Rotary Series Elastic Actuator for Precise Joint Torque Control
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Solution Overview
Problem
Position-controlled robots face limitations in precise force control due to nonlinearities in gear reduction, such as friction and stiction, which hinder their stability and safety when interacting with environments or humans, necessitating a more effective means of controlling rotational force.
Innovation Solution
A rotary series elastic actuator (SEA) is developed, comprising a motor, gear transmission, and a spring assembly with sensors, where the spring deflection is measured to provide torque feedback, enabling precise control of rotational force through a feedback control loop.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Force
If position-controlled actuators with gear reduction are used, then output force is increased, but force control precision deteriorates due to nonlinearities such as friction and stiction
Solution Approach 1:
An elastic element (spring) is introduced as an intermediary between the actuator and the environment. This spring serves as a force sensor by deflecting in proportion to the applied force, enabling direct force measurement without being affected by gear transmission nonlinearities. The spring acts as a mediator that transforms force control into position measurement of the spring deflection, resolving the contradiction between force magnitude and force precision.
Solution Approach 2:
The patent replaces direct mechanical force sensing through gear transmissions with an elastic element-based force sensing mechanism. Instead of relying on position sensors and gear reduction to infer force, the system uses the elastic deformation of a spring element to directly measure force, substituting the mechanical force transmission path with a compliant force sensing path that is insensitive to friction and stiction.
2Measurement precision
If position control is used for robot stability, then position accuracy is improved, but stability deteriorates when physically contacting rigid environments
Solution Approach 1:
The elastic element is positioned beforehand between the actuator and the environment to provide compliance and cushioning during physical contact. This pre-configured elastic element absorbs shocks and prevents rigid interactions, maintaining stability when the robot contacts rigid environments while still enabling precise force control through spring deflection measurement.
3Reliability
If force control is implemented, then safety and stability improve, but control complexity increases without force feedback
Solution Approach 1:
The elastic element provides inherent force feedback by converting force into positional information through its deflection. Sensors measure the spring deflection position, which directly corresponds to the applied force, creating a natural feedback loop. This eliminates the need for complex force sensors or additional feedback mechanisms, achieving reliable force control with simplified system architecture.
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
The SEA enhances the stability and safety of robots by allowing precise control of rotational force, improving their interaction with environments and humans, and providing natural interaction akin to human muscle-powered movements.
Implementation Method 1
The elastic element is usually a spring or a set of springs whose deflection is proportional to the force applied to itself
Implementation Method 2
One or more sensors in an SEA are used to detect the deflection of the elastic element
Data Source
AI summary
A rotary-type series elastic actuator (SEA) for use in robotic applications. The SEA including a motor, gear transmission assembly, spring assembly, and sensors. In one example, a robotic joint may include the SEA as well as two links coupled with each other at the joint assembly. The two links may be designated as input and output links. Each link may have a joint housing body which may be concentrically connected via a joint bearing so that they freely rotate against each other. The housing frame of the SEA may be fixed at the joint housing body of the input link while the output mount of the spring assembly of the SEA may be concentrically coupled with the joint housing body of the output link. The rotation of the motor rotor causes the rotation of the output link with respect to the input link plus spring deflection of the spring assembly. When an external force or torque are applied between the two links, a control action of a control loop may cause a rotation and motive force of the motor that lead to the deflection of the spring assembly to balance with the external force/torque and inertial force from body masses moving together with the links.