AU749057B2 - Sensor arrangement for detecting the physical properties of liquids - Google Patents

Description

(.WO 00/26661 PCT/DE99/03458 Sensor arrangement for detecting physical properties of fluids Prior Art The invention relates to a sensor arrangement for detecting physical properties of fluids according to the preamble of the main claim.

Microacoustic sensor arrangements with so-called acoustic SAW or surface wave components (SAW Surface Acoustic Wave) are applied, for example, as sensors for various physical parameters in fluids. The measurement of electrical parameters forms an important area here, such as dielectric constants and/or conductivity and the measurement of mechanical parameters, such as eg. the thickness and/or viscosity.

In the known sensor arrangement, a principle of measurement is proceeded from, which is described, for example, in the essay "A study of Love-wave acoustic sensors", J. Du, G.L. Harding, P.R. Ogilvy and M. Lake in the journal Sensors and Actuators A56(1996), pp. 211 to 219. With the measurement construction described here, a sensor is realised, in which horizontally polarised acoustic shear waves are worked with, so-called leaky modes or surface skimming bulk waves (SSBW) or Love-waves. These acoustic wave modes are generated and also detected by interdigital transducers known from the mentioned prior art, so that the desired sensor signal can be obtained from the propagation behaviour on a propagation path or measurement path.

In particular with an application of the sensor arrangement for determining the quality of motor or lubrication oil in a vehicle or comparable machines, it is often necessary to obtain a multiplicity of measurement parameters in order to evaluate these as input diameters for corresponding control devices. Here, in particular the viscosity, temperature, electrical conductivity and the dielectric constants of the oil are of WO 00/26661 PCT/DE99/03458 2 importance if a useful statement, eg. of the monitoring of oil renewal intervals, is to be obtained.

Advantages of the invention The sensor arrangement mentioned above for the detecting of physical properties of fluids is advantageously further developed according to the invention, with the characteristic features of the main claim. According to the invention, a compact sensor arrangement is advantageously created with which an integration of various single measured value sensors on a chip is made possible at low manufacturing cost.

In the sensor arrangement of the invention, the electro-acoustic converters which generate and detect the surface waves with predetermined wave modes are arranged with furthermeasuring arrangements. For the measuring of the viscosity of the fluids a measured value corresponding to the propagation behaviour of the acoustic waves along a propagation path is determined in the known way. The basic sensor element is, for example, a substrate of certain quartz, lithium tantalate and lithium niobate sections, which are particularly suited to the acoustic shear modes described above.

As the viscosity of the fluid is as a rule very dependent on temperature, according to the invention, on the substrate a temperature dependent resistor as measuring arrangement will be applied for the measurement of temperature. The conductor track structure on the substrate is constructed of metal coating, preferably of platinum, both the temperature-dependant resistance and the transmission and reception conductor track structures of the electroacoustic converter being able to be manufactured of this platinum layer, so that only one single layer of metal coating is necessary. Platinum is extraordinarily chemically resistant, so that, under certain circumstances, a sealing layer can be disposed of.

A dielectric layer applied to the substrate above the metal coating can, however, also be arranged as acoustic wave duct layer for the electroacoustic converter additional to WO 00/26661 PCT/DE99/03458 3 the chemical retardation layer. It is also possible to provide titanium or silicium or other intermediate layers as adhesive layers between the substrate and the electroacoustic converters as well as between the electroacoustic converters and other layers applied to them.

Alongside the electroacoustic converters and the temperature-dependent resistor, an arrangement for electrical conductivity measurement can be created from the layer of metal coating by means of uncoated electrodes. The conductivity measurement is undertaken simply with a direct current or an alternating current which flows through the fluid.

Moreover, advantageously by means of a capacitor structure from the same layer of metal coating, an arrangement for measuring dielectric constants can be created. The capacitor structure can, in the same way as the electroacoustic converter, have the form of an interdigital capacitor and, if necessary, be covered by an additional insulation layer; it is, however, also possible, for better connection to the fluid to be measured to use this uncovered, the chemical resistance of the layer of metal coating then being able to be used.

In the arrangement of the invention all measuring arrangements for determining individual physical dimensions can be easily contactable individually and therefore all four measurement procedures can be carried out on corresponding evaluation electronics for sequential measurement with a single substrate, which can be used especially for forms of construction inexpensive to manufacture. Moreover, a good thermal coupling on the substrate is advantageous as, in the case of signal evaluation, by this means, with particularly slight deviations, the temperature of the various measured value sensors is available, so that very precise compensation procedures can be applied.

These and other characteristics of preferred further developments of the invention are mentioned in the subclaims, the description and the drawings, the individual characteristics can be realised individually or collectively in the form of sub- WO 000/26661 PCT/DE99/03458 4 combinations in the embodiment of the invention and in other areas and can represent both advantageous and patentable designs for which the patent is here being claimed.

Drawing An embodiment of a sensor arrangement according to the invention is described on the basis of the single Figure of the drawing, which shows a schematic plan view of the substrate with single measured value detection arrangements.

Description of the embodiment The schematic view according to the only Figure shows the main component of the proposed sensor arrangement, a one-sided polished substrate 1 of a piezoelectrical material, in which horizontally polarised acoustic shear modes can be excited by an interdigital transducer 3, which is contactable via electrical terminals 2 and are detectable in an interdigital transducer 4 as electroacoustic converters. The operating principle of this arrangement is described, for example for determining the viscosity of a fluid, eg. in the prior art quoted above, "A study of Love-wave acoustic sensors", J. Du, G.L. Harding, P.R. Ogilvy and M. Lake in the journal Sensors and Actuators A56(1996), pp. 211 to 219.

Y-rotating quartz sections, some lithium niobate and lithium tantalate sections as well as appropriately poled piezoelectric ceramics are suited to use as substrate materials for the substrate plate 1. The previously described measurement arrangement with the electroacoustic converters for measuring the viscosity of a measured fluid over the propagation of a surface wave and the subsequently described measuring arrangements are structured from a layer of metal coating, preferably of platinum and, if appropriate, adhesive layers. A dielectric layer, not visible here, can be arranged over the electroacoustic converters 3 and 4 and also makes chemical retardation WO 000/26661 PCT/DE99/03458 Present also are electrodes 5 on the substrate 1 for the measuring of conductivity which are acted upon by a direct or alternating current. In many cases of application, however, a conductivity measurement can be dispensed with. The measurement of temperature is carried out by a resistance meander 6 also constructed from the layer of metal coating. The arrangement of the resistance meander 6 can also, in a departure from the form depicted, lie between the converters 3 and 4 outside of the propagation path.

A capacitor structure 7, which is either uncovered for better contact with the fluid to be measured or with a covering layer 8 here indicated with a broken line for improving the chemical resistance, is present for the measurement of the dielectric constants. The measurement of the dielectric constants and conductivity can also be undertaken with electroacoustic converters for determining the viscosity of the fluid, only one other excitation frequency being selected as the frequency necessary for the generation of the surface wave.

The measured fluid, eg. the motor oil of a motor vehicle, flows past the substrate surface and, if applicable to detect the physical properties and the measurement signals obtained are tapped off at the contact points and directed to an evaluation device not illustrated here for the generation of the necessary control and/or display signals, eg. for the quality of the motor oil.

Patent Claims 1) Sensor arrangement for detecting physical parameters of fluids, with