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

Transcription

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 CNMAT Sensor Workshop 2008 Capacitive Touch Sensors John Lazzaro CS Division, UC Berkeley CNMAT Sensor Workshop: Capacitive Touch Sensors UC Regents Summer 2008 UCB

6 Today s lecture: Capacitive touch sensing Physics of capacitance Simple touch switches Touch pads and touch screens Novel applications CNMAT Sensor Workshop: Capacitive Touch Sensors UC Regents Summer 2008 UCB

7 Physics of Capacitance CNMAT Sensor Workshop: Capacitive Touch Sensors UC Regents Summer 2008 UCB

8 Capacitor: A part you buy from Digikey. CNMAT Sensor Workshop: Capacitive Touch Sensors UC Regents Summer 2008 UCB

9 Capacitor: Two conductive plates, separated by an insulator (dielectric). Current cannot flow through an insulator. Thus, electrons can t pass from A to B. CNMAT Sensor Workshop: Capacitive Touch Sensors UC Regents Summer 2008 UCB

10 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. CNMAT Sensor Workshop: Capacitive Touch Sensors UC Regents Summer 2008 UCB

11 How many does a 1.5V battery place on plate A? It depends on the 3-D shape of the capacitor, and the material properties of the dielectric. A: Area of plates d: Plate separation ϵ: Dielectric property Q: Number of V: Voltage on plates The ratio Q/V is defined as the capacitance C of the device. Q = CV CNMAT Sensor Workshop: Capacitive Touch Sensors UC Regents Summer 2008 UCB

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. CNMAT Sensor Workshop: Capacitive Touch Sensors UC Regents Summer 2008 UCB

13 Notation alert! From now on we draw net charge on the plates. field lines A capacitance meter counts the number of field lines (and thus, Q). We draw field lines to illustrate the pairing of charges. CNMAT Sensor Workshop: Capacitive Touch Sensors UC Regents Summer 2008 UCB

14 Simple Touch Switches CNMAT Sensor Workshop: Capacitive Touch Sensors (Sources: Analog Devices, Cypress, and Synaptics data sheets and websites). UC Regents Summer 2008 UCB

15 ipod 3G front panel What s on the back of the PC board? Traces on back of PC board form the capacitors! Capacitance meter chip (Synaptics) CNMAT Sensor Workshop: Capacitive Touch Sensors UC Regents Summer 2008 UCB

16 PC Board Top View PC trace design for a Plate A Plate B capacitive button switch What does the finger do? Capacitor Plate A PC BOARD Cross- Section ipod White Plastic Casing Copper field lines Glass Copper field lines Copper Capacitor Plate B FR-4 (insulator) Copper FR-4 CNMAT Sensor Workshop: Capacitive Touch Sensors UC Regents Summer 2008 UCB

17 Some of the field lines will terminate on the iron in the red blood cells of a nearby finger. Recall Capacitance meter counts the number of field lines to determine Q, and then computes C = Q/V R= PCB LA>ER 4 T= PLAST$C CO9ER Capacitor Plate A CNMAT Sensor Workshop: Capacitive Touch Sensors Capacitor Plate B UC Regents Summer 2008 UCB

18 Compact button switch designs CONNECTED TO C SHIELD Capacitor Plate A on the top layer of the PCB. LED LED Capacitor Plate A B1 B Capacitor Plate B Capacitor Plate B Figure 2. Button Layout METAL DOME FLEXIBLE METAL DOME FLEXIBLE Capacitively sensed mechanical switch TO C Capacitor SHIELD CONNECTED Plate A Capacitor Plate B CONNECTED TO CIN Capacitor Plate A PCB BOTTOM LAYER CROSS SECTION

19 Capacitive fader design Capacitor Plate B 0 Plate B 1 Plate B 2 Capacitor Plate A Plate B 3 Plate B 4 Plate B 5 Plate B 6 Super-resolution via interpolation... Plate B 7

20 Touch wheel design B 0 B 1 B 7 B 2 Plate A B2 B 6 B 3 B 5 B 4 Figure 13. Scroll Wheel Sensor Layout B 0 B 1 B 2 B 7 A B 3 B 6 B 4 B 5

21 Keypad design A One switch Switch array B 0 B 0 B 1 B 1 A (connect all As together) B 1 B 0 If row I and column J have big Cs, key Kij is touched

22 Scaling up

23 Typical part Programmable Controller for Capacitance Touch Sensors AD7142 FEATURES Programmable capacitance-to-digital converter 36 ms update rate maximum sequence length) Better than 1 ff resolution 14 capacitance sensor input channels No external RC tuning components required Automatic conversion sequencer On-chip automatic calibration logic Automatic compensation for environmental changes Automatic adaptive threshold and sensitivity levels On-chip RAM to store calibration data SPI -compatible serial interface (AD7142) I 2 C -compatible serial interface (AD7142-1) Separate VDRIVE level for serial interface Interrupt output and GPIO 32-lead, 5 mm x 5 mm LFCSP_VQ 2.6 V to 3.6 V supply voltage Low operating current Full power mode: less than 1 ma Low power mode: 50 µa /-3. G. /-32 G2 /-3< G< /-3G 2 /-3F < /-3= G /-38 F /-3E = /-3D 8 /-3C E /-32. D /-322 C /-32< 2. /-32G 22 / #4-"0) #%/ #%/ $ )%-$" 2< 2= 28 <. FUNCTIONAL BLOCK DIAGRAM #5-!/4 6,!%-7 <=.>4? "7/-!,!-*3 #*@%/" $ %"&' $ %"&( <C <D 289:-!!9" /)/ /*3!%*0,3) ),!, %";-#!"%# #"%-,0-3!"%&,/",3) /*3!%*0 0*;-/!"#! <E /,0-:%,!-*3 "3;-3" /,0-:%,!-*3 %,6 B*5"%9*3 %"#"! 0*;-/ -3!"%%@B!,3) ;B-* 0*;-/ 2G 2F 2E 2D 2C <8,$ //,;3) )$ // );3)2 );3)< ;B-* $3.04 Qu 1 $1.69 Qu 5000 Senses 14 Cs <2 << <G <F <=

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. CNMAT Sensor Workshop: Capacitive Touch Sensors UC Regents Summer 2008 UCB

25 Touchpads and Touchscreens CNMAT Sensor Workshop: Capacitive Touch Sensors (Sources: Apple, Synaptics, and Cirque patents, various websites). UC Regents Summer 2008 UCB

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

27 Touchpad Cross-section Insulating surface Substrate Sensor Traces 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 Insulating surface B Substrate Sensor Traces Capacitor Plate B

30 Sensor Array 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 C ij array (frames) at a constant frame rate. Capacitance video camera.

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

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

33 Before Multi-touch Single-touch: A fast accurate keypad Keypad recap: Switch Array B 0 B 1 adapted the keypad idea to track one finger over a track pad cross-point matrix (1991). One switch Connect all As together If row I and column J have big Cs, key Kij is touched

34 Before Multi-touch B 0 B 1 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...

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... Logic Block Diagram ADVANCED CY8CTMG120 TrueTouch Multi-Touch Gesture Touchscreen Controller

37 Novel applications CNMAT Sensor Workshop: Capacitive Touch Sensors UC Regents Summer 2008 UCB

38 Touch sensing ideas... Techniques not limited to finger capacitance CNMAT Sensor Workshop: Capacitive Touch Sensors UC Regents Summer 2008 UCB

39 Touch pad and touch screen ideas... Techniques are not limited to the Cartesian coordinate space. CNMAT Sensor Workshop: Capacitive Touch Sensors UC Regents Summer 2008 UCB

40 Touch sensing ideas... Techniques can be adapted to flexible printed circuit technologies. Flexibility to meet customers needs Design Guide CNMAT Sensor Workshop: Capacitive Touch Sensors UC Regents Summer 2008 UCB

41 Touch user interfaces on curved surfaces (Apple patent)

42 Touch user interfaces on curved surfaces (Apple patent)

43 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.