I. Overview of an electric actuator
Electric actuator is a kind of actuator that uses electricity as energy in the control system, receives electric signal of regulating instrument and so on, changes the control amount according to the signal size, causes the material quantity or energy change of the input or output control object, achieves the automatic regulation purpose. 

In industrial production, the application of electric control valve is the most widely. The electric regulating valve is composed of the executive mechanism and the regulating mechanism ( valve ). The valve part is introduced in front, so only the electric actuator is introduced in the electric regulating valve.

Advantages and disadvantages of electric actuators
Advantages: The power of the electric actuator is convenient, action is sensitive, signal transmission speed is fast, suitable for long-distance signal transmission, easy to use with the electronic computer. 
Disadvantages: The electric actuator is not suitable for the occasion of fire and explosion - proof, and the structure is complex, the price is expensive. 

II. working principles of electric actuators
The following is a brief introduction to the direct stroke electric actuator and angle stroke electric actuator. 

The working principle of straight stroke and angular travel electric actuator is to receive the standard value stream electric signal from 4 ~ 20 ma sent by the regulator or other instrument, and turn it into displacement thrust or corner moment after the actuator, to operate the switch, valve and so on, to complete the automatic adjustment task. Both of these actuators are composed of servo amplifiers and actuators. Now there are mechatronics intelligent structure, their structure, working principle and use methods are similar, the difference is only, one output displacement ( thrust ), one output angle ( torque ).

( 3 ) When a power failure or regulator fails, it should be able to operate manually on the actuator so as to take emergency measures. To do this, there must be a clutch and handwheel. 

( 4 ) In the manual operation of the actuator, in order to provide the automatic tracking basis for the regulator ( tracking is the need of no disturbance switching ), the valve position tracking signal should be provided on the actuator. This is both the need for the position feedback of the actuator itself, and the need for the valve position indication.

( 5 ) In order to protect the valve and transmission mechanism not to be damaged due to excessive operating force, the actuator shall have a device with limit device and a limiting force or torque. In addition to the above basic requirements, in order to facilitate the combination of various valve characteristics, it is best to have the optional nonlinear characteristics on the actuator. In order to work with the computer, it is best to enter the digital signal directly. In recent years, there are also the PID operation function of the actuator, this is called " digital actuator" and " intelligent actuator. ". At present, the most widely used or analog electric actuator, in which the DKJ ( output angle ) and DKZ type ( output linear displacement ) are the most common in the electric unit of our country.

IV. Classification of electric actuators
1. DKJ and DKZ electric actuators
DKJ electric actuator is the executive unit of DDZ type electric unit instrument cluster, it receives the control signal of the upper instrument, completes the regulating task automatically, and is widely used in power station, chemical industry, petroleum, metallurgy, building material, water treatment, heat supply, light industry and other industries. DKJ electric actuator, with the corresponding electric operators, can complete the " manual - automatic" of the adjustment system, and the remote manual operation. 

The DKZ type direct stroke electric actuator adopts the new technology developed by "S" electric actuator: the WF - S type position transmitter composed of the precision conductive plastic potentiometer and the special integrated circuit of the country. Therefore, the electric actuator not only has a great improvement in reliability, accuracy, load capacity, signal quality coefficient and other performance, but also has a very important significance to the development of environmental conditions.

2. Integral electric actuator

The simplest electric actuator is composed of the capacitor phase separation circuit, without the preamplifier, directly relying on the action of the switch to control the motor. 

3. Rolling - cutting motor type electric actuator
The servo motor based on the basic principle of the common asynchronous motor, high speed and small torque, so the actuator has to use more complex reducer. 

4. Multi - turn electric actuator
The output shaft motion mode of the multi - rotary electric actuator is rotary, and the total stroke exceeds 360 degrees, suitable for gate valve, cut-off valve and other controlled objects, the multi - turn electric actuator is relatively special, it can be equipped with a speed reducer to switch to angular travel electric actuator or straight - stroke electric actuator, or even multi - turn, the output force ( moment ) increased after the addition of one stage deceleration, and the speed of movement is slowed down.

5. Permanent magnet low speed synchronous motor type actuator
With the actuator of permanent magnet low speed synchronous motor, taking the homemade DZAS travel actuator as an example, the output thrust is 400 ~ 2500 n, the stroke is 10 ~ 40 mm, the speed is 0.75 ~ 1.25 mm / s. The domestic SKZ actuator is also composed of permanent magnet low speed synchronous motor. 

6. Digital electric actuator
In order to cooperate with the computer directly, the digital electric actuator has appeared recently, its input signal is pulse form, servo motor is driven by pulse stepping motor. Although the output angle of the stepper motor is strictly proportional to the number of input pulses, the number of pulses may be wrong due to interference, so the actuator should still have position feedback signal, which is usually sent back to the computer from the standard signal of 4 ~ 20ma DC, and then the closed loop is formed by comparing the a / d conversion with the set valve position to ensure the correct opening of valve position.

V. Instructions for selection of electric actuators
1. Determine the output torque of the electric actuator according to the torque required by the valve
The torque required by the valve opening and closing is decided by the user or by the manufacturer itself. the torque is determined by the valve diameter, working pressure and so on. however, the torque of the same valve manufactured by different manufacturers is different from that of the same valve manufacturer, so the torque selection of the actuator is too small to open close valve, so the electric actuator must select a reasonable torque range.
 
2. Determine the electrical parameters according to the selected electric actuator
Because of the different electrical parameters of different manufacturers, the design and selection of the general need to determine the electrical parameters, mainly motor power, rated current, secondary control circuit voltage, etc., often in this aspect of negligence, the result control system and electric actuator parameters do not match the operation of time trip, fuse blowing, thermal overload relay protection and so on.