FI95081B - A method and apparatus for detecting a liquid on the surface of a solid - Google Patents

Description

, 95081

A method and apparatus for detecting a liquid on the surface of a solid

The invention relates to a method and apparatus for detecting a liquid on a smooth surface of a solid permeable to electromagnetic radiation. The invention relates in particular to a method and apparatus for detecting water on the windscreen of a vehicle and to an apparatus based on the method for providing on-demand control automation for the automatic starting and stopping of a windscreen wiper.

In practice, there is a need to measure the proportion of liquid covered by a liquid on the surface of a solid. In many applications, the measurement does not need to be particularly accurate, but simplicity and functionality are required in difficult and variable conditions. A typical solid / gas interface where you want to measure the liquid-covered portion is the wind * or rear window of a vehicle, implement, or the like.

15 Dozens of inventions have been patented in order to solve the vehicle application in question. However, they have not been applied commercially to vehicles. This is due to their technical shortcomings.

Four types of solutions have been proposed for the application: based on capacitance, refraction and reflection of electromagnetic radiation (mainly infrared and visible light), microwave reflection and sound caused by hitting droplets. Capacitive methods do not achieve sufficient accuracy at low water concentrations under such varying conditions (humidity and temperature). Most of the disclosed inventions are based on the utilization of various reflections or refractions of electromagnetic radiation.

25 '. Solutions based on electromagnetic radiation fall into two main groups: in the second group, the radiation is emitted from inside the glass from free air and is reflected / scattered back from water droplets and / or solid dirt on the surface of the glass. In the other, radiation is transmitted inside the glass through a prism at an angle where it is totally reflected inside the glass as it travels.

30 The latter inventions are based on e.g. to the principle set out in Swedish Patent No. 353,497 of 1969. In it, the radiation transmitted inside the glass through a prism propagates in total reflection alternately from both surfaces of the glass. Some of the radiation is taken out by the second prism after a few total reflections. If the radiation meets a drop of water on the surface of the glass, it will escape from the glass instead of being completely reflected. This causes a decrease in the intensity of the radiation coming through the receiving prism, from which the presence of droplets can be detected.

2 95081

The method according to said Swedish patent has the fundamental and essential disadvantage that the intensity of the background radiation entering the detector element is at times so high that it overloads the detector and thus prevents the desired operation. The same applies to all inventions and known techniques based on the same principle, in which the detector is in free air.

In U.S. Patent No. 4,676,638, access of background radiation to a radiation measuring detector is eliminated by two different principles. The first way is to use a multi-surface prism so shaped that the background radiation is exposed to the non-reflecting surface in the path of the radiation after the first total reflection from the surface 10 of the prism. This is not the case for the radiation used for the measurement, which reaches the photosensitive sensor after the second reflection in the prism. Another way of distinguishing the background radiation of the same patent is basically the same thing. It causes the rectangular prism and the air gap to reflect the total background radiation from the end surface of the prism and thus prevents it from entering the photosensitive detector. This is possible with a rectangular prism, because the refractive indices of glass, water and air happen to be suitable for this, as shown in detail in the patent.

The solution according to that patent has many drawbacks. A particularly significant shortcoming is that it does not in any way compensate for changes in the parameters of the radiation transmitting and receiving components, both in terms of environmental conditions and aging. However, in the automation application of vehicle windshield wipers, for example, these changes are essential distractions.

The invention relates to a method and an apparatus in which the above-mentioned patents and *; disadvantages of prior art solutions. The method according to the invention thus provides the detection of a liquid on the surface of a solid, in which the effect of background radiation is eliminated partly automatically and partly by means of the coupling and placement of the detector elements. In particular, the invention relates to a method and apparatus for providing a control signal suitable for automatically starting and stopping the windshield wipers of a vehicle 30.

The method according to the invention for detecting a liquid on the surface of a radiation-transmitting solid is characterized mainly by the features set forth in claim 1. 35

The apparatus according to the invention for detecting a liquid on the surface of a solid permeable to electromagnetic radiation is characterized mainly by the features set forth in claim 6.

n 3 95081

The method and apparatus according to the invention are based on the behavior of radiation as it enters at different angles from inside a solid to its outer surface. If the angle between the beam coming from the inside to the surface and the surface is smaller than the limit angle Θ1 of the total reflection of the substance, the total beam is reflected back inside the solid. However, if the beam hits a liquid droplet at the interface 5, the radiation can pass through the droplet into the surrounding gas space, unless said incident angle is less than the total angle of reflection Θ2 of the solid-liquid interface for that wavelength. The behavior of the radiation is as described above in cases where the refractive indices of the solid, liquid and gas (vacuum) decrease in this order, as is the rule in practice.

10

Thus, liquid droplets on the surface of a solid or a uniform layer of water cause at the angles between the total reflection boundary angles Θ1 and Θ2 the total part of said surface is reflected back to the solid the smaller part of said surface is covered by the liquid, while in the boundary angle Θ1 the incoming radiation is totally reflected regardless of whether there is liquid on the surface. On this basis, changes in the intensity difference of the total beam reflected from the surface indicate changes in the liquid-covered portion of the solid surface. It is advantageous to make the initial arrangement such that, when the measuring surface is free of liquid, the intensities of the radiation reflected are equal. In that case. The difference in intensities is proportional to the proportion of liquid surface covered by the liquid.

According to the invention, at least two detectors 11 and I2 sensitive to the radiation in question, for example a photocell, are used to detect total reflections radiated back from the surface of the solid, producing a direct voltage depending on the amount of radiation and the electrical load on the detectors. According to the invention, the detectors: are short-circuited so that the plus sign of the first detector is connected to the negative sign of the second detector and the minus sign of the first detector is connected to the plus sign of the second detector, respectively. A single pair of detectors can be replaced by groups of detectors, where each detector in one pair of detectors has been replaced by a group of detectors connected in series and / or in parallel. In this case, it is preferred, but not necessary, that identical detectors be used and that the connection be symmetrical between the detectors used for measurement and compensation.

In the method according to the invention, each detector or group of detectors is connected in a state of equilibrium in a state of equilibrium 35 in a way, because they load each other so that the pole voltage of each is zero. In this case, they both carry the same short-circuit current, and the voltage drop caused by the internal resistor with this current cancels out the 95081 4 electromotive force dependent on the radiation coming to the detector. This is the case when there is no liquid in the measuring range. The presence of a liquid, as described above, causes some of the radiation from the second beam to escape and thus the intensity of the radiation impinging on the corresponding detector is lower than that of the radiation impinging on the second detector. In this case, the voltage across the detector member 5 formed by said detectors changes sharply. The intensity of the phenomenon is confirmed by the fact that, for example, as the radiation received by a photocell increases, in addition to the increase in electromotive force, the internal resistance of the cell also decreases sharply.

With the overall procedure used in the invention and as part of it, the connection 10 described above achieves completely superior protection against all external interference factors. This is because in the equilibrium state, for which the changes are detected as changes in the measured value, all the relevant disturbance factors cancel each other out due to the reference principle used. The detectors are set to the same conditions (e.g. temperature), whereby the changes in conditions occur similarly and simultaneously for each detector and affect the same parameters 15 equally, thus maintaining equilibrium (U = 0). In addition, it is the case that the photoelectric cell is least dependent on the temperature of the cell when loaded, which is why the connection used is also the best possible in this respect. Also diffuse radiation caused by e.g. the inaccuracy of the surfaces and which is practically always accessible to the detectors to some extent, is evenly distributed among the detectors, so that the balance is not disturbed either. Also, the aging of the used radiator 20 is completely compensated in the equilibrium state, because the beams of the radiation used for the measurement come from the same or a similar radiator. The aging of the detectors also takes place at approximately the same pace, so that its effects are also reversed. From the above, it is clear that it is advantageous to use identical detectors in the detector element. However, this is not necessary, as a balance can also be achieved between 25 different (sized) detectors by balancing the situation with the relationship between the radiation beams. In this case, however, at least some of the compensatory benefits described above are lost.

The self-compensation described above and the elimination of background radiation are crucial advantages, for example in an application where the method and apparatus according to the invention are used to automatically control vehicle windscreen wipers, i.e. to start wipers when water enters the measuring area. the influence. The in-vehicle application is a rather difficult application to the prior art solutions because it changes the environmental conditions (e.g., temperature and radiation conditions) rapidly and over a wide range.

In the case of a plate-like fixed measuring surface, such as in a car windscreen,

II

5 95 081 can be directed radiation used for the measurement of bringing in the disc surface and the same surface of the second side plate measuring area for introducing the reflected beams for detectors used in the method and apparatus of the invention, the plate attached to the surface and the radiation-permeable pieces or prismatic body, which is sufficiently high refractive indices an attached 5 with medium allows the infrared radiation plate and back from the plate without total reflection. Radiation-transmitting and distributing members as well as radiation-receiving detector members can be attached to the prism member either embedded or attached to the outer surface. When the solid is thicker and / or freely deformable on the opposite side of the measuring surface, a separate body to be attached is not necessary, but the members required in method 10 can be placed directly on the solid itself.

In the case of a plate-like solid, it is also possible to use a separate prism element to transmit radiation inside the plate and from there to the detector element, whereby the total reflections may be more than one and both outer surfaces of the solid plate, increasing the measurable area.

There are no essential requirements for the radiation-emitting members other than that they produce at least two directed beams of radiation hitting the surface of the solid material at its angles as defined above. Thus, such radiation emitting members may be attached to the side surfaces of the prism ground at appropriate angles. It is most advantageous to use a single radiation source, the radiation emitted by which is divided and directed into two beams traveling in certain directions. Such an apparatus consists, for example, of a single radiation source directed at a semipermeable mirror. Some of the radiation is reflected from the mirror in the desired direction and some passes through the mirror. The transmitted radiation of the mirror is reflected in the other direction by means of, for example, 25 impermeable mirror surfaces. Another way to distribute radiation is to use appropriately ·; dimensioned prisms. Any other solutions that can be used to produce at least two beam directed in a particular direction are also useful in accordance with the invention.

The measuring range according to the invention is the area on the surface of a solid on which the beam which hits the surface of the solid is reflected from the impacted surface at such an angle: * that when it hits the liquid on the surface of the solid it is not completely reflected from the solid-liquid interface. It is advantageous but not necessary for the reference beam to hit the same measuring range.

It is also possible to send the reference beam directly to the detector used for comparison. In this case, however, part of the benefit of the compensation is lost.

35 6 95081

The detectors used according to the invention can also be mounted on a prism element, for example on its side surface, at such points that the two beams used for the measurement, which are totally reflected from the measuring area, hit them. In this case, it is advantageous to leave an air gap between the prism and the detectors in order to ensure the total reflection of the background radiation from the inner surface of the prism, thus preventing the background radiation from entering the detectors. However, according to a preferred embodiment of the invention, the detectors are preferably mounted on an auxiliary prism located at the end of the air gap from the prism through which the rays from the measuring area reflected from the plate surface arrive. This makes the direction of the detectors perpendicular to the incoming direction of the beams and the air gap prevents the background radiation from entering the detectors, as described below in connection with the figures.

One way to eliminate background radiation is to delimit and cover the area around the measuring area with an opaque material so that background radiation from outside the solid material cannot reach the detectors because it cannot propagate from the resulting 15 windows to the solid surface the radiation used emits the detectors specifically at angles smaller than the limit of the total reflection.

According to the invention, the radiation can be transmitted continuously, whereby the deviation of the output voltage U from the equilibrium state reflects the liquid-covered part of the measuring area. The dependence is, for example, sufficiently linear in a reasonable range in the vicinity of the equilibrium state for said wiper application.

Alternatively, the radiation can also be transmitted in the form of pulses, in which case a signal directly dependent on the amount of liquid at the height of the pulse is obtained, which can be processed by the electronics of the invention up to the control signal. When using a pulse signal, it is advantageous to continuously transmit a reasonable amount of radiation evenly to each detector with a separate radiator. This avoids any other imbalance in the voltage of the detector element which may otherwise occur in the near-irradiated state, which is otherwise possible because, for example, without radiation, the internal resistance of the photocells 30 becomes very high, so that a negligible radiation imbalance can give an incorrect measuring voltage. Another way to eliminate the possibility of this problem is to .. dimension the input impedance of the electronics handling the voltage coming from the detector element so small that it eliminates such errors based on low total radiation by loading. The advantage of this procedure is that all the significant interference factors due to the environmental factors are compensated for by the same radiation source, the reference principle and the coupling shown, since they apply in the same way to the quantities to be compared. Even changes in the measuring surface, such as rocking in the windscreen, are well compensated for because the interference they cause to both radiation beams is usually the same.

il 7 95081

The invention is described below with reference to the accompanying figures, in which

Figures 1a and 1b show how the radiation behaves when it enters at different angles from inside the solid 5 to its outer surface,

Figure 2 shows a simple embodiment of the apparatus according to the invention.

Figures 3a and 3b show the interconnection of detectors 10 of the apparatus according to the invention, which forms an electronic detector element.

Figures 1a and Ib show how the radiation behaves at different angles from inside the solid, radiation-transmitting substance to its outer surface 1. If the angle α1 between the beam coming from the inside to the surface 1 and the surface is less than the boundary angle Θ1 of the total reflection 15, the beam a is reflected inside the solid. However, if the beam a hits the liquid drop V at the interface, the radiation can pass through the drop to the surrounding medium, e.g. air, as shown in Figure 1b. However, if said angle of incidence is less than the cut-off angle kokonais2 of the total reflection of the solid-liquid interface for that wavelength, the total reflection always occurs regardless of the presence of the liquid. Such. The 20 situations in Figures 1a and 1b are represented by radius b and angle a2.

In the measuring member of Fig. 2, which consists of prism elements P1 and P2 and radiation transmitting, directing and receiving members, measuring radiation is transmitted from one radiation source S1 and is divided into two equally strong radiation bundles by means of a semi-transparent mirror ml, lattice or the like. One of these, beam A in Fig. 2, is guided by a solid: inside a mirror ml to its outer surface 1 at a measuring point and at an angle between the total reflection boundary angles Θ1 and Θ2 described above, and the other beam B in Fig. 2 at mirror at an angle less than than the total angle of reflection between the solid and the liquid Θ2. From the dry surface, both beams 30 are totally reflected at angles equal to the incident angles, but if the interface in the measuring area is completely or partially covered by liquid V, the radiation at a greater angle escapes in a corresponding proportion to the surrounding space, such as radius c in Fig. 2 and attenuated accordingly.

35 In Figure 2, the radius t represents the background radiation coming from the outside through the measuring surface. This background radiation cannot reach the detectors 11,12 because the angle Θ4 between it and the end surface 3 of the prism bounded by the air gap is in all cases smaller than the limit angle of the total reflection, 95081. In the case of Fig. 2, the above-described total reflection of the background radiation requires that the refractive index of the solid and the prism element be at least 1.5, as in glass, because the angle Θ3 between the measuring surface and the end surface 3 of the total background radiation prism element ΡΊ is 90 °. If 5 other materials are used, care must be taken that the angle Θ3 formed by the measuring surface and the end surface of the prism is large enough with respect to the refractive index of the solid and the prism so that said total reflection to background radiation occurs in all cases at the end surface opposite the prism element air gap.

10 In this case, when the photocells 11 and 12 sensitive to the radiation in question, which act as detector means, are connected as shown in Fig. 3a, a voltage U depending on the difference in the intensities of the above-mentioned beam is output.

Figure 3b shows how several detectors can be connected instead of one short-circuited pair 15. It has a k1 detector connected in series to a group connected in parallel with n1. On the other hand, it has a k2 detector connected in series to a group connected in parallel with n2. Said detector groups are then connected to opposite signal poles. Thus, in Fig. 3b, the detectors 11 and 12 of the detector member of Fig. 3a have been replaced by detectors connected in parallel and in series. It is preferred to connect identical detectors as shown in Figure 3b so that kl = k2 and n1 = n2. 20 However, the coupling can also be balanced by adjusting the intensities of the radiation beams A and B. In this case, however, part of the benefit of the compensation is lost.

In the apparatus according to the invention, the electronic part required for the automatic on-demand control of the vehicle wipers can be realized using both direct radiation and pulses by using known techniques so that the signal values describing the presence of water start the wipers from the desired signal level and stop accordingly.

The invention is not limited to the vehicle application described above or to the embodiment shown in the descriptions and drawings, but may be modified within the scope of the appended claims. Said materials have been used only as examples to illustrate the principle and to describe the main application. For the sake of simplicity, it is assumed in the figures shown that the refractive index of the prism elements and the fastener is the same as that of the solid.

II

35

Claims (14)

95081 A method for detecting a liquid 5 in the measuring range on the surface of a radiation-transmitting solid, characterized in that electromagnetic radiation is transmitted in at least two beams through the solid to the measuring range such that the angles (ai) and (a2) between the direction of the beams (A, B) the following condition: the angle (α1) is less than or equal to the cut-off angle (Θ1) of the total internal reflection from the solid surface and greater than or equal to the cut-off angle (Θ2) of the total internal reflection from the solid-liquid interface, and the angle (α2) is less than than the cut-off angle (Θ2) of the total internal reflection from the solid-liquid interface, and that after 15 total reflections from the plate surface in the measuring range, the result of the comparison of the measured intensities of the beams is used to detect the liquid in the measuring range. A method according to claim 1, characterized in that said beams (A, B) totally reflected from the surface of the solid in the measuring range are passed to at least two radiation-sensitive detectors (11, 12) or groups of detectors connected to different poles so that the detectors ( 11,12) the voltage (U) measured across the connected poles is proportional to the difference in intensities of the beams (A, B) impinging on the detector member formed by the detectors (11,12) and at the same time to the liquid-covered portion of the measuring surface. 25 Method according to one of the preceding claims, characterized in that said electromagnetic radiation is transmitted as continuous radiation or pulses and is received and measured, respectively, as a continuous voltage or pulse height generated in the detector element formed by the detectors (11, 12). 30 Method according to one of the preceding claims, characterized in that said radiation-sensitive detectors (11, 12) are additionally emitted background radiation (T) 35 which is low in relation to the measurement radiation, continuous and substantially equally strong between the detectors, in order to maintain the switching balance even with small amounts of measurement radiation. Method according to one of the preceding claims, characterized in that the voltage (U) or the pulse height measured over said short-circuited detector elements 0 95081 is used as a control signal after possible amplification to control, for example, the starting and stopping of the windscreen wipers. An apparatus for detecting a liquid in a measuring range on the surface of a transmitting solid, characterized in that the apparatus comprises at least (a) means for transmitting electromagnetic radiation in at least two beams (A, B) through the solid to the measuring range so that the direction of the beams (A, B) and the angles (ai) and (a2) between the measuring surface 10 satisfy the following condition: the angle (u1) is less than or equal to the limit of the total internal reflection from the solid surface (Θ1) and greater than or equal to the limit of the total reflection from the solid-liquid interface ( Θ2), and the angle (a2) is less than the limit angle of the total internal reflection from the solid-liquid interface (Θ2), and (b) means in the measuring range for measuring the difference or ratio of the intensities of the totally reflected beams 20 on the solid surface. to detect the above liquid. Apparatus according to claim 6, characterized in that said means (b) comprise at least two detectors (11, 12) or 25 groups of detectors sensitive to electromagnetic radiation arranged so that said beams in the measuring range after total reflection from the solid surface * hit the detectors and are short-circuited so that the voltage (U) measured over the detectors (11,12) is proportional to the difference in intensities of the beams hitting the detectors. Apparatus according to claim 7, characterized in that said means (b) further comprise means for amplifying said voltage (U) on the surface of the solid as a control signal depending on the amount of liquid at the measuring point, which can be used, for example, to control the apparatus for removing liquid on the surface. Apparatus according to any one of the preceding claims 6 to 8, characterized in that it comprises means for transmitting said electromagnetic radiation as continuous radiation or pulses and means for measuring totally reflected rays from the solid surface in the detectors (I1, I2) as voltage or pulse height, respectively. Apparatus according to any one of the preceding claims 6 to 9, characterized in that it further comprises means for transmitting a continuous and substantially equal background radiation to the detectors to maintain the balance of the circuit even with small amounts of radiation. Apparatus according to any one of claims 6 to 10, characterized in that it comprises a first prism element (P1) transmissively attached to the measuring side on the opposite side of the measuring point, comprising said means (SI, ml, m2) for transmitting beam beams through the solid. Apparatus according to claim 11, characterized in that said means for transmitting radiation comprise a means (SI) for transmitting electromagnetic radiation in a certain direction and means (ml, m2) for dividing said radiation into two beams of suitable intensity. Apparatus according to claim 11 or 12, characterized in that it also comprises a second prism element (P2) to which said detectors (11, 12) are attached and which are positioned relative to the first prism element (P1) so that there is an air gap ( 2), wherein the rays reflected from the plate surface from the measuring area pass through said first prism (P1) and further through the air gap (2) and the second prism (P2) to the detectors, whereby background radiation from any direction to the solid measuring surface does not enter the air gap (2) and the second prism (P2) to the detector members (11,12), but is totally reflected from the surface of said first prism member (P1) against said air gap back inside it and, if desired, turned off to another, non-reflective surface of the prism (P1). Use of an apparatus according to any one of the preceding claims 6-13 for detecting the liquid on the windscreen of a vehicle and for controlling the automatic start and stop of the windscreen wipers. 35 95081