Abstract : This paper introduces the basic principle and hardware of a control system for an electric multi-axis nut tightening machine. The system adopts a distributed structure, with each axis controlled by a control unit based on the AFC-1100 core, ensuring constant torque tightening. Keywords: Tightening machine; Universal coupling; Servo electric torque fastener; Axis control unit; 1 Introduction Automatic tightening machines are mechatronic devices integrating mechanical transmission, electrical transmission, pneumatic technology, electronic technology, and automatic detection. As the name suggests, a tightening machine is a device unit for tightening workpieces, mainly used for bolt/nut tightening. A single machine may assemble hundreds or thousands of parts using bolt fastening methods, and in mass production, this is often done by multiple people at different workstations. Moreover, dozens or hundreds of machines are assembled daily, resulting in a considerable number of bolts that must be tightened to the torque specified in the product drawings. To improve production efficiency and ensure the tightening torque of bolts, automatic tightening machines were developed. 2. The Original State of Our Automatic Tightening Machines: In 1997, FAW Daewoo established a factory in Yantai. The automatic tightening machines on its transmission assembly line were manufactured by the Korean company DAEWOO, while the electric tightening machines used were from the Japanese company DDK. The tightening machines employ servo electric torque clamps and an AFC-1100 controller, which is compact, powerful, and has a self-testing function. Its tightening function is divided into two control methods: torque method and angle method. For example, the gear shift cover tightening control uses torque method, while the drain bolt tightening control uses angle method. The torque method is the most commonly used bolt tightening method. It controls the preload of the connected parts by the torque value displayed on the torque wrench, and the operation is simple and intuitive. The angle method involves tightening the bolt to a predetermined torque and then rotating it by a predetermined angle. This is based on the approximate proportional relationship between the rotation angle when tightening the nut or bolt and the sum of the bolt elongation and the looseness of the tightened parts. Therefore, a predetermined preload can be achieved by rotating the bolt at a specified angle. 3. Current Status of Our Factory's Product Technology and Requirements for Future Technical Upgrades Our factory's current mainstream product is the D16/D20 series manual transmission. Bolted connections are primarily used to ensure the tight, leak-proof, and reliable long-term operation of flange, housing, and side cover connection systems. Sufficient sealing pressure is required on the gasket surface, especially under high-temperature and vibration conditions where gaskets age, creep, and loosen, and flanges, side covers, and bolts undergo thermal deformation. Therefore, torque has a crucial impact on bolt connections. The application and control of bolt preload are therefore very important; excessive or insufficient preload will adversely affect the seal. Excessive preload will crush the gasket, causing it to lose elasticity, and may even break the bolt; insufficient preload will result in the residual compressive stress on the gasket surface failing to reach the working sealing pressure, leading to leakage in the sealing cavity connection system. Therefore, controlling bolt preload is a crucial issue that must be addressed in actual production. The differential rear cover was initially tightened with 10 bolts; due to technological changes, the differential rear cover bolts have been changed to 11 bolts. For convenience, products using 10 bolts are designated as Model A, and products using 11 bolts are designated as Model B. Model A transmissions will be produced for another two years, after which all production will be switched to Model B transmissions. During these two years, production will be switched between the two models. We will now discuss the differential side cover bolt tightening machine, mainly analyzing it from mechanical and electrical perspectives. 3.1 Mechanical Aspects The original structure of the differential side cover bolt tightening machine is shown in Figure 1. The distribution diagram of the tightening head sleeve of the tightening machine is exactly the same as the bolt distribution diagram of the side cover, and they are coaxial. After the tightening slide is pushed to the designated position by the cylinder, the tightening machine begins tightening. The distribution diagrams of Model A and Model B after technical modification are shown in Figure 2. Where a, b, c, d, e, f, g, h, j, and k are the original 10-axis tightening guns; while A, B, C, D, E, F, G, H, I, J, and K are the dimensions of the equipment after the product drawings were revised, representing 11-axis tightening guns. A comparison of the drawings shows that the 10-axis tightening machine and the 11-axis tightening machine have only one shaft difference in size. Based on on-site investigation, the newly added shaft can be placed at position i (indicated by the dotted line). The following analysis compares two solutions that meet the requirements. Solution 1 involves modifying the equipment by adding a switching template to the existing tightening machine (as shown in Figure 3). This allows the existing tightening machine to be reused, saving some money. However, due to the addition of a shaft, the positions of multiple shafts on the tightening machine are not concentric with the template shafts. We need to use universal couplings to solve this problem. A universal coupling is a type of coupling that allows for a large radial displacement between two shafts, with a maximum shaft angle of 45°, and the shaft angle can be changed as needed during operation. To eliminate the periodic fluctuation problem of the driven shaft speed of the universal coupling, we connect two single universal couplings in series to form a double universal coupling (as shown in Figure 4). The angular relationship between the drive shaft, driven shaft, and intermediate shaft is given by: tanф1 = tanф3cosα1, tanф2 = tanф3cosα2, tanф1/tanф3 = cosα1/cosα2. Here, ф1, ф2, and ф3 represent the angular relationships between the drive shaft, driven shaft, and intermediate shaft, respectively; α1 and α2 represent the inter-shaft angles between the drive shaft, driven shaft, and intermediate shaft. When α1 = α2, ф1 = ф2, eliminating the angular difference between the drive shaft and driven shaft. This eliminates the speed fluctuations in the driven shaft caused by the constant speed rotation of the drive shaft, ensuring that the actual torque of the tightening machine is accurately reflected on the tightening shaft. Furthermore, the double universal joint must meet three conditions during installation: 1) The inter-shaft angles between the drive shaft, driven shaft, and intermediate shaft are equal; 2) The axes of the drive shaft, driven shaft, and intermediate shaft should be in the same plane; 3) The shaft forks at both ends of the intermediate shaft should be in the same plane. Considering the actual situation on the assembly line, without making significant modifications to the existing tightening machine, only one template matching the 11-axis rear cover is needed. When producing model B products (as shown in Figure 5), the tightening machine connected to the left side of axis ω1, via a double universal coupling, outputs power to the sleeve on the template via axis ω2 on the right side, thereby tightening the bolts. When producing model A products, simply remove the template and double universal coupling, install the sleeve onto the existing tightening machine, and it is ready to use (see Figure 6). [align=center] [/align] Scheme Two: The left side of the tightening machine is the tightening mechanism for model A products, and the right side is the tightening mechanism for model B products. Figure 7 shows that different (model A and model B) tightening mechanisms can be selected when producing two types of products. The switching between different models is achieved by using a cylinder to achieve the purpose of processing different models of products. When producing model A products, the corresponding model A tightening machine slides to the corresponding position to tighten the side cover bolts. When producing model B products, the model B tightening machine tightens in the initial position. A switch for selecting between A and B models is located on the control panel. Each tightening mechanism operates independently; a problem in one machine will not affect the other. Advantages and Disadvantages Option 1: Compact equipment structure, small footprint, minimal modification, ample flexibility, and savings on equipment upgrade costs. When producing model B, only the model B template needs to be installed, and all 11 tightening heads will operate; when producing model A, the model B template can be removed, and only one shaft needs to be shielded. Option 2: Loose equipment structure, large footprint, limited flexibility, double the number of tightening heads, and a correspondingly doubled number of controllers (doubling equipment costs). After two years, once all models are switched over, the other set of equipment will be idle, resulting in significant financial waste. 3.2 Electrical Aspects: Given that the tightening machines are packaged as shown in Figure 8 (one tightening machine per controller), all tightening machines can be connected to a computer or PLC via an RS-422 data communication interface to control printing, data transmission, and simultaneous tightening commands. First, we configure the tightening machine's controller, including functions such as tightening mode, maximum and minimum tightening torque. Next, we connect the cables according to the specifications in the instruction manual. It's important to note that the tightening machine requires three-phase 220V voltage and should not be confused with two-phase power. Finally, we power it on. Through centralized control by the PLC, when the workpiece reaches the designated position, the PLC sends a signal to control the slide table to move forward. Upon receiving the instruction, the tightening machine begins tightening according to the system parameters set by the controller. During the tightening process, torque sensors, encoders, and other units feed back tightening data to the controller. After the tightening process is complete, an OK/NG signal is sent to the PLC for judgment and to proceed to the next step. This cycle repeats continuously. 1. Introduction to the Tightening Machine The basic structure of the tightening machine is shown in Figure 9. The tightening machine mainly consists of a drive motor, a planetary gear reduction mechanism, an encoder, and a torque sensor. The drive motor primarily supplies power to the tightening machine, converting electrical energy into mechanical energy; the planetary gear reduction mechanism reduces gear speed and increases torque; the encoder acts as a sensor, outputting the motor's rotational angle (or position) signal to the driver, and promptly feeding back speed, torque, and angular displacement to the controller; the torque sensor converts the analog signal of the rotational torque transmitted from the drive rod to the tightening workpiece into a digital signal, which is then transmitted to the controller. 2. Axis Control Unit: Each tightening machine is equipped with an axis control unit. Its components are independent systems, primarily controlling their respective tightening machines, including setting some parameters. Finally, the axis control units are connected together via an I/O interface unit, ultimately controlled by a PLC. The axis control unit mainly has the following specific functions: (1) Setting the tightening mode, maximum and minimum torque in the controller; (2) Statistical tightening values, storage and printing functions; (3) Connecting with the PLC, judging the tightening results and automatically issuing OK/NG control signals; (4) Data communication to transmit the tightening results to the PLC, automatic alarm and other functions. Through mechanical and electrical analysis and comparison, we finally determined the first scheme. After only two months of modification, the project was successfully completed. So far, we have successfully modified this automatic tightening machine, and all aspects of the indicators have met the requirements. Through our independent research and development, we have solved the problem of upgrading and replacing equipment for new product models. More importantly, it has trained and trained technical personnel, enabling them to fully master this advanced technology and use it for our own purposes, enhancing the strength of technical personnel, and applying the technology to actual production, which has improved our factory's competitiveness and laid a technical foundation for our company's subsequent product development.