Flow recorder controller applications5/30/2023 The answer to this question is any factor requiring the controller to change its output in order to maintain the process variable at setpoint. What accounts for the erratic controller output, then? The fact that the process variable never deviates significantly from the setpoint tells us the control system is operating quite well. The chaotic appearance of the output signal is not really a problem, contrary to most peoples’ first impression. We can see from this trend that the controller is doing exactly what it should: holding the process variable value close to setpoint, manipulating the final control element as far as necessary to do so. Here, the setpoint (SP) appears as a perfectly straight red line, the process variable as a slightly erratic blue line, and the controller output as a moderately erratic purple line. Here is an example of a typical “trend” showing the relationship between process variable, setpoint, and controller output in automatic mode, as graphed by a recorder: This is especially true when the recorder is configured to record not just the process variable, but also the controller’s setpoint and output variables as well. Recorders are extremely helpful for troubleshooting process control problems. The strip chart recorder uses a scroll of paper drawn slowly past one or more lateral-moving pens, while the paperless recorder does away with paper entirely by plotting graphic trend lines on a computer screen: Two more chart recorders appear in the next photograph, a strip chart recorder on the right and a paperless chart recorder on the left. Two such chart recorders are shown in the following photograph, used to record temperatures at a powdered milk processing facility: RecorderĪ circular chart recorder uses a round sheet of paper, rotating slowly beneath a pen moved side to-side by a servomechanism driven by the instrument signal. Recorders usually have indications built into them for showing the instantaneous value of the instrument signal(s) simultaneously with the historical values, and for this reason are usually designated as indicating recorders.Ī temperature indicating recorder for the nuclear reactor system shown previously would be designated as a “TIR” accordingly. Another common “auxiliary” instrument is the recorder (sometimes specifically referred to as a chart recorder or a trend recorder ), the purpose of which is to draw a graph of process variable(s) over time.
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