Basic Control Actions and Industrial Automatic Control An automatic controller compares the actual value of the plant output with the desired value, determines the deviation, and produces a control signal which will reduce the deviation to zero or to a small value.The manner in which the automatic conroller produces the control signal is called the control action.Classifications of industrial automatic controllers Induetrial automatic controllers may be classified according to their control action as:·two-position or on-off controllers;·proportional controllers;·integral controllers;·proportional-plus-integral controllers;·proportional-plus-derivative controllers;·proportional-plus-derivative-plus-integral controllers.Most industrial automatic controllers use eletricity or pressurized fluid such as oil or air as power sources. Automatic controllers may also be classified according to the kind of power employed in the operation, such as pneumatic controllers, hydraulic controllers, or electronic controllers.What kind of controller to use must be decided by the nature of the plant and the operating conditions,including such considerations as safety, availability, reliability, accuracy, weight, and size?Elements of industrial automatic controllers An automatic controller must detect the actuating error signal, which is usually at a very low power level, and amplify it to a sufficiently high level. Thus, an amplifier is necessary. The output of an automatic controller is fed to a power device, such as a pneumatic motor or valve, a hydraulic motor, or an electric motor.The controller usually consists of an error detector and amplifier. The measuring element is a device that converts the output variable into another suitable variable, such as a displacement, pressure, or electric signal, which can be used for comparing the output to the reference input signal. This element is in the feedback path of the closed-loop system. The set point of the controller must be converted to a reference input of the same units as the feedback signal from the measuring element. The amplifier amplifies the power of the actuating error signal, which in turn operates the actuator. The actuator is an element which alters the input to the plant according to the control signal so that the feedback signal may be brought into correspondence with the reference input signal.Self-operated controllers In most industrial automatic controllers, separate units are used for the measuring element and for the actuator. In a very simple one, however, such as a self-operated controller, these elements are assembled in one unit. Self-operated controllers utilize power developed by the measuring element and are very simple and inexpensive. The set point is determined by the adjustment of the spring force. The controlled pressure is measured by the diaphragm. The actuating error signal is the net force acting on the diaphragm. Its position determines the valve opening.The operation of self-operated controller is as follows: Suppose that the output pressure is lower than the reference pressure, as determined by the set point. Then the downward spring force is greater than the upward pressure force, resulting in a downward movement of the diaphragm. This increases the flow rate and raises the output pressure.When the upward pressure force equals the downward spring force, the valve plug stays stationary and the flow rate is constant. Comversely, if the output pressure is higher than the reference pressure, the valve opening becomes small and reduces the flow rate through the valve opening. Such a self-operated controller is widely used for water and gas pressure control. In such a controller, the flow rate through the valve opening is approximately proportional to the actuating error signal.Control actions The following six basic control actions are very common among industrial automatic controllers: two-position or on-off, proportional, integral, proportional-plus-integral,proportional-plus-derivative,andproportional-plus-derivative-plus-integral control action. Note that an understanding of the basic charateristics of the various actions is necessary in order for the control engineer to select the one best suited to his particular application.翻译：自动控制器把对象的实际输出与期望值进行比较，确定偏差，并产生一个使误差为零或微小值的控制信号。

自动控制器产生控制信号的方式叫做控制作用。

工业自动控制器的分类根据控制作用，工业自动控制器可分为：·双位或继电器控制器；·比例控制器；·积分控制器；·比例-积分控制器；·比例-微分控制器；·比例-微分-积分控制器。