DE19706486B4 - Sensor device for determining the aging state of liquid media - Google Patents

Sensor device for determining the aging state of liquid media Download PDF

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DE19706486B4
DE19706486B4 DE1997106486 DE19706486A DE19706486B4 DE 19706486 B4 DE19706486 B4 DE 19706486B4 DE 1997106486 DE1997106486 DE 1997106486 DE 19706486 A DE19706486 A DE 19706486A DE 19706486 B4 DE19706486 B4 DE 19706486B4
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sensor
period
wave
liquid medium
interdigital
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DE19706486A1 (en
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Anton Dipl.-Phys. Leidl
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Fraunhofer Gesellschaft zur Forderung der Angewandten Forschung eV
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    • FMECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F16ENGINEERING ELEMENTS AND UNITS; GENERAL MEASURES FOR PRODUCING AND MAINTAINING EFFECTIVE FUNCTIONING OF MACHINES OR INSTALLATIONS; THERMAL INSULATION IN GENERAL
    • F16HGEARING
    • F16H57/00General details of gearing
    • F16H57/04Features relating to lubrication or cooling or heating
    • F16H57/041Coatings or solid lubricants, e.g. antiseize layers or pastes
    • FMECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F01MACHINES OR ENGINES IN GENERAL; ENGINE PLANTS IN GENERAL; STEAM ENGINES
    • F01MLUBRICATING OF MACHINES OR ENGINES IN GENERAL; LUBRICATING INTERNAL COMBUSTION ENGINES; CRANKCASE VENTILATING
    • F01M11/00Component parts, details or accessories, not provided for in, or of interest apart from, groups F01M1/00 - F01M9/00
    • F01M11/10Indicating devices; Other safety devices
    • GPHYSICS
    • G01MEASURING; TESTING
    • G01NINVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N27/00Investigating or analysing materials by the use of electric, electro-chemical, or magnetic means
    • G01N27/02Investigating or analysing materials by the use of electric, electro-chemical, or magnetic means by investigating the impedance of the material
    • G01N27/22Investigating or analysing materials by the use of electric, electro-chemical, or magnetic means by investigating the impedance of the material by investigating capacitance
    • G01N27/221Investigating or analysing materials by the use of electric, electro-chemical, or magnetic means by investigating the impedance of the material by investigating capacitance by investigating the dielectric properties
    • GPHYSICS
    • G01MEASURING; TESTING
    • G01NINVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N29/00Investigating or analysing materials by the use of ultrasonic, sonic or infrasonic waves; Visualisation of the interior of objects by transmitting ultrasonic or sonic waves through the object
    • G01N29/02Analysing fluids
    • G01N29/022Fluid sensors based on microsensors, e.g. quartz crystal-microbalance [QCM], surface acoustic wave [SAW] devices, tuning forks, cantilevers, flexural plate wave [FPW] devices
    • GPHYSICS
    • G01MEASURING; TESTING
    • G01NINVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N33/00Investigating or analysing materials by specific methods not covered by groups G01N1/00 - G01N31/00
    • G01N33/26Oils; viscous liquids; paints; inks
    • G01N33/28Oils, i.e. hydrocarbon liquids
    • G01N33/2888Lubricating oil characteristics, e.g. deterioration
    • GPHYSICS
    • G01MEASURING; TESTING
    • G01NINVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N2291/00Indexing codes associated with group G01N29/00
    • G01N2291/02Indexing codes associated with the analysed material
    • G01N2291/025Change of phase or condition
    • G01N2291/0258Structural degradation, e.g. fatigue of composites, ageing of oils
    • GPHYSICS
    • G01MEASURING; TESTING
    • G01NINVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N2291/00Indexing codes associated with group G01N29/00
    • G01N2291/02Indexing codes associated with the analysed material
    • G01N2291/028Material parameters
    • G01N2291/02818Density, viscosity
    • GPHYSICS
    • G01MEASURING; TESTING
    • G01NINVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N2291/00Indexing codes associated with group G01N29/00
    • G01N2291/02Indexing codes associated with the analysed material
    • G01N2291/028Material parameters
    • G01N2291/02863Electric or magnetic parameters
    • GPHYSICS
    • G01MEASURING; TESTING
    • G01NINVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N2291/00Indexing codes associated with group G01N29/00
    • G01N2291/04Wave modes and trajectories
    • G01N2291/042Wave modes
    • G01N2291/0422Shear waves, transverse waves, horizontally polarised waves
    • GPHYSICS
    • G01MEASURING; TESTING
    • G01NINVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N2291/00Indexing codes associated with group G01N29/00
    • G01N2291/04Wave modes and trajectories
    • G01N2291/042Wave modes
    • G01N2291/0423Surface waves, e.g. Rayleigh waves, Love waves
    • GPHYSICS
    • G01MEASURING; TESTING
    • G01NINVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N2291/00Indexing codes associated with group G01N29/00
    • G01N2291/04Wave modes and trajectories
    • G01N2291/042Wave modes
    • G01N2291/0427Flexural waves, plate waves, e.g. Lamb waves, tuning fork, cantilever

Abstract

Sensor device for detecting material parameters of a liquid medium, consisting of two interdigital transducers (12, 14) for generating an electroacoustic wave from one (12) of the interdigital transducers to the other (14) of the interdigital transducers, the two interdigital transducers (12, 14) having one first evaluation circuit for determining the viscosity, the viscoelasticity and / or the density of the liquid medium on the basis of the determined resonance frequency and / or the damping of the electroacoustic wave, and wherein one of the interdigital transducers (12) is connected to a second evaluation circuit for determining the dielectric constant and / or the conductivity of the liquid medium is connected on the basis of the complex electrical impedance of the interdigital transducer (12), with the second evaluation circuit determining one of the resonant frequencies of the electroacoustic wave to determine the complex electrical impedance Different frequency is used.

Description

  • The present invention relates refer to a sensor device for determining the state of aging liquid Media, in particular for determining and monitoring the state of aging of oil.
  • It is known that the quality of oils is of one Variety of factors depends. These factors include the total base number, the total acid number, additive degradation, iron content, load capacity and the like. This, the quality For example, factors that determine engine and transmission oils can can only be determined in the laboratory. Continuous or quasi-continuous monitoring the oil quality therefore not in most technical systems.
  • For example, in passenger cars Engine oil usually changed every 15,000 km. Thus the maintenance cycles not determined as required, but the engine oil is in each case changed after a certain number of kilometers driven.
  • For quasi-continuous monitoring the oil condition in technical systems on site, for example the running time and the oil temperature detected, the oil change intervals using models be determined. However, this method has many uncertainties afflicted and says nothing about the actual Properties of the oil.
  • The DE 3701348 C2 relates to a method for checking the quality of an operating fluid of motor vehicles. For this purpose, this document teaches the detection of a generally electrical quantity that is correlated with the quality of the operating fluid, for example the resistance value or the capacity of the operating fluid. This document does not provide any further information about the sensor used.
  • The EP 0080632 A1 is directed to an oil deterioration detection device. A capacitive sensor consisting of a pair of spaced electrodes is used for this. This capacitive sensor generates an output signal as a function of the dielectric constant or the conductivity of the oil. The deterioration of the oil is determined on the basis of the output signal, for which purpose the output signal is compared with a reference voltage which represents a limit value which, when exceeded, generates an optical or acoustic warning.
  • In Sensors and Actuators, 20 (1989), Pages 253-268, is the use of SAW components to record liquid parameters described. This document describes on the one hand oscillator measurements, where changes the liquid properties due to interference the oscillation frequency detected become. This document also describes propagation measurements, at which a phase shift and a damping of the amplitude of the surface waves detected becomes.
  • From Chemical Abstracts, 1994, Vol. 120, quote no. 111 208 is the possibility known, the measurement of the SAW damping of SAW components to determine the deterioration of engine oil.
  • The present invention lies based on the prior art mentioned, the task is based on a sensor device for determining the aging condition of a liquid Medium, especially oil, to create a needs-based exchange of the liquid medium, especially the oil, to ensure.
  • This task is accomplished by a sensor device according to claim 1 solved.
  • According to the present invention preferably the viscosity, the viscoelasticity, the density, the dielectric constant and / or conductivity of the liquid Medium determined.
  • In preferred embodiments points the sensor device of the present invention further includes a temperature sensor on to capture the respective state parameters during the different periods at two different temperatures to enable. Thus a temperature dependency of a respective state parameter can be determined. A comparison of the temperature dependencies of a state parameter that during the first period and the second period can then be recorded in addition to Determination of the aging state of the liquid medium used become.
  • To record the viscosity or viscoelasticity, a liquid Medium can advantageously be a surface wave sensor that consists of at least two Interdigital transducers are used. Furthermore, the permittivity and / or conductivity a liquid Medium preferably through a planar capacitor, the complex of which electrical impedance on the dielectric constant and the conductivity depends on the adjacent medium, be determined. In preferred embodiments of the present According to the invention, a planar interdigital capacitor is used for this purpose.
  • The present invention is particularly suitable for the on-site determination of the oil quality of an engine. To do this, simple and inexpensive measurable oil parameters must be used. With the aging of oils and increasing contamination by abrasion, soot, water, etc., also change the physical material parameters of the oil, such as viscosity, viscoelasticity, density, dielectric constant and conductivity, which are correlated with the measurement signals of an oil quality sensor. However, the current absolute values of these parameters say nothing about the oil quality, since these values show a wide spread from oil type to oil type. As a result, it is only possible to make a statement about the state of oil aging by changing this material data over time. To determine the oil quality, ie the state of oil aging, it is therefore not the instantaneous absolute values of the condition parameters mentioned above that are used, but rather their change over time.
  • The viscosity and viscoelasticity of oils is strongly temperature dependent. This temperature dependency contains information about the viscosity or the viscoelasticity, i.e. additive degradation, oils. Go through the oils while a temperature ramp during operation, for example at start and after the stop of motors, so for the sensor signals with the viscosity or viscoelasticity are correlated from this temperature ramp in addition to the information obtained at a fixed temperature, information about the changes the viscous and viscoelastic oil properties with different Temperatures won. From such a temperature-dependent change the viscosity can then, for example, a deterioration in the load capacity of the oil getting closed.
  • The present invention thus enables inexpensive Determination of the aging state of a liquid medium, for example an engine oil. So you can according to the present Invention the oil change intervals In technical systems, the service life is determined as required can increase and resources can be spared.
  • A sensor device according to the invention is suitable is preferred for the detection of condition parameters of an engine oil. With such an oil quality sensor can the density of the oil as well as viscous, viscoelastic and dielectric oil properties recorded on site become. If these parameters are recorded at different times, point The changes the physical parameters are high Correlation with oil aging on. So you can Statements about the actual oil condition be made.
  • Preferred embodiments of the present Invention are hereinafter made with reference to the accompanying drawings explained in more detail. It demonstrate:
  • 1 a schematic representation of a preferred embodiment of the sensor device according to the invention; and
  • 2 a diagram of recorded sensor signals against the temperature for new oil and different ages of the oil;
  • 3 by means of an interdigital capacitor recorded dielectric properties of an oil at different ages of the same;
  • 4 a schematic representation to explain the sensor device according to the invention.
  • The present invention is described in following using a device for determining the aging condition an engine oil explained in more detail. It however, it is obvious that the present invention further for determining the state of aging other more fluid Media can be used, their physical material parameters change with aging, for example due to increasing contamination.
  • 1 shows a schematic representation of a sensor device according to the invention for detecting the viscous and viscoelastic properties and the density of an oil and also for detecting the dielectric oil properties.
  • As in 1 are on a dielectric substrate 10 , for example a quartz substrate, two interdigital transducers 12 and 14 arranged. The two interdigital converters 12 and 14 form a surface wave sensor (SAW sensor) in the form of a delay line. A resonator or a plate mode sensor can also be used. The interdigital converters 12 and 14 consist of comb-shaped electrodes, for example in thin-film technology on the piezoelectric substrate 10 are upset. Via a high-frequency electrical AC voltage that is sent to a first circuit input 16 is created with the interdigital converter 12 electroacoustic waves excited by means of the interdigital transducer 14 via a first circuit output 18 can be detected. The resonance frequency and the damping are determined by the material of the substrate 10 , the material of the comb electrodes, which may be gold, for example, is determined by the geometric layout of the comb electrodes and a liquid adjacent to the sensor component, as will be referred to later 4 is explained in more detail.
  • Changes in the viscous and viscoelastic properties as well as the density of the oils change the resonance frequency and the damping of the electroacoustic wave between the interdigital transducers 12 and 14 , This change in resonance frequency and / or attenuation can be evaluated electronically, for example in an oscillator or resonator arrangement, using the SAW sensor as the frequency-determining element. As in 1 is shown, the first comb electrode of the interdigital transducer 12 , which serves as a transmitter converter, with the first circuit input 16 electrically coupled. The second comb electrode of the interdigital transducer 12 lies on ground. The first comb electrode of the interdigital transducer 14 is also on Dimensions. The second comb electrode of the interdigital transducer 14 is with the first circuit output 18 coupled.
  • According to a preferred exemplary embodiment, an interdigital transducer of the SAW sensor, in which in 1 shown embodiment of the interdigital converter 12 , used as a planar capacitor, interdigital capacitor IDK. This is the first comb electrode of the interdigital capacitor 12 further with a second circuit input 20 connected. Via this circuit input 20 can the complex electrical impedance of the interdigital transducer 12 formed capacitor can be detected.
  • The complex electrical impedance of this capacitor depends on the dielectric constant and the conductivity of the adjacent medium. In the case of motor oils, ∈ ≈ 2 - 4, the dielectric constant is significantly changed, for example, by the introduction of water, ∈ ≈ 80, which can occur, for example, due to a cooling water ingress into the oil. A gasoline entry, ∈ ≈ 2 - 4, on the other hand, hardly affects the dielectric constant, but a gasoline entry strongly influences the viscosity, so that this is achieved by the two interdigital transducers described above 12 and 14 formed SAW sensor can be detected. The base or acidity of the oil (TBN or TAN) are responsible, among other things, for the corrosion of the engine. They influence the conductivity of the oil and can via the complex electrical impedance of the interdigital transducer 12 be recorded.
  • The surface wave sensor used in accordance with the present invention can, for example, on a 36 ° rotated quartz substrate 10 be manufactured in thin film technology. The two interdigital converters 12 and 14 , which serve as transmitter transducers or receiver transducers of electroacoustic waves, are arranged in such a way that surface shear waves are excited and can consist, for example, of 130 double (double-split) gold electrode pairs with a metallization height of 350 nm and a structure width of 9 μm. This results in a resonance frequency of approximately 70 MHz. In a preferred embodiment, the electrode overlap is 2.376 mm.
  • The resonance frequency and the attenuation of the surface wave sensor are preferably with an oscillator circuit that is connected to the first circuit input 16 and the first circuit output 18 is interconnected, evaluated. The electrical impedance of the interdigital transducer 12 is also preferably via an oscillator arrangement, which via the second circuit input 20 is interconnected, evaluated.
  • The complex impedance of this capacitor is preferably evaluated by the interdigital converter 12 is formed at a frequency below or above the resonance frequency of the surface wave sensor and is therefore independent of the excitation of the electroacoustic wave. For the assessment of the oil quality, the real and / or the imaginary part of the impedance of the capacitor is used in order to obtain information about the dielectric constant and / or the conductivity of the oil. In a preferred embodiment, the frequency at which the electrical impedance of the interdigital transducer 12 is determined to be below 100 kHz. For decoupling the high frequency, ie the resonance frequency of the surface wave sensor, and the low frequency, ie the frequency at which the electrical impedance of the interdigital transducer 12 is determined is between the first circuit input 16 and the first comb electrode of the interdigital transducer 12 preferably a high pass 22 arranged while between the second circuit input 20 and the first comb electrode of the interdigital transducer 12 preferably a low pass 24 is arranged. The high pass 22 can be formed, for example, by a capacitor with a suitable capacitance during the low-pass filter 24 can be formed by a coil with a suitable inductance.
  • The oscillator circuit of the surface wave sensor that is connected to the first circuit input 16 and the second circuit output 18 connected, can have an amplitude control, which is designed such that the control signal thereof is a measure of the damping of the surface wave and thus the viscosity of the oil. The design of such control circuits is known in the art. By decoupling the evaluation circuits for the electrical impedance of the one interdigital capacitor 12 and the evaluation circuit for the surface wave sensor by both interdigital transducers 12 and 14 By means of high and low passes, a parallel evaluation of the dielectric as well as the viscous and viscoelastic properties and the density of the oil can be carried out.
  • For mechanical and chemical protection of the Sensor surface, i.e. the interdigital converter, the component can be preferred embodiments of the present invention, for example using silicon carbide (SiC) be coated. The coating with silicon carbide can be done in thin film technology take place and offers excellent chemical and mechanical Protection.
  • The sensor device according to the invention for detection of material parameters of a liquid Medium, especially oil, preferably also has a temperature sensor.
  • According to the present invention, the temporal change in the material parameters of the oil is now detected, for example by means of the sensor device described above, by these parameters during a first period in which the oil has an initial state and during a second period which is spaced in time from the first period is to be detected. The capture during These two periods can be carried out, for example, at identical temperatures, and an identical temperature can be ensured by using the temperature sensor. Furthermore, it is possible to record the material parameters at two different temperatures in each period in order to obtain a temperature dependence of the same. The change in the temperature dependence between the first period and the second period can then additionally be used to make statements about the aging condition of the oil.
  • 2 shows a sensor signal-temperature diagram of a surface wave sensor. The measured value, ie the amplitude of the surface acoustic wave of the SAW sensor, is plotted against the resistance of a temperature sensor, which is a measure of the temperature.
  • The graph shows the curves obtained when the material parameters of a motor vehicle engine oil are recorded after a different operating time or mileage of the motor vehicle. How out 2 It can be seen that the measured value of the surface wave sensor changes significantly from an initial state, new oil, depending on the mileage, 3,500 km or 23,000 km, and the operating time, 300 hours or 400 hours. Consequently, it can be seen in the diagram for the engine oil when new and in different age groups that the sensor signal changes significantly with increasing oil aging. The change in the measured value of the signal at a fixed temperature, for example 40 ° C. or 100 ° C., corresponds to the change in viscosity of the oil. In contrast, the temperature behavior, ie the slope of the curves, increased only slightly in the illustrated embodiment with increasing oil age and indicates a slight reduction in the viscosity index improvers, ie only a slight reduction in additives.
  • 2 shows a diagram showing the dielectric properties, for example by referring to 1 Sensor device described were detected for the oil in the different aging states of the oil, as they are in 2 are shown. The capacitance was evaluated using an oscillator circuit at approx. 50 kHz. At the in 3 The diagram shown corresponds to a decrease in frequency, an increase in capacity and thus an increase in ∈, as is the rule for aged oils.
  • The following is referring to 4 a preferred embodiment of the surface wave sensor, as used according to the present invention, explained in more detail. 4 shows the schematic diagram of a surface acoustic wave device used in preferred embodiments of the present invention.
  • First, a general description of the in 4 given surface wave component. On a piezoelectric substrate 110 , for example a y-rotated quartz, are comb-shaped electrodes 112 . 114 . 116 and 118 for example applied in thin film technology. The electrodes 112 and 114 form a first interdigital converter 120 , the so-called transmitter interdigital converter (transmitter IDT), while the electrodes 116 and 118 a second interdigital converter 122 , the so-called receiver-in digital converter (receiver IDT). The substrate can consist of any piezoelectric that is suitable for excitation of shear waves. A y-rotated quartz with a quartz cut between 35 ° and 40 ° is preferably used.
  • By applying a high frequency AC voltage to the transmitter IDT 120 are due to the piezoelectricity of the material of the substrate 110 electroacoustic waves that in 4 schematically by that with the reference symbol 124 shown arrows are excited. The distance between the fingers of the interdigital transducers impresses a wavelength λ, the excitation frequency f = ν / λ being determined via the material-dependent propagation speed ν of the wave.
  • As in 4 is shown, so-called double-finger transducers are preferably used to suppress the reflection on the fingers. In these transducers, two fingers of one electrode are arranged side by side and form a finger quartet with two fingers of the other electrode of the interdigital transducer, which are arranged next to the pair of fingers of the first electrode. With the reference symbol 126 is a pair of fingers of the electrode 116 designated. As in 4 further shown, the width of a finger is denoted by b, while the distance between two adjacent fingers is denoted by a. The length of the overlap area of the fingers of two opposite electrodes is denoted by c. The wavelength λ corresponds, as in 4 is the length of a finger quartet, ie four times the width of a finger plus four times the distance between adjacent fingers.
  • With a suitable choice of substrate material and the surface orientation runs one wave excited by the transmitter IDT along this surface and generated in the recipient IDT a high-frequency AC voltage that is evaluated electronically can be. amendments the physical boundary conditions along the component surface liquids or coatings affect the rate of propagation, i.e. the frequency, and the amplitude of the waves and can as Sensor effect can be used.
  • The following explains how the surface wave component described above ver with a high sensitivity to liquid properties and low design-related interference as a liquid sensor and thus as a sensor device according to the present invention can be applied.
  • The relative frequency change of a surface wave as a measurement effect in surface shear wave sensors is calculated as follows when the sensor surface is loaded with a Newtonian liquid:
    Figure 00130001
    where c is a constant of proportionality, η is the viscosity of the Newtonian liquid, ρ is the density of the Newtonian liquid, and f is the frequency. The depth of penetration of the wave into the liquid is proportional to √ ν (2 · η / ρ · f) and increases with decreasing frequency, so that interface effects play a smaller role at low frequencies. In addition to increasing the effective measurement volume, a low frequency therefore leads to improved sensor properties. Therefore, frequencies below 100 MHz are preferably used for surface wave liquid sensors.
  • In addition to the surface shear waves (OFSWs), however in y-rotated quartz volume shear waves, for example the so-called 'surface skimming bulk waves' (SSBWs), be excited as mentioned above has been. The frequencies of the OFSWs and the SSBWs are very close together.
  • In analogy to an optical grating, is determined by the number of finger quartets of the interdigital transducers set a frequency bandwidth. In the usual manufacture of this Sensors with aluminum as the electrode material and the use overlap from low frequencies, which are necessary for the measurement of the viscosity these frequency bands the OFSW and the SSBW, whereby both modes are excited simultaneously. Due to the sensor effect changes the speed and damping of the OFSW (surface shear wave). The Speed and damping the SSBWs changes not, however, or only very slightly. The interference of the different Shear waves therefore become dependent from the sensor effect and disturbing sensitive the evaluation. If a surface component of the above described Kind as a liquid sensor must therefore be used the simultaneous excitation of the surface shear wave and the volume shear wave be prevented.
  • To interference of the above Preventing kind must be done be that the OFSW and the SSBWs have different frequencies. By the physical properties of the metallization of the IDTs the boundary conditions at the interface of the quartz substrate with the metallization of the electrodes. By changing the physical Properties of the metallization of the IDTs can thus, for example the speed and thus the frequency f of the OFSW decreased become. By such a modification of the physical properties the excitation frequency of the SSBWs does not become, or at least becomes, the metallization much weaker than the frequency of the OFSW affects. Through a targeted adjustment the bandwidth of the OFSW the number of fingers of the interdigital converter and by increasing the Frequency distance of the OFSW from the SSBWs by a corresponding Metallization can be achieved that the excitation band of the SSBWs outside of OFSW frequency band. This is a suggestion from the SSBWs prevents and interference between the OFSW and the SSBWs no longer occurs.
  • The relative frequency change of the OFSW due to the electrode material can be calculated approximately using the following formula:
    Figure 00150001
    where γ is the metallization rate (= b / (a + b), width / (width + distance) of the electrodes), ρ m is the density of the metallization, h is the layer height of the metallization, λ is the wavelength, c 66 is the Shear stiffness of quartz is dependent on the cutting angle of the substrate, ν m is the speed of sound of the metallization, and ν c is the speed of SSBW is dependent on the cutting angle of the substrate.
  • The relative bandwidth (Δf / f) N within which the surface shear wave can be excited is determined by the number of finger quartets N and can be calculated as (Δf / f) N = 2 / N.
  • To prevent interference between the OFSW and the SSBWs, the following must apply:
    Figure 00150002
  • Hence N> 2 · (f / Δf) m .
  • With the aluminum usually used for filter technology as the electrode material, the low mass per unit area results in ρ m · h · γ and the small difference in sound velocities between aluminum (ν m ) and SSBWs (ν c ) at a frequency of approx. 50 MHz and a y-rotated quartz section of 36 ° a number of finger quartets in the order of 10 4 . A filter with such a number of fingers is not suitable for technical implementation.
  • For low frequencies, for example below 100 MHz, metallization with high mass per unit area and low speed of sound must therefore be used in accordance with the above equation. Suitable materials for the metallization of the electrodes of the interdigital transducers are, for example, gold, platinum or copper. For example, for gold with a layer height h of 350 nm at 65 MHz and a y-rotated quartz cut of 36 °, there is a minimum number of 75 fingers quartet.
  • Thus, according to the present invention a surface wave liquid sensor with a high sensitivity to liquid properties and minor interference by an interference of the surface shear wave with volume shear waves will be realized.
  • Another problem with using Surface acoustic wave devices as liquid sensors, that by means of an advantageous embodiment of the present Invention to be solved is the triple-pass echo (TTE; TTE = Triple Transit Echo) and electromagnetic crosstalk (EM).
  • An electroacoustic wave that in the area of the receiver interdigital transducer running, generated on the fingers, or the busbars of the fingers, over the connected electrical load impedance a potential difference, which in turn is a surface wave generated that runs back in the direction of the transmitter interdigital transducer and there also over the generator impedance generates a wave in the direction of the receiver running back. The through these effects on the receiver interdigital transducer incoming, twice reflected wave has the measuring section go through three times and thus triple the phase shift like experience the non-reflected wave. This effect is called triple transit Called Echo (TTE). The same leads to a frequency dependent, constructive or destructive interference between direct measurement signal's and the twice reflected signal. This creates a frequency-dependent ripple in transmission behavior the surface acoustic wave device, the amplitude of the ripple from the transmission loss (IL; IL = insertion loss) of the component and the TTE suppression. When evaluating the through a liquid on the surface shear wave generated sensor effect occurs the TTE as a disturbance. The measurement error generated depends on the TTE suppression, transmission loss and the size of the measurement effect from.
  • It is obvious that with increasing transmission loss IL TTE suppression is increasing. Approximately the following applies: TTE [dB] = 2 · IL [dB] + 12dB. So it is for the Advantageous sensors, surface wave components with high transmission loss to use from the sender IDT to the receiver IDT to the influence of the TTE as low as possible to keep. Attenuation> 10 dB, more advantageously> 15 dB, are preferably suitable.
  • In addition to the TTE, the receiver interdigital transducer interferes also the direct electromagnetic crosstalk between the transmitter IDT and the recipient IDT with the surface wave signal and occurs in the form of a frequency-dependent ripple as a disturbance in the measurement effect on. The amplitude of this ripple depends on the amplitude ratio between electromagnetic crosstalk EM and transmission loss IL from. The smaller this ratio is, the less the influence of crosstalk on the measurement result. The electromagnetic crosstalk depends on the structure of the surface acoustic wave device from. In practice, damping can of electromagnetic crosstalk EM in the range of 50 to 80 dB can be achieved. To the influence of the EM to keep small must the transmission loss IL be kept small, i.e. there must be a large amplitude of the surface wave guaranteed his.
  • As is evident from the foregoing, there is a conflict between the high transmission loss required to suppress the TTE and the low transmission loss required to minimize the effect of electromagnetic crosstalk. In order for both effects to be balanced, the following equation must apply: EM [dB] = IL [dB] + TTE [dB] = 3 * IL [dB] + 12dB
  • The result for the transmission loss is: IL [dB] = (EM [dB] - 12dB) / 3
  • Depending on a specific surface component attainable suppression of electromagnetic crosstalk this results in a range of 10 dB to 30 dB, preferably from 15 dB to 25 dB, as transmission attenuation for surface wave sensors.
  • The transmission loss of surface shear wave devices depends on the quartz cut of the substrate used, on (Δf / f) m , on the number and overlap length c of the fingers of the IDTs (see 4 ) and from an electrical adaptation to the impedance of the connected measuring device. These parameters must be taken into account when designing the surface component in order to achieve a corresponding transmission loss that meets the above requirements.
  • The following are based on a concrete embodiment exact parameters specified for the implementation of a surface wave liquid sensor according to the invention are suitable. The substrate is a y-rotated quartz with a quartz cut used by 36 °. As a metallization for the electrodes of the interdigital transducers are gold with a layer height h of 350 nm used. The metallization rate γ, i.e. the ratio b / (b + a), is 0.5, i.e. 50%. With an excitation frequency f of 65.6 MHz this results in a wavelength λ of 76 μm. The number the finger quartet in a so-called 'double split' converter is 90. The finger overlap c is 25 λ. The surface wave filters dimensioned in this way exhibits a transmission loss of 20 dB on.
  • The present invention provides thus devices and methods for determining the state of aging a liquid Medium, especially an engine oil, which meets the needs Determination of oil change intervals enable in many technical systems, thereby increasing their lifespan elevated and resources can be saved. The simple construction of a sensor device according to the invention for detection the needed The present invention can also implement material parameters at low cost become. The present invention can be used for quasi-continuous monitoring of oil aging in many technical systems, for example in internal combustion engines, transmissions, Hydraulic systems and the like advantageous for needs-based Determination of the maintenance cycles can be used, which increases the lifespan and resource conservation can be achieved. It’s obvious to professionals that at certain applications to detect the change in a state parameter a liquid medium may suffice while it is beneficial in a variety of applications, a plurality the parameters, such as viscosity, viscoelasticity, density, the dielectric constant and conductivity, all by means of the preferred embodiment of the sensor device according to the invention detected can be take.

Claims (8)

  1. Sensor device for recording material parameters of a liquid medium, consisting of two interdigital transducers ( 12 . 14 ) to generate an electro-acoustic wave from one ( 12 ) the interdigital converter to the other ( 14 ) the interdigital transducer, the two interdigital transducers ( 12 . 14 ) are connected to a first evaluation circuit for determining the viscosity, the viscoelasticity and / or the density of the liquid medium on the basis of the determined resonance frequency and / or the damping of the electroacoustic wave, and one of the interdigital transducers ( 12 ) with a second evaluation circuit for determining the dielectric constant and / or the conductivity of the liquid medium on the basis of the complex electrical impedance of the interdigital transducer ( 12 ) is connected, a frequency different from the resonance frequency of the electroacoustic wave being used to determine the complex electrical impedance by means of the second evaluation circuit.
  2. Sensor device according to claim 1, in the determination of the aging condition of a liquid medium, especially of oil, starting from an initial state of the same, at least one State parameters of the liquid Medium during a first period in which the liquid medium returns to its initial state has, and while at least one second period following in time is recorded, and which further includes a means for comparing the at least one while the first period and the at least one second period detected state parameters and a device for determining the state of the liquid medium based on the comparison.
  3. Sensor device according to claim 1 or 2, which also has a temperature sensor for detecting the temperature of the liquid Has medium.
  4. Sensor device according to claim 3, in which the detection device has the at least one state parameter while the first period and the at least one second period at identical temperature detected.
  5. Sensor device according to one of claims 2 to 4, in which the detection device has the at least one state parameter while the first period and the at least one second period each detected at at least two different temperatures, wherein the device further includes a device for determining a temperature dependency, that of the at least one state parameter during the first period and which has at least a second period, the The comparison device also determines those determined for a state parameter temperature dependencies compares and the condition determination device the aging condition of the medium further on the basis of the temperature dependency comparison determined.
  6. Sensor device according to one of claims 1 to 5, in which the first evaluation circuit is a first oscillator circuit to determine the resonance frequency and / or the damping of the through the surface wave sensor generated electro-acoustic wave.
  7. Sensor device according to one of Claims 1 to 6, in which the second evaluation circuit comprises a second oscillator circuit for determining the complex electrical impedance of the interdigital transducer ( 12 ) having.
  8. Sensor device according to claim 6 or 7, in which the first oscillator circuit is an amplitude control device whose control signal is a measure of the damping of the surface wave, from which the viscosity of the liquid Medium is determinable.
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