Weighing scales – Self-positioning – Electrical current generating or modifying
Reexamination Certificate
1999-09-08
2001-12-04
Gibson, Randy W. (Department: 2859)
Weighing scales
Self-positioning
Electrical current generating or modifying
C177S229000, C073S862639, C073S001150
Reexamination Certificate
active
06326562
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention concerns a force-measuring apparatus, particularly a weighing cell, with a load receiver for receiving the force that is to be measured and a force-transmitting device for transmitting at least a partial amount of the force to be measured from the load receiver to a measuring transducer that delivers a signal corresponding to the force to be measured. The load receiver is guided in planar translatory motion in relation to a stationary part of the force-measuring apparatus by two parallelogram guides (guide links) that extend in two mutually parallel planes, are rigid with regard to deformation within their respective planes and have elastic flexibility in the transverse direction of the planes. Each of the two parallelogram guides is connected at one end (with respect to its lengthwise direction) to the load receiver and at the opposite end to the stationary part of the force-measuring apparatus. A parallelogram plane is defined by the lengthwise direction of the guides and the path of motion traveled by the load receiver. The force-transmitting device has at least one force-transmitting lever that receives its input force through a coupling from the load receiver and is rotatable in relation to a fulcrum axis that is fixed on a support portion of the stationary part extending between the two parallelogram guides in a plane that is parallel to the common plane of the parallelogram guides.
2. Description of the Related Art
It is a known design concept for weighing cells of this kind to be assembled from numerous individual components that need to be either rigidly attached to or movably pivoted at the stationary part. In particular, the pivot points of the parallelogram guides and the lever are located on the stationary part. Positional changes of these pivot points that are caused by the force to be measured will impair the measuring accuracy. Therefore, the stationary part needs to have a particularly high degree of structural rigidity.
It is also a known concept (DE 41 19 734 A1) to configure the stationary part, the parallelogram guides, the load receiver and the lever as a continuum of interconnected material portions of a monolithic material block, whereby in particular the assembly process of the corresponding separate components is eliminated. However, the separation of the individual material portions from the material block, e.g., by the method of spark erosion, represents a relatively exacting procedure, particularly in the case where the apparatus will have to meet a high level of measuring accuracy and, therefore, the thinned-down material domains by which the material portions are movably connected have to be formed with a commensurate degree of precision.
SUMMARY OF THE INVENTION
The object of the present invention is to provide a force-measuring apparatus of the kind described at the beginning, composed of only a small number of simple parts and offering a satisfactory level of measuring accuracy.
According to the invention, this problem is solved by configuring the stationary part in the shape of a base plate that forms the support portion. Two side plates, perpendicular to the plane of the base plate, extend along the borders of the base plate that run in the lengthwise direction of the parallelogram guides. The parallelogram guides at their stationary ends (i.e., the far ends from the load receiver) are anchored on the side plates.
The concept of a base plate and transversely extending side plates results in an exceptional degree of rigidity in the stationary part. At the same time, this shape of the stationary part offers a simple way of anchoring the ends of the parallelogram guides. The latter extend on both sides of and parallel to the base plate from their stationary ends (where they are anchored to the base plate) to their opposite, movable ends (where they are connected to the load receiver). Due to their elastic flexibility, they form a parallelogram linkage to guide the load receiver. The parallelogram plane (i.e., the kinematic plane of the motion of the load receiver) is perpendicular to the planes of the parallelogram guides and of the base plate and runs in the lengthwise (end-to-end) direction of the parallelogram guides. Given that the stationary part has a high resistance to deformation, the corner points of the parallelogram linkage will not be displaced under a load, which is beneficial for the measuring accuracy. At the same time, the stationary part has a simple shape so that its manufacture is relatively uncomplicated.
In an advantageous embodiment of particular simplicity, the base plate and the associated side plates are formed as a section of an integral, monolithic H-profile in which the transverse web segment of the H-profile represents the base plate and the two parallel segments of the H-profile represent the side plates. An H-profile of this kind may be produced by a simple process, e.g., as an extruded profile or a pressure casting.
Further in the interest of a simple design configuration, the side plates in a practical embodiment have border surface areas parallel to the planes of the parallelogram guides where the stationary ends (i.e., the far ends from the load receiver) of the parallelogram guides are attached. This is also particularly advantageous if, as in most cases, the bending flexibility of the parallelogram guides is realized by means of flexural pivots (flexures, for short) that form the ends of the parallelogram guides and have a virtual pivotal axis located between two flat attachment terminals. In this case, the parallelogram guides each have two flexures at the ends where the parallelogram guides are attached to the stationary part, with the attachment terminals extending parallel to the plane of the parallelogram guide. The attachment terminals on the side of the stationary part lie flat against and are attached to the parallel border surfaces of the side plates. The attachment terminals of the same flexures but on the opposite side of the virtual pivotal axis are attached to a rigid, plate-shaped portion of the parallelogram guide.
It is also within the scope of the invention that at least one of the side plates has a slit extending at least through the portion next to the place where one of the parallelogram guides is attached, the width of the slit being adjustable in the direction transverse to the plane of the parallelogram guides. By adjusting the width of the slit, e.g., by means of an adjustment screw acting on the slit, it is possible to vary the distance between the parallelogram corner next to the respective point of attachment and its neighboring corner in the transverse direction to the plane of the parallelogram guides. This allows the corrective adjustment of so-called corner-load errors that occur in particular if the force to be measured is introduced asymmetrically into the load receiver, i.e., not centered along a symmetry axis extending in the direction of the displacement.
In a further practical embodiment of the invention, the lever is rotatably constrained on the stationary part by at least one flexure with two attachment terminals and a virtual pivotal axis located between them, where one attachment terminal is fastened to the stationary part, the other attachment terminal is fastened to the lever, and the virtual pivotal axis constitutes the lever fulcrum. In this arrangement, the flexure constitutes an immediate rotatable constraint of the lever to the stationary part where, due to the shape of the stationary part, a suitable attachment surface can be made available without a problem.
This fulcrum constraint of the lever is configured to particular advantage in an embodiment of the invention where the two side plates of the stationary part each have a frontal attachment area extending in a plane perpendicular to the lengthwise direction of the parallelogram guides, with the fixed, stationary end of a flexure fastened to each attachment area, and where the lever has a pivotal portion extending between and faste
Burkhard Hans-Rudolf
Emery Jean-Christophe
Hess Luzi
Kaderli Erwin
Köppel Thomas
Gibson Randy W.
Laubscher Lawrence E.
Mettler-Toledo GmbH
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