Lead-lag algorithm

The lead-lag algorithm has three tuning constants - gain (AT), lead (71) and lag (T2). It should not be necessary for the engineer to know how the DCS vendor has defined the algorithm, but if Y is the output and X the input, then strictly it should be of the form... 

Like the FID algorithm, there are a variety of versions of the lead-lag algorithm depending on the method used to convert the analog version into its closest discrete equivalent. Few DCS vendors disclose exactly how the algorithm is coded. The approach to tuning must therefore be to assume that it is theoretically correct and, if it does not behave as exactly as expected, to adjust the tuning constants by trial and error. 

We should also remember that the tuning has been based on the assumption that the process is first order plus deadtime. It is theoretically possible to implement a second order equivalent of the lead-lag algorithm but this would require the identification of second order models for the DV and MV, and the calculation of additional tuning constants. It is unlikely therefore to be practical. It would be easier to fine tune the dynamic compensation. This also takes account of any abnormalities in the way in which the DCS vendor may have coded the lead-lag algorithm. 

Figure 8.10 shows the first of the decouplers. When PIDi takes corrective action, the decoupler applies dynamic compensation to the change in output (AOPi) and makes a change to MV2 that counteracts the disturbance that the change in MVi would otherwise cause to PV2- Dynamic compensation is provided by a deadtime/lead-lag algorithm. 

We apply dynamic compensation in the form of a deadtime/lead-lag algorithm. This is tuned in exactly the same way as described in Chapter 6 covering bias feedforward. By performing open loop steps on the MV we obtain the dynamics of both the inferential and... 

Dynamic compensation is likely to be necessary to ensure that the reflux and steam flows are adjusted at the right time. The method for tuning these deadtime/lead-lag algorithms is described in Chapter 6. Part of this procedure involves steptesting the DV, in this case feed rate, to obtain the dynamic response of the PV, in this case tray temperature. This can present a problem on some columns. 

On the other hand, conventional control approaches also rely on models, but they are usually not built into the controller itself. Instead the models form the basis of simulations and other analysis methods that guide in the selection of control loops and suggest tuning constants for the relatively simple controllers normally employed [PI, PID, I-only. P-only, lead-lag compensation, etc. (P = proportional, PI = proportional-integral, PID = proportional-integral-derivative)]. Conventional control approaches attempt to build the smarts into the system (the process and the controllers.) rather than only use complex control algorithms. 

Traditional Advanced Control (TAC) employs the use of advanced control algorithms combined with regulatory control functions (i.e., lead/lag, ratio, high/low selectors, etc) to implement a control strategy. 

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