BLT tuning

One of the major questions in multivariable control is how to tune controllers in a diagonal multiloop SISO system. If PI controllers are used, there are 2N tuning parameters to be selected. The gains and reset times must be specified so that the overall system is stable and gives acceptable load responses. Once a consistent and rational tuning procedure is available, the pairing problem can be attacked. 

The method should be viewed in the same light as the classical SISO Ziegler-Nichols method. It gives reasonable settings which provide a starting point for further tuning and a benchmark for comparative studies. 

Calculate the Ziegler-Nichols settings for each individual loop. The ultimate 

FORTRAN program with a subroutine ZNT which does this iterative calculation using simple interval halving. 

A detuning factor F is assumed. F should always be greater than. 1. Typical values are between 1.5 and 4. The gains of all feedback controllers are calculated by dividing the Ziegler-Nichols gains by the factor F. 

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Table 17.1 gives a FORTRAN program that calculates the BLT tuning for the Wood and Berry column. The subroutine BLT is called from the main program where the process parameters are given. The resulting controller settings are compared with the empirical setting in Table 17.2. The BLT settings usually...

Using the multivariable DMC produces some improvement for setpoint disturbances. However, as Fig. 17.3 shows, there are only slight differences between DMC and the diagonal structure (with BLT tuning) for load disturbances. Thus the real economic incentive for the more complex multivariable configuration has yet to be demonstrated by impartial evaluators. 

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