This is an example of a factory modification that originally used a servo amplifier electric actuator to control without servo control. The role of the servo amplifier is to achieve PID adjustment. The essence of the transformation is to move the PID adjustment function into the DCS system and use DCS to implement PID adjustment. According to Lanpu Electronics has many years of experience in developing electric actuators (electric actuators), and the author did not mention the premise of the transformation. The pink text in the article below is the comment I added.
In the cement industry, electric actuators are widely used. However, the original control mode has a high failure rate (this is the original text of the author, which is very biased. The modified control circuit of the author may not be more stable than the control circuits of many electric actuator products now). The adjustment is extremely unfavorable and the control effect cannot be exerted well. This article introduces the use of the "servoless" control concept to control electric actuators (actually, the PID adjustment function of the servo controller is moved into the DCS control system).
1. Reason for transformation
Our company's wet production line has dozens of electric actuators, and most of them use manual controllers and servo amplifiers to control the process valves. Due to poor working conditions and easy aging of electronic components, servo amplifiers are prone to failure, affecting the performance of the entire actuator. For example, the servo amplifier oscillates (perhaps because the dead zone is not adjusted or the component is aging), causing the actuator to oscillate (not necessarily caused by the servo amplifier, it may be caused by the aging of the brake pads of the motor and the increase of the dead zone caused by the increase in mechanical interval) , Resulting in failure of the servo motor holding brake (mainly causing oscillation, which can be solved by increasing the dead zone, but the control accuracy is reduced), increasing the failure rate of the actuator servo motor, causing fluctuations in process parameters and affecting production.
2 Control principle of servo amplifier with electric actuator (electric actuator)
In fact, you can take a look at the article "Structure and Working Principle of Electric Actuators" written by Lamp Electronics, which explains this principle in more detail.
The block diagram of the original control is shown in Figure 1. The hand controller gives a 4-20mA signal of the front servo amplifier. After the given signal is compared with the 4-20mA signal fed back by the electric actuator position transmitter, the electronic switch and trigger (generally the electronic switch and trigger are built in In the servo amplifier), the servo motor of the electric actuator is controlled to rotate forward or reverse. At the same time, the 4-20 mA signal fed back by the electric actuator position transmitter also changes, and the feedback signal is sent back to the front magnetic amplifier for comparison. Until the reference is balanced with the feedback signal. Because the feedback signal is connected in series with the display of the handheld programmer and the servo front servo amplifier, the feedback signal is compared with the given signal, and the change process of the feedback signal is also displayed on the handheld programmer.
Fig. 1 Servo amplification control principle of electric actuator (electric actuator)
"Zero adjustment" potentiometer and "stable" potentiometer in servo pre-amplifier (there may be no potentiometer in intelligent products, set with parameters), is the key to whether the electric actuator can produce oscillation, and it is also prone to failure. Point (the key to damage is easy to oscillate if it is not adjusted properly, and its essence is easy to damage the power module); thyristors, diodes, current-limiting inductors, and single crystal tubes in servo electronic switches and trigger circuits are also prone to failure .
3 Use "no servo" technology to control the actuator (essentially move the servo function into the DCS system)
The DCS system of the No. 3 kiln system was established during the technical renovation of No. 3 kiln, which began in November 1999. At the same time, according to our long-term experience of using actuators and servo-less transformation, 5 actuators including bag dust extraction fan inlet valves, cold air valves and other actuators in the No. 3 coal mill were servolessly controlled with RC arc extinguishing and RV varistor. Retrofit (most current electric actuators have similar protection circuits).
The ladder diagram program (Figure 2) is used to simulate the comparison and amplification functions of servo amplifiers for 220V and 380V electric actuators, and the drive output control of electric actuators is implemented with switching output modules (the corresponding switching control signals must be The type of digital input signal required by the electric actuator is consistent, while the adjustable electric actuator has a built-in servo amplifier and can only accept analog input, so it cannot be modified.) Use the analog input module to complete the process valve position feedback amount acquisition (not Understand how to use the module input module to complete the valve position signal acquisition. Here I think that only those actuators with valve position feedback are suitable for transformation.)
Figure 2 Ladder program without servo control
The value of 400101 in the ladder program is given by the operator, and is assigned to 400135 after division and multiplication are rounded; similarly, 300019 is the feedback value, and is assigned to 400140 after division and multiplication rounded. The comparison between 400135 and 400140 is respectively Drive the corresponding 000033 coil to make the intermediate relay K1 in the hard line (see Figure 3) engage, the AC contactor KM1 operates, and the motor rotates forward; otherwise, drive the 000034 coil to make the intermediate relay K2 in the hard line engage and AC contact KM2 operates and the motor rotates in reverse, thereby realizing the non-servo forward or reverse rotation of the servo motor of the on-site electric actuator (electric actuator).
Figure 3 Control principle of electric actuator (electric actuator)
DCS: Control system; ESR: Actuator position transmitter; Q: Power low-voltage circuit breaker; Q1, Q2: Maintenance and repair switches; RC: Arc-extinguishing device; RV: Varistor
"# 100" in the ladder program is a rounded operation value. "100" is actually the accuracy of controlling the opening degree of the valve, that is, the control range of the dead zone. In the high-precision control, "# 50" or less is preferable, which is equivalent to the role of a "stable" potentiometer in the servo front magnetic amplifier. 000581 is the reset coil in the program. "# 40" means that the output is automatically turned off after the drive output is held for 40s.
After construction, the hard line can be applied to 220V and 380V at the same time.
Select 220V or 380V actuators. Due to the centralized control, each electric actuator (electric actuator) must be added with two intermediate relays and a control box next to the machine; if it can be negotiated with the manufacturer, the contactor and RC arc extinguishing The device and varistor are installed inside the electric actuator (electric actuator), which can be easily installed and operated. Our company has modified the Siemens actuators on the production line as above.
4 Use effect
After the transformation, the five servo-less actuators (electric actuators) have been put into operation for more than half a year, and the automatic control performance has been greatly improved, fully adapting to the harsh working environment of the cement production site, without human intervention, reliable operation, low failure rate, and large maintenance Reduce, reduce the maintenance cost, and save the investment costs of the communicator and servo amplifier. The RC arc extinguishing device and RV varistor can suppress the occurrence of arcing in the drive circuit and ensure the safe and effective long-term operation of the control circuit. Provides stable and accurate parameters for process operation, facilitates operation, and improves system functions.
In summary, the above transformation method requires that the electric actuator (electric actuator) is a switch-type electric actuator (electric actuator) with valve position feedback.
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