WO2009031083A1

WO2009031083A1 - Sensor device for solid-liquid phase transition

Info

Publication number: WO2009031083A1
Application number: PCT/IB2008/053485
Authority: WO
Inventors: Rainer Hilbig; Stefan Schwan; Arnd Ritz; Theodorus P. C. M. Vos; Bernd Engelbrecht
Original assignee: Philips Intellectual Property & Standards Gmbh; Koninklijke Philips Electronics N.V.
Priority date: 2007-09-03
Filing date: 2008-08-28
Publication date: 2009-03-12

Abstract

A sensor device is described being designed to detect the phasetransition ofat least a part of a test material from the solid phase to the liquid phase. The sensor device comprises a light source (1) for initiating the phase transition for example by heating up the test material (10) by means of the emitted transition light (7). Further, the sensor device comprises a light detector (8) detecting detection light (6) thrown back by the solid phase of the test material (10) and subsequently detection light (6) thrown back by the liquid phase of the test material (10) afterat least a part of the test material (10) is melted by the transition light (7). The comparison of the signals received by the light detector (8) enables the detection of a phase transition of the test material (10) heated up by means of the light source (10). Examples of test materials (10) are ice and fat. Possible applications are the detection of ice in the automotive or aircraft industry.

Description

SENSOR DEVICE FOR SOLID-LIQUID PHASE TRANSITION

FIELD OF THE INVENTION

The current invention is related to a sensor device for detecting the transition between the solid phase and the liquid phase of a solid material, and to a method of initiating and detecting the transition between the solid phase and the liquid phase of a solid material.

BACKGROUND OF THE INVENTION

In GB 1 364 845 A an ice detector is described detecting ice by means of splitting laser light emitted by a laser into at least two laser beams being detected by two separate photo diodes. The signals provided by both photo diodes due to the incident laser beams are compared. Ice being present in the path of one of the laser beams attenuates the laser beam, whereas the other laser beam is not attenuated. The difference between the signals provided by the photo diodes increases and the ice is detected. The ice detector is insensitive with respect to dirt being present in one of the paths of the two laser beams. A heater that is optionally integrated in the ice detector and that may be used to prevent false measurements due to dirt strongly restricts the construction and the use of the ice detector.

SUMMARY OF THE INVENTION It is an objective of the current invention to provide a reliable and cost- effective sensor device for detecting a solid material.

The objective is achieved by means of a sensor device comprising at least one light source and at least one light detector, the light source being designed to initiate an at least partial phase transition of a test material from solid phase to liquid and/or vapor phase by means of transition light emitted by the light source and the light detector being designed to detect the phase transition of the illuminated test material by means of detection light. A light source with narrow band emission, such as lasers in general, laser diodes, light emitting diodes or other suitable lamps may be used. The light detector may be a photodiode or any kind of spectroscopic device. The detection light being thrown back by the test material comprises scattered, diffusely reflected and/or specularly reflected light. Mostly, but not necessarily, an optical device such as a lens will be used in order to focus the light emitted by the light source on the test material. The test material may be, for example, ice or fat. Initiating the phase transition by means of the light source and measuring the difference in the reflection, absorption or scattering properties of the test material caused by means of the phase transition from solid to liquid and/or vapor enables a more reliable operation than for example the ice detector known from prior art, because the measurement is directly related to the properties of the test material such as for example the enthalpy of fusion of ice. If there is no ice on a substrate, which may be any kind of surface of a test object, the properties of the substrate where the ice might have been present and the properties of the detection light essentially are not changed by means of the transition light. Dirt being present on the surface of the test object, such as a wing of an airplane, would clearly cause a different signal than ice at the same place of the wing. Further, no additional wiring is needed to heat up for example a certain place of the wing in order to be sure that the measurement signal is not caused by dirt as would be necessary when using an ice detector as described by the prior art. Adding an adaptive optical element such as, for example, a moveable mirror to the sensor device would enable the detection of a test material at different places with one laser sensor instead of a number of costly laser sensors being required at different places. Using different intensities of the light source may enable the detection of different test materials. In another embodiment according to the current invention, the at least one light source is a laser. A laser may have the advantage that the laser light may be collimated to a narrow parallel beam by means of an optical device such as, for example, a lens. The narrow parallel beam may have the advantage that the energy density stays essentially constant along the beam. The distance between the laser source and the test object may be less important and costly adaptive focusing of the transition and/or the detection light with respect to the test object and consequently the test material (if present) on the surface of the object may be prevented. The laser may be operated continuously at constant intensity and the light detector may continuously receive reflected or scattered light from the test object and/or the test material in order to detect the melting of the test material.

In another embodiment according to the current invention, the laser is designed to emit test light and transition light, the transition light having a higher intensity than the test light and the detection light is test light thrown back by the test material. Using a laser source such as a solid state laser at different power levels may enable the emission of test light and transition light having different frequencies. Different temperatures of the gain medium at different power levels may cause this. Different frequencies may enable a more sensitive sensor device. Further, the intensity of the test light may be adapted to the light detector in order to improve the sensitivity of the sensor device.

In an alternative embodiment according to the current invention, the sensor device comprises a transition laser and a test laser, the transition laser being designed to emit transition light and the test laser being designed to emit test light, the transition light having a higher intensity than the test light and the detection light being test light thrown back by the test material. Two independent lasers, i.e. a transition laser and a test laser, may increase the flexibility of the sensor device. Each laser can be selected in a way that maximum performance with respect to initiating the phase transition on the one side and determining the phase transition on the other side is enabled. Parameters that can be chosen nearly independently are, for example, emission frequencies, intensities and polarization.

In one embodiment according to the current invention, the at least partial phase transition of the test material from solid phase to liquid phase is detected by self- mixing interferometry (SMI). Self mixing interferometry may enable a more sensitive detection of phase transitions, because the phase relation between emitted laser light and thrown back laser light re-entering the laser cavity of the laser source may comprise more information than the intensity of the thrown back light alone. Especially suited for self-mixing interferometry are Vertical Cavity Surface-Emitting Lasers (VCSEL), Vertical Extended Cavity Surface-Emitting Lasers (VECSEL) or side-emitting laser diodes. Especially VCSELs and VECSELs can be produced and tested on wafer level, which may reduce the cost of the sensor device. Further, VCSELs and VECSELs can easily be arranged as arrays. The whole array may be used to emit the transition light, wherein, for example, only one VCSEL or VECSEL may be used to detect the phase transition. VECSELs do have the advantage that the detection range may be increased to more than 10 meters. In one embodiment according to the current invention, the transition laser is designed to emit transition light having a first frequency and the test laser is designed to emit test light having a second frequency different from the first frequency. Adapting the transition light to the test material by using a frequency that is preferably absorbed by the test material may accelerate the detection of the phase transition. Adapting the test light to the solid or alternatively the liquid and/or vapor phase of the test material by using, for example, a frequency preferably interacting with the solid or alternatively the liquid and/or vapor phase of the test material may increase the sensitivity of the sensor device. The frequency of the test laser may be adapted to absorption bands of the solid phase of the test material or absorption bands of the liquid and/or vapor phase of the test material. If the test material is totally removed from the surface of the test object

(vaporization), the test laser may also be adapted to the properties of the surface of the test object by selecting the frequency of the test laser according to the absorption bands of the surface of the test object if the liquid phase of the test material is transparent, but this approach may be prone to error due to dirt, which may be present on the surface of the test object.

A sensor device according to the current invention may be used in any application selected from the group of automotive, aircraft or food industry applications. The sensor device may be used in a vehicle in order to detect ice on a road. Further, the sensor device may be used in an aircraft to detect ice on the wings. The sensor device may be used in the food industry to detect the correct amount of a certain fat in the food.

It is further an object of the current invention to provide a method of detecting a solid material.

The object is achieved by means of a method of detecting the at least partial phase transition of a test material from solid phase to liquid and/or vapour phase, the method comprising: providing transition light for melting at least a part of the test material by means of a light source, and detecting the at least partial phase transition of the test material from solid phase to liquid phase by means of detection light.

Using the active method of initiating the phase transition and measuring an effect based on the phase transition improves the accuracy and reliability of the method in comparison to known methods. Additional steps that may be integrated in the method are providing first test light before providing transition light, detecting first detection light before providing transition light, - providing second test light after providing transition light and detecting second detection light after providing transition light.

These additional steps may further improve the accuracy and reliability of the method. Additional features will be described below which can be combined together and combined with any of the aspects. Other advantages, especially over other prior art, will be apparent to those skilled in the art.. Numerous variations and modifications can be made without departing from the claims of the present invention. Therefore, it should be clearly understood that the form of the present invention is illustrative only and is not intended to limit the scope of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be explained in greater detail with reference to the figures, in which the same reference signs indicate similar parts, and in which: Fig. 1 shows a principal sketch of a first embodiment of a sensor device according to the current invention. Fig. 2 shows a principal sketch of a second embodiment of a sensor device according to the current invention.

Fig. 3 shows a principal sketch of a third embodiment of a sensor device according to the current invention.

Fig. 4 shows a possible timing scheme for driving a sensor device according to the current invention. Fig. 5 shows measured signals of a sensor device according to the current invention.

DETAILED DESCRIPTION OF EMBODIMENTS The first embodiment according to the current invention shown in Fig. 1 comprises a laser as a light source 1 and a light detector 8 for detecting ice 10 on the surface of a target object 20. The laser emits transition light 7 being a laser beam directed to the surface of the target object 20 which is at least partly covered with ice 10. A light detector 8 that may be for example a photodiode detects detection light 6 being in this case transition light 7 thrown back by the ice. The laser beam melts at least a part of the ice 10, building a water spot 11. The melting of the ice 10 changes the properties of the detection light 6, which is detected by means of the photodiode. In a stationary application, that means an application with a defined distance between the laser and the target object 20, a lens or any other suitable optical device may focus the laser beam on the surface of the target object 20. In dynamic applications with a varying distance between the laser and the target object 20, the laser beam may be collimated to a narrow but parallel beam by means of a lens. The collimated laser beam may be more suitable because the energy density of the collimated laser beam is essentially independent of the distance between the laser and the target object 20. Alternatively, an auto focus device may be used to adapt the focus of the laser beam 5 to the surface of the target object 20, but this solution may be expensive and prone to error.

There are at least two modes of operation that may be used to drive the laser sensor. In a first discrete mode, a first low intensity laser beam is used to emit test light 5 and the test light 5 is thrown back by the ice, resulting in detection light 6 being measured by means of the photodiode 8, and subsequently the high intensity laser beam is used to heat up and melt a part of the ice 20 by means of transition light, building a spot of water 11. After melting a part of the ice, the laser emits a second low intensity test light 5 and again the ice throws back the test light 5, resulting in detection light 6 being measured by means of the photodiode 8. The difference between both signals measured by means of the photodiode is an indication whether ice 10 was present on the surface of the target object 20. In a second continuous mode, the detection light 6 of a high intensity laser beam, the transition light 7, is measured preferably continuously by means of the photodiode 8 and a significant change of the measured signals is an indication of ice 10 being present on the target object 20.

Fig. 2 shows a principal sketch of a second embodiment of a sensor device according to the current invention. In contrast to the first embodiment, the sensor device comprises two light sources: a test laser 2 and a transition laser 3. The transition laser 3 may be optimised to melt ice 10 on the target object 20, and the test laser 2 may be optimised with respect to the sensitivity of the light detector 8 in order to improve the sensitivity of the sensor device. The transition laser 3 may emit transition light 7 of a first frequency and the test laser 2 may emit test light 5 of a second frequency being different from the first frequency. The first frequency may be in the infrared frequency range and adapted to an absorption band of ice at, for example, a wavelength of around 2.6μm. The second frequency may be adapted to the sensitivity of the light detector 8, which sensitivity may be further improved by coupling a filter to the light detector 8 with maximum transmission at the second frequency of the test light 5. In addition or alternatively, the second frequency of the test light 5 may be selected in a way that influence by ambient light is minimized in order to improve the signal to noise ratio. Further, the second frequency may be adapted for example to the absorption bands of water and or water vapour in order to improve the sensitivity of the sensor device. As discussed in the context of the first embodiment shown in Fig. 1, the sensor device can be driven in a discrete or a continuous mode.

In the embodiment of the sensor device according to the current invention shown in Fig. 3, the light detector 8 is coupled to the test laser 2. The test laser 2 acts as a light source and the light detector 8 receives a signal due to self-mixing interference. The test light 5 emitted by the test laser 2 is thrown back by the target object 20 or ice 10 on the surface of the target object 20. This thrown back detection light 6 partly re- enters the laser cavity of the test laser 2, and a lens may be used to focus the detection light 6 with respect to the laser cavity of the test laser in order to increase the corresponding signal in the laser cavity of the test laser 2. The frequency of the detection light 6 entering the laser cavity of the test laser 2 is shifted with respect to the frequency of the test light 5 emitted by the test laser 2. The detection light 6 re-entering the laser cavity interferes with the test light 5, resulting in a beat frequency that can be detected by the light detector 8. The light detector 8 may be a photodiode providing an electrical signal to an analyzer (an ASIC or programmable processor, not shown) in order to analyze the signal. Melting of the ice 10 induced by transition light 7 emitted by the transition laser 3 (not shown) changes the properties of the detection light 6, causing a change of the electrical signal provided by the photodiode. This change in the electrical signal provided by the photodiode is analyzed by the analyzer and corresponds to the melting of the ice 10. Analogous to the embodiment discussed in combination with Fig. 2, the transition laser 3 may be optimised to melt ice 10 on the target object 20, and the test laser 2 may be optimised with respect to the sensitivity of the light detector 8 in order to improve the sensitivity of the sensor device. The transition laser 3 may emit transition light 7 of a first frequency and the test laser 2 may emit test light 5 of a second frequency being different from the first frequency. The first frequency may be in the infrared frequency range and adapted to an absorption band of ice at for example a wavelength of around 2.6μm. The second frequency may be adapted for example to the absorption bands of water and or water vapour in order to improve the sensitivity of the sensor device. As discussed in the context of the first embodiment shown in Fig. 1, the sensor device can be driven in a discrete or a continuous mode.

Fig. 4 shows a timing scheme for driving a sensor device according to the second embodiment of the current invention. In comparison to the embodiment shown in Fig. 2, the light detector 8 is placed at a different location. At time tl shown on the left side of Fig. 4, the test laser 2 emits test light 5 of a first frequency and of low intensity hitting ice 10 on the surface of the target object 20. Detection light 6 thrown back by the ice 10 is detected by means of the light detector 8 and converted to a first electrical signal stored in a memory device not shown. At time t2, later in time than tl, shown in the middle of Fig. 4, the transition laser 3 emits transition light 7 of a second frequency and of sufficient intensity to melt at least a part of the ice 10, resulting in a water spot 11. The first frequency is different form the second frequency and the light detector 8 comprises a filter filtering out the second frequency. Consequently, no light caused by the transition laser 3 is detected by means of the light detector 8. At time t3, later in time than t2, shown on the right side of Fig. 4, the test laser 2 again emits test light 5 of the first frequency and of low intensity hitting the water spot 11 on the surface of the target object 20. Detection light 6 thrown back by the water spot 11 is detected by means of the light detector 8 and converted to a second electrical signal also stored in a memory device, not shown. An analyzer (not shown) compares the first signal and the second signal and the ice is detected by means of the difference between the first signal and the second signal.

In Fig. 5, a principal sketch of measured signals of a sensor device according to the current invention is shown. On the abscissa the time t is depicted and on the ordinate the relative signal strength S is depicted.

The experiment was performed in the following way: A small water droplet was placed on a metallic copper surface (target object 20) and the temperature of the surface was decreased below 0° C. The test laser 2 (He-Ne laser) and the transition laser 3 (Ar-ion laser emitting at the wavelength λ = 488 nm) spatially overlap on the droplet. The overlap surface was about 2 mm². Both lasers are operated in the continuous wave mode. The reflected light of the test laser 2 was monitored by a first photodiode being the light detector 8; the thrown back light of the transition laser, which was mainly used as a status monitor of the heating process, was detected with a second photodiode. Curve 17 in Fig 5 shows the time dependence of the signal strength resulting from the thrown back light of the test laser 2 detected by the first photodiode; curve 15 shows the time dependence of the signal strength resulting from the thrown back light of the transition laser 3 detected by the second photodiode.

The transition laser 3 was switched on at time t_a, shown by the increased signal strength measured by the second photo diode as depicted in curve 15. While the test laser was continuously operating, the signal strength measured by the first photo diode showed a strong increase that stareted no sooner than tb, which is later in time than t_a, as depicted in curve 17. At the same time tb, the signal strength measured by the second photo diode slightly decreased as depicted in curve 15. Consequently, the ice- water transition could be detected by means of the thrown back laser light of the transition laser and the thrown back laser light of the detection laser, both of which can be used as detection light 6 in conformity with the different embodiments according to the current invention.

The present invention will be described with respect to particular embodiments and with reference to certain drawings, but this is not to be construed in a limiting sense, as the invention is limited only by the appended claims. Any reference signs in the claims shall not be construed as limiting the scope thereof. The drawings described are only schematic and are non-limiting. In the drawings, the size of some of the elements may be exaggerated and not drawn to scale for illustrative purposes. Where the term "comprising" is used in the present description and claims, it does not exclude other elements or steps. Where an indefinite or definite article is used when referring to a singular noun, e.g. "a" or "an", "the", this includes a plural of that noun unless specifically stated otherwise.

Furthermore, the terms first, second, third and the like in the description and in the claims are used for distinguishing between similar elements and not necessarily for describing a sequential or chronological order. It is to be understood that the terms so used are interchangeable under appropriate circumstances, and that the embodiments of the invention described herein are capable of operation in other sequences than described or illustrated herein.

Moreover, the terms top, bottom, first, second and the like in the description and the claims are used for descriptive purposes and not necessarily for describing relative positions. It is to be understood that the terms so used are interchangeable under appropriate circumstances and that the embodiments of the invention described herein are capable of operation in other orientations than described or illustrated herein.

Other variations of the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims.

Claims

CLAIMS:

1. Sensor device comprising at least one light source (1) and at least one light detector (8), the light source (1) being designed to initiate an at least partial phase transition of a test material (10) from solid phase to liquid and/or vapor phase by means of transition light (7) emitted by the light source (1), and the light detector (8) being designed to detect the phase transition of the illuminated test material (10) by means of detection light (6).

2. Sensor device according to claim 1, wherein the at least one light source (1) is a laser.

3. Sensor device according to claim 2, the laser being designed to emit test light (5) and transition light (7), the transition light (7) having a higher intensity than the test light (5) and the detection light (6) being test light (5) thrown back by the test material (10).

4. Sensor device according to claim 2, wherein the sensor device comprises a transition laser (3) and a test laser (2), the transition laser (3) being designed to emit transition light (7) and the test laser (2) being designed to emit test light (5), the transition light (7) having a higher intensity than the test light (5) and the detection light (6) being test light (5) thrown back by the test material (10).

5. Sensor device according to claim 3 or 4, wherein the at least partial phase transition of the test material from solid phase to liquid phase is detected by self-mixing interferometry (SMI).

6. Sensor device according to claim 4, the transition laser (3) being designed to emit transition light (7) having a first frequency and the test laser (2) being designed to emit test light (5) having a second frequency different from the first frequency.

7. Use of a laser sensor according to any of the preceding claims in any application selected from the group of automotive, aircraft or food industry applications.

8. Method of detecting the at least partial phase transition of a test material from solid phase to liquid and/or vapour phase, the method comprising: - providing transition light (7) for melting at least a part of the test material by means of a light source (1) and detecting the at least partial phase transition of the test material from solid phase to liquid phase by means of detection light (6).

9. Method according to claim 8, the method further comprising the steps of: providing first test light (5) before providing transition light (7), detecting first detection light (6) before providing transition light (7), providing second test (5) light after providing transition light (7) and detecting second detection light (6) after providing transition light (7).