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Cascade ControlControl System Analysis Test Procedure

By definition, cascade control systems consist of two or more controlloops in series. Cascade loops are invariably installed to prevent outsidedisturbances from entering the process, and to put the principlenonlinearities in a much faster responding inner loop controller.

The primary loop controller adjusts the secondary loops controllersetpoint. A cascade control system controls a single variable, and theinner loop controller is used only to assist in achieving this.

In order for cascade control to be successful, the secondary loop dynamicresponse must be much faster than the primary loop. A good rule ofthumb is that the primary loop should be three times slower than thesecondary. Otherwise, the primary loop will need to be de-tuned to sucha degree that cascade control will degrade the overall quality.

The following example illustrates a typical cascade system to control aheat exchanger.

Coolant Flow = Fc

Process FluidFlow = Fp

TIC

TT2

TT1

FT2

FT1

FIC

RSP

Protuner1600PC

Test 1Secondary

Test 2 -Primary with Secondary in Auto

PV1 PD1

PD2

PV2

Figure 7.1 - Preferred Protuner Connection to Test Cascade Control Loops

CHAPTER

7

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The preferred Protunertm connection to test loops configured in cascade,is to record the controller output (PD signal) and process variable (PVsignal) of both the primary and the secondary controllers as illustrated inFigure 7.1. In many DCS systems, the PD signal from the primarycontroller, which is the remote setpoint (RSP) to the secondary controlleris in software and not available as an analog signal. One solution, is toprogram the DCS system to retransmit the controller output (PD) signalas an analog signal to a spare output point. If for some reason, it is notpossible to connect the Protunertm to record the PD signal of the primarycontroller, an alternate connection diagram is illustrated in Figure 7.2.

Coolant Flow = Fc

Process FluidFlow = Fp

TT2

TT1

FT2

FT1

Test 1Secondary

PV1PD1 PV2

Protuner1600PC

FIC

TIC Note: No connection point for

PD2 is present. When analyzing TIC step test results use data

PV1 in place of PD2

Figure 7.2 - Alternate Protuner Connection to Test Cascade Control Loops

As illustrated in Figure 7.2, the alternate Protunertm connection to testcascade control loops is to simply record the three available signals.

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Control System Analysis Test Procedure

Regardless of the connection type used, the Control System Analysisprocedure to test and optimize the closed loop operation of a cascadecontrol system is the same. The following outlines the proper procedure:

Test and tune the secondary control loop first.

1. The secondary control loop in this example, is the flow control loop.Follow the test and analysis procedures in Chapter 3. Enter theProtunertm tuning parameters in the secondary and place the loopback in automatic in remote setpoint mode.

2. Next test and tune the primary controller. The primary loop in this example is the temperature control loop. If theProtunertm is connected using the alternate cascade connection as shownin Figure 7.2, you will need to select the signals for analysis as shown:

Process Demand Signal: FIC-PV(In the alternate connection, TIC-PD was not recorded.)Process Variable Signal: TIC-PV

Table 7.1 - Signal Analysis

Protection Against Windup

In a single loop controller, reset windup or integrator saturation canoccur if the output saturates and the controller continues to integrate theerror. That is, if the controller output reaches 100% and the valve is fullyopen, or if there is an output limit on the controller and the controllercontinues to integrate the error. In these cases, the integrator portion ofthe PID calculation can calculate some very large values for the controlleroutput, even though the actual controller output is limited. If thishappens, it can take sometime for the calculated controller output to getback to an actual controller output value again, this can result in a verylarge overshoot.

To prevent this, the controller PID algorithm must be written toautomatically stop the calculation of the controller integral action whenan output limit is reached. Most single loop PID controller algorithmsincorporate some form of anti-reset windup protection.

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In cascade control loops, reset windup or integrator saturation can occurif the output of the secondary controller saturates and the primarycontroller continues to integrate the error. To prevent this, it is necessaryto employ external anti-reset windup protection to automatically stop thecalculation of the primary controller integral action when an output limitis reached on the secondary controller. In most DCS systems, cascadecontrollers have external anti-reset windup protection as part of thenormal cascade control loop configuration. We have found in actual fieldtesting, that when single loop controllers are configured for cascadecontrol, external anti-reset windup protection is not setup properly,Thus, large overshoots occur during startup in response to large setpointchanges, and following large load disturbances, which could beprevented.