The first example is that of a flow control loop on a liquid flow. Figure 1 shows a closed loop test with a step change of setpoint.
Figure 1.
Three problems immediately stand out:
1. The tuning is terribly slow.
2. The valve is sticking badly. It takes a long time to move then sticks several more times as it opens.
3. The valve is probably extremely oversized which can be seen by the huge step in PV as compared with small step in the controller output.
Figure 2 shows the open loop stepping test. This confirms deductions 2 and 3 made from the closed loop test. In particular it shows that the valve is at least 7 times oversized. In addition another extremely serious problem has been uncovered, namely that the valve overshoots really badly, particularly in the closing direction, and the positioner takes an extremely long period to correct the overshoot. These factors make good control almost impossible.
Figure 2.
This is confirmed in Figure 3 which is closed loop and shows a setpoint step change made with much better tuning, which was kept as slow as possible to try and overcome the problems of sticking and overshoot. However, it did not help as the valve stuck and then hugely overshot. The controller tried to bring it back, but the same thing happened in reverse and the valve is now in what could be described as a terribly slow stick-slip cycle.
Figure 3.
Problems on a pressure control loop
The second example is an interesting pressure control loop. The operators were always complaining about this loop, which they had to try and control in manual as it did not control in automatic.
A closed loop test with the existing tuning parameters was tried, but nothing seemed to happen when a setpoint step change was made. Very fortunately there was a flowmeter in series with the valve, which allowed us to examine the valve performance. The open loop stepping test showing the flow versus the controller output is shown in Figure 4.
Figure 4.
The following extremely serious valve problems were revealed:
1. There is no valve movement until the controller output gets above 30%.
2. The valve is very sticky.
3. When making the same size steps on the controller output the valve response is very erratic and non-repeatable with big movements in some places, and small in others. At times it didn’t move at all. This makes any reasonable control impossible.
However, further testing of the actual pressure loop showed it to have extremely slow dynamics, and in fact crude but reasonable pressure control was obtained, by running the loop with “ON/OFF” control, that is with the valve either fully open or closed. This is equivalent to controlling the temperature in a bath, as opposed to “throttling” control when controlling the temperature of a shower.
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 gives training courses which can be held in clients’ plants, where students can have the added benefit of practising on live loops. His work takes him to plants all over South Africa and also to other countries. He can be contacted at Michael Brown Control Engineering cc, +27 (0)82 440 7790, [email protected], www.controlloop.co.za
Connecting every transport node RJ Connect
Editor's Choice Data Acquisition & Telemetry
Stockholm's bus system strategically links urban mainline, suburban mainline, non-mainline routes, community service buses and night buses. To acquire and process data from multiple sources and analyse onboard information on their moving buses, Transdev sought a dependable and powerful onboard computer. It teamed up with CatAB, Moxa’s local representative, known for delivering top-notch industrial data communication boards and equipment since 1988.
Read more...Local range of planetary units SEW-EURODRIVE
Editor's Choice Motion Control & Drives
As SEW-EURODRIVE South Africa actively extends its offerings to customers, the SEW PPK and SEW P2.e industrial gearbox ranges are good examples of solutions that are well suited to the local business environment.
Read more...Case History 195: Unstable reboiler steam flow Michael Brown Control Engineering
Editor's Choice
A high-pressure steam flow control in a reboiler on a column in a petrochemical refinery continually cycled when placed in automatic. Several attempts had been made to tune the controller, but these had been unsuccessful.
Read more...Open control system for retrofit of conveyor control system Beckhoff Automation
Editor's Choice
For every online retailer, warehouse logistics is part of the critical infrastructure. An Australian office equipment supplier has retrofitted the warehouse logistics installation of its central warehouse, and replaced the proprietary decentralised controllers of the conveyor lines with PC-based control from Beckhoff, based on powerful EtherCAT communication.
Read more...Digital industrial platforms and why they are important
Editor's Choice
One of the most significant trends driving digital transformation is the emergence of digital industrial platforms. This article will briefly explore what digital industrial platforms are, why they are important, and how they might shape the future of industrial automation.
Read more...Celebrating 65 years: rebuilding and redefining its legacy
Editor's Choice News
Founded in 1959 by Neill Simpson, Axiom Hydraulics has grown into one of South Africa’s elite hydraulic companies. Over the past six and a half decades they’ve weathered many challenges, but none as devastating as the fire of 2023.
Read more...Young robotics team takes world title igus
Editor's Choice News
In an inspiring demonstration of innovation and teamwork, Texpand, a young South African robotics team, recently made history by winning the 2024 FIRST Tech Challenge World Championships.
Read more...SAIMC: It’s not black and white SAIMC
Editor's Choice SAIMC
Grey imports are a problem worldwide, not least in the automation industry in South Africa. The Supplier Advisory Council (SAC) operates under the umbrella of SAIMC, and is tackling this problem head-on.
Read more...Loop signature 25: Tuning part 3 - Results of tuning a particular simple self-regulating process by several different methods. Michael Brown Control Engineering
Editor's Choice
A couple of SWAG methods of tuning were given in the previous Loop Signature article. I have tuned a simple self-regulating process using those methods, and two other tuning methods, one of them being the sophisticated Protuner tuning package, which is the system I employ. The tests were performed on a very accurate and powerful simulation package, and the results are compared below.
Read more...PC-based control for advanced hydrogen storage technology Beckhoff Automation
Editor's Choice PLCs, DCSs & Controllers
The proportion of renewable energies from solar, wind and water is rising continuously. However, sufficient storage options are of the essence to use these energies as efficiently as possible. GKN Hydrogen offers a particularly compact and safe option, low-pressure metal hydride hydrogen storage systems with PC-based control from Beckhoff.
printer friendly version