Multivariable control applications: turbine and chemical plant examples

Two applications of the inverse Nyquist array design method are described. One concerns an automotive gas turbine of the two shaft variety, with a set of variable geometry nozzles fitted upstream of the power turbine and a heat exchanger fitted in the exhaust system. This form of design was adopted by most manufacturers in an attempt to improve the efficiency and overall performance of the turbine, especially at part load, and also to allow a less complicated transmission to be used. The variable geometry nozzles enable the power turbine inlet temperature to be held at a reasonably high value giving an improvement in the fuel consumption at low fuel flows. The heat exchanger is fitted to recover some of the energy from the exhaust gases and feed this back to the air leaving the compressor. The second problem considered concerns the stabilisation and control of a chemical reactor. The original system description has more outputs than inputs. However, the design specifications allow the problem to be recast as a regulator design, and a simple satisfactory controller using only proportional-plus-integral action elements is determined. The problems presented show the insight which the engineer can gain into the design of such systems by using multivariable frequency-domain techniques