November 2006 Articles & Features

Advance Auto


adjust text size: Eye On Electronics

By Mike Dale | November 2006

There's a long list of vehicle systems that do things for us that we're capable of doing on our own. Rain-detecting wiper systems would seem to fit the description, but there's more to the story.

It just doesn't seem like it should be that hard to do. Rain falling onto a windshield is detected by a sensor that sends commands to the wipers, the electric windows and the sunroof motors. The car's interior is protected and the windshield is cleaned to maintain visibility. No actions are required from the driver. How hard could this be?

The truth is that since GM first displayed such a system on a concept car called the LeSabre in 1951, there have been hundreds of patents and thousands of attempts to do a really effective job of detecting rain and correctly responding to it. Most of them have had issues that prevented wide-scale adoption of the technology.

What makes this difficult is the many forms that rain can take. Rain can vary in intensity from a very light mist to a heavy downpour, from fully liquid to all the 212 forms of frozen and semifrozen ice, slush, sleet and snow the Eskimos talk about. Rain can have nearly the purity of distilled water or it can be contaminated with industrial pollution, mud or even pollen. The rain may not, in fact, be rain at all, but rather the splash from a passing truck, slush from a winter pothole or a squirt from a windshield washer system.

Currently there are two primary sensor technologies in common useï¿½ optical and capacitive. The sensor output alone is not the whole story. For these sensors to work, their output has to be both signal- and software-processed. The result is that a great deal of information can be determined about the exact nature of the rainfall, which can then be turned into control actions that can and do go way beyond just turning the wipers on and off.

The best way to understand how these systems work is to look at what information is really needed, then at what sort of reactions might really be wanted. To automatically control the wiper function, it's necessary to know not only that it's raining, but how heavy it's raining and then whether the rain is increasing or decreasing. The software needs to be able to determine from the sensor information that it really is rain or snowï¿½that is, a liquidï¿½that needs to be removed from the windshield (the system shouldn't be triggered by small bugs, for example). There needs to be a way to correctly respond to water that really isn't â€œrain.â€

Many concepts for rainwater detection didn't meet even these basic requirements. The conductivity type of sensor used a couple of metal strips printed on the outside of the windshield. It turned out that these were subject to oxidation. Additionally, the sensor was easily misled by bug splats, contaminated rainwater and just plain old dirt. Abrasion from the windshield wipers kept the effective life of this type of sensor short.

Another often-tried concept concerned sensors that detected the vibration in the glass caused by the impact of raindrops. Piezoelectric and sonic detectors detect rain by â€œlisteningâ€ to the drops hitting the windshield. Hard-shelled bugs, sprays of gravel, sleet and hail are just a few causes for the false triggers that plagued these systems. Work continues to be done on both resistive and vibration systems.

Let's look at the two most successful current technology sensorsï¿½the optical and the capacitive type. The optical sensor works on the principle of refraction. The sensor uses a light-emitting diode to send a cone of light through the windshield glass. When the windshield is dry, almost the entire amount of light is reflected onto a light sensor. As the raindrops begin, a portion of the light is deflected, causing a change in the amount of light received by the sensor. The larger the area available to sense the reflected light, the greater the detail of information that can be seen. It's possible to literally count the raindrops that fall onto the sensed area.

The software comes into play because it can do extensive, real-time analysis of the sensor output. One Denso patent says â€œthe condensation on the windshield is recognized as a constant increase of water because of the even dispersion of small waterdrops in the atmosphere.â€ Another Denso comment is that â€œeach drop of the rain changes its shape from a flat form just after landing to a spherical form because the surface tension minimizes the surface area of the waterdrop.â€ As complicated as all that sounds, what it means is that the sensor can not only count the number of raindrops, it also can monitor their shape and flow pattern as they dribble down the windshield. Using this information, it's possible to determine that the water is in fact rain, in addition to how hard it's raining and whether or not the intensity of the rain is increasing or decreasing.

The capacitive sensor depends on the dramatically different dielectric constants of water (E=80) and glass (E=2). The standard practice is to form a capacitor by building fingerlike plates of metallic material into the layer of plastic between the inner and outer glass of the windshield. One set of fingers interlaces but does not touch the other fingers. When the windshield is dry, the outer layer of glass forms a dielectric between the surfaces of each set of fingers. As the windshield gets wet, the dielectric constant changes as a function of the amount of water that contacts the windshield. It would be advantageous to have the sensor on or just below the surface of the glass, as that would give it the greatest sensitivity. Unfortunately, capacitive metallizations on the surface of the glass have much the same problem as the resistive sensors: The wipers wipe them away pretty quickly.

The capacitance of the water-sensing capacitor is typically monitored by connecting it to a high-frequency balanced bridge detector like a Wheatstone bridge. It's also possible to directly measure the capacitance change. Hella has developed a capacitive version where an oscillatory circuit containing the aerial structure is detuned when a raindrop changes the capacitance.

One advantage of the capacitive system is that the glass layers are already there. The glass serves not only as the base dielectric layer, but also to mount and locate both the sensor and the connecting wires. The cost to build the sensor really involves just the printing of the metallic fingers and the connecting of the wires out to the signal-processing portion of the system. Since optics are not an issue, the capacitive sensor can be mounted in that dark area at the edge of the windshield the automakers call the â€œshade band.â€ The driver usually doesn't even know the sensor is there.

So how much does the capacitance change as a result of rain falling on it? A patent issued to ITT says that a sensor developed by them has a nominal capacitance of 100 picofarads (pf). When a uniform layer of .5mm (.020-in.) droplets covers the sensed area, the capacitance changes by 10 pf. While not large, it's enough of a change that the electronics can use it to determine both the presence of rain and the rate of the rainfall.

As mentioned, there are opportunities to do more than simply control the wipers. The capacitive sensor developed by Hella has elements integrated into it that can also measure light, humidity and solar load. This allows the cost of one sensor to be spread out over what would otherwise be three separate sensors, their wiring and the space they would take up.

There are other technological methods of rain detection on the horizon, such as very short wavelength radar and improved ultrasonic/vibration systems. Rain-sensing technology could also be adapted for an effective black ice detection system that could improve winter driving safety.

It's not only the wipers that may need to respond to an oncoming rainstorm. There has been much discussion of the use of cruise contr