Design of Multi-axis Ultrasonic Testing System Based on Hybrid Control System
“The control system has developed from the first generation control system of PCS (PneumatiCControl System) more than 50 years ago, through ACS (AnalogouSControl System), CCS (ComputerControl System) and DCS (Distributed Control System), to today’s fifth generation Control system — Fieldbus control system FCS (FieldbuSControl System).
The control system has developed from the first generation control system of PCS (PneumatiCControl System) more than 50 years ago, through ACS (AnalogouSControl System), CCS (ComputerControl System) and DCS (Distributed Control System), to today’s fifth generation Control system — Fieldbus control system FCS (FieldbuSControl System).
Distributed control system DCS is composed of operation station, control station (monitoring station), application module and historical module, communication network and engineer station. In addition to high-reliability software and hardware, as well as redundancy measures such as communication networks and control stations, DCS also uses fault detection and diagnosis engineering software to monitor production conditions, so as to detect faults early and take timely measures to improve production. security.
In the late 1980s, on the basis of DCS, it began to develop a network structure and network protocol suitable for the industrial environment, and realized the communication of the sensor and controller layers, which is the field bus. But research has been slow and there are no international standards to follow. The International Electrotechnical Commission (IEC) officially announced all technical standards of the IEC61158 fieldbus in January 2000. But this standard accommodates 8 kinds of communication protocols that are incompatible with each other. Due to the compromise of the existing interests of various parties, the original intention of completely unifying international standards has not been realized.
CAN (Controller Area Network) bus is a kind of field bus. It generally uses a typical serial bus topology for communication. The bit rate of this method is lower than that of the ring structure, but the structure is simple, the cost is low, and the communication is reliable. CAN bus has short transmission time and low probability of being interfered, which ensures real-time communication. CAN can correctly locate the source of errors and separate permanent hardware errors from software errors. Its protocol adopts CRC check and provides corresponding error handling functions, while ensuring the reliability of data communication.
Proposition of Hybrid Control System
The field bus control system FCS should be compatible with the distributed control system DCS. First of all, from the perspective of engineering cost and benefit, the advantages of field bus are good interoperability, simple structure, low wiring cost; decentralized control functions, flexible and reliable, and rich on-site information. However, these advantages are based on the premise of the initial installation of the FCS system. If the enterprise has established a complete DCS and wants to transition to the FCS, it must carefully consider the rate of return of the existing investment to the existing investment. It is the first choice to make full use of the existing DCS facilities, the existing DCS wiring and mature DCS control and management methods to realize FCS. Although fieldbus has advantages over existing digital field protocols, the transition to it is costly and risky. Furthermore, from the technical inheritance and control means, FCS and DCS are also required to be compatible with each other. FCS realizes that the control function is moved down to the field layer, so that the multi-layer network of DCS is flattened, and the independent function of each field device node is strengthened. Therefore, it is necessary to increase and improve the data communication function between field sub-layer devices in FCS.
The integration of FCS and DCS mainly includes three aspects:
(1) Integration of FCS and DCS input and output buses
The integration of fieldbus and DCS input and output bus has the following three characteristics:
①In addition to installing the fieldbus interface board or the fieldbus interface unit, there is no need to make other changes to the DCS;
② Make full use of the operation and control function blocks of the DCS control station, because the number and types of function blocks in the fieldbus instruments developed at the initial stage are limited;
③Using the existing DCS technology and resources, the investment is small, the effect is quick, and it is convenient to promote the application of the field bus.
(2) Integration of FCS and DCS networks
Integrating the field bus on the I/O bus of the DCS control station is the most basic primary integration technology, and it can also be integrated at a higher level of the DCS, that is, integrating the field bus on the DCS network (DCSnet), as shown in Figure 2 Show. Fieldbus server (FieldbuSServer, FS) is connected to DCS network, FS is a complete computer, and has installed fieldbus interface card and DCS network interface card.
The field device or field instrument communicates with its interface card through the field bus, and the input, output, control and calculation function blocks in the field instrument can independently form a control loop on the field bus without borrowing the functions of the DCS control station. The field bus server communicates with the DCS network (DCSnet) through its DCS network interface card, and the FCS can also be regarded as a node on the DCS network or a device of the DCS. In this way, FCS and DCS can share resources with each other. FCS directly borrows the operator station or engineer station of DCS.
The integration of FCS and DCS network has the following four characteristics: ①In addition to installing the fieldbus server, there is no need to make other changes to the DCS; ②The control loop can be independently formed on the fieldbus to achieve complete decentralized control; ③In the fieldbus server Some advanced function blocks can be configured in a unified manner with the basic function blocks in the field instrument to form a complex control loop; ④Using some resources of the existing DCS, the investment is low and the effect is quick, which is convenient to promote the application of the field bus.
(3) System integration of FCS and DCS
There are two ways for system integration of FCS and DCS: one is that the FCS network (FCSnet) integrates with the DCS network (DCSnet) through the gateway (Gateway), and exchanges information directly on their respective networks, as shown in Figure 3; the other is It is the FCS and the DCS that are respectively attached to the enterprise network (Intranet), and exchange information indirectly through the enterprise network.
The integration of FCS and DCS has the following four characteristics: ① FCS is installed independently, almost no changes are made to DCS, and only a gateway needs to be connected to DCSnet; ② FCS is a complete system, and there is no need to borrow more resources than DCS; ③ Both It is beneficial to the development and promotion of FCS, and it is also beneficial to make full use of the resources of the existing DCS; ④The system has a large investment and is suitable for new installations.
Multi-axis Ultrasonic Inspection System Based on Hybrid Control
The main functions to be completed by the multi-axis ultrasonic inspection system are as follows: multi-axis linkage ultrasonic automatic scanning inspection; helical imaging, cross-sectional imaging and longitudinal cross-sectional imaging of bars; plane scanning of forgings and carbon fibers; quasi-3D scanning imaging; A/ B/C imaging and analysis; defect quantification and defect image analysis; defect location and marking, etc.
The data connection between each I/O component of the ultrasonic testing system and each module inside each main component adopts a traditional parallel network, and the system control network introduces CANbus, through which the direct control of the on-site servo controller can be realized. At the same time, data exchange between the system and other ultrasound equipment can also be realized. If the multi-master mode is adopted, it can also be integrated into the system network upward. The schematic diagram of the structure of the multi-axis ultrasonic detection system is shown in Figure 4.
In order to complete the multi-axis linkage ultrasonic automatic scanning inspection, the CAN bus is used to control multiple servo motors. In the system, the CAN bus adapter card installed in the control station is selected to be connected to the CAN bus to realize the integration of the field bus and the DCS input and output bus. The field nodes are 6 servo controllers with CAN control ports, which are completed by the servo controllers. Control the peripheral servo motor to realize various point movements. It adopts TCL6 series controller of German Baigra company. In addition to all the basic functions of servo controller, this type of controller also has a built-in programmable logic controller (PLC), which can be programmed and controlled by adding CAN communication on the basis of TCL6. interface for remote control.
Communication and control strategy based on field bus: firstly initialize the CAN bus adapter card and CAN communication interface; realize the communication between the bus adapter card and the controller. Secondly, the data is transmitted to the controller through the bus. The PC first writes the data that needs to be downloaded into the PCCAN card according to a certain data structure, and the PCCAN card transmits the data to the bus, which is consistent with the purpose of the bus data. The controller with the same address detects the data on the bus, starts to accept the data and checks whether the data is corrupted during transmission, if it is complete, it will execute the command and send a positive response to PCCAN. When the communication is established, the motor completes the action according to the command, and the fixed movement mode can be solidified in the controller through the PLC. In order to reduce the communication pressure of the bus, the main motion control work is completed through PLC programming. For example, in the case of a known motion track, when using the Electronic gear ratio mode, the known motion track is divided into segments and the electronic gear ratio of each segment is programmed into a specific PLC program segment to complete the spindle motor-based operation. Follow-up control.
In practical application, only a start broadcast signal is sent from the bus, and the motion control of the subordinate motor is completed by the PLC alone. The PLC calculates the signal fed back by the photoelectric encoder and timely calls the prefabricated electronic gear ratio to complete the complex motion. This processing method is simpler than that the host computer accesses the signal of the photoelectric encoder in real time through the bus to change the electronic gear of each motor. The bus only needs to read the motor position status according to the monitoring interface. Adopting such a design can reduce the communication tasks of the bus by 50%, and also reduce the work intensity of the upper computer, so that the upper computer has more resources to solve other computing tasks.
The system can better fulfill the requirements of the multi-function detection system. Due to the selection of bus control, the reliability of the system is greatly improved compared with the previous control methods, and the connection is simplified. At the same time, due to the use of the servo controller with built-in PLC, part of the work of the upper computer is moved to the lower computer for processing, and the upper computer has more resources to perform corresponding image processing and other work.
According to the characteristics of FCS and DCS in the measurement and control system, this paper analyzes the control mechanism and application status of FCS and DCS, and proposes “integration of FCS and DCS input and output bus”, “integration of FCS and DCS network” and “integration of FCS and DCS”. Three hybrid control models were integrated, and the control strategy and communication mechanism adopted by the hybrid control model were studied. The feasibility of the integrated model of FCS and DCS is verified through the research and field implementation of the multi-axis ultrasonic testing system project, and the technical difficulties encountered in the process of project implementation and application are expounded in detail.
The Links: LB064V02-TD01 LTM150XH-L04