16 August 2023 www.instrumentation.co.za CONTROL SYSTEMS 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, michael.brown@mweb.co.za, www.controlloop.co.za Loop signatures 19 Noise – nuisance or enemy? This Loop Signature series of articles would not be complete without sections on the problems of noise and filtering, which are two of the most misunderstood areas in process control. For the purpose of this discussion, noise can be defined as a random variation in the process variable (PV) signal around a mean level. There can be various causes for this, but generally it is caused by the process itself, or by the measuring method. Typically, noise in flows may be caused by turbulence, and in tank levels by ripples and disturbances on the surface of the liquid in the tank. Figure 1 shows a portion of a closed loop test on a coarse ore feed conveyor belt. It can be seen that the signal coming in from the weightometer is extremely noisy. The amplitude of the noise is approximately 25%, and the noise, when put through a spectrum analyser displays more the characteristics of white noise, and does not show any significant individual harmonic frequencies. The noise in this case is mostly caused by the different sizes of rocks passing over the weightometer. What are the detrimental effects of noise? There are really only two. Firstly, as far as the control itself is concerned, noise can be transmitted through the controller from input to output and may cause the output to start jumping about. If the final control element (e.g. valve) can follow these movements, it will lead to increased wear on the mechanical components in the final control element, leading to a shorter life, and also the control variance itself may be increased. In the example shown in Figure 1 it can be seen that the noise on the output of the controller is so small that it will not cause the ore feed system to respond to it. Secondly, the noise can create problems for the human beings who are using the control system, like operators, process people and control and instrument personnel. Generally the more serious of the two problems is the latter – the effect of noise on humans! To understand what I am getting at, refer to Figure 2, which shows two people in the plant looking at a pressure reading. The operator is looking at a digital signal on his screen, and sees the pressure as 3,532 bar. The other man is out in the plant looking at the pressure on the dial of a pressure gauge. He sees the position of the needle on the scale relative to the range, and also that the pressure is about 3.5 bars. The operator reading the digital signal, is completely reliant on the actual value displayed. He has no reference to a relative value, and has no immediate idea if the value is high or low, whereas the person looking at the gauge can immediately see roughly what and where the value is. For control purposes, on most loops one is not too worried about absolute accurate values. It is generally more important that the process be kept at the setpoint value. In the old days of analogue instruments, operators often used to make a mark with a pen on the PV scale on the controller at the value where they wanted the PV to be controlled. You normally are not interested in controlling to three decimal places, and even with the best control in the world, your PV on most industrial processes is always jumping around a certain amount due to noise and minor load variations going on in the process. Figure 1. Figure 2.
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