accelerometer supported tilt as an input method for mobile

Abhishek Chaturvedi Rev01

Accelerometer supported Tilt as an input method for Mobile devices

INTERACTION WITH MOBILE DEVICES Text entry into mobile devices is an increasingly important factor in mobile devices in this quickly progressing electronics market. For desktop computers and laptops, keyboards are feasible devices for text entry. However, when the size of the device is reduced to something that fits in the palm of your hand, entering data becomes more complex. Hence a lot of unique methods of data entry have become prevalent in contemporary handheld devices. Soft keyboards, handwriting recognition, are some of methods predominant in today’s cell phones and PDAs. Most cell phones use the twelve number keys for text entry. Smaller devices, such as two-way pagers use the scroll and select method, as the interface is even smaller. The scroll and select technique requires the user to browse the entire alphanumeric character list and make selections. Some of the common issues a lot of data entry techniques of this type have to deal with are slowed data entry, operator fatigue.

ACCELEROMETER ASSISTED TILT-TEXT ENTRY As more features and functionality are added to mobile devices there is more interaction required between the user and the mobile device. More interaction begets more I/O keys for efficient data input, and more keys on an ever-shrinking device is an oxymoron.

Hence more unique and creative approaches to mobile device data input deserve being looked into; as these mobile devices move into the next generation. The intent of this note is to familiarize the reader with the unique approach to tilt text and how an accelerometer IC from Memsic can be incorporated in to this application. Memsic accelerometer chip is available in a standard Leadless chip carrier (LCC) package, which is (5x5x2) mm in dimensions, well suited for a mobile application.

TILT - DATA ENTRY PROCESS

Fig. 1 Accelerometer placed flat surface of the

PCB.

As most tilt data entry applications can be accomplished in the +/-60˚ range. The user can enter data by physically tilting the device, observing the screen for visual feedback, and listening for audio feedback. The user can navigate through various tilt menus to select different modes of operation such as tilt scrolling operations (for navigation through call list and maps) and other specialized operations such as gaming, pedometer, motion alarm etc. Tilt mode of data entry is well suited for interface with smaller handheld devices but not


limited to them and can also be applied to larger devices.

ACCELEROMETER SPECIFIC FUNCTIONALITY Once an accelerometer type device is included in a cell phone type system, how it is used is completely dependant on the manufacturer making it. However, when once an accelerometer is incorporated in to the design, making the most use of it would be a way to increase the cost benefit from the accelerometer. The author of the note has attempted to put together a few conceptual ideas to provide the reader a better understanding of accelerometer supported input methods and perhaps instigate designers to incorporate some more genuine and novel ideas into their own applications. Introducing the Conceptual Cell phone The conceptual 3G-cell phone will have minimal number of keys, besides a few key to execute functionality such ‘enter’ (after the user has highlighted the function he/she wishes to select) and the power/on off switch. Figure 2 shows an artists concept of a new 3 G phone with very few keys. In addition to the advantages discussed before, the lack of keys also gives the opportunity to have a larger screen in the same face area of the cell phone. The larger screen displays more information at a faster pace to the user, thus increasing the quality of the interface, keeping the overall size of the cell phone in check.

Incremental Vs Continuous angular measurement Most applications in this note are divided into incremental and continuous measurement applications. Incremental measurement applications have fixed threshold levels between which no change occurs when the angular change is between the specified thresholds. Continuous angular measurement has no set thresholds and measures angle with a resolution of less than a degree. Therefore a slight change in orientation of the device would effect the operation in continuous angular measurement operation.

Fig2. Conceptual cell phone faceplate with no

keys


Navigating through menus (incremental tilt) Figure 3 shows an example tilt sensitive screen in a cell phone. The concept screen shows a few menu items but more can be added as needed. The user would simply navigate through the items simply by tilting the phone; the selected item would appear as a different color as the rest of the items. To execute the item the user would simply click one of the buttons on the side of the cell phone. This feature would be more an incremental tilt feature where the threshold would be approximately +/-15˚ of change in inclination, which would cause a correlated change in the menu item.

Fig. 3Concept cell phone with tilt sensitive menu navigation

Scrolling through number lists (incremental measurement) This is another example of incremental measurement, where +/-15˚ of inclination would cause the selected item to scroll through the list; the user can stop the selection by holding the phone level (or within +/-15˚ of angular inclination). Higher degrees of inclination could modulate the speed of the scroll through the list. The user can then tilt the cell phone left or right horizontally to make the call. The call activate thresholds can be set to higher levels (ex. +/-90˚).

Fig. 4 Concept cell phone tilt sensitive call list

navigation Dialing numbers not in call list (incremental measurement) To dial independent numbers (not in the call lists) the user can use the same approach earlier discussed for menu


selection. See fig.5. With incremental tilt movements the user can highlight a number and select it by clicking one of the ‘enter’ keys on the side of the cell phone. To place the call, the user must highlight the call key and click the enter button the side. The audio beeps and the clicking provide hap-tic and kinesthetic feedback to the user.

Fig.5 Concept cell phone showing Tilt-dial

Map scrolling (continuous tilt) One major usage of an accelerometer inside a cell phone would to navigate through web pages and maps. This will enable the user to seamlessly navigate through maps by the tilting the cell phone. When the cell phone is held level, the map would not move, but as one tilts the cell phone, the map would seemingly slide into that direction. The

slide speed would depend on the angle of tilt. Fig.6 shows a conceptual visualization of this particular kind of feature.

Fig. 6 Concept cell phone showing tilt map

feature


Gaming One other application of an accelerometer would be gaming. The games can be of traditional variety (button input type) or specially designed games, which use the accelerometer. The tilt effect would enhance tactile responses of the user. Fig. 7 shows such an application.

Fig. 7 tilt sensitive phones for games.

Special Applications Specialty applications can be developed for use of accelerometer functionality. Man-down applications, which presents the opportunity to sense absence of motion and sends a beacon for assistance for user, this would apply to the elderly,

people involved in hazardous occupation such as miners, policemen, firemen etc. Pedometer applications are newly developing, which use the accelerometer to measure speed and distance traveled by the user on foot. The measurements algorithms would depend on items like, accelerometer placement inside the cell phone, measurement accuracy etc. One simple way of measuring distance traveled is to count number of steps taken in a fixed amount of time and multiplying this value with the user's stride length. This can be done by implementation of a peak detection algorithm. Warranty detect algorithms are other innovative applications that are developing after the arrival of lower cost accelerometers. Essentially the handheld mobile device detects drops or harsh use. This information is valuable when the field failures are encountered and the mobile devices are returned.


Fig. 8 Various acceleration signatures during different types of movement activity

Fig. 8 displays different kinds of output signatures acquired from the accelerometer during different kinds of movement activity. Experiments can be performed to measure and characterize movements for other additional applications. Movement signatures will predominantly depend on accelerometer placement in the cell phone in addition to physical characteristic of the cell phone such as dimensions, weight etc.

ACCELEROMETER PLACEMENT Accelerometer placement inside the cell phone or the mobile device is an important decision; that will depend on intended usage of the accelerometer in various applications, physical dimensions of the accelerometer and axis positioning of the accelerometers. Fig. 9 displays two possible placements

of the accelerometer. Memsic accelerometers are dual axis accelerometers where the sensitive axes are parallel to the plane of placement of the IC. Therefore to achieve X and Y-axis sensitivity flat placement of a single accelerometer is sufficient. However to achieve complete X, Y and Z axis measurement, two dual axis accelerometers will be required with one placed on edge. Please refer to the Fig.9 for more details.


MemsicAccelerometer

+Y

-Y

-X

+X

Mem

sicac

celer

omet

er

Flat Placement

On edge placement for Z axis(daugter board)

-X

+Y

+X

Main PCB

Tilt Right

Tilt Left

Tilt top

Tilt Bottom

Fig. 9 Accelerometer placement


ACCELEROMETER SELECTION In order to select the right accelerometer for a cell phone application or any other handheld device, some considerations have to be kept in mind. Some considerations might be more important than others: Size Physical dimensions of the accelerometer package should be consistent with the other components used in the cell phone. Size of the accelerometer is limited by the actual real estate available on the cell phone PCB, which is usually very densely populated. Durability Durability and physical robustness are very important considerations when selecting an accelerometer in a hand held device. Despite the fact that handheld devices are very prone to being dropped quite frequently, more durable device also leads to lower ppm failures in assembly lines. An accelerometer’s internal structures, which are in the order of submicrons, can experience forces in excess of several thousands of g’s when dropped on a hard surface. In many capacitive type accelerometers available in the market today, this can lead to catastrophic failures. These failures are primarily due to movable microstructures that can fracture or stick at such g levels. Memsic is a pioneer and leader of thermal accelerometer technology that is devoid of any moving parts. Therefore Memsic devices are able to withstand 50,000 g’s of force (missile

firing). Thus without any moving parts Memsic accelerometers offer the very robust and reliable solution in the accelerometer market today. In addition, this durability allows the manufacturer to assembly the accelerometer IC just like any other IC in the system without taking extra precautionary measures. Special methods of assembly usually add to cost of the finished product. Cost Cost is another major factor that drives component selection in general for many consumer devices. Memsic uses a standard 0.6µm process and does not need any special micromachining technique to manufacture its sensor structure. This results in lower end costs. Memsic is one of the most cost effective accelerometer solutions in the market today. Power Consumption In a handheld portable device, lower power consumption is a key factor in the selection of devices to ensure a competitive battery life. However, a lot of this depends on how the designer decides to operate the device. Clever design methodologies can dramatically save power consumption and still provide seamless operation to the end user of the cell phone. Power cycling is one method of operating devices at lower power. Other methods of operation involve shutting down the device completely during standby more and to enable it in power cycling mode only when the user activates the device. Memsic is constantly pursuing the path to lower power devices. (Please contact Memsic for recent product announcements.)


Measurement Resolution Resolution is defined, as the smallest change in the output the system will display. For most human tilt applications a resolution of +/-1˚ of inclination is almost undetectable. The noise floor of the accelerometer device and resolution of the controller interfacing with the accelerometer limits the resolution. Filtering the output for smaller bandwidths improves the resolution of the device. Signal Bandwidth Human motion is usually under 10Hz. Tilt/scroll is in the range of 0.5-2hz; pedometer is around in the range of 7-10Hz. The bandwidth of Memsic devices is specified to 30hz, which is well over human motion limits. Please refer to the application notes on the website for more design assistance The Memsic application department can provide design assistance in addition to the application notes. MEMSIC INC. 800 Turnpike Street, Suite 202 North Andover, MA 01845 TEL : (978)738-0900 FAX : (978)738-0196 KEYWORDS: Tilt, scroll, accelerometer, angular measurement, text entry.

accelerometer supported tilt as an input method for mobile

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