When designing a robot joint, simply producing a large force is not enough. Even if the same force is realized, if the drive unit becomes excessively large or heavy, it can burden the robot’s profile, joint layout, motion, and overall structure. Especially in systems where many joints must fit into a confined space, such as humanoids or lightweight robots, the size and weight of a single actuator accumulate and affect the entire design. The concept worth examining here is torque density, which describes how much output torque can be realized relative to the volume or weight an actuator occupies — a criterion for seeing how efficiently the required force can be configured within a limited space and weight.
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The Definition of Torque Density and the Elements That Form It
Key Summary Torque density is a concept that addresses the level of output torque relative to the volume and weight an actuator occupies.
Torque density is used to describe how much torque an actuator can produce relative to its size and weight. For a drive unit of the same size, a structure that can realize greater force can be advantageous in design, and to realize the same force, a smaller and lighter structure can help with system design.
However, it is not a concept determined by the performance of a single motor or a single reducer alone. Many elements act together, such as the motor configuration, the reducer structure, the housing design, the bearing arrangement, the way internal space is used, and the way the control section and sensor are integrated. How these elements are arranged and combined changes the overall size and weight of the actuator and its configuration efficiency as a drive unit.
For this reason, torque density is closer to a perspective for examining how efficiently a single drive module is designed than a concept for viewing the performance of individual components separately. An actuator for a robot joint must generate force, transmit it, and perform repeated motion within a confined space. It is therefore an important design indicator that helps in understanding the balance between size and force when reviewing a robot drive unit.
Why Torque Density Matters in Humanoid Design
Key Summary A humanoid robot must arrange many joints within a limited profile. Because the size and weight of the actuator in each joint can affect the robot’s profile, joint layout, and motion range, torque density is treated as an important design criterion.
A humanoid robot uses many joints to realize movements similar in form to a human’s. Drive units go into various areas such as the shoulder, elbow, wrist, hip, knee, and ankle, and each joint must generate the needed motion within a limited profile. At this point, the space and weight the actuator occupies become important conditions for designing the entire structure.
If the actuator that goes into a joint becomes larger, the profile of the robot’s arm or leg can grow along with it. If the drive unit becomes heavier, load can concentrate in a particular area, and this can also affect the design of other joints or the frame. In the end, the size and weight of a single actuator becomes not a single-component issue but a matter of the balance and arrangement of the entire robot structure.
This is exactly where the importance of torque density lies. An actuator that can realize the needed force within a confined space helps configure the joint section more concisely. When the space around the joint can be used efficiently, the designer can review the robot’s form, component arrangement, and range of motion more flexibly.
Of course, humanoid performance is not determined by torque density alone. In actual robot design, various elements such as the control method, structural rigidity, battery, sensors, software, and safety conditions must be considered together. Still, because the actuator is a core element that generates joint motion, torque density is a criterion well worth examining together when reviewing a humanoid drive unit.
Combining the Slim Package with Torque Density
Key Summary A slim package is an integrated design that reduces the profile thickness while maintaining motion stability and rigidity. Because it can integrate motion components more efficiently within the same profile, it is favorable for forming torque density.
Viewing torque density from an actual design perspective leads naturally to the importance of the package structure. An actuator is not a simple component made up only of a motor and reducer, but a drive unit in which various elements such as the housing, bearings, sensors, and control elements gather within a single space. Even when the same function is realized, the overall size and weight can change depending on how concisely these elements are arranged.
A slim package has important meaning from this perspective. A slim package means a slim structure that reduces the actuator’s thickness along the rotation axis. However, it does not mean simply making it thin. The needed elements must be arranged appropriately even within a thin structure, and the operating conditions required in the actual use environment must be considered together.
The reason a slim package connects to torque density is the space constraints a robot joint has. Many components must go together inside the robot’s arms, legs, and torso, and space for rotation and wiring is also needed around the joint. Being able to reduce the actuator’s thickness here helps configure the joint section’s profile more concisely.
A slim package also has meaning from the system designer’s perspective. When the drive unit is thin and concise, it can be easier to adjust the arrangement relationship with other components within the overall robot structure. Because the joint profile can be reduced and design space secured, it also works positively toward configuring the product form and motion more naturally.
The slim package design that Bonsystems pursues is also in line with this current. The BSR and BCSA lineups consider a direction of configuring the drive unit efficiently within a confined space, and can be reviewed to suit a variety of application environments such as robot joints and automation equipment.
The Bonsystems Lineup
Key Summary The Bonsystems BSR cycloidal reducers and BCSA cycloidal actuators are a product lineup that can be applied to the drive units of robot joints and automation equipment, focusing on configuring the drive unit efficiently within a confined space.
Viewing torque density from the perspective of drive-unit design, the structure and packaging method of the actuator ultimately become important, because you must consider size, weight, internal arrangement, and the application environment together with realizing the needed force. The Bonsystems BSR cycloidal reducers and BCSA cycloidal actuators are a product lineup that can be applied to the drive units of robot joints and automation equipment from this perspective.
The BSR is a cycloidal-reducer-based drive component that can be used to configure the reduction structure needed for a rotary drive unit or robot joint compactly. The BCSA is an actuator concept that integrates the reducer, motor, and control elements within a single package, oriented toward configuring the elements needed for a robot joint drive unit concisely.
The two product families focus on configuring the drive unit efficiently within a confined space, based on a slim package, a Pinless cycloidal structure, and a part-count reduction design direction. This design direction connects naturally with the concept of torque density. If the size and weight of the actuator can be managed efficiently, more diverse design possibilities can be secured in the process of reviewing the profile, arrangement, and motion of a robot joint.
Actual applicability can differ depending on the product model and use environment. The required torque, installation space, load conditions, repeated-motion method, control configuration, and the robot’s overall structure must be reviewed together. Torque density is therefore appropriately understood not as a criterion for judging a product on its own, but as a design indicator to examine together when applying an actuator to an actual system.
Based on the BSR and BCSA lineups, Bonsystems reviews applicability suited to a variety of drive environments, such as robot joints, lightweight robots, and automation equipment. Examining the actuator’s size, structure, and configuration direction together, based on the purpose of use and the requirements, helps in finding a more realistic drive solution.
FAQ
Q: What exactly is torque density?
A: It is a concept that describes how much output torque can be realized relative to the volume or weight an actuator occupies. It is used to understand how efficiently the needed force can be configured within the same size and weight.
Q: How is torque density formed?
A: It is formed by the combination of the motor design, the reducer structure, and the package efficiency of the actuator as a whole. Integrated design such as a slim package and minimized part count directly affects torque density.
Q: Why does torque density matter in humanoid design?
A: A humanoid robot is a structure in which many joints are arranged within a limited profile. If the actuator in each joint becomes larger or heavier, it can affect the overall profile and motion. Torque density is therefore an important criterion when reviewing a joint drive unit.
Q: Can I choose an actuator based on torque density alone?
A: It is an important reference indicator, but not a criterion for deciding an actuator choice on its own. For actual application, the required torque, installation space, load conditions, motion method, control configuration, and overall system structure must be reviewed together.
Q: What information should I prepare for a Bonsystems actuator consultation?
A: Consultation goes more smoothly if you prepare the purpose of use of the robot or equipment to be applied, the required motion method, the installable space, the expected load, the operating environment, and the drive method. Even if specific figures are not yet decided, sharing the target motion and structural direction lets us review applicability together.
