Measuring and testing – Dynamometers – Responsive to force
Reexamination Certificate
1999-12-22
2002-04-09
Fuller, Benjamin R. (Department: 2855)
Measuring and testing
Dynamometers
Responsive to force
Reexamination Certificate
active
06367338
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a force-measuring apparatus, particularly a weighing cell, with a stationary part and a load receiver that serves to receive the force to be measured. The load receiver is coupled to the stationary part so that, in response to the force, the load receiver is movable in relation to the stationary part. The force-measuring apparatus includes a lever that is connected to the load receiver through a first connector element and to the stationary part through a second connector element. The first and second connector elements are located at a distance from each other in the lengthwise direction of the lever and are part of an arrangement that allows the lever ratio to be changed. The second connector element forms the fulcrum axis for the rotation of the lever as the latter transmits the displacement of the load receiver to a lever output area located at a distance from the first and second connector elements in the lengthwise direction of the lever.
Force-measuring apparatus of this kind are described, e.g., in the Swiss patent CH 591 684 and incorporate the concept that, in many cases, a base model type of a weighing cell is followed by one or more derivative models that differ from the base model in regard to the weighing range or the measuring resolution. According to the aforementioned patent, the number of components required to manufacture weighing cells of this kind can be reduced by changing the arm ratio of the lever that serves to transmit the displacement motion of the load receiver to a compensation coil immersed in the gap of a permanent magnet assembly that is attached to the stationary part. As proposed in that patent, this can be accomplished by connecting a one-armed lever to the stationary part through two tensile flexures so that the lever is rotatable about a fulcrum axis formed by the tensile flexures in an arrangement where the lever arm length between the fulcrum axis and an attachment area for connecting the lever to the load receiver can be changed by means of spacer blocks. The spacer blocks can be arranged on the one hand between the lever and the tensile flexures and on the other hand between the stationary part and the tensile flexures. It is further proposed in the aforementioned patent that appropriate spacer blocks be inserted also at the respective attachment areas of the load receiver and/or the lever where a tensile flexible coupling element is attached that connects the load receiver with the lever. A similar weighing cell, in which the concept of changing lever ratios with spacer blocks is applied to a two-armed lever, is described in the Swiss patent 591 685.
The weighing range of these known load cells can be selected from at least two possible values by changing the position of the spacer blocks and thereby changing the lever ratio between the displacement of the load receiver and the displacement of the compensation coil.
However, with the known weighing cells, it has been found that changing the positions of the spacer blocks in order to change the weighing range is generally a time-consuming process and can also be harmful to the measuring accuracy.
SUMMARY OF THE INVENTION
In view of the aforementioned problems inherent in state-of-the-art devices, it is therefore the object of the present invention to provide a force-measuring apparatus that allows the measuring range to be changed quickly and reliably.
According to the invention, this goal is accomplished through a design advancement of the known force-measuring apparatus. The essential characteristic feature of the inventive design is that the lever has at least three attachment areas for fastening one of the first and second connector elements, and that the attachment areas are set apart at a distance from each other in the lengthwise direction of the lever.
The invention is based on the observation that the problems with the state-of-the-art solutions mentioned above are essentially caused by the way the spacer blocks are handled for changing the measuring range. Namely, in a first step, the tensile and pivotal flexures have to be disconnected from the lever, the load receiver, and the stationary part. Next, the spacer blocks are removed from their original positions and arranged in their new positions. In a final step, in which the tensile and pivotal flexures are reattached, the spacer blocks have to be kept precisely fixed in their new positions. This complicates the handling of the tensile and pivotal flexures in the assembly process because of the additional time required to change the measuring range. In addition, it is possible for the spacer blocks to be dislodged from their intended new positions, which is detrimental to the measuring accuracy.
The force-measuring apparatus with the inventive design advancements is free of the problems associated with the handling of the spacer blocks, because the lever itself has at least three attachment areas for fastening a connector element, which are set apart at a distance from each other in the lengthwise direction of the lever. In each measuring range, only two of the attachment areas are used, so that the measuring range can be set or changed by simply selecting or changing the attachment areas used for fastening the connector elements without the need for the time-consuming and error-prone process of handling spacer blocks. As a result, the time required to change measuring ranges is shortened, while at the same time the functional reliability of the force-measuring apparatus is increased.
In addition, from a manufacturing point of view, the force-measuring apparatus according to the invention has the advantage that one and the same lever component can be used to manufacture balances with different weighing ranges by fastening the connector elements to different respective attachment areas. This manufacturing advantage is also particularly useful even in the case where no change in measuring range has to be made during the actual operation of the force-measuring apparatus.
The process of changing measuring ranges of a force-measuring apparatus in accordance with the invention can be further simplified through an arrangement where at least two attachment areas are set apart at a distance from each other in the transverse direction of the lever, because this allows a connector element to be fastened to one of the attachment areas without spatial interference by a connector element already fastened to the other attachment area.
The fulcrum axis formed by the second connector element can be defined with particular accuracy while, at the same time, the lever is stabilized against tilting in relation to its longitudinal axis, if at least one of the attachment areas for the second connector element has at least two attachment area segments at a distance from each other along the fulcrum axis. In this arrangement, the second connector element can be, for example, a pair of tensile pivotal flexures.
The process of changing measuring ranges of a force-measuring apparatus in accordance with the invention can be further simplified through an arrangement where at least one attachment area comprises an attachment surface approximately perpendicular to the longitudinal axis of the lever and located at a frontal end surface of the lever at the opposite end from the lever output area, because this arrangement allows a connector element to be fastened to the attachment surface without spatial interference by a measuring transducer, e.g., an electromagnetic force-compensation system, which is normally arranged at the output area of the lever. In particular, the inventive concept includes the possibility that the aforementioned attachment surface consists of a plurality of attachment surface segments set apart at a distance from each other in the direction of the fulcrum axis. The inventive concept further includes an embodiment where the attachment surface for the first connector element is arranged parallel to the fulcrum axis between two attachment area segments for the second connector
Baltisberger Stephan
Burkhard Hans-Rudolf
de Waal Hans
Hess Luzi
Reber Daniel
Fuller Benjamin R.
Kueffner Friedrich
Mettler-Toledo GmbH
Thompson Jewel V.
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