Humanoid robot rotary actuator，the role of precision bearings

Humanoid robot rotary actuator: the role of precision bearings

If a humanoid robot is compared to a dynamic dancer, then the rotary actuator is the "engine" in its joints, driving every agile movement. As the core component of humanoid robots, rotary actuators are mainly composed of motors, reducers, sensors, and controllers. Their working principle is to convert electrical energy into mechanical energy, output power through the motor, adjust torque and speed through the reducer, and monitor the motion status in real time through sensors. Finally, the controller accurately controls the direction, speed, and position of the motion according to a preset program.

Essentially, rotary actuators provide crucial degrees of freedom for robots. Taking the human arm as an example, our shoulders, elbows, and wrists have multiple degrees of freedom, allowing us to perform complex movements such as waving, grasping, and writing. The same goes for humanoid robots, whose number and distribution of rotating actuators determine their degree of freedom and range of motion. Generally speaking, the more complex the tasks that robots need to complete, the more rotary actuators are required, and the higher the degrees of freedom. For example, Tesla's Optimus humanoid robot is equipped with 14 rotating actuators throughout its body, which are distributed in various joint parts of the robot, enabling it to perform various humanoid actions such as walking, running, bending over, and lifting hands.

In the process of implementing complex actions in humanoid robots, rotary actuators play an irreplaceable key role. In industrial production, robots need to accurately grasp and place various components, which requires rotary actuators to have high-precision positioning capabilities and stable torque output to ensure the accuracy and reliability of operations. In the service field, robots may need to interact naturally with humans, such as dancing, guiding, etc., which requires rotating actuators to quickly respond to instructions, achieve smooth and natural movements, and enhance user experience. It can be said that the performance of rotary actuators directly affects the work efficiency, accuracy, and flexibility of humanoid robots, and is a key factor determining whether humanoid robots can operate efficiently in various scenarios.

In the precise and complex system of rotary actuators, bearings play a crucial role as "invisible pillars" and play a decisive role in the performance of rotary actuators. From the most basic function perspective, the primary task of bearings is to support rotating components and ensure the stability of motor shafts, gearbox input and output shafts, etc. during rotation. Taking a common motor as an example, when the motor is running at high speed, the rotor will generate significant centrifugal force and vibration. Without high-quality bearing support, the rotor will shake, leading to unstable motor operation and even potential damage to the motor. And the bearing is like a sturdy "base", firmly fixing the rotor to maintain precise axial position during high-speed rotation, thereby ensuring the normal operation of the motor.

In terms of reducing friction, the role of bearings is irreplaceable. When the components of the rotary actuator move relative to each other, friction can cause energy loss, heat generation, and wear, seriously affecting the efficiency and lifespan of the equipment. The bearing greatly reduces the friction coefficient through its special structural design, such as the rolling friction between the rolling elements (balls, rollers, etc.) and the raceway. Compared with direct sliding friction, the energy loss of rolling friction can be reduced several times or even tens of times. This not only improves the energy conversion efficiency of the rotary actuator, reduces energy consumption, but also reduces the degree of component wear and extends the service life of the equipment.

Bearings have a crucial impact on the motion accuracy and stability of rotary actuators. When performing high-precision operations on humanoid robots, such as grasping small objects and performing fine assembly, the motion accuracy of the rotary actuator directly determines the success or failure of the operation. High precision bearings can control the jumping and deviation of rotating components within a very small range, ensuring precise and error free motion output by the actuator. At the same time, the stiffness and load-bearing capacity of the bearings ensure that the actuator can still operate stably under various external forces and moments. If the accuracy or stiffness of the bearings is insufficient, the robot will experience shaking, deviation, and other problems during motion, making it unable to complete precise tasks.

1， High Load Capacity: Supporting the Robot's' Force Responsibility

In daily operation, humanoid robots need to complete various complex actions, which puts extremely high demands on the load capacity of bearings. Taking the handling of heavy objects as an example, during the process of extending and lifting objects, the bearings in the rotating actuators at the joints of the robot's arm not only bear the weight of the arm itself, but also the weight of the heavy object and the inertial force generated during the movement. If the load capacity of the bearing is insufficient, it may cause deformation and damage to the bearing, thereby preventing the robot from working properly.

Bearings with high load capacity can ensure stable operation of robots in various movements. In terms of design, these types of bearings usually use special materials and structures, such as increasing the number and size of rolling elements, optimizing the shape and contact angle of the raceway, etc., to improve their load-bearing capacity. In some large industrial humanoid robots, heavy-duty cross roller bearings or tapered roller bearings are used, which can withstand large radial, axial, and overturning moment loads, providing solid support for the stable operation of the robot. For example, in the automotive manufacturing workshop, humanoid robots responsible for handling large automotive parts require strong load-bearing capacity at their joint bearings to cope with heavy parts and frequent handling movements, ensuring the reliability and stability of the robot under high-intensity work.

2，High precision and high repeatability: ensuring the "precise dance steps" of robots

When performing tasks, humanoid robots often need to complete some delicate operations, such as assembling electronic components, surgical assistance, etc. These tasks require extremely high precision and repeatability of robot motion, and high-precision and high repeatability bearings are the key to achieving this goal.

Taking electronic component assembly as an example, robots need to accurately place tiny electronic components in designated positions on circuit boards, which requires the motion accuracy of rotary actuators to reach the micrometer or even nanometer level. High precision bearings can control the jumping and deviation of rotating components within a very small range, ensuring precise and error free motion output by the actuator. At the same time, high repeatability ensures that the robot can achieve the same accuracy standard every time it performs the same action, avoiding operational errors caused by error accumulation. In the medical surgical assistance scenario, when robots assist doctors in minimally invasive surgery, the high precision and high repeatability of bearings can ensure the precision of robot operation, reduce damage to patient tissues, and improve the success rate of surgery.

High precision and high repeatability bearings not only enhance the robot's ability to complete fine tasks, but also significantly improve the quality of the robot's work. In industrial production, high-precision robot operation can reduce the defect rate of products, improve production efficiency and product quality; In the field of services, high-precision robot movements can provide a better service experience, enhance the friendliness and reliability of human-computer interaction.

3，Low friction and low torque: endowing robots with a "dynamic posture"

Low friction and low torque bearings are like endowing humanoid robots with agile "dance moves", enabling them to move more efficiently and agilely. When the friction and torque of the bearings are low, the resistance that the robot needs to overcome during motion will decrease, greatly reducing energy loss.

In terms of energy utilization, low friction and low torque bearings can improve the motion efficiency of robots. Taking a continuously working humanoid robot as an example, adopting low friction bearings may reduce its energy consumption by 20% -30% when completing the same task, which not only reduces operating costs but also extends the robot's endurance time. In terms of response speed, low torque bearings enable robots to start, stop, and change direction of motion more quickly. When robots perform dance, their fast response speed allows them to complete various complex combinations of movements more smoothly, showcasing beautiful dance moves; In industrial production, fast response speed can improve production efficiency and meet the demand of production lines for fast operations.

In order to achieve low friction and low torque, bearing manufacturers typically use advanced materials and lubrication technologies. Using special ceramic materials or high-performance synthetic materials as rolling elements and raceways, which have low friction coefficients; Meanwhile, by using high-quality lubricating grease or oil, a uniform lubricating film can be formed between the rolling elements and the raceway, further reducing friction and wear.

4，High reliability and long lifespan: loyal companions accompanying robots

The working environment of humanoid robots is often complex and varied, and may face harsh conditions such as high temperature, high humidity, dust, and vibration. In such an environment, bearings must have high reliability to ensure the stable operation of robots. In high temperature environments, the material of the bearing may undergo thermal expansion, resulting in changes in clearance and decreased lubrication performance. If the reliability of the bearing is insufficient, it is prone to failure; In environments with high levels of dust, dust particles may enter the interior of bearings, exacerbating wear and affecting their performance and lifespan.

Long life bearings are of great significance in reducing the maintenance costs of robots. Frequent replacement of bearings not only increases maintenance time and costs, but also affects the normal use of robots. Taking an industrial humanoid robot as an example, if it uses bearings with a short lifespan, it may need to replace the bearings 2-3 times a year. The cost of each replacement includes the cost of the bearings themselves, the labor costs of maintenance personnel, and production losses caused by downtime. The cumulative cost is quite considerable. Long life bearings can extend the replacement cycle to several years or even longer, greatly reducing maintenance costs and downtime, and improving the efficiency and economic benefits of robots.

In order to improve the reliability and lifespan of bearings, manufacturers usually put effort into material selection, structural design, and manufacturing processes. Using high-strength and corrosion-resistant materials, optimizing the sealing structure of bearings to prevent pollutants from entering; Through advanced manufacturing processes, the accuracy and surface quality of bearings are improved, reducing stress concentration and wear.

1，Cross roller bearings: the "best partner" for rotary actuators

The rollers of the cross roller bearing are arranged in a 90 ° perpendicular cross arrangement, with spacers or isolation blocks installed between the rollers to prevent tilting or friction, effectively reducing the increase in rotational torque. The inner and outer rings adopt a segmented structure, and the gap can be adjusted. If preloading is applied, high-precision rotational motion can be achieved. In the rotating actuators of humanoid robots, cross roller bearings are mainly used to withstand large axial, radial, and torque loads. For example, in the shoulder joints of robots, complex multi-directional movements are required. Cross roller bearings can stably support the motor shaft and reducer output shaft, ensuring that the joints can still rotate accurately and smoothly under the weight of the arm and external loads, achieving actions such as waving and grabbing heavy objects. Its high rigidity and compact design not only greatly saves installation space, but also improves the motion accuracy and stability of robot joints, making it an indispensable key component of rotary actuators.

2，Flexible ball bearings: intimate comrades in harmonic drive

Flexible ball bearing is a special type of thin-walled ball bearing, mainly used for wave generators in harmonic drive mechanisms. Its cross-section is very thin, and when assembled with a camshaft, it becomes elliptical to achieve the function required for harmonic transmission. In harmonic drive, the wave generator causes elastic deformation of the flexible wheel and interacts with the rigid wheel to achieve motion transmission. Flexible ball bearings play a crucial role in supporting and transmitting, capable of withstanding radial and limited axial loads, ensuring the stable operation of the wave generator. In the forearm joints of humanoid robots, harmonic drive is often used to achieve high-precision motion control. Flexible ball bearings are like "close comrades", helping the harmonic drive mechanism efficiently convert the motor's rotational motion into precise joint motion, enabling the robot to perform fine actions such as threading needles and writing.

3， Four point contact bearings: multifunctional "practical experts"

The inner and outer raceway of a four point contact bearing has a peach shaped cross-section. When there is no load or pure radial load, the steel ball and the ring form a four point contact, while under pure axial load, the steel ball and ring form a two-point contact. This allows it to withstand bidirectional axial loads and torque loads, and has the functions of both single row angular contact ball bearings and double row angular contact ball bearings. In rotary actuators, four point contact bearings are commonly used as input bearings to handle radial and axial load combinations in two directions. In the waist joints of robots, both body weight and torsional force need to be borne simultaneously. Four point contact bearings, with their multifunctional characteristics, can stably support rotating parts, ensuring that the waist joints can still rotate flexibly and accurately under complex loads, achieving robot turning, bending and other actions. It is a multifunctional, compact and economically efficient solution.

4，Angular contact ball bearings: experts in handling bidirectional loads

Angular contact ball bearings consist of an inner ring, an outer ring, rolling elements (steel balls), and a cage. Their working principle is that when the inner or outer ring is subjected to rotational torque, the rolling elements roll between the inner and outer raceway, achieving force transmission and motion conversion. Single row angular contact ball bearings can only withstand axial loads in one direction, which can cause additional axial forces when subjected to radial loads. Therefore, they are usually used in pairs to achieve bidirectional axial load capacity. As the contact angle increases, its load capacity also increases. In the neck joints of humanoid robots, it is necessary to achieve flexible rotation in multiple directions while bearing the weight of the head and various forces generated during movement. Paired use of angular contact ball bearings can effectively cope with these bidirectional loads, ensuring stable and precise motion of the neck joint, enabling the robot to achieve movements such as turning, lifting, and lowering, providing more flexible and natural motion performance for the robot.

5，Fully loaded cylindrical roller bearings: the 'stability guard' under heavy loads

Fully loaded cylindrical roller bearings adopt a very compact design and have strong load-bearing capacity. It has a large number of rollers and no cage, which greatly improves the radial bearing capacity of the bearing. In the leg joints of humanoid robots, they need to bear the weight of the entire body as well as the enormous impact and vibration generated during movements such as walking and running. Fully loaded roller bearings can stably support the rotating actuators of the legs under these heavy load conditions, reduce vibration, and improve control force and rigidity. Taking the Atlas robot as an example, its leg joints bear tremendous pressure and impact when walking on complex terrain. The application of RNN bearings enables the leg actuators to operate stably, ensuring the stability and flexibility of the robot's walking, making it an ideal choice for heavy-duty application scenarios.

With the continuous expansion of application scenarios and the increasing complexity of functional requirements for humanoid robots, the demand for bearings in rotary actuators has also brought a series of challenges. At the technical level, how to further improve the comprehensive performance of bearings, such as maintaining high precision and low friction under high loads, while meeting the design requirements of miniaturization and lightweight, is the key issue currently faced. The complexity of manufacturing processes and cost control are also major challenges. High precision and high-performance bearings often require advanced manufacturing processes and precision machining equipment, which not only increases production costs but also limits capacity improvement. Challenges and opportunities coexist, and the future development direction of bearing technology also provides solutions to these problems. In terms of material innovation, the development of new high-performance materials such as ceramic materials, high-strength alloys, etc. is expected to further improve the performance of bearings. Ceramic bearings have the advantages of low density, high hardness, high temperature resistance, and corrosion resistance, which can significantly improve the operational performance of bearings under extreme conditions. They have broad application prospects in the field of humanoid robots in the future.

In terms of design innovation, by optimizing the structural design of bearings, such as adopting new raceway shapes and rolling element arrangements, the load capacity, accuracy, and lifespan of bearings can be improved. Some new bearing designs can also achieve functions such as self-lubricating and adaptive adjustment, further improving the performance and reliability of robots.

With the development of technologies such as the Internet of Things and artificial intelligence, intelligent bearings will also become a future trend. Intelligent bearings can monitor their own operating status in real time, such as temperature, vibration, wear, etc., and predict and diagnose faults through data analysis, achieving preventive maintenance. This will greatly improve the operational stability and maintenance efficiency of humanoid robots.

In the long run, the advancement of bearing technology will bring tremendous impetus to the development of humanoid robots. More advanced bearings will enable robots to achieve more complex, precise, and efficient movements, expanding their applications in medical, rescue, aerospace, and other fields. In medical surgery, high-precision robot operation can improve the success rate of surgery and the rehabilitation effect of patients; In disaster relief, robots are able to perform complex tasks in hazardous environments, reducing casualties.

Bearings, seemingly insignificant small components, play an indispensable and crucial role in the rotating actuators of humanoid robots. From supporting rotating components to reducing friction, from ensuring motion accuracy to bearing various loads, the performance of bearings directly determines the efficiency, accuracy, and reliability of rotary actuators, which in turn affects the application performance of humanoid robots in various fields. Although current bearing technology still faces many challenges in meeting the needs of humanoid robots, these challenges also provide broad space for technological innovation. With the continuous advancement of materials science, design concepts, and manufacturing processes, we have reason to expect that future bearings will inject strong impetus into the development of humanoid robots with higher performance and lower costs. At that time, humanoid robots will be able to play a greater role in more fields, bringing more convenience and surprises to human life and work. Let us look forward to a bright future where bearing technology and the humanoid robot industry mutually promote and develop together.