SCALE WITH EVACUABLE HOUSING

Description

Field of invention

[0001] The invention relates to a scale comprising an evacuable housing with a base, side walls and a ceiling, and a weighing device arranged in the housing with a load carrier, a weighing system and a weighing sensor, wherein the weighing system has a housing-fixed base and a load transducer connected to the load carrier, which is vertically movable to the base via a linkage assembly articulated to the base and is coupled to the base-fixed weighing sensor via a lever assembly. State of the art

[0002] Precision balances, particularly those used as mass comparators for comparative weight measurements for calibration purposes, are well known to those skilled in the art. The tolerable measurement uncertainties for such precision balances are extremely low and strictly standardized (e.g., OIML R111.1). To reduce these measurement uncertainties accordingly, it is necessary to suppress potential external disturbances as completely as possible. One such disturbance is the density of the air surrounding the test weight, since each weight experiences a buoyant force that depends on the density of its surrounding air and counteracts its weight. It is therefore known to perform weight measurements that are to meet the highest precision standards under vacuum conditions.For this purpose, the actual weighing device (and of course the weight to be weighed) is placed in an airtight housing, which is evacuated before the measurement. The influence of the aforementioned buoyancy can thus be completely eliminated.

[0003] However, evacuating the housing presents additional problems. Depending on the degree of evacuation, i.e., the negative pressure inside the housing relative to the surrounding environment, considerable forces can act on the housing. These forces can lead to mechanical stresses and distortions, which in turn can be transferred to the delicate components of the weighing system itself, which is fixed to the housing at its base. As a result, additional measurement uncertainties can arise.

[0004] In the context of this description, a weighing system is understood to be a complex assembly of levers and links by means of which a load cell, into which the weight of the test weight is introduced during the weighing process, is coupled, on the one hand, to the housing-fixed base of the weighing system and, on the other hand, to the base-fixed weighing sensor, allowing for vertical deflection. In precision scales that typically operate on the principle of electromagnetic compensation (EMF scales), said weighing sensor is typically a moving-coil arrangement, with the coil current required to compensate for the deflection caused by the weight force acting on the load cell serving as a measure of the acting weight force. The specific nature of the weighing sensor is irrelevant in the context of the present invention.In any case, such weighing systems, with their long, highly delicate and partly folded lever, link and joint arrangements, are highly susceptible to mechanical stresses and distortions.

[0005] Another problem associated with evacuating the housing is the temperature drop that accompanies the reduction in air pressure. This can lead to changes in dimensions within the weighing system due to thermal expansion, which in turn generate further measurement uncertainties.

[0006] From DE 10 2011 000 429 A1 and its English-language equivalent US 2013 / 0333957 A1, a weighing booth is known with a work surface designed as a continuous, flat plate and an integrated scale comprising a weighing pan extending approximately flush with the work surface and at least one operating surface integrated into the work surface. The weighing booth is equipped with an extraction device that can extract air contaminated by toxic substances and replace it with clean air. This makes it possible to work with toxic or hazardous substances in the weighing booth. Task

[0007] The object of the present invention is to further develop a scale with an evacuable housing in such a way as to reduce the measurement uncertainties related to evacuation as explained above. Description of the invention

[0008] This problem is solved in conjunction with the features of the preamble of claim 1 by having a vertical column rise from the center of the base of the housing, on the free end of which the base of the weighing system is fixed.

[0009] Preferred embodiments of the invention are the subject of the dependent claims.

[0010] Traditionally, the weighing system is mounted to the housing by fixing its base directly to the housing's bottom surface. The present invention departs from this approach. Instead, the mounting is achieved indirectly via a stable column that rises from the center of the housing base. Any warping of the housing base, particularly manifest as curvature, is deflected into a purely vertical movement by the column. The weighing system, whose base is fixed at the top of the column, thus also experiences only a vertical displacement, which affects all its components equally. In this way, internal stresses within the weighing system, which can lead to the aforementioned measurement uncertainties, are eliminated.The person skilled in the art will recognize that it is irrelevant whether a (plate-like) base of the weighing system is fixed directly to the column tip or whether the free end of the column supports a platform projecting beyond its outer circumference, on which the base of the weighing system is fixed. What is crucial is the reduction of the (indirect) contact between the weighing system base and the housing bottom to a minimal area defined by the column's cross-section, within which no stresses in the horizontal plane, but only harmless vertical movements, can be transmitted.

[0011] This approach works particularly well when the base of the housing is circular and surrounded by the side walls in a ring shape. Such rotational symmetry of the base ensures that any bulging caused by negative pressure leads exclusively to a vertical lifting of the column (and thus the weighing system), while any lateral tilting is prevented.

[0012] A related approach proposes that the base and side walls of the housing be formed in one piece from a single material. In particular, the base and side walls of the housing can be machined rotationally symmetrically from a single block of material, especially a metal block, preferably by milling, drilling, and / or electrical discharge machining (EDM). Alternatively, additive manufacturing techniques are also conceivable. In any case, the aim of this approach is to design any deformations occurring during evacuation in such a rotationally symmetrical manner that they are all redirected into a purely vertical movement of the column according to the invention.

[0013] However, an unavoidable asymmetry is introduced by the weighing system itself, which cannot possibly be manufactured rotationally symmetrical. In a preferred embodiment of the invention, therefore, the weighing system projects beyond the outer circumference of the column, at least in the area of ​​the load cell, and—where present—beyond the edge of the support plate carrying the weighing system. The weighing platform, designed as a pendulum-like pan, is hinged to the load cell below it. In other words, the actual weighing device is designed according to the concept of an under-pan scale. This ensures that, with a suitably articulated coupling of the weighing platform to the load cell, the weight force to be measured is always applied to the load cell in a precisely vertical manner.Furthermore, this approach offers optimal use of the free space created by the column according to the invention in the housing below the weighing system, which thereby becomes the weighing space.

[0014] In precision balances with an evacuable housing, changing test weights, which involves re-ventilation and re-evacuation, is always time-consuming. It is therefore known to provide a test weight magazine with an automatic load-changing device within the evacuable housing, allowing several test weights to be weighed successively from the magazine without having to re-ventilate and re-evacuate the housing in between. In the context of the present invention, it is considered particularly advantageous if a rotary table with a test weight magazine, arranged concentrically to the column, is rotatably mounted on it. The rotary table can have several positions for storing test weights distributed around its circumference.To load the weighing carrier, the test weight to be weighed can be rotated towards the weighing carrier and transferred from the magazine to the weighing carrier by means of a suitably designed load transfer device. Such a transfer can be effected, in particular, by a vertical adjustment of the rotary table and / or elements arranged on it. The specific design of such a load transfer device is not relevant in the context of the present invention. However, those skilled in the art will understand that in the context of precision measurements, any movements or degrees of freedom should be minimized, and distances and gap dimensions should therefore be dimensioned as small as possible, so that a change in the relative height between the weighing carrier and the rotary table due to warping of the housing would be detrimental. However, such problems are not to be feared with regard to the present invention and its further development described herein.As explained at the outset, according to the invention, all housing distortions are redirected into a vertical movement of the column. The rotary table mounted on the column itself is therefore raised or lowered with it and thus experiences no change in its relative height to the weighing system fixed at the top of the same column or to the load carrier connected to it at a fixed height.

[0015] From a practical and design perspective, however, it can be just as advantageous, or even more so, to mount the rotary table externally, i.e., on the inner wall of the housing. While this may lead to a vertical relative displacement of the rotary table and the weighing platform when the housing is evacuated, a stable mounting of the rotary table is structurally much easier to implement radially further out than radially further in, namely on the column. Therefore, the person skilled in the art must weigh the actual suboptimalities against each other in each individual case.

[0016] It is generally considered advantageous to minimize the volume of the housing that needs to be evacuated. In a particularly preferred embodiment of the invention, the side walls of the housing do not extend beyond the column, or at least not by more than the height of the weighing system, and the weighing system is fitted into a corresponding recess in the housing's top. In other words, the rotational symmetry of the housing, described above as preferential, is deliberately broken in the area of ​​the housing's top. This is harmless, however. Due to the non-rotationally symmetrical shape of the weighing system itself, a symmetry break already exists in this area, to which the asymmetry of the housing's top merely adapts. Furthermore, the housing's top is not in contact with the weighing system, so any warping of the top cannot negatively affect the weighing system and thus the measurement result.

[0017] The housing, which, as mentioned above, is preferably made of metal, is, in its particularly preferred embodiment, designed with thick walls. This serves not only to maximize mechanical stability but also to maximize the heat capacity and thermal conductivity of the housing, which in turn reduces temperature differences within the housing during evacuation. Therefore, after evacuation, a stable temperature required for measurements will be established more quickly than with thin-walled housings. Consequently, measurement series can be carried out more rapidly.

[0018] The housing surfaces are preferably unpainted and polished. This prevents the evaporation of dyes under vacuum inside the housing and thus avoids corresponding contamination of the test weights. On the housing exterior, this maximizes reflectivity, meaning that external electromagnetic radiation causes less or slower temperature changes inside the housing.

[0019] Further details and advantages of the invention will become apparent from the following specific description and the drawings. Brief description of the drawing

[0020] It shows: Figure 1: a schematic representation of a scale according to the invention. Description of preferred embodiments

[0021] Figure 1Figure 1 shows a highly schematic representation of a scale 10 according to the invention. The scale 10 shown comprises an evacuable housing 12 and a weighing device 14 arranged in the housing 12. In the embodiment shown, the scale 10 is fixed to a vibration-isolated foundation 18 by means of supports 16. The specific method of fixing the scale 10 to the foundation 18 and its vibration isolation are not relevant in the context of the present invention.

[0022] The housing 12 essentially consists of a base 121, which can be, in particular, circular in shape. Side walls 122 rise around the edge of the base 121; in the illustrated embodiment, these side walls are integrally connected to the base 121 and made of the same material. The housing 12 is provided with a cover 123, designed as a removable lid, which can be fixed airtight to the free edges of the side walls 122. This creates a cavity completely enclosed by walls, which can also be referred to as a weighing chamber 124.

[0023] The weighing chamber 124 can be evacuated via evacuation means not shown. Those skilled in the art will understand that absolute evacuation is practically impossible and will therefore recognize that evacuation here refers to creating a significant negative pressure in the weighing chamber relative to the surrounding environment.

[0024] According to the invention, a vertical column 125 rises in the center of the base 121 into the weighing chamber 124. In the illustrated embodiment, the column 125 is formed integrally with the base 121 and is made of the same material.

[0025] A support plate 126 is fixed at the top of the column 125, i.e. at its free end, and projects on one side beyond the outer circumference of the column 125 and into the weighing chamber 124.

[0026] The weighing system 141 of the weighing device 14, in particular its base, is fixed to the support plate 126. A weighing carrier 142, designed as a pendulum pan, is articulated to the load cell of the weighing system 141 (not shown separately). In the illustrated embodiment, the articulated connection of the weighing carrier 142 to the load cell passes through a recess in the support plate 126. The weighing carrier 142 hangs freely pendulum-like next to the column 125 in the weighing chamber 124. A test weight 20 is shown on the pan of the weighing carrier 124.

[0027] The negative pressure created during the evacuation of the weighing chamber 124 generates considerable forces acting on the walls of the housing 12. These can, as shown in Figure 1The effects of the dotted lines, which are shown to a considerably exaggerated degree, lead to a temporary, elastic deformation of the housing walls. Particularly in the area of ​​the circular base 121, this deformation occurs essentially as a rotationally symmetrical bulging. This results in a non-uniform distribution of heaves and slopes across the entire base, which would cause a weighing system fixed flat on the base to warp. However, in the center of the base 121, this bulging only manifests as a vertical lifting. Accordingly, the column 125 located in the center of the base 121 moves purely vertically. The weighing device 14 fixed to the column 125 is also displaced purely vertically. This vertical displacement is shown in Figure 1indicated by dashed lines. It concerns the entire weighing device 14, including its weighing system 141, its load carrier 124 (including test weight 20) and its in Figure 1 The weighing sensor is not shown separately. Therefore, all these elements do not undergo any relative movement to each other during evacuation, so that the deformation of the housing 12 caused by evacuation has no influence whatsoever on the measurement result of the weighing device 14.

[0028] In the illustrated embodiment, in order to minimize the weighing chamber volume, the side walls 122 do not extend beyond the upper edge of the weighing system 141. Instead, the ceiling 123 of the housing 12 has a recess 127 in the area of ​​the weighing system 141 that corresponds to the shape of the weighing system 141, into which the weighing system 141 projects without contact.

[0029] Of course, the embodiments discussed in the specific description and shown in the figures represent only illustrative examples of the present invention. In light of this disclosure, a wide range of variations is available to those skilled in the art. In particular, they can provide further components within the weighing chamber 124. Specifically, a test weight magazine and an automated load-changing device can be arranged within the weighing chamber 124, for example, in the form of a rotary table mounted on the column 125. For the sake of clarity, however, these elements are shown in Figure 1 not shown. Reference symbol list

[0030] 10 Scale 12 Housing 121 Base 122 Side wall 123 Ceiling 124 Weighing chamber 125 Column 126 Support plate 127 Recess 14 Weighing device 141 Weighing system 142 Load carrier 16 Support 18 Foundation 20 Test weight

Claims

1. Balance (10), comprising an evacuable housing (12) having a bottom (121), side walls (122), and a top (123), and a weighing device (14) arranged in the housing (12) and having a load carrier (142), a weighing system (141) and a weighing sensor, wherein the weighing system (141) has a base fixed to the housing and a load receptor connected to the load carrier (142), which load receptor is coupled vertically movably to the base via a link arrangement articulated to the base on the one hand and is coupled force-transmittingly to the weighing sensor fixed to the base via a lever arrangement on the other hand, characterized in that a vertical post (124) protrudes from the center of the bottom (121) of the housing (12), the base of the weighing system (141) being fixed on a free end of the post.

2. Balance (10) according to claim 1, characterized in that the bottom (121) of the housing (12) is shaped as a circular disk and is surrounded by the side walls (122) in an annular manner.

3. Balance (10) according to any one of the preceding claims, characterized in that the bottom (121) and the side walls (122) of the housing (12) are formed in one piece from a common material.

4. Balance (10) according to any one of the preceding claims, characterized in that the free end of the post (125) carries a platform (126) which projects beyond an outer circumference of the post and on which the base of the weighing system (141) is fixed.

5. Balance (10) according to any one of the preceding claims, characterized in that the weighing system (141), at least in the region of the load receptor, projects beyond the outer circumference of the post (125) and - if present - beyond the edge of the platform (126), and the load carrier (142), which is configured as a pendulum tray, is hinged to the load receptor in a suspended manner below the load receptor.

6. Balance (10) according to claim 5, characterized in that a turntable with a test weight magazine is concentrically and rotatably mounted to the post (125).

7. Balance (10) according to any one of the preceding claims, characterized in that the side walls (122) of the housing (12) do not or at most by less than the height of the weighing system (141) project beyond the post (125), and the weighing system (141) is fitted into a corresponding recess (127) in the top (123) of the housing (12).

8. Balance (10) according to any one of the preceding claims, characterized in that the housing (12) is made of preferably thick-walled metal and has an unpainted, polished surface on the outside as well as on the inside.