Electric Actuators and Smart Actuators Overview

1.1. Electric Actuator

Electric actuators, like pneumatic actuators, are an important part of the control system. It receives the 0 ~ 10mA or 4 ~ 20mA DC current signal from the controller and converts it into the corresponding angular displacement or linear displacement to control the valves, baffles and other control mechanisms to achieve automatic control.

Electric actuators are available in angular travel, linear travel and multi-turn type. The angular stroke electric actuator uses a motor as a power element to convert the input DC current signal into a corresponding angular displacement (0° to 90°). This actuator is suitable for controlling rotary valves such as butterfly valves and baffles. After the linear actuator receives the input DC current signal, it rotates the motor, then decelerates through the reducer and converts it into a linear displacement output, to control various control valves such as single-seat, double-seat, and three-way and other linear control mechanisms. Multi-rotary electric actuators are mainly used to open and close gate valves, globe valves and other multi-turn valves. Due to its relatively large electric power, the largest ones are several tens of kilowatts, which are generally used as local operation and remote control.

Several types of electric actuators are basically the same in electrical principle, except that the reducer is different. The following is a brief introduction of the electric actuators for quarter-turn.

The electric actuator consists of servo amplifier and actuator. The actuator also includes a two-phase servo-blue reducer and position transmitter. The block diagram of the electric actuator.

Ii from the regulator as the input signal of the servo amplifier, which is compared with the position feedback signal If, the difference is amplified to control the two-phase servo motor forward or reverse, and then after the deceleration of the reducer, change the output shaft adjustment Valve opening or baffle angular displacement). At the same time, the displacement of the output shaft is converted into a current signal by the position transmitter as a valve position indication and feedback signal If. When If and Ii are equal, the two-phase motor stops rotating. At this time, the opening of the regulating valve is stabilized at a position proportional to the signal (Ii) of the regulator output (ie, the actuator input).

The relationship between the output shaft angle θ and the input signal Ii is

In the formula, the ruler is the proportional coefficient.

From the above formula, the output shaft angle is proportional to the input signal, and the electric actuator can be regarded as a proportional link. The electric actuator can also realize the automatic switching of the control system and the manual operation through the electric operator. When the switch of the operator is switched to the “manual” position, the power of the motor is directly controlled by the forward and reverse operation buttons to realize the forward rotation and reverse rotation of the output shaft of the actuator, and the remote control manual operation is performed.

The following describes the servo amplifiers and actuators of the electric actuator.

Servo amplifier

The servo amplifier is composed of a pre-magnet amplifier, a trigger, a thyristor main circuit and a power supply. As shown in Figure 4-5. Its function is to integrate the input signal and the feedback signal, and to amplify the resulting signal so that it has sufficient power to control the rotation of the servo motor. Based on the polarity of the resultant signal after synthesis, the amplifier should output signals of corresponding polarity to control the forward and reverse operation of the motor.

In order to meet the requirements for composing a complex control system, the servo amplifier has three input signal channels and one position feedback channel - _ Therefore, it can input three signals and one position feedback signal at the same time. A simple control system uses only one input channel and position feedback channel.

The pre-stage magnetic amplifier is a high-gain amplifier. The input signal from the controller and the position feedback signal are compared in the magnetic amplifier. When the two are not equal, the amplifier amplifies the deviation signal according to the input signal and the feedback phase. The positive and negative polarities of the subtracted deviation and the amplifier generate two-level input voltage at two points, a and b, and control one of the two transistor trigger circuits to work and one is off, so that the thyristor of the main circuit is turned on, and the two-phase servo motor is connected. Rotate through the power supply to drive the adjustment mechanism for active control. The thyristor plays a role of no electric shock switch in the circuit. The servo amplifier has two sets of switching circuits, namely two sets of the trigger and the main circuit, each of which receives an input signal of positive or negative deviation, respectively, to control the forward rotation or reverse rotation of the servo motor. At the same time, the position feedback signal changes with the change of the motor rotation angle. When the position feedback signal is equal to the input signal, the pre-feedback signal changes with the change of the motor rotation angle. When the position feedback signal is equal to the input signal, the preamplifier Without output, the servo motor stops.

2. Executing agency

The actuator consists of servo motor, reducer and position transmitter. It receives the output signal of the servo amplifier or electric operator, controls the positive and negative rotation of the servo motor, and then becomes the output torque to reduce the speed of the servo motor. At the same time, the position transmitter converts the angular displacement of the adjustment mechanism into a corresponding DC current signal to indicate the valve position and feed it back to the input of the preamplifier to balance the input current signal.

(1) Servomotor

The servo motor is the power part of the actuator. It is a two-phase servo motor yoke composed of a stator and a squirrel-cage rotor laminated with grooved silicon steel plates: the stator has the same winding, and the phase-shifting capacitor makes the two-phase winding The currents are 90° out of phase, and the two-phase windings also differ in space by 90°, thus constituting a stator rotating magnetic field. The direction of rotation of the motor depends on the advance or lag in the phase of the current in the two sets of windings.

Considering that the motor in the actuator is frequently in the process of starting and braking, when the controller output overload or other reasons make the valve stuck, the motor may also be in a state of stall for a long time, in order to ensure that the motor is not in this situation. Burned due to overheating, this type of motor features high starting torque and low starting current. In addition, in order to minimize the servo motor in the power to continue "inactive" after power off, and to prevent the motor from being powered off by the load force to promote the phenomenon of reversal, in the servo motor is also equipped with neodymium magnetic brake Mechanism to ensure that the rotor is immediately braked when the motor is de-energized.

(2) Reducer

Servo motor speed is high and output torque is small. The rotation speed is generally 600~900r/min, and the speed of the adjusting mechanism is relatively low, and the output torque is large. The total stroke (90°) of the output shaft is generally 25s, that is, the output shaft speed is 0.6r/min. Therefore, there must be a speed reducer between the servo motor and the adjustment mechanism, which turns the high speed and low speed into low speed and high torque. The servo motor and the adjustment mechanism are generally equipped with a two-stage speed reducer. The reduction ratio is generally (1000). ~1500): 1.

The reducer uses a gear mechanism that is a combination of a flat gear and a planetary reducer. The flat gear has simple processing and high transmission efficiency, but the decelerator has a large volume, and the planetary deceleration mechanism has the advantages of small size, large reduction ratio, large bearing capacity, and high efficiency.

(3) Position transmitter

The position transmitter is based on the working principle of the differential transformer and uses the displacement of the output shaft to change the position of the iron core in the differential coil to generate a feedback signal and a position signal. In order to ensure that the voltage and feedback signals of the position transmitter are linearly related to the displacement of the output shaft, the differential transformer power supply of the position transmitter adopts LC series resonant magnetic saturation regulator and a zero compensation circuit is set in the position transmitter to ensure the Location transmitter good feedback characteristics.

The actuator of the quarter-turn actuator is connected to the output shaft of the reducer through a cam. The iron core of the differential transformer is pressed against the inclined surface of the cam by a spring. The output shaft rotates from 0° to 90°, and the differential transformer core generates. Axial displacement, Z set i send H output current is 4~20mADC.

The position of the linear motor actuator The connection and adjustment between the transmitter and the speed reducer is achieved by levers and springs. When the output shaft of the reducer moves up and down, one end of the lever is pressed against the end surface of the output shaft by spring force, so that the differential transformer putter produces axial displacement, thereby changing the position of the iron core in the differential transformer winding to achieve change The purpose of the position transmitter output current.

(4) Manipulator

The operator is used to complete tasks such as switching between manual and automatic, remote operation, and automatic tracking of bumpless switching. According to its different functions, there are three types: the first one is with switching, valve position indication, tracking current indication and midway limit; the second is with switching operation, valve position indication and tracking current; the third is There are switching operations, valve position indication and tracking current, but no tracking current indication.

As the degree of automation continues to increase, more demands are placed on electric actuators, such as digital input electric actuators that require direct connection to a computer, automatic holdover, and no digital/analog conversion. Servomotors are used at low speeds. After the electric motor, it helps to simplify the structure of the electric actuator and improve the performance. It needs to be further promoted.

3. Overall characteristics of electric actuator

Figure 4-6 shows the block diagram of the characteristics and signal transmission of each part of the electric actuator.

The servo amplifier is a non-linear circuit with relay characteristics and has a dead zone (<150μA). When |Ii-If|< Δ/2, no output; when |Ii-If| ≥ △/2, the output is a constant AC voltage (about 215V).

Reducer and position transmitter are proportional links.

Servo motor is connected to the power supply: The servo motor operates at a constant speed, so it is an integral link. Therefore, the dynamic characteristics of the electric actuator mainly depend on the characteristics of the servo motor, that is, it has an integral characteristic.

Because the insensitivity zone of the servo amplifier is very small, when the system is balanced, Ii-If=Δ≈0 can be considered, so Ii=If=kfθ, so the static relational expression of the electric actuator is: where kf is the position to send The scale factor of the device, that is, the feedback coefficient (kf=10mA/90°). The above formula shows that there is a good linear relationship between the input and output of the electric actuator.

1.2 Smart Executive Agency

The construction principle of the intelligent electric actuator is the same as that of the analog electric actuator. However, the intelligent electric actuator adopts a novel structural component. A micro-processing system is used in the servo amplifier. All control functions can be programmed, and it also has a digital communication interface, so that it has a HART protocol or fieldbus communication function and becomes a node in the field bus control system. Some servo amplifiers also use frequency conversion technology to more effectively control servo motor operation. Reducer uses a novel transmission structure, smooth operation, high transmission efficiency, no creep, and low friction. The position transmitter adopts new technology and new methods. Some use Hall effect sensors and directly sense the longitudinal or rotational motion of the valve stem to achieve non-contact positioning detection. Some also use special potentiometers and potentiometers. There are ball-bearings and resistance sheet made of special conductive plastic material; there are non-contact rotary angle sensors that use magnetoresistance effect.

Compared with conventional electric actuators, smart electric actuators have the following features.

1) With intelligent and high-precision control functions. The intelligent electric actuator can directly receive the transmitter signal, automatically adjust the PID according to the set value, and control the process variables such as flow, pressure and temperature. The configuration can be used to create a variety of nonlinear flow characteristics in accordance with the polyline, 'to achieve compensation for the process of nonlinear characteristics, in order to improve the system's control accuracy, but also get rid of the long-term change in the shape of the valve to change the flow characteristics of the backwardness situation.

2) Integrated g design ideas. Position controllers, PID controllers, servo amplifiers, electro-pneumatic converters, valve position transmitters, etc. are installed in the + field instrument to reduce the influence of factors such as leakage and interference in the signal transmission on the system control accuracy; Electric d-action and intermittent adjustment technology are also used instead of mechanical friction braking technology to improve the reliability of the whole machine.

3) With intelligent communication capabilities. Between the host computer and the control system, two-way digital communication can be performed according to the specified communication protocol through the field bus, and constitute the required control system. This is one of the important differences between the smart actuator and the conventional electric actuator.

4) With intelligent self-diagnosis and protection. When power supply, pneumatic components, mechanical components, control signals, communication or other aspects fail, they can quickly identify and effectively take protective measures to ensure the safety of the control system and the production process.

5) With flexible configuration function, "a multi-purpose machine" improves economic efficiency. For example, for the input signal, the appropriate signal source can be selected via the software configuration; for the operating speed and stroke of the actuator, it can also be set arbitrarily by the configuration software, all without the need to replace the hardware. In this way, as long as a small number of smart actuators can be used to meet the different requirements of various industrial processes, the economic benefits of manufacturers and users are greatly improved.