Table of Contents
A quadruped walking robot is designed to move not only across flat surfaces but also over slopes, stairs, uneven terrain, and other challenging environments by coordinating the movement of four legs. Each leg consists of multiple joints, and the robot controls these joints individually to adjust foot placement and maintain body posture.
During walking, the robot repeatedly lifts its legs and places its feet on the ground. While one leg is moving, the remaining legs must support the robot’s body, and impact generated when a foot contacts the ground is transmitted through the leg to the joints.
For this reason, joint design for quadruped walking robots must consider more than the ability to generate sufficient torque. The size and weight of the drive unit, its arrangement within the leg, and its ability to operate under repeated loads must all be considered together.
Operating Environments for Quadruped Walking Robots
Key Summary Quadruped walking robots are designed for environments that may be difficult for wheeled mobile robots to navigate, requiring joint systems capable of adapting to varying terrain and load conditions.
Quadruped robots are being considered for a wide range of environments, including factories, logistics facilities, construction sites, agricultural areas, and facility inspection applications. These environments may include stairs, slopes, uneven ground, narrow passages, and obstacles that make conventional wheeled movement difficult.
Wheeled mobile robots are generally well suited to efficient and stable movement on relatively flat surfaces. Quadruped walking robots, however, can individually adjust the position of each leg, allowing them to adapt their posture and foot placement to changes in ground height and terrain conditions.
However, as terrain conditions change, the loads applied to each joint also vary. Impact is generated when a foot contacts the ground, and the amount of torque required at each joint can change depending on factors such as the robot’s weight, posture, movement, and payload.
Because of these operating characteristics, the joints and drive units of a quadruped robot need to be designed with repeated and varying loads in mind.
The Load the Drive Unit Receives During Walking
Key Summary The joints of a quadruped robot are repeatedly subjected to both the load of supporting the robot’s body and the impact generated when its feet contact the ground.
During walking, the four legs of a quadruped robot do not always carry the same load. When one leg is lifted, the remaining legs in contact with the ground share the task of supporting the robot’s weight.
As walking speed, posture, and payload change, the load and torque requirements at each joint continuously vary.
When a foot contacts the ground, external impact is transmitted through the leg to the joints. On hard or uneven surfaces, both the magnitude and direction of this impact can vary. The drive unit must therefore provide the required torque while maintaining reliable operation under repeated movement and changing loads.
For this reason, component selection should not be based solely on maximum output specifications. Repeated loads, impact, operating duration, and the actual motion patterns expected during walking should also be considered.
How the joint responds to forces transmitted from the ground is another important design factor that can affect the overall stability and performance of the walking system.
Multi-Joint Structure and Actuator Design
Key Summary Because a quadruped robot integrates multiple joints within a limited leg structure, actuator size, weight, and arrangement directly influence the overall robot design.
The leg of a quadruped walking robot generally consists of multiple joints that perform different functions, such as connecting the leg to the body, adjusting the direction of the leg, and bending or extending the leg in a motion similar to that of a knee.
The specific joint configuration varies depending on the robot’s design and intended application, but quadruped locomotion is generally achieved through the coordinated movement of multiple joints.
Each joint uses an actuator to generate movement. The actuator converts the motor’s rotational motion into the speed and torque required at the joint and operates according to commands from the robot’s control system.
As the number of joints increases, so does the number of motors, reducers, sensors, control components, and cables that must be integrated into the robot.
If each drive unit is excessively large or thick, the leg becomes bulkier and the distance between joints may need to increase. A thinner and more compact drive unit, on the other hand, can provide greater flexibility when arranging joints, sensors, and wiring within a limited space.
For this reason, actuator selection for a quadruped walking robot should consider not only output performance but also dimensions, weight, cable routing, and mounting structure.
Component Requirements for Quadruped Robots
Key Summary Joint components must provide sufficient torque while keeping the leg structure from becoming unnecessarily large or heavy.
The drive system of a quadruped robot must provide sufficient torque to support the robot’s body and move its legs. However, increasing component size and weight simply to achieve higher output can also increase the energy required to move the legs.
For this reason, actuator and reducer selection requires an appropriate balance between output performance, size, and weight.
Space for sensors, cables, and other components must also be considered when designing each joint. Features such as hollow structures or dedicated cable-routing paths can provide additional flexibility when integrating these components into the leg.
Durability under repeated walking and landing motions is another important consideration.
Unlike equipment that operates under relatively constant conditions, a quadruped walking robot experiences continuously changing joint loads depending on terrain, posture, speed, and movement patterns.
Drive units should therefore be selected according to the actual operating environment and expected load conditions, and repeated-motion performance should be evaluated during the prototype and testing stages.
Bonsystems Drive Solutions for Quadruped Walking Robots
Key Summary The Bonsystems BCSA is a slim, compact actuator designed to support different joint requirements in quadruped walking robots through high-torque output and multiple reduction ratio options.
The Bonsystems BCSA is a slim and compact robot actuator based on cycloidal reduction technology.
By combining a compact drive-unit structure with the ability to deliver the torque required for body support and leg movement, the BCSA can be considered for quadruped robot designs in which multiple joints must be arranged within limited leg space.
The motion and load requirements of a quadruped walking robot vary depending on the position and function of each joint.
A joint located close to the body may need to control the direction of the entire leg while supporting significant loads. A knee-type joint, in contrast, repeatedly bends and extends the leg during walking.
Because the BCSA is available with different reduction ratio configurations, designers can select drive characteristics according to the torque and rotational requirements of individual joints.
Repeated walking and landing also generate changing loads and impact at the joints. The cycloidal reduction structure used in the BCSA is designed to distribute load across multiple contact areas, supporting stable force transmission under changing load conditions.
This structural characteristic can be beneficial in quadruped robot applications where multiple joints operate repeatedly while the robot moves across uneven terrain.
The BCSA also incorporates Bonsystems’ Pinless cycloidal design, which reduces some of the coupling elements used in conventional pin-based configurations.
By simplifying the reducer’s internal component configuration, the Pinless structure can help reduce the space required for internal components and support the development of thinner, more compact drive units. A simplified structure may also help reduce assembly complexity during the manufacturing process.
The appropriate actuator configuration for a quadruped robot depends on multiple factors, including robot size and weight, joint position, leg length, walking pattern, and required joint motion.
With its slim structure, high-torque capability, multiple reduction ratio options, and design considerations for repeated loads, the BCSA can support drive-unit configurations tailored to the role and installation conditions of individual joints.
Key Takeaways
The joints of a quadruped walking robot are repeatedly exposed to the load of supporting the robot’s body as well as impact generated when the feet contact the ground.
Because multiple joints must be integrated into each leg, actuator thickness and weight affect not only the overall leg profile but also the amount of mass the robot must repeatedly accelerate and move during walking.
Selecting components based only on maximum output can overlook important operating conditions. Repeated loads, impact, operating duration, wiring paths, mounting methods, and available installation space should all be considered as part of the joint design process.
The Bonsystems BCSA is designed to support different torque and rotational requirements across quadruped robot joints through a slim structure and multiple reduction ratio configurations.
Rather than applying exactly the same drive configuration to every joint, designers can consider different actuator specifications according to the role, load conditions, and installation space of each joint.
Frequently Asked Questions (FAQ)
Q1. Why is a slim joint design important for quadruped walking robots?
A. Multiple joints, sensors, cables, and drive components must be integrated within each leg. Reducing the thickness of the drive unit provides greater flexibility when arranging these components in limited space and can help create a more compact overall leg structure.
Q2. Why is durability important in a quadruped walking robot?
A. During walking, the joints are repeatedly subjected to the load of supporting the robot’s body and to impact when the feet contact the ground. Because these loads occur continuously during operation, the drive system must be designed and evaluated for repeated load conditions.
Q3. What should be considered when selecting an actuator for a quadruped robot?
A. Factors such as robot weight, joint position, required torque, range of motion, operating speed, and available installation space should be considered together. Repeated loads, impact, operating duration, wiring layout, and mounting conditions should also be evaluated based on the robot’s actual operating environment.
Q4. Can the same actuator be used for every joint of a quadruped walking robot?
A. Not necessarily. Each joint performs a different movement and experiences different load conditions, so its torque, speed, reduction ratio, and installation requirements may also differ. The motor, reducer, and actuator configuration should therefore be selected according to the role and design conditions of each joint.
Q5. How is a quadruped walking robot different from a wheeled mobile robot?
A. Wheeled mobile robots are generally efficient on relatively flat surfaces but may face limitations when navigating stairs, steps, obstacles, or uneven terrain. Quadruped walking robots can adjust the position of each leg independently to adapt to changes in ground height and terrain, although their multiple joints make drive-system and control design more complex.
References
• [1] Bonsystems Official Website — https://www.bonsystems.com
• [2] International Federation of Robotics (IFR) — https://ifr.org
• [3] Korea Institute for Robot Industry Advancement (KIRIA) — https://www.kiria.org
• [4] Korea Institute of Robotics & Technology Convergence (KIRO) — https://www.kiro.re.kr
Bonsystems
A specialist in Pinless cycloidal reducers and actuators
