Load cell with reduced sensitivity to thermal shock

Details Load cell with reduced sensitivity to thermal shock Load cell with reduced sensitivity to thermal shock

2000-10-30

2002-10-29

Noori, Max (Department: 2855)

Measuring and testing

Dynamometers

Responsive to force

C073S795000

Reexamination Certificate

active

06470759

ABSTRACT:

CROSS-REFERENCES TO RELATED APPLICATIONS (IF ANY)
None
STATEMENT AS TO RIGHTS TO INVENTIONS MADE UNDER FEDERALLY-SPONSORED RESEARCH AND DEVELOPMENT (IF ANY)
None
BACKGROUND
1. Field of the Invention
This invention relates to load cells and more particularly to a load cell that provides more strain at lower weights and has reduced sensitivity to thermal shock.
2. Description of Prior Art
A load cell used to measure applied loads often uses resistance strain gauges wired into a Wheatstone bridge on the inside of the gage-holes. These strain gauges measure the strain placed on the load cell by changing resistance in response to the change in strain by an applied load. This resistance is measured. This measured resistance in then converted into a weight figure.
There is a desire and need to achieve lower weighing capacities in a larger load cell package size. This gives a company the ability to stretch a standard package size downward in capacity, while using the same barstock, processes, methods, etc., for the larger package size. Using the same package size for more capacities gives the company more buying power of barstock (buying more of one size) and reduces the amount of new processes that are needed. It is cheaper to have the ability to minimize the number of differences in a product line.
To create the ability of a load cell to read at lower weight capacities, material is removed from the load cell to increase the strain on the load cell to allow for accurate readings by the strain gauges.
The current art is to remove the material from the exterior of the load cell. This sculpting requires an elaborate machining operation and renders the cell more sensitive to thermal transients. The cross sectional area of the remaining material becomes very small at low capacities and this small cross sectional area is a poor heat conduit, creating a large temperature gradient across the sculpted region. The poor heat conduction also causes this gradient to last for a longer time than it would if the cross sectional area were larger.
Milling is a cheap process and a preferred method to be used, but the current art weakens the outside of the cell; this has the load cell so weakened that it is too flimsy for milling so an expensive EDM operation must be used. Some additional fixturing may be used to stiffen the cell enough for milling, but the cost of production goes up.
The current art also makes it difficult to create lower weight capacity hermetically sealed load cells. A round through-hole for the load cell is convenient, as it allows easy gauging and an inexpensive round cover for hermetic sealing. To achieve a 250 lb capacity, the current art abandons the round hole, as there isn't enough material left on the cell's exterior to remove. The 250 lb cell uses an elliptical hole with potting for environmental protection. Part of the reason for the potting is that it's difficult to obtain good performance for a round hermetic cover in a 250 lb cell, but another reason is that an elliptical cover is more expensive. It's harder to make an elliptical cover and it's more expensive to weld the elliptical shape.
The addition of the ability to achieve lower weighing capacities for a load cell while reducing sensitivity to thermal shock and the need for hermetic sealing leaves room for improvement within the art.
1. Field of the Invention
U.S. Class 177/211
2. Description of Related Art Including Information Disclosed Under 37 CFR §1.97** > and 1.98<.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a means to achieve lower weighing capacities in a relatively large load cell barstock size while reducing the load cell's sensitivity to thermal shock.
The inventor of the present invention has increased the measurable strain on a load cell package increasing its ability to accurately measure lower weights while reducing the load cell's sensitivity to thermal shock. According to one aspect of the present invention, it has been discovered that the removal of material from the interior of the load cell produces the ability of the load cell to accurately measure strains at lower weights and applied loads while reducing the sensitivity of the load cell to thermal shock.
The current invention produces a stiffer load cell than the current methods with thick, stiff sections next to thin sections. The thin sections reduce the overall stiffness enough to get the strain levels that are desired, but the thick section have enough moments of inertia to keep the cell from becoming flimsy. This allows the load cell to be milled, which is a less expensive process than those currently being used.
The present invention is less sensitive to thermal shock. This is due to the fact the material under the strain gauges is thicker than the current art. The current art has the strain gauges mounted on a section of reduced stiffness by design, as the strains are higher in reduced sections. However, a reduced section conducts heat poorly, causing a large temperature difference between the strain gauges which lasts for a long time. The present invention has gauges mounted on a thicker section which conducts heat better, causing a smaller temperature difference which lasts for a shorter time.
The size and depth of the grooves produce the desired overall stiffness to get the strain levels while counterbalancing the need to reduce thermal sensitivity.
The current invention allows for the lower weight measurement hermetically sealed load cells. It allows for the maintaining of a round gage-hole for low-cost strain gauging and for low-cost hermetic sealing.
By removing material from the inside of the load cell within the gage-hole, the Inventor has added the ability to achieve lower weighing capacities for a load cell while reducing sensitivity to thermal shock and allowing for hermetically sealing all at a reduced cost.


REFERENCES:
patent: 4064744 (1977-12-01), Kistler
patent: 4125168 (1978-11-01), Ormond
patent: 4520679 (1985-06-01), Hatamura
patent: 4546838 (1985-10-01), Ormond
patent: 4558756 (1985-12-01), Seed
patent: 4785673 (1988-11-01), Aumard
patent: 6122846 (2000-09-01), Gray et al.

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