FIELD

[0001] The invention relates to an object, such as a vehicle tire, comprising elastomer material. BACKGROUND

[0002] In order to develop better safety procedures for elastic friction contact materials, for example in automotive tires, shoe soles etc., obtaining information about the rubber-contact surface interaction (friction) is paramount.

[0003] Currently, in automotive tires, pressure sensors are fitted into all tires and are used to collect data about tire pressure and, hence, indirectly, drive safety. Sensors that are able to sense tire performance and safety with other variables than pressure are less common.

[0004] There exist several publications describing prototypes and experimental arrangements of acceleration sensors or strain gauges or similar attached inside the tire that measure tire deformations, thus the mechanical loading such as the friction forces. Such sensors are usually located in areas further away from the friction contact areas, that is where the severity of the strains and the forces are potentially less and/or accelerations are not excessively high thereby limiting the sensitivity and increasing the durability of the sensors. [0005] The lack of suitable sensors that can be attached close to the tire tread is due to the limited durability of the sensing instrumentation, such as the sensing element, electrical connectors and electronics in the extreme cyclic loading conditions in the tread of rolling tires.

[0006] The lack of real-time information about road conditions and tire performance and condition leads to a serious inadequate conception about tire-road interaction and the grip. Several safety concerns arise from this. The poor ability to properly predict tire performance and road conditions leads to safety hazards during driving, especially with the increase in driving automation. Similar problems relating to the grip between a moving object and its contact surface may also exist in connection with other objects made from elastomer material, such as bicycle tires and shoes.

SUMMARY OF THE INVENTION

[0007] The aim of the present invention is to reduce the drawbacks mentioned above.

[0008] This aim can be achieved by an object according to claim 1. The object according to the invention comprises elastomer material and a strain sensor for measuring strain of said elastomer material. The strain sensor comprises a body comprising or made of piezoresistive elastomer and arranged in contact with said elastomer material of the object. The body has an elastic modulus equal to or differing 10 % or less from the elastic modulus of the elastomer material of the object.

[0009] Significant benefits can be achieved by means of the invention.

[0010] By means of the strain sensor the strain of the object can be measured, which provides information on conditions between the moving object and its surface. The elastic properties of the piezoresistive body material of the strain sensor exactly or closely correspond to those of the elastomer material of the object which is in contact with the strain sensor body. This increases the operating life of the object and/or the strain sensor since the elastomer materials of the object and the body behave in a similar manner under strain. Therefore, no cracks or other damages caused by the unequal strains are present. Moreover, the temperature dependency of the viscoelastic properties of the sensor material is very similar to the corresponding material properties of the object, thus minimizing the elasticity mismatch over a broad temperature range.

[0011] According to an embodiment of the invention, the object comprises a vehicle tire and the strain sensor is placed within the tire so that the sensor body is in contact with elastomer material of the tire. The robust elastomeric strain sensor(s) can be located very near the tire-road contact, for example embedded in the tire tread rubber, to directly measure tire tread deformations laterally and radially, in real time during driving. The sensors can also be used to measure strains perpendicular to the surface. The measured strains give information on the slip at various slip angles, that is, while driving along straight line, cornering, and also in the zero acceleration situations, e.g. steady driving at constant velocity, when it has been previously difficult to gather any in situ information of the friction potential, which can be understood as the maximum attainable friction between the tire and road surface in simultaneous road conditions.

[0012] The data measured can be used to gather information about tire behaviour and aging, tire-road friction and road conditions, generating useful and easily interpreted sensor data thus providing information on the prevailing friction potential, setting conditions for safe driving speed, for example. Moreover, the friction potential data can be made directly adequate for data sharing to serve the other road users.

[0013] According to another embodiment of the invention, the object comprises a vehicle wheel and the strain sensor is arranged in contact with elastomer material of the tire. The strain sensor is coupled to a detection unit configured to detect changes in electric resistance/impedance of the body the strain sensor. The detection unit includes a wireless transmitter configured to transmit measurement data to a desired remote location. The detection unit is located outside a space defined by the inner surface of the tire and the wheel rim. This ensures undisturbed data transmission from the signal processing unit. Usability view of the information that can be generated using the tire sensor data and to join with other available data is huge. Digitally adopted information can further be shared to other road users and/or other applications.

BRIEF DESCRIPTION OF THE DRAWINGS [0014] In the following, the invention will be described in detail by the aid of an example with reference to the attached drawings, wherein

[0015] Fig. 1 shows a strain sensor for measuring strain of an object according to an embodiment of the invention, and

[0016] Fig. 2 shows as a cross-sectional view a vehicle wheel provided with the strains sensor of fig. 1.

EMBODIMENTS

[0017] Fig. 1 shows a strain sensor 2 for monitoring and/or measuring strain of an object 1 that comprises elastomer material, such as rubber. The object 1 can be a traction device which is in contact with a load bearing surface or other contact surface. The object 1 comprises typically a vehicle tire, such as a bicycle tire, car tire or heavy vehicle (bus, truck) tire, and the load bearing surface is a road surface. The vehicle can be an electric vehicle. Alternatively, the object 1 can be an axle sealing, a bumper or a shoe having a sole made of elastomer material or any other object comprising elastomer material exposed to a mechanical load and significant strain. The strain sensor 2 is arranged in contact with or in physical communication with the elastomer material of the object 1. The strain sensor 2 can be integral with or embedded in the elastomer material of the object 1.

[0018] The strain sensor 1 comprises a body 3 comprising or made of piezoresistive elastomer. The body 3 is arranged in physical contact with or in physical communication with the elastomer material of the object 1. The body 3 is attached to the elastomer material of the object 1. Electric resistance/impedance of piezoresistive material changes in response to changes in mechanical stress applied to said material.

[0019] The piezoresistive elastomer of the body 3 contains suitable rubber, e.g. styrene butadiene rubber (SBR) or solution-styrene butadiene rubber (S-SBR). Suitable styrene butadiene rubber materials for the body 3 are described in ASTM standard D3191-06. Any other elastomer that can be utilized in tire and that can be made piezoresistive can be used as well. Moreover, rubbers and/or mixtures based on other elastomers such as natural rubber (NR), polybutadiene rubber (PB), ethylene propylene diene monomer rubber (EPDM), recycled rubber and/or their compounds can be used. Thermoplastic elastomers, such as cross-linked ethylene vinyl acetate (EVA), thermoplastic polyurethanes (TPUs), styrene based thermoplastic elastomers (e.g. SEBS, SIS, SEPS, SEEPS etc.), ionomer thermoplastics, and any related physically or chemically cross-linked elastomers can be used.

[0020] Additionally, the piezoresistive elastomer material of the body 3 contains or is impregnated with electrically conductive particles, such as carbon black and/or fine graphite particles, or single layer of few layers of graphene powders or flakes for making the body 3 electrically conductive or increasing the electric conductivity of the body 3. The said carbon fillers have the benefit of being chemically very stable, having low density compared to metals and inorganics, and are well compatible with the rubber matrix. Moreover, it has been observed that the sheet-like shaped carbon particles are useful in generating the linear piezoelectric response in the composite. However, other types of fillers can be used, such as carbon nanotubes, and inorganic semiconductive metal oxide, metal nitride, metal boride, metal carbide, metal sulphide, boron carbide, and metal oxynitride fine powders, as they provide similar electrical conductivities and chemical inertness comparable to conductive carbon.

[0021] The piezoresistive elastomer material of the body 3 may also contain silica by which the hardness and other elastic properties of the body material can be adjusted. In a typical rubber composition of the body 3 the sulfur curing SBR rubber formulation according to ASTM D3191-06 contains silica filler (10-30 phr), which serves to adjust the elastic properties of the body material. Moreover, the insulating silica particles, which are present in excess amount as compared to the conductive particles, form obstacles to the formation of electrically percolating pathways between the carbon particles. Thus, the network of silica particles will have an effect on the piezoelectric response of the sensor rubber composite.

[0022] The piezoresistive material of the body 3 can be prepared by adding electrically conductive carbon particles into silica filled base rubber. The silica content of the base rubber is 10-30 phr. Suitable conductive fillers are the combinations of graphite nanoplatelets (GNPs) and carbon black (CB). The content of GNPs and CB is between 3 and 11 phr. Preparation of the base rubber and compounding the fillers can be done by using the conventional methods used in rubber industry, such as batch mixing using 2-roll mills. Also further processing, such as the formation of sheet-like conductive SBR rubber strain sensors, and lamination with insulating silica-SBR rubber can be done using conventional calendars and rubber extruders. Within certain range of filler composition the compounded rubber has very linear piezoresistive response and elastic dynamic range up to 30-50% strains, which is far higher than most of common types of strain sensors.

[0023] The material of the body 3 is selected so that the elastic properties of the body 3 exactly or closely correspond to the elastic properties of the elastomer of the object 1. The elastic modulus (Young’s modulus) of the body 3 is equal to or differs 10 % or less, for example 5 % or less, typically 2 % or less from the elastic modulus of the elastomer material of the object 1 which is arranged in contact with the body 3. The elastic modulus is the ratio of stress, below the proportional limit of the material, to the corresponding strain. The greater the elastic modulus, the stiffer the material, or the smaller the elastic strain that results from the application of a given stress. [0024] Shore A hardness of the body 3 is also equal to or differs 10 % or less, for example 5 % or less, typically 2 % or less from the Shore A hardness of the elastomer material of the object 1 which is arranged in contact with the body 3.

[0025] In the following, some examples of piezoresistive materials of the body 3 and the preparation thereof are presented.

[0026] Example 1. Compounding of piezoresistive SBR rubber material of the body 3 with highly linear response and high strain. First, sulphur containing SBR-silica (20-25 phr, typically 20 phr) rubber base was compounded using a laboratory 2-roll mill. The composition was the test formula presented in the ASTM D3191-06 standard, containing the necessary curing additives and precipitated silica filler. Initial Shore A hardness of the base rubber compound was 54. Into the silica-SBR was added between 5-8 phr conductive CB and GNPs fillers and the composition milled using a laboratory scale 2-roll mill. Finally, the compound was calendared into suitable thickness (0.5-1 mm) sheet for the assembly of the strain sensor 2. The laminated strain sensors 2 with the electrodes, wiring and thin (0.3 -0.5 mm) SBR-silica rubber insulation layers were cured at temperatures between +160 °C and +180 °C for 5 to 25 minutes. The elastic modulus (in the range of 20+10 MPa at small strains) and the Shore A hardness (ShA 65+5) of the body 3 rubber material was similar to a typical tread rubber compound of a vehicle tire.

[0027] Example 2. The strain sensor body 3 rubber compound based on SBR-silica rubber base with slightly higher silica content (22-27 phr, typically 25 phr) having initial Shore A 58 hardness was utilized in the same procedure as explained in Example 1. The sensor body 3 rubber compound having the higher silica content but the similar conductive filler loadings as in example 1 had higher elastic modulus and Shore A hardness, yet still presenting linear piezoresistive properties for the strain sensor use. Thus, the elastic properties of the sensor body 3 material could be adjusted by tuning the silica loading in the base rubber composition.

[0028] Example 3. Sensitivity of the piezoelectric response on the composition and content of the carbon based conductive fillers. Varying the loading of the conductive fillers, and varying the ratio between the CB and GNP both resulted in significantly different piezoresistive response. Linear piezoresistive response and material resistivity in the range between few hundred to few thousand Ohm*m was achieved only for the filler compositions range where the CB loading was at least twofold compared to the loading of the GNPs.

[0029] The strain sensor 2 is provided with a detection unit 6 for detecting changes in electrical resistance/impedance of the piezoresistive elastomer material of the body 3. The changes in resistance/impedance of the piezoresistive elastomer material of the body 3 can be detected directly, e.g. by Wheatstone bridge or other measurement circuit, or indirectly by detecting a property that is proportional to or indicative of the changes in resistance/impedance of the piezoresistive body 3 material, for example electric current passing through the body 3 and/or voltage drop across the body 3. The detection unit 6 comprises a power unit 15 for supplying AC or DC voltage and current to the body 3. The power unit 15 can be a constant voltage unit that supplies a constant voltage or a constant current unit that supplies a constant current. The detection unit 6 further comprises a measuring unit 16 for measuring electric current flowing through the body 3 or voltage drop across the body 3. The detection unit 6 may comprise necessary hardware and software for processing the measurement data and/or wireless transmission means 17 for transmitting measurement data from the detection unit 6 to a remote location, such as a display or control system of the object 1.

[0030] The strain sensor 2 comprises a first electrode 4 and a second electrode 5 for supplying voltage and current to the body 3. The first electrode 4 and the second electrode 5 are attached to the body 3. The body 3 is between the electrodes 4, 5. The first electrode 4 and the second electrode 5 are attached to the body 3 by a layer of non- vulcanized or vulcanized elastomer material arranged on the electrodes 4, 5 so that the electrode 4, 5 is between the body 3 and the layer. The electrode 4, 5 can be a wire end, which can be twisted, curled, bent or folded.

[0031] The electrodes 4, 5 are coupled to the detection unit 6 by means of connecting wires 14. According to an embodiment of the invention the electrodes 4, 5 and/or the connecting wires 14 of the strain sensor 2 are made of twisted metallic wires minimizing the stresses on the adhesive interface between the rubber material of the body 3 and the electrodes 4, 5, and providing mechanical interlocking for the electrodes 4, 5 embedded in the rubber matrix of the body 3. Sufficient adhesion of the electrodes 4, 5 during the cyclic loading is important for measuring the correct strain sensor signal when the electrode rubber interface may experience high tensile, shear or compressive loading. To achieve the high level of adhesion to the body 3 material, the connecting wires 14 and/or the electrodes 4, 5 can be manufactured from surface coated stainless steel wire, e.g. surface coated steel wire designed for the steel cords in the tires.

[0032] The connecting wires 14 are twisted, curled, bent or folded. The connecting wires 14 are made of flexible and/or electrically conductive materials, e.g. copper, aluminium, steel, manganese, electrically conducting oxides or ceramics, e.g. ITO, or electricity conducting plastics, such as PEDOT:PSS, electricity conducting compounds or composites, such as conductive inks or pastes that are used and developed for printed electronics. The connection between the connecting wires 14 and the electrodes 4, 5 and/or electrodes 4, 5 and the body 3 is strain tolerant at least up to 15% relative strain or repeated relative strain.

[0033] The connecting wires 14 can be made by printing, spraying, pressing, melting, welding, sintering, soldering, plating or by using commercial or specifically/generally made wires with standard or non-standard methods. The wires 14 may be single stranded or have multiple strands. The connecting wires 14 can be electrically insulated or non- insulated.

[0034] The piezoresistive elastomer of the body 3, electrodes 4, 5 and/or the connecting wires 14 are electrically isolated from the elastomer material of the object 1.

The electrical isolation of the body 3, the metallic electrodes 4, 5 and/or the connecting wires 14 can be done using laminated layers of the dielectric rubber compounds, such as silica filled SBR strand /tape or by applying solutions of the rubber matrix (S-SBR) containing no electrically conductive fillers.

[0035] Fig. 2 shows an embodiment of the invention in which the object 1 comprises a vehicle tire 9 or a vehicle wheel 7 comprising a wheel rim 8 and a tire 9 mounted on the rim 8. The tire 9 can be any kind of tire having a wearing layer - threaded or non-threaded. In the embodiment of fig. 2 the tire 9 comprises two sidewalls 10 that are spaced apart in an axial direction of the tire 9, and a central portion 11 joining the sidewalls 10. The sidewalls 10 can comprise a strengthening ply. The central portion 11 comprises a tread portion 12 and a cord layer/belt/ply of steel or other strengthening material 13. The strain sensor 2 is placed within the tire 9, typically within the central portion 11, for example between the thread portion 12 and the strengthening belt 13. The strain sensor 2 may form an extra layer between the tread portion 12 and the strengthening belt 13. The body 3 of the strain sensor 2 is arranged in contact with elastomer material of the tire 9. The strain sensor

2 is placed in a tire blank before the vulcanization process of the tire 9.

[0036] The detection unit 6 is placed outside of a space defined by a periphery of the wheel rim 8 and an inner surface of the tire 9. The detection unit 6 can be mounted on the side of the wheel rim 8, a wheel hub, a wheel cap or a dust cap. The connecting wires 14 coupling the electrodes 4, 5 to the detection unit 6 are placed in the tire sidewall 10 or sidewalls 10, for example under the inner ply of the sidewall 10 or on the outer or inner surface of the sidewall.

[0037] The detection unit 6 can be in wireless connection with the electrical power steering (EPS) system of the vehicle via wireless transmission means 17. The connecting wires 14 are also placed the tire blank before the vulcanization process of the tire 9.

[0038] In the embodiment of fig. 2 the elastic moduli of the body 3 and the elastomer material of the tire 9, which is in contact with the body 3, are equal or differ 10 % or less, for example 5 % or less, typically 2 % less from each other in the circumferential and/or radial and/or axial direction of the tire 9. The tire 9 may comprise several strain sensors 2 placed at different locations within the tire 9. The strain sensors 2 may be aligned to different directions to enable strain measurement in multiple directions.

[0039] As the elastomer material of the object 1 is stretched or compressed, the body 3 of the strain sensor 2 is also stretched or compressed which causes changes in electric resistance/impedance of the body 3. The changes, such as an amount of change and/or rate of change, in electric resistance/impedance of the body 3 are detected by the detection unit 6. The changes in electric resistance/impedance of the body 3 can be detected indirectly by detecting/measuring a property that is proportional to or indicative of the changes in resistance/impedance of the body 3, for example electric current passing through the body

3 and/or voltage drop across the body 3. This can be done by applying a voltage from the detection unit 6 to the body 3. As the electric resistance/impedance of the body 3 changes, the amount of electric current passing through the body 3 and/or the voltage drop across the body 3 also changes. The electric current flowing through the body 3 or the voltage drop across the body 3 is proportional to the resistance/impedance of the body 3. The change in resistance/impedance of the body 3 in turn correlates with the magnitude of the changes in the strain of the body 3. Thus, the magnitude of change in the strain can be defined from the change in the current flowing through the body 3 or the voltage drop across the body 3. The measurement data can be processed as desired in the detection unit 6. Further, the measurement data can be wirelessly transmitted from the detection unit 6 to a desired remote location, such as a display or control unit of the object 1.

[0040] The strain sensor 2 can be calibrated individually and even automatically by using applicable measurement methods, electronics and software when required and by using appropriate measurement protocol including maneuvers.