VI.1 Introduction

When more than one input and output exist in a process, several control configurations are possible depending on which output is controlled by manipulating which input. In order to arrive at the best possible configuration, one needs to answer the following crucial design questions:

•  What are the control objectives, i.e . how many and which process variables are needed to be controlled?

•  Which outputs should be measured and whether they are primary measurements (outputs to be controlled are measured directly, eg . temperature, pressure etc.) or secondary measurements (outputs to be controlled are estimated from auxiliary measurements, eg . concentrations etc .)?

•  Which inputs should be manipulated? Manipulations should be direct, fast and should have strong effects on controlled outputs. Some inputs are quite reliable in manipulation ( e.g. liquid flow) and some are not ( e.g. solid/slurry flow).

•  What are the possible control configurations (input/output mapping) and what is the best among them? How to measure its superiority quantitatively?