Sensor Element

Abstract

The invention relates to a sensor element that is capable of sensing dynamic loads and/or vibrations in a machine component, the sensor element comprising a multilayer coating (302) deposited on a substrate (300′). The multilayer coating comprises a sensitive layer (304) of a piezoelectric material and a first electrode layer (308) of a metallic material, which electrode layer serves as a first electrode of the sensor element. The substrate (300′) may serve as the second electrode. In order to seal any pinholes in the sensitive layer (304) and thereby prevent short-circuiting between the first and second electrodes, the multilayer coating further comprises a pinhole sealing layer (306) deposited on top of the sensitive layer (304), so as to be sandwiched between the sensitive layer and the first electrode layer (308).

Claims

1. A sensor element comprising a multilayer coating deposited on a substrate, the multilayer coating including a piezoelectric layer of a piezoelectric material, a first electrode layer of a metallic material configured to function as a first electrode of the sensor element, and a pinhole sealing layer of a dielectric material deposited on top of the piezoelectric layer so as to be sandwiched between the piezoelectric layer and the first electrode layer.

2. The sensor element according to claim 1, wherein the piezoelectric layer includes one of aluminium nitride and zinc oxide, is deposited with a crystallographic orientation predominantly lying in the c-axis, and has a thickness of less than 10 microns.

3. The sensor element according to claim 1, wherein the piezoelectric layer has a thickness of less than 2 microns.

4. The sensor element according to claim 1, wherein the pinhole sealing layer consists of aluminium oxide and has a thickness of less than 10% the thickness of the piezoelectric layer.

5. The sensor element according to claim 1, wherein the thickness of the pinhole sealing layer is less than 5% the thickness of the piezoelectric layer.

6. The sensor element according to claim 1, wherein the multilayer coating further includes an insulation layer deposited on top of the first electrode layer.

7. The sensor element according to claim 1, wherein the piezoelectric layer is deposited on the substrate.

8. The sensor element according to claim 1, wherein the multilayer coating further includes a metallic bond layer deposited on top of the substrate immediately prior to the piezoelectric layer.

9. The sensor element according to claim 1, wherein the substrate is formed by a material that is an electrical conductor.

10. The sensor element according to claim 1, wherein the substrate is a surface of a sheet-metal carrier with a thickness of less than 500 microns.

11. The sensor element according to claim 9, wherein the sheet-metal carrier is formed from one of a steel material including up to 0.2 wt. % carbon, nicked alloy, titanium, and titanium alloy.

12. The sensor element according to claim 1, wherein the substrate is formed by at least one surface of a mechanical component.

13. The sensor element according to claim 1, wherein the mechanical component is a component of one of a rolling element bearing and a hub bearing unit.

14. The sensor element according to claim 1, wherein the substrate is formed by a material that is an electrical insulator.

15. The sensor element according to claim 1, wherein the multilayer coating further includes a second electrode layer configured to function as a second electrode of the sensor element.

16. The sensor element according to claim 1, wherein the substrate is configured to function as a second electrode of the sensor element.

17-18. (canceled)

19. A method of producing a sensor element, the sensor element including a multilayer coating deposited on a substrate, the method comprising the steps of: (i) depositing a piezoelectric layer; (ii) depositing a pinhole sealing layer on top of the piezoelectric layer; (iii) depositing an electrode layer on top of the pinhole sealing layer.

20. The method according to claim 19, wherein the piezoelectric layer includes one of aluminium nitride and zinc oxide, is deposited with a crystallographic orientation predominantly lying in the c-axis, and has a thickness of less than 10 microns.

21. The method according to claim 19, wherein the piezoelectric layer has a thickness of less than 2 microns.

22. The method according to claim 19, wherein the pinhole sealing layer includes aluminium oxide deposited in a thickness of less than 10% of the thickness of the piezoelectric layer.

23. The method according to claim 19, wherein the pinhole sealing layer has a thickness of less than 5% of the thickness of the piezoelectric layer.

24. The method according to claim 19, wherein the step of depositing the piezoelectric layer includes depositing the piezoelectric layer on one of the substrate and a metallic bond layer.

25. (canceled)

26. The method according to claim 19, further comprising a step of depositing an insulating layer on top of the electrode layer.

27. The method according to claim 19, wherein at least one of: at least one of the layers of the multilayer coating is deposited by means of a physical vapour deposition process; and each layer of the multilayer coating is deposited by means of a physical vapour deposition process.

28. (canceled)

29. The method according to claim 19, wherein the piezoelectric layer and the pinhole sealing layer are deposited by means of reactive sputtering.

30. The method according to claim 19, wherein the pinhole sealing layer is deposited by means of atomic layer deposition.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0031] The invention will now be described in more detail for explanatory and in no sense limiting purposes with reference to the following figures, in which

[0032] FIG. 1 illustrates an example of a rolling element bearing;

[0033] FIG. 2 illustrates a sectional view of part of a mechanical component provided with a sensor element according to a first embodiment of the invention;

[0034] FIG. 3 illustrates a layer-by-layer composition of a multilayer coating according to a second embodiment of the invention.

[0035] FIG. 4 illustrates a flowchart of a method by which the multilayer coating of FIG. 3 may be produced.

DETAILED DESCRIPTION

[0036] FIG. 1 illustrates an example of a rolling element bearing. The bearing comprises an inner ring 100 and an outer ring 102 rotatably coupled by means of rolling elements 104. The rolling elements are disposed between the inner and outer rings on at least one set of opposing raceways. Many applications exist where it is desirable to determine the loads acting on a bearing. A common way of doing this is to mount one or more displacement sensors on e.g. an outer circumference of the outer ring 102, to measure deformation of the outer ring and calculate the bearing loads therefrom. Such sensors would be destroyed if mounted on, for example, a bearing raceway. However, sensor elements placed on a bearing raceway would provide valuable information about contact stresses that occur when a bearing is in operation. This information can be obtained by means of a sensor element according to the invention, which comprises a wear-resistant multilayer coating that can be deposited directly on a mechanical component in a thickness of less than 50 microns.

[0037] FIG. 2 shows a sectional view of part of a mechanical component 200 that has been provided with a sensor element according to a first embodiment of the invention. The mechanical component could be, for example, an inner ring of a rolling element bearing, where the sensor element is intended to measure contact stresses on an inner ring raceway. The sensor element comprises a multilayer coating 202, which coating comprises a piezoelectric layer 204 deposited on substrate formed by a raceway surface 200′ and a perpendicular surface 200″ of the inner ring 200. The multilayer coating 202 further comprises an electrode layer 208, which serves as a first electrode. The substrate (200′, 200″) serve as the second electrode in the illustrated example. Suitable, the first electrode has a circumferential length of less than the distance between the points at which two adjacent rolling elements make contact with the raceway surface 200′, to ensure that only one rolling element at a time can cause the piezoelectric layer to deform. To insulate the first electrode 208 from contact with electrical conductors, an insulation layer 210 may be deposited on top of the electrode layer 208. The measurement application in this example entails that the sensor element will be subject to high tribological stresses, in which case it may be advantageous to provide the sensor element with an anti-wear coating (not shown) on top of the insulation layer 210.

[0038] During operation of the bearing, the rolling elements overroll the coated raceway surface 200′ on the inner ring, causing mechanical deformation of the piezoelectric layer 204. This mechanical deformation produces a proportional change in the electric polarization of the piezoelectric layer. The sensor output is therefore an electric charge generated between the first and second electrodes, which charge is proportional to the mechanical deformation. One way of measuring the sensor output is by coupling the first and second electrodes to an appropriate charge amplifier by means of sensor connections 212. The charge amplifier suitably convents the electric charge output to a voltage output. Thus, a voltage signal may be produced during bearing operation which, with the aid of suitable processing means, can be used to determine the contact stresses.

[0039] Because the coating may be subject to high stresses, the piezoelectric layer 208 in this embodiment of the invention preferably consists of aluminium nitride that is deposited with a crystallographic orientation which predominantly lies in the c-axis, <002> direction, i.e. perpendicular to the surface on which it is deposited. This orientation ensures optimal piezoelectric properties, and aluminium nitride also possesses the necessary robustness. Another advantage of aluminium nitride is that it is self-polarizing and does not require subsequent polarization to enhance the piezoelectric effect. A drawback of aluminium nitride is that it is porous. To be suitable for integration on e.g. a bearing raceway, the piezoelectric layer 204 is preferably deposited in a thickness of less than 5 microns. At such small layer thicknesses, there is a risk that the piezoelectric layer 204 will contain pinholes. Such pinholes could allow short-circuiting between the first and second electrodes, leading to signal failure.

[0040] According to the invention, this difficulty is overcome by depositing a pinhole sealing layer 206 on top of the piezoelectric layer 204, prior to deposition of the electrode layer 208. The pinhole sealing layer 206 is an ultra-thin film of a dielectric material, which seals the pores in the piezoelectric layer without substantially adding to the thickness of the piezoelectric layer. Suitably, the thickness of the pinhole sealing layer 206 is less than 10% of the thickness of the piezoelectric layer 204 and is preferably less than 5% of the thickness of the piezoelectric layer. The pinhole sealing layer 206 preferably consists of aluminium oxide deposited in a thickness of between 0.01 and 0.1 micron. A sensor element according to the first embodiment of the invention therefore takes up very little space and produces a highly reliable signal.

[0041] As shown in FIG. 2, the multilayer coating 202 may be provided such that it extends over more than one surface of the component 200. With reference to the example of a bearing inner ring, the coating extends from the raceway surface 200′ of the inner ring to a perpendicular surface 200″ of the inner ring, where the sensor connections 212 are suitably attached to the first and second electrodes. In the case of a bearing ring, realization of the sensor connections between the first and second electrodes and e.g. the charge amplifier is much more straightforward when the connections are made at this perpendicular surface 200″. This is particularly true if the bearing ring rotates during operation. In this case, the sensor connections between the first and second electrodes and the charge amplifier may be achieved by means of e.g. slip rings.

[0042] Thus, the multilayer coating 202 of a sensor element according to the invention may be deposited on surfaces of rotatable components. These surfaces include the inner and/or outer circumference of a bearing outer ring, the inner and/or outer circumferences of a bearing inner ring and even the surfaces of a rolling element. When deposited on a rolling element or on a bearing raceway, the piezoelectric layer 204 of the sensor element is subjected to compression and the piezoelectric effect is generated in d33 mode, meaning that the applied force is parallel to the direction of polarization. When deposited on an outer circumference of a bearing outer ring or the bore of a bearing inner ring, the piezoelectric layer bends as a result of deformation of the bearing ring due to the passage of rolling elements. Thus, the local bending component of the total movement of the piezoelectric layer will cause a difference in the strains induced on either side of the piezoelectric layer. The resulting forces act in a direction transverse to the direction of polarization, and piezoelectric effect is thus generated in d31 mode.

[0043] As well as being suitable for direct integration on a component, a sensor element according to the invention may also be integrated on a carrier element, which carrier element is then attached to the component on which measurements are to be performed. FIG. 3 illustrates a second embodiment of a sensor element according to the invention, where the multilayer coating is deposited on a surface 300′ of a metallic carrier 300, such as a thin sheet of stainless steel e.g. AISI 304 or AISI 316L. The carrier 300 may serve as the second electrode. The multilayer coating 302 shown in FIG. 3 comprises a piezoelectric layer 304, a pinhole sealing layer 306, a first electrode layer 308, and an insulation layer 310, where the individual layers are deposited in this sequence. In this embodiment, and in the first embodiment, the multilayer coating 302 may further comprise a metallic bond layer 301, which is deposited immediately prior to deposition of the piezoelectric layer 304. The bond layer 301 improves the adhesion of the piezoelectric layer 304, thereby improving the reliability and accuracy of the sensor element. Suitable materials for the bond layer include molybdenum and chromium.

[0044] The multilayer coating 302 may also be deposited on a non-metallic substrate, in which case a second electrode layer of e.g. copper or nickel would be deposited prior to deposition of the piezoelectric layer 304, to serve as the second electrode. The multilayer coating may also comprise a second electrode layer to serve as the second electrode when the coating is deposited on a metallic substrate (on a metallic carrier element or on a component surface). Suitably, a further insulation layer is then deposited between the substrate 300′ and the second electrode layer. Further layers of shielding may also be provided.

[0045] A carrier element 300 made of a sheet of e.g. stainless steel, with a thickness of less than 500 microns is flexible, strong and corrosion resistant. A sensor element that comprises such a thin metal sheet is therefore also flexible and resilient. This is particularly advantageous if the sensor element is to be mounted on a curved component, such as a bearing outer ring. The sensor element can then be made to follow the curvature of e.g. the bearing ring. A further advantage of a carrier element 300 made from stainless steel or other steel material is that the sensor element is then suitable for welding to a steel component. When the steel component is a bearing, suitable materials for the carrier element 300 include steels with no more than 0.2 wt. % carbon, nickel and nickel alloys, titanium and titanium alloys. For example, a sensor element according to the second embodiment of the invention could be mounted in a circumferential direction on a bearing outer ring and affixed by means of spot welding a first and second longitudinal end of the carrier to the surface of the bearing outer ring. Such a joining process is much faster and more economical than e.g. adhesive bonding.

[0046] Thus, a multilayer coating of a sensor element according to the invention may be deposited on one or more surfaces of a machine component or on a surface 300′ of e.g. a thin and flexible metallic carrier. The multilayer coating may have a thickness of less than 50 microns, with typical values of between 10 and 20 microns. The individual layers of the coating, of course, have even smaller thickness values. A suitable method of achieving such thin-film layers is by means of a physical vapour deposition process. A preferred method of producing one example of a multilayer coating of a sensor element according to the invention will now be described with reference to FIG. 3 and the flowchart of FIG. 4.

[0047] In a first step 410, the metallic bond layer 301 is deposited on the metallic substrate 300 (which substrate can also be a surface of e.g. a bearing ring). A molybdenum target is sputtered in an inert gas environment, leading to the formation of a molybdenum film on the metallic substrate 300. The film preferably has a thickness of less than 1 micron, where 0.1 micron is a typical value.

[0048] In a second step 420, the piezoelectric layer 304 is deposited on the bond layer 301 by means of reactive sputtering. A pure aluminium target is sputtered in a nitrogen-containing environment, leading to the formation of an aluminium nitride film on the bond layer 301. The aluminium nitride film 304 has a thickness of less than 10 microns, with a preferred value of between 0.5 and 2 microns. During this step of the process, the process parameters are controlled to ensure that the aluminium nitride is formed with a crystallographic orientation which predominantly lies in the c-axis.

[0049] In a third step 430, the pinhole sealing layer 306 is deposited on the piezoelectric layer 304 by means of reactive sputtering. A pure aluminium target is sputtered in an oxygen-containing environment, leading to the formation of an aluminium oxide film on the piezoelectric layer 304. The aluminium oxide film may have a thickness of less than 0.5 micron, with a preferred value of between 0.01 and 0.1 micron.

[0050] In a fourth step 440, the electrode layer 408 is deposited on the pinhole sealing layer 306. A nickel target is sputtered in an inert gas environment, leading to the formation of a nickel film on the pinhole sealing layer 308. The nickel film may have a thickness of less than 5 microns, with a preferred value of between 0.1 and 2 microns. During the deposition, suitable masks are used to enable an electrical connection from the electrode layer 408 to be made.

[0051] In a fifth step 450, the insulation layer 410 is deposited on the electrode layer 308 by means of reactive sputtering. A pure aluminium target is sputtered in an oxygen-containing environment as described above for the third step. The resulting aluminium oxide insulation layer has a thickness of less than 10 microns, with a preferred value of between 0.1 and 1 micron.

[0052] The above process may be carried out in commercially available sputter deposition equipment. Magnetron sputtering or RF sputtering, for example, may be applied. An advantage of the sputtering technique is that the entire multilayer coating of the sensor element may be deposited in the same chamber of the process equipment, thereby saving time and costs. According to an alternative method and embodiment of the sensor element, the aluminium oxide sealing layer may be provided by means of atomic layer deposition.

[0053] A sensor element according to the invention can be used to determine loads in a component such as a rolling element bearing. The sensor element produces a strong signal with an excellent linear response over a wide frequency range. Loads/deformations of a bearing ring—due to the passage of the rolling elements—may be measured by applying the sensor element on e.g. an inner circumference of the inner ring and suitably filtering the signal produced. The cut-off frequency is selected depending on the application conditions, e.g. speed of rotation. The inventive sensor element could also be used on a hub bearing unit. For example, an inboard side of the hub mounting flange could be provided with a multilayer coating according to the invention, to detect loads between the flange and the part to which it is affixed (e.g. a steering knuckle). In a further example, the coating could be provided on the flange radius at the outboard side of the hub mounting flange. This radius refers to an arcuate portion between the hub outer ring and the hub mounting flange, and a sensor element located here could be used to determine bending moment due to cornering forces.

[0054] A sensor element according to the invention may also be used for condition monitoring purposes, e.g. to monitor high-frequency vibrations in a rolling element bearing. For the sensing of vibrations, a higher cut-off frequency is suitably selected. As will be clear to persons skilled in the art, suitable pass-band filters and processing circuitry may be employed to process the sensor signal in the desired frequency range or ranges. Depending on the stiffness, damping and mass characteristics of the system in which the e.g. bearing is mounted, it may also be advantageous to filter out certain resonances from the system

[0055] Thus, a sensor element according to the invention may be used to sense dynamic loads and/or vibrations in a component. A sensor element according to the first embodiment of the invention is suitable for direct integration on one or more surfaces of the component. A sensor element according to the second embodiment of the invention may be quickly and easily affixed to the surface of a component.

[0056] A number of aspects/embodiments of the invention have been described. It is to be understood that each aspect/embodiment may be combined with any other aspect/embodiment. Moreover the invention is not restricted to the described embodiments, but may be varied within the scope of the accompanying patent claims.

REFERENCE NUMERALS

[0057] FIG. 1 illustrates an example of a rolling element bearing, [0058] 100 bearing inner ring, [0059] 102 bearing outer ring, [0060] 104 rolling elements. [0061] FIG. 2 illustrates a sectional view of part of a mechanical component provided with a sensor element according to a first embodiment of the invention, [0062] 200 mechanical component (bearing inner ring), [0063] 200′, 200″ component surfaces (substrate), [0064] 202 multilayer coating, [0065] 204 piezoelectric layer, [0066] 206 pinhole sealing layer, [0067] 208 electrode layer, [0068] 210 insulation layer, [0069] 212 sensor connections. [0070] FIG. 3 illustrates a layer-by-layer composition of a multilayer coating according to a second embodiment of the invention, [0071] 300 thin metallic sheet, [0072] 300′ substrate, [0073] 301 bond layer, [0074] 302 multilayer coating, [0075] 304 piezoelectric layer, [0076] 306 pinhole sealing layer, [0077] 308 electrode layer, [0078] 310 insulation layer. [0079] FIG. 4 illustrates a flowchart of a method by which the multilayer coating of FIG. 3 may be produced, [0080] 410 deposit layer of molybdenum by means of sputtering, [0081] 420 deposit layer of aluminium nitride by means of reactive sputtering, [0082] 430 deposit layer of aluminium oxide by means of reactive sputtering, [0083] 440 deposit layer of nickel by means of sputtering, [0084] 450 deposit layer of aluminium oxide by means of reactive sputtering.