The automobile industry is now undergoing great changes. The product life cycle is constantly shortened, the update speed is faster, the more personalized order customized production, and the mixed production of multiple models and configurations has become the major trend of the automobile industry. The production of main auto parts, such as engine, gearbox, suspension system and so on, has also changed greatly from the traditional assembly process.
The traditional automobile rear axle assembly line mostly uses the power roller line as the conveying equipment to convey the rear axle components in each assembly station according to the process flow. The main disadvantage of the assembly line composed of power roller table is that the working position process cannot be changed easily after the design, construction and installation.
1. Background analysis
1.1 rear axle characteristics of BEV new energy vehicle
The rear axle of the new energy vehicle is more complicated than that of the fuel vehicle than that of the traditional vehicle in terms of the battery inverter, controller and other devices, and the assembly process is more complex than that of the traditional fuel vehicle. There will be more working procedures. After assembly, the test link will be more strict.
1.2 with the promotion and implementation of the principle of average fuel consumption of domestic CAFC enterprises, vehicle enterprises need to produce more new energy pure electric and hybrid vehicles to balance the fuel consumption index of enterprises. Countries have also announced a timetable for banning the sale of fuel vehicles. In this context, a foreign first-line brand also started to release the pure electric (Bev) model of its SUV and introduce it into domestic production. The AGV rear bridge assembly line introduced in this paper is built for its matching.
2. AGV assembly line design
The rear axle of this pure electric vehicle weighs about 350kg and the clamp is about 400kg. One AGV is equipped with one set of fixture, one set of fixture is used to hold one rear axle workpiece, and each assembly station is moved in sequence according to the process sequence.
The loop line is arranged with 19 stations, as shown in Figure 1, including 1 bypass branch station, 1 standby station and 1 wa (wheel alignment) station. The maintenance area and maintenance branch line shall be arranged on the east side of the loop line. Operation station length: 4.5m production time: 18u / h, assembly of 18 workpieces per hour.
2.1 AGV model design
According to the size and weight of the product, the load parameters of AGV are determined, and the driving motor power and follower wheel are selected to determine the gear train. Design the car body structure and arrange the electrical layout. The owner is a first-line brand customer in Europe. The design standards refer to CE and TUV certification standards and issue TUV certification report.
2.2 AGV power system design
The most important part of AGV design is the power system design. Its flow chart is shown in Figure 2. It is subdivided into gear train design, drive power calculation and motor selection.
2.3 gear train design
According to the final assembly weight of the rear axle and the weight of the fixture, the deck AGV with a design load of 1 ton is selected. The arrangement route of the rear axle assembly process is a circular route, without sideshift and spin motion modes, and without Omni moving gear train structure. In the assembly process, the wa station is connected with the wheel alignment equipment, which has a requirement of ± 3mm for AGV parking accuracy. In the horizontal comparison between the main driving wheel mode of AGV and the differential steering wheel mode, the single steering wheel mode can ensure higher running and parking accuracy, so the single steering wheel drive + 2 directional follower mode is selected for AGV wheel system.
2.4 calculation of driving power
At present, the mainstream electrical system voltage of AGV system includes 24V, 48V and 96V, and 24V system voltage is generally selected for medium and low load. Select the drive motor and calculate the motor power.
Calculate the motor power P = 600W.
According to the statistics of other electrical power consumption of AGV vehicle body electrical system, the current is about 5A in the parking standby state and 20A in the full speed operation state. According to the working hours of the loop station, the layout of the charging station is arranged, and four charging stations are preliminarily selected to be arranged in the station with long parking working hours. Combined with the beat calculation, the battery power loss and replenishment are calculated. According to the charging efficiency and the reduction of charging efficiency when the battery is nearly full and other factors, the battery power replenishment margin is taken as 20%, and the 60ah lithium-ion power battery is selected. Four charging stations can meet the requirements of charging and discharging.
2.5 calculation of AGV number
According to the length of the station, the production beat and the operation process time of each station. The number of AGV vehicles required can be calculated according to the output beat and the comprehensive equipment efficiency.
2.6 layout design of AGV body
Appearance structure: AGV body is generally divided into three main functional compartments, driving wheel compartment, battery compartment and electrical compartment (see Figure 2). AGV chassis layout is divided into driving wheel, follower wheel and other functional modules. The function of driving wheel cabin is to install AGV driving wheel and its accessories to provide driving force and control the direction of AGV travel. The internal structure includes driving wheel body, travel motor, rudder motor, reducer and travel code wheel, rudder code wheel and limit sensor, power cable and control cable. The function of battery compartment is to install AGV power battery, charging contactor, etc., to provide power supply for AGV walking, and to charge the power battery with the action of ground charging mechanism.
2.7 AGV body structure mechanics simulation
SolidWorks (SW) software is used to build a three-dimensional solid model for car body design, which can effectively determine the mass, volume, center of gravity and other parameters of car body, facilitate subsequent model analysis and manufacturing, and provide accurate parametric solid modeling and engineering design drawings.
2.8 AGV control system
AGV owner control adopts Siemens SIMATIC s7-1200 series PLC. It wirelessly connects with AGV console through AGV on-board wireless router, receives console command and reports vehicle status. The main controller of AGV car body is connected with the lower motion control unit (MCU) through CAN bus. MCU controls each motion axis on AGV car body, such as travel motor, rudder motor, etc. The main controller of AGV car body is connected with each sensor and controller on AGV car body through CAN bus, such as the safety relay and laser anti-collision sensor pls, and the navigation sensor, RFID and battery power supply system.
2.9 upper control structure
Ips-x, the control system of AGV upper automobile factory, exchanges data, receives production instructions, and reports the status of AGV system and production process data.
3. Implementation effect of AGV assembly line
Through the understanding and design of the assembly process of BEV rear axle, the new assembly line mode of replacing the traditional conveyor roller line with AGV is implemented, which completely changes the occupation of the roller line, and is not easy to change the disadvantages of the working position process. It better adapts to the upgrading requirements of the model in the future, and enables the assembly line to have flexible production capacity. It is in line with the intelligent development direction of production equipment of the owner group in the next few years. It establishes a model project for the subsequent engineering planning. More in line with the national intelligent manufacturing development strategy, leading the domestic industry development direction. Its application is very representative of the advanced in this field, and the field production and use effect after implementation is also very ideal, as shown in Figure 3. It is worth popularizing in the same industry.
 Wang Junjian, Shang Xiaohui. AGV car technology and its application in automobile manufacturing industry [J]. Automotive practical technology, 2016, (5): 187-189199.doi: 10.16638/j.issn. 1671-7988.2016.05.061
 Cheng Jianwei, Zhang Changyong, Chu Haibo, Wang xingcai. Design of AGV control system based on S7-200 [J]. Electromechanical engineering technology, 2016,45 (6): 80-84.doi: 10.3969/j.issn.1009-9492. 2016.06.021