Weighing scales – Self-positioning – Electrical current generating or modifying
Patent
1986-02-11
1988-05-17
Miller, Jr., George H.
Weighing scales
Self-positioning
Electrical current generating or modifying
177DIG1, G01G 314
Patent
active
047444290
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
This invention relates to apparatus for the measurement of forces, and has particular application to weighing apparatus. Although primarily developed for the weighing of large and awkward loads, such as livestock and large industrial loads, the invention will have applicability in other areas as well, and indeed a feature of the invention is its applicability in a wide variety of load measuring applications.
The invention is particularly concerned with techniques for the application of forces to bending beams, so that forces may be measured by the use of strain measuring devices
BACKGROUND ART
U.S. Pat. No. 2,899,191 discloses a weighing scale in which a weighing platform is supported on a pair of beams. The beams are supported adjacent their outer ends on knife edges, and the platform is supported on the beams by journals comprising pins mounted in lugs, the pins being spaced inwardly equal distances from the knife edges. A strain gauge is mounted at the centre of each beam.
Such an arrangement has the advantage that, in principle, the bending moment at the centre of the beam will be proportional to the sum of the forces applied to the journal pins, so that measuring the strain at the centre of each beam will provide a measure of the total load on the platform.
An arrangement such as that described in the Hunt U.S. patent, is found to suffer from serious disadvantages as a result of the manner of support and of application of load to the beams on which the strain gauges are mounted. Thus, for example, any misalignments, and slope or uneveness in the supporting surface will be transmitted as twisting or bending forces to the beam, and significant inaccuracy may result.
In U.S. patent specification No. 4,287,958 another approach to load summation using a similar configuration of forces and strain gauge is disclosed, in this case in the form of a load measuring vehicle suspension. In this specification the load receiving journals are located so that their axes intersect the beam axis, and are provided by elastomeric bushings on stub shafts which extend on either side of the beam at each of the load application points. As the journals are located outwardly of the beam, however, the arrangement is prone to the production of twisting forces which will interfere with accurate measurement. Furthermore, the manner in which the ends of the beam in this disclosure are mounted, encourages the transmission to the beam of torsional forces, as well as spurious bending moments, from these mountings wherever misalignment occurs.
DISCLOSURE OF INVENTION
Although the prior art devices discussed above approach the problem of load measurement in a way which has an elegant mathematical basis which is employed in the preferred embodiments of the present invention, the realisation of this approach in those prior art devices has involved structures which allow considerable inaccuracy to arise from misalignment of the apparatus. Thus, for example, the structures disclosed in the U.S. patents discussed above, will produce considerable inaccuracies where loads are applied to the beam in such a way as to produce twisting effects, or where the beam is supported so as to produce twisting, or misalignment of the applied loads.
The prior art approach has involved emphasis on the maintenance of those dimensions in the geometry of the apparatus which mathematically determine the operation of the equipment, such as the distance between points of application of forces. The present invention however, is based on the recognition that in the use of strain determination for load measurement, errors caused by spurious strain arising from forces which result from misalignment of the apparatus or the angular application of load forces, may be much greater than errors which may result from dimensional inaccuracies or dimensional changes, and therefore that a system which reduces such spurious strain at the cost of dimensional accuracy may give more accurate load measurement in practical conditions of use.
In one aspect the inv
REFERENCES:
patent: 4078623 (1978-03-01), Ohta et al.
patent: 4315554 (1982-02-01), Williamson
patent: 4611678 (1986-09-01), Andriewsky
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