From a transmission perspective, multi-jointed industrial robots can also be described as 4- to 6-joint industrial arms. As an extension of the human arm, they are widely used in tasks such as material handling, grinding, welding, and stamping. Multi-joint industrial robots can work independently or in coordinated groups. They can function as the main body of production machinery to complete tasks alone, or they can work in conjunction with machine tools, plastics machinery, automated production lines, and other machinery to perform loading and unloading operations. Initially, China lacked the manufacturing capacity for such equipment, and the limited production demand relied entirely on European and Japanese manufacturers or as part of imported production lines. With China's industrial upgrading and continuous advancement towards automation and reduced human labor, the domestic demand for multi-joint robotic arms has increased significantly. Consequently, research and development and manufacturing enterprises specializing in multi-joint industrial robotic arms have sprung up across China.
For robotics manufacturers, a comprehensive approach is needed, considering everything from the mechanical body and transmission mechanisms to the reduction gears, energy storage mechanisms, servo motors, servo drives, and control computer systems, to design and manufacture a suitable product. As an industry, it's difficult for a single company to handle all the critical components required for a robotic arm, and even a single specialist cannot master all the key technologies due to the highly specialized division of labor. My company and I specialize in the research, design, and manufacturing of servo motors. Since 2009, our company has been researching the motion characteristics of robotic arms, paying particular attention to the characteristics of the servo motors required. After eight years, we developed the GK series servo motors suitable for such equipment. The newly launched R-GK9 series permanent magnet servo motors, based on the R-GK6 and R-GK8, are specifically developed for 6-joint robotic arm applications. Compared to previous series, the new generation uses new materials and the latest magnetic field design technology, resulting in higher power density and a more compact size.
The following is a brief discussion of my views and experiences regarding robot motors, focusing solely on their application in 6-joint robots. Due to my limited expertise, errors and omissions are inevitable; I hope readers will forgive any shortcomings and consider this merely a starting point to spark further discussion on this topic.
Robotic arm mechanism structure
Multi-joint robotic arms have up to 7 degrees of freedom. The seventh axis is usually added during secondary development of applications, and the manufacturing of the robotic arm body often involves 4 or 6 axes. Its structure is shown in the figure below.
As shown in the diagram, the inertia characteristics of the robotic arm motors are as follows: axes J1 and J2 bear the weight of the entire mechanism, which randomly opens and closes during arm operation. From the application perspective of motors J1 and J2, the mechanical inertia of the load changes in real time. Based on past experience, the range of load inertia variation is within 7 times. The mechanical inertia of loads J3, J4, and J5 also varies, but the range of variation is not as large as that of J1 and J2. Therefore, from a control perspective, the motor requirement is that the inertia must be large enough for the system to easily enter the stable region. Thus, high-inertia motors should be selected for axes 1 and 2, while medium-inertia motors should be selected for axes 3, 4, and 5. The motor for axis 6 is generally installed inside the robotic arm, prioritizing small size and low-heat power density characteristics. The above outlines the inertia requirements for motor selection for different axes of a 6-joint robotic arm.
Working characteristics and requirements of articulated arm motors
Multi-joint robotic arms operate with multiple motors working in tandem along multiple axes, generally operating on an S3 intermittent duty cycle. From the perspective of completing a single action, it exhibits constant torque and high acceleration/deceleration characteristics. To reduce the size of the working mechanism, a small-volume, high-ratio RV gearbox is often used to connect the transmission mechanism. Therefore, for high-speed responsive robots, high-speed, high-response, and small-volume motors are required. To accommodate the high inertia, the motors on axes 1 to 3 are inevitably relatively short and stubby in appearance.
The demand for safety mechanical brakes
Most 6-joint robotic arms require mechanical brakes for safety. Due to the influence of gravity and inertia, the safety factor of the brake torque must be larger than that required for general mechanical applications, ranging from 1.5 to 2.0 depending on the operating environment. In short, it is crucial to prevent slippage under any circumstances. Furthermore, to ensure the overall safety of the robotic arm, the brake signal is provided directly from the control system, rather than from the servo driver. The specific coordination of control signals and brake protection will not be discussed further here.
Servo motor encoder and reliable repeatability
Due to the spatial motion characteristics of multi-axis linkages, multi-turn absolute position encoders are the best choice for articulated motors. Currently, two types are popular: multi-turn position memory encoders with gear mechanisms and battery-powered encoders that memorize multi-turn positions via circuitry. From a long-term maintenance perspective, the first type of encoder is better, but considering the initial investment cost, the second type is more suitable.
Tables 1 and 2 summarize the selection of typical robot servo motors and list commonly used Dengqi GK9 servo motors. These products are widely used in robotic arms from well-known domestic manufacturers and some foreign robot manufacturers. Specific rotational speeds and moments of inertia are adjusted based on different mechanical designs. In short, a robotic arm is a highly mechatronic integrated mechanism, requiring close communication between mechanical engineers and servo motor suppliers to develop the most suitable product.
