Measuring and testing – Dynamometers – Responsive to force
Patent
1997-02-28
1998-10-27
Dougherty, Elizabeth L.
Measuring and testing
Dynamometers
Responsive to force
73862632, G02L 504
Patent
active
058279814
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to devices for measuring force.
2. Summary of the Prior Art
In many industrial applications it is commonplace to measure force by applying the force to an elastic member, and measuring the deflection, or strain, of that member. It is normally required to keep the sensing member as compact as possible, and particularly for loads of the order of tonnes, the conventional solution is to measure the local strains at the locations of the maximum stresses caused by the applied load. Typically this is achieved by electrical resistance strain gauges applied to a cylinder in compression, or to a block in shear. The design and fitting of such strain gauges is a matter of compromise: for example, if the adhesive securing them is too flexible, they are inaccurate, and if it is too rigid, they may fall off. The electrical power which can be dissipated in them is limited, and to achieve an accuracy of 0.02%, as is commonly required, signal levels of the order of microvolts have to be measured. This is not easy in an industrial situation, where severe interference can be caused by heavy fluctuating electrical loads in the vicinity. Additionally, in order to achieve maximum accuracy and sensitivity, the maximum strain in the elastic members has to be as high as possible; this introduces problems of creep under a sustained force, and hysteresis in response to a varying force.
An alternative to the use of the elastic member discussed above, in which relatively small strains are measured by strain gauges, is the proving ring: a torus or "anchor ring" to which the force to be determined is applied across a diameter, and the deflection is measured across this same diameter, or the diameter at right angles to it. For a given force, a proving ring can be designed to yield almost as large a deflection as is desired, but it suffers from the disadvantage that the combination of tensile, shear, and bending stresses due to the shape causes it to be physically large, making it awkward to use and also more expensive than alternative devices.
EP-A-0003685 and DE-A-3725535 each disclose an arrangement involving a proving ring. A ring has clamps at opposite ends of a diameter to which forces are applied. In DE-A-3725535, the deformation of the ring (which is circular) is measured in the direction of application of the forces. In EP-A-0003635 the deformation of the ring is measured in a direction perpendicular to the direction of application of the forces.
U.S. Pat. No. 5115680 discloses another apparatus for sensing force, in which two elongate members are mounted between a pair of blocks. One of the blocks is end fixed and the force to be sensed is applied to one end of a spring, the other end of the spring being fixed relative to the first block. The deflection of the spring permits the other block to move, thereby deforming the elongate members. The deformation is measured to measure the force.
SUMMARY OF THE INVENTION
The present invention seeks to provide an elastic member capable, within a compact size, of producing a deflection under the action of an applied force much larger than is normally measured by strain gauges, so that an alternative linear displacement transducer can be employed.
Accordingly, a first aspect of the present invention provides a force sensing device comprising a ring composed at least partially of elastically deformable material, two connection portions located at respective ends of a diameter of said ring, and measurement means for determining the distortion of said ring when a force is applied between said connection portions of the device, the ring in its rest configuration being of greater dimension in a first direction along said diameter than in a second direction perpendicular thereto in the plane of the ring; connection portions and at least two elongate members extending between the two intermediate portions and being integral with the intermediate portions, the widths of the two elongate members increasing gradual
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Adrian March Limited
Dougherty Elizabeth L.
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