|Title of Invention
PRECISION FORCE SENSOR COMPRISING STRAIN GAUGE ELEMENTS
|Disclosed is a precision force transducer comprising a spring element (1) whose load-dependent deflection is converted into an electrical signal by means of strain gauge elements (10). The spring element (1) is made of a precipitation-hardening basic nickel alloy that has a nickel content ranging from 36 to 60 percent and a chromium content ranging from 15 to 25 percent while the strain gauge elements are composed of a polymer-free layer system, thus making it possible to produce a precision force transducer which features great accuracy, a low amount of creeping, and low sensitivity to moisture.
THE PATENT ACT 1970 (39 of 1970)
The Patents Rules, 2003 COMPLETE SPECIFICATION
(See Section 10, and rule 13)
1. TITLE OF INVENTION
PRECISION FORCE SENSOR COMPRISING STRAIN GAUGE ELEMENTS
WEENDER LANDSTRASSE 94-108,
3 7075 GOETTINGEN,
3. PREAMBLE TO THE DESCRIPTION
The following specification particularly describes the invention and the manner in which it is to be performed : -
ENGLISH TRANSLATION VARIFICATION CERTIFICATE u/r. 20(3)(b)
I, Mr. HIRAL CHANDRAKANT JOSHI, an authorized agent for the applicant, SARTORIUS AG do hereby verify that the content of English translated complete specification filed in pursuance of PCT International application No. PCT/EP2006/011272 thereof is correct and complete.
The invention relates to a precision force sensor with a spring element, whose load-dependent deflection is converted in an electrical signal by the strain gauge elements.
Precision force sensors of this kind are known in general and are described, for instance in the DE195 11 353 CI.
If one wants to enhance the accuracy of this precision force sensor, then creep and hysteresis present a major problem in this. One approach for improvement here was to use steel-types having low creep properties and subject them to a special heat treatment, for instance, the so-called Maraging-steels. Even austenitic steels with nano-structure and blocked dislocations have also been suggested (DE 198 13 459 Al). Another approach suggests the use of aluminum alloys, whereby the creep of this material is compensated by a counteractive creep of the conventional strain gauges. The creep of the conventional strain gauges arises from the polymer foil, which forms the base layer of the strain gauges, and from the adhesive used between the strain gauges and the spring element. Since both the creep effects have different temperature dependencies, this compensation succeeds only at the most in a low temperature range. However, all these known solutions only permit a useful resolution of precision force sensor of approximately 50,000 steps.
If the precision force sensor is used for scales verifiable for legal metrology, then in this case only approx. 3 x 3000 verification scale intervals are possible.
Another fault appearing in the case of the conventional strain gauges is the moisture sensitivity of the adhesive layer and the carrier foil. Owing to the force bypasses that occur, the high-resolution precision force sensors can be encapsulated only to a limited extent against the effect of moisture. As such, the moisture sensitivity of the conventional strain gauges presents a factor restricting the resolution in designing the precision force sensors.
The task of the invention, therefore, is to specify a precision force sensor of the type mentioned in the beginning, which permits a clearly higher accuracy.
As per the invention, this is achieved by the fact that the spring element comprises an exudation-tempered nickel-base alloy with a nickel content between 36 and 60% and a chromium content between 15 and 25% and that the elongation measurement elements comprise a polymer-free layer system.
The use of exudation-tempered nickel-base alloy as the spring element is known. For instance, a force sensor for a brake is described in the DE 103 50 085 Al, in which an exudation-tempered steel - preferably of the type 17-4 PH or Inconel 718 - is used as the material of the spring element and in which semi-conductor strain gauge elements made of silicon are used as strain gauge elements, which are connected with the spring element by means of lead borate glass-solder. However, the strain gauge elements based on semi-conductors show a high temperature coefficient, so that this force sensor cannot achieve a high accuracy in a higher temperature range. Moreover, a force-related coupling via the glass solder leads to very high inner stresses in the silicon chip, because the spring materials mentioned above have a different thermal expansion coefficient than silicon. Since the glass materials have a tendency to flow under application of force, very high creep effects can be expected in a force transmission system containing glass, which make it impossible to design a precision force sensor.
Only through a combination of exudation-tempered nickel-base alloy, which has a very low creep, an almost constant E-modulus over large temperature ranges and a high strength, with the polymer-free layer system as strain gauge element, the accuracy of the precision force sensor can be enhanced considerably. As a result of the omission of the polymer base layer and the adhesive, this layer system also has very less creep and low sensitivity to moisture. In this way, a useful resolution of over 200,000 steps can be realized. The difficult processability of this class of material is thereby accepted.
As exudation-tempered nickel-base alloy preferably an alloy with a nickel-content between 50 and 55% and a chromium-content between 17 and 21% is used. For instance, the alloys standardized under the material number 2.4668 in accordance with EN 10027-2 belong to this class of alloys.
The polymer-free layer system is applied on to the spring element by means of a thin-layer process, preferably a PVD or CVD process; PVD thereby stands for Physical Vapor Deposition and CVD for Chemical Vapor Deposition. The layer system thereby shows the following sequence of layers: Insulation layer comprising SiC>2, AI2O3 or a similar alloy made of an insulating material, layer made of a ternary alloy with Ni and Cr as the main components sensitive to strain, and finally an optional protective layer made of SiC»2, AI2O3 or a similar alloy made of an insulating material. The ternary NiCr-alloy thereby may be adjusted by selecting the third alloy component and through suitable process execution in such a way that a possibly lowest temperature coefficient of the complete precision force sensor is the result.
In order to be able to manufacture as many spring elements as possible in one work process when applying the strain gauge elements, for instance by means of sputtering, the actual spring element is purposely designed as small as possible. This actual spring element is then supplemented at the ends through end-pieces, in order to enable good fixing possibilities of the precision force sensor as well as force transmission elements adjusted to the individual application. The connection between the actual spring element and the end pieces can be made through welding or adhesive binding.
In case of end pieces made of plastic, these can also be injection molded directly on to the spring element (the so-called Incert-molding).
In a further advancement, the spring element shows the shape of a parallel guide. The precision force sensor is, in this case, insensitive to fluctuating force transmission points. When using the precision force sensor as a load cell, a weighing
pan can then be fixed directly on to the force transmission area of the precision force sensor or at the related end piece.
The invention is described below with the help of a design example along with illustrations. The different illustrations thereby show the following:
Figure 1: shows an overview of the precision force sensor in a perspective view,
Figure 2: a sectional view of the layer system of the strain gauge elements, and
Figure 3: a profile view of the precision force sensor with the end pieces
The precision force sensor in Figure 1 possesses a spring element 1, which has an area fixed to the housing 2, an upper rod 3, a lower rod 4, and a force transmission area 5. The springy areas of the spring element 1 are mainly the thin areas 6, whereas the remaining areas are mostly stiff owing to their geometric design. The complete spring element is worked out from a single piece through the inner cavity 7. The material thereby preferably an exudation-tempered nickel-base alloy with a nickel-content between 50 and 55% and a chromium content between 17 and 21%. Because of the difficult processability of this material, the geometry is selected in such a way that also production methods for materials that are difficult to process, such as wire eroding, can be used. At the thin points 6, strain gauge elements 10 are located, whose structure is explained in more detail with the help of Figure 2. The spring element 1 is fixed to a housing 8 indicated only schematically. The transmission of the force to be measured, which is indicated in Figure 1 by a force arrow 9', takes place via an application-specific force transmission part 9, which is also indicated only schematically in Figure 1. Since the depicted spring element 1 has the shape of a parallel guide, a weighing pan can be fixed directly on to the force transmission part 9 (not shown), when using the precision force sensor as load cell.
The details of the polymer-free strain gauge elements 10 used are given in Figure 2: The strain gauge elements 10 comprise a thin-layer structure, which is deposited preferably by means of PVD or CVD process. The insulation layer 11 deposited directly on the spring element preferably comprises AI2O3, Si02 or Si2N3, which is deposited by means of plasma-deposition free of pores. The exact composition varies during the deposition so that the exact stochiometric composition is frequently not present in the finished insulation layer. Instead of a single insulation layer, a combination of several layers is also possible. The aim thereby is to achieve a reliable insulation between the spring element and the following layer sensitive to strain at the lowest possible layer thickness. For the layer 12 sensitive to strain preferably ternary NiCr-alloys are used, which, by a suitable process design of the sputter-process and the suitable selection of the third alloy component, can be modified in their composition in such a way that a zero temperature dependence of the apparent strain on the spring element is the result. Optionally, one more cover layer made of the insulation materials given above can be deposited as an additional reaction-less protection. Since the polymer-free thin-layer structure is composed exclusively of materials that do not absorb any water, the additional cover layer mentioned above can be omitted in a lot of application cases.
All layers shown in Figure 2 are not drawn to scale: The individual layers of the strain gauge elements 10 have thicknesses in the um-range, while the thickness of the thin areas 6 lies in the mm-range depending upon the load range of the precision force sensor.
Figure 2 shows only the layers essential for the function. For instance, the structures necessary for contacting can easily be added by any expert. A layer system made of sputtered gold and nickel is used preferably for the contact structures. The nickel-layer thereby also acts as a diffusion barrier, in order to ensure the long-term stability of the ternary NiCr-layer sensitive to strain.
Frequently, sensor structures made of materials, which show a high temperature coefficient of the electrical resistances, are also laced. In this way, a possible present temperature coefficient of the entire precision force sensor can be corrected.
Figure 3 shows a precision force sensor, which has the end pieces 21 and 22 to the side of the spring element 1. The end pieces are preferably made of a material that can be processed easily. In this way, simpler fixing possibilities and more complex shapes can be designed. The end piece 21, for instance, shows a tapped bore 23, by means of which the precision force sensor can be screwed on to a part 25 of the housing (screw 24). The end piece 21 is designed in the lower area a little longer than the spring element 1, so that a protrusion 26 arises. In this way, the precision force sensor can be screwed properly to a flat housing part 25, such as to a flat floor panel and this gives rise to a gap 27, which restricts the maximum deflection of the spring element 1. The other, rectangular end piece 2 shows a round shaft 28 in the upper area with a conical end 29. By means of this, a conventional round weighing pan (not shown) can be placed directly upon this. The connection between the spring element 1 and the end pieces 21 and 22 is done preferably by welding. However, an adhesive connection can also be made, since this adhesive binding has a low specific load owing to the relatively large adhesive area. A possible creep of this adhesive area is not critical, because it does not influence the accuracy of the precision force sensor and the width of the gap 27 — and thus the overload limit — changes only negligibly. It is also possible to manufacture the end pieces from plastic and to inject this plastic directly on to the spring element 1. This process is known by the name of Incert-molding. Naturally, it is also possible to select a different material for the end piece 21 and/or a different connection method for the end piece 22. It is also possible to provide only one end piece, 21 or 22.
List of reference numerals
1.' spring element
2. area fixed to the housing
3. upper rod
4. lower rod
5. force transmission area
6. thin part
7. inner cavity
9. force transmission component
10. strain gauge element
11. insulation layer
12 layer sensitive to strain
13 cover layer
21,22 end pieces
23 tapped bore
25 housing part
29 conical end
1. Precision force sensor with a spring element (1), whose load-dependent
• displacement is converted in an electrical signal by the strain gauge element
(10), characterized by the fact that the spring element (1) comprises of an exudation-tempered nickel-base alloy with a nickel content between 36 and 60% and a chromium content between 15 and 25% and that the strain gauge elements (10) are made of a polymer-free layer system.
2. Precision force sensor as per claim 1, characterized by the fact that as exudation-tempered nickel-base alloy an alloy with a nickel content between 50 and 55% and a chromium content between 17 and 21% is used.
3. Precision force sensor as per claim 2, characterized by the fact that as exudation-tempered nickel-base alloy the alloy with the material number 2.4668 in accordance with EN 10027-2 is used.
4. Precision force sensor as per claim 1, characterized by the fact that the layer system for the strain gauge elements (10) is sputtered.
5. . Precision force sensor as per claim 4, characterized by the fact that the layer
system sputtered on the spring element for the strain gauge elements (10) has the following layer sequence: insulation layer made of SiCh or AI2O3, a layer sensitive to strain made of a ternary NiCr-alloy, a cover layer made of Si02 or AI2O3.
6. Precision force sensor as per claim 4 or 5, characterized by the fact that in
addition at least one layer strongly dependent on temperature is sputtered.
7. Precision force sensor as per claim 1, characterized by the fact that the spring element (1) shows an end piece (21, 22) made of a different material on at least one of the ends.
8. Precision force sensor as per claim 7, characterized by the fact that the end piece(s) (21,22) is/are connected to the spring element (1) through welding.
9. Precision force sensor as per claim 7, characterized by the fact that the end piece(s) (21, 22) is/are connected to the spring element (1) through adhesive binding.
10. Precision force sensor as per claim 7, characterized by the fact that the end piece(s) (21, 22) comprise a plastic material and is/are injected on to the spring element (1).
11. Precision force sensor as per claim 1, characterized by the fact that the spring element (1) shows the form of a parallel guide.
Disclosed is a precision force transducer comprising a spring element (1) whose load-dependent deflection is converted into an electrical signal by means of strain gauge elements (10). The spring element (1) is made of a precipitation-hardening basic nickel alloy that has a nickel content ranging from 36 to 60 percent and a chromium content ranging form 15 to 25 percent while the strain gauge elements are composed of a polymer-free layer system, thus making it possible to produce a precision force transducer which features great accuracy, a low amount of creeping, and low sensitivity to moisture.
The Controller of Patents,
The Patent Office,
|Indian Patent Application Number
|PG Journal Number
|Date of Filing
|Name of Patentee
|SARTORIUS WEIGHING TECHNOLOGY GMBH
|WEENDER LANDSTRASSE 94-108, 37075 GOETTINGEN
|PCT International Classification Number
|PCT International Application Number
|PCT International Filing date