1. Introduction With the continuous expansion of production scale and continuous improvement of production efficiency, cranes are developing in the direction of large tonnage, large span, high speed, and high intelligence, which guarantees the safety, reliability, and intelligence of the overall control of cranes. The requirements on other aspects are also getting higher and higher, especially for large-span bridge and gantry cranes. Due to the large span, the low stiffness of the main beam, and the fact that there are many factors such as partial load or track deviation in actual work, the large vehicles are The phenomenon of non-synchronization will occur during the operation process, which will cause great damage to the mechanical structure and even cause serious accidents. Therefore, it is of great significance to explore the causes of out-of-synchronization and prevent and eliminate them.
The crane operation system consists of a frequency converter and an AC motor. As the overall control object, the transfer function is a first-order inertial link.
The torque balance equation on the motor shaft in the transmission system is
The quality of the crane itself is large, and the weight of a large gantry crane is several thousand tons. Different types of cranes have great differences in their movement parameters, and the control parameters of the system are also different. During the operation of the crane, the wheel pressure of the wheel is uncertain because of different loads and different positions of the trolley. In addition, other factors such as wind resistance, slope of the track, deviation, etc. will also cause the characteristics of the crane drive system to change. Therefore, the crane operating mechanism is a large-lag, time-varying and non-linear system. As the load changes or interference factors, its object characteristic parameters and structure change. It is very difficult to express its characteristics with an accurate mathematical analytical formula.
With the constant maturity and wide application of frequency conversion technology, cranes have increasingly adopted frequency conversion speed control solutions. The use of variable frequency speed control technology can enable the vehicle operating mechanism to have good mechanical properties, good starting and braking performance, and to compensate for deficiencies in machining, making the operation more stable and more reliable.
The commonly used solution in engineering is to adopt a PID control synchronization scheme. Absolute encoders are installed on the rigid leg and flexible legs of the cart. Absolute encoders are used to measure the displacement of the rigid leg and flexible leg relative to the standard position. To PLC, PLC uses PID algorithm to get offset, and the revised frequency command sends to two inverters of rigid leg and flexible leg. In this frequency command, the inverter adjusts the motor speed of the flexible leg side to make the flexible leg side motor reduce driving. speed. In order to achieve the purpose of synchronization of the cart.
For a controlled object such as a crane synchronization system that is difficult to establish an accurate control model, the traditional PID controller, a linear controller that is sensitive to control parameters, cannot achieve a good control effect. The most prominent point is the PID parameters. The problem is that traditional PIDs do not have the ability to adapt. This is mainly reflected in three aspects:
(1) The parameter setting of the PID controller must be relative to a known model of a certain model, and the system parameters are known to the system; in the actual debugging process, the adjustment of the control parameters of the system is difficult and the overshoot phenomenon tends to occur, resulting in the occurrence of the operation process. Writhing and oscillating, the operation is not smooth. The randomness of the control algorithm is relatively large, and the control parameters of the system cannot be adapted to cranes with different levels of rigidity.
(2) Once the PID controller parameters have been set, they can only be applied in a fixed condition. However, in reality most production processes are non-linear and their characteristics change with load and time. Obviously a fixed set of parameters cannot satisfy this change.
(3) The parameters of the traditional PID controller can only be set to meet the requirements of a certain aspect of the production process control objectives. In the process of designing the control system, the main concerns of the people are "setting value tracking characteristics" and "interference suppression characteristics", while the traditional PID controller can only meet one requirement by setting a group of PID parameters. Therefore, the controller parameters are often set using a compromise method. The control effect thus obtained is obviously not optimal.
(4) Although some improved digital PID controllers now improve the response speed, control accuracy, and control effectiveness of the system, its controller core still needs to be based on the exact mathematical model of the controlled process. The control effect is limited when the precise mathematical model is difficult to obtain or the controlled process has high-order, nonlinear, and other characteristics.
2. Analysis of common schemes In the general electric drive system, the system operation is required to be rigid, and the ideal state is that the change of the load does not affect the change of the speed. However, it can be known from the characteristics of the asynchronous motor that, within the maximum output torque of the motor, the change of the load will affect the rotation speed of the motor. Generally, the larger the load is, the larger the rotation speed is. One to two drives (with two motors as an example), the two motors can only be synchronized under ideal conditions. In fact, it is impossible to achieve the same slip ratio of the two motors. Even if the power supply is the same frequency, the rotation speeds of the two motors are not exactly the same. This must use 2 sets of frequency converter to drive separately, adopt the real speed feedback of the electric motor, two sets of frequency converters communicate with each other, and set up to run synchronously, one master one slave mode.
The common synchronization schemes in engineering are the following:
One, two encoders on each of the two motor shafts. Then the encoder's signal is connected to the respective controlled inverter. Through the inverter's own PID adjustment function, adjust the operating speed of the respective motor. The actual operating speed of each motor tends to a theoretical set value. When this method is used to adjust the synchronization, debugging and maintenance are simple, but the accuracy is not high. Sometimes there are dangers. Poor anti-interference. Installation trouble.
Second, an encoder is connected to each side of the two motors and the encoder signal is sent to the PLC's high-speed counting module. The PLC reads the two encoder signals for comparison. The PID algorithm is implemented programmatically in the PLC. The speed of the two inverters is controlled separately by the PLC. To achieve the speed synchronization of the two motors. In this way, if the signal is read without error, the calculation formula is correct and the programming is accurate, the synchronization of the motor can be achieved. However, in the actual operation, there are the following problems:
(1) The PID formula is difficult to find accurately.
(2) The encoder signal is susceptible to interference.
(3) Long debugging time. (In the debugging process, it is necessary to grasp the setting start time. How to ensure the smooth running of each setting time.)
(4) Maintenance is more difficult. (There are a lot of PLC calculations and debugging difficulties.)
(5) Higher costs (including production costs, maintenance costs.)
Third, on the basis of the second method, the use of robust control, fuzzy control, adaptive control and other algorithms in the PLC. The speed of the two inverters is controlled separately by the PLC. To achieve the speed synchronization of the two motors. In this way, if the signal is read without errors, the calculation formula is correct, and the program is written accurately, the synchronization problem can be solved very well. However, in the actual operation, there are the following problems:
(1) High commissioning requirements. (Mastering these control algorithms requires a very high theoretical basis, and ordinary debuggers are difficult to handle.)
(2) Long debugging time. (In the debugging process, it must be run repeatedly under various conditions to get the appropriate parameters.)
(3) It is difficult to maintain. (If field equipment needs to be adjusted, it is difficult for general maintenance personnel to complete.)
Fourth, the use of synchronous cards, synchronization card is installed in the inverter inside a highly integrated PCB board, equipped with a synchronous card inverter will form a programmable servo controller. It has a built-in modern control algorithm such as robust control. The user only needs to set the corresponding parameter on the inverter. The synchronous control can operate in master/slave mode between multiple drives. The master station is responsible for communication and control signals in synchronous control. Can realize the corresponding control function through the built-in programming. This program has the following features:
(1) Easy commissioning. (According to the actual equipment requirements, set the appropriate parameters.)
(2) Short debugging time. (Set the appropriate parameters to get a good result.)
(3) It is easy to maintain. (If field equipment needs to be adjusted, general maintenance personnel can also set parameters.)
In the engineering practice, the scheme of using the synchronous card occupies a great advantage in synchronous control because of high cost performance, convenient debugging and maintenance.
3. Practical Application Harvest The inverter is a high-performance inverter from Korea Harvest Electric Co., Ltd. Harvest synchronous control has nearly ten years of production and application experience, mature and reliable technology, widely used in similar applications, fully meet the technical requirements of crane synchronization, and provide reliable post-maintenance and technical support, so we use this product to achieve synchronization Features. It has a variety of control methods: V/F control, non-inductive vector control (open loop), vector control (closed loop).
Free programming features: diverse mathematical equations, multiple logic operations, multiple timing operations, arbitrary programming combinations, well-defined functions.
â— Multi-drive & multi-motor control: Synchronous control between multiple drives, parallel operation of PWM inverters and multi-motor control.
With the synchronous card (Sychron-Card) used by SEOHO-VD series inverters, it is possible to realize highly precise and stable torque and speed control without encoders, thus making the application of inexpensive inverters to synchronous control systems become a reality. An example will be used to illustrate the multiple drive & multi-motor control of the SEOHO drive.
SEOHO inverter control master-slave motor is to set more than one inverter as a host and multiple slaves, through the mutual communication between the master-slave inverter to control the two motor shaft output torque and speed synchronization. The use of this function of SEOHO inverter to realize the scheme of realizing synchronous speed between multiple motors, compared with the traditional scheme of using the encoder to realize the synchronous speed function on the motor, has the following characteristics:
(1) Low cost. (Compared to the usual synchronization scheme, no encoder is required to meet the control requirements).
(2) High control accuracy.
(3) Commissioning is simple and convenient.
(4) The signal is stable. (Without an encoder, the system reduces one source of interference.)
(5) Diversification of control methods.
The main station is responsible for communication and control signals. RS-422 acts as a physical layer for synchronization control, which allows multiple drivers to be connected to the bus. In order to ensure the communication speed, there is only one-way data flow from the master station to the slave station, and the slave station simply receives data from the bus. In other words, the slave station did not send data to the master station. The data between the master station and the slave station is transmitted and received with the clock signal. In this way, high communication quality is guaranteed.
Step Control System Block Diagram Application Example: A car control system for a type A door machine for Weihai Rongcheng Gaya Shipyard.
The span of this door machine is 52 meters, using four 7.5KW frequency conversion motors. It is required that the door machine has multiple speeds, and four motors must run synchronously at each speed. Design: Two 15KW SEOHO inverters (model: SOHO15VD4Y, each driving two 7.5KW motors), two Option-Card (expansion cards), and two Sychron-Card (synchronous cards).
Twisted pair (8 wire) as RS422 synchronous serial communication medium. To ensure stability, terminals must be added at both ends of the communication bus. For multiple drive systems, the master uses one terminal and the last slave inserts the other terminal. There are two connection plugs on the communication card, one for connecting the twisted pair and the other for connecting the terminal (or twisted pair for connecting to the next drive). The specific connection method is shown in the figure below.
Synchronous Card Communication Connection Master monitors the RUN/STOP status of the slave via the "Digital Input" and sets the "Speed" and "Torque" settings via the data line based on the externally given reference value and the RUN/STOP signal. The RUN/STOP signal is sent to the slave. Messages are sent in two parts: one is a control message, and the other is a feedback signal. In each communication cycle, the master station issues control commands. The slave station performs the corresponding operation according to the command value sent from the master station and outputs a RUN/STOP signal from the "Digital Output Relay Output" to the binary input terminal of the master station. The master station starts receiving the signal and starts operation. If it is not detected that the slave station is in the running state, the master station will remain in the initial operation state and will not perform the next operation. For example, the master station does not control the motor to increase the speed.
During the master and slave operations, the master station monitors the operation of the slave station at any time. If the slave station exits an abnormal condition, the master station will receive an abnormal signal from the slave station through the digital output signal of the slave station. Therefore, the master station will control the entire system to stop and alarm. When the host fails, the slave machine reports a fault (F39).
After tuning through, set the following parameters:
The system synchronous parameter setting of this door machine's vehicle running system realizes the synchronous speed control of the AC frequency conversion technology and the PLC control frequency converter, realizes the precise control of the synchronization of each master-slave unit and the online detection and monitoring of the main process parameters, from the completion of debugging to the Now, the operation is stable. No abnormalities occurred.
It can be seen from this example. The use of SEOHO frequency converter to achieve synchronous control has a simple control mode (only via a single network cable), and is easy to operate (through a few simple parameters). Reliable operation (no interference). High degree of integration of equipment, control of diversification and other advantages. The control of the master station is realized through the field bus, so that various information instructions can be fed back to the host computer such as the PLC and the touch screen conveniently through the bus to realize the automatic control function. Can also be achieved through the relay circuit, to meet the requirements of various industries and various control methods.
other instructions
The synchronous control function of SEOHO frequency converter can be used not only in the operating mechanism but also in the hoisting mechanism of the crane and the hoisting grab control. (There are many times when the hoisting mechanism is controlled by two motors. One motor controls one hook and requires the double hooks to lift the weights synchronously. The grab control requires the opening and closing of the grab to be synchronized.) For the master-slave connected to a single-axis device, the components used in the system and the parameters are adjusted in the same way as the synchronous control of the cart.
In many applications, the master station and the slave station can be controlled independently from each other when an emergency such as a failure occurs in the master station or the slave station. The control bit "Sync_Ctrl_Bypass" can be set via "binary input". When this bit is enabled, synchronous control is canceled and the master/slave stations operate independently of each other. The input signals (RUN, Speed_Set, Trq_Set) are accessed by "digital input terminals". It can also come from the PROFIBUS bus. In this way, other control signals (Sync_Ctrl_Bypass, Trq_Ctrl_Option_Bypass) can be conveniently implemented together with the input signals (RUN, Speed_Set, Trq_Set) to control the motor.
When the control bit "Sync_Ctrl_Bypass" is set, the slave will no longer follow the master action. In this case, we can use the free function block to solve. Call this free function module. The output signal "RUN" is the result of logical AND of the control signal "Sync_Ctrl_Bypass" and "Sync_Ctrl_RUN" or DI1 signal (ie: (!X & Y) | (X & Z)). Similarly, the speed and torque settings are also set.
4 Conclusion Synchronized transmission is the basic technology of the crane. Since the era of inverter drive, synchronous transmission has become a typical inverter application technology. The extensive application of the harvesting synchronization card and SEOHO-VD in the lifting industry shows the ability to harvest frequency converters in advancing technological progress in the lifting industry.
About the Author:
He Xiaomin, male, graduated from Taiyuan University of Science and Technology, School of Electronic Information Engineering, Control Theory and Control Engineering, Now, Qingdao Bangbang Control Technology Co., Ltd. is engaged in technical work. References [1] Korea SEOHO Converter - Synchronization Control Function Manual [2] SEOHO-VD Frequency converter synchronous control function crane application example tutorial