Hsiao and chen (1995) also reported promising results using an improved PID model . their algorithm includes a form of gap action control whereby the simple PID output is modified in the face of excessive variations or unusual operating conditions. The control of syrup brix was improved by allowing a variable recycle back to the last effect. As with the throughput control used at amatikulu, this system would limit the capacity of the station, and would not be able to anticipate and handle input constraints.