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.

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Recently, the analyses of sampled data control systems are carried out by means of the Nyquist plot or the root locus method, but they are not always simple for calculation. Therefore, the author tries to extend the merits of the Bode diagram to the sampled data control systems. The open loop pulse transfer function W(z) is represented as a rational polynomial in z by z-transformation.
Then, a system is amenable to the graphical calculation, representing its components by log modulus and phase shift. In continuous data control systems, the Bode diagram is plotted for ω=0 to ∞, while in this case, it is enough to plot only for ω=0 to π. Then we can easily find the stability criterion of the system and its design will be performed on the concepts of the gain margin and the phase margin. In designing systems by z-transform method, there is a possibility of missing the hidden instability even in the conservative design. But this can be evaded, if we are scrupulous to design the system for the worst operating condition.

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About one year ago we proposed a controller which has a higher grade control action than ordinary controllers. The controller has two components ; the first one acts as the ordinary controllers but the second one monitors the first one so as to achieve the better transient behavior. This control function resembles to cascade control but is non-linear.
This paper deals with a electric model of the control system including the controller with higher grade action. In this model the monitoring component selects two values for either of a parameter of the first component according to the magnitude of error.

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The direct type governer used for the speed control of Diesel Engine, has a dynamical unstable property. This is a very interesting problem as a stability problem of dynamical system including self excited oscilation, and has been studied by Routh and other investigators ever since the invention of governer by Watt. The governer is sometimes used safely with no damping device. This is not explained by linear theory, and the reason of the fact is thought to be due to the effect of time lag and frictional resistance existing in the controlling system. The author analysed the stability of governer with constant time lag in the paper “Stability of Retarded Control” and got a conclusion that the time lag gives a stabilising effect to the system. This paper presents a theoretical consideration on the stability of governer with frictional registance without viscous damping. The conclusion is that, a certain amount of frictional registance is necessary for the stable action of the control system, and the critical amount of it is given in fig 9 and 10 in a standard hon dimensional form.

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