Abstract
The paper describes some criterions on the selection of control equipments. Contrellers of three term action would not almighty because of so many noisy control variables exist in a continuous process. It is not favorable to select a controller with reset action for a process with poor self regulation and asymmetrical nonlinearity. Simple proportional action with nallow or moderate proportional band is generally suitable to pressure and liquid level processes. Rate action gives very good result when it is equipped in a primary element or in a transmitter for a process with excessive lags. In the case of flow control, simple proportional speed floating control is best and has similar characteristics as proportional plus reset plus inverse-derivative action (Fig. 3). The author offers a new flow control arrangement which consists of a quick response flow transmitter, a proportional or proportional-reset action controller with nallow band and slow reset rate and a R-C lag element on the control valve end (Fig. 4). The time constant of the lag element is selected to be approximately equal to the reset action time so that transmission lags of the air tubing are made harmless. This is acceptable to modern graphic panel instrument systems to be disused of tedeous filed adjustments of controllers.
