1. 1975: Capacitive touch switches in use

2. 1994:
1st laptop
capacitive touchpad

3. 2003:
1st iPod
capacitive touchwheel

4. Today:
Multi-touch
capacitive touch screens
How do
they
work?

5. Capacitive Touch Sensors John Lazzaro CS Division, UC Berkeley
CNMAT Sensor Workshop 2008
Capacitive Touch Sensors
John Lazzaro CS Division, UC Berkeley
Greet class

6. Today’s lecture: Capacitive touch sensing
Physics of capacitance
Simple touch switches
Touch pads and touch screens
Novel applications

7. Physics of Capacitance

8. A part you buy from Digikey.
Capacitor:
A part you buy from Digikey.

9. Current cannot flow through an insulator
Current cannot flow through an insulator. Thus, electrons can’t pass from A to B.
Capacitor: Two conductive plates, separated by an insulator (dielectric).

10. Battery pumps electrons from plate A to plate B
Battery pumps electrons from plate A to plate B. We notate each electron pumped from A as and refer to it as a positive charge.

11. battery place on plate A?
How many
does a 1.5V
battery place on plate A?
A: Area of plates
d: Plate separation
ϵ: Dielectric property
Q: Number of
V: Voltage on plates
It depends on the 3-D shape
of the capacitor, and the
material properties of the dielectric.
The ratio Q/V is defined as the capacitance C
of the device.
Q = CV

12. We can use a capacitance meter to measure C.
LED Display
Conceptually, the meter puts a V across the C, and then drains off the charge and counts Q.
C = Q/V.

13. net charge on the plates.
Notation alert!
From now on we draw
net charge on the plates.
We draw field lines
to illustrate the
pairing of charges.
field lines
A capacitance meter counts the number of field lines (and thus, Q).

14. (Sources: Analog Devices, Cypress, and Synaptics data sheets and websites).
Simple Touch Switches

15. Traces on back of PC board form the “capacitors”!
What’s on the
back of the PC board?
iPod 3G front panel
Traces on back of PC board form the “capacitors”!
Capacitance meter chip (Synaptics)

16. PC trace design for a capacitive button switch
PC Board Top View
PC trace design for a capacitive button switch
Plate A
Plate B
What does the finger do?
iPod White Plastic Casing
PC BOARD Cross- Section
field lines
Glass
Copper
Capacitor Plate A
Capacitor Plate B
FR-4 (insulator)
FR-4

17. meter counts the number of field lines
Some of the field lines will terminate on the iron in the red blood cells of a nearby finger.
Capacitor Plate A
Capacitor Plate B
Recall
Capacitance
meter counts the number of field lines
to determine Q,
and then
computes
C = Q/V

18. Capacitively sensed mechanical switch
Compact button switch designs
Capacitor Plate A
LED
Capacitor Plate A
Capacitively sensed mechanical switch
Capacitor Plate A
Capacitor Plate B
Capacitor Plate B
Capacitor Plate B

19. Super-resolution via interpolation ...
Capacitive fader design
Capacitor Plate A
Capacitor Plate B0
Plate B1
Plate B2
Plate B3
Plate B4
Plate B5
Plate B6
Plate B7
Super-resolution via interpolation ...

20. Touch wheel design B0 B1 B7 B2 B6 B3 B5 B4 B1 B0 B2 B7 B3 B6 B4 B5
Plate A B6 B3 B5 B4 B1 B0 B2 A B7 B3 B6 B4 B5

21. Keypad design One switch Switch arrayB0 B1
One switch
Switch array B0 B1
(connect all As together)
If row I and column J have big Cs, key Kij
is touched

22. Scaling up

23. Typical part
$3.04
Qu 1
$1.69
Qu 5000
Senses
14 Cs

24. Interesting AD7142 facts ... Pad C a few pF. Finger ΔC a few fF !
Measured pad C is always drifting. Sensor chip tracks it adaptively.
Sense time per pad: 3ms. If all 14 channels in use: 36ms “frame rate”.
Plastic thickness over PCB: 2-4mm.

25. Touchpads and Touchscreens
(Sources: Apple, Synaptics, and Cirque patents, various websites).

26. Top view - finger surface
Touchpad: A circuit board
Back
view:
Capacitance meter
chip

27. cross-points are sensor capacitors.
Touchpad
Cross-section
Grid
cross-points are sensor capacitors.

28. Touch screens use a transparent wire matrix
(ITO, Indium-Tin Oxide) under glass or plastic.

29. The Sensor Array Cross-Point close-up Recall ... Capacitor Plate A
Capacitor Plate B
Recall ...

30. “Capacitance video camera”.
Some of the field lines will
terminate on the iron in the
red blood cells of a nearby finger.
Sensor Array
If many fingers may touch at once (multi-touch), scan out Cij array (frames) at a
constant frame rate.
“Capacitance video camera”.

31. Like a camera chip readoutTo
parallel
A/D
channels
Excite one row with a sense waveform
Circuit from
Apple patent
teaching the
iPhone touchscreen controller chip.
Read capacitance of all columns in parallel ...

32. One A/D readout channel
Looks a lot like a radio receiver ...
Which may be why Apple partnered
with Broadcom for the design!
Not feasible in

33. adapted the keypad idea to track one finger over a track pad
Before Multi-touch
Single-touch: A fast accurate keypad
One switch
Switch Array
Connect all As together
If row I and column J
have big Cs,
key Kij is touched
Keypad recap: B0 B1
adapted the keypad idea
to track one finger over a
track pad
cross-point matrix (1991).

34. First, drive all Y lines simultaneously,
Before Multi-touch
First, drive all Y lines simultaneously,
and measure X capacitance vector ... B0 B1
Then, drive
all X lines simultaneously,
and measure
Y capacitance vector.
Track peaks over time, etc ...

35. All-analog computation !
Analog background C normalization
Analog peak interpolation
A/D at
very end.
To identical circuits for Y.

36. For your design: A typical part
Most 2-D products are sold as per-customer custom modules (Synaptics) or done as in-house ASICs (Apple).
Recently, standard parts have started to appear ...

37. Novel applications

38. Touch sensing ideas ...
Techniques not limited
to finger capacitance

39. Touch pad and touch screen ideas ...
Techniques
are not
limited to the
Cartesian coordinate space.

40. Touch sensing ideas ...
Techniques can be adapted to flexible printed circuit technologies.

41. Touch user interfaces on curved surfaces (Apple patent)

42. Touch user interfaces on curved surfaces (Apple patent)

43. Today, sensor chips and Internet
Final thought
Touch Sensing = Materials + Electronics + Product Design
A decade ago, a design team needed experts in all 3 disciplines to succeed.
Today, sensor chips and Internet
PC board services change the equation.
Sensing on curved surfaces await their
iPod moment -- a product design concept that brings them mainstream.

44. Final thought Split-adjusted stock prices
Touch Sensing = Materials + Electronics + Product Design
Split-adjusted stock prices
for DuPont, Synaptics, and Apple
since 2003 (3G iPod introduction date)