Top-pan balance with an overload safety mechanism and a corner load sensor

A top-pan balance having a pan, a weighing system, and an overload safety mechanism. The load cell (4) of the weighing system is connected to fixed points (1) on a housing of the weighing system by an upper connecting rod (2) and a lower connecting rod (3) as a parallel guide so as to be movable in the vertical direction. The pan is attached to a pan support (8/9) for securing against overload, the support being connected to the load cell through an auxiliary parallel guide (15/16) and through a biased spring element (17/18), whereby the pan is quasi rigidly coupled to the load cell in the permissible weighing range, but is only resiliently coupled to the load cell when the permissible weighing range is exceeded. At least one limit stop is fixed to the housing and limits the elastic deflection of the pan and the pan support in case of overload. An additional corner load sensor is provided between the pan support and the load cell, and the corner load sensor and the overload safety mechanism form a common assembly (7), wherein the corner load sensor is disposed behind the overload safety mechanism in the force flow direction from the pan to the load cell. Furthermore, the assembly is attached on the side of the load cell facing the connecting rods and extends into the space between the connecting rods.

FIELD OF AND BACKGROUND OF THE INVENTION

The invention relates to a top-pan balance having a weighing pan, a weighing system, the load cell of which is connected to fixed points on the housing of the weighing system by an upper connecting rod and a lower connecting rod as a parallel guide so as to be movable in the vertical direction, and an overload safety mechanism. The weighing pan is attached to a pan support for securing against overload, the pan support being connected to the load cell through an auxiliary parallel guide and through a pre-tensioned spring element, whereby, in the permissible weighing range, the weighing pan is rigidly coupled to the load cell, and outside the permissible weighing range, is resiliently coupled to the load cell. At least one limit stop is fixed to the housing and limits the elastic deflection of the weighing pan and the pan support in case of overload.

Balances with overload safety mechanisms of this type are known, for example, from DE 28 30 345 A1 (U.S. Pat. No. 4,273,203). However, the embodiment described there takes up a substantial amount of space, so that the balance housing is made larger due to the overload safety mechanism. The overload safety mechanism also comprises a large number of parts, which makes assembly complex.

A significantly more compact embodiment made from fewer parts is known from DE 101 61 517 B4. This embodiment has proved to be successful.

OBJECTS AND SUMMARY OF THE INVENTION

It is an object of the invention to equip a balance of the aforementioned type with a further capability without necessarily increasing the structural volume of the weighing system.

According to one formulation of the invention, an additional corner load sensor is provided between the pan support and the load cell, the corner load sensor and the overload safety mechanism form a common assembly, wherein the corner load sensor is arranged behind the overload safety mechanism in the force flow direction from the weighing pan to the load cell, and the assembly is attached on the side of the load cell facing the connecting rods and extends into the space between the connecting rods.

Corner load sensors in balances are, in principle, already known. For example, DE 30 03 862 C2 discloses a corner load sensor with strain gauges on a vertical support element directly under the weighing pan. However, this corner load sensor must be dimensioned for the maximum overload that the balance is to withstand without damage. But this does not allow adequate corner load signals to be obtained from the strain gauges. The same applies for the corner load sensor in DE 10 2006 031 950 B3 (US 2009/0114455A1), which can be retrofitted between the weighing pan and the bottom pan of a balance.

In contrast thereto, in the combination according to the invention of corner load sensor and overload safety mechanism in a common assembly, the corner load sensor is arranged behind the overload safety mechanism, so that the corner load sensor is loaded no further than the response threshold of the overload safety mechanism. This means that the thin material sites of the corner load sensor can be made significantly thinner and the strain gauges applied can supply a significantly larger corner load signal.

DE 30 03 862 C2 also discloses—like DE 196 32 709 C1—that corner load sensors with strain gauges at the support sites of the connecting rods of the weighing system that are fixed to the housing detect the horizontal forces where the position of the weighed object is off-center. However, in order for such corner load sensors to be able to deliver a usable signal, the support sites of the connecting rods fixed to the housing must have a certain amount—if only very little—of resilience. However, this leads to a change in the geometry of the parallel guide and thus influences the corner load of the parallel guide. Here also, the problem arises that stable behavior of the parallel guide in the event of corner loading requires that the connecting rods have the most stable possible support points, whereas the desire for a sufficiently large output signal from the corner load sensors requires more resilient support points.

In contrast thereto, the corner load sensor provided, according to one aspect of the invention, together with the overload safety mechanism, between the pan support and the load cell does not influence the parallel guide of the weighing system in any way. The configuration of the parallel guide can be optimized without regard to the corner load sensor; and the corner load sensor can be dimensioned without regard to the parallel guide.

Since the common assembly made from corner load sensor and overload safety mechanism is fastened to the side of the load cell facing the connecting rods and extends into the region between the connecting rods, a particularly space-saving arrangement is produced. In this way, the outer dimensions of the weighing system are not changed by the overload safety mechanism and the corner load sensor.

In the case of weighing systems with a gearing lever, the lever is usually situated in the plane of symmetry of the weighing system. There is therefore only a little space available in the plane of symmetry. Advantageously therefore, the spring element of the overload safety mechanism is divided into two parts arranged on either side of the plane of symmetry. For example, the spring element may consist of two helical springs.

The corner load sensor advantageously consists of at least three vertically arranged thin material sites to which strain gauges are applied. The thin material sites are advantageously arranged in the force flow such that these sites are tension-loaded when the weighed object is placed approximately centrally on the weighing pan. Therefore, even with the weighed object arranged off-center (producing a corner load), the thin material sites experience only a relatively small bending load. If four thin material sites with strain gauges are used, arranged on the sides of a square, then a half bridge or full bridge can be connected for the X-direction and the Y-direction, respectively, and an output signal can be generated directly for the corner load in the X-direction and the Y-direction, respectively. The strain gauges can be most easily placed if they are positioned on the outside of the thin material sites.

Particularly good reproducibility of the corner load signals is achieved if the thin material sites are formed monolithically from a single component. This prevents slippage in the contact regions.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The weighing system ofFIG. 1functions according to the principle of electromagnetic force compensation and comprises a connecting rod parallel guide and a multiplier lever. Mounted on a region1fixed to the housing are an upper connecting rod2and a lower connecting rod3which together guide a load cell4movably in the vertical direction. The thin material sites which act as hinges are indicated at reference numeral5. The force to be measured is transmitted via a coupling element (not shown) and a multiplier lever6(of which only a small part is shown) to a coil (not shown) which extends into a permanent magnet system (also not shown). The parts1. . .6are formed monolithically from a single block of material. Weighing systems of this type are generally known and therefore do not need to be described in detail here. Specifically the assembly7, with the corner load sensor and the overload safety mechanism, which is emphasized inFIG. 1with a thicker outline and is shown again alone and enlarged inFIG. 2, embodies the invention.

The assembly7essentially consists of the pan support8/9, which is constructed from the pan support upper part8and the pan support lower part9and the corner load sensor part10. The pan support lower part9is shown again alone inFIG. 4for clarity, and similarly the corner load sensor part10is shown again alone inFIG. 3.

The corner load sensor part10of the assembly7is firmly fixed to the load cell4of the weighing system by screws12(FIG. 1) and threaded holes13. The pan support upper part8supports the weighing pan (not shown) of the balance at the bore14. Situated on the pan support lower part9is an extension arm9′ to which a hook for suspended weighing can be attached. The pan support8/9and the corner load sensor part10are connected to one another via two spring plates15and16in the form of an auxiliary parallel guide and via two pre-tensioned helical springs17and18. The helical springs17and18are hooked onto a projecting hook19on the pan support lower part9and onto a projecting hook20on the corner load sensor part10. The helical springs therefore pull the scale pan lower part9upwardly and the corner load sensor part10downwardly, so that the underside21of the projecting hook20is pressed against the stop11on the pan support lower part9. The pre-tensioning force of the helical springs is large enough such that the contact force against the stop is approximately 150% of the nominal load of the balance. Up to this threshold value, the pan support8/9and the corner load sensor part10are semi-rigidly coupled to one another, so that the weight force of the weighed object is transmitted via the weighing pan, the pan support8/9and the corner load sensor part101:1 to the load cell4of the weighing system. Only in the event that the threshold value is exceeded is the pre-tensioning force of the helical springs17and18no longer sufficient. Given a more than 50% overload, the stop11lifts off the projecting hook20, the pan support8/9sinks further, whilst the corner load sensor part10, and therefore also the load cell4, do not move therewith. The descent of the pan support8/9and of the weighing pan is stopped by a limit stop (not shown) which is suitably arranged directly between the weighing pan and the housing of the balance and transmits relatively large overloads directly from the pan to the housing. Therefore, in the event of an overload, only slightly more than the threshold value is transmitted to the weighing system and the weighing system is effectively protected against overload. By virtue of the auxiliary parallel guide made from the spring plates15and16, the response threshold of the overload safety mechanism is independent of the location of the overload on the weighing pan. Even when the hook is used for suspended weighing on the extension arm9′, the overload safety mechanism remains operational, and also if a suitable limit stop is provided on the extension arm9′ or on the pan support lower part9.

The operation of the assembly7as a corner load sensor is realized in the corner load sensor part10, as shown inFIG. 3. The forces transmitted from the weighed object on the weighing pan, including the torques caused by an off-center position of the goods being weighed, are transmitted through the pan support lower part, via the two projecting hooks20and the spring plates15,16to the approximately H-shaped rear part22of the corner load sensor part10. From this rear part22, the forces and torques are transmitted via four vertically oriented thin material sites23with strain gauges24applied, to the front part25of the corner load sensor part10and therefrom via the screw connection12/13to the load cell4of the weighing system. InFIGS. 2 and 3, only two thin material sites23are shown, the others lying symmetrically thereto, specifically such that the four thin material sites23, seen in plan view, are positioned on the sides of a square. The weight force of the weighed object put the four thin material sites essentially under tensional load and therefore generates a common mode signal at the strain gauges, whereas with an off-center position, the torques lead to a difference signal at the two respectively opposite strain gauges. This difference signal can then be separated from the common mode signal in a Wheatstone Bridge circuit, in known manner, and an output signal is obtained from the corner load sensor for the X-direction and the Y-direction, respectively. This corner load sensor signal can then be used in known manner, together with the stored corner load correction factors, to calculate a correction value in the electronics of the balance. With the arrangement of the four thin material sites at the sides of a square (seen in plan view), the sensitivity with regard to corner load torques in the X and Y-directions is equal, with the result that the mathematical evaluation is simple. The overall corner load sensor part10is machined monolithically from a single block of metal. In this way, distortions during assembly of the individual parts and slipping effects at connection sites are prevented, with the result that the corner load signals have a high degree of reproducibility.

With the arrangement described, the corner load sensor is situated behind the overload safety mechanism, seen in the force flow direction from the weighing pan to the load cell. In this way, overload forces do not reach the thin material sites23of the corner load sensor, so that the thin material sites do not have to be dimensioned for overload forces. This means that the thin material sites can be made thinner and the strain gauges emit a larger signal. The same applies given the presence of a plurality of correctly oriented overload limit stops, including for overload torques.

In the advantageous embodiment of the balance shown, four thin material sites23are provided with strain gauges for the corner load sensor. This results in the possibility of particularly simple electronic evaluation of the strain gauge signals. Naturally, three thin material sites23with strain gauges also suffice in order to obtain the corner load signals in the X and Y-directions. The three thin material sites must then be arranged, for example, on the sides of an equilateral triangle. Each strain gauge would have to have a fixed resistor added to make a half-bridge and thus be evaluated. The X and Y-corner signals would then have to be calculated from the three signals using known mathematical operations.

AsFIG. 1shows, the assembly is screwed onto the side of the load cell4facing the connecting rods and extends into the space between the connecting rods2and3. The assembly7therefore needs no additional space in the balance housing. In order to be able to guide the multiplier lever6located in the plane of symmetry of the weighing system through the assembly7, the pan support lower part9and the corner load sensor part10, each has a free space26. For the same reason, two helical springs17and18are provided, arranged at the sides of the assembly7.