Where am I? Sensors and Methods for Mobile Robot Positioning

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6 42 Part I Sensors for Mobile Robot Positioning In summary, the open-loop IFOG is attractive from the standpoint of reduced manufacturing costs. Additional advantages include high tolerance to shock and vibration, insensitivity to gravity effects, quick start-up, and good sensitivity in terms of bias drift rate and the random walk coefficient. Coil geometry is not critical, and no path length control is needed. Some disadvantages are that a long optical cable is required, dynamic range is limited with respect to active ring-laser gyros, and the scale factor is prone to vary [Adrian, 1991]. Open-loop configurations are therefore most suited to the needs of low-cost systems in applications that require relatively low accuracy (i.e., automobile navigation). For applications demanding higher accuracy, such as aircraft navigation (0.01 to 0.001(/hr), the closed-loop IFOG to be discussed in the next section offers significant promise. 2.3.4 Closed-Loop Interferometric Fiber Optic Gyros This new implementation of a fiber-optic gyro provides feedback to a frequency or phase shifting element. The use of feedback results in the cancellation of the rotationally induced Sagnac phase shift. However, closed-loop digital signal processing is considerably more complex than the analog signal processing employed on open-loop IFOG configurations [Adrian, 1991]. Nonetheless, it now seems that the additional complexity is justified by the improved stability of the gyro: closed-loop IFOGs are now under development with drifts in the 0.001 to 0.01(/hr range, and scale-factor stabilities greater than 100 ppm (parts per million) [Adrian, 1991]. 2.3.5 Resonant Fiber Optic Gyros The resonant fiber optic gyro (RFOG) evolved as a solid-state derivative of the passive ring resonator gyro discussed in Section 2.1.2.2. In the solid-state implementation, a passive resonant cavity is formed from a multi-turn closed loop of optical fiber. An input coupler provides a means for injecting frequency-modulated light from a laser source into the resonant loop in both the clockwise and counterclockwise directions. As the frequency of the modulated light passes through a value such that the perimeter of the loop precisely matches an integral number of wavelengths at that frequency, input energy is strongly coupled into the loop [Sanders, 1992]. In the absence of loop rotation, maximum coupling for both beam directions occurs in a sharp peak centered at this resonant frequency. If the loop is caused to rotate in the clockwise direction, of course, the Sagnac effect causes the perceived loop perimeter to lengthen for the clockwise-traveling beam, and to shorten for the counterclockwise-traveling beam. The resonant frequencies must shift accordingly, and as a result, energy is coupled into the loop at two different frequencies and directions during each cycle of the sinusoidal FM sweep. An output coupler samples the intensity of the energy in the loop by passing a percentage of the two counter-rotating beams to their respective detectors. The demodulated output from these detectors will show resonance peaks, separated by a frequency difference f given by the following [Sanders, 1992]: f = D (2.8) n where f = frequency difference between counter-propagating beams D = diameter of the resonant loop
Chapter 2: Heading Sensors 43 6 = rotational velocity = freespace wavelength of laser n = refractive index of the fiber. Like the IFOG, the all-solid-state RFOG is attractive from the standpoint of high reliability, long life, quick start-up, and light weight. The principle advantage of the RFOG, however, is that it requires significantly less fiber (from 10 to 100 times less) in the sensing coil than the IFOG configuration, while achieving the same shot-noise-limited performance [Sanders, 1992]. Sanders attributes this to the fact that light traverses the sensing loop multiple times, as opposed to once in the IFOG counterpart. On the down side are the requirements for a highly coherent source and extremely low-loss fiber components [Adrian, 1991]. 2.3.6 Commercially Available Optical Gyroscopes Only recently have optical fiber gyros become commercially available at a price that is suitable for mobile robot applications. In this section we introduce two such systems. 2.3.6.1 The Andrew “Autogyro" Andrew Corp. [ANDREW] offers the low-cost Autogyro, shown in Figure 2.11, for terrestrial navigation. It is a single-axis interferometric fiber-optic gyroscope (see Sec. 2.1.2.3) based on polarization-maintaining fiber and precision fiber-optic gyroscope technology. Model 3ARG-A ($950) comes with an analog output, while model 3ARG-D ($1,100) has an RS-232 output for connection to a computer. Technical specifications for the 3ARG-D are given in Table 2.1. Specifications for the 3ARG-A are similar. A more detailed discussion of the Autogyro is given Table 2.1: Selected specifications for the Andrew Autogyro Model 3ARG-D. (Courtesy of [Andrew Corp].) Parameter Value Units Input rotation rate ±100 (/s Minimum detectable rotation rate ±0.05 ±180 (/s (/hr Rate bandwidth 100 Hz Bias drift (at stabilized temperature) — RMS Size (excluding connector) 0.005 18 77 dia × 88 3.0 dia × 3.5 Weight (total) 0.63 1.38 Power 9 to 18 630 (/s rms (/hr rms mm in kg lb VDC mA Figure 2.11: The Andrew Autogyro Model 3ARG. (Courtesy of [Andrew Corp].)
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CHAPTER 5 ODOMETRY AND OTHER DEAD-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 Global N
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Systems-at-a Glance Tables Beacon N
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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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258 References 349. SPSi - Spatial
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