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Kevin John Wang An Ultrasonic Compass for Context-Aware Mobile Applications MASSACHUSETTS INSTITUTE OF TECHNOLOGY An Ultrasonic Compass for Context-Aware Mobile Applications by Kevin John Wang MASSACHUSETTS INSTITUTE OF TECHNOLOGY June 2004 c Massachusetts Institute of Technology 2004. All rights reserved. 2 Abstract If we are to realize the everyday benefits promised by pervasive computing and contextaware applications, we must first develop the infrastructure to provide contextual location and orientation information through pervasive computing elements. I lay the foundations for leveraging the Cricket indoor location system to supply orientation information. I first characterize the use of ultrasound in Cricket for distance and orientation measurements. I then propose a set of methods to calculate 3-DOF orientation from an array of well placed ultrasonic sensors operating in the Cricket system. I design and implement a prototype of this Cricket Compass using a combination of hardware and software and demonstrate end-to-end functionality of the system. 3 4 Acknowledgments First, I thank Professor Seth Teller for giving me the opportunity and support to pursue this research. His enthusiasm and never-ending stream of ideas have been both educational and of great encouragement to me while under his supervision. I also thank the rest of the Cricket Team for making this entire endeavor possible. I especially thank Bodhi Priyantha for his help with everything Cricket and Compass related. I braved graduate student life together with my office-mates Roshan Baliga and Jonathan Wolfe. Thanks to both for making the lab fun, even during those times when it should not have been. Finally, I am deeply grateful to my family for always being there. Without them I would have never have made it this far. 5 6 Contents 1 Introduction 1.1 Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2 The Design Space . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.3 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 14 16 18 2 Steps Towards Orienting the Cricket Compass 2.1 Existing Orientation Systems . . . . . . . . . . 2.1.1 Constellation . . . . . . . . . . . . . . . 2.1.2 HiBall . . . . . . . . . . . . . . . . . . . 2.1.3 Whisper . . . . . . . . . . . . . . . . . . 2.1.4 Commercial Systems . . . . . . . . . . . 2.2 Groundwork for the Cricket Compass . . . . . . 2.2.1 Calculating Planar Orientation . . . . . 2.3 Challenges in Orienting the Cricket Compass . 2.3.1 Characterizing Cricket Ultrasound . . . 2.3.2 The Differential Distance Problem . . . 2.3.3 Disambiguating θ . . . . . . . . . . . . . 2.3.4 Implementation Experience . . . . . . . . . . . . . . . . . . . 19 19 20 20 21 21 22 22 24 25 28 31 32 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 Theory of Operation 3.1 Cricket Infrastructure . . . . . . . . . . . 3.2 Three-Dimensional Local Orientation . . . 3.3 Obtaining Accurate Differential Distances 3.4 Registration to Global Orientation . . . . 3.4.1 Centroid Relative Coordinates . . 3.4.2 Rotation of the Planes . . . . . . . 3.4.3 Rotation in the Plane . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37 37 38 42 42 43 44 45 4 The Cricket Compass Prototype 4.1 Hardware Design Parameters . . . . 4.1.1 Sensor Array . . . . . . . . . 4.1.2 Analog-to-Digital Conversion 4.2 Software Design Parameters . . . . . 4.3 Testing and Modifications . . . . . . 4.3.1 Correlation . . . . . . . . . . 4.3.2 Filtering . . . . . . . . . . . . 4.3.3 Pulse Shaping . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47 48 48 51 52 52 53 55 56 7 . . . . . . . . . . . . . . . . . . . . . . . . 4.3.4 Error Detection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 Results 5.1 Localizing Beacons . . . . . 5.1.1 Setup and Procedure 5.1.2 Analysis . . . . . . . 5.2 End-to-End Orientation . . 5.2.1 Setup and Procedure 5.2.2 Analysis . . . . . . . 5.3 Future Work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59 61 61 61 63 65 66 67 69 6 Contributions 71 A Compass Hardware Design A.1 Analog Ultrasound Gain Circuit . . . . . . . . . . . . . . . . . . . . . . . . A.2 Sensor Array Specifications and Naming . . . . . . . . . . . . . . . . . . . . A.3 Murata MA40S4R Ultrasonic Sensor Information . . . . . . . . . . . . . . . 73 73 74 75 B Compass Functionality B.1 Setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . B.2 Demonstration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . B.3 MATLAB functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77 77 77 78 8 List of Figures 1-1 Block diagram overview of the Cricket Compass. . . . . . . . . . . . . . . . 17 2-1 Determining the angle of orientation along the horizontal plane. . . . . . . . 2-2 A rotated compass leads to a difference in distances between the beacon and each of the receivers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-3 An amplified ultrasound pulse on a Cricket listener. . . . . . . . . . . . . . 2-4 A photo of the version 2 Cricket, which can function as a listener or a beacon. 2-5 Receivers R1 and R2 can measure the differential distance from a far-away beacon. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-6 An observed phase ∆ can actually correspond to an infinite number of possible real phases, all separated by 2π. . . . . . . . . . . . . . . . . . . . . . . 2-7 θ is ambiguous; there are two beacon positions B1 , B2 that result in the same θ at the compass. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-8 Two ultrasound receivers mounted on a precision rotating platform. . . . . 2-9 A comparison of ultrasound receiver compass performance. . . . . . . . . . 23 3-1 Determining the angle θ between the vector to the beacon and the axis formed by the receiver pair. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-2 The intersection of two circles presents two possible beacon positions. . . . 3-3 Determining the position of a beacon using an array of receivers. . . . . . . 3-4 The second rotation step in determining absolute orientation. . . . . . . . . 4-1 4-2 4-3 4-4 4-5 4-6 4-7 4-8 4-9 4-10 A block diagram of the Cricket Compass implementation. . . . . . . . . . . A few possible ultrasound sensor array geometries. . . . . . . . . . . . . . . A comparison of sensor pair coverage for different geometries. . . . . . . . . A photo of the actual compass hardware prototype. . . . . . . . . . . . . . . A plot of four ultrasound waveforms coming from the Compass hardware. . A plot of the six inter-sensor normalized cross correlations for varying time delays. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A plot of error in calculated orientation using two different methods for the differential distance measurement. . . . . . . . . . . . . . . . . . . . . . . . Plots of different types of ultrasound signals and resulting normalized cross correlations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The standard Cricket ultrasound pulse and the modified pulse for the Compass. A visualization of the differential distances on sensor pairs. . . . . . . . . . 5-1 Annotated photos of beacon localization experiment. . . . . . . . . . . . . . 5-2 Illustration of Compass rotation in the beacon localization experiment. . . . 5-3 A plot of beacon localization using the Compass. . . . . . . . . . . . . . . . 9 24 26 27 29 30 31 32 34 38 39 40 45 47 49 50 51 53 54 55 57 58 59 62 63 64