Differential Screw Force Applicator for In-Situ Load Cell Calibration
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Solution Overview
Problem
Force measuring transducers require periodic calibration, but existing methods often introduce errors due to the limitations of testing machines, particularly in systems where dynamic force capabilities exceed static forces, or when in-situ calibration is not feasible, and current coupling and fixturing technologies fail to meet high accuracy standards.
Innovation Solution
A precision force applicator assembly with a differential screw assembly and a coupling assembly that includes a retainer assembly with three-dimensional curved surfaces, allowing for accurate and repeatable force application to load cells, enabling in-situ calibration without relying on the testing machine's reaction structure.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If the testing machine is used as the force applicator for calibration, then the calibration can be performed using existing equipment, but the fixturing introduces unacceptable error and the dynamic force capability cannot be maintained for the required duration
Solution Approach 1:
The force applicator assembly is divided into separate components: a stationary member secured to the fixed structure, a moving member with the load cell, and a differential screw assembly connecting them. This segmentation allows independent optimization of each component for precision force application while isolated from the testing machine's reaction structure errors.
Solution Approach 2:
The differential screw assembly acts as an intermediary mechanism between the stationary and moving members. It provides precise control over the force applied to the load cell through differential thread pitches, enabling accurate calibration forces independent of the testing machine's dynamic force variations.
2Adaptability or versatility
If in-situ calibration is performed using current coupling and fixturing technologies, then the calibration can be done on the installed transducer, but the accuracy does not meet the latest high accuracy standards
Solution Approach 1:
The coupling assembly features locally optimized three-dimensional curved surfaces (spherical or ellipsoidal) at the contact points between the moving member and load cell. These specialized local geometries ensure precise force transmission along the intended axis while eliminating moment errors, achieving the high accuracy required for modern calibration standards.
Solution Approach 2:
The coupling assembly uses spherical or ellipsoidal curved surfaces instead of flat or cylindrical contact surfaces. This curvature allows for self-alignment and ensures that forces are transmitted purely axially through the load cell during calibration, eliminating bending moments and improving measurement accuracy.
3Force
If external loads are applied using existing methods, then force can be applied to the transducer, but repeatable results with high accuracy cannot be achieved
Solution Approach 1:
The differential screw assembly provides dynamic control over the force application through rotation of the rotatable member. By adjusting the rotation amount, precise incremental forces can be applied and controlled, enabling repeatable calibration at multiple force levels while maintaining high accuracy throughout the calibration range.
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 solution provides high accuracy and repeatability in force application, allowing for precise calibration of load cells in-situ, meeting international standards and eliminating errors associated with existing methods.
Implementation Method 1
a differential screw assembly connecting the moving member to the stationary member
Implementation Method 2
The differential screw assembly can include a rotatable member threadably connected to the moving member with a first set of threads having a first thread pitch. The rotatable member is also threadably connected to the stationary member with a second set of threads having a second thread pitch, the second thread pitch being different than the first thread pitch.
Data Source
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AI summary
A force applicator assembly (10; 10') is disclosed to calibrate an in-situ force transducer (or load cell) (104) in a force (load) applying test machine (125). The force applicator (10) includes stationary member (22; 22') configured to be secured to fixed structure, a moving member (20; 20'), a load cell (12) operably coupled to an end of the moving member (20; 20'); and a differential screw assembly (24; 24") connecting the moving member (20; 20') to the stationary member (22; 22'). A coupling assembly (14) can be used to assure that only tension or compression loads are applied. The coupling assembly (14) can be configured if desired such that no tension or compression loads can be applied. A method to calibrate an in-situ force transducer (104) in a force applying test machine (205) is also provided and uses a force generator (16; 16'; 125) and the coupling assembly (14).