The first windshield wipers were operated manually by moving a lever inside the car back and forth. Today, most of us take our electric windshield wipers for granted. The wipers faithfully keep the window clear, moving back and forth across the windshield countless times as they sweep the water away. On their highest speed, they move impressively fast, sometimes shaking the car from side to side. What kind of a mechanism can move the wiper arms so effectively and so reliably?
Windshield wipers are found on car windshields, some car headlights, airplanes and even on the space shuttle. In this article, we'll take a look inside windshield wipers, learn about the blades and the controls and then explore a new rain-sensing wiper control system!
Inside the Wipers
The wipers combine two mechanical technologies to perform their task:
Motor and Gear Reduction
It takes a lot of force to accelerate the wiper blades back and forth across the windshield so quickly. In order to generate this type of force, a worm gear is used on the output of a small electric motor.
The worm gear reduction can multiply the torque of the motor by about 50 times, while slowing the output speed of the electric motor by 50 times as well. The output of the gear reduction operates a linkage that moves the wipers back and forth.
Inside the motor/gear assembly is an electronic circuit that senses when the wipers are in their down position. The circuit maintains power to the wipers until they are parked at the bottom of the windshield, then cuts the power to the motor. This circuit also parks the wipers between wipes when they are on their intermittent setting.
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A short cam is attached to the output shaft of the gear reduction. This cam spins around as the wiper motor turns. The cam is connected to a long rod; as the cam spins, it moves the rod back and forth. The long rod is connected to a short rod that actuates the wiper blade on the driver's side. Another long rod transmits the force from the driver-side to the passenger-side wiper blade.
Now let's talk about some details of the wiper blades.
Wiper blades are like squeegees. The arms of the wiper drag a thin rubber strip across the windshield to clear away the water.
When the blade is new, the rubber is clean and has no nicks or cracks. It wipes the water away without leaving streaks. When the wiper blades age, nicks or cracks form, road grime builds up on the edge and it doesn't make as tight a seal against the window, so it leaves streaks. Sometimes you can get a little extra life out of your wiper blade by wiping the edge with a cloth soaked in window cleaner until no more dirt comes off the blade.
Another key to streak-free operation is even pressure over the length of the rubber blades. Wiper blades are designed to attach in a single point in the middle, but a series of arms branch out from the middle like a tree, so the blade is actually connected in six to eight places. If ice or snow forms on these arms, it can make the distribution of pressure uneven, causing streaks under part of the blade. Some wiper manufacturers make a special winter blade with a rubber boot covering the arm assembly to keep snow and ice out.
Most cars have pretty much the same wiper design: Two blades move together to clean the windshield. One of the blades pivots from a point close to the driver's side of the car, and the other blade pivots from near the middle of the windshield. This is the Tandem System in the figure below. This design clears most of the windshield that is in the driver's field of view.
There are a couple of other designs on some cars. Mercedes uses a single wiper arm that extends and retracts as it sweeps across the window -- Single Arm (Controlled) in the figure above. This design also provides good coverage, but is more complicated than the standard dual-wiper systems. Some cars use wiper blades that are mounted on opposite sides of the windshield and move in the opposite direction, and some vehicles have a single wiper mounted in the middle. These systems don't provide as much coverage for the driver as the standard two-blade system.
Most wipers have a low and a high speed, as well as an intermittent setting. When the wipers are on low and high speed, the motor runs continuously. But in the intermittent setting, the wipers stop momentarily between each wipe. There are many different kinds of switches for wipers. Some cars have just one intermittent speed, others have 10 discrete settings and still others have a sliding scale that can be set for almost any time interval.
Whichever kind of controls your car has, setting them just right can be tricky -- too fast and the windshield gets dry and the wipers squeak; too slow and your visibility is blocked by raindrops. Compounding this is the fact that the amount of water hitting the windshield changes as your car speeds up and slows down. It can require almost constant attention to keep the wipers operating properly. Carmakers may finally have conquered this problem with the holy grail of wiper technology -- the rain-sensing wiper.
In the past, automakers have tried to either eliminate the wipers or to control their speed automatically. Some of the schemes involved detecting the vibrations caused by individual raindrops hitting the windshield, applying special coatings that did not allow drops to form, or even ultrasonically vibrating the windshield to break up the droplets so they don't need to be wiped at all. But these systems were plagued by problems and either never made it to production or were quickly axed because they annoyed more drivers than they pleased.
However, a new type of wiper system is starting to appear on cars that actually does a good job of detecting the amount of water on the windshield and controlling the wipers. One such system is made by TRW Inc., here is a PDF describing their rain sensor system. TRW Inc. uses optical sensors to detect the moisture. The sensor is mounted in contact with the inside of the windshield, near the rearview mirror.
The sensor projects infrared light into the windshield at a 45-degree angle. If the glass is dry, most of this light is reflected back into the sensor by the front of the windshield. If water droplets are on the glass, they reflect the light in different directions -- the wetter the glass, the less light makes it back into the sensor.
The electronics and software in the sensor turn on the wipers when the amount of light reflected onto the sensor decreases to a preset level. The software sets the speed of the wipers based on how fast the moisture builds up between wipes. It can operate the wipers at any speed. The system adjusts the speed as often as necessary to match with the rate of moisture accumulation.
The TRW system, which is found on many General Motors cars, including all Cadillac models, can also be overridden or turned off so the car can be washed.
For more information on windshield wipers and related topics, check out the links on the next page!
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