Maintaining good user experience as touch screen size increases
Capacitive touchscreens in consumer electronics to took off with the launch of Apple’s iPhone in 2007. The 3.5” screen introduced a multi-touch user experience that changed the way we interact with our electronics. Touchscreen displays are now a standard in consumer electronic products such as DSCs (Digital Still Cameras), PNDs (Portable Navigation Devices), e-readers, tablets, Ultrabooks and AIO (All-In-One) PCs.
A key trend in all of these devices is the move to larger screen sizes. Not only are capacitive touchscreens growing to address new market segments such as Ultrabooks or notebooks, they are also increasing within their current product segment. For example, smartphone OEMs are making the move from smartphones to superphones, providing larger screen sizes as a key differentiation in the market.
The main product segments for touch-enabled devices today are smartphones with screen sizes between 3” to 5”; super-phone or phablet in the range of 5” to 8”; tablets 8” to 11.6;, Ultrabooks 11.6” to 15.6”; and notebooks ranging as high as 17”. Tablets are considered one of the fastest ramping mobile devices in its five years of product history; sales are predicted to overtake PC sales by 2015 (Figure 1). This is causing PC vendors to shift their focus to adopting touch-friendly designs such as convertible notebooks that can function as notebooks or tablets.
As screen sizes of touch-enabled devices grow larger, the main challenge for designers is maintaining the same high performance users have come to expect from a cell phone but over a larger screen. This means scanning more intersections over more surface area in the same amount of time. In addition, the processor has to work with less signal and more noise while still maintaining the speed, precision, and responsiveness required for a desirable user interface experience.
Users expect large screen devices to have similar performance and touch experience to that of their smartphones, but large screen devices often deal with different use cases than what is typical on a smaller phone. Notebooks or PCs are more likely to be used while plugged into a power source, there is more surface area to rest palms or other large objects on the screen when typing, and users are more likely to set larger devices on a table or in their lap instead of holding it in their hands.
All of these conditions and circumstances change the electrical properties of a device. The key ingredients to a robust and responsive user experience include sensitivity, tracking multiple moving touch objects, recognizing and tracking fingers in different noise environments, recognizing and tracking fingers under different environmental conditions, and maintaining acceptable power consumption to achieve the desired battery life.
Capacitive touchscreens operate by driving a transmit voltage into the sensor panel on the device that creates a signal charge. This signal is then received by the touchscreen controller, which is able to determine the sensor capacitance by measuring the change of the sensor charge. The current received by the chip is equivalent to the capacitance of the panel multiplied by the voltage of the transmit drive (Q1 = C * VTX). A baseline circuit is able to remove the nominal non-touch sensor charge so the system can focus on measuring the change of sensor charge due to finger touch. This improves touch measurement, resolution and sensitivity.
The main problem with larger screens is that the transmit voltage has more surface area to cover and the resistance and capacitance of the sensor increases. The touch panel is limited by the higher parasitic capacitance and resistance, affecting the RC time constant, which results in slower transmit frequency. The transmit operating frequency affects signal settling, refresh rate and power consumption. The goal is to determine the highest transmit operating frequency conditions for a consistent touch response across the panels while minimizing scan time and power.
Refresh rates versus user interface needs
Refresh rate is the number of times in a second that the touchscreen controller can measure a touch on the screen and report it back to the host processor. A higher refresh rate will provide a responsive user experience by collecting more x/y data coordinates in a shorter amount of time. Most consumer electronics devices require a touch controller refresh rate of greater than 100 Hz, or about 10 ms. Certain applications, such as digital drawing pads or Point of Sale (POS) terminals require even higher refresh rates to capture and recognize signatures and quick pen strokes.
It is challenging for large screens to maintain fast refresh rates because the touch controller needs to sweep greater surface area, gather data from all the intersections, and then process that data. The two main components that effect refresh rate are how fast the screen is scanned and how fast the scanned data is processed. A 17” screen has 11 times more intersections than a 5” screen with the same sensor characteristics (3108 vs. 275). In order to maintain the user experience of the 5” screen, the 17” screen requires more scanning and processing power.
One technique to help solve the scanning problem is to make sure the touch controller has enough receive channels to sweep the screen in a single pass. Most touchscreen stack-ups are composed of sensor patterns under the cover glass in an array of ‘unit cells’ that run in the x and y direction, with x being transmit and y being receive or vice versa. The receive channel will collect the data and use analog to digital converters (ADC) to convert the change in mutual capacitance of each unit cell into digital data for the host to interpret where the finger touch coordinates are located. If the number of receive channels or ADCs are inadequate, then it will take multiple scans and more time to sweep the entire panel. This results in fewer samples that can be taken in a given time period, leading to an unsatisfactory user experience.
A technique to help solve the processing problem is to add a bigger processor to the touch controller or offload some of the computing to the system’s main processing unit. This means sending capacitive data to the host side and running algorithms on the applications or graphics processor. One implementation would be to use the touchscreen controller to scan the sensor, search for first touch, and then transfer the image to the host processor. The host will then process the full array, filter noise, find touch coordinates and track finger IDs. This use of parallel processing allows the heavy number crunching to be done in the multi-GHz, multi-core processors that serve as a host for the touchscreen and display.