Tendon-driven solution for dexterous hands: The key path and ideal choice for achieving high dexterity and humanoid manipulation
This article aims to deeply analyze the core reasons why the tendon-driven solution has become the mainstream and shown excellent potential in the field of modern humanoid robot dexterous hands. First, the article defines the strategic position of the dexterous hand in the humanoid robot system and the technical challenges it faces, and establishes an evaluation system with the "human hand" as the ultimate benchmark. Subsequently, it will focus on the core advantages of the tendon-driven solution compared with traditional transmission methods, including its breakthroughs in dynamic optimization, spatial layout, bionics, and the application of new materials. By analyzing industry benchmark cases such as Tesla's Optimus and combining the practices of Jiuzhangzhu Company in the fields of intelligent prosthetics and industrial dexterous hands, this article will verify the engineering feasibility and market prospects of the tendon-driven solution. Finally, this article will extend its perspective to the disabled user market, explore how the technology can evolve from an auxiliary tool to a partner empowering life, and tap into its deep humanistic value and market opportunities.
I. Technical Requirements and Ultimate Challenges of the Dexterous Hand
On the journey of humanoid robots towards general intelligence, the dexterous hand plays a crucial role. It is not only the main medium for the robot to interact with the physical world but also the core component for it to perform delicate operations and complete complex tasks.
The human hand's motion system consists of 22 degrees of freedom (excluding the internal and external rotation of the forearm). Each of the five fingers has 4 degrees of freedom (20 in total), and the wrist has 2 degrees of freedom. These degrees of freedom are distributed in parts such as the metacarpophalangeal joint (MCP), proximal interphalangeal joint (PIP), and distal interphalangeal joint (DIP). The dexterous hand must simulate this complex structure to achieve the delicate operation ability of the human hand, such as 13 basic functions including hanging, holding, touching, pushing, hitting, dynamic operation, spherical grasping, spherical fingertip grasping, cylindrical grasping, hooking, two-fingertip pinching, multi-fingertip pinching, and lateral pinching.
To fulfill this role and perform these actions, the design of the dexterous hand must meet a series of demanding technical requirements:
High degrees of freedom: Simulate the multi-joint and multi-finger coordinated motion ability of the human hand to adapt to the grasping and operation of objects of different shapes, sizes, and materials. Use new materials to simulate the soft palm surface, increase the contact area during the grasping action to enhance the stability of grasping, and further reduce energy consumption to increase the energy efficiency ratio.
Lightweight: As the end of the robot's limb, the weight and inertia of the hand directly affect the energy consumption, speed, and stability of the overall motion.
Fast response: Have a millisecond-level response speed to achieve dynamic grasping, rapid reaction, and smooth human-machine interaction. Ensure the repeatability and consistency of actions through motion calculation and algorithm compensation.
However, the only ultimate competitor to measure the success of the dexterous hand is the human hand, which has been perfected through millions of years of evolution. Therefore, we must establish a multi-dimensional evaluation system whose criteria are not limited to engineering parameters but should also include a comprehensive consideration of "human-like" characteristics:
Function and performance: Not only look at indicators such as strength, speed, and accuracy but also see if it can perform a series of complex "in-hand manipulations" such as unscrewing bottle caps, using tools, and tying shoelaces.
Size and bionics: The size should be comparable to that of the human hand to work seamlessly in an environment designed for humans. Its motion form and grasping posture should have biological naturalness.
Impact on the body: How the design scheme of the dexterous hand affects the overall dynamic characteristics, energy consumption, and control complexity of the humanoid robot is the key to evaluating its advantages and disadvantages.
Functional performance in structure: The human hand integrates three major functions of motion, support, and sensation. Therefore, the design of the dexterous hand should not only consider the drive but also integrate a rich sensor array (force, position, temperature, etc.) to achieve closed-loop perception and control.
In the current discussion, a common view is that the performance of the humanoid dexterous hand in specific situations is not as good as that of professional three-jaw or parallel grippers. This view confuses the fundamental difference between general-purpose equipment and special-purpose equipment. Just as the human hand is not as efficient as a torque wrench when tightening high-torque bolts, this does not undermine the versatility and value of the human hand. In the example, the three-jaw chuck, as a special-purpose tool on the industrial automation assembly line, focuses on the ultimate efficiency of a single task; while the dexterous hand is a general-purpose tool, aiming to achieve adaptability and versatility in unstructured environments. This is essentially a matter of "each has its own advantages," rather than a debate about the correctness of the dexterous hand's technical direction. The application goal and scenario of the dexterous hand are to achieve general functionality in the human-machine environment on the basis of ensuring sufficient performance and cost.
II. Core Advantages of the Tendon-Driven Solution: The Perfect Combination of Engineering Feasibility and Bionics
In the exploration of the "human-like" dexterous hand, the choice of the transmission scheme is the key starting point for determining the overall performance of the system. Traditional gear transmissions, linkages, or point-push rod schemes have mature applications in scenarios such as industrial robotic arms. However, in the design of high-degree-of-freedom and miniaturized dexterous hands, they are indeed restricted by volume, weight, and structural complexity. Meanwhile, the tendon-driven solution, with its lightweight and flexible layout, provides new possibilities for the realization of the dexterous hand. It is worth noting that different transmission methods are not mutually exclusive. In the torso and large joints of the humanoid robot, traditional transmissions still have important value; while in the "hand" with extremely high requirements for finger flexibility and integration, the tendon-driven solution shows relatively unique adaptability.
1. Remote Drive Layout and Dynamic Optimization
One of the greatest advantages of the tendon-driven solution is that it supports long-distance indirect transmission. This means that the drive units (actuators) such as motors and reducers, which are sensitive to space requirements, can be moved out of the smaller execution space and installed at a relatively spacious remote location. This "remote drive" layout provides convenience in terms of dynamics:
Figure: Schematic diagram of the tendon-driven transmission solution
Concentrating the mass near the center of motion of the robot can greatly reduce the inertia caused by the end of the arm during motion. This makes the arm move faster when swinging, reduces the overall energy consumption of the body, and improves the dynamic response performance. Research shows that the finger flexion and extension speed of an optimized tendon-driven hand can reach the level of 30 milliseconds, and the motor response delay can be as low as 18 milliseconds. Although the flexibility of the tendon may pose challenges in control, it has great speed potential under low loads.
2. Friendly Spatial Layout, Empowering High-Degree-of-Freedom Design
Integrating more than 20 degrees of freedom in a limited space the size of a human hand. The tendon-driven solution greatly simplifies the mechanical structure at the end of the finger by replacing rigid gears and linkages with flexible "tendons," releasing the internal space of the finger. One or more tendons can pass through multiple joints to achieve coupled motion or independent control, making it possible to realize complex multi-joint linkages in a high-density space and paving the way for the design of a humanoid dexterous hand with more than 20 degrees of freedom.
3. Simulating Human Tendons, Achieving Flexible and Natural Movements
The tendon-driven solution is highly similar in structure to the human "skin-muscle-bone" system. On the one hand, it realizes the transmission of flexible movements through tendon control, which is the fundamental characteristic that distinguishes it from traditional rigid transmissions. This bionic design not only looks more natural in appearance but also brings "compliance" in function. On the other hand, when the dexterous hand comes into contact with an object, the filling material between the motion mechanism and the epidermis can absorb a part of the impact energy due to its elasticity, avoiding damage to the motor or the object caused by rigid collisions. This inherent compliance is crucial for safe interaction with humans or fragile objects in an unstructured environment.
4. Breakthrough in Key Materials: Empowerment of Ultra-High Molecular Weight Polyethylene Fiber (UHMWPE)
The long-term reliability of the tendon-driven solution was once its main shortcoming, but the emergence of new materials has completely changed this situation. High-performance fibers represented by ultra-high molecular weight polyethylene (UHMWPE) fibers (such as Dyneema® or Spectra®) provide a solid foundation for the engineering application of the tendon-driven solution.
Figure: Advantages of ultra-high molecular weight polyethylene fiber materials
Ultra-high strength and light weight: The specific strength of UHMWPE fiber is more than 15 times that of high-quality steel, and its density is only 0.97 g/cm³, reducing the total weight of the dexterous hand by 30%-40% and the moment of inertia by 45%, thereby improving the grasping speed and flexibility and making it an ideal material for achieving lightweight and high-load transmission.
Excellent wear resistance and fatigue resistance: This material has excellent wear resistance, corrosion resistance, and bending fatigue resistance. After 2 million cycles of tensile testing, the strength retention rate is >95%, which is significantly better than that of traditional polyester fibers, whose performance decays by 30% after 1 million cycles. It ensures that high strength can be maintained after millions of flexion and extension cycles, meeting the requirements of high-frequency use of the robot hand.
Coping with the creep problem: Although creep (irreversible elongation under continuous load) was once the main challenge faced by UHMWPE fibers and may affect the long-term accuracy of the system, its impact has been controlled within an acceptable range through advanced weaving processes, pre-tensioning technology, and compensation in the control algorithm. Solving the creep problem remains a key research direction for improving long-term reliability.
Overall, the tendon-driven solution can approach the size of a human hand. In terms of performance, although it may not be as good as heavy-duty gear-driven systems in terms of ultimate torque output, its comprehensive advantages in speed, flexibility, and lightweight make it more suitable for the application scenarios of general dexterous hands.
III. Industry Applications and Verification of Existing Cases
The superiority of theory ultimately needs to be tested by practice. The tendon-driven solution has proven its value in multiple cutting-edge fields and is gradually becoming the mainstream choice in the industry.
Benchmark Product Establishes the Technical Route: Tesla Optimus
Tesla uses a hybrid drive structure of "planetary gearbox + screw + tendon" in the dexterous hand of its second-generation humanoid robot, Optimus Gen 3. Among them, the motor and reduction mechanism are also integrated in the forearm. The screw converts the rotational motion into linear motion, and finally, the tendon transmits the power to each finger joint.
The external placement of the driver increases the degrees of freedom. Replacing the worm with a screw improves the accuracy and load capacity, and replacing the torsion spring with a tendon enhances the flexibility. This design gives the dexterous hand 22 degrees of freedom, approaching the level of a human hand, and the response time reaches 0.2 seconds, which is at least 0.3 seconds faster than traditional solutions.
This design choice, made by one of the most eye-catching robot companies in the world, strongly proves the comprehensive advantages of the tendon-driven solution in balancing performance, cost, and engineering complexity, laying the foundation for the mainstream position of this technical route.
Verification of Reliability in Medical and Industrial Scenarios
In the medical scenario, the tendon-driven dexterous hand has been verified to be suitable for applications such as precision surgery and rehabilitation treatment due to its high flexibility, lightweight, and delicate manipulation ability. For example, the Da Vinci surgical robot uses a similar linear drive solution with tendons, which can achieve high-precision multi-degree-of-freedom operations and reduce the trauma to patients in minimally invasive surgeries. The Shadow Hand series launched by the UK's Shadow Company combines tendon-driven transmission and is suitable for micro-manipulation tasks in the fields of scientific research and medicine, such as cell manipulation and prosthetic control.
In the industrial scenario, the tendon-driven solution has shown excellent adaptability. For example, the Dex5-1 dexterous hand of Unitree Technology uses a gear transmission solution, but its H2 robot with 31 degrees of freedom may combine a hybrid transmission of tendons and gears to achieve more flexible operations. The 19DOF five-finger dexterous hand of Zhiyuan Robotics integrates 12 motors in the palm in three rows and uses tendon-driven transmission to achieve high degrees of freedom. It can grasp objects weighing 5 kg and self-lock to lift 30 kg, making it suitable for industrial-level heavy-load scenarios. The Lingxin Qiaoshou L30 has switched from the previous two generations of linkage transmission to tendon-driven transmission, increasing the degrees of freedom to 25. Zhiyuan Robotics uses a hybrid transmission method (a combination of linkages and tendons) to improve transmission efficiency and reduce the size.
IV. Practice of Jiuzhangzhu Company: From Intelligent Prosthetics to Inclusive Technology
Under the general trend of tendon-driven technology, innovative companies represented by Jiuzhangzhu Company are bringing this technology from the laboratory to a broader market, especially in the field of intelligent prosthetics closely related to people's livelihood.
1. Intelligent Prosthetic Hand Meeting Size, Performance Expectations, and Reliability
The intelligent prosthetic hand released by Jiuzhangzhu is built based on a deep understanding of the key indicators of the dexterous hand.
Size and weight: By adopting a highly integrated tendon-driven solution, the prosthetic hand is close to an adult hand in terms of size and outer contour constraints, making users hardly feel any foreign body sensation when wearing and using it and making it easier to be psychologically accepted.
Performance and function: This prosthetic hand can simulate the human hand to perform most daily grasping actions. Its lightweight design brings lower wearing energy consumption and faster response speed, allowing users to interact with life more naturally and intuitively.
Excellent tactile sensors: Sensors are distributed on the surface such as the fingertips to sense information such as the pressure and temperature of objects. At the same time, the application of flexible materials enables it to fit the irregular surface of the dexterous hand, improving durability and operational stability.
2. Inclusive Advantage Based on Economic Cost
Currently, high-performance myoelectric prosthetic hands are extremely expensive, often costing tens of thousands or even hundreds of thousands of yuan. The tendon-driven solution of Jiuzhangzhu shows great potential in terms of economic cost. The mechanical structure of the tendon-driven solution is more simplified. Combining the integrated micro gearbox and reducer of precision machining with additive manufacturing technologies such as 3D printing can significantly reduce the manufacturing cost of components.
This article is from the WeChat public account "IPO Assistant", author: Tang Yipao. It is published by 36Kr with authorization.