Radially Stacked Series Elastic Actuator for Compact Humanoid Torque
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Solution Overview
Problem
Humanoid robots face a tradeoff between actuator size and performance due to limited internal volume, necessitating actuators that provide a better performance-to-volume ratio.
Innovation Solution
The implementation of radially stacked series elastic actuators with a motor, gearbox, and actuator output circumferentially contained within an actuator ground, featuring a spring that deflects with rotational movement, and sensors to measure displacement and torque, optimizing torque density, specific torque, and bandwidth.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Power
If larger actuators are used to improve performance, then torque density and bandwidth are improved, but the volume occupied increases beyond the human envelope
Solution Approach 1:
The actuator employs a radially stacked configuration where the motor, gearbox, and actuator output are circumferentially contained within the actuator ground. The motor is nested within the actuator housing, the gearbox is nested within the motor ground, and the actuator output is nested within the gearbox structure. This nesting arrangement maximizes space utilization and achieves high torque density without increasing external volume.
Solution Approach 2:
The invention transitions from a conventional linear arrangement of actuator components to a radial stacking configuration. By organizing components in concentric circles around a central axis rather than in a linear sequence, the design exploits the radial dimension to pack more components into the same envelope volume, thereby improving torque density without increasing overall size.
2Volume of moving object
If actuators are made smaller to fit within the human envelope, then volume is reduced, but performance including torque density and bandwidth deteriorates
Solution Approach 1:
The actuator incorporates a series elastic element (spring) between the motor and gearbox that can deflect radially. This flexible element allows the motor to operate at higher speeds and torques while the spring compliance provides mechanical filtering and protection. The thin-walled motor ground and actuator housing are designed to accommodate radial deflections while maintaining structural integrity, enabling high performance within compact dimensions.
Solution Approach 2:
The series elastic actuator design introduces dynamic compliance through the spring element, allowing the system to adapt its mechanical impedance during operation. The spring deflects dynamically in response to load variations, enabling the small actuator to maintain high bandwidth and performance by actively managing energy storage and release, rather than relying solely on rigid mechanical connections.
3Measurement precision
If a spring is added to measure displacement and improve torque resolution, then measurement precision is improved, but device complexity increases
Solution Approach 1:
The spring serves as a mechanical intermediary between the motor and gearbox, providing both force transmission and displacement measurement functionality. By measuring the spring's radial deflection, the system can infer torque with high resolution without requiring separate sensors in the torque path. This intermediary approach simplifies the overall measurement system while maintaining high precision.
Solution Approach 2:
The spring element performs multiple functions simultaneously: it transmits torque from the motor to the gearbox, provides mechanical compliance to protect against shocks, enables displacement measurement through its deflection, and contributes to the series elastic actuation mechanism. This multi-functionality reduces the need for separate components, thereby limiting the increase in device complexity while achieving high measurement precision.
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
These actuators achieve superior cumulative performance metrics in a given volume, enhancing torque density, specific torque, and torque resolution while fitting within the human envelope of humanoid robots.
Implementation Method 1
A spring, such as a planar torsion spring, can be coupled between the motor ground and the actuator ground such that the spring deflects as the motor ground rotates relative to the actuator ground.
Implementation Method 2
A spring displacement sensor can be provided to measure the displacement of the motor ground relative to the actuator ground. In an example embodiment, the spring displacement sensor is a capstan-reduction encoder that amplifies torque resolution.
Implementation Method 3
The motor output is coupled to the input of the gearbox and the output of the gearbox is coupled to the actuator output. In some embodiments, the gearbox is circumferentially contained within the motor ground. The gearbox can be a harmonic drive in some embodiments.
Implementation Method 4
a wave generator bearing into which the wave generator is inserted and that contacts the flex spline based on the position of the wave generator to cause the flex spline to mesh with the circular spine in limited areas.
Implementation Method 5
The actuator output sensor, according to an example embodiment, comprises a read head that is fixed relative to the motor ground and magnet that spins with the actuator output. The read head reads the magnetic field of the magnet to output a signal indicative of the position of the actuator relative to the motor ground.
Data Source
AI summary
A radial stacked actuator includes an actuator ground; a motor including a motor ground constrained to the actuator ground with one or more rotational degrees of freedom; a spring coupled between the motor ground and the actuator ground; a gearbox coupled to the motor at an input of the gearbox; and an actuator output coupled to an output of the gearbox. The spring is configured to deflect as the motor ground rotates relative to the actuator ground.