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Comparison of Cascade and Parallel Cascade Control

M.Bharathi, C.Selvakumar

Cascade control systems are widely used in industry for improving the dynamic response of systems. Cascade systems are particularly useful in reducing the effects of load disturbances that are introduced into the secondary or slave loop. The majority of these systems are of the series type i.e., the output of the secondary loop process transfer function is the input to the primary loop process transfer function. Typical example (Figure 1) is tray temperature control in a distillation column cascaded to reboiler steam flow control. Steam flow rate is the output of the secondary loop process transfer function, and steam flow is the input to the primary loop process transfer function (temperature/steam flow). These series cascade systems were quantitatively studied by Franks and Worley (1956).