Abstract:
An apparatus and a method for operating an optoelectronic rain sensor for detecting the wetting of a window ( 14 ) with moisture for wetting-dependent triggering of a system, in particular a windshield wiper system for motor vehicles, is proposed in which the component of the ambient beam ( 18 ) of the sensor signal ( 22 ) in a first interval is virtually eliminated and therefore need no longer be taken into account in the ensuing detection of the sensor signal ( 22 ) in a second interval relevant to triggering of the windshield wiper system. The apparatus includes first means ( 26, 40, 42 ), which in the first interval detect the sensor signal ( 22 ) and ascertain an interfering light signal ( 44 ) corresponding to the ambient beam ( 18 ), the latter signal being delivered to a regulator ( 50 ), which as a function of the interfering light signal ( 44 ) regulates compensation means ( 30 ) disposed in the current circuit of the receiver ( 16 ), and second means ( 26, 60, 62 ), which in the second interval, directly following the first, detect the sensor signal ( 22 ) and ascertain a useful light signal ( 64 ) corresponding substantially to the transmitter beam ( 12 ).

Description:
BACKGROUND OF THE INVENTION 
     The invention is based on an apparatus for operating an optical rain sensor. 
     From European Patent Disclosure EP 0 460 180 B1, an apparatus for triggering a wiper system as a function of moisture on a window is already known that has an optoelectronic rain sensor whose sensor signal, on the one hand, has one component based on the presence of moisture on the window (useful light component) and one component based on the ambient light reaching the rain sensor (interfering light component), and on the other includes only a component based on the ambient light (interfering light component). In a first interval, the sensor signal composed of the useful light component and the interfering light component is sampled by first means. Second means sample the sensor signal in a second interval, in which only the interfering light component based on the ambient light is present. The first and second means are sample and hold elements, for example. The signals sampled during the first and second intervals, respectively, are subtracted linearly from one another in a differential amplifier, so that an output signal is obtained that corresponds to the useful light component of the sensor signal. The first and second intervals are assumed to be close together in time. The length of the first interval for detecting the wetting of the window is on the order of magnitude of microseconds, while the second interval for detecting the ambient light is on the order of magnitude of milliseconds. 
     A disadvantage here is that two signals each have to be processed. This is attained at the cost of resolution of the useful light component. This will now be explained, taking as an example the evaluation of the sensor signal with a microcontroller, but it is also relevant to an analog subtractor: In rain sensors, under certain prerequisites regarding the ambient light, such as the sun going down, it is known that the interfering light component of the sensor signal can far exceed the useful light component. As an example, the interfering light component can be greater than the useful light component by a factor of 10. For an eight-bit microcontroller under such light conditions with maximum amplification of the sensor signal, only about 25 bits are available for the useful light component. In principle, the minimum detectable change in the sensor signal is one bit, and in this case therefore corresponds to a detectable relative change in the useful light signal of four percent. Vehicle manufacturers, however, require that relative signal changes of less than one percent be detected; that is, the signal dynamics of the rain sensor should be better than one percent, so that wiper operation can be attained that is attuned to the wetting of the window and that meets the wishes of the vehicle driver. 
     Another disadvantage is that the useful light component of the sensor signal can be evaluated, for the sake of triggering a windshield wiper, only at isolated times after the two intervals have elapsed. Dynamic evaluation or continuous evaluation of the useful light signal in real time during the longer first interval is accordingly impossible. 
     From German Patent Disclosure DE 42 17 390 A1, an apparatus for controlling a windshield wiper system is also known that has an optoelectronic rain sensor that decouples the ambient light component from the sensor signal by modulating the transmitter with a frequency in the range of greater than two kHz. The detected sensor signal, in the form of an alternating signal with a direct current component generated by the ambient light is separated by means of a known circuit, such as a phase-selective rectifier, into the useful light component and the interfering light component. 
     Other interfering factors of the ambient light, which arise for instance from the nonlinear characteristic curve of the radiation receiver of the rain sensor and cause a nonlinearity of the useful light signal under different ambient light conditions, are also precluded. To that end, the voltage dropping at a working resistor of the radiation receiver is detected and processed in a first operational amplifier to a correction variable, which is delivered to a second operational amplifier along with the useful light signal. The second operational amplifier eliminates the changes in the sensor signal caused by the ambient light by inputting the correction variable and outputs a useful light signal that has been linearized by the correction variable, and the useful light signal is delivered to a control stage for triggering a windshield wiper system of a motor vehicle. 
     A disadvantage here is that only the nonlinearity of the sensor signal caused by the radiation receiver is precluded. The nonlinear characteristic curve of the amplifier that is also present is not taken into account. 
     Another disadvantage is that the sensor signal is not linearized directly on being generated in the rain sensor but instead is linearized only after its further processing in a current to voltage converter, in the phase-selective rectifier, and in the amplifier, so that the sensor signal is amplified including the extraneous light component. This in turn contributes to worsening of the resolution of the useful light component of the sensor signal, as described above. 
     SUMMARY OF THE INVENTION 
     The apparatus of the invention is designed to eliminate the disadvantages of the prior art. It has the advantage that in a first interval, an optical receiver in the rain sensor detects only the ambient radiation, and an interfering light signal corresponding to this ambient radiation is ascertained, so that a regulator, as a function of the interfering light signal, regulates compensation means for compensating for the ambient light in the current circuit of the receiver, so that in a second interval, immediately following the first, in which the receiver detects the transmitter radiation and ambient radiation, a useful light signal that substantially corresponds only to the transmitter radiation is ascertained directly. 
     It is advantageous here that the influence of the ambient radiation on the triggering of a windshield wiper system is virtually eliminated in the first interval; that is, it is compensated for by the regulator in a closed-loop control circuit and therefore in the chronologically successive detection of the sensor signal (that is, still before the first amplification of this signal, such as preamplification with a fixed amplification factor), is not detected in the second interval that is relevant to triggering a system, in particular a windshield wiper system, and therefore need no longer be taken into account in an evaluation circuit for triggering this system. 
     From this, the further advantage is obtained that the input of the amplifier device cannot be overloaded by ambient radiation, since when the amplification factor is set, taking into account the maximum detectable transmitter radiation in the first interval, the ambient radiation is compensated for, and in the second interval the ambient radiation is eliminated, and the maximum transmitter radiation is barely less than what would overload the input. 
     As a result, the operating point of the input circuit, which essentially has the receiver and the first amplifier device, is independent during the second interval from the interfering light component and is therefore constant. Any change in the interfering light component would otherwise, given a nonlinear characteristic curve, lead to a change in the detected useful light component even though the useful light component was actually constant, and thus would make the detection of the useful light incorrect. This could possibly cause tripping of the wipers at the wrong time. The influence of the ambient radiation on the operating point is eliminated by the apparatus of the invention. 
     Any shift in the operating point caused by the useful light component is not critical, since changes in the useful light component caused by rain drops falling on the window, for instance, are typically greater than the changes in the useful light signal that are caused by the shift in the operating point. 
     If the operating point of the apparatus is adjusted in a further amplifier, which is disposed in the evaluation circuit, then this adjustment of the operating point again takes place independently of the interfering light component. 
     Another advantage resides in the closed-loop control circuit, which continuously compensates for the interfering light component of the sensor signal, that is, in real time during the first interval and which thus reacts directly and quickly to changes in the ambient radiation, so that wiper operation of the windshield wiper is not tripped undesirably, for instance when driving through tunnels. 
     Another advantage is that if a microcontroller is used in an evaluation assembly for evaluating the useful light signal in the second interval, there is no need to blank out the interfering light component, and a less-powerful microcontroller power can be used, which has cost advantages. 
     A further advantage is that during the second interval, the useful light signal can be evaluated continuously and isochronously, that is, in real time, in the evaluation assembly. 
     Advantageous further refinements of and improvements to the characteristics recited in the main claim are obtained by the provisions recited in the dependent claims. It is especially advantageous that the regulator regulates the interfering light signal down to a low desired value, which in particular assumes the value of 0. This minimizes the ambient light component in the second interval. 
     Two possible designs of the exemplary embodiment with respect to the amplifier device for the sensor signal and the compensation means that are disposed in the current circuit of the receiver are especially advantageous. If the compensation means are in the form of a controlled current source, which picks up the interference current generated by the ambient radiation and thus prevents the interfering current from influencing the amplifier device, then the amplifier device has a current-controlled input. 
     However, if the compensation means are in the form of a regulatable resistor, at which the interfering voltage to be measured, which is generated by the interfering current and is to be delivered to the amplifier device, drops then the amplifier device has a voltage-controlled input. If the ambient light component is increasing, that is, if there is an increasing interference current and an increasing interference voltage, then the resistance is conversely reduced proportionally. The resistor can be realized in the form of a controllable MOSFET, for example. 
     First and second means are also provided for generating an interfering light signal and a useful light signal, respectively, and are embodied as sample and hold elements, for example. This has the particular advantage that second means furnish the evaluation assembly with a continuous signal for evaluation in the first interval as well. 
     A further advantage is considered to be that the apparatus of the invention can easily be constructed on the basis of an integrated circuit (ASIC). 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     One exemplary embodiment of the invention is shown in the drawing and described in further detail in the ensuing description. 
     FIG. 1 is a schematic block circuit diagram of the apparatus of the invention in accordance with the exemplary embodiment. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 shows a block circuit diagram of the apparatus of the invention. An optoelectronic rain sensor is shown, whose transmitter  10  feeds transmitter radiation  12  into a window  14 . This window  14  is for example a front window of a vehicle that is wiped by a windshield wiper, not shown, and in whose wiping field the rain sensor is disposed. The transmitter radiation, or beam,  12  is directed in the window  14  to a region in which radiation  20  is coupled out and carried to a receiver  16  of the rain sensor. Along with the component of the transmitter radiation  12 , the out-coupled radiation, or beam,  20  includes a component of an ambient beam  18 , which originates for instance in sunlight or other light sources outside or inside the motor vehicle. 
     The receiver  16  is by way of example a luminous diode (LRD or LED), which generates a receiver current  24  as a function of the detected radiation  20 . 
     The receiver  16  is connected on the one hand to ground and on the other, via compensation means  30 , to the battery voltage Ub or on-board electrical system voltage of the motor vehicle. Thus the compensation means  30  are disposed in the current circuit of the receiver and for instance have a controlled current source or a regulatable resistor, in particular a controllable MOSFET. 
     A sensor signal  22  of the rain sensor is picked up between the receiver  16  and the compensation means  30 . This sensor signal  22  can be either a voltage or a current signal, which is carried to an amplifier device  26  with a voltage- or current-controlled input. 
     The amplifier device  26  includes at least one preamplifier, which amplifies the sensor signal  22  by a constant factor. The sensor signal  22 . 1 ,  22 . 2  amplified in this amplifier device  26  is delivered in alternation to a closed-loop control circuit for regulating the compensation means  30  or to an evaluation circuit for evaluating the sensor signal  22 . 2  with regard to wiper operation. 
     The closed-loop control circuit has a first switch  40 . If the first switch  40  is closed, then the sensor signal  22 . 1  charges a first capacitor  42 . The voltage picked up at the capacitor corresponds to an interfering light signal  44  and is carried on as an actual value or controlled variable of the closed-loop control circuit to a comparator  46 . Via a second input, a desired value  48  is specified to the comparator  46  as a guide variable. The differential signal  52  ascertained in the comparator  46  is delivered as a standard deviation to a regulator  50 , which as a function of the differential signal  52  outputs a controlling variable  54  to the compensation means  30 . 
     Analogously to the closed-loop control circuit, the evaluation circuit has a second switch  60  and a second capacitor  62 . The useful light signal  64  ascertained in them is delivered via a further amplifier  66  to an evaluation assembly  70 , which in turn evaluates the useful light signal  64  with a view to triggering a wiper motor  80 . 
     Below, first and second means are introduced that encompass among others the function of sample and hold elements and that essentially include the first switch  40  and the first capacitor  42 , and the second switch  60  and the second capacitor  62 , respectively, and preferably the amplifier device  26  for each as well. 
     The evaluation assembly  70  is typically embodied by a microcontroller, but it can also include an analog circuit. 
     The mode of operation of the apparatus of the invention shown in FIG. 1 will now be described in further detail. The sensor signal  22  now is a voltage signal which is applied to a voltage-controlled input of the amplifier device  26 , and the compensation means  30  are embodied by a controllable MOSFET. 
     By means of the transmitter beam  12  of the rain sensor, the wetting of the window  14  with moisture or water is detected by the component that reaches the receiver  16 . This component of the full radiation  20  detected by the receiver  16  is therefore called the useful light component. However, because of the functional principle of the optoelectronic rain sensor, ambient radiation  18  can also get from outside into the window  14  and from there can reach the receiver  16 . This ambient radiation, or beam,  18  is superimposed on the transmitter beam  12  and therefore interferes with the detection of the wetting of the window  14 , so that the component of the radiation  20  that originates in the ambient beam  18  will hereinafter be designated the interfering light component. 
     In the input circuit of the circuit, that is, the receiver  16  and the amplifier device  26 , it is possible with the apparatus of the invention to detect the useful light component without the interfering light component of the radiation  20 . To that end, the transmitter  10  is operated in clocked fashion, so that it is off in a first interval and is on in a second interval. Synchronously to this, the switches  40 ,  60  are also switched. The length of each interval ranges on the order of magnitude of milliseconds and is for instance 5 milliseconds for the first interval and one millisecond for the second interval. 
     During the first interval, the following now happens: The radiation  20  detected by the receiver  16  now includes only one component, originating in the ambient beam  18 . As a function of the intensity of the radiation  20 , the receiver  16  generates a current  24 . The receiver current  24  causes a voltage drop at the resistor of the MOSFET  30 , and this drop is amplified as a sensor signal  22  in the amplifier device  26 . 
     During the first interval, the first switch  40  is closed and the second switch  60  is opened, so that the amplified sensor signal  22 . 1  charges the first capacitor  42 . The first means  40 ,  42  consequently generate an interfering light signal  44 , which reproduces only the ambient beam  18  detected by the receiver  16 . Theoretically, the desired value  48  specified in the comparator  46  is assigned a value of zero. However, since this is difficult to achieve in actuality, a desired value  48  of virtually 0 is specified. The interfering light signal  44  is typically greater than 0. In a known way, the comparator  46  of the closed-loop control circuit forms the difference between the desired value  48  and the interfering light signal  44 , so that the regulator  50  regulates the resistance of the MOSFET  30  during the interval in such a way, in particular reducing it, that the sensor signal  22  decreases, and in the process the capacitor  42  is discharged via the sensor signal  22 . 1 , until consequently the interfering light signal  44  becomes virtually 0. 
     The resistance of the MOSFET  30 , which is regulated in the first interval, is now maintained in the second interval. 
     During the second interval, the first switch  40  is opened and the second switch  60  is closed. Thus the closed-loop control circuit is switched off, and no further regulation of the compensation means  30  takes place during the second interval. Since during the second interval now, in addition to the ambient beam  18 , the component of the transmitter beam  12  that varies with the moisture on the window is also detected by the receiver  16 , but the compensation means  30  compensate for the ambient beam  18  on the sensor signal  22 , the sensor signal  22  includes only the useful light component, originating from transmitter beam  12 , in the radiation  20 . Analogously to the first means  40 ,  42  of the closed-loop control circuit, the sensor signal  22  is processed in the evaluation circuit by second means  60 ,  62  to form a useful light signal  64 . The useful light signal  64  is then amplified and the further amplifier  66 , in which an operating point can for instance can also be adjusted, and is evaluated in the evaluation assembly  70 , for instance being compared with turn-on and turn-off thresholds stored in memory there, so that if one of these thresholds is exceed or undershot, wiper operation, such as continuous wiper operation, is turned on or off. 
     As a result, the first and second intervals cannot be transposed chronologically, because the transmitter  10  is first turned on and then after that is turned off. 
     In a preferred version, the ON times of the transmitter  10  are short, being on the order of magnitude of one millisecond, for instance. In this case, during the short second interval, the second capacitor  62  is charged by the sample and hold element in the form of the second means  60 ,  62  via the sensor signals  22 . 2 , and the useful light signal  64  is ascertained, which is then buffer-stored in the subsequently opened second switch  60  in the capacitor  62 , and is processed in the evaluation assembly  70 . The next time the second switch  60  closes, the capacitor  62  can be charged or discharged via the sensor signal  22 . 2 . 
     This has the advantage that the transmitter  10 , which is typically embodied as a light-emitting diode (LED), is supplied with current only briefly, and therefore only a relatively slight power loss drops at the transmitter  10 . 
     If the second interval lasts longer, then continuous values of the useful light signal  64  can be ascertained at the second capacitor  62  and evaluated by the evaluation assembly  70 , since the second capacitor  62 , with the second switch  60  closed, is charged and discharged again via the sensor signal  22 . 2  during the second interval. A dynamic evaluation can consequently be achieved in this way. 
     The actual interval lengths that are realized, however, depend on the demands made of the apparatus according to the invention in an individual case. 
     As mentioned at the outset, the compensation means  30 , as an alternative to the version described above, include a current source, controlled by the regulator  50 , which picks up the current  24  generated by the ambient beam  18  and thus compensates for it. In this case, the sensor signal  22  is a current signal, which is delivered to a current controlled input of the amplifier device  26 . This does not change anything in the above-described mode of operation of the apparatus of the invention. 
     In an equivalent version of the exemplary embodiment, the receiver  16  and the compensation means  30  are disposed in reverse order in the receiver current circuit, so that the compensation means  30  are polarized not toward battery voltage Ub, or the positive pole, but rather toward ground. A regulatable resistor or a controllable current sink can for instance be used as the compensation means  30 .