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Top1. Introduction
Ultrasonic sensors have been widely used for distance measurement and obstacle avoidance because of their small size, low price and simple hardware interface. To obtain 360° panorama distance information, an ultrasonic ranging system consisting of multichannel ultrasonic sensors is (Moravec, 1988). One problem with such sensors in an ultrasonic ranging system operating in close proximity is the well-known crosstalk phenomenon, where one ultrasonic sensor receives echoes transmitted by another ultrasonic sensor (Meng, Yao, & Wu, 2006). Usually, the ultrasonic receiver cannot judge whether the received echo is created from its own transmission or not, so false time-of-flight (TOF) measurement often occurs. In order to avoid ultrasonic crosstalk, most ultrasonic ranging system triggered their sensors sequentially (Hori, Nishida, Kanade, & Akiyama, 2003). which limit the work efficiency of the ultrasonic ranging system.
To eliminate the ultrasonic crosstalk, some researchers adopted different code and modulation schemes constructing excitation sequence to assign each ultrasonic sensor a recognizable signature. Jörg and Berg (1998) first applied pseudorandom sequences to give each ultrasonic sensor a marker. A matched filter in the receiving circuit then identified the associated source sensor. Ureña, Mazo, García, Hernández, and Bueno (1999) used a 13-bit Barker code to construct their excitation sequences for ultrasonic sensors. While the available Barker codes limit their application. Golay codes (Hernández, Ureña, Hernanz, García, Mazo, Dérutin et al., 2003; Ding & Payne, 1990; Hernández, Ureña, García, Mazo, Hernanz, Dérutin, & Sérot, 2004; Alvarez, Ureña, Mazo, Hernandez, García, Jimenez, & Donato, 2004; Diaz, Ureña, Mazo, García, Bueno, & Hernández, 1999; Marziani, Ureña, Hernández, Mazo, Alvarez, García, & Donato, 2007) were applied in ultrasonic ranging system to avoid crosstalk and increase the signal to noise ratio (SNR). Chaotic codes (Fortuna, Frasca, & Rizzo, 2003; Yao, Meng, Li, & Lin, 2008) were also used to eliminate crosstalk in multichannel simultaneously triggered ultrasonic ranging System. Fortuna et al. (2003) exploited chaotic pulse position modulation (CPPM) to fire the ultrasonic sensor. Yao et al. (2008) proposed chaotic pulse position width modulation (CPPWM) excitation sequences as the transmission sequences of ultrasonic sensors. Nakahira, Kodama, Furuhashi, and Maeda (2005) used binary-coded frequency-shift keyed signals to drive ultrasonic sensors to eliminate crosstalk. Meng, Zhang, Liang, and Wang (2005) and Meng, Liang, and Li (2006) proposed pseudorandom frequency modulation and frequency-hopping pseudorandom pulse-width modulation sequences to excitation ultrasonic sensor, and the result of avoiding crosstalk was not good. The signals with linear frequency modulation (LFM) (Pollaskowski, 1994) were used as the transmission signals of ultrasonic sensors, which could avoid crosstalk. But the spectra of LFM excitation sequences are not totally matched to that of the ultrasonic ranging system. And the number of available LFM excitation sequences is limited because of the narrow bandwidth of the ultrasonic ranging system. In order to make the spectrum of excitation sequence match to that of the ultrasonic ranging system, Pollatowski and Ermert (1994) applied transmitter signals with a constant amplitude level and nonlinear frequency modulation. However, they did not research how to obtain the good correlation characteristics of the excitation sequences.