Deadtime and Lead-Lag Algorithms

An increase in the flow of stream A will cause an immediate increase in the flow of stream B. But, because the additional stream A is largely accumulated in the drum, this will cause the combined temperature to change. The feedback controller will detect this and will 

The feedforward controller therefore made the correct change it just did so too soon. The dynamics of the PV with respect to the DV are now much slower that its dynamics with respect to the MV. We therefore need to include some dynamic compensation that, in this case, delays the feedforward correction. Failure to properly include such compensation can result in the addition of feedforward causing the scheme to perform less well than the standalone feedback controller. 

Of course, this is a contrived example. Had the process design department consulted the plant s control engineer, the drum would have been installed upstream of the measurement of the flow of stream A Further the averaging level controller could then properly be 

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 

While one might think that the formula for the algorithm could be derived from Equations (2.23) to (2.26) by setting Y=PV, X = MV, Ti = T2, T2 = 0 and T3 = 7 1. However the performance of the result only partially matches that of the analog version. Equation (6.10) was derived by making the same changes to the Laplace form before converting it to the 

---