linear action. To convert the linear action to a rotary action, manufacturers employ various techniques, typically geartrains. Therefore the hysteresis and/or apparent stickiness may actually be due to play in the linkages.

On the steps where the valve, and hence the flow PV, did follow the PD, we were able to see that there was a slight non-linear installed characteristic in the valve, and we were able to establish the process dynamics to allow us to get good tuning parameters. The original tuning parameters were actually too fast and were contributing to the cycling, so it was not only due to the valve problem, and the tuning was also causing instability.

Final Closed Loop test with the new parameters is shown in Figure 2 and is interesting. On the SP step down it can be seen how the valve started moving and then stuck, and the PD then integrated down well over 10% before the valve could move again and bring the PV back to SP. On the SP step up the valve initially didn’t move at all until the PD had integrated up the same amount, and then it moved to get the PV to SP.

This is another example of how important it is to analyse a loop before tuning and to be aware of the various problems that can exist. As I have said many times in the past, the old adage that tuning can solve all problems is nonsense. I have seen the same problem of slippage in the valve linkages many times in the past, and it somis ething that not many C&I; practitioners seem to be aware of.

About Michael Brown


Michael Brown.

Michael Brown is a specialist in control loop optimisation, with many years of experience in process control instrumentation. His main activities are consulting and teaching practical control loop analysis and optimisation. He now presents courses and performs optimisation over the internet.

His work has taken him to plants all over South Africa and also to other countries. He can be contacted at: Michael Brown Control Engineering CC, +27 82 440 7790


Credit(s)



Share this article:
Share via emailShare via LinkedInPrint this page

Further reading:

A South African legacy in telemetry
Editor's Choice Industrial Wireless
Telemetry is becoming a vital component of industrial strategy, allowing companies to harness real-time data to optimise processes and reduce waste. One company leading this technological shift is Interlynx.

Read more...
Case History 199: Another example of the effectiveness of cascade control
Michael Brown Control Engineering Editor's Choice Fieldbus & Industrial Networking
In my last article I wrote about how cascade control systems can effectively overcome valve problems. This article gives another example of how a temperature control was able to perform well, in spite of really severe valve problems.

Read more...
Upgrading legacy automation
Omron Electronics Editor's Choice Fieldbus & Industrial Networking
Legacy automation is characterised by technology in the later stages of its useful life. As new automation technologies continue to emerge and interconnect at an exponential rate, failing to integrate these technologies can widen the gap between the competitive and the obsolete.

Read more...
Planetary gear units for high torque requirements
SEW-EURODRIVE Editor's Choice Motion Control & Drives
Packing a compact design, along with high torque and low-speed outputs, the new SEW PPK and SEW P2.e planetary gear units from SEW-EURODRIVE offer new capabilities in continuous heavy-duty applications where space is at a premium.

Read more...
These robots crawl into every nook and cranny
DNH Tradeserve t/a DNH Technologies Editor's Choice Motion Control & Drives
Inuktun's small crawler robot magnetically sticks to metal walls and is able to move in all directions. It carries cameras, sensors and tools for inspection or maintenance work in tight pipes and on the outer hulls of tanks or ships. All crawler modules and cameras are equipped with brushed DC motors from Swiss drive specialist, maxon using various motor-gearhead combinations.

Read more...
Swiss watchmaking meets hypercar power
Horne Technologies Editor's Choice
The display of Bugatti’s upcoming luxury model, Tourbillon will be something truly special. Instead of a digital version, the driver will see a genuine Swiss timepiece behind the steering wheel.

Read more...
Reinventing the wheel
Editor's Choice
Once a curiosity in the early automotive age, in-wheel motors are now re-emerging with real promise. From electric cars to commercial vehicles and even aircraft, they are on the verge of transforming transportation engineering.

Read more...
Creating new magnets for electric motors
Editor's Choice
Innomotics, a global specialist in electric motors and large drive systems, is coordinating a consortium for a research project on ‘Integrated Product and Process Innovation for Electric Drives’.

Read more...
Sustainability is transforming fluid power
Editor's Choice Motion Control & Drives
Sustainability is reshaping the future of fluid power. With the growing demand for cleaner, more efficient technologies and tightening global regulations, fluid power systems are being re-engineered for higher efficiency, lower emissions and reduced material usage.

Read more...
The power of water
Editor's Choice Electrical Power & Protection
The Alpenglow Hy4 is the world’s first water-based hydrogen combustion engine, offering a convincing alternative to traditional battery-electric vehicles and established hydrogen fuel cell designs.

Read more...









While every effort has been made to ensure the accuracy of the information contained herein, the publisher and its agents cannot be held responsible for any errors contained, or any loss incurred as a result. Articles published do not necessarily reflect the views of the publishers. The editor reserves the right to alter or cut copy. Articles submitted are deemed to have been cleared for publication. Advertisements and company contact details are published as provided by the advertiser. Technews Publishing (Pty) Ltd cannot be held responsible for the accuracy or veracity of supplied material.




© Technews Publishing (Pty) Ltd | All Rights Reserved