Force Sensor Isotropic Compliance via Caged Ball Elastic Deformation
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Solution Overview
Problem
Existing force sensors and machines for Human-Robot Interaction (HRI) face challenges in providing isotropic planar compliance, which is essential for safe and effective interaction. Current solutions often result in high costs, mechanical constraints, and inability to handle high-frequency interaction forces.
Innovation Solution
A compliant device with an elastic element and caged balls, allowing the elastic element to laterally deform and generate an isotropic centering force in the x-y plane. This device is designed to be compact, low-weight, and easily mountable, with displacement sensors to measure interaction forces effectively.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional force sensors are used, then measurement precision is achieved, but isotropic compliance and safety are compromised
Solution Approach 1:
The force sensor is segmented into multiple independent elastic elements (springs) arranged in a specific geometry. Each spring independently contributes to the overall compliance, allowing the system to achieve isotropic compliance through the collective behavior of multiple segments rather than relying on a single stiff structure.
Solution Approach 2:
The patent changes the stiffness parameter of the force sensor by introducing elastic elements with controlled mechanical properties. By selecting appropriate spring constants and geometric configurations, the sensor achieves a balance between measurement precision and compliance, transforming it from a stiff conventional sensor to a compliant safe sensor.
2Ease of operation
If actuators are mounted close to the end-effector, then back-drivability and safety are improved, but device complexity and mounting constraints increase
Solution Approach 1:
The elastic compliance function is extracted from the actuator assembly and implemented as a separate force sensor module with its own elastic elements. This allows the actuator to remain simple and the compliance to be provided by the dedicated sensor, eliminating the need for complex integrated compliant actuators while maintaining back-drivability.
Solution Approach 2:
The force sensor acts as an intermediary element between the actuator and the end-effector. It provides the necessary compliance and safety features without requiring the actuator itself to be complex or mounted in constrained locations, mediating the interaction forces in a controlled manner.
3Ease of manufacture
If actuators are controlled to reduce interaction force, then cost is reduced, but high-frequency interaction forces cannot be effectively cancelled
Solution Approach 1:
The elastic elements in the force sensor provide beforehand cushioning by physically absorbing and attenuating high-frequency interaction forces before they reach the actuator and user. This passive mechanical cushioning complements the active control system, enabling effective high-frequency force cancellation without requiring expensive high-bandwidth actuators.
4Ease of operation
If elastic elements are used for compliance, then back-drivability is achieved, but manufacturing precision and stiffness control become challenging
Solution Approach 1:
The patent employs a specific asymmetric geometric arrangement of multiple elastic elements (springs) with different orientations and lengths. This asymmetric configuration compensates for manufacturing tolerances and material property variations, ensuring that the overall stiffness matrix achieves the desired isotropic compliance characteristics even when individual elements have slight variations.
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 device achieves isotropic stiffness in the x-y plane, allowing for controlled interaction forces, even in non-back-drivable machines. It provides overload protection, self-centering capability, and low hysteresis, making it suitable for pHRI applications while maintaining low costs and simplicity in design.
Implementation Method 1
at least one elastic element (6) is positioned over the extended portion of the floating body, compressed between and connecting the floating body and the first frame
Implementation Method 2
At least a row of caged balls (5) support the floating body over the first and second frames
Data Source
AI summary
Device with isotropic planar compliance comprising a floating body (2) positioned between a first (3) and second frame (4), said floating body having an extended portion (21) protruding through the first frame, at least a row of caged balls (5) supporting the floating body, at least one elastic element (6) positioned over the extended portion of the floating body, compressed between and connecting the floating body and the first frame. Said caged balls allow the at least one elastic element to laterally deform, so that their axis bends, while the ends of said at least one elastic element lay on two parallel planes, which remain at constant distance while sliding one with respect to the other. The device comprises also at least one displacement sensor (10), for measuring the displacement of the floating body with respect to the first or second frame.