In the field of agricultural production, the handling and transportation of agricultural products and materials are indispensable. Employing humanoid robots to replace traditional manual labor has become a key focus in the current research and development of intelligent agricultural equipment. Humanoid bionic robots, by mimicking biological movement mechanisms and integrating high-precision sensors and control systems, can achieve human-like motions and operational functions. Based on this premise, this research conducted a survey on anthropomorphic dimensions to determine the size of a bionic bipedal robot. With reference to the skeletal structure and joints of the human lower limb, a degree-of-freedom (DOF) analysis was performed for the robot. Subsequently, systematic designs for the thigh, calf, and foot were developed, leading to the fabrication of a physical prototype. Finally, finite element modeling and analysis were employed to validate the feasibility of the design scheme.
The concept of morphological bionics originates from a profound insight into and abstraction of natural biological forms. Agricultural operations are inseparable from human labor—moving seeds, fertilizers, pesticides, and other agricultural inputs from storage to the field, and transporting harvested produce from the field to vehicles. In the evolution of technology, the bionic robot, as an outstanding representative of intelligent technology, is gradually moving from laboratories into people’s lives. In terms of the integration of artificial intelligence and mechatronics, the application of networking and big data, sustainable development and energy conservation, as well as bionic design and the application of new materials, the humanoid bionic robot for agricultural material transfer operations holds immense potential. Compared to robots of other morphologies, an intelligently designed bionic robot with material handling functions can liberate farmers from arduous agricultural tasks.
Design Requirements and Research
Anthropomorphic Dimension Research
In the development of robotic systems for field operations, the operational parameters must conform to ergonomic standards. The advantages of a bipedal bionic robot are primarily reflected in its bionic design, enabling it to simulate human movements and maintain balance and stability in complex environments. Human legs must account for more than 50% of total body height to achieve the biomechanical efficiency of a human bipedal gait. Determining the optimal robot configuration involves comparing different body structures from perspectives such as stride length, gait, and turning flexibility. Anatomical studies indicate that for an average adult male, the thigh length is approximately 0.25 times the body height, the calf length is about 0.2 times, the foot length is about 0.15 times, and the foot width is roughly 0.4 times the foot length. Based on this, target dimensions for a bionic robot with a total height of approximately 1.6 meters were established, as summarized in the table below.
| Body Segment | Proportion to Total Height | Target Length (mm) for ~1.6m Robot |
|---|---|---|
| Total Leg Length | > 0.5 | > 800 |
| Thigh | ~0.25 | ~400 |
| Calf | ~0.2 | ~320 |
| Foot Length | ~0.15 | ~240 |
| Foot Width | ~0.4 x Foot Length | ~96 |
Analysis of Robot Lower Limb Degrees of Freedom and Dimensions
The design approach was developed from three main aspects: modular design, bionic design, and lightweight design. Research on human anatomy provides crucial information regarding the degrees of freedom of each joint in the lower limb, which guides the design of robots, especially those that mimic human motion characteristics. The human lower limb’s hip, knee, and ankle joints are composed of four connected rigid bodies. Each joint can be modeled using three single-axis rotational joints, providing the lower limb with 12 degrees of freedom in total. The skeletal structure of the human lower limb includes 30 bones. The spherical head of the femur and the acetabulum of the hip bone form the hip joint, connecting the lower limb to the torso. The tibia and fibula are the two bones of the lower leg, supporting and moving the body. The tarsals, metatarsals, and phalanges are the short, tubular bones that make up the foot, providing stability and flexibility.
Based on literature review, the hip joint has 3 degrees of freedom (flexion/extension, internal/external rotation, and abduction/adduction), the knee joint has primarily 1 degree of freedom (flexion/extension with limited rotation), and the ankle joint has 2 degrees of freedom (dorsiflexion/plantarflexion and inversion/eversion). These degrees of freedom allow the human lower limb to adapt to different terrains and activities. For the initial design of the agricultural bionic robot, a simplified but functionally adequate DOF configuration was targeted, as shown in the following table comparing human and robot joint mobility.
| Joint | Human DOF (Primary) | Target Robot DOF | Description | Target Motion Range (°) |
|---|---|---|---|---|
| Hip | 3 | Pitch | Flexion/Extension | -30 to 150 |
| Roll | Abduction/Adduction | -50 to 50 | ||
| Yaw | Internal/External Rotation | -40 to 40 | ||
| Knee | 1 (2 with coupled rotation) | Pitch | Flexion/Extension | 35 to 180 |
| Ankle | 2 | Pitch | Dorsiflexion/Plantarflexion | -53 to 45 |
| Roll | Inversion/Eversion | -20 to 20 |
Overall System Design
Mechanical System
The design of the mechanical structure followed a scheme that prioritizes lightweight construction while ensuring sufficient strength. It also emphasized ease of maintenance and repair. Innovative structural designs were adopted to reduce the overall weight without compromising the structural integrity of the bionic robot. Particular attention was paid to the design of the bipedal system, balancing weight distribution and joint actuation forces to ensure efficiency and safety during agricultural material transportation tasks.
Electronic Control System
The electronic control system required a highly integrated, modular design. The goal was to minimize weight and simplify the system architecture while ensuring it could be housed within a sealed enclosure. By utilizing high-speed AD/PWM signal converters and a powerful embedded computer, the system’s processing speed was optimized. This enhancement improves the response time of control algorithms and the dynamic performance of the hydraulic actuators, which is critical for the stable walking of the bionic robot.
Hydraulic Servo System
For the hydraulic servo system, the focus was on designing compact, high-power-density hydraulic actuators to improve dynamic response speed and reduce their own weight. Simultaneously, the hydraulic circuits were optimized to ensure precise and efficient operation of the joints, which is crucial for the stable and adaptive gait required in uneven agricultural fields.
Structural Design
Through scientific planning and human-machine synergy, the healthy development of agricultural mechanization can be promoted. During the development of the bionic robot, efforts were made to maintain the strength of the mechanical structure while minimizing the weight of the hydraulic actuators and the robot’s leg components as much as possible. A lightweight design not only reduces energy consumption but also enhances the flexibility and efficiency of the robot in farming operations.
Detailed Structural Design
The simplest structural and actuator configuration scheme was adopted. The following table details the final designed motion ranges for each joint of the bipedal bionic robot.
| Joint | Degree of Freedom | Designed Motion Angle Range (°) |
|---|---|---|
| Hip | Pitch (Flexion/Extension) | -12 to 101 |
| Roll (Abduction/Adduction) | -15 to 15 | |
| Yaw (Rotation) | -13 to 13 | |
| Knee | Pitch (Flexion/Extension) | 70 to 180 |
| Ankle | Pitch (Dorsiflexion/Plantarflexion) | -53 to 45 |
| Roll (Inversion/Eversion) | -15 to 15 |
Foot System Design
The foot plate was constructed from 10mm thick 6061 aluminum plate. A finite element analysis (FEA) was performed by applying a force equivalent to twice the total robot weight (approximately 1569 N) to the structure. The analysis yielded a maximum stress of 88.7 MPa, which is well below the yield strength of 5515 MPa for 6061 aluminum alloy, indicating a high safety factor. The foot’s rubber sole was designed using a Mount rubber material with a Shore A hardness of 78±3, capable of withstanding a surface pressure of up to 17 kg/cm², providing necessary grip and damping.
Calf System Design
The primary structure of the calf was fabricated by welding 10mm thick 6061 aluminum plates. A force equal to twice the system’s weight under full load (1548 N) was applied at all pin joint locations in the FEA simulation. The maximum stress calculated was 40.98 MPa, again significantly lower than the material’s yield strength. The parallel hydraulic cylinder arrangement was designed with reference to bionics, mimicking the tibialis anterior and gastrocnemius muscles of the human calf. Each hydraulic cylinder has a diameter of 50 mm and a stroke of 130 mm, with a fully extended total length of 480 mm.
Thigh System Design
A hollow shell-type skeletal structure was adopted for the thigh. The knee joint’s flexion/extension degree of freedom was realized using a four-bar linkage mechanism actuated by a piston hydraulic cylinder. The three hydraulic cylinders in the thigh assembly were arranged in a concentrated, opposing layout for compactness. The thigh shell skeleton, hip joint, and knee joint components were primarily made from 10mm thick 6061 aluminum. Applying a 1548 N load (2x system weight) at the critical axis holes resulted in maximum stresses of 30.29 MPa, 49.8 MPa, and 12.55 MPa for the thigh assembly, hip joint, and knee linkage components respectively, all safely below the yield limit.
The structural integrity was verified using static finite element analysis. The governing equation for stress ($\sigma$) under load ($F$) over area ($A$) is fundamental, but for complex geometries, FEA solves the system of equations derived from elasticity theory:
$$ \nabla \cdot \boldsymbol{\sigma} + \mathbf{F} = 0 $$
where $\boldsymbol{\sigma}$ is the stress tensor and $\mathbf{F}$ is the body force vector. The von Mises stress criterion was used to evaluate the yield condition:
$$ \sigma_{vm} = \sqrt{\frac{(\sigma_{1}-\sigma_{2})^2 + (\sigma_{2}-\sigma_{3})^2 + (\sigma_{3}-\sigma_{1})^2}{2}} $$
where $\sigma_{1}, \sigma_{2}, \sigma_{3}$ are the principal stresses. The factor of safety ($n$) is then calculated as:
$$ n = \frac{\sigma_{yield}}{\sigma_{vm,max}} $$
The results confirmed a high factor of safety for all major components.
The physical prototype of the lower limb assembly was successfully fabricated based on the design, demonstrating the manufacturability of the proposed bionic robot structure.
Experimental and Manufacturing Evaluation Example
The knee joint’s range of motion was selected as a case for evaluation. To achieve a large angular displacement at the knee joint, a four-bar linkage mechanism was implemented to provide motion amplification, enabling a greater range of joint rotation.
The piston hydraulic cylinders used for joint actuation are typically cylindrical. The four-bar linkage components were made from 6061 aluminum plate. Links AB and BC act as two-force members. Forces were applied at their connection points along the directions of the thigh and calf extension lines, each with a magnitude equal to the hydraulic cylinder’s rated output force of 20580 N. The analysis showed that the deformation of the parts did not exceed 0.3 mm, and the maximum stress remained below 600 MPa, which is acceptable for the chosen aluminum alloy with proper heat treatment.
The relationship between the hydraulic cylinder displacement ($\Delta L$) and the change in joint angle ($\Delta \theta$) for the knee and hip joints was derived. The calculated slope ($k$) for the knee joint mechanism is approximately 0.247°/mm. This amplification allows the legs to achieve stronger and more effective motion. To calculate the knee joint’s rotational speed ($\omega$), the hydraulic cylinder’s piston stroke is known to be 130 mm (0.13 m), and the piston speed ($v$) is approximately 0.064 m/s. The rotational speed can be obtained using the following kinematic relation:
$$ \omega = v \cdot k $$
Substituting the values:
$$ \omega = 0.064 \, \text{m/s} \cdot 0.247 \, \text{°/mm} $$
Note that 1 mm = 0.001 m, so $k = 0.247 \, \text{°} / 0.001 \, \text{m} = 247 \, \text{°/m}$. Therefore:
$$ \omega = 0.064 \, \text{m/s} \cdot 247 \, \text{°/m} = 15.808 \, \text{°/s} $$
Thus, the calculated rotational speed of the knee joint is approximately 15.81 °/s.
A summary of key performance parameters from the design and analysis phase is provided below.
| Parameter | Value | Unit | Notes |
|---|---|---|---|
| Target Robot Height | ~1.6 | m | Based on anthropomorphic survey |
| Total Leg DOF (per leg) | 6 | – | Hip(3)+Knee(1)+Ankle(2) |
| Primary Structural Material | 6061 Al | – | 10mm plate, Yield Strength 5515 MPa |
| Max Stress in Foot (FEA) | 88.7 | MPa | Under 2x body weight load |
| Hydraulic Cylinder Stroke (Knee) | 130 | mm | |
| Knee Joint Motion Amplification Factor (k) | ~0.247 | °/mm | From four-bar linkage design |
| Calculated Max Knee Rotational Speed | ~15.81 | °/s | Based on piston speed of 0.064 m/s |
| Safety Factor (Typical Component) | > 60 | – | σ_yield / σ_vm_max |
Conclusion
Agricultural mechanization plays a pivotal role in the construction of modern agriculture. In the field of agricultural intelligent equipment innovation, research on the design of humanoid robots for agricultural material transfer operations is still in a stage of tackling key technologies. This work systematically explored the core design aspects of a bipedal bionic robot, including its overall system architecture and internal structure, aiming to provide theoretical support and practical reference for the construction of smart farms. Although the current operational accuracy and terrain adaptability of robots in farming material handling have not yet reached an ideal level, breakthrough progress has been made by research teams in key technical areas such as the design of bionic motion mechanisms and the optimization of energy utilization efficiency. However, significant challenges remain regarding performance indicators like motion stability and operational continuity in complex environments, urgently calling for the establishment of an interdisciplinary collaborative innovation mechanism to advance the field of the agricultural bionic robot.