In simple terms, a robot's internal bus is a communication line that connects the various components of a robot, enabling the transmission of data and control signals between different hardware components. Through this "information superhighway," the robot's brain (controller) can send instructions to various actuators (such as motors and sensors) and also receive feedback information from these components, thereby achieving precise control and stable operation of the robot.
As shown in the figure, complex robotic systems require support for multiple communication interfaces, and different interfaces may be used in combination. The figure above illustrates a distributed robotic system with multiple communication interface paths, each with different specifications.
The function of bus
Data transmission: Ensuring that the various components of the robot can exchange data quickly and accurately. For example, environmental information collected by sensors (such as distance, temperature, and light) needs to be transmitted to the controller in a timely manner via the internal bus so that the robot can make corresponding decisions.
Synchronization and Coordination: Coordinating the working rhythm of different components. When a robot performs complex tasks, the motors of multiple joints need to work together. The internal bus can ensure that each motor operates in a predetermined order and speed, achieving smoothness and precision in the robot's movements.
Control signal transmission: The control commands issued by the controller are transmitted to each actuator, enabling the robot to complete various actions according to the preset program, such as walking, grasping, and carrying.
As designers continuously explore new methods to achieve shorter cycle times and higher throughput to meet big data demands, enable wider bandwidth systems to operate at ultra-high efficiency, and minimize downtime, they also seek to minimize the impact of upgrades by reusing existing cabling infrastructure. Most systems also incorporate advanced features such as smarter diagnostics, higher safety specifications, and faster, better real-time characteristics for motor control.
Control system communication involves the transmission of information between the motion controller and other components. For example, sensor data needs to be sent to the controller, and controller commands need to be sent to the actuators. Currently, the main control buses include serial buses RS232/RS485, CAN, EtherCAT, PROFINET, POWERLINK, PROFIBUS, SERCOS, Ethernet, Modbus, CC-Link, MECHATROLINK, etc. Considering factors such as bandwidth, stability, and cost-effectiveness of legged robot control systems, legged robots currently primarily use three types of buses: serial buses RS232/RS485, CAN, and EtherCAT.
Bus, Protocol and Interface
To understand buses, you first need to understand these three different concepts: bus, protocol, and interface.
Bus: Its main function is to provide a physical channel for data transmission, enabling electrical connections and signal transmission between devices. It determines the physical characteristics of data transmission, such as bandwidth, speed, and direction.
Protocols focus on standardizing the logical rules for data transmission, ensuring accurate and reliable data exchange between devices. Protocols define data formats, transmission processes, and error handling methods.
An interface combines physical and logical characteristics. Physically, an interface manifests as various plugs and sockets; logically, it defines the rules governing data transmission formats and electrical characteristics.
The bus provides a data transmission channel, the protocol specifies the data transmission process, and the interface performs signal conversion and adaptation.
Common types of robot internal buses
CAN bus, or Controller Area Network, boasts advantages such as high reliability, strong anti-interference capabilities, and low cost. Developed by Bosch in the early 1980s for automotive electronic control systems, the CAN bus addressed the growing demand for electronic devices in automobiles at the time. Traditional wiring methods were insufficient to meet these needs, leading to the development of the CAN bus. Its multi-master communication, high reliability, and real-time performance effectively solved this problem, initially achieving success in Mercedes-Benz vehicles. Over time, in 1991, Bosch released the CAN 2.0 protocol specification, dividing it into CAN 2.0A (standard frames) and CAN 2.0B (extended frames), further enhancing data transmission capabilities and flexibility. The International Organization for Standardization (ISO) also adopted it as the international standard ISO 11898 in 1993. Subsequently, the CAN FD (CAN with Flexible Data Rate) protocol emerged, significantly increasing data transmission rates and meeting the growing demand for large data volume transmissions.
Beyond the automotive industry, the CAN bus also performs exceptionally well in industrial applications. In industrial automation, it connects programmable logic controllers (PLCs), sensors, and actuators, enabling real-time data communication and collaborative operation between devices, such as robot control and material handling systems in automated production lines. In power systems, the CAN bus is used to monitor and control electrical equipment, such as monitoring the switch status of substations and acquiring power parameters, ensuring the stable operation of the power system. In elevator control systems, the CAN bus connects various elevator components, such as the car controller, door controller, and floor sensors, achieving efficient and reliable operation control and fault diagnosis. Furthermore, in smart building applications, the CAN bus can connect lighting systems, air conditioning systems, and security systems, enabling intelligent centralized management and control. In short, the CAN bus is ideally suited for connecting motor drivers and sensors at various joints, ensuring the stable operation of robots.
EtherCAT bus: Ethernet for Control Automation Technology is a fieldbus system based on Ethernet. First proposed by Beckhoff Automation GmbH in 2003, EtherCAT is a fieldbus technology based on Ethernet. At its inception, the industry urgently needed a high-speed, efficient, and low-cost communication solution, and EtherCAT emerged to meet this need. Its ability to overcome the limitations of traditional Ethernet in industrial automation applications quickly gained attention. Its most significant feature is its extremely high data transmission speed, achieving nanosecond-level synchronization accuracy. EtherCAT uses only three layers of protocol: physical layer, data link layer, and application layer, similar to most traditional fieldbuses. However, compared to other real-time Ethernet protocols such as PROFINET and EtherNet/IP, its protocol stack is more streamlined. Therefore, it possesses ultra-high-speed data transmission capabilities, enabling the exchange of large amounts of data in a very short time, meeting the real-time control requirements of robots, allowing robots to respond quickly to commands and achieve high-precision motion control. Its distributed clock technology ensures precise clock synchronization of all devices in the network, ensuring coordinated movements of robot joints and avoiding motion errors caused by time deviations.
In robotic applications with extremely high real-time requirements, such as collaborative robots and high-speed sorting robots, the EtherCAT bus enables robots to respond quickly to external signals, achieving more flexible and precise operations. A prime example is the "Rain Dance" system at Singapore Changi Airport. Based on its high performance, high real-time capability, and flexible topology, EtherCAT technology perfectly solves the challenges of real-time synchronization from 1216 servo axes.
Serial bus: There are many types of serial buses, among which RS-485 bus is perhaps the most widely used in robotics. It uses differential transmission, has strong anti-interference capabilities, long transmission distances, and relatively low cost. It is suitable for simple robot systems where communication speed requirements are not high, but cost and reliability are important, such as small educational robots or simple service robots. RS-485 itself does not have a strictly defined communication protocol; in practical applications, it needs to be used with specific communication protocols to achieve orderly data transmission and coordinated operation between devices. A common RS-485 bus-based communication protocol is the Modbus RTU protocol, which defines the data frame format, communication rules, function codes, etc., enabling interoperability between devices from different manufacturers on the RS-485 bus. In addition, users can also customize communication protocols according to their needs.
Summarize
With the development of robots, more specialized protocols for robots may emerge, or certain standard buses from other industries may be modified and integrated into the robot system, such as the historical evolution of CAN or EtherCAT.
In addition, given the complexity of cables, SPE (Single Pair Ethernet) may make significant progress in the robotics field, replacing existing dual-pair Ethernet, thereby saving wire harnesses, simplifying cabling, and enhancing reliability.
