Housingless load cell

The invention relates to a housingless load cell whose resistance and measuring accuracy can be improved with respect to environmental influences such that at least the predominant part of the surface (12) and/or the surface (14) has a transversal path, such that water hitting the body of the load cell (1) can flow off when the load cell is in a built-in state. The angle α1 between the transversally extending part of the surface (12) and the perpendicular peripheral surface of the body (3) and/or the angle β2 between the transversally extending part of the surface (14) and the peripheral surface of the leg (4) is respectively greater than 90° and is, more particularly between 93°- 120°. A housingless load cell comprising a compensation electronics system is provided in one of the transversal holes (7). Finally, a load cell is further developed such that the peripheral surface of the leg (4) comprises at least one essentially perpendicular groove (24) forming a rotational lock piston with a protruding element on the lower plate.

The invention relates to a non-encased weighing cell with a weighing-cell body, comprising a head, a trunk and a foot, wherein the head is set apart from the trunk by at least one top recess, which extends in a rotationally symmetrical manner around the longitudinal axis of the weighing-cell body, wherein the recess comprises a surface that tapers off towards the trunk and a surface that tapers off towards the head; the trunk is essentially cylindrical in shape, and the trunk is set apart from the foot by at least one bottom recess that extends in a rotationally symmetrical manner around the longitudinal axis of the weighing-cell body, wherein the recess comprises a surface that tapers off towards the foot and a surface that tapers off towards the trunk.

As a rule, weighing cells, for example pendulum weighing cells for use in weighing machines for road vehicles, comprise a measuring element to which a wire strain gauge has been glued, which measuring element hereinafter is referred to as a weighing-cell body.

The weighing-cell body is made of a high-strength metal material, as a rule of steel; it comprises a head, a foot, and a trunk arranged in between. In a suitable position on the surface of the weighing-cell body, usually on the essentially cylindrical trunk, wire strain gauges are arranged. These wire strain gauges, as a rule Konstantan or Karma wire strain gauges, are usually electrically wired as a Wheatstone full bridge.

To protect the wire strain gauges from water, humidity or dirt, a separate housing is arranged around the weighing-cell body. Furthermore, a balancing chamber has been affixed to this housing, which balancing chamber comprises electronics for electrically balancing the weighing cell, for example for determining the zero point, the characteristic value, the temperature behaviour etc., and if necessary a signal processing device. This balancing chamber is also used for protecting the electronic components from humidity and dirt.

Furthermore, weighing cells are known in which in the trunk, on an axis perpendicular to the longitudinal axis of the weighing-cell body, a transverse through-hole is provided, in whose centre a membrane has subsequently been installed, onto which membrane the wire strain gauges are glued. Instead of providing one through-hole and a separate membrane, it is also known to provide two transverse holes that are positioned on the same axis, wherein the depth of said transverse holes is less than the radius of the trunk, and wherein said transverse holes form a web between themselves, which web assumes the function of the membrane, and onto which web the wire strain gauges are glued.

In order to achieve a targeted load introduction from the end surfaces of the head and of the foot into the trunk, in particular in the direction of a web or of a membrane, recesses that are aligned so as to be rotationally around symmetrical the longitudinal axis of the weighing-cell body are provided, through which recesses the head is set apart from the trunk, and the trunk is set apart from the foot.

Actually, in those cases where the wire strain gauges are arranged on a web in the interior of the trunk said wire strain gauges are better protected when compared to those cases where they are arranged on the perpendicular circumferential surface of the trunk so that such a weighing cell can be used even without a separate housing that encloses the entire weighing cell, which results in a considerable cost reduction. However, it is still possible for humidity and dirt to collect in various positions on the weighing-cell body, including in the transverse holes, and in this way damage the weighing-cell body, for example through corrosion. In particular, in the region of the rotationally symmetrical recesses that set the trunk apart from the head or the foot, moisture can collect.

Furthermore, even if no separate housing that encloses the entire weighing cell is provided, a balancing chamber to accommodate the balancing electronics is still required.

The known weighing cell is associated with a further disadvantage in that it can be subject to torsion during operation, which results in measuring errors.

Starting with the problems shown above, it is the object of the present invention to create a non-encased weighing cell which is particularly resistant to environmental influences, and which features very good measuring accuracy.

According to the invention the above-derived and shown object is first of all met in that in a non-encased weighing cell with the characteristics of the precharacterising part of claim1at least the predominant part of the surface of the top recess, which surface tapers off towards the trunk, and/or the predominant part of the surface of the bottom recess, which surface tapers off towards the foot, is inclined such that in the installed state, as intended, of the weighing cell any water hitting the weighing-cell body can flow away, wherein the angle α1between the inclined part of the surface of the top recess, which surface tapers off towards the trunk, and the perpendicular circumferential surface of the trunk and/or the angle β2between the inclined part of the surface of the bottom recess, which surface tapers off towards the foot, and the circumferential surface of the foot is larger than 90°, and in particular is between 93° and 120°. A value of 117.5° has been shown to be particularly suitable.

In this way a situation can be achieved in which in the region of the recesses almost no plane surfaces perpendicular to the longitudinal axis of the weighing-cell body are present any longer, on which surfaces moisture and dirt can accumulate. It is quite possible for a narrow annular region in the end region of the surface tapering off towards the trunk or towards the head to be present, which annular region is not inclined but extends so as to be perpendicular in relation to the longitudinal axis of the weighing-cell body, which annular region, however, due to its small size when compared to the much larger inclined part of the surface of the respective recess does not have any effect on the outflow of the water. Thus, in longitudinal section along the longitudinal axis of the weighing-cell body, the surface line of the top recess and/or of the bottom recess in the respective regions tapering off downwards always have an incline which ideally extends right to the edge of the trunk or of the foot. Water reaching this region, or moisture collecting in this region drains in this way, wherein dirt particles are also flushed away. Thus the danger of corrosion in the region of the recesses is clearly reduced.

In addition it can be provided for at least the predominant part of the surface of the top recess, which surface tapers off towards the head, and/or the predominant part of the surface of the bottom recess, which surface tapers off towards the trunk, is inclined, such that in the installed state, as intended, of the weighing cell any water hitting the weighing-cell body can flow away, wherein the angle α2between the surface of the bottom recess, which surface tapers off towards the trunk, and the perpendicular circumferential surface of the trunk and/or the angle β1between the surface of the top recess, which surface tapers off towards the head, and the circumferential surface of the head is larger than 90°, and in particular is between 93° and 120°, wherein in the upper region of the recesses, too, an inclined surface is formed from which surface water can drain off more easily than from a horizontal surface. A value of 117.5° has been shown to be particularly suitable.

If the diameter of the head and/or the diameter of the foot is smaller than the diameter of the trunk, the surface of the top recess, which surface tapers off towards the trunk, can be inclined also in that region that is not covered by the head, and/or the surface of the bottom recess, which surface tapers off towards the trunk, can be inclined also in that region that is not covered by the foot. The designation “covered” refers to the region of the inclined surfaces, which region in a top or bottom view of the weighing-cell body is covered by the head or the foot and is thus not visible.

Advantageously, the longitudinal section of the top recess and/or of the bottom recess has at least in part a shape of the surface line that is curved, in particular shaped in a circular arc, an ellipsis or a parabola. However, other shapes of the surface line are imaginable, provided the gradient of the surfaces in the region of the recess is sufficient for the moisture to drain off optimally.

In a further advantageous embodiment the transition region between the surface of the top recess, which surface tapers off towards the trunk, and the perpendicular circumferential surface of the trunk and/or the transition region between the surface of the bottom recess, which surface tapers off towards the foot, and the circumferential surface of the foot, is inclined or rounded. Accordingly, it is imaginable that in each case the transition region between the surface of the top recess that tapers off towards the head and the circumferential surface of the head is inclined or rounded. In this way edges are prevented and instead a shape is created from which water can drain off well.

Also in the transition region between the interior surface of the transverse holes and the perpendicular circumferential surface of the trunk, edges can be avoided by using inclined or rounded shapes.

According to the invention, the object derived and shown above is met in that in a non-encased weighing cell with the characteristics of the precharacterising part of claim11wire strain gauges are arranged on the web, which wire strain gauges are electrically connected to balancing electronics that are arranged in one of the transverse holes. In this way there is no longer a need to provide a separate balancing chamber to accommodate the balancing electronics, which balancing electronics can comprise a printed circuit board which on one side can comprise a device for temperature compensation, in particular a meandering layer of nickel. In this way the balancing electronics are arranged directly in the region of the wire strain gauges.

Advantageously the wire strain gauges are enclosed by a casting compound, for example a casting compound made of plastic, in particular flexible plastic, so that said wire strain gauges are now fully protected from humidity or dirt in the transverse holes. At the same time the casting compound can serve as an attachment for the printed circuit board in that said printed circuit board is at least partially embedded in the casting compound.

A particularly good measuring result is achieved if on each side of the web at least one wire strain gauge is arranged. In this case, to establish an electrical connection between the wire strain gauges and the balancing electronics a borehole can be provided in the web, through which web the connecting lines lead. Furthermore, the weighing-cell body can comprise a cable bushing through which the signal lines and current supply lines lead from one of the transverse holes to the outside into a weighing-cell cable. Advantageously, this cable bushing is sealed off against humidity and dirt.

Particularly good protection from humidity and dirt is achieved if the transverse holes are covered by covers. Sealing off the covers can take place by means of welding, in particular microplasm welding. In this way optimal sealing action of the covers is achieved. It is also possible, as an alternative or in addition, to glue, screw and/or tighten the covers.

Finally, according to the invention, the previously derived and shown object is met in that in a weighing cell with the characteristics of the precharacterising part of claim23the circumferential surface of the foot comprises at least one groove that is essentially perpendicular, which groove in each instance can interact, in the installed state of the weighing cell, with an element that is provided on the base plate and that projects therefrom. Such a projecting element can for example be a pin, which in particular extends perpendicularly to the longitudinal axis of the weighing-cell body. Also imaginable are several grooves that are circumferentially arranged at even spacing, as well as several corresponding elements in the base plate. In this way a device to prevent torsion of the weighing cell can be created, which device ensures permanent positioning of said weighing cell, thus clearly reducing the risk of measuring errors occurring.

FIGS. 1a) andb) show a non-encased weighing cell for two different load stages, namely inFIG. 1afor a load stage of 50 t and inFIG. 1bfor a load stage of 25 t. In each instance the weighing cell shown comprises a weighing-cell body1, which in turn comprises a head2, a trunk3and a foot4. The head2is set apart from the trunk3by a top recess5, which extends in a rotationally symmetrical manner around the longitudinal axis of the weighing-cell body1. Towards the bottom the trunk3is set apart from the foot4by a corresponding recess6. In its perpendicular circumferential surface the essentially cylindrical trunk3comprises two transverse holes7and8, of which only one transverse hole7is shown in the view selected inFIGS. 1a) andb). In each case the transverse holes are tightly sealed off by a welded-on cover9.

Furthermore, a cable bushing10is provided in the weighing-cell body1, through which cable bushing10the signal lines and current supply lines are led from the front transverse hole7towards the outside in a weighing-cell cable11.

Since the weighing cell depicted does not comprise a separate casing and is thus directly exposed to environmental influences, in particular to water, humidity, dirt etc., there are no flat surfaces extending perpendicularly to the longitudinal axis. It is clearly illustrated that the surface12that tapers off towards the trunk3, and the surface13of the top recess5, which surface13tapers off towards the head2, are inclined so that water or moisture drains off directly and cannot collect. The surface14that tapers off towards the foot4, and the surface15that tapers off towards the trunk3, of the bottom recess6, are designed correspondingly. Similarly, the transition region16between the internal surface of the transverse holes7and8and of the perpendicular circumferential surface of the trunk3is somewhat inclined so that here, too, water can drain off directly.

The principle on which the non-encased weighing cell according to the invention is based is further illustrated inFIG. 2.FIG. 2shows a longitudinal section of the weighing cell fromFIG. 1a). Here, too, the illustration shows that there is no horizontal surface that extends perpendicularly to the longitudinal axis, on which surface the humidity could collect. Thus the two recesses5and6are designed such that the angle α1between the surface12of the top recess5, which surface tapers off towards the trunk3, and the perpendicular circumferential surface of the trunk3exceeds 90°. The same applies to the angle β2between the surface14of the bottom recess6, which surface tapers off towards the foot4, and the circumferential surface of the foot4. Finally, both the angle α2between the surface15of the bottom recess6, which tapers off towards the trunk3, and the perpendicular circumferential surface of the trunk3, as well as the angle β1between the surface13of the top recess5, which surface tapers off towards the head2, and the circumferential surface of the head2are larger than 90°. In the region of the recesses5and6the surface line is approximately parabolic.

FIG. 2further shows the two transverse holes7and8, which are sufficiently deep for a web17to be formed between them, at both sides of which web17a wire strain gauge18has been glued on. In each instance the wire strain gauge18is enclosed by a casting compound made of flexible plastic. The relatively soft casting compound19ensures that the wire strain gauges18are permanently protected against mechanical and climatic influences, in particular also during the balancing procedure.

In each instance a separate cover20, which has been welded on, seals the two transverse holes7and8off towards the outside.

Furthermore, the transverse hole7comprises the balancing electronics comprising a printed circuit board21and a meandering layer22, made of nickel, for temperature compensation. The balancing electronics are affixed in that one face of the printed circuit board21is embedded in the casting compound19. In this way there is no need whatsoever for a separate balancing chamber since the latter is formed by the transverse hole7.

FIG. 2further shows a perpendicular groove24with which a pin that extends perpendicularly to the longitudinal axis of the weighing cell can interact, wherein said pin is firmly connected to a base plate which in the installed state carries the weighing cell.

This device to prevent torsion ensures permanent positioning to reduce the sensitivity error when the weighing cell is at an inclined position. Of course it is also possible to provide several grooves, for example three grooves, which advantageously are distributed on the circumference of the weighing-cell body so as to be regularly spaced apart.

FIG. 3shows a cross section of the weighing-cell fromFIG. 1a). The illustration clearly shows that a borehole23extends through the web17that is in place between the transverse holes7and8, through which borehole23the lines for electrical connection of the wire strain gauges18to the balancing electronics lead. Furthermore, the cable bushing10is shown, through which the signal lines and current supply lines lead from the transverse hole7towards the outside into the weighing cell cable11.

FIGS. 4a) andb) show a longitudinal section of two embodiments of the weighing cell according to the invention.FIG. 4a) shows a weighing cell in which the predominant part of the surface12of the top recess5, which surface tapers off towards the trunk3, as well as the predominant part of the surface15of the bottom recess6, which surface tapers off towards the trunk3, is inclined, and that only in the end region of the surfaces12and15is there a narrow annular region which extends perpendicularly to the longitudinal axis of the weighing-cell body1.

FIG. 4bshows an embodiment according to the solution according to the invention, which embodiment is to be preferred over that shown inFIG. 4a, in which preferred embodiment the surfaces12and15are inclined up to the upper or lower edge of the trunk3.FIGS. 4a) andb) further show that the surface12and the surface15are inclined also in that region that is not covered by the head2or by the foot4. In each instance the covered region is to the right of the dashed line, while the non-covered region is to the left of the dashed line.