CONTROL LOOP DIAGRAMS Process ControlsControl loops can be either stable or unstable. Instability is caused by a combination of processtime lags discussed earlier (i.e., capacitance, resistance, and transport time) and inherent time lagswithin a control system. This results in slow response to changes in the controlled variable.Consequently, the controlled variable will continuously cycle around the setpoint value.Oscillations describes this cyclic characteristic. There are three types of oscillations that canoccur in a control loop. They are decreasing amplitude, constant amplitude, and increasingamplitude. Each is shown in Figure 10.Decreasing amplitude (Figure 10A). These oscillations decrease in amplitude and eventually stopwith a control system that opposes the change in the controlled variable. This is the conditiondesired in an automatic control system.Figure 10 Types of OscillationsIC-07 Page 14 Rev. 0
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