Force-Torque Sensor Overload Structure for Deflection Limiting
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Solution Overview
Problem
Silicon-based strain gauges used in force/torque sensors exhibit high temperature sensitivity and hysteresis, leading to sensor drift and potential damage due to strain hardening in robotic applications with high velocities and loads, where thick load beams are not sufficiently rigid to prevent deflection.
Innovation Solution
Incorporating overload structures with wire-cut electrical discharge machining to limit deflection of sensor structures, allowing the use of metal-foil strain gauges and preventing excessive motion beyond a predetermined amount, thereby enhancing sensor durability and reducing temperature sensitivity issues.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If thick load beams are used to improve durability and maximum load rating, then sensor durability is improved, but beam rigidity is reduced leading to excessive deflection and strain hardening damage in high velocity robotic applications
Solution Approach 1:
The load beam is segmented into a sensor structure portion and an overload structure portion. The sensor structure portion contains the strain gauges and is designed for measurement, while the overload structure portion is designed to fail first under excessive loads, protecting the sensitive sensor structure. This segmentation allows the sensor structure to be optimized for rigidity without compromising overall system durability.
Solution Approach 2:
The overload structure acts as an intermediary element between the external loads and the sensor structure. It absorbs and dissipates excessive energy through controlled deformation, preventing direct transmission of damaging forces to the strain gauges while allowing normal operational loads to pass through to the sensor structure.
2Measurement precision
If silicon-based strain gauges are used to detect small deflections, then measurement sensitivity is improved, but temperature sensitivity and hysteresis increase causing sensor drift
Solution Approach 1:
The overload structure is designed as a sacrificial, disposable element that protects the expensive and sensitive strain gauges. By providing a controlled failure path in the overload structure, the strain gauges are protected from damage and excessive stress that would cause drift, extending their operational life and measurement stability.
3Strength
If thin beams are used to maintain rigidity under load, then beam rigidity is improved, but sensor durability is reduced due to increased stress concentration
Solution Approach 1:
The beam is divided into functional segments: a thin sensor structure portion optimized for rigidity and measurement, and a thicker overload structure portion optimized for durability and load absorption. This allows each segment to be optimized for its specific function without compromise.
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 overload structures effectively inhibit excessive deflection, preventing damage to strain gauges and reducing sensor drift, while enabling the use of thinner beams and metal-foil strain gauges for improved durability and accuracy in force/torque measurements.
Implementation Method 1
overload structures with wire-cut electrical discharge machining
Data Source
AI summary
A force/torque sensor includes an outer body structure, an inner body structure, and three sensor structure extending between the outer structure and the inner structure. Overload structures are positioned between the sensor structures. An overload structure includes an outer portion that has a plurality of outer facets, and an inner portion that has a plurality of corresponding opposing inner facets. The outer facets and the inner facets are configured to permit a limited predetermined amount of relative motion between the inner body structure and the outer body structure in at least six degrees of freedom from a neutral position, and to inhibit relative motion between the inner body structure and the outer body structure beyond the predetermined amount. An overload structure may be formed by separating a bridge using wire-cut electrical discharge machining, optionally performed in a single pass.