Ultrasonic TOF Systems

Ultrasonic TOF Systems
Ultrasonic TOF ranging is today the most common technique employed on indoor mobile robotics systems, primarily due to the ready availability of low-cost systems and their ease of interface. Over the past decade, much research has been conducted investigating applicability in such areas as world modeling and collision avoidance, position estimation, and motion detection. Several researchers have more recently begun to assess the effectiveness of ultrasonic sensors in exterior settings [Pletta et al., 1992; Langer and Thorpe, 1992; Pin and Watanabe, 1993; Hammond, 1994]. In the automotive industry, BMW now incorporates four piezo ceramic transducers (sealed in a membran for environmental protection) on both front and rear bumpers in its Park Distance Control system [Siuru, 1994]. A detailed discussion of ultrasonic sensors and their characteristics with regard to indoor mobile robot applications is given in [Jörg, 1994]. Two of the most popular commercially available ultrasonic ranging systems will be reviewed in the following sections. 4.1.1.1 Massa Products Ultrasonic Ranging Module Subsystems Massa Products Corporation, Hingham, MA, offers a full line of ultrasonic ranging subsystems with maximum detection ranges from 0.6 to 9.1 meters (2 to 30 ft) [MASSA]. The E-201B series sonar operates in the bistatic mode with separate transmit and receive transducers, either side by side for echo ranging or as an opposed pair for unambiguous distance measurement between two uniquely defined points. This latter configuration is sometimes used in ultrasonic position location systems and provides twice the effective operating range with respect to that advertised for conventional echo ranging. The E-220B series (see Figure 4.2) is designed for mono static (single transducer) operation but is otherwise functionally identical to the E-201B. Either version can be externally triggered on command, or internally triggered by a free-running oscillator at a repetition rate determined by an external resistor (see Figure 4.3).
Selected specifications for the four operating frequencies available in the E-220B series are listed in Table 4.1 below. A removable focusing horn is provided for the 26- and 40-kHz models that decreases the effective beam width (when installed) from 35 to 15 degrees. The horn must be in place to achieve the maximum listed range. Ultrasonic TOF Systems Ultrasonic TOF ranging is today the most common technique employed on indoor mobile robotics systems, primarily due to the ready availability of low-cost systems and their ease of interface. Over the past decade, much research has been conducted investigating applicability in such areas as world modeling and collision avoidance,position estimation, and motion detection. Several researchers have more recently begun to assess the effectiveness of ultrasonic sensors in exterior settings [Pletta et al., 1992; Langer and Thorpe, 1992; Pin and Watanabe, 1993; Hammond, 1994]. In the automotive industry,BMW now incorporates four piezo ceramic transducers (sealed in a membrane for environment alprotection) on both front and rear bumpers in its Park Distance Control system [Siuru, 1994]. A detailed discussion of ultrasonic sensors and their characteristics with regard to indoor mobile robot applications is given in [Jörg, 1994].Two of the most popular commercially available ultrasonic ranging systems will be reviewed in the following sections.4.1.1.1 Massa Products Ultrasonic Ranging Module Subsystems Massa Products Corporation, Hingham, MA, offers a full line of ultrasonic ranging subsystems with maximum detection ranges from 0.6 to 9.1 meters (2 to 30 ft) [MASSA]. The E-201B series sonar operates in the bistatic mode with separate transmit and receive transducers, either side by side forecho ranging or as an opposed pair for unambiguous distance measurement between two uniquely defined points. This latter configuration is sometimes used in ultrasonic position location systems and provides twice the effective operating range with respect to that advertised for conventional echo ranging. The E-220B series (see Figure 4.2) is designed for mono static (single transducer) operation but is otherwise functionally identical to the E-201B. Either version can be externally triggered on command, or internally triggered by a free-running oscillator at a repetition rate determined by an external resistor (see Figure 4.3).Selected specifications for the four operating frequencies available in the E-220B series are listed in Table 4.1 below. A removable focusing horn is provided for the 26- and 40-kHz models that decreases the effective beam width (when installed) from 35 to 15 degrees. The horn must be in placeto achieve the maximum listed range.

4.1.1.2 Polaroid Ultrasonic Ranging Modules

The Polaroid ranging module is an active TOF device developed for automatic camera focusing, which determines the range to target by measuring elapsed time between the transmission of an ultrasonic waveform and the detected echo [Biber et al., 1987, POLAROID]. This system is the most widely found in mobile robotics literature [Koenigsburg, 1982; Moravec and Elfes, 1985; Everett, 1985; Kim, 1986; Moravec, 1988; Elfes, 1989; Arkin, 1989;

Borenstein and Koren, 1990; 1991a; 1991b; 1995; Borenstein et al., 1995], and is representative of the general characteristics of such ranging devices. The most basic configuration consists of two fundamental components:

1) the ultrasonic transducer, and 2) the ranging module electronics. Polaroid offers OEM kits with two transducers and two ranging module circuit boards for less than $100 (see Figure 4.4).

A choice of transducer types is now available. In the original instrument-grade electrostatic version, a very thin metal diaphragm mounted on a machined backplate formed a capacitive transducer as illustrated in Figure 4.5 [POLAROID, 1991]. The system operates in the mono static transceiver mode so that only a single transducer is necessary to acquire range data. A smaller diameter electrostatic transducer (7000-series) has also been made available, developed for the Polaroid Spectra camera [POLAROID, 1987]. A more rugged piezoelectric (9000-series) environmental transducer for applications in severe environmental conditions including vibration is able to meet or exceed the SAE J1455 January 1988 specification for heavy duty trucks. Table 4.2 lists the technical specifications for the different Polaroid transducers.

The original Polaroid ranging module functioned by transmitting a chirp of four discrete frequencies at about of 50 kHz. The SN28827 module was later developed with reduced parts count, lower power consumption, and simplified computer interface requirements. This second-generation board transmits only a single frequency at 49.1 kHz. A third-generation board (6500 series) introduced in 1990 provided yet a further reduction in interface circuitry, with the ability to detect and report multiple echoes [Polaroid, 1990]. An Ultrasonic Ranging Developer’s Kit based on the Intel 80C196 microprocessor is now available for use with the 6500 series ranging module that allows software control of transmit frequency, pulse width, blanking time, amplifier gain, and maximum range [Polaroid, 1993]. The range of the Polaroid system runs from about 41 centimeters to 10.5 meters (1.33 ft to 35 ft). However, using custom circuitry suggested in [POLAROID, 1991] the minimum range can be reduced reliably to about 20 centimeters (8 in) [Borenstein et al., 1995]. The beam dispersion angle is approximately 30 degrees. A typical operating cycle is as follows.

1. The control circuitry fires the transducer and waits for indication that transmission has begun.

2. The receiver is blanked for a short period of time to prevent false detection due to ringing from residual transmit signals in the transducer.

3. The received signals are amplified with increased gain over time to compensate for the decrease in sound intensity with distance.

4. Returning echoes that exceed a fixed threshold value are recorded and the associated distances calculated from elapsed time.

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