Backdrivability is the property by which, when an external force is applied to the output shaft of an actuator, that force is transmitted into the internal drivetrain, including components such as the reducer and motor, allowing the output shaft to move in the reverse direction in response to the external force. Put simply, it describes how naturally and flexibly a robot joint or actuator output shaft responds when a user pushes it by hand. This property does not merely mean that a shaft moves easily; from the standpoint of a robot system, it is a design element connected to external-force sensing, shock mitigation, contact response, and control stability. It is therefore treated as an important review indicator for robots that move in environments close to users or perform complex contact motions, such as collaborative robots, humanoid robots, and service robots.
Table of Contents
The Definition and Operational Meaning
Key Summary Backdrivability is the property by which an external force applied to the output shaft is transmitted into the internal drivetrain, allowing the output shaft to move in response. It is connected to how naturally the actuator responds when a user moves the output shaft by hand.
Backdrivability refers to the property by which, when an external force is applied to the output shaft of an actuator, that force is transmitted toward the reducer and motor, allowing the output shaft to move in the reverse direction in response. Intuitively, it indicates how smoothly and naturally the output shaft responds when a user moves it by hand.
Generally, an actuator is designed for forward power transmission, conveying the motor’s rotation to the output shaft. However, in applications that can come into contact with the external environment, such as collaborative robots or humanoid robots, the response to externally applied force also becomes an important design variable. This is the response characteristic that backdrivability describes.
The higher the level of backdrivability, the more naturally an externally applied force is transmitted from the output shaft into the drivetrain. As a result, a robot joint can comply more flexibly with external force, and the entire system can respond more stably in contact situations.
The Connection to Collaborative Robot Safety
Key Summary Because a collaborative robot is operated alongside users, the possibility of external contact always exists. An actuator that takes backdrivability into account can respond more flexibly to external contact, so it is reviewed as an important element in the safety design of collaborative robots.
A collaborative robot is designed to work in the same space as users. Unlike equipment that repeats a set path within a separated space, a collaborative robot moves objects or assists assembly at a close distance to the worker, and must adapt to a variety of work environments. In this process, accidental contact can occur between the robot and a person, the robot and a workpiece, or the robot and surrounding equipment.
In such environments, how the actuator responds to external force matters. If the joint hardly responds to an external force applied to the output shaft and only resists with high rigidity, the load transmitted to a person or workpiece at the moment of contact can increase. Conversely, an actuator designed with external-force response in mind can react more flexibly to contact and can contribute to mitigating contact impact.
Significance in Humanoid Motion
Key Summary A humanoid robot performs complex motions such as walking, object manipulation, and interaction with the human body. Backdrivability contributes to complying naturally with external force in these motions and realizing smooth, human-like movement.
A humanoid robot must perform complex motions such as walking, maintaining balance, manipulating objects, and interacting with the human body. Most of these motions involve contact with the external environment. As a foot meets the ground, a hand grasps an object, and an arm or body comes close to the surroundings, various external forces are transmitted to the robot’s joints.
Taking walking as an example, the moment a foot meets the ground, the joint must respond to the ground reaction force. While grasping an object, additional force can be transmitted to the joint if the object is heavier than expected or positioned differently than anticipated. When moving in spaces close to people, even small contact can affect the robot’s posture and control. Thus, it is not enough for a humanoid robot to simply move its joints to set angles.
Backdrivability is the characteristic that helps a robot joint comply with external force in these contact situations. When the output shaft can move to a certain degree in response to external force, the robot can more flexibly accommodate contact with the ground or objects. This can act as an important element in improving walking stability, the naturalness of object manipulation, and the quality of interaction with users.
In a humanoid robot, the actuator is not simply a component that moves a joint. It is a core module that jointly determines the robot’s posture, power transmission, external-force response, and motion stability. For this reason, the actuator’s reverse-drive characteristics are an important design element to review in order to make a humanoid robot’s movement more natural.
The Bonsystems Lineup
Key Summary The Bonsystems BCSA lineup is an actuator that combines the strengths of the company’s cycloidal reducer with integrated motor and control elements, connecting naturally with the backdrivability requirements of collaborative and humanoid applications.
The Bonsystems BCSA lineup is an actuator solution that integrates motor and control elements based on cycloidal reducer technology. In applications where external contact matters, such as collaborative robots or humanoid robots, not only simple output performance but also the joint’s external-force response characteristics must be considered together. From this perspective, the structure and control configuration of an integrated actuator become important review elements in robot drive-unit design.
The BCSA is an actuator lineup that considers strong force, a thin structure, and customizability together. These characteristics can help guide application discussions in projects where the drive unit must be configured within confined joint spaces, as in collaborative and humanoid robots, or where the mechanical design and actuator specification must be matched together.
Based on cycloidal reducer and actuator technology, Bonsystems is developing drive solutions needed for a variety of applications, such as robot joints, collaborative robots, humanoids, and autonomous mobile robots. The motion characteristics and applicability of each specific model can differ depending on the robot’s structure, required torque, installation space, control method, and operating environment. In projects where backdrivability matters, it is therefore necessary to review the target motion, joint layout, load conditions, and control configuration together.
Based on these conditions, Bonsystems can provide consultation on the direction of robot drive-unit design and the applicability of the BCSA. For projects where contact with the external environment matters, such as collaborative or humanoid robots, drive-unit design that considers not only the actuator’s output performance but also its external-force response characteristics is needed.
FAQ
Q: What is backdrivability?
A: Backdrivability refers to the property by which, when an external force is applied to the actuator output shaft, that force is transmitted into the internal drivetrain and the shaft can move in response. It is related to how naturally the actuator responds when a user pushes a robot joint or output shaft by hand.
Q: How is this characteristic connected to collaborative robot safety?
A: Because collaborative robots operate in close proximity to users, the possibility of external contact is always present. An actuator with good external-force response can react more flexibly to contact, making this characteristic important from a safety-design perspective.
Q: What role does it play in humanoid motion?
A: It helps the robot respond naturally to ground reaction forces during walking, react smoothly to unexpected contact during object manipulation, and achieve smoother, more human-like movement.
Q: How is it connected to Bonsystems actuators?
A: The BCSA lineup combines the strengths of Bonsystems’ cycloidal reducer with integrated motor and control elements. This makes it suitable for system-level consideration of the backdrivability and external-force response characteristics formed by the combination of motor, reducer, and control.
Q: What elements form backdrivability?
A: It is formed by the combination of the reducer’s structure and friction characteristics, the motor’s control design, the response characteristic of the sensor and controller closed loop, and the integrated design of the actuator as a whole. It is not determined by a single component specification.
