Automatic Wiper Control IC from Melexis Sets New Reference Standard

July 12, 2011 - Tessenderlo, Belgium. Melexis: Intelligent controls in automobile convenience and safety systems continue to enhance the driving experience. Automatic windshield wipers bring increased driver convenience and enhanced vision quality by reacting autonomously when mist, fog, rain or road spray begin to obstruct light transmission through the front windshield. Early generation reference designs rely on discrete sensing schemes or ASIC based sensor modules. Melexis new MLX75308 RainLight Sensor Control IC brings much needed improvement in performance, flexibility and complexity.

The MLX75308 controls all the major components found in a rain and light sensor module typically applied behind the front windshield of an automobile. The sensor system consists of Near Infrared LEDs, Photodiodes, an MLX75308, a microcontroller and, in many instances, a Network Bus interface IC. The significant challenge addressed by the MLX75308 is the ability to provide sensitive detection of raindrops or mist in the presence of a highly dynamic light load from the sun. Previous solutions to this challenge involved expensive optical waveguides or elaborate algorithm which reduced responsivity and increased occurrence of false wipes. The MLX75308 establishes a new standard for sensitivity and sunlight rejection by separating and suppressing the sun signal from the rain signal being measured by the photodiodes.

Automotive OEMs often prefer to optimize automatic wiper systems to appeal to their typical consumer’s preferences and their vehicles market segment positioning. Some manufacturers prefer a more relaxed rate of wiping, prioritizing that over a perfectly clear windshield. Others will activate the wipers at the slightest suggestion of rain. Running the headlights whenever the wipers activate is desired by some OEMs while others will consider the ambient light level input to determine when or if to automatically switch on the headlamps. By presenting the signal data to the ECU on-demand and in a digital form via SPI the MLX75308 enables the system integrator or OEM to determine how the sensor data will affect system level behavior.

The need for customization of performance and system features also favors the Melexis IC. Systems need to be flexible enough to cope with a wide variety of windshield angles (from 40° to 90°) as well as different tinting and shading. Sensor implementations may demand a larger area of sensing coverage, more LED’s and photodiodes to improve rain detection thresholds or less and fewer to optimize cost and size. Systems also need to work over a wide current range from picoamps to milliamps as the difference between light levels on a bright, sunny day are vastly different than when driving at night. The MLX75308 can address all these options with its programmable analog signal control chain and 16 bit ADC and digital signal processing on chip. This combination allows the device to address a very high dynamic range on both input and output. The IC enables the module designer to make a standard module with a flexible bill of materials that can be adapted easily for a range of platforms.

Rain and sun detection requires a sensor to be located in a position that mimics the driver’s real time view on the road. Minimizing sensor size is critical to avoid obstructing driver view of the road. The MLX75308 is configured in a 4x4 QFN package and the integration levels and signal processing on-board result in a low BOM for the sensor. This location presents unlimited opportunities for a sensor module to be able to share the automatic wiper control data. The data can as well serve for headlight control, sunlight load sensing in HVAC controls, mist and condensation controls for automatic climate controls. The flexibility of the MLX75308 can allow for creative applications circuits to bring about highly versatile sensor modules.

The mounting location of rain light sensor modules lends itself to further expansion in functionality. With existing Bus connection to the chassis controls the rain light module can easily become a sensor cluster feeding many types of information to several different systems. Far Infrared non-contact temperature sensors could be included to provide inside and outside temperature measurement.

When it comes to Rain Light sensing and automatic wiper systems controls, Melexis has raised the performance bar and now establishes the reference design standard. Samples of the Melexis MLX75308 are currently available.