Method for producing a base body of a weighing sensor and base body

Abstract

The invention relates to a method for producing a base body (100) of a weighing sensor of a high-precision balance, characterised in that a base body (100) is machined from a monolithic metal block, said base body having at least one arm (3, 4), which is articulated to the rest of the metal block via integral flexible spring joints (30, 40) in the form of thinned points. The base body (100) is then exposed to a chemical solution (104) at least at the flexible spring joints (30, 40), in order to bring about material removal at these points. The invention also relates to a base body (100) produced by means of said method.

Description

[0001] Method for producing a base body of a weighing sensor and base body

[0002] The invention relates to a method for producing a monolithic or partially monolithic base body of a weighing sensor and a base body produced by the method according to the invention.

[0003] In particular, the invention relates to a method for producing a monolithic or partially monolithic base body for a high-resolution balance with a resolution of up to 0.0001 mg and several million weighing steps, e.g. a high-precision balance.

[0004] Such scales typically operate on the principle of electromagnetic force compensation, meaning the weight is converted into an electrical signal by a force sensor. The core of such a high-precision scale is a typically monolithic base body, which is milled from a block of material and has a section known as the load receptor, which is linked to the rest of the base body via one or more arms, also called transmission levers. An alternative to such a monolithic base body is a so-called partially monolithic base body, which is composed of a few monolithic parts. The arms, for example, form one or more parallelogram guides. The force to be measured is transmitted via one or more arms and compensated by means of electromagnetic force compensation (EMF) using a position-controlled coil.The corresponding arms of the parallelogram guide are connected to the rest of the metal block via bending spring joints in the form of thin sections in a monolithic or partially monolithic base body. Other parts of the base body are also connected via bending spring joints and can be used to reduce disruptive effects caused by off-center loads through adjustment, or they can be used to transmit force between sections of the base body (e.g., levers and shell couplings) in the form of so-called coupling elements. This means that all parts and bending spring joints are machined, particularly milled, from the metal block blank, and all these components merge into one another as a single piece. This applies to both monolithic and partially monolithic base bodies.What these monolithic or semi-monolithic base bodies have in common is that each arm, and thus also the two flexible spring joints of the arm, merge into one another as a single piece. They are originally machined from the same part and also merge into the adjacent part of the metal block as a single piece, and are also machined from a single metal block. This means that the arms, the joints, and the immediately adjacent parts of the metal block are made of the same material and only a single material. They are not assembled from multiple parts, glued together, or welded together. The invention relates to such a monolithic base body.

[0005] This system of manufacturing as many components of the weighing system as possible from a single blank results in a very complex three-dimensional geometry.

[0006] In addition to milling, other technologies such as EDM and grinding can be used for manufacturing. The subsequent achievable weight performance of the base body in relation to changing environmental influences (e.g., temperature) depends on the accuracy and dimensional stability of the arms, but especially the flexible spring joints, as well as the absence of stress in the material. These flexible spring joints have thicknesses of a few tenths of a millimeter at most, which is very challenging for manufacturing, especially since the workpiece support force during milling is negligible at such thicknesses. Therefore, manufacturing in these areas of the monolith can only function if the feed rate and milling forces in the area of ​​the flexible spring joints are very small, which in turn slows down the manufacturing process and increases costs and the risk of scrap.

[0007] The object of the invention is to create a method for manufacturing a monolithic or partially monolithic base body of a weighing sensor that enables faster production with greater process reliability, achieves at least the previous manufacturing accuracy, and, above all, improves the weighing behavior of a base body manufactured in this way. The invention achieves this through the following steps:

[0008] A) A base body or a part of a base body of a weighing sensor is machined, in particular milled, from a monolithic metal block, with at least one arm which is connected to the rest of the metal block via integral bending spring joints in the form of thin sections, and

[0009] B) the manufactured base body is exposed to a chemical solution at least at the bending spring joints, which causes material removal at the bending spring joints.

[0010] In step A), at least one arm, the adjacent bending spring joints, and the so-called remainder of the metal block are machined from the metal block. The bending spring joints are made of the same material as the rest of the base body, since, like the arms, they are machined from a monolithic metal block, which represents the starting body for these sections. In the process according to the invention, no current is applied to the workpiece during its chemical treatment; thus, it is a purely chemical process and not an electrochemical removal process. Specifically, this means that no electrical contact to the base body is required.

[0011] It has been discovered that internal stresses in the material cause disruptive long-term effects in the area of ​​the bending spring joints, which are not always consistent (process variation) and thus negatively influence the weighing behavior. Such stresses can be generated by machining, but also by the production of the blank during extrusion or rolling. Tests have shown that these stresses can be significantly reduced by minimal chemical removal of the outer layers of the material.

[0012] The stresses in the rest of the metal block are also reduced. These are caused by the extrusion or rolling of the starting material. Since mechanical or, more generally, external forces are exerted on the base body, especially in the area of ​​the flexible spring joints, the chemical removal has exclusively positive effects. The reduced roughness, which results in the area of ​​the arms or in the rest of the metal block, also ensures more reproducible behavior of the load cell.

[0013] Another effect, which positively influences production speed and scrap rates in particular, is that, thanks to chemical material removal, the flexible spring joints no longer need to be manufactured as thinly as before during machining or milling. This is a huge advantage that positively impacts processing speed. This allows base bodies to be manufactured with the same final dimensions, but with a different, i.e., greater, thickness during machining by material removal, especially milling, because these bodies are then chemically post-processed. Alternatively, thinner thicknesses can be achieved through chemical machining.

[0014] The entire substrate can be brought into contact with the chemical solution, e.g., immersed in the solution, or just a portion of it. This depends on the geometry of the substrate in question and which areas are to be positively modified by the chemical solution.

[0015] Alternatively, the base body is sprayed with the solution, either completely or only in sections.

[0016] The chemical solution is preferably an etchant, i.e. it etches the surface at least in the area of ​​the bending spring joints.

[0017] In step B), at least 2 to 50 μm of material should be removed from the flat sides of the flexible spring joints. This means that the thinnest point becomes twice as thin, i.e., 4 to 100 μm thinner. It has been shown that these small machining operations alone are sufficient to significantly reduce the stresses in the surface area and also shorten the machining time during milling.

[0018] The base body, including the spiral spring elements, is preferably made of aluminum or a suitable alloy. The chemical composition of the solvent must be matched to the material of the base body, as must the solvent temperature, solvent concentration, and exposure time, i.e., the removal time. Aluminum has proven to be a suitable material for the base bodies of load cells.

[0019] After immersion in the chemical solution in the area of ​​the flexible spring joints, the base body is preferably not subjected to further mechanical treatment. However, it is common practice to rinse the base body or immerse it in one or more solutions to control or interrupt the chemical process, or to remove insoluble components that have accumulated on the surface. The latter step is typically performed with nitric acid for aluminum alloys, also known as pickling.

[0020] Preferably, after machining, the base body should only be reworked by contact with liquid, in particular by immersion in liquid, i.e. in the chemical solution and possibly other solutions, or by spraying, so that no further stresses enter the component.

[0021] Optionally, the chemical solution can be forced along the milled base body by a generated flow to ensure that a certain amount of chemical solution flows over the surface to be treated within a certain period of time. The flow can be generated, for example, by a pump, air bubbles, or by swinging the container containing the solution and the base body.

[0022] As has been shown, an exposure time of at least 10 minutes, particularly at least 30 minutes, at room temperature in a 20% NaOH solution (sodium hydroxide solution) is usually sufficient, during which the milled base body is immersed in the chemical solution. However, according to the tests conducted, an exposure time of more than 60 minutes is not necessary to achieve further significant property improvements.

[0023] A further significant reduction in stress and a shortening of the immersion time were achieved in experiments by heating the chemical solution, 20% NaOH (caustic soda), to a temperature of 40°C to 70°C while immersing the substrate for 1-10 minutes. However, these slightly elevated temperatures are sufficient to produce a so-called aging effect as a side effect, i.e., an aging or tempering effect occurs in the surface and internal material regions.

[0024] An optional subsequent drying process at an elevated temperature of at least 50°C, particularly for a period of 20 to 60 minutes, also has a positive effect. This further reduces internal stresses in the material.

[0025] Another option is to immerse areas of the base body in the chemical solution for different lengths of time, i.e. to allow them to be processed by the chemical solution for different lengths of time. This applies in particular to the areas of the flexible spring joints. For example, depending on the installation state, upper flexible spring joints of arms can be processed for longer or shorter periods of time than underlying flexible spring joints of the same parallelogram guide. Furthermore, if several arms or parallelogram guides are provided on a base body, they can of course remain immersed in the chemical solution for different lengths of time. This makes it possible to achieve different thicknesses of the flexible spring joints and, for example, to allow an area that is more difficult for a milling cutter to access to remain thicker after milling, but can then remain in the chemical solution for longer and be reworked.

[0026] Areas can also be covered so that no chemical solution reaches certain areas that should not be treated with the chemical solution or should be treated for a shorter time than other areas.

[0027] The process according to the invention can be fully automated, i.e., appropriate fully automated handling systems pick up the base body and transfer it into the solutions, and then remove it from the solutions. For example, the base body is first treated with NaOH and then pickled.

[0028] When we talk about different processing times in the chemical solution, this does not include the time required for moving the substrate into and out of the liquid, because then an upper section would always be in the liquid for a shorter time than a lower section of the substrate. To achieve these different treatment times, it is therefore necessary that the otherwise preferably continuous speed for moving the substrate into and out of the liquid becomes discontinuous, or that the substrate remains at a certain absorption depth before being immersed further or, if necessary, rotated and reinserted into the liquid at a different location.

[0029] Not only is it possible to immerse the base body alone in the chemical solution, but at least one additional component can also be already attached to it when immersed in the chemical solution (which is part of the scale to be manufactured with the corresponding base body). This additional component is either covered, not immersed in the solution, or is insoluble (made of a different material). However, it can potentially change the stress of the base body slightly, so it can be advantageous to have a pre-assembled assembly, of which the base body is a part, attached to the base body during processing through the chemical solution. Pre-assembled assemblies include, for example, mechanical stops or detents.

[0030] The invention also relates to a base body of a weighing sensor which is manufactured according to the method according to the invention.

[0031] Further features and advantages of the invention will become apparent from the following description and the accompanying drawings, to which reference is made. In the drawings:

[0032] - Figure 1 is a side view of a base body of a high-precision balance produced by the method according to the invention according to a possible embodiment,

[0033] - Figure 2 is a plan view of the base body according to Figure 1, and

[0034] - Figure 3 shows a device with which the method according to the invention is carried out.

[0035] Figures 1 and 2 show a monolithic base body 100 of a load sensor for a high-precision scale, milled from a single metal block. The base body 100 has several sections, namely a support 1, a load sensor 2, and several pivotably mounted arms 3 and 4. Behind the upper arm 3, an identical upper arm 3' is provided, visible in Figure 2. Similarly, behind the lower arm 4, an identical lower arm 4' is provided, concealed in Figure 1. These arms 3, 3', 4, and 4' are also called guide rods. For example, the upper and lower arms can each be V-shaped, see Figure 2.

[0036] The total of four arms 3, 3', 4 and 4' result in a parallelogram guide for the load receptor 2.

[0037] The four arms 3, 3', 4, and 4' are each supported by a flexible spring joint 30, 40 at the opposite ends. They merge integrally into the adjacent sections of the base body 100 and are produced, for example, by milling. Accordingly, the flexible spring joint 30, 40 is also an integral part of the base body 100.

[0038] A weighing pan (not shown) may be attached directly or indirectly to the load receptor 2.

[0039] The base body 100 further comprises a transmission lever 5, which is separated from the carrier 1 by a groove 6. Furthermore, one or more flexible spring joints 7 are provided for mounting the transmission lever 5 on the carrier 1.

[0040] The flexible spring joints 7 extend upwards into a projecting area 8 of the beam 1 and downwards into a crossbeam 9. The transmission lever 5 extends downwards from area 8. The connection between a front end 11 of the transmission lever 5 and the load receiver 2 is established by a coupling element 12, which is also integrated into the metal block. This coupling element 12 is articulated to the end 11 by a flexible spring joint 13 and to the lower part of the load receiver 2 by another flexible spring joint 14.

[0041] In the center of the coupling element 12 is another bending spring joint 15, which is perpendicular to the two other bending spring joints 13 and 14, thus achieving decoupling between the load sensor 2 and the transmission lever 5 in both directions. To complete the load sensor, a coil (see coil center 16) must be attached to the transmission lever 5 from below, and a cylindrical permanent magnet must be inserted from below into a designated opening 17 (see Figure 2) and secured there.

[0042] A slot 37 for an optical position sensor is also incorporated into the transmission lever 5. A round hole 20 is provided in a projection 21 on the carrier 1 for a light-emitting diode for the optical position sensor, as well as a hole 19 on the opposite side of the projection 21 for a differential photodiode for the optical position sensor.

[0043] A wide slot 22 at the end of the transmission lever 5 serves to limit the movement of the transmission lever 5. A horizontal pin (not shown) mounted eccentrically in the projection 21 extends through this slot 22 and limits the movement of the transmission lever 5 to the difference between the slot width and the diameter of the pin.

[0044] Also milled in one piece from the base body 100 is a device for reducing the effects of off-center loads. A fastening point 23 of the upper arm 3, 3' is connected to the rest of the support 1 by two horizontal arms 24 and 25, which form a parallelogram guide.

[0045] The fastening point 23 is separated from the rest of the support 1 by a slot 38. The area of ​​the fastening point 23 is supported by a vertical web 26 and a corner load adjustment lever 27 on an area 28 firmly connected to the support 1.

[0046] Due to the lateral offset of the bending spring joints 32, 33, a tilting of the corner load adjustment lever 27 leads to a slight vertical movement of the attachment point 23 for the upper arms 3, 3'. By adjusting the vertical distance of the arms 3, 3', 4, and 4' in the area of ​​their support-side attachment point, an adjustment of the parallelogram guide formed by them with respect to corner load freedom is possible.

[0047] Figure 3 depicts the base body 100 in a highly stylized and simplified manner. Also shown is a container 102 containing a chemical solution, in this case a caustic solution 104.

[0048] An example of a caustic solution is 20% NaOH solution (aqueous sodium hydroxide solution).

[0049] After machining, in particular milling, the base body 100 is usually washed and degreased without further mechanical processing, after which it can be immersed fully automatically into the solution 104 by a gripper 106, either partially or completely.

[0050] The chemical solution 104 is heated to above 20°C, preferably to a range of 40°C to 70°C, while the base body 100 is immersed in it.

[0051] The time during which the base body 100 is in the solution and undergoes surface removal due to the solution varies depending on which chemical solution is chosen and how much removal is desired.

[0052] In particular, a material removal of 2 to 50 pm should be carried out on the flat sides 120, 122 of the bending spring joints 30, 40.

[0053] To allow the chemical solution to flow into all areas of the base body 100, a pump 114 may be provided, which generates a flow inside the container 102. However, this is not absolutely necessary.

[0054] Typically, it has been found that the base body 100 should be immersed in the chemical solution 20% NaOH at elevated temperatures for at least 1 minute, in particular at least 10 minutes, in order to have undergone sufficient surface treatment.

[0055] During immersion, only a part of the base body 100 can enter the chemical solution 104, so that it is also possible to mount an additional component such as the coil on the base body 100 when it is immersed in the solution in sections.

[0056] If, for example, threads have already been cut, which should be the case, they can be covered, for example with a plug, to prevent the chemical solution from penetrating them. It can also be advantageous if, for example, the flexible spring joints are treated with the chemical solution to varying degrees, for example, flexible spring joints 30 and 40. Then, the base body 100 is first only partially immersed in the solution 104, namely in the area of ​​the flexible spring joints 40. The gripper 106 then remains in this position for a certain period of time before finally the flexible spring joints 30 are also immersed.

[0057] After withdrawing the base body 100 from the chemical solution, the base body 100 must be pickled, e.g., either immersed in another solution to rinse off the etching solution or sprayed to stop the etching process.

[0058] Subsequent mechanical processing preferably does not take place.

[0059] However, after rinsing, the base body 100 is dried at elevated temperatures of over 50°C, for example, over a period of 20-60 minutes, which additionally reduces internal stresses in the material.

[0060] Even though only the flexible spring joints 30 and 40 have been described here with regard to the processing by the chemical solution 104, it is understood that all or some of the other described flexible joints and their arms can also be processed in this way.

Claims

Claims 1. A method for producing a base body (100) of a weighing sensor of a scale, characterized by the following steps: A) a base body (100) or a part of a base body (100) is machined, in particular milled, from a monolithic metal block to form at least one arm (3, 3', 4, 4') which is connected to the rest of the metal block via integral bending spring joints (30, 40) in the form of thin sections, and B) the manufactured base body (100) or part of a base body is exposed to a chemical solution (104) at least at the bending spring joints (30, 40), which causes material removal at the bending spring joints (30, 40).

2. Method according to claim 1, characterized in that the entire base body (100) or a partial area of ​​the base body (100) is exposed to a chemical solution (104), in particular immersed or sprayed.

3. Method according to claim 1 or 2, characterized in that the chemical solution (104) etches the surface at least in the region of the flexible spring joints (30, 40).

4. Method according to one of the preceding claims, characterized in that in step B) at least a defined material removal of 2-50 pm takes place on flat sides (120, 122) of the bending spring joints (30, 40).

5. Method according to one of the preceding claims, characterized in that the chemical solution (104) is heated above 20° C, preferably in the range of 40° C to 70° C, when the base body (100) is immersed.

6. Method according to one of the preceding claims, characterized in that the base body (100) is made of aluminum or an aluminum alloy.

7. Method according to one of the preceding claims, characterized in that the base body (100) after being subjected to the chemical solution (104) in the area of ​​the bending spring joints (30, 40) is no longer mechanically reworked.

8. Method according to one of the preceding claims, characterized in that the base body (100) is reworked after the machining exclusively by contacting with liquid.

9. Method according to one of the preceding claims, characterized in that the chemical solution (104) flows along the machined base body (100) by means of a generated flow.

10. Method according to one of the preceding claims, characterized in that the machined base body (100) is absorbed in the chemical solution (104) for 30 to 10 minutes, in particular for at least 2 minutes.

11. Method according to one of the preceding claims, characterized in that the chemical solution (104) is heated to a temperature of 40°C to 70°C when the base body (100) is acted upon by it.

12. Method according to one of the preceding claims, characterized in that regions, in particular flexible spring joints (30, 40), are processed for different lengths of time in the chemical solution (104).

13. Method according to one of the preceding claims, characterized in that at least one additional component, which is part of the scale to be manufactured, is mounted on the base body (100) when it is immersed in the chemical solution (104).

14. Method according to one of the preceding claims, characterized in that the base body (100) is dried at a temperature of at least 50°C after removal of the chemical solution (104).

15. Base body (100) manufactured by a method according to one of the preceding claims.

Images