Sensors and Methods for Mobile Robot Positioning

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Part I Sensors for Mobile Robot Positioning CARMEL, the University of Michigan's first mobile robot, has been in service since 1987. Since then, CARMEL has served as a reliable testbed for countless sensor systems. In the extra “shelf” underneath the robot is an 8086 XT compatible single-board computer that runs U of M's ultrasonic sensor firing algorithm. Since this code was written in 1987, the computer has been booting up and running from floppy disk. The program was written in FORTH and was never altered; should anything ever go wrong with the floppy, it will take a computer historian to recover the code... 12
CHAPTER 1 SENSORS FOR DEAD RECKONING Dead reckoning (derived from “deduced reckoning” of sailing days) is a simple mathematical procedure for determining the present location of a vessel by advancing some previous position through known course and velocity information over a given length of time [Dunlap and Shufeldt, 1972]. The vast majority of land-based mobile robotic systems in use today rely on dead reckoning to form the very backbone of their navigation strategy, and like their nautical counterparts, periodically null out accumulated errors with recurring “fixes” from assorted navigation aids. The most simplistic implementation of dead reckoning is sometimes termed odometry; the term implies vehicle displacement along the path of travel is directly derived from some onboard “odometer.” A common means of odometry instrumentation involves optical encoders directly coupled to the motor armatures or wheel axles. Since most mobile robots rely on some variation of wheeled locomotion, a basic understanding of sensors that accurately quantify angular position and velocity is an important prerequisite to further discussions of odometry. There are a number of different types of rotational displacement and velocity sensors in use today: & Brush encoders. & Potentiometers. & Synchros. & Resolvers. & Optical encoders. & Magnetic encoders. & Inductive encoders. & Capacitive encoders. A multitude of issues must be considered in choosing the appropriate device for a particular application. Avolio [1993] points out that over 17 million variations on rotary encoders are offered by one company alone. For mobile robot applications incremental and absolute optical encoders are the most popular type. We will discuss those in the following sections. 1.1 Optical Encoders The first optical encoders were developed in the mid-1940s by the Baldwin Piano Company for use as “tone wheels” that allowed electric organs to mimic other musical instruments [Agent, 1991]. Today’s corresponding devices basically embody a miniaturized version of the break-beam proximity sensor. A focused beam of light aimed at a matched photodetector is periodically interrupted by a coded opaque/transparent pattern on a rotating intermediate disk attached to the shaft of interest. The rotating disk may take the form of chrome on glass, etched metal, or photoplast such as Mylar [Henkel, 1987]. Relative to the more complex alternating-current resolvers, the straightforward encoding scheme and inherently digital output of the optical encoder results in a lowcost reliable package with good noise immunity.
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CHAPTER 5 ODOMETRY AND OTHER DEAD-R
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R e t r o r e f l e c t i v e m a r
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CHAPTER 8 MAP-BASED POSITIONING Map
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218 Appendices, References, Indexes
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220 Appendices, References, Indexes
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Systems-at-a-Glance Tables Odometry
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Systems-at-a-Glance Tables Landmark
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REFERENCES 1. Abidi, M. and Chandra
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238 References 31. Boltinghouse, S.
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240 References MOBOT-IV.” Proceed
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252 References 249. Sanders, G.A.,
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254 References 278. Turpin, D.R., 1
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256 References 309. EATON - Eaton-K
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