WO1995027894A1

WO1995027894A1 - Method and apparatus for detecting liquid on a solid material surface

Info

Publication number: WO1995027894A1
Application number: PCT/FI1995/000171
Authority: WO
Inventors: Matti Kolari
Original assignee: Matti Kolari
Priority date: 1994-04-07
Filing date: 1995-03-30
Publication date: 1995-10-19
Also published as: AU2139395A; FI95081C; FI941605A0; FI95081B

Abstract

In practice a number of situations arise which indicate a need to measure the proportion of the surface of a solid that has been covered with liquid appearing on the surface. Such measurements do not always require great precision, but rather simplicity of operation and the guarantee of operation under a wide range of varying environmental conditions. The invention presented here comprises a method and an apparatus for the measurement of liquid appearing on the surface (1) of a solid substance. The method is based on the comparison of the intensity of radiation which is totally reflected back into the interior of a solid from the boundary between the surface (1) of a solid and liquid appearing on this surface, and the intensity of radiation which passes through this solid/liquid boundary on the surface, using a photoelectric cell connected in a short circuit to measure these intensities. The difference in intensities is measured and applied to the control of e.g. the windshield wipers of a motor vehicle.

Description

Method and apparatus for detecting liquid on a solid material surface

The present invention relates to a method and an apparatus for detecting liquid on the smooth surface of a solid material transparent to electromagnetic radiation. One particular aim of the invention is to provide a method and an apparatus for detecting water on the windshield of a motor vehicle, combined with a need-based control system that would recognize the conditions necessary for automatically turning windshield wipers on and off.

There are a number of practical applications in which there is a need to measure how much of a solid surface has been covered by a liquid. Measurement does not need to be particularly accurate for many of these applications, rather the need is for a simple system that would work well under difficult and varying conditions. One of the most common places where it would be useful to measure the relative area covered by liquid at the solid/gas interface is the windshield or rear window of a motor vehicle, heavy- duty machines, and other machines such as a tractor.

Dozens of patents have been obtained for inventions designed to solve the automatic windshield wiper problem. However, none of these inventions has been commercially applied in motor vehicles, because of technical deficiencies in the inventions.

The solutions proposed for the problem of automatic windshield wipers can be classified into four types on the basis of the method applied: capacitance, refraction and reflection of electromagnetic radiation (primarily infrared and visible light), microwave reflection, and acoustic emissions from the impact of raindrops. The capacitive methods fail to achieve sufficient accuracy under conditions of light rainfall at varying degrees of temperature and humidity. Most of the inventions previously patented are based on the reflection or refraction of electromagnetic radiation.

Solutions based on electromagnetic radiation can be further divided into two types: 1 )

The first type emits radiation from inside the window through the air, and the radiation is reflected or backscattered by the water droplets or by solids deposited on the surface of the glass. 2) the second type allows radiation to pass into the solid glass at an angle which causes it to be continually totally reflected back into the solid from the surfaces of the glass. The second type of inventions are those based on a principle presented i.a. in Swedish Patent No. 353 497 filed in 1969. In this invention, the radiation passes through a prism to the inside of the solid glass, where it proceeds through a series of total reflections from both surfaces of the glass. Part of the radiation is recaptured by another prism after a few reflections. If the radiation encounters a water droplet on the surface of the glass, it escapes. This causes a drop in the intensity of the radiation recaptured by the radiation receiving prism: this drop in intensity is used to detect the presence of water droplets.

The method described in the above-mentioned Swedish Patent has a critical defect, however: the strength of background radiation is at times so great that it overloads the detector and thus inhibits the desired system functions. The same defect appears in all similar inventions employing the principle described in the above Swedish Patent, and in all known techniques in which the sensor is mounted above an air gap.

US Pat. No. 4,676,638^' describes an invention in which background radiation is prevented from affecting the radiation receiving unit by two different methods: 1)The first method uses a multifaceted prism so formed that background radiation is reflected back toward a non-reflecting surface. The radiation used as part of the sensing system is reflected a second time toward a photodetector. 2)The second method is based on the same principle. A rectangular prism and an air gap create total reflection of background radiation at the back face of the prism, thus preventing it from reaching the photodetector. This is made possible with a rectangular prism because of the favourable combination of the refractive indices of glass, water, and air, a combination described in great detail in the patent text.

There are, however, several problems in the methods described in the above US Patent. On particularly critical defect is that changes in the parameters of either the radiation emitting or the radiation receiving unit, caused by environmental conditions or by ageing, are not compensated. However, it is precisely these changes that are critical in applications of the method to an automatic windshield wiper system in motor vehicles.

The object of the present invention is a method and an apparatus which will eliminate the disadvantages inherent in the above-mentioned patents and in technical solutions based on currently available techniques. This new method senses the presence of liquid appearing on a solid surface, and eliminates the effect of background radiation. This elimination is achieved automatically and by the arrangement of the circuit and the location of the detecting devices. Central to the invention is the method and apparatus which is used to create the control signal that automatically starts and stops the windshield wipers of a motor vehicle.

The invention comprises a method to detect the appearance of liquid on the surface of a solid which is permeable to radiation; the primary distinguishing characteristics of this method are presented in Patent Claim 1.

This invention further comprises an apparatus to detect the appearance of liquid on the surface of a solid which is permeable to electromagnetic radiation; the primary distinguishing characteristics of this apparatus are presented in Patent Claim 6.

The method and apparatus of this invention are both based on the behaviour of the electromagnetic radiation as it comes from different angles within the solid substance to reach the outside surface. Whenever the angle between the surface and the radiation coming from within the solid to the surface is smaller than the critical angle Q1 of the total reflection of the substance of the solid, the radiation will be totally reflected back into the inside of the solid. If however the radiation strikes a drop of liquid at the solid/gas boundary surface, the radiation may move through this drop into the surrounding gas, unless the angle of arrival at the surface is smaller than the critical angle Q2 of total reflection at the boundary surface between the solid and the liquid at the wave length in question. The behaviour of the radiation further includes instances like those described above, in which the refractive index of the solid substance, that of the liquid, and that of the gas decrease in this order, as is the behavioural rule.

Therefore, the greater the amount of liquid (either as drops or as a continuous sheet) there is on the surface of the solid, the smaller the amount reflected back into the solid of radiation arriving at the surface at angles between the critical angles Q1 and

Q2 of total reflection. At angles less than critical angle Q1 , however, radiation arriving at the surface will be completely reflected back into the solid, regardless of whether there is liquid on the surface or not. On this basis, it can further be said that the changes in the differences in intensities of the total reflection beams from the surface will indicate changes in the relative part of the total surface of the solid that is covered with liquid. It would be advantageous to have a starting point in which the intensities of the reflected radiation from the measured surface would be equal when the surface is free of liquid. Under such conditions, the difference in intensities would thus correspond to the area of the surface of the solid which would be covered with liquid.

The invention requires the use of at least two radiation sensitive detectors, 11 and 12, to measure the radiation totally reflected back from the surface of the solid. Such detectors could be, for example, photocells which would produce direct voltage dependent on the amount of radiation which hits the detector and the loading of the cells. In the present invention, the two detectors are connected through a short circuit so that the positive pole of the first cell is connected to the negative pole of the second, and vice-versa. A single pair of detectors could be replaced with a plurality of detectors in which a group of sensors is connected in series and/or in parallel to take the place of the single^' pair. In such case, it would be advantageous, but not indispensable, to use identical detectors, and to make the circuit symmetrical between detectors used for measurement and for compensation.

In the method used by this invention, both detectors or groups of detectors are connected in balance in such a manner that they form a short-circuit, because they load each other so that the terminal voltage of each is zero. In this way the same short circuit current flows through both of the detectors, and the internal resistance to this current causes a drop in voltage which nullifies the electromotoric force, depending on the strength of the radiation coming to the detector. This is the situation when there is no liquid in the measurement area. The appearance of liquid, as described above, partly allows the escape of one of the beams, so that the amount of radiation reaching one of the detectors is lower than the amount reaching the other detector. At this, the voltage over the detector unit changes sharply. The strength of the phenomenon is increased by the fact that the increase in the radiation received by the photocell increases the electromotoric force of the cell, in addition to which the internal resistance of the cell decreases sharply.

The total method used by the invention, and the above-mentioned circuit which forms a part of it, works to eliminate all external disturbance factors by providing a completely insurmountable protective shield. This is because in a system in which the relative changes are expressed as changes in measurement values from the balanced state, all critical disturbance factors will nullify each other due to the comparative principle used in the method. Both the detectors are subject to the same conditions (of e.g. temperature), so that changes due to environmental conditions will be the same and occur simultaneously in both detectors. This means that the effect of changes in these parameters will be the same on both sides, and the balance will be maintained (U = 0). In addition, the loading of the photocells minimizes the temperature dependence of the cells, for which reason the connection used here is the best possible in regard to this factor, too. Further, scattered radiation, which is caused by i.a. unevenness in the surface of the solid and which in practice always passes to some extent into the detector, is evenly distributed between the two detectors in the pair, so that this factor is also unable to disturb the balance. Even the ageing of the radiation source is completely compensated for evenly, since the beams used as the measuring radiation are emitted from the same or a plurality of the same emitters. Similarly, the ageing of the detectors occurs at more or less the same rate, so that the effects of this factor are also eliminated. On the basis of the above, it is clear that the sensor should use identical detectors throughout the system. Nonetheless, this is not absolutely necessary, because balance may also be achieved using different detectors, balancing the system by using a suitable ratio between the intensities of the beams. This will however inevitably result in at least a partial decrease in the compensating advantage just described.

The self-compensation and elimination of background radiation described above are crucial advantages in e.g. those applications in which the method and apparatus of the invention are used to automatically control the windshield wipers of a motor vehicle.

In other words, in applications in which the wipers are automatically turned on when water appears on the measuring area of the windshield, either as water droplets or as a continuous sheet of water, and correspondingly turned off when the water has been removed from the glass by the wipers. Motor vehicle applications are rather difficult to achieve successfully, particularly with the techniques now available, due to rapid changes which may take place within a wide range of extremes of environmental conditions (such as e.g. temperature and ambient radiation).

When the measuring surface is a plate-like fixed surface, as for example the windshield of an automobile, the method and the apparatus of the invention use a prism unit that is transparent to radiation, and which is mounted on the plate surface, to transmit the measurement radiation to the inside of the solid and to direct the radiation reflected back from the measurement area ultimately to the detector. This prism or unit must be mounted with material having a sufficiently large index of refraction, so that radiation is transferred through the interface between the prism and the plate of glass, and is recaptured without total refraction. A radiation emitter, a radiation refractor, and a radiation receiver, can be mounted either by sinking their components into the prismatic unit, or by mounting them on its surface. If the solid substance is thicker and/or free-formed on the side opposite the measuring surface, a separately attached unit is not necessary. In this case, the components required by the method can be placed directly onto the solid substance itself.

In the case of a plate-like solid, it is also possible to use one separate prismatic unit to emit radiation into the plate, and another to direct it back to a detector unit, allowing for a plurality of total reflections from both exterior surfaces of the solid.

This would permit an increase in the size of the measured surface.

The only essential requirement for the radiation emitting units is that they produce at least two beams that are separately directed so as to pass through the solid to touch the surface at the prescribed angles. Thus, radiation emitting units could be mounted on the face of a prism ground to the correct angle. Most advantageous would be the use of only one radiation emitting source, the radiation from which would be divided into two beams aimed in the two directions required. Such a device could for example be composed of one radiation emitting source that sent beams through a semi-transparent mirror. Part of the radiation would be reflected by the mirror to the desired direction and part would pass through the mirror. The radiation that passed through the mirror could then be reflected in another direction, e.g. by a non-transparent mirror surface. Another possibility for dividing the radiation would be to use suitably dimensioned prisms. Almost any solution which would produce at least two beams aimed in the prescribed direction, would serve the purposes of the invention.

The measurement area of the invention is that area on the surface of the solid to which the measurement radiation is directed. When the measurement radiation hits the surface at the correct angle, it will be totally reflected back into the solid except where the solid surface is covered by liquid, either in the form of droplets or in a sheet. At these points, the beams will be transferred into the liquid. For purposes of comparison, it would be an advantage to also direct a second beam to the measurement area but this is not a necessity.

For purposes of comparison, it is also possible to send a comparison beam directly to the detector. This would mean a partial loss of the advantage provided by the compensating effect described above, however.

The detectors required by the invention may also be mounted on the prism units, for example on the face of the prism at such a point where the two beams totally reflected by the measurement area will strike them. In such case, it would be advantageous if an air gap was left between the detector and the prism, to prevent background radiation from the interior surface of the prism from entering the detector. One particularly efficient method of implementing the invention would be to mount the detectors on to an auxiliary prism, separated by an air gap from the prism which receives the radiation reflected from the measurement area on the plate surface. This would create a detector in which the cells would be in at right angles to the direction of the incoming beams, with an air gap to prevent background radiation from entering the detectors, as explained further below in connection with the figure.

One method of eliminating background radiation is to limit and cover the area around the measurement area with a radiation impermeable material so that background radiation from outside the solid reaching the measurement area cannot enter the device. This is because background radiation could not then enter the window thus formed to reach the surface of the solid at an angle smaller than that of total reflection. But the beams used for measurement and for control are specifically emitted to reach the detector at precisely those angles smaller than the critical angles of total reflection.

The radiation required by the invention may be sent continuously, so that the voltage

(U) deviation from a state of balance reflects the proportion of the surface of the measuring area that is covered by liquid. In e.g. the windshield wiper application mentioned above, this correlation is sufficiently linear close to the state of balance in the circuit. Alternatively, the radiation may also be transmitted in pulses, in which case the height of the pulse will be a signal directly dependent on the amount of liquid. This signal may be handled electronically to become the control signal. When the pulse signal is used, it is an advantage to continually send from a separate emitter a reasonable amount of radiation evenly to both detectors. This will eliminate the possibility of imbalances of the voltage of the sensor in the nearly radiation-free states between pulses of radiation. This possibility exits because e.g. without radiation (light beam) the internal resistance of the photoelectric cell becomes very high, so that even a very small imbalance in radiation may give a false voltage reading. Another way to eliminate this problem is to make the input impedance of the measurement electronics so low that when by loading the detector it eliminates the errors resulting from very low total radiation. The advantage of this solution is that all environmental factors that create critical disturbances are compensated for by the same radiation source. This is the result of the comparison principle and the method of mounting described, because disturbances will thus affect equally the variables to be compared. Even changes in the measurement area surface, such as a stone hitting the windshield, are well compensated, because the disturbance which results is usually of the same magnitude in both beams.

The invention is described in the accompanying figures in which

Figures 1a and 1b represent the behaviour of radiation when it reaches the outside surface after travelling through the solid at various angles.

Figure 2 shows the apparatus of the invention in a simple embodiment.

Figures 3a and 3b show the connections of the detectors in the apparatus of the invention: these detectors form the sensor.

Figures 1 a and 1 b show the behaviour of the radiation as it comes from various angles through a radiation permeable solid to the outer surface 1. Whenever the angle a1 between the outer surface 1 and the radiation beam a coming to the surface is smaller than the critical angle Q1 of total reflection, beam a will be totally reflected back into the interior of the solid. If, however, beam a hits liquid droplet V at the surface boundary, the beam can pass through the droplet into the surrounding intermediary substance, e.g. ambient air, as shown in Figure 1 b. If however the angle of incidence just mentioned is smaller than the critical angle of total reflection Q2 at the liquid/solid interface at the wavelength in question, total reflection will take place regardless of the presence or absence of liquid. This situation is represented in Figures 1a and 1b by beam b and angle a2.

The measuring unit shown in Figure 2 is formed from prism units P1 and P2, and the radiation source, directional guidance, and receiving units. The measuring unit sends the measuring beam of radiation from one emitter unit S1 ; the beam is divided by e.g. a semitransparent mirror ml , or a grating, or a similar device, into two beams of equal strength. One of these beams, labelled beam A in Figure 2, is guided through the solid by mirror ml to the measurement site on the outer surface 1 , at an angle which is between the above described critical angles of total reflection Q1 and Q2. The other beam, labelled beam B in Figure 2, is guided by mirror m2 to the same point at an angle which is smaller than the critical angle of total reflection Q2 at the solid/liquid interface. With a dry surface, both beams are totally reflected at angles which are equal to the angle of incidence, but if the solid/liquid interface of the measuring area is totally or partly covered with liquid V, beams coming at greater angles will escape in a corresponding ratio into the space around the solid, as does beam c in Figure 2, and that part of the beam which is totally deflected will be correspondingly damped.

In Figure 2, beam t represents background radiation coming from outside through the measuring surface. This background radiation cannot reach detectors 11 and 12 because the angle Q4 between the detectors and the surface of the prism face 3, which is bounded by an air gap, is in all cases smaller than the critical angle of total reflection. In the situation shown in Figure 2, it is a prerequisite for the total reflection of background radiation just described that the solid and the prism unit are made of a material such as glass that has a refractive index of at least 1.5, since the angle Q3 between the surface of the measuring area and face 3 of the prism unit P1 is 90∞. If some other material than glass is used as the solid, care must be taken to ensure that the angle Q3 formed by the measuring surface and the prism face is sufficiently large in relation to the refractive index of the solid and the prism that the above-mentioned total reflection of background radiation occurs in all cases at the prism face on the side of the prism next to the air gap. When the means of detection is the photoelectric cells 11 and 12, connected in the manner shown in Figure 3a, the differences in intensities of the above-mentioned beams will produce dependent voltage U to the output.

Figure 3b shows how a plurality of detectors can be connected in a short circuit instead of a single pair of detectors. The figure shows groups of detectors connected both in series k1 and in parallel n1. In addition there are also groups of detectors connected in series k2 and in parallel n2. These groups of detectors are thus connected with the opposing poles together. Thus, the detectors 11 and 12 in the sensor in Figure 3a are replaced with detectors connected in parallel and in series. It would be an advantage to connect identical detectors in the manner shown in Figure 3b so that k1 = k2 and n1 = n2. However, the circuit can also be maintained in balance by adjusting the intensities of beams A and B. This will nonetheless result in a reduction in the advantage produced by compensation as described above.

The electronics required for the control system for the application of the invention to an apparatus for automatic windshield wipers, either when utilizing a continuous beam of radiation or when utilizing pulses, may be realized using currently available techniques, so that the presence of water will produce a signal that will automatically start the windshield wipers at a preset level, and conversely stop the wipers at another preset level.

The invention is not restricted to the above described motor vehicle application, nor to the embodiment described in the figures and their accompanying text. The invention can be adapted in many ways to many situations, within the framework of the patent requirements. The materials mentioned here are used only as an example to make the principles of the invention more concrete and more readily observable, and to describe one of the most important possible applications.^' The figures presented here have been simplified by assuming that the refractive index of the prism unit and the mounting substance is the same as that of the solid.

Claims

Patent Claims

1. A method for detecting liquid on the measurement area of the surface of a solid permeable to radiation, characterized in that the electromagnetic radiation is emitted as at least two beams to pass through the solid to reach the measurement area, so that the angles

(a1) and (a2) between the directions of the beams (A, B) and the measuring surface meet the following requirements:

angle (a1) is equal to or smaller than the critical angle (Q1 ) of internal total reflection occurring at the surface of the solid, and equal to or greater than the critical angle (Q2) of internal total reflection occurring at the solid/liquid boundary, and

angle (a2) is smaller than the critical angle (Q2) of internal total reflection taking place at the solid/liquid boundary, and that after the total reflection occurring at the measuring area on the plate surface, the measured intensities of the beams are compared to obtain a result used as the indication of liquid on the surface of the measuring area.

2. A method based on the method described in Patent Claim 1, characterized in that the beams (A,B) totally reflected back from the above-mentioned measuring area on the surface of the solid are led to at least two radiation-sensitive detectors (11 ,12) or a group of detectors, which have been connected with their opposite poles together so that the voltage (U) over the connected poles of the detectors (11 ,12) corresponds to the difference in the intensities of the beams (A,B) reaching the sensor formed by the detectors (11 ,12), and thus to the proportion of the surface of the measuring area covered by liquid.

3. A method based on the above methods mentioned in Patent Claims 1 and 2, characterized in that the above-mentioned radiation is emitted either continuously or in pulses, and received and measured similarly either as a continuous voltage or as the height of pulses reaching the sensor formed by the detectors (11,12).

4. A method based on the above methods mentioned in Patent Claims 1 , 2, and 3, characterized in that in addition to the measuring radiation, a continuous background radiation (T) is also sent to the above-mentioned radiation-sensitive detectors (11 ,12), in equal strength to both detectors, in order to maintain balance even at very low quantities of measuring radiation.

5. A method based on the above methods mentioned in Patent Claims 1 , 2, 3, and 4, characterized in that the voltage (U), or the height of the pulses, measured between the poles of the above-mentioned short circuit connection of the sensor unit, or the height of the pulses, is used (after any necessary amplifying) as the control signal to e.g. automatically turn windshield wipers on and off.

6. An apparatus to detect liquid on a measuring area on the surface of a radiation permeable solid, characterized in that the apparatus contains at least

a) unit(s) to emit electromagnetic radiation in at least two separate beams (A,B) through the solid to the measuring area so that the angles (a1) and (a2) between the direction of the beams (A,B) and the measuring surface fulfill the following criteria:

angle (a1) is equal to or smaller than the critical angle (Q1) of internal total reflection occurring at the surface of the solid, and equal to or greater than the critical angle (Q2) of internal total reflection occurring at the solid/liquid boundary, and

angle (a2) is smaller than the critical angle (Q2) of total internal reflection occurring at the solid/liquid boundary, and

b) components to measure the differences or ratios between the intensities of the beams totally reflected back from the surface of the solid, to indicate the presence of liquid on the measuring area on the surface of the solid.

7. An apparatus based on Patent Claim 6, characterized in that the components mentioned in Patent Claim (6,b) comprise at least two detectors (11 ,12) or groups of detectors sensitive to electromagnetic radiation, located so that the above-mentioned radiation reaches the detectors and that the detectors are connected in a short-circuit so that after the occurrence of total reflection at the measuring area on the surface of the solid, the measured voltage (U) between the poles of the detectors (11 ,12) corresponds to the differences in the intensities of the beams of radiation striking the detectors.

8. An apparatus based on Patent Claim 7, characterized in that the unit mentioned in Patent Claim (6,b) comprises in addition a component to amplify the above-mentioned voltage (U) as a control signal dependent on the amount of liquid on the measuring area on the surface of the solid, which signal may be used to control e.g. an apparatus to remove the liquid from the surface of the solid.

9. An apparatus of some type based on the above Patent Claims 6-8, characterized in that it comprises components to emit the above-mentioned electromagnetic radiation either continuously or in pulses, and components (11 ,12) to detect and measure the radiation totally reflected from the measuring area on the surface of the solid correspondingly either as continuous voltage or the height of the pulses.

10. An apparatus of some type based on the above Patent Claims 6-9, characterized in that it contains in addition components to emit a small amount of background radiation, which is weaker in strength than the measurement radiation, in equal quantities to both detectors, in order to maintain balance in the circuit even in cases of low amounts of radiation.

11. An apparatus of some type based on the above Patent Claims 6-10, characterized in that it comprises at the measurement area, on the opposite side of the measurement surface, a prism unit (P1 ) mounted in such a way as to allow the radiation to pass through the interface between the prism and the solid, and which comprises the previously mentioned components (S1 ,m1 ,m2) for sending beams through the solid.

12. An apparatus based on Patent Claim 11 , characterized in that the above- mentioned components for the emission of radiation comprise a unit (S1) to send the radiation in a predetermined direction, and components (m1 ,m2) to divide this radiation into two beams of suitable strength.

13. An apparatus based on Patent Claims 11 and 12, characterized in that it also contains a second prism unit (P2) on which the above-mentioned detectors (D1 , D2) are mounted, and which is located in respect to the first prism unit (P1) so that between the two is an air gap (2), which further is located so that the radiation reflected back from the measurement surface passes through the above-mentioned first prism unit (P1) and thence through the air gap (2) and the second prism unit (P2) to reach the detectors; as a result the apparatus is thus arranged so that no background radiation coming from any direction from the measurement surface passes through the air gap (2) and the second prism unit (P2) to reach the detectors

(11 ,12), but that this background radiation is totally reflected back into the first 'prism from the surface of the prism facing the air gap, and if desired extinguished by another, non-reflecting, face of the prism (P1).

14. Some type of apparatus based on the above Patent Claims 6-13, for use as an control system to detect the presence of liquid on the windshield of a motor vehicle and turn on and off windshield wipers.