1975: Capacitive touch switches in use
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Transcript of 1975: Capacitive touch switches in use
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1975: Capacitive touch switches in use
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1994:1st laptop capacitive touchpad
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2003:1st iPod capacitive touchwheel
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Today:Multi-touchcapacitive touch screens
How do
they work?
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UC Regents Summer 2008 © UCBCNMAT Sensor Workshop: Capacitive Touch Sensors
2008-7-23
John Lazzaro CS Division, UC Berkeley
CNMAT Sensor Workshop 2008
Capacitive Touch Sensors
www.cs.berkeley.edu/~lazzaro
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UC Regents Summer 2008 © UCBCNMAT Sensor Workshop: Capacitive Touch Sensors
Today’s lecture: Capacitive touch sensing
Physics of capacitance
Touch pads and touch screens
Simple touch switches
Novel applications
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UC Regents Summer 2008 © UCBCNMAT Sensor Workshop: Capacitive Touch Sensors
Physics of Capacitance
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UC Regents Summer 2008 © UCBCNMAT Sensor Workshop: Capacitive Touch Sensors
Capacitor: A part you buy from Digikey.
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UC Regents Summer 2008 © UCBCNMAT Sensor Workshop: Capacitive Touch Sensors
Capacitor: Two conductive plates, separated by an insulator (dielectric).
Current cannot flow through an insulator. Thus, electrons can’t pass from A to B.
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UC Regents Summer 2008 © UCBCNMAT Sensor Workshop: Capacitive Touch Sensors
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.
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UC Regents Summer 2008 © UCBCNMAT Sensor Workshop: Capacitive Touch Sensors
How many does a 1.5Vbattery place on plate
A?
The ratio Q/V is defined as the capacitance C of the device.
Q = CV
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.
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UC Regents Summer 2008 © UCBCNMAT Sensor Workshop: Capacitive Touch Sensors
We can use a capacitance meter to measure C.
LED DisplayConceptually, the meter puts a V across the C, and then drains off the charge and counts Q.
C = Q/V.
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UC Regents Summer 2008 © UCBCNMAT Sensor Workshop: Capacitive Touch Sensors
Notation alert! From now on we draw
net charge on the plates.
A capacitance meter counts the number of field lines (and
thus, Q).
We draw field lines to illustrate the
pairing of charges.
field lines
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UC Regents Summer 2008 © UCBCNMAT Sensor Workshop: Capacitive Touch Sensors
Simple Touch Switches
(Sources: Analog Devices, Cypress, and Synaptics data sheets and
websites).
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UC Regents Summer 2008 © UCBCNMAT Sensor Workshop: Capacitive Touch Sensors
iPod 3G front panel
Capacitance meter chip (Synaptics)
Traces on back of PC board form the
“capacitors”!
What’s on the back of the PC board?
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UC Regents Summer 2008 © UCBCNMAT Sensor Workshop: Capacitive Touch Sensors
PC BOARD Cross- Section
Plate A
Plate B
PC Board Top View PC trace design for a capacitive button switch
What does the finger do?
FR-4 (insulator) FR-4
Glass
iPod White Plastic Casing
Copper CopperCopper
Copper
Capacitor Plate A
Capacitor Plate B
field linesfield lin
es
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UC Regents Summer 2008 © UCBCNMAT Sensor Workshop: Capacitive Touch Sensors
Capacitor Plate A
Capacitor Plate B
Some of the field lines will terminate on the iron in the red blood cells of a nearby finger.
RecallCapacitance meter counts the number of field lines to determine
Q, and then computesC = Q/V
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Compact button switch designsCapacitor Plate
A
Capacitor Plate B
Capacitor Plate A
Capacitor Plate B
LED LED
Capacitively sensed mechanical switch
Capacitor Plate A
Capacitor Plate ACapacitor
Plate B
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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 ...
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Touch wheel design
Plate A
B0 B1
B2
B3
B4B5
B6
B7
A
B0B1
B2
B3
B4
B5
B6
B7
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A
B0
B1
B0
B1
A
One switch
(connect all As together)
Keypad design
If row I and column J have big Cs, key Kij
is touched
Switch array
B0 B1
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Scaling up
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Typical part
$3.04 Qu 1
$1.69 Qu 5000
Senses 14 Cs
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UC Regents Summer 2008 © UCBCNMAT Sensor Workshop: Capacitive Touch Sensors
Interesting AD7142 facts ...
Pad C a few pF. Finger ΔC a few fF !
Sense time per pad: 3ms. If all 14 channels in use: 36ms “frame rate”.
Measured pad C is always drifting. Sensor chip tracks it adaptively.
Plastic thickness over PCB: 2-4mm.
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UC Regents Summer 2008 © UCBCNMAT Sensor Workshop: Capacitive Touch Sensors
(Sources: Apple, Synaptics, and Cirque patents, various websites).
Touchpads and Touchscreens
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Touchpad: A circuit board Top view - finger surface
Back view: Capacitance meter chip
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Touchpad Cross-section
Grid cross-points are sensor capacitors.
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Touch screens use a transparent wire matrix (ITO, Indium-Tin Oxide) under glass or plastic.
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The Sensor Array
Capacitor Plate A
Capacitor Plate B
Recall ...
Cross-Point close-up
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Some of the field lines will terminate on the iron in the
red blood cells of a nearby finger.
If many fingers may touch at once (multi-touch), scan out Cij array (frames) at a
constant frame rate.
“Capacitance video camera”.
Sensor Array
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Like a camera chip readout
Circuit from Apple patent teaching the iPhone touchscreen controller chip.
To parallel
A/D channels
Excite one row with a sense waveform
Read capacitance of all columns in parallel ...
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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 1994 ...
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Before Multi-touch Single-touch: A fast accurate keypad
adapted the keypad idea to track one finger over a track pad cross-point matrix (1991).
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
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B0 B1
Before Multi-touch First, drive all Y lines simultaneously, and measure X capacitance vector ...
Then, drive all X lines simultaneously, and measure Y capacitance vector.
Track peaks over time, etc ...
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All-analog computation !
A/D at very end.
Analog peak interpolatio
n
Analog background C normalization
To identical circuits for
Y.
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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 ...
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UC Regents Summer 2008 © UCBCNMAT Sensor Workshop: Capacitive Touch Sensors
Novel applications
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UC Regents Summer 2008 © UCBCNMAT Sensor Workshop: Capacitive Touch Sensors
Touch sensing ideas ...
Techniques not limited to finger capacitance
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UC Regents Summer 2008 © UCBCNMAT Sensor Workshop: Capacitive Touch Sensors
Touch pad and touch screen ideas ...
Techniques are not limited to the Cartesian coordinate space.
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UC Regents Summer 2008 © UCBCNMAT Sensor Workshop: Capacitive Touch Sensors
Techniques can be adapted to flexible printed circuit technologies.
Touch sensing ideas ...
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Touch user interfaces on curved surfaces (Apple patent)
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Touch user interfaces on curved surfaces (Apple patent)
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Today, sensor chips and Internet PC board services change the equation.
A decade ago, a design team needed experts in all 3 disciplines to succeed.
Sensing on curved surfaces await theiriPod moment -- a product design
concept that brings them mainstream.
Final thought
Touch Sensing = Materials + Electronics + Product Design