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March/April 2002

Rain Sensors: 
A Thing of the Past ... or Future?


I imagine that the first vehicle windshield wiper was powered by the same forces that drove the car. Literally. Some driver clutching his kerchief stretched his arm out from behind the steering wheel and wiped the mist and road gunk off of his windshield. After all, glass windshields were used on vehicles for the first time in 1903. It wasn’t until 1916 that the mechanical windshield wipers started to appear on select cars and it was a full ten years later before wipers became standard automotive original equipment. 

Rain Sensors Emerge
Both rain-sensor and intermittent wipers are significant milestones that incorporate the windshield wiper as part of an overall design system. These systems were developed with the end user in mind and were not last-minute considerations. In fact, the rain-sensor system took years of development efforts by many talented individuals from several major companies including Pilkington, General Motors, DuPont and the 3M Company. Then it was several more years before it actually went into production. 

The rain sensor is really on the verge of being one of those “Great idea, but who really needs it?” items. Think about it. Research and development is full of individuals finding cures for which there are no diseases. And then they leave it up to the marketing department to find a fit and function for these “cures” in real life applications. I’m speaking from experience about my first encounter with a prototype rain-sensor system on display in the Futures Exhibit area in the bowels of Libbey-Owens Ford Tech Center a dozen years ago. I followed the demo instructions and used a squirt bottle to spray water onto a windshield mounted in the front part of a chopped-up vehicle. My companions and I all thought aloud, “What kind of driver would be so inclined to want an automatic wiper system?” Years later when the rain sensor was introduced on the 1996 Cadillac, I had my answer.

Are Rain Sensors Necessary? 
In reality, the rain sensor is partly a safety product and in the automotive industry, safety sells. I must admit that there are occasions when passing semi-trucks on the road in the right weather conditions that a rain sensor would be a benefit. 

Traditionally the rain sensor was available only on the high-priced luxury vehicles, but the popularity is growing and it is becoming more mainstream and is appearing on several average-priced cars. The list of manufacturers that are using a rain sensor on 2002 models includes BMW, Ford, General Motors, Jaguar, Lexus, Mercedes Benz, Peugeot and Volkswagen. Each is designed on the same basic principles.

How They Work
The majority of the rain sensors are mounted on the inside surface of glass near the rear view mirror and are attached directly to the glass with an adhesive or a mechanical clip-on locking mechanism to a bracket that is attached to the glass. This keeps the sensor out of the primary vision area of the driver while giving it the opportunity to be in the standard wiping path of the traditional windshield wiper.

However, the system is flexible enough that the sensor can be placed in various alternate windshield areas when used with the non-standard wiper configurations and travel paths. 

The rain sensor is actually based on an optical sensor unit that emits infrared (IR) light to the windshield. The system, when turned on, monitors for changing conditions constantly. Some of the light is reflected back to the IR detectors. The amount of light reflected back to the sensors changes depending on the outside-surface conditions of the windshield. When the unit detects moisture, less light is reflected back to the photo cells in the unit. The system detects a change in the intensity of the reflected light and that change is interpreted by a unique microprocessor contained in the control module. This activates the wipers as needed. These systems are sensitive enough to pick up mist, wet snow and rain. 

Working Around Rain Sensors
The rain sensor was designed to provide low production costs and to minimize added costs in windshield replacement because the system was not an integral part of the windshield. But auto glass manufacturers have implemented several aesthetic changes to rain-sensor versions of windshields that do not promote interchangeability with the non-rain-sensor versions. The current trend is to include a third visor pattern area on the glass around the rearview mirror. To accommodate positioning of the rain sensor on the glass in the required optimal area, a clear patch needs to be incorporated in the third visor. Right now, there are more than 60 basic replacement part numbers for either rain-sensor-specific or rain-sensor-optional windshields, not including all the color and hardware combinations. A familiarity with the various design styles of rain sensors available will save a lot of headaches when trying to order the right part(s).
There are a couple of basic rain-sensor designs that attach to the glass in a similar method. The bonded-on varieties are attached to the glass with an optically clear grade double-faced tape manufactured by the 3M Company. This style is typical of the rain sensors used by BMW, General Motors and Mercedes. The sensor lens constantly is attached to the glass with the tape. The replacement glass may or may not contain the replacement lens already attached to the glass. 

If it is not attached, there are several varieties of attachment kits in the marketplace that contain everything needed to remove the lens from the original windshield and reattach it to the replacement windshield. One such kit from General Motors even contains quite a few items, such as cleaning cloths and a cleaning solution that may be helpful in removing the residual adhesive from the lens. It also has a detailed four-page instruction sheet complete with pictures and diagrams. Other kits have been put together with the minimalist in mind and contain only a couple pieces of the adhesive tape and the glass primer. Yet a third option is the replacement lens available for the BMW and Mercedes designs, which has the adhesive tape applied with a peal liner for ease of use. Regardless, the key is to make sure the glass is clean and that the tape is applied without any fingerprints and air bubbles on either the glass or the lens. 

Remember how the sensor works. It must “see” through the glass to read the outside surface moisture. If it sees air bubbles, dirt or fingerprints, it will cause the wipers to malfunction. It can detect the defect as moisture that can’t be eliminated, which causes the wipers to constantly activate. If enough dirt is present, the sensor will not activate the wipers at all because the IR light reflected back to the unit remains constant and is not affected by the outside glass surface conditions. 

Also note that super glue does not work. Most super-glue-type adhesives are not meant for long-term weathering exposure. The glue might work initially, but eventually the adhesive will degrade and cause the sensor to malfunction since it can not see through the adhesive.

Sensor Attachments
Ford, Jaguar, Lexus and Volkswagen all have the secondary type of rain sensors that attach to the glass in some sort of mechanical fashion. In these sensor designs, the service and windshield replacement does not involve any adhesive tape to bond on the sensor lens. The sensor attaches to a bracket that is already bonded to the glass. The brackets need to be designed precisely and positioned so the rain sensor lens can make contact with the glass, so it is common that these replacement windshields have the bracket already attached. The Ford and Jaguar design reminds me of a hammock swing because there is a pair of brackets and the sensor lens nests in between them. A mechanical slide clips the lens to the brackets. The Lexus rain sensor actually slides into and snaps on to the bracket that is bonded to the glass. Volkswagen has a rain sensor that is integral to the mirror bracket. The rain sensor is incorporated into the rearview mirror stem and, when correctly in place, the lens sees out though the middle of the rear view mirror bracket. 

Oddly enough, I have not seen any rain sensor designs called out on SUV, van or wagon backlites. Perhaps it’s a matter of more time. Certainly these vehicles can be equipped with all the high-tech gadgetry and safety features customers have been demanding. Wouldn’t it make sense that a rain sensor in combination with the back window wiper would keep the back window clean for when you need it? 

Unlike the windshield, which the driver knows immediately when it needs to be wiped, the backlite becomes a secondary, less-viewed through portal to the road behind. This is one reason why it is not watched continuously. For the driver to reap the benefits of the rain sensor, mounting it on the backlite would keep the window clean so that it could be clearly seen through as quickly as it was necessary. 

Its taken more than ten years since the rain sensor was first introduced to come this far. Perhaps in a few more years time it will not only migrate to other vehicle makes and models, but it will also move to the rear of the vehicle. This way the rain sensor would not only benefit the drivers of the world from seeing the road clearly ahead of them, but also to clearly see what is catching up from behind. 

Monica Matthews works in the auto glass replacement division of Pilkington North America in Toledo, Ohio, as a technical services specialist.



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