The key performance indicators of a robot joint actuator are made up of various elements such as torque density, motion accuracy, responsiveness, back-drivability, and durability, and which indicator is prioritized differs depending on the application environment. This article explains the core performance indicators of joint actuators qualitatively, what each indicator means for system design, and how the Bonsystems lineup connects to these indicators, step by step.
Table of Contents
Robot Joint Actuator Performance Indicator 1 — Torque Density and Package Efficiency
Key Summary Torque density addresses how much output torque can be realized relative to the volume and weight an actuator occupies. It is an especially important criterion in humanoid applications where many actuators are placed in confined joint spaces.
Torque density is an indicator that addresses how much output torque can be realized relative to the volume and weight an actuator occupies. A robot joint has structural constraints that prevent its profile from being enlarged without limit. Especially in structures where many joints are arranged in series, such as the arm, leg, wrist, and ankle, the size and weight of each actuator directly affect the balance and design direction of the entire robot.
When many joints are configured within a single system, as in a humanoid robot, the size and weight of a single actuator go beyond a simple component specification and become a baseline for the design of the entire structure. In areas that bear large loads, such as the shoulder or knee, sufficient torque is needed, while in areas with limited space, such as the wrist or ankle, it is important to fit the required drive performance within a small profile.
Robot Joint Actuator Performance Indicator 2 — Motion Accuracy
Key Summary Motion accuracy addresses how stably a robot actuator moves in line with the target position and speed. It directly affects repeated motion, automation tasks, interaction with users, and the realization of humanoid motion.
Motion accuracy is an indicator that addresses how stably a robot actuator moves in line with the target position and speed. A robot actuator is not simply a rotating component but a core drive unit that turns control commands into actual motion. Motion accuracy is therefore an important criterion for a robot to take a desired posture, perform repeated tasks, and interact naturally with the external environment.
Motion accuracy is formed by the combination of the motor’s rotation characteristics, the reducer’s transmission structure, the sensor’s feedback, and the controller’s closed-loop design. It is not determined by the specification of a single component, but by how well-balanced the entire actuator is designed as a single drive module. Especially in robot joints, small errors can accumulate and lead to differences in the motion of the end effector, so stable motion transmission at the drive-unit stage is treated as important.
Robot Joint Actuator Performance Indicator 3 — Responsiveness
Key Summary Responsiveness addresses how quickly an actuator reacts when a control command is input and how naturally it follows through to the target motion.
Responsiveness is an indicator that shows how quickly an actuator reacts when a control command is input and how naturally it follows through to the target motion. A robot joint repeatedly faces situations of starting motion from a standstill, changing direction mid-motion, or responding to external forces. If responsiveness is low, the motion can feel sluggish or the control quality can drop.
In systems operated in close proximity to users, such as collaborative robots or humanoids, responsiveness is also connected to safety. When external contact, load changes, or posture changes occur, the drive unit must react quickly for the entire system to maintain smooth and predictable motion. In this process, simply moving fast is not the only thing that matters. Control stability, in which the motion continues steadily without shaking, must also be considered together.
Robot Joint Actuator Performance Indicator 4 — Back-Drivability and Safety
Key Summary Back-drivability indicates how naturally a joint can be pushed back or move when an external force is applied. For collaborative robots and humanoids, where a person can contact the robot, it is an important criterion for judging safety and smooth motion.
Back-drivability, put simply, is a characteristic that shows how smoothly a robot joint can move when it receives an external force. For example, if the joint resists rigidly and does not move at all when a person lightly pushes the robot arm by hand, the impact can be large in a collision. Conversely, if it can move naturally while accepting external force to some degree, the robot can interact more smoothly with its surroundings.
This characteristic is especially important for robots that move in spaces close to people, such as collaborative robots or humanoids, because the robot can unexpectedly touch a person’s hand, arm, or an object during work. If the joint resists external force too strongly at that moment, the burden of contact can grow. A joint structure that takes back-drivability into account can move while accepting external force in part, helping make the robot’s motion safer and more natural.
Robot Joint Actuator Performance Indicator 5 — Durability
Key Summary Durability indicates whether an actuator can stably maintain its motion characteristics even under wear, vibration, temperature change, and repeated load over long-term operation. The Bonsystems BSR and BCSA lineups pursue structures suited to long-term operation through a slim structure, Pinless structure, and a design direction that reduces the part count.
Durability is an indicator that addresses whether an actuator can stably maintain its motion characteristics even when affected by wear, vibration, temperature change, and repeated load over long-term operation. A robot joint is not a component that operates once and stops, but a drive unit that must continuously withstand repeated motion and load changes. Durability should therefore be examined not only by initial specifications but also by how consistent the performance remains over actual operating time.
The more complex the structure of an actuator applied to a robot joint, the more assembly elements and potential wear points can increase. Conversely, when the part composition is simple and the load path is designed stably, a more favorable structure can be created in terms of long-term operation. For this reason, when designing a joint actuator, you must consider not only output performance but also the simplicity of the internal structure, the load-distribution method, and stability under repeated motion.
The Pinless structure that Bonsystems applies is connected to a direction that structurally reduces the wear variables that can arise in conventional pin configurations. In addition, cycloidal reduction technology has the structural characteristic of distributing loads widely as it transmits them, so it can be a meaningful design basis in robot joint applications that need repeated motion and high-load environments. This structural approach goes beyond simply producing strong force and plays an important role in realizing a drive unit that moves stably over the long term.
Ultimately, the key performance indicators of a robot joint actuator have different priorities depending on the application environment. In humanoid multi-joint systems, torque density that can realize the needed force within a confined space can be reviewed as important, and in environments where user contact occurs, back-drivability and control stability can be treated as important. For long-term operating equipment, durability and the consistency of repeated motion should also be examined together.
Based on the BSR and BCSA lineups, Bonsystems supports technical consultation in which the application direction of robot reducers and actuators can be reviewed together. Examining the application purpose, required torque, installation space, drive conditions, and control environment in an integrated way lets you review a drive structure suited to the system more concretely.
FAQ
Q: What are the key performance indicators of a robot joint actuator?
A: Torque density, motion accuracy, responsiveness and control stability, back-drivability, and durability are the main indicators. However, the same criteria are not prioritized in every application; the indicators viewed as important can differ depending on the robot’s structure, operating environment, load conditions, and the possibility of user contact.
Q: What does torque density mean?
A: Torque density is an indicator that addresses how much output torque can be realized relative to the volume and weight an actuator occupies. It is reviewed as especially important in humanoid or multi-joint robot applications where many actuators are placed in confined joint spaces.
Q: Why is back-drivability important for collaborative robots and humanoids?
A: Back-drivability is a characteristic of how smoothly a robot joint can move when it receives an external force. In environments where people are close to the robot, unexpected contact can occur, so a joint reacting naturally rather than resisting external force too rigidly helps with safety and smooth motion.
Q: How is durability formed?
A: It is formed by the combination of integrated design elements such as a slim package, minimized part count, the simplification of potential wear points, and securing rigidity. Bonsystems pursues a direction that structurally reduces pin-level wear variables through the Pinless structure.
Q: Which of the performance indicators is most important?
A: The most important indicator differs depending on the application environment. In automation environments with much repeated motion, motion accuracy and control stability can be important, and in environments with the possibility of user contact, back-drivability and safety can be treated as important.
