The touch screen technology is widely used in PDA, smart phone, PMP, ATM, information kiosk and many other types of equipment in industrial, medical and commercial environment. Actually the technology enabling these devices is not new, since it was invented by Dr. Samuel C. Hurst in 1971. But it becomes hotter after the release of popular iPhone and iPod touch. With new patents filed for the touch screen technology, Apple brings a new wave to this mature segment and more companies are involved in this revolution with improved interactive UI, ICs, assembly modules and software components.
Conventional Touch Screen
The implementation of a touch screen includes resistive, capacitive, infrared, surface acoustic wave, optical imaging, acoustic pulse, and other emerging technologies. The kit usually is made up of a touch module, and a controller, which measures the touch events in frequency, voltage or current, encodes and transmits to the drivers running in host controller. Different touch screen technologies are suitable for different environments. The selection criteria are:
- - Light Transmission
- - Response Time
- - Touch Accuracy
- - Environment Requirement
- - Lifecycle
- - Surface Hardness
- - Resolution
- - Input Mode (Bare or gloved finger, styles, pen)
- - Display Size (Small, Middle or Large Screen)
- - Multi-touch
Currently most of the portable devices are using resistive touch screen modules, because this technology has perfect balance between low cost and required performance. Both stylus and finger operation are supported. With the resistive touch screen, most of the technical innovations take place on the GUI system in the host. For example, HTC S1 released a new UI called touchFLO. This new technology works efficiently with single-touch screen. For example, the clockwise and counterclockwise finger event on specific screen area will zoom in/out this part of picture or web page. Sometimes it is more convenient because the user just need one hand to hold, and the thumb of same hand to operate. The iPhone requires both hands to operate on-the-go anyway. Yes, even the user can use two fingers of one hand to operate, but he/she should use the other hand to hold it, unless the device can be installed somewhere. Besides, HTC S1 supports handwriting and virtual keyboard, iPhone uses virtual keyboard. Typing European characters on the virtual keyboard might be a good idea, but not for Asian languages. Some other human machine interactivity research organizations also invented another new symbolic operation UI system with single-touch screen. For example, by handwriting an "h" to represent home page, the computer will show up the index for whole system. This method is deployed in an Automotive PC platform (VIA x86 design), it is very effective and attractive. The driver can handwrite on the screen while looking at the road without staring at the screen and touching some buttons. Therefore this UI improved the safety on the road.
I still remember the replacement of B/W LCD on mobile phones. The color LCD is more expensive and wasting more power, the engineers are arguing about if the idea of using color LCD in mobile phone is stupid or not. The sales of mobile phones with B/W LCD is dropping so quickly, now over 90% mobile phones are equipped with color LCD, in bigger size and dual LCD configuration. This story tells us that consumer electronics' product design is marker driven, instead of technology driven. So my conclusion is, even if single-touch screen is fully capable of every operation with some advantages, the multi-touch screen is a major trend of new touch-screen devices, because the consumers love it. Let us check it out and find resources to implement it by ourselves.
If anyone is just trying to clone Apple's design, he will be disappointed. The multi-touch is a system design, not just a small improvement on touch screen itself. Apple's initiative can not be cloned and it is protected by more than 200 patents. I found that Mezu was going to launch its Mini-One on CeBit 2008, but finally it was enforced to be close down on that exhibition because of another MP3 infringement issue. And the Mezu Mini-One does not support multi-touch as well. I am surprised that Apple didn't comment on the design issue for Meizu's MiniOne officially, even they seem like twins. (Please check left picture. ) Ironically Meizu issued a design patent in China. A clone design asks to be protected from being cloned again? I really hope they can use their resource on other proper projects.
Traditional resistive and capacitive can only support one touch hot area, i.e. one time to track one touch event. If two fingers are put on touch screen or touch pad, the result returns to host is the position of the last touch or somewhere between these fingers. In order to setup multiple touch hot areas, the designer must upgrade all of the three parts of the touch screen sub-system. That means innovation should take place on panel input modules, panel controller (ASIC or MCU) and device drivers in host, of course the application running the host should support multi-touch as well. The capability of touch screen controller limits the tracking number of touch events. I am not so sure about the application software support. The application software should track multi-touch events by themselves in some operations, for example, multiple objects should be tracked individually in a multi-user game. In another case, if the multi-touch can be translated into high-level zoom command, or rotate command, the regular software should be able to support multi-touch operation already. In that case, the device driver should support multi-touch event and translate into high-level commands. It is up to system architect to design the whole software stack.
IBM and Logitech have already issued some patents for multi-touch technology. Apple's iPhone is first consumer products deployed multi-touch. So far Apple has filed two patents for multi-touch, one is self-capacitance, and the other is mutual capacitance. These technologies have a new name as projection capacitive. The multi-touch has already deployed in iPhone, iPod, Mac Book Air. The implementation is described on some patent search sites (Please check reference). More competitors like LG, Samsung, and Microsoft are going to release new models and new operation systems with improved touch screen modules and drivers. Even MTK, a well-known supplier for OEM mobile phones, is going to improve their reference design of touch screen and try to mimic the operation of iPhone. Besides projection capacitive touch, there are resistive and surface capacitive implementation for multi-touch.
To allow people to operate with multiple fingers, the iPhone uses a new arrangement of existing technology. It includes a layer of capacitive material, just like many other touch screens. However, the iPhone's capacitors are arranged according to a coordinate system. The coordinate system does not require very high precision, because it is finger based operation. But I have not idea about the detail accuracy of it. Anyway its accuracy can not support handwriting. Its circuitry can sense changes at each point along the grid. In other words, every point on the grid generates its own signal when touched and relays that signal to the iPhone's processor. This allows it to determine the location and movement on the capacitive material. It won't work if you use styles or wear non-conductive gloves.
In mutual capacitance, the capacitive circuitry requires two distinct layers of material. One houses driving lines, which carry current and other houses sensing lines, which detect the current at nodes. Self capacitance uses one layer of individual electrodes connected with capacitance sensing circuitry. Both of these possible setups send touch data as electrical impulses.
After all, Apple's projection capacitive touch screen works like a big grid of keyboards. Each key is driven by row and column pluses, and encoded with its location and address. That working method and programming skills are very common in microcontrollers. The improvement are materials, these keys are almost transparent (Apple calls them transparent electrodes) and on a single film (maybe ITO film or ITO glass) can be produced by advanced process. Because the surface could be glass, so you will find iPhone's glass is quite hard and anti-scratch. According to Unwired View, iPhone also use force-sensing mechanism to filter the touch events by accident.
Actually Apple's iPhone only works dual touch event, it can support more events if Apple improves the capability of the controller, driver and application software.
Because its nature similarity with LCD drivers, some LCD panel manufacturers are integrating these feature into LCD modules. AUO released in-cell multi-touch LCD modules (480*272) on Oct, 2007. More and more LCD with single-/multi-couth screen will be available. I personally think AUO’s approach maybe more competitive because of the overall BOM cost and performance. Neither AUO in-cell and Apple projection capacitive multi-touch screen have large size LCD deployment, because capacitive is very sensitive to EMC noise, while large size HDTV LCD itself is a big noise generator!
Because capacitive touch screen is very sensitive with environment, even with the innovation on input module, multi-touch requires more improvement on controller to work properly. Maybe that is another reason why Apple keeps the method back cover on iPhone/iPod?
According to report from howtostuffs.com, the iPhone's processor and software are central to correctly interpreting input from the touch-screen. There are many processors in iPhone, Infineon GSM processor, Samsung ARM11 as the application processor, and a dedicated screen controller BCM5974 from Broadcom. I can not find any information from Broadcom site, I guess it is a custom chip for Apple. So I checked other products, I guess this chip is a DAC as touch screen digitizer. If so, then the DAC is used to sample the grid instead of switching method. It also means ARM11 acts as the processing for multi-touch. The processor uses software to interpret raw data as commands and gestures.
- Signals travel from the touch screen to the processor as electrical impulses.
- The processor uses software to analyze the data and determine the features of each touch. This includes size, shape and location of the affected area on the screen. If necessary, the processor arranges touches with similar features into groups. If you move your finger, the processor calculates the difference between the starting point and ending point of your touch.
- The processor uses its gesture-interpretation software to determine which gesture you made. It combines your physical movement with information about which application you were using and what the application was doing when you touched the screen.
- The processor relays your instructions to the program in use. If necessary, it also sends commands to the iPhone's screen and other hardware. If the raw data doesn't match any applicable gestures or commands, the iPhone disregards it as an extraneous touch.
There are some other suppliers for capacitive touch technology. The implementation of controller includes ASIC and MCU based. ASIC is very low cost for mass-production, while MCU has the advantage of custom design but its price is higher. The following vendors are delivering touch solutions for markets. Some of them have already supported multi-touch. More vendors are coming to develop new parts to support. Mentioned vendors have their own patents on touch technology.
Synaptics is a leading worldwide developer of custom-designed user interface solutions for mobile computing, communications and entertainment devices. Synaptics is focusing on touch pad solutions, and offer ASIC with I2C/SMBus.
Taiwan ELAN Microelectoncis Company (EMC) also a patent with trademark of eFinger. It claims that patent is competitive on touch screen application. But it requires registration on specification and has an open lawsuit with Synaptics.
Cypress offers CapSense in many applications. For example, the keypad part of V3/V8 from Motorola, LG chocolate mobile phones. Cypress CapSense is based upon its PSoC mixed signal array with embedded RISC M8 microcontroller. It is very easy to use. The new PSoC CapSense touch screen solution also offers designers the ability to implement multiple additional functions beyond touch screens. The same devices can implement capacitive buttons and sliders simultaneously, replacing their mechanical counterparts, as well as proximity sensing. Engineers can also take advantage of the PSoC mixed signal array to implement functions beyond CapSense. Such functions include, driving LEDs, backlight control, motor control, power management, I/O expansion, accelerometers and ambient light sensors. These functions, in conjunction with flexible communication (I2C and SPI), allow for unparalleled system integration. Cypress's CapSense touch screen solution is available using projected capacitance and surface capacitance sensing techniques.
Leadis is dedicated to creating compelling touch solutions focused on a strategy of Innovation and Integration. Leadis' line of PureTouch (TM) capacitive touch controller solutions will begin sampling in the first half of 2008.
ATLab is a Korean company, co-operates with ST to offer touch screen touch ICs for capacitive technology.
Quantum Research Group, offers QProx(TM) touch control and sensor ICs. As same as Cypress, its solution offers more features for QSlide and QWheel. Atmel acquired Quantum already.
Thanks to Apple, more and more players are trying to compete on this market.
ST has two lines for touch pads, one is licensed from Quantum (MCU based), the other is licensed from ATLab (ASIC based). TI is trying to promote its MSP430 in this market. NXP also has its companion chip for PDA with touch screen DAC inside. Some other vendors such as Microchip and Maxim have similar product lines.
Driver and Application Software
I am not quite sure about how Apple implement these, I would rather to use software to refer to both driver and application software part. Apple also filed a complementary patent called "multi-finger gesture" for presentation of the finger movement. But that part requires more know-how on system software, I am not going to discuss here. Anyone who is interested in please visit some professional sites of MIT or NYU.
Latest Development for Alternative UI
Microsoft is developing surface computer, which can track up to 52 touches.
Besides multi-touch, some more topics regarding touch screen is rising on horizontal. They are photo sensor in pixel, polymer waveguide, distributed light, strain gauge, dual-force touch, laser-point activated touch and 3D touch.
Most the multi-touch related patents are held in 3M, Nitto Denko, Oike-Kogyo, Dupont, Apple, IBM for materials and panels, but more Taiwanese, Korean industries are trying to file their own patents on the touch panels. As a system developer, we just make sure our products will not infringe these patents. It is tricky for those mobile phones manufacturers who are trying to clone Apple design on their own products.
The full text for Apple multi-touch patent.
Touch screen definition from Wikipedia
A touch screen kit selection table from EZscreen.
A teardown BOM list for iPhone
A web site reveals internal information and patents for Apple's products.
The wireless technology oriented site offers many Apple iPhone information.
How the iPhone Works from howstuffworks.com
Optera has JV with TPK in Xiamen, China.
Synaptics, the biggest touch pad suppliers in the world.
Indium Tin Oxide, the "Multi-Finger" transparent touch screen from Taiwanese supplier, EMC (Elan Microelectronics Company). The application notes requires registration.
Touch screen controller from ADI.
Cypress PSoC based CapSense(TM) application note.
Touch solution from Leadis.
Quantum Research Group
News: AUO in cell multi-touch panel.
Perceptive Pixel, Inc. was founded by Jeff Han in 2006 as a spin-off of the NYU Courant Institute of Mathematical Sciences to develop and market the most advanced multi-touch system in the world.
Jeff Han's old project site.