WO2012175256A1

WO2012175256A1 - Method for producing a pressure sensor

Info

Publication number: WO2012175256A1
Application number: PCT/EP2012/059347
Authority: WO
Inventors: Ulrich Buder; Anh Tuan Tham
Original assignee: Endress+Hauser Gmbh+Co. Kg
Priority date: 2011-06-20
Filing date: 2012-05-21
Publication date: 2012-12-27
Also published as: DE102011077829A1

Abstract

The invention relates to a method for connecting a first component of a pressure sensor to a second component of a pressure sensor by means of a circumferential weld joint. The first component has a first substantially rotationally symmetrical connecting region, and the second component has a second substantially rotationally symmetrical connecting region that is to be connected to the first connecting region by means of the circumferential weld joint. The first component has a transducer element which is either not symmetrical or which has a symmetry that is broken with respect to the axis of symmetry of the rotationally symmetrical connecting regions. The method has the following method steps: positioning the first and second connecting regions relative to each other; and producing the weld joint which is sequentially prepared along the periphery of the components, the connecting regions to be welded being locally melted starting from at least one starting point. The at least one starting point is specified with a defined orientation relative to the transducer element.

Description

Method for producing a pressure sensor:

The present invention relates to a method for producing a pressure sensor, in particular a pressure sensor with a (piezo) resistive pressure transducer.

Resistive pressure transducers usually comprise a pressurizable measuring diaphragm which has at least two resistance elements with different dependencies on a pressure-dependent deformation of the measuring diaphragm. In most cases, four resistance elements are provided with pairwise approximately equal pressure dependence, wherein the resistance elements are connected to a full bridge. From the number and pressure dependence of the resistance elements described it follows that the measuring diaphragm has, at best, a twofold rotational symmetry.

Pressure sensors in process measuring technology have, for example, a hydraulic measuring device which comprises a metallic housing which has in its interior a chamber into which a transducer element which comprises the measuring diaphragm is inserted. The transducer element is often held on a support which closes an opening of the chamber pressure-tight. Furthermore, especially in modular manufacturing concepts after the

Completion of sensor modules, which a hydraulic measuring mechanism with

comprise welded transducer element, a process connection to the

Sensor module, in particular attached to the housing of the measuring unit by means of a circumferential weld. This weld is made sequentially, the joining partners circumferentially with a local melting of the area to be welded when driving around the joint partners, with several distributed around the circumference

Welding sources can be used in parallel. A variable characterizing the welding process is, in particular, the amount of heat introduced and removed per time and area.

It is known that sequential welding processes make thermomechanically stress-free joining of the workpieces impossible due to the local and temporally staggered production and solidification of the melt. In the joined workpiece, therefore, there is a dependent on the welding process after cooling

Stress state, which in the extreme case is completely asymmetrical, but in any case one compared to the rotational symmetry of a substantially at least partially cylindrical rotary part has a broken symmetry with an asymmetric state or a symmetrical state with significantly reduced symmetry. This manifests itself in a state of stress corresponding shrinkage of the joined component.

Depending on the degree of mechanical coupling of the sensor element to the housing, the mechanical stresses induced by the welding process to a greater or lesser extent also reach the transducer element which is sensitive to mechanical stresses and cause it to become distorted. This leads to a change of the

Sensor output signal or sensor behavior, which is referred to as welding reaction.

It is the object of the present invention to make the weld reactions more controllable or to minimize them.

The object is achieved by the method according to the independent claim. 1

The approach of the invention is based on the consideration that both the transducer element and the housing after the connection with the

Process connection have a broken symmetry, and that the

Interaction between the two components, which take the form of

Welding reactions manifested by the orientation of the components depends on each other.

According to the invention, the scattering of welding reactions between different pressure sensors is now minimized by always aligning the components in the same orientation with respect to each other. Insofar as the here considerable orientation of the housing is given by the welding process, this means that the welding process for the production of a pressure sensor type always runs with the same orientation with respect to the transducer element. This means, in particular, that the starting point and the direction of the welding process relative to the transducer element for the pressure sensors of one type are set substantially uniformly.

The inventive method for connecting a first component of a Pressure sensor with a second component of a pressure sensor by means of a circumferential weld, wherein the first component has a first, substantially rotationally symmetrical connection region, wherein the second component has a second, substantially rotationally symmetrical connection region, which is to be connected to the first connection region by the circumferential weld wherein the first component comprises a transducer element which has no or one with respect to the axis of symmetry of the rotationally symmetrical connection regions of a broken symmetry, is a method which comprises the following steps:

Positioning the first and second connection areas with each other; and

Producing the welded joint, which is prepared from at least one starting point, the components sequentially circulating, with a local melting of the joint areas to be welded; wherein according to the invention the at least one starting point is predetermined with a fixed orientation with respect to the transducer element.

According to one embodiment of the invention, the direction is further specified, in which the weld is prepared from the starting point.

According to one embodiment of the invention, the defined orientation is given by an angle φ ₀ + j ^■ 2 ^■ π / K, wherein the angle φ _{0 is} predetermined, where K is a symmetry number of the resistance elements of the transducer element element and / or a symmetry number of the stresses of the transducer element due of weld pains, where K <4, and where j = 0, 1, ..., K-1. According to one embodiment of the invention, the defined orientation is given by an angle φ ₀ + j ^■ 2 ^■ π / (L ^■ M), wherein the angle φ _{0 is} given, where L a symmetry number of the resistive elements of the transducer element element and M is a symmetry number of the strains of the transducer element due to welding perturbations, where L and M are prime, where (L ^■ M) <12, in particular (L ^■ M) <6, and where j = 0, 1, ..., (L ^■ M) - 1.

According to one embodiment of the invention, the same angle φ ₀ for the

Prescribed pressure sensors of one type or a series of a type.

According to one embodiment of the invention, the first component comprises a housing in which the transducer element is arranged.

According to one embodiment of the invention, the housing with respect to the

Symmetryeachse the first connection area substantially

rotationally symmetrical.

According to one embodiment of the invention, the first component has a

Mark on, based on which the orientation of the first starting point is determined with respect to the transducer element. According to one embodiment of the invention, the production of the

Weld joint simultaneously from several starting points each with a predetermined orientation with respect to the transducer element. In particular, N starting points can be provided whose orientation is offset from one another by 2 ^■ π / N, where N> 2 and N <12, preferably N <8, in particular N <4.

In one development of the invention, the orientation of the welding process is selected such that due to the welding process introduced into the transducer element mechanical stresses that act on the resistance elements of the transducer element, are minimal or as homogeneous as possible.

This can be achieved by keeping the orientation of the starting point of the welding process constant not only in an arbitrary orientation with respect to the measuring cell, but by first determining in which

Orientation the welding reactions in the above sense are minimal.

According to one embodiment of the invention, the transducer element has a Measuring diaphragm whose surface normal runs parallel to the axis of symmetry of the first connection region, wherein the transducer element more

Having resistive elements, which are each arranged at least in pairs on a circle around the center of the measuring diaphragm wherein the at least one starting point for preparing the welded connection is selected so that for a measuring diaphragm in equilibrium, the average of the amounts of

Radial voltage quotient of predominantly radial stresses sensing resistive elements less than 1/2, in particular less than 1/4 and preferably less than 1/8, wherein the radial stress quotient of a

Resistance element is defined as the amount of radial stress of the

Averaged over the surface area of the resistance element divided by the amount of the radial stress of the measuring membrane, a measuring membrane is averaged over a circular ring around the center of the measuring membrane in which the surface area of the resistance element lies.

According to one embodiment of the invention, the transducer element has a

Measuring diaphragm whose surface normal runs parallel to the axis of symmetry of the first connection region, wherein the transducer element more

Resistance elements, which are each arranged at least in pairs on a circle around the center of the measuring diaphragm, wherein the at least one starting point for preparing the welded connection is selected so that for a measuring diaphragm in equilibrium, the average of the amounts of

Tangential voltage quotient of predominantly tangential voltage sensing resistive elements less than 1/2, in particular less than 1/4 and preferably less than 1/8, wherein the Tangentialspannungsquotient a resistive element is defined as the amount of tangential stress of the measuring membrane averaged over the surface area of the resistive element divided by the Amount of radial stress of the measuring membrane averaged over a circular ring around the center of the measuring membrane in the area of the

Resistive element is located.

A transducer element comprising monocrystalline silicon usually comprises a first pair of resistive elements which predominantly sense radial stresses and a second pair of resistive elements which predominantly detect tangential voltages, wherein the pairs of resistive elements can be arranged in particular on a circle which in particular in FIG Edge region of the deflectable region of the measuring diaphragm are arranged.

Transducer elements comprising polycrystalline silicon or strain gages usually have two pairs of resistive elements

arranged in different radii.

Carbon nanotube-based transducer elements allow the detection of radial or tangential stresses by suitable positioning of the CNT. CNT-based transducer elements may only evaluate the resistance of a CNT. Accordingly, it applies here that the at least one starting point for preparing the welded joint is chosen so that for a measuring membrane in equilibrium the mean value of a voltage quotient of the CNT is less than 1/2, in particular less than 1/4 and preferably less than 1/8, wherein the voltage quotient of the CNT is defined as the amount of stress of the measuring diaphragm averaged over the area of the CNT divided by the amount of stress of the measuring diaphragm averaged over a circular ring around the center of the measuring diaphragm in which the CNT lies.

A measuring diaphragm is then in equilibrium if the same pressure prevails on both sides of the measuring diaphragm, which in particular is not more than the atmospheric pressure, wherein a temperature of approximately 300 K is preferably present.

The above requirements for the various stress quotients relate to the pressure sensor after preparation of the welded joint in thermal equilibrium at room temperature.

In a further development of the invention, the transducer element in the measuring operation in equilibrium of the measuring diaphragm outputs a transducer zero point signal, wherein the transducer element outputs a transducer voltage signal at the upper limit of the measuring range, wherein the starting point is selected so that the zero point signal due to welding reactions by the Preparing the welded joint by not more than 0.5%, preferably not more than 0.2%, more preferably not more than 0.1% and more preferably not more than 0.05% of the difference between the converter zero point signal and the Transducer voltage signal changes. The following presentation is merely plausibility of the effect of the invention, in no way intended that the outlined mechanism to a

restrictive interpretation of the scope of the invention is used. The transferred due to the welding process on the transducer element

Stress state generates in this local stress maxima, minima and zero crossings, their position as a function of the welding process relative to

Starting point and direction of rotation of the weld can be described by means of an orientation of the welding start point with respect to the transducer element.

If a resistance element of the transducer element is in such a maximum / minimum or zero crossing, then its transducer output signal changes particularly strong or weak, i. the welding reaction is particularly strong or weak. If the position of the resistance elements of the

Transducer element to the welding induced voltage maxima, minima and

Zero crossings in the production of multiple pressure sensors each randomly, it is to be expected on the pressure sensor production a strong dispersion of welding reaction. If the position of the voltage maxima, minima and zero crossings in the transducer element relative to the weld start point is not known, it can still be determined by setting the orientation between the weld start point and transducer element for all sensors of one type or a series of one type

Welding process be added, the dispersion of welding reaction can be significantly reduced. The welding reaction thus achieved, which can also correspond to the maximum welding reaction for this process, can, as known and repeatable, be suitably taken into account in the further production process.

If the position of the voltage maxima, minima and zero crossings in the measuring element is known relative to the welding starting point, it is possible by a suitable definition of the angle between the welding starting point and the resistance elements of the

Transducer element possible to minimize not only the scattering but even the absolute value of the welding reaction. For this purpose, the welding starting point is selected so that the resistance elements of the transducer element for all sensors of one type, or a series of a type, which are joined with the same welding process, in the areas of the transducer element are present in which local zero crossings of the voltage distribution. Theoretically, an adaptation of the position of the transducer element or a change in the position of the resistance elements on em transducer element to achieve the effect of the invention is conceivable. The invention will now be described with reference to one of the figures

Embodiment explained. It shows: an end face of a housing of a pressure sensor to be produced according to the invention with a welded-on process connection;

Fig. 1 b: a side view of a housing of an inventive

to be produced pressure sensor with a welded process connection;

2 shows a representation of the shrinkage due to the welded joint as

Function of the angle; a representation of a coordinate system for describing the orientation of the welding process with respect to a transducer element; and

FIGS. 4a-c: Representations of voltages in the sensor element due to

Welding repercussions.

The one in Fign. 1 a and 1 b includes a rotationally symmetrical stainless steel housing 10, on the end face with a circumferential weld 14 which begins at a starting point in an orientation 16 and runs in the drawing counterclockwise, a process connection 20 is attached. Due to the sequential welding process, the axial shrinkage of the housing 10 over the weld is inhomogeneous, as shown in Fig. 1a, the shrinkage the greater the darker the dyeing. As shown in Fig. 1 b, a transducer element 30 is mounted in the housing 10 in a transducer chamber with a pressure-bearing joint, wherein over the joint mechanical stresses due to the

Welding process can be initiated in the transducer element. Fig. 2 shows the axial shrinkage along the circumference of the weld. The maximum here is about 0.25 mm and is about ^π / ₄ behind the

Welding start point. The minimum of the shrinkage is about 5 ^π / ₄ , that is to say the maximum, and is about 0.21 mm. It is obvious that these different shrinkage leads to considerable mechanical stresses in the

Housing 10 leads.

3 shows a coordinate system for describing the welding process with respect to a transducer element 30. The transducer element, which may in particular be a piezoresistive Si transducer element, comprises a stiff edge region 32 which surrounds a measuring diaphragm 34, wherein the measuring diaphragm has two inner

Resistor elements 36, 38 and two outer resistance elements 37, 39 which are operated in a full bridge circuit. As an axis for describing the orientation of the transducer element is a reference axis from the center of the diaphragm selected by an outer resistive element 37, which are arranged in a deflectable edge zone of the measuring diaphragm 34, in which occur at a pressure-dependent deflection of the measuring diaphragm, the largest radial stresses. Insofar as the housing is rotationally symmetric, so that an excellent starting point for the welded joint is not readily given, the radius on which the starting point lies can be relative to the reference axis of the weld

Transducer element take any angle φ ₀ . The effects of uncontrolled selected starting points will now be explained with reference to Figs. 4 ac explains:

At a first angle φ ₀ for a starting point of the welded joint, the stress distribution shown in FIG. 4 a may arise due to welding reactions in the measuring membrane, the "+" for a tensile stress at the top of the measuring membrane, the "-" for a compressive stress Here, in the area of the outer resistance elements 37, 39, tensile stresses occur and in the area of the inner resistance elements, compressive stresses occur, such a voltage distribution would obviously lead to a zero shift of the bridge circuit, the would simulate a pressurization of the measuring membrane from the viewer's perspective. At a second angle φ ₀ for a starting point of the welded joint which is offset by 90 ° with respect to the first angle, the stress distribution shown in FIG. 4b may follow, which would result in a zero offset with a reversed sign.

At a third angle φ ₀ for a starting point of the welded joint, which is offset by 135 ° with respect to the first angle, the stress distribution shown in FIG. 4c may follow, in which mechanical stresses due to welding reactions in the area of the resistance elements have zeros. There should be no zero shift here.

Now it is obvious that a missing determination of the angle φ ₀ for the

Starting point of welding a previously made calibration of the pressure sensor destroyed because it is not clear whether and in what direction the zero point is shifted.

Any determination of the angle φ ₀ for the starting point for all pressure sensors of a type or a series of a type, however, leads to a reproducible displacement of the zero point, which can then be uniformly compensated. In a more refined embodiment of the invention can be tried to determine the value for the angle φ ₀ first for which angle φ _0, the lowest zero shift and possibly also the lowest scatter of the

Zero offset occurs. If this angle is found, then a series of pressure sensors with this angle φ ₀ for the starting point can be made.

In order to find the starting point, a suitable marking can be attached to the housing.

Claims

claims

A method of connecting a first component of a pressure sensor to a second component of a pressure sensor by means of a circumferential weld joint, wherein the first component comprises a first, substantially

having rotationally symmetrical connection region, wherein the second component is a second, substantially

Has rotationally symmetrical connection region, which is to be connected to the first connection region by the circumferential weld joint, wherein the first component comprises a transducer element which has no or one with respect to the axis of symmetry of the rotationally symmetric

Compound regions has a broken symmetry, the method comprising:

Positioning the first and second connection areas with each other; and

Producing the welded joint, which is prepared from at least one starting point, the components sequentially circulating, with a local melting of the joint areas to be welded; characterized in that the at least one starting point is predetermined with a defined orientation with respect to the transducer element.

2. The method of claim 1 wherein the defined orientation is given by an angle φ ₀ + j ^■ 2 ^■ π / K, wherein the angle φ _{0 is} predetermined, where K is a symmetry number of the resistance elements of the

Transducer element element and / or a symmetry number of the stresses of the transducer element due to the welding reactions, where K <4, and where j = 0, 1, K-1.

The method of claim 1 wherein the defined orientation is given by an angle φ ₀ + j ^■ 2 ^■ π / (L ^■ M), wherein the angle φ _{0 is} predetermined, where L is a symmetry number of the resistance elements of

Transducer element element and M is a symmetry number of the stresses of the transducer element due to the weld perturbations, where L and M are prime, where (L ^■ M) <12, in particular (L ^■ M) <6, and where j = 0, 1, (L ^■ M) - 1.

A method according to claim 2 or 3, wherein the same angle φ ₀ is predetermined for the pressure sensors of one type or a series of a type.

Method according to one of claims 1 to 4, wherein the first component comprises housing, in which the transducer element is arranged.

The method of claim 5, wherein the housing relative to the

Symmetry axis of the first connection area substantially

is rotationally symmetric.

Method according to one of the preceding claims, wherein the first component has at least one mark, based on which the orientation of the first starting point is determined with respect to the transducer element.

Method according to one of the preceding claims, wherein the production of the welded connection takes place simultaneously from a plurality of starting points each with a predetermined orientation with respect to the transducer element. 9. The method of claim 8, wherein N starting points are provided, whose

Orientation against each other by 2 ^■ π / N is offset, where N> 2 and N <12, preferably N <8, in particular N <4.

10. The method according to any one of the preceding claims, wherein the

Transducer element has a measuring diaphragm whose surface normal runs parallel to the axis of symmetry of the first connection region, wherein the transducer element has a plurality of resistance elements which are each arranged at least in pairs on a circle around the center of the measuring diaphragm, wherein the at least one starting point for preparing the welded connection is selected such that for a measuring diaphragm in the

Balance of the mean value of the amounts of the radial stress quotients of predominantly radial stresses sensing resistive elements less than 1/2, in particular less than 1/4 and preferably less than 1/8, wherein the radial stress quotient of a resistive element is defined as the amount of the radial stress of the measuring diaphragm averaged over the Area of the resistance element divided by the amount of radial stress of the measuring membrane averaged over a circular ring around the center of the measuring membrane in the area of the

Resistive element is located.

Method according to one of the preceding claims, wherein the

Transducer element has a measuring membrane, the surface normal parallel to the axis of symmetry of the first connection region, wherein the transducer element comprises a plurality of resistance elements, which are each arranged at least in pairs on a circle around the center of the measuring diaphragm wherein the at least one starting point for preparing the welded connection is selected such that for a measuring membrane in the

Balance of the mean of the amounts of tangential stress quotient of predominantly tangential stress sensing resistive elements less than 1/2, in particular less than 1/4 and preferably less than 1/8, wherein the Tangentialspannungsquotient a resistive element is defined as the amount of tangential stress of the measuring membrane averaged over the Surface area of the resistance element divided by the amount of radial stress of the measuring membrane averaged over a circular ring around the center of the measuring membrane in the area of the

Resistive element is located.

Method according to one of the preceding claims, wherein the pressure sensor has a specified measuring range, wherein the transducer element outputs a transducer zero-point signal in equilibrium of the measuring diaphragm, and wherein the transducer element at the upper limit of the measuring range

Transducer voltage signal outputs. wherein the starting point is selected so that the zero point signal due to welding effects by the preparation of the welded joint by not more than 0.5%, preferably not more than 0.2%, more preferably not more than 0, 1% and more preferably not more than 0.05% of the difference between the transducer zero point signal and the transducer

Tension signal changes.

Series of multiple pressure sensors of one type, with the series off

Pressure sensors, which are made by the method according to one of claims 1 to 1 1, and wherein for all pressure sensors of the series of at least one starting point of the welded joint is defined the same.