Abstract:
A device for monitoring the state of a window pane includes an optical emitter which emits a light beam on to the window pane. The device also contains an optical receiver which detects light of the light beam modulated by the window pane and, as a result, generates a received signal. The emitter and the receiver are arranged at a distance from the window pane. An evaluation circuit evaluates the received signal in order to determine the state of the window pane.

Description:
BACKGROUND 
     The invention pertains to a device for monitoring the state of a window pane, in particular, an automobile window pane, consisting of at least one optical transmitter which emits a light beam onto a pane, at least one optical receiver which receives the light of the light beam modulated by the pane and subsequently generates a reception signal, and an evaluation circuit that evaluates the reception signal to determine the state of the pane. 
     The comfort and safety of a motor vehicle can be enhanced by automatic operation of the windshield wiper system. Usually, optical rate sensors located directly on the inside of the pane are used for this purpose. The state of the window pane on the outside of the pane is detected by the sensor through the pane. The monitored region in this case is limited by the size of the sensor and is less than the field of view of the driver. 
     SUMMARY 
     The present invention is based on the task of creating a device for monitoring the state of a window pane which can be flexibly adapted to different operating situations and which allows a dependable determination of the state of the window pane. 
     The task is achieved in that the transmitter and the receiver are located at a distance from the window pane. 
     Due to this distant placement of both the transmitter and the receiver from the pane, the advantage achieved is that the size of the monitored region can be selected independently of the dimensions of the device. An additional advantage consists in that the location of the monitored region can be largely user-selected and can be located, for example, in the immediate field of view of an automobile driver. Due to the potential for prior specification of a suitable size and location of the monitored region, the dependability of the attainable evaluation result can be improved significantly. 
     Even though the measurement can also theoretically take place during transmission, the receiver preferably detects the light of the transmitter through reflection or scattering off the pane. A sufficiently large monitored region with simultaneously good intensity of the reflected light can be attained when the transmitter and the receiver are located at a distance of about 10 to 30 cm from the pane. 
     Preferably the light spot projected onto the pane by the light beam has a surface area of at least 25 cm 2  and preferably about 100 cm 2 . The general state of the pane will then be represented in sufficient measure by the reflected light, and local changes in the state of the pane due to, for example, an area of dirt or a single, large rain drop, are not problems because of the size of the monitored region and they cannot falsify the monitored result, as is the case for sensors which are located directly on the glass and thus necessarily have a small monitored region. 
     The value of the monitored result will be increased when the state of the window pane that is being monitored is in the region of the immediate field of view of a driver. 
     Preferably, the transmitter operates in the infrared range, because this will prevent a driver or another person that is looking through the pane from being adversely affected by interfering reflections. Furthermore, it is an advantage that low-cost optical-electronic components operating in the infrared range, such as Si-photodiodes (as the receiver) and infrared LEDs (as the transmitter) are available. 
     According to one preferred design format of the present invention, the device is composed of a number of optical transmitters. Due to several optical transmitters, a greater transmission power can be achieved, so that the distance between the sensor and the pane can be increased. This makes possible more favorable monitoring geometries and, in addition, allows the angle between the optical axis of the transmitter and receiver, respectively, and the pane to be selected as more flat. 
     Even though basically several optical receivers can be provided, one preferred embodiment is characterized in that the device is composed of only one optical receiver. 
     In the case of several optical transmitters and one single receiver, one favorable embodiment of the invented evaluation circuit is characterized in that the circuit is composed of a discriminator stage which uses the reception signal to derive a first and a second reception signal according to the acquired modulated light from one or more first and one or more second optical transmitters. Due to this property, a transmitter-specific and, thus, also light-spot-specific, evaluation of the reception signal provided by the single receiver will be possible, which in practice allows a simultaneous monitoring of different regions of the window pane. 
     One simple possibility for forming the first and the second reception signals consists in that the first and second optical transmitter is controlled by a pulse signal of different phase and the discriminator stage is composed of a phase-synchronous demodulator. 
     Preferably, the evaluation circuit is composed of a difference stage which forms a difference signal from the first and the second reception signals. Different states of the window pane can be recognized from the temporal change in behavior of the difference signal. Whereas adhered dirt or even damage to the window pane will cause sudden, static changes, fast changes are caused by large raindrops and slow changes by smaller raindrops. Based on the state of the window pane detected in this manner, additional measures can be undertaken, such as, for example, the regulation of the speed of the windshield wipers. 
     A particularly compact design of the invented device is attained when the transmitter and the receiver are located in a common module or housing. 
     According to one preferred configuration of the present invention, on the input side of the optical receiver there is a reception lens which concentrates the light modulated by the pane onto the receiver. Due to this property, the sensitivity of the reception device will be increased and thus a greater distance between the sensor and the pane will be possible. 
     Due to the reception lens, a reception zone will be defined on the window pane. With regard to attaining the greatest measurement sensitivity, it is preferable to design the reception lens so that a reception zone defined on the pane by the reception lens matches the illuminated zone formed on the pane by the light spots of the light beam. 
     Due to the optical filter located on the input side in front of the optical receiver, interference caused by incident, secondary light can be prevented as long as the secondary light has a different wavelength than the transmitted light. In the case of a sensor operating in the infrared range, visible, scattered light can thus be effectively excluded as a potential source of interference. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The invention will be described in greater detail below based on the following figures and examples. 
     FIG. 1 is a schematic illustration of one circuit design of one embodiment of the present invention; 
     FIG. 2 is a detailed view of the signal processing state shown in FIG. 1; 
     FIG. 3 is a first example of a transmitter/receiver device; 
     FIG. 4 is another example of a transmitter/receiver device; 
     FIG. 5 is a representation of a first monitored region with an essentially square shape; 
     FIG. 6 is a representation of a second monitored region with an essentially square shape; 
     FIG. 7 is a representation of an additional monitored region with an essentially square shape; and 
     FIG. 8 is a representation of a monitored region with elongated shape. 
    
    
     DETAILED DESCRIPTION 
     According to FIG. 1, a device according to this invention has an internal voltage supply  1  which is connected by means of power supply lines  2 , 3  to an external power supply (not illustrated in FIG.  1 ), for example, the on-board power supply of a motor vehicle. Reference number  4  denotes an opening in the housing. The internal power supply  1  powers two switching power sources  6  and  7  via a power supply line  5  and powers a microcontroller  9  via a power supply line  8 . The switching power supplies  6  and  7  are connected electrically on the output side to a first optical transmitter  10  and to a second optical transmitter  11 , respectively. The optical transmitters  10 , 11  operate in the infrared range and can be designed, for example, with III-V semiconductor LEDs. 
     The control of the optical transmitters  10 , 11  takes place by means of the microcontroller  9 . A pulse signal with a frequency of 50 kHz, for example, is available to a digital output  12  of the microcontroller  9 . The pulse signal is sent directly to the switching input of the power supply  7  via a control line  13 , but it reaches the switched input of the power supply  6  only after passing through a 180° inverter  14 . The switching signal output from the inverter  14  is thus phase-shifted by 180° to the switching signal in the control line  13 , which means that the switching power supplies  6 , 7  are alternately switched on and off and the optical transmitters  10 , 11  are thus operated alternately. 
     The light emitted from the transmitters  10 , 11  moves to a window pane (not illustrated in FIG. 1) and is reflected from the window pane (in a manner to be described below) onto a receiver  15 . The receiver  15  can be designed with a Si-photodiode, for example. 
     The reception signal  16  output from the receiver  15  is amplified by an amplifier  17  and then passes through a bandpass filter  18 . The output signal  19  of the bandpass filter  18  is sent to a signal processing circuit  20 . 
     The circuit design of the signal processing circuit  20  is illustrated in FIG.  2 . The output signal  19  is sent to the inputs of two switches  21 , 22 , which are alternately driven by switch supply lines  23 , 24  corresponding to the pulse signal or the inverted pulse signal. In this manner, the signal fraction of the output signal  19  provided by the first transmitter  10  is available at the output of the switch  21 , and the signal fraction provided by the second transmitter  11  is available at the output of the switch  22 . These signal fractions are integrated in outlet-connected integrators  25 , 26  and are sent as first and second output signals  27 , 28 , respectively, to the inverting or non-inverting input of a differential amplifier  29 . Due to the differential amplifier  29 , the signal difference between the first and the second output signal  27  and  28  will be amplified with high sensitivity and output as an analog difference signal  30 . 
     Therefore, the amplitude of the difference signal  30  is a measure of the difference between the quantities of light received from the first optical transmitter  10  and the second optical transmitter  11 . Since the two optical transmitters  10 , 11  illuminate different regions of the window pane, the amplitude of the difference signal  30  represents a measure for local differences in the reflectivity and/or scattering behavior of the window pane. 
     The difference signal  30 , as illustrated in FIG. 1, is sent to an A/D-converter  31  and is converted into a digital signal  32 . The digital signal  32  is sent to a digital input  33  of the microcontroller  9 . 
     The digital signal  32  is evaluated by the microcontroller  9  with respect to the signal amplitude and the temporal change in signal amplitude. The temporal change in the signal is used for recognition of different states of the window pane, such as may be caused, for example, by raindrops (fast changes), fine foggy mist (slow changes) or damage and adhered dirt (sudden, static changes). The information obtained from the microcontroller  9  is sent via a bidirectional data line  34  to a driver  36 , which is connected over a serial data link on line  35  to an external data bus (not illustrated) of the motor vehicle. 
     A reset of the microcontroller  9  into a defined, initial state occurs when starting the motor vehicle by means of a control line  37  between the internal power supply  1  and the microcontroller  9 . The initial state, as a rule, is a default value set by the manufacturer and can be reprogrammed by the user by means of the driver  36  and the bidirectional data lines  34 ,  35  as desired. 
     FIG. 3 shows a first example of a rain sensor module  50  and its positioning with respect to a window pane  51 . The module  50  features as first optical transmitter, an IR-LED  10   a  and as second optical transmitter, an IR-LED  11   a . The two IR-LEDs  10   a ,  11   a  generate illuminating light cones  52   a  and  53   a  each with identical opening angle α. The opening angle α can be defined by a suitable LED lens or additional lenses. FIG. 3 makes clear that the illuminating light cones  52   a ,  53   a  light up different light spots  54   a ,  55   a  on the window pane  51 . The light spots  54   a ,  55   a  can partly overlap, as in the example illustrated in FIG.  3 . 
     Furthermore, the module  50  has a reception lens  56 , an optical IR-filter  57  and the IR receiver  15  already described in conjunction with FIG.  1 . The receiver  15  is located on the optical axis of the reception lens  56  and is set at a distance from it. Due to the distance between the receiver  15  and the reception lens  56  and due to the power of the lens  56  and the shape of the lens  56 , a reception zone E will be defined on the pane  51 . Only reflected light (i.e., back-scattered or reflected) in the region of the reception zone E can be detected by the receiver  15 . 
     The mode of operation of the module  50  is as follows: by means of the control of the IR-LEDs  10   a  and  11   a  explained in FIG. 1, the light spots  54   a  and  55   a  are illuminated alternately. The receiver  15  detects the scattered or reflected light coming back from the alternating two light spots  54   a  and  55   a . Then, due to the switching structure described in FIG. 1, a possibly different reflection and scattering behavior of the pane  51  will be detected in the regions of the light spots  54   a  and  55   a  and evaluated with respect to temporal changes. Since the reception zone E of the two light spots  54   a  and  55   a  overlap, all lighted regions of the pane  51  contribute to a signal, so that a maximum light yield and thus a maximum sensitivity will be attained. Furthermore, the measuring sensitivity is also determined by the relative size of the light spots  54   a  and  55   a  to each other. In the case of light spots of identical size, a maximum sensitivity will be achieved, because in this case the signal tuning can be carried out with the greatest possible accuracy in the phase-synchronous demodulator formed of the switches  21 ,  22  and the integrators  25 ,  26 . 
     In the case of a pane  51 ′ or  51 ″ located at an angle to the optical axis of the reception lens  56 , basically comparable conditions are present, but with identical opening angles α of the LEDs, different sizes of the light spots  54   a  and  55   a  and also different distances from the receiver  15  are used. This situation can be taken into account by different configurations of the LEDs  10   a ,  11   a  with respect to their optics and/or lighting intensity, and also by an asymmetrical signal evaluation. Furthermore, it should be taken into account that, in the case of an inclined pane  51 ′ or  51 ″, reflected quantities of the reflected light will be increasingly reflected out of the beam path of the reception lens  56 , which can be mostly compensated for by a suitable, axial and asymmetrical arrangement of the LEDs and also by a greater lighting intensity. 
     FIG. 4 shows an example of a second module  50 ′ with four IR-LEDs  10   a ,  10   b  and  11   a ,  11   b . In this case, the IR-LEDs  10   a ,  10   b  and also the IR-LEDs  11   a ,  11   b  are jointly controlled according to FIG.  1 . The light spots  54   a ,  54   b  generated by the jointly controlled IR-LEDs  10   a ,  10   b  together form a first illuminated zone I, while the light spots  55   a ,  55   b  of the IR-LEDs  11   a ,  11   b  are combined into a second illuminated zone II. A different reflection or scattering behavior of the pane  51  is measured in the illuminated zones I and II. In this case, the reception zone E′ is located within the total lighting region formed by the two illuminated zones I and II. 
     Based on the increase in the number of IR-LEDs  10   a ,  10   b ,  11   a ,  11   b  in comparison to the first module  50  shown in FIG. 3, a greater distance between the rain sensor module  50 ′ and the pane  51  is possible, and furthermore, a flatter angle can be used between the optical axis of the lens  56  and the pane  51 ′,  51 ″. 
     FIGS.  5 , 6 , 7  and  8  show various possibilities for lighting of the pane  51 , and also the selection of the reception zone E. 
     According to FIG. 5, four light spots form intersecting, diagonally arranged illuminated zones I and II. The reception zone E overlaps the overall illuminated zone I and II well. 
     According to FIG. 6, two illuminated zones I and II located above each other are formed by four light spots. Here, too, there is a good overlap between the overall illuminated zone I and II anal the reception zone E. 
     The geometries shown in FIGS. 5 and 6 can be produced with the module  50 ′ described in FIG.  4 . 
     FIG. 7 shows illuminated zones I and II, which are formed from a total of eight light spots. The illuminated zones I and II are arranged in a diagonal cross, similar to FIG.  5 . The reception zone E′ is configured in the shape of a circular disk as in FIGS. 5 and 6. 
     In FIG. 8, the illuminated zones I and II are formed from six light spots located side by side, and neighboring light spots are allocated to different illuminated zones I or II. Here, the elliptical or cigar-shaped reception zone E″ nearly circumscribes the entire illuminated surface and can be attained in a suitable manner by a combination of a spherical lens with a cylindrical lens, or by an astigmatic lens. The lighting geometry shown in FIG. 8 corresponds to the field of view of a driver of a motor vehicle, wherein the measured values can be ascertained in a highly accurate manner and thus control of the windshield wipers can be obtained. 
     In the case of abrasion, such as scratches on the window pane, the two areas detected by the sensor will not be identical. Therefore, in this case the transmitter regions I and II will not be alternated at a sensing ratio of 50:50, but rather of an asymmetrical sensing ratio of, e.g., 70:30. Due to the use of an integrated phase synchronous demodulator, this asymmetrical behavior can be taken into account. 
     Due to this variable sensing ratio, the sensor will thus compensate for faults, dirt and other static events. The sensor will recognize a change as static when it exceeds a characteristic time constant. lens, or by an astigmatic lens. The lighting geometry shown in FIG. 8 corresponds to the field of view of a driver of a motor vehicle, wherein the measured values can be ascertained in a highly accurate manner and thus a control of the windshield wipers can be obtained. 
     In the case of abrasion such as scratches on the window pane, the two areas detected by the sensor will not be identical. Therefore, in this case the transmitter regions I and II will not be alternated at a sensing ratio of 50:50, but rather of an asymmetrical sensing ratio of, e.g., 70:30. Due to the use of an integrated phase synchronous demodulator, this asymmetrical behavior can be taken into account. 
     Due to this variable sensing ratio, the sensor will thus compensate for faults, dirt and other static events. The sensor will recognize a change as static when it exceeds a characteristic time constant.