01-11-2012, 05:04 PM
AUTOMATIC RAIN SENSING WIPER
AUTOMATIC RAIN.doc (Size: 884.5 KB / Downloads: 79)
wipers.doc (Size: 28.5 KB / Downloads: 41)
AUTOMATIC.pptx (Size: 385.71 KB / Downloads: 48)
INTRODUCTION
Windshield wipers, which have been standard equipment on most vehicles since 1916, were developed to improve visibility during inclement weather. For nearly 100 years, drivers operated the wipers with manual controls mounted on the dashboard. Today, technology has made possible motorized windshield wipers that automatically recognize a developing visibility problem and clear the glass before the driver can react to the problem.
The term “rain-sensing wipers” is commonly used for a system designed to clear the windshield of rain, snow, and debris without driver intervention. The system is programmed to automatically activate and alter the speed and rate of the wiper blades to keep outward visibility unobstructed at all times. A rain-sensing wiper system does not clean the windshield any better than a manual system; it simply monitors outward visibility and automatically activates the wipers.
A rain sensor for an automotive wiper control system detects rain conditions to produce a rain condition indicative signal. The rain sensor includes a vibrator, the outer surface of which is exposed to rain drops and which vibrates or oscillates at an amplitude and frequency corresponding to the energy applied by rain drops. The rain sensor is adapted to produce the rain condition indicative signals corresponding to the vibration amplitude of the vibrator. The vibrator is resiliently suspended within a sensor casing by means of a resilient member which is adapted to allow vibration of the vibrator as rain drops hit the outer surface and to absorb vibrational noises transmitted through the vehicle body. The resilent member simultaneously serves to waterproof the components within the sensor casing.
Rain-sensing Wipers
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 was 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 do a good job of detecting the amount of water on the windshield and controlling the wipers. One such system is made by TRW Inc., 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 Working Principles Of Wiper
The wiper combines two mechanical technologies to perform their task:
1) A combination electric motor and worm gear reduction provides power to the wipers.
2) A neat Iinkage converts the rotational output of the motor into the back-and forth motion of the wipers.
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, and then cuts the power to the motor. This circuit also parks the wipers between wipes when they are on their intermittent setting.
Linkage
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.
Wiper Blades
Wiper blades are like sponge. 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 cracks. It wipes the water away without leaving streaks. When the wiper blades age, cracks form, stain builds up on the edge and it doesn't make as tight as a seal against the window, so it leaves streaks.
Sometimes car drivers have to wipe the edge with a cloth soaked in window cleaner until no more dirt comes off the blade.
Another way to have a streak-free operation is to ensure the 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, so the blade is actually connected in six to eight places. In the foreign country such as in the Europe, 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.
Rain Detection Unit
The system detects rain by using two sorts of sensors. One of them is the impedance grid sensor shown in Fig.5.2. The grid is made of two comb-like copper plates separated by a minimum distance of in. The sensor is glued to the windshield glass with the help of a strong adhesive material. The thin configuration of the plates allows the wiper to slide over without peeling them off. When the plates are dry, the resistance between the two plates is very high, but when water is between the plates, current can flow between the plates, thus decreasing the resistance. This operation allows this design to be used as a rain sensor. The sensor becomes operational when one plate is connected to a power source, and the other plate is taken as the sensor output.
Optical Sensors
The optical sensors are used to bounce beams of light through the windshield, and look for disturbances in the beams caused by raindrops at the outside surface of the windshield. The rain sensor has an emitter that emits pulses of light, coupled into the windshield with a lens. These beams travel through the windshield at about 45 degrees. Through research it was anticipated that the infra-red beams were to be totally reflected by the outside surface of the windshield into the receiver. However, when testing the analog IR sensor supplied by Optek Inc., it was determined that the infra-red beams were not totally reflected by the windshield, but that the infra-red beams were approximately 30% reflected by the outside surface of the windshield. Troubleshooting this issue included using different types of glass to reflect the IR beams and comparing that output with the results of reflecting the IR beams off a white sheet of paper. In conclusion, it was determined that the light beams from the IR sensors were not totally reflected by any type of glass and therefore the design approach was modified. Although the glass did not reflect 100 percent of the light emitted, there was enough light reflected by the glass to detect the change in reflectivity due to a raindrop. The downfall is that the rain threshold for the sensor was lowered and it was not as easy to determine when moisture was present. If rain drops are present on the outside surface of the windshield, some of the beams escape and this reduces the intensity of the beams. The detector will measure this reduction in intensity and communicate that to the rest of the system that actuates the windshield wipers.Fig.5.2b.shows a diagram of the operation.