ECE 480 – Team 6In Order of Presentation:

Ishaan SandhuDannY Kang

Arslan QaiserEric Otte

Anuar Tazabekov

Capacitive Rain Sensor for Automatic Wiper Control

Agenda

• Introduction

• Background

• Design Specifications

• Conceptual Design

• Proposed Design Solution

• Hardware Specifications

• Reflection

Introduction

Ipod/MP3 GPS

In-Dash DVD PlayerSafetySatellite Radio

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 LayoutFinal 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

Reflection

• Background

• Design Specifications

• Conceptual Design

• Proposed Design Solution

• Hardware Specifications

• Questions?