Touch Sensing: NextGeneration User Interface for the Automotive Market

1
Touch SensingNext-Generation User Interface
for the Automotive Market?

• By
• Vivien Delport
• Microchip Technology Inc.
• Applications Engineering Manager
• Security, Microcontroller Technology
  Development Division

2
Objectives

• Understanding the variety of uses for capacitive
  touch sensing
• Gain a better understanding of how capacitive
  touch sensing works
• Understand design trade-offs when choosing a
  solution for your next touch-sensing design
• Understand what is needed to implement a
  touch-sensing application

3
Traditional Interfaces

• Human Machine Interfaces (HMIs) traditionally
  consist of mechanical systems that allow drivers
  to control the vehicle
• HMI feeds status from the vehicle back to the
  driver
• Switch position (on/off)
• Light or LED turns on
• Sound-effect feedback
• Reaction from subsystem (e.g. window
  moving)

4
Capacitive Touch Sensing

• Already used in automotive applications
• Replaces bulky mechanical user-control switches
• Proximity detection using a capacitive sensor
• Passive Keyless Entry systems
• Automatically turn dome lights on

5
Why Capacitive Touch?

• Auto control panels get more complex
• Harder to place all the controls in the limited
  available space
• Requires no mechanical movement
• Create completely sealed,
  modern-looking designs
• Limited space on the surface of the steering
  wheels
• Where drivers airbag is mounted
• No mechanical switches that can disassemble
  into the drivers face or chest

6
Other Applications for Capacitive Touch Sensing

• Slider controls
• Proximity activation
• Fluid-level detection

7
Basic Principles
8
How Does Capacitive Touch Sensing Work?

• Introduction of finger produces a parallel
  capacitance
• Equivalent Circuit
• So, we need to measure a capacitance and detect a
  shift in its value

Iron in Blood
9
Finger Introduces Additional Ground Path
CF
CP
Cover plate provides dielectric between charged
pad and ground introduced by finger
10
Capacitance Equation
A
A
e
e
r
0
C

d
d
e0 Permittivity of Free Space (8.854
Pico-Farad/meter) er Relative Dielectric Constant
(unit-less)A Area of Plates (meters) C Capacitanc
e (Farads) d Distance Between Plates (meters)
11
Dielectric Constant (er)
-
-
-
-
-
-
-
Covering Plate







Copper Pad
PCB
erglass 4 ? 8 erplexiglas 2.25 ? 3.5
12
Pad Shape and Size
½ x ½ (12.7mm x 12.7mm)
13
Sensors and Traces
14
Effects of Water
e
e
A
r
0

C
d
Cover Plate
C
C
F2
F1
Target Sensor
Nearby Sensor
Printed Circuit Board
15
Methods for Capacitive Touch Sensing

• Simple RC circuit
• Measure time (Timer) or Voltage
  Analog-to-Digital Converter (ADC) or Comparator
• RC oscillator
• Measure frequency
• Integrates simple RC over time
• Charge transfer/switch capacitor
• Transfer charge from touch sensor to storage
  capacitor repeatedly
• Effectively a step response to a switch capacitor
  RC filter
• Measuring the transfer function of a
  capacitor-based circuit
• Applying a frequency and measuring the output

16
High-Level Overview

• Touch system is segregated into blocks
• Each needs to be optimized for specific challenges

17
Some Design Challenges

• Hardwaredetecting a 5 to 20 shift in
  picoFarads
• Touchwith good sensors 3 to 20
• Heat, humidity, adjacent metal and environmental
  shift 5 to 25
• People sometimes wear gloves, touch with
  fingernails, etc.

18
Factors Driving Capacitance

• Size of sensor pad
• Amount of finger covering the sensor
• Material covering the sensor
• Thickness of material covering the sensor
• Protective coverings
• Materials on fingers or hands
• Gloves and paint

19
LIN Slave With RGB Control and mTouch Sensing
Solution
20
(No Transcript)
21
Summary

• Understand the variety of uses for capacitive
  touch sensing
• Understand how capacitive touch sensing works
• Understand design tradeoffs when choosing a
  solution for your next touch-sensing design
• Touch sensing can complement or expand the
  functionality of traditional mechanical
  pushbutton switches

22
Any Questions?

• For more information, please visit
  www.microchip.com/mtouch
• www.microchip.com/automotive
• Thank you for attending

Note The Microchip name and logo, and PIC are
registered trademarks of Microchip Technology
Inc. in the U.S.A. and other countries. mTouch is
a trademark of Microchip Technology Inc. in the
U.S.A. and other countries. All other trademarks
mentioned herein are property of their respective
companies.