Functional requirements of the hottest MES for the

2022-08-14
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The functional requirements of MES on the field control system

MES (Manufacturing Execution System) technology is popular all over the world with its new concept, perfect system architecture and powerful functional modules, and is also applied in the high-level automobile production line introduced in China. At the same time, MES also attracts domestic automobile manufacturers with its excellent performance in reliability, monitoring and digital management. At present, whether it is a new or upgraded project, the planning or function expansion goal of its control system is centered on MES. As the field level control system is the foundation and support of MES, whether its function is perfect directly affects the role of MES. Therefore, it is very necessary and urgent to explore how to plan or expand the function of the field control system according to the concept and functional requirements of MES. Then, on the basis of the well-known functions under the conventional control mode, how can MES be expanded to give full play? The author briefly expounds his own experience in learning and practice through the following

function expansion

1. In communication

mes, the information between systems and control layers must be able to flow freely, so it is necessary to build a communication network between them. At present, the conventional practice is to exchange information between the management layer (monitoring layer) and the control layer through industrial Ethernet, and between the control layer and the field device layer through fieldbus. See Figure 1 for the network topology diagram. In order to avoid unnecessary communication conversion, the PLC of the control system should preferably have the function of communicating between Ethernet and Fieldbus (PROFIBUS, DeviceNet, etc.). The frequency converter should preferably have a communication interface corresponding to the field bus. Devices such as intelligent instruments should preferably have a fieldbus or Ethernet communication interface. If they belong to a system or are closely related to a system, local analog transmission can also be carried out in the system. If PROFINET technology is applied in the workshop, the communication will become simpler, but it is hardly applied in the automotive industry at present

Figure 1 simple structure of network topology

2. Equipment operation status detection

in order to make the monitoring computer or human-machine interface in MES more vividly and accurately display the operation status of the whole process equipment and its devices, the field control system must detect and judge the operation or on-off status of motors, pumps and sensors. This requires the addition of corresponding detection elements, such as encoders, which can directly detect whether the motor is running; Photoelectric switch, which can detect whether the workpiece passes or occupies the position; Distance measuring switch, which can detect the travel distance of mobile equipment; Pressure switch, which can detect whether it is overloaded; Anemometer, which can indirectly detect whether the fan is running or not; Proximity switch, which can indirectly reflect the synchronous state between devices; Ammeter, which can indirectly reflect the running state of motor, etc. When the indirect method is adopted, it needs to cooperate with the software to realize its function. For example, by detecting the running current of the motor (excluding the starting process), it can indirectly judge whether it is in the running state. The coordinates shown in Figure 2 reflect the running current of the motor. The upper and lower limits are determined according to the variation range of the load in the actual work of the motor. When the motor running current is between the upper and lower limits, it can be considered that the motor is in the running state, the current is zero, it is in the stop state, and other sections are in the fault state

Figure 2 Relationship between motor running state and current as the largest lithium battery manufacturer in the world

3. Parameter detection and control

mes detection and adjustment of field parameters are mainly realized by Taihe control system. Therefore, it is suggested to establish a digital or analog parameter transmission channel between the on-site PLC and the on-site equipment, and try not to adopt the direct communication mode between the on-site equipment and the upper computer. Taking temperature measurement and control as an example, there are many schemes. At present, the more popular scheme is to directly detect and control through the local or remote analog i/o of PLC. Figure 3 takes the remote analog i/o measurement and control as an example. The upper computer and the on-site man-machine interface can realize parameter adjustment and dynamic display, and the upper computer can also record and draw parameter change curves. Analog i/o controls the opening of the electric control valve according to the transmitter signal to achieve temperature regulation

Figure 3 temperature principle of remote analog i/o measurement and control

4. Position detection of workpiece or mobile equipment

if the upper system wants to accurately display the travel position of workpiece or mobile equipment, the control system should configure laser ranging sensor, encoder or bar code switch according to different display objects. The laser ranging sensor can directly detect the moving distance of the equipment; The encoder scheme is to detect the change of the rotation angle of the output shaft and then convert it into displacement; The bar code switch determines the position of the equipment by detecting the change of the bar code on the bar code belt, which must be arranged and installed along the moving path of the equipment. See Figure 4 for the approximate shape of the bar code strip. In addition, when the accuracy requirement is not high, in order to save cost, the moving distance can also be calculated according to the formula s=vt. Where, s is the moving distance, V is the actual running (rotation) speed of the equipment detected or converted, and t is the running time

Figure 4 bar code belt shape

5. Integrated into the recognition system

car body recognition is an important part of MES. At present, the common practice is to use RFID technology to integrate the identification system into the control system. The typical network structure of RFID is shown in Figure 5. The data carrier is written into the processing and installation information at the entrance of the workshop and moves with the workpiece. Each station completes the process operation according to the information read from the data carrier, and then writes the necessary process information into the data carrier. Generally, read-write stations must be set before and after important process sections and before classified turnouts

Figure 5 typical network structure of RFID

6. Redundant setting of sensors

redundant setting of sensors is very necessary in the production workshop where MES is built. Redundancy can not only improve the reliability and efficiency of equipment operation, but also create convenient conditions for fault diagnosis. Generally, the position detection sensors should be redundantly set in key parts such as elevators, transporters and overturners to reduce the probability of collision, dropping, etc., and reduce the shutdown caused by sensor failure. In addition, they can also become a reference for fault diagnosis. Take the elevator as an example. In Figure 6, a and B are redundant sensors that detect the rise in place. Under normal circumstances, a and B act at the same time, and the workpiece is transported horizontally. If a and B do not act at the same time, it can be diagnosed that a or B is in a fault state, and the transportation should be suspended

Figure 6. The elevator transports workpieces up and down

7. Information prompt

mes pays attention to improving the quality and efficiency of each process. For this reason, the information prompt board set in the station and production line can display the processing (installation) information from the production management system and the help seeking information from each station of the production line. Figure 7 shows a simple workstation installation prompt board, indicating the models of accessories to be installed on the current vehicle and subsequent vehicles. Figure 8 shows the simple concrete pressure testing machine, which is a frequently used detection instrument. The station status prompt board displays the fault (red) or help (blue) information of each production station

Figure 7 simple installation prompt board

8. Manual reading and writing of on-site information

since the manufacturing information of the workpiece must accompany the workpiece all the time, in order to avoid affecting the follow-up operation for a long time until the sample is broken due to data loss, it is necessary to properly set up manual reading and writing stations on the site, such as industrial computer, HMI, etc. The reading and writing station shall be able to read the relevant information of the upper system and the control system through the communication network and write the information into the data carrier or the upper system. Generally, such reading and writing stations should be set at least at the entrance, exit and workpiece transfer place of the workshop. In addition, it is also very necessary to manage the permissions of the read-write station

figure 8 simple station status prompt board

9. Fault diagnosis

fault diagnosis is also an important function of MES. The on-site PLC will complete many fault diagnosis logics and transmit the results to the upper system, which will prompt in order according to the possibility. There are many methods for PLC software diagnosis, such as trend analysis method, direct reference method and standard time delay method. Taking the standard time delay method as an example, figure 9 shows the mechanized conveying equipment. The time interval between the workpiece running from sensor a to sensor B is t, and the allowable range of change is T1 ≤ t ≤ T2. When t exceeds T2, sensor B has no signal change, and the conveying equipment should stop running, and it should be diagnosed as a possible fault of sensor B. Of course, the cause of the failure needs to be comprehensively analyzed according to more detailed information

Figure 9 conveyor transports workpieces horizontally

10. Error proofing

different workshops or different sections adopt different error proofing methods. Taking the assembly section of some interior trim parts in the general assembly workshop as an example, in order to ensure the accuracy of the assembly work pieces, the accessories must be placed in zones according to the type and category, and then the light curtain or photoelectric switch should be installed on the channel where the accessories are taken for action detection. As shown in Figure 10, a light curtain is set between people and accessories. When taking accessories through the light curtain, the light curtain will output signals to PLC, and PLC will confirm whether the model of the accessories taken out matches the current model through software. If not, an audible and visual prompt to stop assembly will appear

conclusion

as mentioned above, whether MES can fully demonstrate its strong advantages depends largely on the effective support of the field control system. From another perspective, the new requirements of MES for the field control system will greatly promote the improvement and perfection of the field control system. The above expansion functions are conceived and realized under the inspiration of MES, but they are not completely limited to the framework of MES. In addition, due to the author's limited experience, the above description fails to cover all the functions supporting MES. (end)

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