Comparative Analysis of Dexterous Hand Structural Solutions: Linkage, Cordless Braces, and Direct Drive Solutions

As an important end effector in robotics, the dexterous hand has broad application prospects in industrial automation, medical surgery, and service robots. The structural design of a dexterous hand directly determines its grasping ability, flexibility, reliability, and cost-effectiveness. Linkage, chord, and direct drive are the three most commonly used structural designs for dexterous hands, each with its unique advantages and limitations. This article will provide a comprehensive comparative analysis of these three designs from multiple perspectives. As the “hand” of a robot, the dexterous hand needs to meet the following core functional requirements:

Grasping ability: capable of stably grasping objects of different shapes, sizes, and weights.

Flexibility: Possesses sufficient degrees of freedom to achieve human-like grasping postures.

Adaptability: It can adapt to the shape and surface properties of different objects.

Precision: Achieving precise position and force control

Reliability: Maintains stable performance during long-term use

Cost-effectiveness: Reasonable manufacturing and maintenance costs

01 Basic Principles and Characteristics of Three Structural Schemes

1. Linkage structure scheme

A linkage structure is a mechanical structure formed by rigid links and joints. It is typically driven by a motor through gears, worm gears, or other transmission mechanisms, enabling the bending and extension of the fingers. The linkage structure is one of the most traditional and commonly used dexterity hand structures.

Working principle: The motor drive is converted into rotational or linear motion of the linkage through a reduction mechanism, and then the motion is transmitted to the finger joints through the linkage mechanism to realize the opening and closing action of the fingers. Each joint is usually driven by an independent motor, or underactuated control is achieved through a coupling mechanism.

Structural features :

• Rigid connecting rod and revolute joint connection
• The transmission path is direct, and the structure is relatively simple.
• It can achieve high gripping force and rigidity.
• Modular design facilitates maintenance and upgrades

2. Tendon and chord structure design

The chordal structure (also known as rope-driven or tendon-driven structure) mimics the transmission principle of biological tendons, transmitting the driving force of the motor to the finger joints through flexible ropes (chordal structures) to achieve finger movement.

Working principle: The motor winds up and down the tendon chords via pulleys or a winding mechanism, converting the linear motion of the tendon chords into rotational motion of the finger joints. A pre-tensioning mechanism is typically required to maintain the tension of the tendon chords and ensure reliable transmission. The tendon chord structure can be fully actuated or underactuated; underactuated designs usually achieve coordinated movement of multiple joints through a coupling mechanism.

Structural features :

• Flexible tendon ropes are used as the transmission medium.
• The motor can be externally mounted, reducing the weight on the fingers.
• Compact structure, allowing for a high degree of freedom
• It has a certain degree of flexibility and cushioning effect.
• Chordae need regular maintenance and tensioning.

3. Direct drive structure scheme

A direct-drive structure refers to a motor directly driving the finger joint, eliminating intermediate transmission mechanisms (such as gears, belts, tendons, etc.) and achieving a direct connection between the motor shaft and the joint. Direct-drive solutions typically require the use of high torque density motors, such as torque motors or custom-designed motors.

Working principle: The motor output shaft is directly connected to the finger joint, and the motor’s rotational motion is directly converted into joint rotation. Because there is no intermediate transmission link, control is more direct, and the response speed is faster. Direct drive solutions can be fully driven or underdriven designs, but fully driven designs are more common.

Structural features :

• The motor is directly connected to the joint, without any intermediate transmission mechanism.
• High transmission efficiency and fast response speed
• High control precision and good position repeatability
• Compact structure, reducing mechanical wear
• The motor needs to be built inside the finger, increasing the finger’s weight and size.
• High-performance requirements for motors result in higher costs.

02 Performance Comparison Analysis of Three Structural Schemes

1. Comparison of gripping force and stiffness

Grasping force is one of the core performance indicators of dexterous hands, directly affecting their ability to be used in industry and daily life.

• Linkage structure

• • Advantages: Due to the use of rigid links and direct drive, the linkage structure can achieve a high gripping force. By properly designing the link length and transmission ratio, a large output force can be generated at the fingertips.
  • Disadvantages: Due to the large number of components and kinematic pairs, cumulative errors and frictional losses will occur, affecting the precise control of the gripping force.
  • Typical data: The fingertip force of a linkage-type dexterous hand is usually between 20-50N.

• Chordae tendon structure

• • Advantages: The biomimetic structure resembles a human hand, resulting in smoother and more fluid movement; through the rational design of the pretension force and transmission path of the tendon rope, the tendon rope structure can achieve a moderate level of gripping force.
  • Disadvantages: Due to the flexibility and frictional wear of tendon chords, the grasping force control precision of tendon chord structures is not as good as that of linkage structures.
  • Typical data: The force of chord-driven dexterous fingertips is usually between 10-30N.

• Direct drive structure

• • Advantages: Since there is no intermediate transmission link, the direct drive structure has the highest transmission efficiency and can theoretically achieve the maximum gripping force.
  • Disadvantages: Due to limitations in motor output torque and heat dissipation capacity, the gripping force of a direct-drive structure may not be superior to that of a linkage structure in practical applications.
  • Typical data: The fingertip force of a direct-drive dexterous hand is usually between 30-60N.

Stiffness comparison :

• Linkage structure: highest, rigid linkages and direct drive provide the greatest structural stiffness.
• Direct drive structure: Medium. Direct motor drive provides high rigidity, but is limited by the rigidity of the motor shaft and bearings.
• Cone structure: Lowest, the flexibility of the tendons results in low overall stiffness.

2. Comparison of dexterity and degrees of freedom

Dexterity refers to the diversity of grasping postures and maneuvers that a dexterous hand can perform, and is usually related to the number of degrees of freedom and the range of motion of the joints.

• Linkage structure
  • Advantages: By designing different linkage mechanisms, various grasping modes and complex joint movements can be achieved.
  • Disadvantages: Due to limitations in link length and range of motion, the joint’s degrees of freedom and range of motion may be restricted.
  • Degrees of freedom: A single finger usually has 2-3 degrees of freedom, but 6 degrees of freedom are commonly found on the market, that is, 1 degree of freedom for each of the four fingers and 2 degrees of freedom for the thumb.
• Chordae tendon structure
  • Advantages: The flexibility and adaptability of chordae tendons allow joints to achieve more complex movement trajectories.
  • Disadvantages: The pretensioning and path planning of the tendons are complex and may affect the accuracy of the movement.
  • Degrees of freedom: usually refers to 3-4 degrees of freedom.
• Direct drive structure
  • Advantages: Since there is no intermediate transmission mechanism, the joint movement is more direct and precise, enabling more flexible movement.
  • Disadvantages: The motor is built into the finger, which increases the weight and size of the finger and may limit the range of motion of the joint.
  • Degrees of freedom: usually refers to 3-4 degrees of freedom.

Comparison of dexterity :

• Cone structure: The highest, flexible chords and flexible wiring provide the greatest freedom of movement.
• Direct drive structure: Medium. Direct drive provides precise motion control, but is limited by the size of the motor.
• Linkage structure: at its lowest, the rigidity and range of motion of the linkage limit dexterity.

3. Comparison of accuracy and control performance

Precision refers to the accuracy of positional and force control that a dexterous hand can achieve during grasping and manipulation.

• Linkage structure
  • Advantages: By rationally designing the length of the connecting rod and the transmission ratio, high positional accuracy can be achieved.
  • Disadvantages: Due to the large number of components and kinematic pairs, cumulative errors and frictional losses will occur, affecting control accuracy.
  • Control performance: Traditional PID control can achieve good position control, but the force control accuracy is relatively low.
• Chordae tendon structure
  • Advantages: By precisely controlling the length and tension of the tendon rope, a high degree of position and force control accuracy can be achieved.
  • Disadvantages: The flexibility and nonlinearity of the tendon ligaments increase the complexity of control.
  • Control performance: More complex control algorithms are required, such as model-based control or adaptive control, to compensate for the nonlinear characteristics of the tendon chord.
• Direct drive structure
  • Advantages: Due to the absence of intermediate transmission links, the direct drive structure has the highest control precision and the fastest response speed.
  • Disadvantages: High performance requirements for motors and controllers, resulting in higher costs.
  • Control performance: It can achieve high-precision position and force control, and is suitable for delicate operation tasks.

Accuracy comparison :

• Direct drive structure: highest, direct drive provides the smallest transmission error and the highest control precision.
• Linkage structure: Medium, high positional accuracy can be achieved through precise machining and assembly.
• Cone structure: Lowest, the flexibility and non-linearity of the tendon result in lower control precision.

4. Comparison of response speed and dynamic performance

Response speed refers to the speed at which a dexterous hand reacts to a control signal, usually measured in milliseconds.

• Linkage structure
  • Advantages: By rationally designing the mass distribution and transmission ratio of the connecting rod, a faster response speed can be achieved.
  • Disadvantages: Due to the large number of components and kinematic pairs, the system has greater inertia and friction, which affects dynamic performance.
  • Typical data: Response time is usually between 50-100ms.
• Chordae tendon structure
  • Advantages: Due to the flexibility of the tendon chords, they can achieve better dynamic response and impact cushioning.
  • Disadvantages: The elasticity and damping properties of the tendons may cause system response delays and oscillations.
  • Typical data: Response time is usually between 30-80ms.
• Direct drive structure
  • Advantages: Since there is no intermediate transmission link, the direct drive structure has the fastest response speed and the best dynamic performance.
  • Disadvantages: Due to the limitations of motor torque and inertia, dynamic performance under heavy loads may be limited.
  • Typical data: Response time is usually between 10-50ms, such as the response time of a certain direct-drive dexterity hand which can reach 20ms.

Dynamic performance comparison :

• Direct drive architecture: highest performance, direct drive provides the fastest response speed and the best dynamic performance.
• Wire rope structure: Moderate; its flexibility provides good dynamic response but may lead to oscillations.
• Linkage structure: lowest, inertia, and frictional losses affect dynamic performance.

5. Size and weight comparison

Size and weight refer to the physical dimensions and mass of a dexterous hand, which directly affect its installation and use on a robot.

• Linkage structure
  • Advantages: Compact structure, allowing for a high degree of freedom within a limited space.
  • Disadvantages: Linkages and kinematic pairs occupy space, which may increase the overall size.
  • Typical data: The weight of a linkage-type dexterous hand is usually between 2-3 kg.
• Chordae tendon structure
  • Advantages: External motor, lighter finger part, and overall size can be designed to be more compact.
  • Disadvantages: It requires additional chord tensioning mechanisms and cable routing space, increasing system complexity.
  • Typical data: The weight of a chord-driven dexterous hand is typically between 0.8-2 kg.
• Direct drive structure
  • Advantages: The absence of intermediate transmission links simplifies mechanical design and may reduce overall size.
  • Disadvantages: The motor is built into the finger, increasing the weight and size of the finger.
  • Typical data: The weight of a direct-drive dexterous hand is usually between 2-4 kg.

Size and weight comparison :

• The tendon ligament structure is the lightest, with an external motor and a lightweight finger section.
• Linkage structure: Medium, compact structure, but the links and kinematic pairs occupy a certain amount of space.
• Direct drive structure: Heaviest, with the motor built into the finger, increasing weight and size.

03 Application Scenarios Analysis of the Three Structural Schemes

• Industrial manufacturing and assembly scenarios

Industrial manufacturing and assembly are among the main application areas of dexterous hands, requiring them to have high grasping force, precision, and reliability.

• Linkage structure
  • Advantages: Simple structure, high reliability, suitable for long-term, high-load industrial applications.
  • Disadvantages: Limited dexterity and freedom, making it difficult to adapt to complex and ever-changing industrial environments.
  • Applicable scenarios: Suitable for repetitive grasping and assembly tasks in structured environments, such as automobile manufacturing and electronic assembly.
• Chordae tendon structure
  • Advantages: High dexterity, many degrees of freedom, and able to adapt to workpieces of different shapes and sizes.
  • Disadvantages: Lower reliability, high maintenance requirements, and unsuitable for use in harsh environments.
  • Applicable scenarios: Suitable for high-precision assembly and complex operations in flexible production lines and variable environments.
• Direct drive structure
  • Advantages: High precision, fast response, enabling precise operation and force control.
  • Disadvantages: High cost, complex maintenance, and unsuitable for use in dusty and oily environments.
  • Applicable scenarios: Suitable for precision assembly and testing tasks in high-precision and high-cleanliness environments.

• Medical surgery and rehabilitation scenarios

Medical surgery and rehabilitation are another important application area for dexterous hands, which require high precision, lightweight, and safety.

• Linkage structure
  • Advantages: Simple structure, high reliability, easy to disinfect and maintain.
  • Disadvantages: Limited dexterity and flexibility, making it difficult to adapt to complex surgical environments.
  • Applicable scenarios: Suitable for simple grasping and manipulation in minimally invasive surgery and rehabilitation training.
• Chordae tendon structure
  • Advantages: High dexterity, high degree of freedom, enabling precise surgical procedures.
  • Disadvantages: Tendon chords are easily affected by body fluids and chemicals, requiring special protection and maintenance.
  • Applicable scenarios: Suitable for delicate operations and tissue grasping in microsurgery and minimally invasive surgery.
• Direct drive structure
  • Advantages: High precision, fast response, enabling precise force control and operation.
  • Disadvantages: High cost, complex maintenance, and not suitable for frequent use in sterile environments.
  • Applicable scenarios: Suitable for precise operation and force feedback control in high-precision surgery and rehabilitation training.

• Service robots and home scenarios

Service robots and home applications require dexterous hands to have high dexterity, safety, and adaptability.

• Linkage structure
  • Advantages: Simple structure, high reliability, and low cost.
  • Disadvantages: Limited dexterity and freedom, making it difficult to adapt to diverse family environments.
  • Applicable scenarios: Suitable for simple item picking and placement in service robots.
• Chordae tendon structure
  • Advantages: High dexterity, high degree of freedom, and can adapt to items of different shapes and sizes.
  • Disadvantages: Lower reliability, higher maintenance requirements, and unsuitable for use in dusty environments.
  • Applicable scenarios: Suitable for various object grasping and manipulation in home service robots.
• Direct drive structure
  • Advantages: High precision, fast response, enabling precise operation and force control.
  • Disadvantages: High cost, complex maintenance, and unsuitable for use around children and pets.
  • Applicable scenarios: Suitable for fine operations and human-computer interaction in high-end service robots.

• Special environments and dangerous scenarios

Special environments and hazardous scenarios include extreme environments such as space, deep sea, nuclear radiation, and fire, which require dexterous hands to have high reliability and adaptability.

• Linkage structure
  • Advantages: Simple structure, high reliability, and able to maintain stable performance in harsh environments.
  • Disadvantages: Limited dexterity and freedom, making it difficult to adapt to complex and ever-changing environments.
  • Applicable scenarios: Suitable for simple sampling and operations in space and deep-sea exploration.
• Chordae tendon structure
  • Advantages: High dexterity, high degree of freedom, and ability to adapt to complex environments and tasks.
  • Disadvantages: The tendon chord is susceptible to extreme temperatures and chemicals, which reduces its reliability.
  • Applicable scenarios: Suitable for complex rubble clearing and life detection in fire and earthquake rescue operations.
• Direct drive structure
  • Advantages: High precision, fast response, and stable performance in extreme environments.
  • Disadvantages: High cost, complex maintenance, and unsuitable for use in high temperature and radiation environments.
  • Applicable scenarios: Suitable for precise operation and detection in environments with nuclear radiation and chemical contamination.

04 Comprehensive Evaluation and Selection Recommendations for the Three Structural Schemes

Based on the preceding comparative analysis, a comprehensive evaluation of the three structural schemes is conducted:

Selection Recommendation

1. Industrial manufacturing and assembly scenarios

   • Recommended structure: linkage structure or direct drive structure.
   • Reasons: Industrial manufacturing and assembly typically require high gripping force and reliability. Linkage structures are low-cost and highly reliable; direct-drive structures offer high precision and fast response, making them suitable for high-precision assembly tasks.
   • Specific recommendations: For general industrial applications, a linkage structure is recommended; for high-precision, high-speed assembly tasks, a direct-drive structure is recommended. Furthermore, depending on the complexity of the scenario, the dexterous hand needs to have a high degree of freedom; the more refined and complex the scenario, the greater the need for a dexterous hand with high degrees of freedom.

2. Medical surgery and rehabilitation scenarios

   • Recommended structure: chordae tendon structure.
   • Reason: Medical surgery and rehabilitation usually require a high degree of dexterity and precision. The tendon chord structure has high dexterity and many degrees of freedom; the direct drive structure has high precision and is suitable for delicate operations.

3. Service robots and home scenarios

   • Recommended structure: tendon ligament structure or linkage structure.
   • Reasons: Service robots and home applications typically require high dexterity and adaptability. The tendon cable structure offers high dexterity and multiple degrees of freedom, while the linkage structure offers low cost and high reliability.
   • Specific recommendations: For high-end service robots, a tendon-and-string structure is recommended; for economical service robots, a linkage structure is recommended.

4. Special environments and dangerous scenarios

   • Recommended structure: linkage structure or direct drive structure.
   • Reasons: Special environments and hazardous scenarios usually require high reliability and adaptability. Linkage structures have high reliability and low cost; direct drive structures have high precision and fast response.
   • Specific recommendations: For general special environments, a linkage structure is recommended; for high-precision, high-risk tasks, a direct-drive structure is recommended.

Future Development Trends

With the continuous advancement of materials science, electrical technology, and control algorithms, the structural designs for dexterous hands are also constantly evolving and innovating:

• Linkage structure
  • Future trends: Development towards lightweight, high precision, and modular design, combined with new materials and manufacturing processes to improve performance and reliability.
  • Innovation directions: Develop reconfigurable linkage mechanisms to achieve dynamic switching of gripping modes; adopt smart materials, such as shape memory alloys, to achieve more flexible motion control.
• Chordae tendon structure
  • Future trends: Development towards higher precision, higher reliability, and intelligence, combining new tendon ligament materials and sensing technologies to improve control accuracy and reliability.
  • Innovation directions: Develop a self-tensioning tendon cable system to reduce maintenance needs; combine flexible electronics and distributed sensing technology to achieve more accurate tactile feedback.
• Direct drive structure
  • Future trends: Development towards high torque density, high efficiency, and low power consumption, combined with new motor technologies and heat dissipation designs to improve performance and reliability.
  • Innovation directions: Develop integrated direct drive modules to reduce size and weight; combine artificial intelligence and machine learning technologies to achieve more intelligent grasping and operation control.

When selecting a structural solution, dexterous hand companies should comprehensively consider factors such as grasping force, dexterity, precision, reliability, and cost based on product positioning, application scenarios, and market demands to make the most suitable choice. Linkage structures, with their advantages of high grasping force, high reliability, and low cost, are suitable for industrial manufacturing and assembly scenarios. Tendon-and-wire structures, with their advantages of high dexterity, multiple degrees of freedom, and light weight, are suitable for medical surgery and service robot scenarios. Direct-drive structures, with their advantages of high precision, fast response, and excellent control performance, are suitable for high-end applications and special environments. Different technical approaches have different application characteristics and requirements. With continuous technological advancements and innovations, these three structural solutions will continue to merge and develop to meet the needs of different markets and application scenarios.

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