Capacitive Rain Sensor For Automatic Wiper Control
Design Issues
Danny Kang
Eric Otte
Arslan Qaiser
Ishaan Sandhu
Anuar Tazabekov
Dr. John R. Deller - Facillitator
April 13th, 2010
Introduction
Design Team 6 faces the challenge of developing a capacitive rain sensor for automatic wiper control in vehicles. During the design of the capacitive sensor, many design issues have been taken into consideration. Most prominent/significant among these design issues are: Accuracy, Standards, and Product Lifecycle Management.
Accuracy
Perhaps the most critical design issue during the development phase of this project has been accuracy. Without exceptional accuracy in detecting rain, and differentiating between rain and other objects, the sensor is useless to the sponsor company, HATCI. The primary goal was to develop a capacitive rain sensor for automatic wiper control in vehicles to replace current optical sensors used for that role. The optical sensors have been refined over the past two decades to the point where they exhibit fairly good accuracy in detecting rain. Unfortunately, the optical sensor design is quite complicated as it requires many components and relies on the emission and reflection of an infrared beam for operation. This complexity drives the price of each optical unit to $16 - $20, which HATCI would like to reduce to be able to make it standard-issue in many of its vehicles. In addition, the optical sensor has a very narrow sensing area on the windshield, and is prone to errors from dirt and dust on the windshield.
The proposed capacitive rain sensor needs to not only be cheaper than the current optical unit, but it needs to improve the accuracy and reliability of the rain detection to the point where it never needs to be turned off or become a source of frustration for the driver. The switch to capacitive sensing from optical detection works to solve many of these issues directly. Unlike optical sensors, capacitive sensors are relatively immune to dirt and dust on the windshield, as these particles will not affect the electric field strength (and likewise the capacitance) significantly. The sensing area is also drastically improved, growing from the width of an IR beam in the optical design to a rectangular size of 20 mm tall by 50 mm wide in the prototype capacitive sensor. On the average, this increase in sensing area will result in much quicker detection of sparse rain.
The core of Design Team 6’s capacitive rain sensor is the Analog Devices AD7745 capacitance-to-digital converter. This IC comes in a small, 16-lead surface mount package, but provides the highest accuracy capacitance-to-digital conversion in its class. It outputs a capacitance range of -4.096 pF to 4.096 pF as a 24-bit digital signal, providing resolution down to 4 aF and accuracy of +-4 fF. This digital capacitive data is relayed to a PIC microprocessor using an I2C two-wire communication interface, and the PIC then compares the incoming data to known signatures of rain, found through testing. The PIC can then determine not only the presence of rain, but the approximate amount of it as well. If the incoming data is too high or too low, the PIC will not recognize it as rain. Additionally, the incoming data must stay within the signature range for at least 3 cycles before any wiper activation to prevent false positives.
Using the software COMMSOL, Design Team 6 refined a sensor trace layout to best detect rain through the approximately 6 mm of windshield glass. The latest results indicate a response of approximately 40 fF change in capacitance per raindrop, a response much larger than the 4 fF accuracy of the AD7745 and large enough to be immune from noise. Because dielectric strengths change with temperature, the capacitive rain sensor will auto-calibrate to the temperature of the vehicle upon startup. It will then re-calibrate every three minutes to account for changes in temperature to maintain accuracy. To protect against false positives caused by user or electrical noise interference on the interior of the vehicle, a ground shield is placed directly behind and around the sensing area, between the sensor layout and the IC’s. A plastic overlay will then cover the exposed circuitry to maintain a sleek profile.
Standards
All electronic vehicle components must comply with international and federal standards with regards to safety, quality, and electromagnetic compatibility (EMC). During the development of the prototype capacitive rain sensor, Design Team 6 did not design to comply with these standards, and these would need to be accounted for as the production level design moves forward.
In the United States, the Society of Automotive Engineers (SAE) is the standards-writing committee in the automotive product field for EMC. The SAE J1113 is the official standards for EMC emissions and immunity testing of vehicle components in US automotive applications, other than the “Big Three” of Ford, Chrysler, and General Motors. As HATCI is concerned mostly with the American automotive market, the SAE J1113 would be the most accepted and reliable standards to follow in a production design.
SAE J1113 involves over 40 tests to determine the electronic vehicle component’s conducted immunity, transient immunity, electrostatic discharge immunity, radiated immunity, AC power frequency immunity, and radiated and conducted emissions. These tests can be performed at an approved test facility, either in-house at HATCI or through a third party.
The design of the capacitive rain sensor is such that Design Team 6 foresees no problems with any immunity or emissions tests according to SAE J1113. This is because the sensor only utilizes pre-existing IC’s such as the AD7745 and PIC16F1826, which have already passed standards testing. For example, the AD7745 features simultaneous 50 Hz and 60 Hz rejection, increasing its immunity to AC power line noise. No new circuitry has been designed with the exception of the addition of interconnects, passive components, and the sensor trace area adhered to the windshield.
Electromagnetic compatibility testing should be performed on the PCB layout to ensure that the traces are routed in a manner such as to not effectively pick up stray RF signals, and not to emit them as well. Long wires running from the windshield-mounted device to the body control module should be properly shielded. The ground shield mounted directly above the sensor trace area will need to be tested for effectiveness, and modified if necessary. In the prototype unit, no grounded shield exists around the top portion of the PCB containing the circuitry. This could be problematic, and a grounded shield may need to be placed in the plastic covering of the entire system as well.
Product Lifecycle Management
The product’s lifecycle was a key consideration in the development of the capacitive rain sensor. The design and consumption aspects were the most important in determining product specifications during prototype development and the production, distribution and end-of-life aspects will become key considerations in further development.
The system uses a variety of different components, each carefully chosen in the design phase to ensure a cost effective and accurate design as well as a system which uses very little power. The AD7745 Capacitance-to-Digital converter was chosen due its high accuracy, low power consumption, small form factor, and minimal cost. The ADP3301 Voltage Regulator was chosen due to its low cost, small form factor, and accurate 5 Volt output which is required for both the microcontroller and the Capacitance-to-Digital converter. The microcontroller PIC16F1826 was chosen to due its I2C compatible interface, its small form factor, and its low cost.
Various production aspects were also taken into account during the development of the system. The goal of the project was to create a higher accuracy, lower cost alternative to current optical rain sensors. Each component, as mentioned above, was chosen carefully to minimize cost as well as energy consumption. The system only contains 3 Integrated-Circuits, 1 Sensor Module and a few resistors and capacitors. This keeps the product very simple minimizing production time, and minimizing the materials used.
When considering the distribution aspects of the product lifecycle the transportation cost, transportation time, inventory, and sales network need to be considered. Since the design has a very small form factor, the transportation cost will be minimized. The design uses various off-the-shelf components readily available so there should be plenty of available inventories. The sales network will be a combination of Hyundai Kia Motors and its affiliates and transportation time will be determined by the affiliated transportation company.
The consumption of the product will involve a few key stages. The first and most important stage would be training the installers, and the maintenance workers about the product. The installation will be simple and involve connecting one wire to the rest of the automobiles body control module as well as clipping the sensor housing to the windshield. The repair stage will involve removing the clipped on sensor and the wire connection to the body control module and replacing it with another module. Over the lifetime of the product, various upgrades will be made including the design of a new capacitive sensor and the use of various other microcontrollers all attributing to a lower energy use down the line.
Conclusion
The issues of accuracy, standard compliance, and product lifecycle management were and will be key factors in a production-level design of the capacitive rain sensor developed by Design Team 6. While the accuracy and cost of the sensor system have been top design issues from the start of the prototype phase, much work remains to be done in complying with all electronic component vehicle standards, and improving product lifecycle management concerns such as ease of manufacture, integration into the vehicle system, and maintenance/repair.
References
http://www.analog.com/static/imported-files/data_sheets/AD7745_7746.pdf
http://www.dtbtest.com/EMI-EMC-testing.aspx
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