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Rev. 0.1 1/15 Copyright © 2015 by Silicon Laboratories AN580 AN580 INFRARED G ESTURE S ENSING 1. Introduction Touchless user interfaces are an emerging trend in embedded electronics as product designers seek out innovative control methods and more intu itive ways for users to interact with electronics. Active infrared proximity motion sensing can solve this challeng e. Silicon Labs Si114x proximity and ambient light sensor products are ideally suited to touchless gesturing ap plications such as page turning on an e-reader, scrolling on a tablet PC, or GUI navigation. The Si114x features up to three LED drivers and ha s the ability to sense gestures within a 7 to 15 cm product interaction region, assuming a hand as th e detectable object. This document will discuss in detail how Silicon Labs implements motion se nsing using infrared technology. Ther e are two primary methods used for gesture sensing – position-based and phase-based. Position-based gesture sensing involves finding gestures based on the calculated location of an object while phas e-based gesture sensing is based on the timing of the changes in signal to determine the direction of an object’s motion. 2. Hardware Considerations This application note focuses on detecting gestures made by a user’s hand. It is important to recognize that the concepts introduced in this application note can be applied to targets other than the hand, as long as the hardware is designed appropriately. The end application and i ndividual system constraints will each dictate the range requirements for IR gesture sensing. Since object refl ectance is the main measurable component for touchless gesturing, a hand is presumed to be the detectable object for the examples in this document. Whereas a hand can achieve gesture sensing up to 15 cm away from the Si114x sensor, a finger, with dramatically lower reflectance, can achieve gesture sensing at a range of < 1 cm for thumb-scroll type applications. The general guideline for designing a gesture sensing system with multiple LEDs is to make sure that there is no “dead spot” in the middle of the detectable area. When a target is placed above the system and is not detected, the target is in a reflectivity dead spot . To avoid dead spots in the system, the LEDs must be placed such that the emitted infrared light can reflect off the target and onto the sensor from the desired detection range. Figure 1 shows systems designed to detect a hand or a finger. The most susceptible area for a dead spot is directly above the sensor in between the two LEDs. The two LEDs are plac ed as close to the edge of the target as possible to provide feedback in the middle while also keeping enough distance between the LEDs so that it can be detected when the finger or hand moves to the left or right.

Figure 1. Ideal Hardware Formations—Hand versus Finger Detection between the fingers, and the shape/curve of the outline of the fingers also makes for unpredictable measurements. For the finger detecting system, the tip of the finger is curved and reflects less light than the middle of the finger. Si114x Proximity Applications”. counts also mean that there is a similarly reflective hand approximately 10 cm away from the system. will need its own counts to distance equation.

over the sensing device and IR LED 1, the measured feedback will correlate to a distance D1 above the system. The same is true for Target 2, IR LED 2, and D2. Figure 2. Distance Approximation

Figure 4. Intersection of Two Circles Implementation possible and will mean that the target is on the left side of P1. gesture can be considered a pause gesture. Designs use a quarter of a second for the gesture time requirement.

three-LED system with a hand about to swipe over the system. Figure 5. Swiping Left over a 3-LED System LED’s measurement and record a timestamp for this as well. First D2 will return back to normal, then D3, then D1. hand approached from directly above (Z-axis) and was retracted to indicate a “select” gesture. LEDs at the same time when swiping up or down.

Figure 6. Gesture Data

8 Rev. 0.1 3. Position-based Method Advantages and Drawbacks The advantage of the position-based method over phas e-based is that the position-based method can offer information on the location of the target. This will allow for ratiometric co ntrol of systems. For example, to scroll through a book several pages you could suspend your hand over the right side of the detectable area instead of having to make several right swipe gestures. The main drawback of the position-based algorithm is the accuracy of the position calculations. The positioning algorithm assumes a spherical output from the LEDs, but in practice LED output is more conical than spherical. The algorithm also assumes uniform light intensity across the entire output of the LED, but the light intensity decays away from the normal. Another issue is that this algorithm does not account for the shape of the target. A target that is uniquely shaped will cause inconsistencies with the po sitioning output. For example, the system cannot tell the difference between the hand and the wrist, so any gestures involving movement that puts the wrist in the area of detection will be less accurately located. The result is that the positioning information provided in this algorithm is good enough for low resolution systems that only need a 3x3 grid of detection, but the curr ent positioning algorithm is not well-suited for a pointing application. This algorithm’s output is not an ideal touchscreen replacement. 4. Phase-based Method Advantages and Drawbacks For applications not requiring position information, th e phase-based method provides a very robust way of detecting gestures. Each gesture can be detected on either the entry or exit from the detectable area, and the entry and exit can be double-checked with each other to provide much higher certainty for each gesture observed. The drawback of this method when compared with the posi tion-based method is that no positioning information is provided. This means that the number of gestures that can be implemented are more limited than the position- based method. The phase-based method can only tell the di rection of entry and exit from the detectable area so any movement in the middle of the detectable area is not detected. 5. Improved Gestures through Combination of the Two Methods These two methods can be implemented alongside one an other to help mask the other’s deficiencies. The position-based algorithm can provide some positional information for ratiometric control, and the phase-based algorithm can be used for detection of most gestures. These two algorithms working together can provide a strong solution for gesture sensing, but the drawback here is the code space requirements of implementing two separate algorithms as well as the CPU cycles to process both algorithms at all times.

Rev. 0.1 9 NOTES:

Silicon Laboratories intends to provide customers with the latest, accurate, and in-depth documentation of all peripherals and modules available for system and software implementers using or intending to use the Silicon Laboratories products. Characterization data, available modules and peripherals, memory sizes and memory addresses refer to each specific device, and "Typical" parameters provided can and do vary in different applications. Application examples described herein are for illustrative purposes only. Silicon Laboratories reserves the right to make changes without further notice and limitation to product information, specifications, and descriptions herein, and does not give warranties as to the accuracy or completeness of the included information. Silicon Laboratories shall have no liability for the consequences of use of the information supplied herein. This document does not imply or express copyright licenses granted hereunder to design or fabricate any integrated circuits. The products must not be used within any Life Support System without the specific written consent of Silicon Laboratories. A "Life Support System" is any product or system intended to support or sustain life and/or health, which, if it fails, can be reasonably expected to result in significant personal injury or death. Silicon Laboratories products are generally not intended for military applications. Silicon Laboratories products shall under no circumstances be used in weapons of mass destruction including (but not limited to) nuclear, biological or chemical weapons, or missiles capable of delivering such weapons. Trademark Information Silicon Laboratories Inc., Silicon Laboratories, Silicon Labs, SiLabs and the Silicon Labs logo, CMEMS®, EFM, EFM32, EFR, Energy Micro, Energy Micro logo and combinations thereof, "the world’s most energy friendly microcontrollers", Ember®, EZLink®, EZMac®, EZRadio®, EZRadioPRO®, DSPLL®, ISOmodem ®, Precision32®, ProSLIC®, SiPHY®, USBXpress® and others are trademarks or registered trademarks of Silicon Laboratories Inc. ARM, CORTEX, Cortex-M3 and THUMB are trademarks or registered trademarks of ARM Holdings. Keil is a registered trademark of ARM Limited. All other products or brand names mentioned herein are trademarks of their respective holders. http://www.silabs.com Silicon Laboratories Inc.

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