Dual-Motor Differential Speed Reducer for Robot Joint Back Drivability
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Solution Overview
Problem
Service robots face challenges with low back drivability in their joint units due to high static friction in differential speed reducers, leading to increased wear and noise when dithering is used to mitigate contact forces, which is critical for safety in human-robot interactions.
Innovation Solution
A method utilizing two motors with differential speed reducers, where the internal, intermediate, and external gears are coupled differently to each motor, allowing for controlled speed differences to minimize static friction, enabling high back drivability and reducing noise and wear.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If dithering is performed to improve back drivability, then static friction is reduced, but mechanism components wear dramatically and noise occurs
Solution Approach 1:
The patent applies the Dynamics principle by continuously rotating the internal gear and external gear at different speeds even when the joint is stationary. This creates a dynamic state within the differential speed reducer where gears are constantly moving relative to each other, preventing static friction from taking over. The intermediate gear remains stationary while the internal and external gears rotate, ensuring continuous motion without actual joint movement, thereby improving back drivability without causing wear or noise associated with dithering.
2Device complexity
If a single motor with differential speed reducer is used, then the structure is simple, but back drivability is low due to high static friction
Solution Approach 1:
The patent applies the Segmentation principle by dividing the single motor system into two separate motors: one driving the internal gear and another driving the external gear. This segmentation allows independent control of each gear's rotation, enabling the internal and external gears to rotate at different speeds while the intermediate gear remains stationary. The segmentation creates a differential speed mechanism that prevents static friction by maintaining continuous relative motion between gears, thereby improving back drivability while maintaining reasonable structural complexity.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This approach achieves reduced friction in the differential speed reducer, allowing for improved back drivability and safer human-robot interactions by minimizing static friction, even when the joint is stationary, thus enabling effective force mitigation during contact.
Implementation Method 1
a large static friction acts on mesh of gears in the differential speed reducer. Accordingly, unless extremely large torque is applied from an output side, it is impossible to rotate the joint from the output side against the static friction
Data Source
AI summary
Provided is a method of driving a joint device suitably used for a robot joint with improved back drivability. A first motor and a second motor are fixedly connected to a first link. An output shaft of the first motor is coupled to an internal gear of a differential speed reducer. An output shaft of the second motor is coupled to an external gear of the differential speed reducer. A second link is coupled to the internal gear of the differential speed reducer. When the first link and the second link are relatively stopped to keep a joint angle fixed, both of the first motor and the second motor are rotated with a rotational speed larger than zero, and the rotational speed of the first motor and that of the second motor are differentiated to stop the second link.