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ECE 480 – Team 6
In Order of Presentation:Ishaan Sandhu
DannY KangArslan Qaiser
Eric OtteAnuar Tazabekov
Capacitive Rain Sensor for Automatic Wiper Control
Agenda
• Introduction
• Background
• Design Specifications
• Conceptual Design
• Proposed Design Solution
• Hardware Specifications
• Reflection
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• Sense water through windshield
• Communicate with microcontroller
• Control Wiper System
• ACCURACY• Differentiate between various objects
• Differentiate varying rain levels
• COMPATIBILITY• Fit in existing housing (1250 mm2)
• Mount via adhesive
• 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
• Overall Design uses 3 layers
• Flex Layer – Capacitive Sensor
• C-D Converter (AD7746)
• Buck Converter Circuit
• Microcontroller (PIC18F4520)
• Wiper Switch
DC Power Source
Keep wipers off.
Capacitive Sensor Traces
on PCB
Turn the wiper on.Water detected on
the windshield.
Capacitive-to-Digital
Converter Circuit
Microcontroller
YES NO
Output Voltage Comparison
Is voltage betweenX Voltage Y ≤ ≤ ?
BudgetPart 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