Cascade control external reset

In selective and cascade control loops, external feedback is the most-often-applied solution. Here, instead of looking at its own output, which can be blocked, the integral mode of the controller looks at an external feedback signal (such as the opening of the valve), which cannot be blocked. In surge control or reactor heat-up applications, the chosen solution usually is to use the slave measurement as the external reset signal to prevent saturation. 

Providing external reset for the cascade master from the slave measurement is always recommended. This guarantees bumpless transfer when the operator switches the loop from slave control to cascade control (Figure 2.45). The internal logic of the master controller algorithm is such that as long as its output signal (m) does not equal its external reset (ER), the value of m is set to be the sum of the ER and the proportional correction (Kc(e)) only. 

Part (c) in Figure 2.85 illustrates a triple cascade loop, where a temperature controller is the slave of an analyzer controller while the reflux flow is cascaded to temperature. Because temperature is an indicator of composition at constant pressure, the analyzer controller serves only to correct for variations in feed composition. Cascade loops will work only if the slave is faster than the master, which adjusts its set point. Another important consideration in all cascade systems (not shown in Figure 2.85) is that an external reset is needed to prevent the integral mode in the master from saturating, when that output is blocked from reaching and modulating the set point of the slave (when the slave is switched to local set point). 

Selective control configurations also require external feedback to protect them from reset windup. Part (d) has a combination of selective and cascade systems and in such configuration, the external reset (ER) signal (not shown) is taken from the measurement of the slave controller (FRC). 

However the situation becomes more complex with cascaded controllers. The situation can arise where the secondary is controlling at SP but with its output at minimum or maximum. It is important therefore that the primary makes no changes to the secondary s SP which will cause it to saturate. DCS controllers have external anti-reset windup protection, sometimes described just as external reset feedback, to prevent this. 

A true cascade system (see Figure 12.3) has at least two controllers a primary or master one and a sectMidary or slave one, whose set point is normally set by the primary controller. An example would be column-top temperature control cascaded to reflux flow control. C5ur preferred scheme uses the secondary variable for external reset feedback to the primary ccxitroller. This gieady minimizes the need for manual—automatic switching—none is needed in the primary control station. 

In order to make sure that the pressure controller (PC-01) set point is changed slowly and in a stable manner, the valve position controller (VPC-02) is provided with integral action only, and its integral time is set to be about 10 times that of PC-01. In order to prevent reset windup when PC-01 is switched to manual or local control from cascade, the valve position controller is also provided with an external feedback signal off the pump speed. 

Chapter 9 points out that an analog PI or PID controller saturates when an override takes over control of the valve unless the controller reset is made to track the valve signal in simple loops. The secondary variable measurement is the proper feedback for a primary controller in a cascade loop. Similarly, for sample-data algorithms, the algoritlun is structured to track an external fee ck signal. "... 

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