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ECE 480 – Team 6 Capacitive Rain Sensor for Automatic Wiper Control In Order of Presentation: Ishaan Sandhu DannY Kang Arslan Qaiser Eric Otte Anuar Tazabekov Agenda • Introduction • Background • Design Specifications • Conceptual Design • Proposed Design Solution • Hardware Specifications • Reflection Introduction GPS Ipod/MP3 In-Dash DVD Player Satellite Radio Safety Background Optical Rain Sensors • Transmits/Receives IR Beams • Expensive • Inaccurate (False Readings) • Uneven Sensing Area Background Why Capacitive? • Higher Accuracy – Less False Positives • Smaller Size – Sleek Design • Cheaper • Fewer Components – Less Complex Background Basics of Capacitive Sensors • Three Main Types: • Displacement • Accelerometer • Pressure • Capacitance • Fringe Fields • Dielectric differences Design Specifications • FUNCTIONALITY • COMPATIBILITY • Sense water through windshield • Fit in existing housing (1250 mm2) • Communicate with microcontroller • Mount via adhesive • Control Wiper System • ACCURACY • Differentiate between various objects • Differentiate varying rain levels • COST • Cheaper than optical sensor • Overall cost < $12 Conceptual Design Sensor Traces • Not Your Typical Capacitor! • Sensing Area – Copper Traces • Designed for Base Capacitance ≈ 5 pF • Creates E-field When AC Voltage Applied • Objects Interfere with E-field – Change Cin • Design Parameters: Size, Spacing, Pattern • Dielectric Insulators are Vital! Conceptual Design Capacitance Monitoring Circuitry • Need Circuitry to Monitor the Capacitance Value of the Sensor Traces • Possible Design: RC Multi-vibrator - Change in C = Change in Time Constant • Better Alternative: Dedicated IC’s – Capacitance to Digital Conversion • Interface to Microcontroller for Software Processing Conceptual Design Microcontroller / Processor • Inputs Capacitance Data from C-D IC • Sensor Response to Rain Can Be Characterized • Software Algorithms To Discriminate Rain from Others • Varying Wiper Speed In Response to Amount of Rain • Prototype – Microcontroller • Production – Body Control Module Proposed Design Solution Capacitance-to-Digital Converter • Use Analog Devices AD7746 • Measures 24-bit capacitance • Accurate to the femto-Farad • Built in temp and humidity sensor for auto-compensation • AD7151 and AD7747 models can also be used Proposed Design Solution Differential Sensor Trace Design • Three Separate Traces • Source Excitation Voltage Applied To Center Trace • Two Differentially Connected Traces • Test Results to Determine Best Design (spacing, patterns, etc) Proposed Design Solution PIC18F4520 Microcontroller • Why we chose the PIC: • Variety of I/O Ports • Easy to use interface • C++ programming • Free! • Can compare voltages Proposed Design Solution Power Supply and Requirements • Prototype will use batteries (9V Batteries) • Production Design will use car battery • PIC needs steady 5V • AD7746 (C-D) needs steady 5.6V • Buck Converter Circuit Proposed Design Solution PCB Layout Final design has two parts: • Flex PCB: • Sensor Traces • Mounts via 3M Adhesive • Standard 2-layer FR4 PCB: • C-D Converter • Microcontroller Hardware Specifications DC Power Source • Overall Design uses 3 layers • Flex Layer – Capacitive Sensor Capacitive-toDigital Converter Circuit Capacitive Sensor Traces on PCB • C-D Converter (AD7746) • Buck Converter Circuit Microcontroller • Microcontroller (PIC18F4520) • Wiper Switch Output Voltage Comparison Is voltage between X ≤ Voltage ≤ Y ? YES Turn the wiper on. Water detected on the windshield. NO Keep wipers off. Budget Part Name Quantity Cost Analog Devices AD7151 Cap-to-Dig Converter 4 $12.68 Analog Devices AD7745 Cap-to-Dig Converter 4 $38.00 Analog Devices AD7746 Cap-to-Dig Converter 4 $34.32 Analog Devices AD7747 Cap-to-Dig Converter 4 $38.00 Analog Devices AD7746 Evaluation Board 1 $136.62 468-MP Adhesive 8 $42.40 Microcontroller 1 $0 Coaxial Cable Assembly 3 $57.08 Fabricate with Flexible PCB 2 $180.00 Total 29 $539.10 Reflection • Background • Design Specifications • Conceptual Design • Proposed Design Solution • Hardware Specifications • Questions?