With the continuous growth of China's economy and the progress of science and technology, robot technology has developed rapidly. From general industrial production, such as assembly and welding, to special application fields, such as medical and aerospace, robots have been widely used in all aspects of modern society. Currently, the performance requirements for robots are becoming increasingly higher. They must not only have fast response characteristics and high tracking accuracy, but also good versatility and scalability. Therefore, increasingly higher demands are placed on robot control systems. SERCOS (Serial Real-time Communication System) bus is an open motion control bus, and its interface protocol has become an international standard for open motion control. This bus has a completely open communication structure, a strict synchronization mechanism , and excellent anti-interference capabilities. It can reliably achieve real-time synchronous control of multi-axis motion in harsh environments, adapting to the development of robot technology. This article will introduce SERCOS bus technology and its application in robot control systems. I. Development of Robot Control System Architecture Traditional robot control systems adopt a centralized architecture, as shown in Figure 1. The host computer is connected to the servo drivers and sensors of each joint through motion control boards. As the complexity of control systems increases, the inherent drawbacks of this architecture gradually become apparent, such as: • Difficulty in system debugging and maintenance due to excessive wiring; • Poor system reliability. When the number of nodes requiring control and the amount of sensor signals requiring feedback increase (e.g., in humanoid robots), if the central processing unit (CPU) of the host computer handles both information processing and control signal generation, it becomes overwhelmed. A CPU failure will severely impact the entire system; • The use of analog signal-based data transmission results in poor noise immunity; • The numerous and complex connections between controller modules, coupled with mutual constraints, make it difficult to achieve synchronized and coordinated motion control of more than a dozen axes; • Robot controllers are typically developed based on independent architectures, using dedicated microprocessor chips, dedicated robot languages, and dedicated operating systems. Therefore, to develop another application system, developers must design the controller from scratch, resulting in long development cycles and huge costs. This dedicated, closed architecture hinders the development of robot controllers and fails to meet the requirements of modern industrial development. Figure 1. Centralized Robot Control System Architecture II. Fieldbus Distributed Architecture With the development of robot control technology, developing "modular and standardized robot controllers with open architectures" has become a current development direction for robot controllers. In recent years, hardware and software technologies have developed rapidly, and high-performance, low-cost DSPs have begun to be widely used in robot controllers. In addition, the development of various high-performance bus technologies has greatly improved the real-time control effect. In a fieldbus distributed architecture, various switching and analog quantities are converted into digital signals locally, and all bus devices communicate using digital signals, reducing transmission errors and improving measurement and control accuracy. The application of fieldbus significantly reduces the number of wires and connecting accessories, greatly reducing installation, debugging, and maintenance costs, and enabling the system to have excellent remote monitoring and fault diagnosis functions, improving system reliability. Fieldbus also makes it easier to expand CNC system hardware; when the number of control axes and I/O points increases, it has no impact on the system's hardware structure, facilitating system expansion and reduction. Because the fieldbus protocol is open, devices from different manufacturers can be interconnected and interchanged as long as they conform to the corresponding standards. Under these conditions, designing an open real-time robot controller based on fieldbus becomes both possible and necessary. According to incomplete statistics, there are currently over 60 types of fieldbuses internationally, with commonly used ones including Interbus, Profibus, Modbus, DeviceNet, ControlNet, CAN, CC-link, and SERCOS. In the robotics field, a distributed architecture based on the CAN bus, as shown in Figure 2, is commonly used. In the architecture shown in Figure 2, the tasks of processing feedback information and generating control signals are assigned to the joint controllers of each node (e.g., implemented using a DSP). The host computer only needs to connect to each joint controller via the CAN bus to perform functions such as task scheduling, human-machine interface, kinematic calculation, and trajectory planning. The use of a distributed architecture simplifies the control equipment, reduces the complexity of system control, lowers costs, and improves system stability, facilitating further system expansion. The CAN bus has advantages such as mature technology, low cost, flexibility, and reliability; however, its data transmission rate is limited, with a maximum of only 1 Mbps, which restricts further improvements in system real-time performance. The SERCOS bus offers data transmission rates up to 16 Mbps. As an international serial real-time communication standard in the motion control field, it comprehensively describes the technical parameters of digital drives from various manufacturers worldwide, offering greater versatility and openness. III. Introduction to the SERCOS Bus The high-speed serial real-time communication system SERCOS was proposed by German industry in the mid-1980s. It is a fieldbus interface and digital exchange protocol for high-speed serial real-time communication between digital servos and controllers, primarily designed for multi-axis motion control systems in automation systems. It was adopted as the IEC 61491 (1995) international standard in 1995 and the European standard EN 61491 in 1998. Currently, it has evolved to the third generation of SERCOS (SERCOS III). It is currently the only international standard for digital servo and drive data communication. China also officially promulgated its national standard for the SERCOS protocol in 2002. Because the SERCOS bus uses fiber optic transmission, it boasts high data transmission rates and features standardization, openness, compatibility, and real-time performance. Therefore, it is particularly suitable for multi-axis linkage control, enabling real-time data communication between industrial control computers and the I/O ports of digital servo devices, sensors, and programmable controllers. It has been widely used in CNC machine tools and other digital control equipment, becoming one of the most promising industrial buses internationally. Compared to other buses, the main advantages of the SERCOS bus are: • High data transmission rates, currently reaching 2, 4, 8, and 16 Mbps, which can be set by the user; • Extremely high effective data transmission efficiency. According to tests, due to the different amount of information added during transmission, the 16Mbps SERCOS bus has the same data transmission performance as a 100Mbps Ethernet system; • Using optical fiber as the transmission medium provides high resistance to electromagnetic interference and electrical isolation; • When servo data and command data are sent and received at different times, their sampling and valid times can be precisely specified through control parameters, ensuring system synchronization and accuracy; • SERCOS communication uses NRZI (Non-Return-to-Zero) encoding and HDLC protocol to ensure transmission reliability and provides rich fault diagnosis information, which is beneficial for system installation and maintenance; • As an international standard, it provides detailed interface specifications for open control units and intelligent digital drives. All its low-level operations, communication, scheduling, etc., are designed according to international standards, harmoniously combining simplicity and precision, simplifying the process of controlling motors. IV. Topology The SERCOS bus adopts a ring topology. The host computer can drive one or more SERCOS loops through the SERCOS driver card, as shown in Figure 3. Each loop consists of one master station and multiple slave stations. The master station connects the host computer to the loop, while the slave stations connect servo devices, I/O modules, AD modules, etc., to the loop. Each slave station can connect one or more servo devices. Theoretically, a master station can control up to 254 servo devices. However, in practical applications, the total number of servo devices controlled by each master station is affected by many factors such as communication cycle time, operating mode, and data transmission rate. The SERCOS interface supports position control, speed control, and torque control, and different axes can adopt different operating modes. Each axis can have one main operating mode and three auxiliary operating modes, and the operating modes can be dynamically switched during operation. Using the SERCOS interface greatly simplifies wiring and control hardware, making debugging more convenient. The use of fiber optic connections completely eliminates electromagnetic interference during transmission, enabling long-distance control. With plastic fiber optics, the maximum distance between adjacent stations can reach 80m; with glass fiber optics, the maximum distance between adjacent stations can reach 240m. Figure 3 shows the topology of the SERCOS bus. Figure 4 shows the structure of a single-joint robot control system based on the SERCOS bus. Through the SERCOS bus, the host computer can send position, speed, and torque commands, as well as operating parameters such as reference offset and forward/reverse limit values, to each joint of the robot. It can also extract the actual position, speed, and torque status information of each joint in real time. The joint controller continuously samples the position, speed, or torque loops of the motors based on the commands received from the SERCOS bus, using algorithms such as integral separation PID control to ensure the controlled variable tracks the given information. The core processing chip of the joint controller can be implemented using a DSP. The driver mainly amplifies power. To make the structure more compact and easier to install inside the robot, the driver and controller can be designed as a single module. Of course, appropriate protection measures such as isolation must be taken between the driver and controller. The SERCOS interface connects the single-joint control system to the SERCOS loop, completes the SERCOS physical layer and data link layer protocol parsing, and realizes data storage and forwarding functions. The SERCOS interface consists of an optical receiver, an optical transmitter, a DIP switch, and a SERCOS interface control chip. The optical receiver and optical transmitter are used for signal transmission and reception, the DIP switch is used for SERCOS address selection, and the SERCOS interface control chip can be STMicroelectronics' SERCON410B or SERCON816[5]. The SERCOS interface software runs in the DSP. V. Communication Method The working timing of one communication cycle of the SERCOS bus during normal operation is shown in Figure 5. The cycle time Tscyc can be (0.062, 0.125, 0.25, 0.5, 1, 2, 3, ..., 65) ms, which is mainly determined by the control method and the number of slave stations. The SERCOS protocol defines three telegram types: Master Station Synchronization Telegram MST, Servo Telegram AT, and Master Station Data Telegram MDT. The Master Synchronization Message (MST) is sent by the master station to all slave stations at fixed intervals, indicating the start of a communication cycle. All slave stations will receive this message simultaneously, and the master station uses it to control the synchronous operation of each slave station. The Servo Message (AT) is sent by each servo slave station to the master station, providing real-time feedback of various servo information, such as the actual position, speed, torque, alarm signals, diagnostic signals, status response signals, servo parameters, and motor parameters of the servo axis. The Master Data Message (MDT) is sent by the master station to the slave stations to issue control commands, such as servo axis command position, speed, torque, operating mode selection, servo parameters, and motor parameters. Each slave station can receive this message and locate its own data at the specified location. Figure 5 shows the working timing of the SERCOS bus during normal operation. The SERCOS protocol specifies that before normal operation, the master station needs to check if the SERCOS loop is closed, identify the servo devices on the loop, and complete the configuration of network communication parameters. These mainly include the system communication cycle Tscyc, the transmission times of each servo telegram ATx (T1.1, T1.2, ..., T1.N), the transmission time of the master station data telegram MDT (T2), the time when each servo device begins sampling feedback data (T4), the position of each slave station's control data in the MDT data area, and the length of the MDT. It is evident that the initialization process of the SERCOS interface is quite complex. To address this, in 1995, Rexroth Indramat developed an active SERCOS interface master control card called "SERCANS". The card has a microprocessor, and the loaded software performs SERCOS loop initialization and communication management. It shares data with the outside world through a dual-port RAM. Developers only need to perform read and write operations based on interrupt signals, greatly reducing the difficulty of master station interface development. In 1999, with the trend of hardware becoming software-defined, Rexroth Indramat transferred the main control function of SERCANS to a software abstraction layer and successfully developed SoftSERCANS. It only requires a passive SERCOS interface main control card. This card is very simple, without a microprocessor, and is much cheaper than active SERCOS interface cards. It provides a dynamic link library (DLL) that can be called by both Win32 and real-time operating systems. For developers, there is no need to deal with hardware address and timing issues; they only need to master the definitions of DLL functions and system parameters, thus minimizing the effort and overhead spent on SERCOS interface implementation. Motion control applications written with SoftSERCANS are hardware-independent, giving the system greater openness and software portability. VI. Applications of SERCOS Bus in the Robotics Field Today, more than 70 companies worldwide offer digital products with SERCOS interfaces, including digital servo drives, controllers, and SERCOS interface I/O modules. Many suppliers also provide hardware and software interface module technology, consulting, and product design services. Figure 6 shows the Beckhoff SERCOS master control cards FC7501 and FC7502 with PCI interfaces. Figure 7 shows the plastic optical fiber used for SERCOS bus signal transmission. Figure 6: Beckhoff SERCOS master control card with PCI interface. Figure 7: Plastic optical fiber used for SERCOS signal transmission. SERCOS bus is also widely used in various printing presses, food packaging machines, assembly robots, and other special-purpose machines manufactured abroad. Figure 8 shows the AMI GP1 pallet stacking robot, which can control up to 40 driven axes through a SERCOS loop. This product has been widely used in medical, automotive, container, glass manufacturing, and canning industries. Figure 8: AMI GP1 pallet stacking robot. Figure 9 shows the LWRI lightweight robotic arm developed by the German Aerospace Center (DLR). It is 1338mm long, weighs only 14.5kg, has a maximum joint rotation speed of 120°s, and has 7 axes (RPRPRPR). Using optical fiber and the SERCOS protocol for communication, it successfully achieves position control, torque control, and impedance control of the robotic arm. Figure 9. German LWR-I lightweight robotic arm. In recent years, China has also begun to try applying the emerging SERCOS bus, and related research is increasing. Since 1997, my country has purchased controllers and starters with SERCOS interfaces from the German company Indramat. The earliest importers included Dalian Machine Tool Plant and Beijing University of Technology. Beijing University of Technology also established a SERCOS interface technology qualification center authorized by the German SERCOS Association, developing LINSERCANS software (porting softSERCANS to a real-time Linux operating system) based on SERCOS interface technology, as well as CNC cross-cutting machines. This center has hosted international conferences related to SERCOS technology for three consecutive years. Currently, there are more than 20 domestic users of the SERCOS interface, including Tsinghua University, Beijing Institute of Mechanical and Electrical Engineering, Beijing 625 Institute, Beiren Group, Shanghai Gauss Printing Machinery Plant, Shenyang No. 1 Machine Tool Plant, Huazhong Numerical Control Group, Beijing University of Aeronautics and Astronautics, Second Automobile Works Equipment Manufacturing Plant, Shanghai Volkswagen Automotive Plant, and Shanghai General Motors, among other well-known domestic enterprises and institutions. Currently, the application of SERCOS bus in the robotics field in China is still in its early stages, but there are already some successful cases of robot systems based on SERCOS bus. For example, the large-scale complex curved surface water-fire forming intelligent robot developed by Tsinghua University's Department of Computer Science and other institutions uses the SERCANS interface card from Bosch Rexroth, Germany, and INDRAMAT servo motors with SERCOS interfaces. This is the first industrial robot in China based on the SERCOS bus, and it has now passed the acceptance review by the National 863 Program expert group. Because the servo motors are connected via the SERCOS bus to control the movement of the robot's five axes, the system wiring is greatly simplified, anti-interference capabilities are enhanced, and data transmission rates are improved. In addition, Tsinghua University's Department of Automation proposed a design scheme for a bipedal walking robot based on the SERCOS bus, and demonstrated that for a SERCOS bus with a transmission rate of 4Mbps, considering data gaps and sampling, gait, and posture planning times, the control cycle of the main computer can be guaranteed to be within 10ms, which is sufficient to complete the static and dynamic walking control of the walking robot. Figure 10 shows the DLR-I intelligent robot dexterous hand system developed by the Robotics Institute of Harbin Institute of Technology and the Institute of Robotics and Systems Dynamics of the German Aerospace Center. The manipulator arm has only one fiber optic loop and four power interfaces. The global hand controller and local finger controllers are connected via fiber optics and communicate using SERCOS. Data exchange of all sensor and control signals from the four fingers can be achieved within 1 ms, making the entire system highly adaptable and scalable. Figure 10 shows the DLR-I intelligent robot dexterous hand. Beijing University of Aeronautics and Astronautics has also developed an ISA bus SERCOS master station card and SERCOS master station driver package, capable of controlling 1 to 36 servo motors, and has applied the SERCOS bus to a teaching robot control system. These research results provide valuable reference for the development of a new generation of open intelligent robot controllers. VII. Conclusion Modern industrial production and robot research increasingly demand openness from robot controllers. The manufacturing industry requires industrial robots with greater flexibility and more powerful programming environments to adapt to different application scenarios and multi-variety, small-batch production. As the only international standard for digital servo and drive data communication, the SERCOS bus represents the development direction of digital servo interfaces and has become a research hotspot in current CNC technology. This bus uses fiber optic connections to connect control components, forming a closed loop to achieve real-time data transmission. Its communication mechanism ensures synchronous operation and control accuracy of the system, and it also possesses high reliability and openness. Therefore, utilizing the SERCOS bus for multi-axis control will be the future development trend of robot systems.