Measuring and testing – Specimen stress or strain – or testing by stress or strain... – By loading of specimen
Reexamination Certificate
2000-09-19
2002-03-05
Noori, Max (Department: 2855)
Measuring and testing
Specimen stress or strain, or testing by stress or strain...
By loading of specimen
Reexamination Certificate
active
06351998
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a method of designing load cells with unmodified vehicle components as the flexible member of the load cell.
BACKGROUND OF THE INVENTION
Load cells are made by gluing strain gages to a flexible member and wiring the strain gages into a Wheatstone bridge. When the member is loaded and flexes, the gage resistance changes and the Wheatstone bridge output changes. When this output correlates well to the primary load, the flexible member becomes a load cell.
The traditional method used for collecting vehicle loads is to cut and re-fabricate the vehicle component so an off-the-shelf load cell can be installed. In some cases an instrumented bolt can be used to collect loads through a joint. If a component is very simple in shape, sometimes it can be made into a load cell using traditional “mechanics of materials” knowledge or experience to determine a strain gage location and orientation.
Cutting and re-fabricating a vehicle component changes the stiffness of the vehicle in the area around the load cell, causing the collected data to be from a non-representative cobbled vehicle. This change in vehicle stiffness will change the loads and frequency content seen at the load cell. Likewise an instrumented bolt changes the stiffness of the joint through which the load of interest is reacted. Again, changing the stiffness changes the load. Using traditional mechanics to design a load cell can be a time consuming trial and error process. It also will only result in an optimal load cell if the designer is very lucky.
Software is available from several companies to design standard load cells. However, no software exists that can design a load cell out of a vehicle component based upon a Finite Element Analysis model.
What is needed is a process that can design a load cell out of a vehicle component. By making load cells directly out of vehicle components, the integrity of the component and surrounding hardware is not changed or compromised, which is ideal for the testing of vehicles put into customers hands, barrier vehicles, and data acquisition vehicles.
SUMMARY OF THE INVENTION
The present invention is a process for designing load cells with articles, as for example unmodified motor vehicle components, as the flexible member of the load cell. The present invention is implemented as a computer program utilizing vehicle components as flexible members that are virtually built and loaded using Finite Element Modeling simulation. The resulting output can be used to determine the best location to lay the strain gages on the vehicle component based upon their corresponding Wheatstone bridge output. This information can be used to determine the quality of a load cell that a certain flexible member will make before any strain gages are actually glued to the article.
The present invention described herein involves loading a Finite Element Model of the article desired to be used for a load cell, and using the model's output therefrom to design the load cell. This process assumes that a strain gage will be laid at an individual element location at a specific orientation relative to the principal strain angle by which the strain gage sees the principal strain in the given element. In this process, combining the strain gage output from multiple elements in the model makes a mathematical transducer. The correct set of elements are chosen by searching every possible combination of elements in the model and determining their Wheatstone bridge output for a given input load. Elements are selected which produce high output with respect to the primary load of interest and low output due to off axis (cross-talk) loads. The element combinations are then listed in order of decreasing sensitivity within a user specified cross-talk tolerance, and displayed graphically to the user. The user may select the element combinations that look the most promising and get additional information on them. Information available includes a Mohr's circle diagram graphically depicting the different loadcases and an indication of how sensitive the location is to strain gage placement. Once a location is chosen, the elements are indicated on a picture of the model, which is printed out to assist in actual strain gage placement. The gages are laid and the load cell is completed and calibrated.
Accordingly, it is an object of the present invention to provide a method for utilizing vehicle components as flexible members that are built and loaded mathematically such that the output can be used to determine the best location, and orientation to lay the strain gages on the vehicle component based upon their corresponding Wheatstone bridge output.
REFERENCES:
patent: 4567774 (1986-02-01), Manahan et al.
patent: 5492002 (1996-02-01), Higgins et al.
patent: 5569857 (1996-10-01), Miyazaki
Davis John D
Hohnstadt Frederick P.
General Motors Corporation
Hargitt Laura C.
Noori Max
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