Abstract:
A device ( 10 ) for operating a windshield wiper ( 12 ) is proposed, with a moisture sensor ( 22 ) for detecting the wetness of a window that in addition to detecting rain and droplets also detects fog and drizzle. The measured values (Us) of the moisture sensor ( 22 ) are allocated incremental values (Ink) by the control unit ( 16 ). The difference between two successive incremental values (Ink) is to be added, with a sign (+ or −), in the memory ( 28 ) to a sum (Σ) of differences formed previously in the same way, and when a threshold (S) is reached by the sum (Σ) stored in the memory ( 28 ), the control unit ( 16 ) trips a wiper operating mode.

Description:
BACKGROUND OF THE INVENTION 
     The invention is based on a device for operating a windshield wiper. 
     A device for controlling a windshield wiper system is already known (from German Patent Disclosure DE 41 41 348 A1), which automatically adapts the wiping cycle frequency to qualitatively and/or quantitatively varying states of the precipitation on the window. To that end, the signals of an optical rain sensor are evaluated by a circuit arrangement. A wiping cycle is initiated when the signals undershoot a particular switching threshold. Each time the wiper sweeps over the rain sensor, the signal increases again, up to a maximum signal value, which is then stored in memory as a reference value for the next wiping cycle. The switching threshold is defined at 95%, as a function of this reference value. 
     A substantial disadvantage of this control arrangement is that after a defined period of time has elapsed, a new wiping cycle is tripped if during this time period no new precipitation has formed to a sufficient extent, and if the signal value of the sensor device does not undershoot the switching threshold. A new reference value is thus formed, because the old one in the meantime may no longer be usable, because of the temperature drift of the sensor signal. Yet this means that the wiper will be wiping a dry or nearly dry window. 
     Another resultant disadvantage is that signals generated by fog or drizzle cannot be distinguished from the signal change caused by an increase in temperature of the sensor: Both effects cause the signal value to undershoot the switching threshold. 
     Still another disadvantage is that the reference value is written in anew for each wiping operation and used to determine the switching threshold for the next wiping cycle. Heavy soiling of the window, for instance, leads to a lower reference value and thus a lower switching threshold, so that the wiping operation is not tripped until there is a heavier total coating of both soil and moisture on the window. This heavier coating impairs the driver&#39;s vision and is a safety risk. 
     It is therefore an object of the present invention to provide a device for operating a windshield wiper, which avoids the disadvantages of the prior art. 
     In keeping with these objects and with others which will become apparent hereinafter, one feature of present invention resides, briefly stated, in a device for operating a windshield wiper, in which incremental values are allocated to the measured values by a control unit, and a mean value of at least two successive measured values is formed as the incremental value, and the respective difference between two successive incremental values is to be added with a sign + or − in the memory to a sum of differences formed previously in the same way, and finally when the threshold is reached by the sum stored in the memory, the control unit trips a wiper operating mode. 
     SUMMARY OF THE INVENTION 
     The device of the invention has the advantage that in addition to detecting rain and droplets, it also detects fog and drizzle. The moisture on the window is detected, and a wiper operating mode appropriate for the incident precipitation is tripped. 
     The allocation of the maximum measured value to a predetermined limit value when the wiper is put into operation is especially advantageous. After that, the allocation is corrected only if maximum values rise, for instance because the window has then been cleaned better. 
     The advantage of forming a mean value from the measured values resides in the quieter response of the device, because individual measured values do not lead to tripping of the wiper activity; only the measured values averaged over a short period of time, or from a predetermined number of values, does so. 
     Another advantage resides in the signal evaluation, which because of the device of the invention is independent of the signal amplification in the control unit, or its increase. 
     Along with program-controlled compensation for the temperature drift of the moisture sensor signals, measuring the temperature and taking it into account in the signal evaluation is also advantageous. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     Exemplary embodiments of the invention are shown in the drawing and described in detail below. Shown are 
     FIG. 1, a device for operating a windshield wiper, shown in the form of a block circuit diagram; 
     FIG. 2, a graph on which the exemplary measured values of a moisture sensor are plotted over time; 
     FIG. 3, a table for calculating the difference and sum Σ of the measured values; and 
     FIG. 4, a graph on which the values of the sum Σ are plotted over the measured values of the moisture sensor. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 shows a device  10  for operating a windshield wiper  12  for a window, not shown, of a motor vehicle, the device being driven by a motor  14 . A control unit  16  triggers the motor  14  via control signals  18 . The control unit  16  receives measured values Us, for instance from an optical moisture sensor  22 , which operates on the principle of total reflection at the window, or in other words the out-coupling of light rays from the window by water droplets located on the surface of the window. For evaluating the measured values Us, the control unit  16  has an incremental value transducer  24 , a differential value former  26 , a memory  28 , and an evaluation stage  30 . A control element  36  is provided for turning the control unit  16 , and thus the wiper operating mode, on and off. 
     In an expanded feature of the exemplary embodiment, a temperature sensor  32  furnishes input signals  34  to the control unit  16 . These signals are evaluated by a program controller  38  and are written into the memory  28 , in order to correct the measured values Us arriving from the sensor  22 . 
     FIG. 2 shows an example of a hypothetical course of a moisture sensor signal that initially represents a dry window and decreases again in its further course because of increasing moisture on the window. The measured values Us of the signal are plotted over the time t in incremental values Ink, in accordance with the respective amplified signal variable. The measured values Us of the moisture sensor  22  are received, with an interval time ti of 6 ms, for example, by the control unit  16  and evaluated. The greater the degree of reflection at the moisture sensor  22 , the cleaner the vehicle window is, and therefore the higher the measured value Us. The arrow T indicates the signal attenuation of the measured values Us upon an increase in temperature of the moisture sensor. 
     FIG. 3 shows a table for calculating differences from the measured values Us and a resultant sum Σ in the memory  28 , taking as the example the measured values Us from FIG.  2 . All the values entered are shown in increments Ink. The sum Σ begins with the starting value 105. 
     FIG. 4 shows the sum Σ, formed of the differences in the measured values US, starting with the starting value, as a function of the measured values Us of the moisture sensor  22 , indicated in incremental values Ink. The values of the sum Σ are restricted to an arbitrary range, which is less than the range of the measured values Us, specifically by means of a first limit value Σ1, which here corresponds to the starting value 105 Ink, and a second limit value Σ2, which here is 95 Ink. Approximately in the middle of this range, there is a threshold S that can be arbitrarily predetermined by the control unit  16  and in this case is 100 Ink. 
     The first limit value Σ1=105 Ink is allocated to the highest attained measured value Us=207 Ink, for a clean window without any moisture coating it. The course of the curve is linear, until the second limit value Σ2=95 Ink is reached. If the measured value Us drops below the second limit value Σ2, then the value of the sum Σ is kept constant by the control unit  16 . The curve shown in FIG. 4 applies to both decreasing and increasing measured values Us of the moisture sensor  22 . 
     The mode of operation of the device of the invention will now be described in conjunction with FIGS. 2,  3  and  4 . 
     The driver of a motor vehicle can turn the windshield wiper on and off by actuating the control element  36 . If it begins to rain, he will turn the wiper on, so that the control unit  16  carries a first control signal  18  for activating the wiper  12  to the wiper motor  14 , and the wiper  12  wipes for at least a first wiping cycle. During it, but no later than after the last brief sweep over the moisture sensor  22  by the wiper  12 , the control unit  16  receives measured values Us from the moisture sensor  22 , shown here for a greatly truncated (hypothetical) period of time of t=84 ms. These measured values are evaluated by the control unit  16  as follows: 
     As seen in FIG. 2, after every interval time ti, the incremental value transducer  24  allocates discrete incremental values Ink to the measured values Us. A timer can for instance be used as the incremental value transducer  24 . The measured values US or incremental values Ink are then, after prior inverse value formation and multiplication by a factor to obtain values greater than one, delivered to the differential value former  26 . There the current measured value Us or incremental value Ink is subtracted from the previous measured value Us or incremental value Ink, and the difference is written in the memory  28 , with the applicable sign (+ or −). In the memory  28 , which is preferably RAM, the differences are added together and yield a sum Σ. 
     As shown in the example in FIG. 3, when the device  10  is put into operation by the driver, the first limit value Σ1 is written into the memory  28  as a starting value (in this case, 105 Ink) for the sum Σ. 
     After the first wiping cycle of the wiper  12 , the condition of the window improves, and the measured values Us of the moisture sensor  22  rapidly increase. Because of the evaporating streaks on the window, the increase in the measured values Us takes longer than it takes to wipe the active sensor area. The positive differences formed are not added to the starting value in the memory  28 , and they therefore allocate the first limit value Σ1 to the cleanest condition of the window (in this case Us=207 Ink) that has been achieved by this wiping cycle. At the same time, the threshold S and the second limit value Σ2, as well as any other value of the sum Σ, are re-allocated to the measured values Us; that is, with the allocation of the first limit value Σ1, the entire range of the sum Σ shifts toward the maximum measured value Us—toward the left, as indicated by dots in FIG.  4 . 
     Any other value Σ1, Σ2 of the sum Σ can also be used as the starting value, here including the values between 95 Ink and 105 Ink. In that case, the positive differences formed are added to the sum Σ, until the first limit value Σ1 is reached. It must be taken into account that starting values below the threshold S cause an immediate tripping of wiper activity once the device  10  has been activated. The most logical solution is therefore to use the first limit value Σ1 as the starting value. 
     Thus when the device  10  is put into operation, the allocation is assigned once and for all. Putting it into operation means that the driver has closed the ignition contact of the motor vehicle, for instance, and thereupon actuates the control element  36  for the first time. No further allocations need be assigned during the ensuing drive, because within this short period of time, aging of the moisture sensor  22  is not apparent. However, if later on, when the device  10  is put into operation again, or at a later time in wiper operation a measured value is ascertained that is higher than the previously highest measured value Us, or in other words if the window is cleaner or drier, or if there is greater signal amplification, then that measured value is allocated to the first limit value Σ1, by the method described above. 
     With an increasing coating of moisture on the window, the measured values Us of the moisture sensor  22  and the sum Σ in the memory both decrease. If the value of the sum Σ drops below the threshold S, then the evaluation stage  30  downstream of the memory  28  detects this as rain and outputs a control signal  18  for turning the motor  14  back on. The wiper  12  cleans the window and in the process sweeps over the moisture sensor  22 . The measured values Us therefore rise again, so that the sum Σ goes above the threshold S again. If precipitation continues to fall, the measured values Us drop again, and when they fall below the threshold S, a wiper operation is performed again. Depending on the intensity of the precipitation, an intermittent wiper mode with a variable length of time between sweeps, for instance, is attained. 
     The threshold S can preferably also be made variable, so that under various weather conditions the sensitivity of the moisture sensor  22  can be regulated. Day/night detection can for instance also be used in adjusting the sensitivity. 
     In the first few minutes after the system is put into operation, the temperature of the moisture sensor  22  rises rapidly, and after that it continues to rise only slowly. When there is a constant film of moisture on the window, the temperature increase causes an initially rapid, and then slowly decreasing, attenuation of the measured values Us. In FIG. 2, the curve for the measured values Us therefore shifts in the direction of solid arrow T, and the unchanged threshold S in FIG. 4 would already have been reached earlier, or in other words with even less moisture on the window. In an extreme case, the wiper  12  would be put into operation even though there was actually no moisture on the window. 
     To compensate for this temperature effect, additional correction increments, from the program controller  38  embodied as a temperature compensation means, are added to the sum Σ in the memory  28 . Since the course of the temperature change is known per se, the program controller  38  can be programmed accordingly. The value of the sum Σ is augmented by the applicable correction increments by which the measured values US are dropping because of the temperature effect, so that when the moisture film on the window is constant, the sum Σ remains constant. For example, to compensate for the temperature drift at the beginning of the turn-on phase, one increment is added to the sum Σ every 5 minutes. A total of about 80 correction increments is added to the sum Σ to correct an about 30% change in level dictated by the temperature. If after about 5 to 10 minutes the moisture sensor  22  is in a state of thermal equilibrium, then no further correction increments of this kind are added. 
     When the ambient temperature warms up, this is taken into account by relying on experience, which tells that the maximum possible change in ambient temperature is about 20 C. per 5 minutes, and a fixed correction is made after the turn-on phase, as follows: If the value of the sum Σ is below the first limit value Σ1, for instance, then one increment is constantly added to the sum Σ every six seconds. 
     When the ambient temperature cools down, the measured values US rise (signal amplification), and the allocation of the maximum measured values US to the first limit value Σ1 is done by the method described at the outset and need not be addressed again. 
     In an alternative exemplary embodiment, the temperature of the sensor  22  and/or of the surroundings is measured by the temperature sensor  32 , so that when the operating or ambient temperature rises, the influence of the temperature on the measured values Us is compensated for by the evaluation stage  30 . 
     In an improved exemplary embodiment, mean values are first formed from the measured values Us. In forming the mean values, measured values Us are averaged over a short time (in this case 24 ms) or for a predetermined number of measured values (in this case, 4). It does not matter whether the mean values are formed directly upon detection of the first measured value Us, or only after the predetermined number of measured values or the elapsed period of time is detected. The mean values are then used instead of the measured values Us for calculating the differences, as already discussed above for the individual values. The calculated difference is added to the sum Σ in the memory  28  with the correct sign (+ or −). The evaluation of the sum Σ is done by the evaluation stage  30 . In FIGS. 3 and 4, instead of the measured values Us in Ink, the mean value in Ink is plotted. Additionally analyzing the individual measured values Us in the evaluation stage  30  makes it possible to determine any trend that may be occurring. 
     In a further exemplary embodiment, instead of the calculated differences, weighted differences are added to the sum Σ in the memory  28 . If the difference is from 1 to 5 Ink, for example, then the same value is added to the sum Σ, but for differences of 5 to 50 Ink or more, only a proportionate, lesser value of the difference is added. As a result, the moisture detection remains sensitive when there is little moisture, and becomes less sensitive when there are major changes in the amount of moisture.