Conventional Polaroid time-of-flight (TOF) sonars provide object range measurements, but their wide beam widths limit the object bearing accuracy. We describe a first-order model that predicts the TOF readings from an object at a given range, bearing, and reflecting strength produced by the Polaroid 6500 series sonar ranging module connected to a 600 series electrostatic transducer. The model provides a simple method for solving the inverse problem of estimating these object parameters from conventional TOF data. Experimental results are given to demonstrate the model performance. Our model provides a reasonable fit for main-lobe data observed for objects with reflecting strengths that vary by 25 dB. An example of an inverse problem solution demonstrates the model can improve object range and bearing measurements, and can also estimate the object reflecting strength, a landmark feature useful for robot navigation tasks.

This paper analyzes a novel sonar mapping procedure that displays the amplitude of the echoes using the conventional Polaroid 6500 ranging module. By resetting the module immediately after an echo indication is observed, the number of indications produced by the module is related to the echo amplitude. We then generate pseudo-amplitude scan (PAS) sonar maps that display more information than conventional maps. The ranging module operation is described and verified experimentally. The improved information content of PAS sonar maps is illustrated by scanning simple objects having different reflecting strengths. Artifacts resulting from interactions between objects are discussed.

A novel sonar array for mobile robots is presented with applications to localization and mapping of indoor environments. The ultrasonic sensor localizes and classifies multiple targets in two dimensions to ranges of up to 8 meters. By accounting for effects of temperature and humidity, the system is accurate to within 1 mm and 6.1 degrees in still air. Targets separated by 10 mm can be discriminated. Targets are classified into planes, corners, edges and unknown, with the minimum of two transmitters and two receivers. A novel approach is that receivers are closely spaced to minimize the correspondence problem of associating echoes from multiple targets. A set of templates is generated for echoes to allow the optimal arrival time to be estimated, and overlapping echoes and disturbances to be rejected.>

A physical model-based analysis of unstructured environments is presented using sonar as a sensing device. Previous methods have relied only on time-of-flight (TOF) methods and have examined only homogenous environments consisting of either smooth or rough surfaces. In this paper, a forward model for the reflection from a class of surfaces with varying degrees of roughness is presented based on the Kirchhoff approximation method. This model integrates different types of environments into a single analytical framework. The echo intensity is parametrized in terms of its energy content and duration, which are functions of the surface roughness, distance, and orientation. The echo-energy and echo-duration maps are introduced to display these parameters. A systematic and robust procedure (ENDURA method) is presented to analyze the reflections and to differentiate and localize the reflecting surfaces. The methodology is verified with experimental results obtained in our laboratory. The results indicate a significant improvement over conventional TOF systems.>

The authors investigate the prey capture problem in an uncluttered environment for a mobile robot equipped with a wide-beam sonar system. An analytical lower bound for the mean capture time is derived by assuming complete information about the prey. Algorithms using qualitative and quantitative information are implemented for prey capture, and compared in terms of mean capture time and capture probability. Although qualitative information is sufficient, quantitative information assures more efficient prey capture to be achieved over a broader range of system parameters. Experimental results verifying the analysis and the simulations are provided.>

The problem of docking two mobile robots using a wide-beam sonar system is considered. Be- cause of the similarity to biological bats, the prob- lem is discussed in terms of prey capture in two di- mensions. The basics of the bat-like sonar system are described. Two measures of performance are consid- ered: the capture probability and the mean capture time when capture occurs. These measures are com- puted in two ways: as the ratio of speeds of the prey to the pursuer is varied and as the strategies employing either qualitative information (prey is to the left or right) or quantitative information (range and azimuth to prey) are employed. The lower bound for the mean capture time is determined from game theory, which assumes complete information about the prey. The analysis is verifled by performing experiments with a real mobile robot. Both capture probability and mean capture time are inversely related to the prey/pursuer speed ratio. It is also observed that, while qualitative information is sumcient for docking to occur, quan- titative information allows successful docking over a larger range of speed ratios.

An active wide-beam sonar system that mimics the sensor configuration of echolocating bats is described for applications in sensor-based robotics. Obstacles in a two-dimensional (2-D) environment are detected and localized using time-of-flight (TOF) measurements of their echoes. The standard threshold detector produces a biased TOF estimate. An unbiased TOF estimate is derived by a parametric fit to the echo waveform, motivated by experimental observations of actual sonar signals. This novel method forms a tradeoff between the complexity of the optimum estimator and the biased threshold detector. Using the TOF information from both methods, the range and azimuth of an obstacle are estimated. Localization is most accurate if the obstacle is located along the system line-of-sight and improves with decreasing range. Standard deviations of the range and azimuth estimators are compared to the Cramer-Rao lower bounds. The parabolic fit method has large variance but zero bias at large deviations from the line-of-sight. The system operation is generalized from isolated obstacles to extended obstacles.>

A sonar system that simulates the sensor configuration of echolocating bats is described. Two estimators for range and azimuth of an obstacle are developed, based on the received signal envelope. The standard threshold detector produces a biased time-of-flight (TOF) estimate. An unbiased TOF estimate is derived by using the echo waveform shape to estimate the object location. The estimates are most accurate if the obstacle is located along the line-of-sight, and improve with decreasing range.>

The reflection of wideband pulse signals from rough surfaces is analyzed. A Kirchoff approximation method is employed to describe scattering from rough surfaces, where the usual plane-wave assumption is relaxed to include spherically diverging waves. The signal intensity detected at a point is expressed as an inverse Fourier transform of the product of the frequency correlation function of the channel and the transmitted pulse power spectrum. Coherent and incoherent components of the received signal are determined. The theory is compared with experimental results obtained from the two rough surfaces using the Polaroid transducer operating in air. Good agreement is obtained over the range of angles of incidence from 0 degrees to 60 degrees .>

Conventional time-of-flight (TOF) systems generate sonar maps based on the range estimate obtained from the TOF measurement. This paper describes a novel characterization of sonar reflections from a single sensor based on the physical principles of reflection and scattering from surfaces. Considerable improvement over conventional TOF systems is achieved. Different types of environments are integrated into a single framework. A new parametrization is developed based on the energy content and duration of the echo. These parameters are functions of the surface roughness and orientation. Echo-energy and echo-duration maps are generated that allow the identification of smooth, moderately rough and rough surfaces.>

The physical properties of acoustic sensors are exploited to obtain information about the environment for sonar map building. A theoretical formulation for interpreting the sensor databases on the physical principles of acoustic propagation and reflection is presented. A characterization of the sonar scan that allows the differentiation of planes, corners, and edges in a specular environment is described. A single sensor mounted on an autonomous vehicle in a laboratory verifies the technique. The implications for sonar map building and the limitations of differentiating elements with one sensor are discussed.>

A multitransducer, pulse/echo-ranging system is described that differentiates corner and plane reflectors by exploiting the physical properties of sound propagation. The amplitudes and ranges of reflected signals for the different transmitter and receiver pairs are processed to determine whether the reflecting object is a plane or a right-angle corner. In addition, the angle of inclination of the reflector with respect to the transducer orientation can be measured. Reflected signal amplitude and range values, as functions of inclination angle, provide the motivation for the differentiation algorithm. A system using two Polaroid transducers is described that correctly discriminates between corners and planes for inclination angles within +or-10 degrees of the transducer orientation. The two-transducer system is extended to a multitransducer array, allowing the system to operate over an extended range. An analysis comparing processing effort to estimation accuracy is performed.>

A spatial sampling criterion for sonar systems that allows all obstacles within a given radius from the sensor to be detected is described. The environment considered is a two-dimensional floor plan that is extended into the third dimension, in which the scanning is performed in the horizontal plane. In this environment, edgelike reflectors, such as edges of doors or doorways, and oblique surfaces are the most difficult to detect. By considering the physics of sound propagation, the sonar scanning density required to detect these objects is determined. An experimental verification is included. The limitations of detecting objects with sonar in a more general environment are discussed. These results can be used to determine the necessary spacing in a transducer ring array and the maximum step size that a mobile robot can translate without danger of collision.>

A procedure to navigate a vehicle from a source to a destination through an unstructured environment using only a sonar sensor is described. The environment considered is a two-dimensional floor plan, consisting of line segments of arbitrary size that is extended into the third dimension. To ensure that a sonar-guided vehicle does not collide with an obstacle, it is necessary to consider an obstacle that produce the weakest echoes and determine the conditions under which these obstacles can be detected. In this environment, the smallest echo is produced by the line that defines the boundary between the two planes, or the edge; and distant edges become invisible. This physical principle then becomes the basis for the navigation strategy, indicating the necessary scanning pattern and maximum step size that guarantee that no collision will occur. The approach is illustrated with results produced by a vehicular robot equipped with a Polaroid sensor. A representative floor plan and the automatically determined path are shown; a blind alley was included to observe the behavior of the algorithm.>

This paper presents the results of an experimental study on the errors associated with determining the shape of a vocal tract from the acoustic output signal. A vocal tract model was constructed from plexiglas sections of constant thickness, each having a hole of a specified area, that closely approximated a lossless autoregressive (AR) system. A small wide-band speaker acted as the excitation source. Linear prediction procedures were applied to the observed signals to estimate the shape of the tube. These values were then compared to the actual values to determine the error. The results indicate the experimental and signal processing conditions required for the estimates of the vocal tract shape to approximate the actual shape.

A physically-based simulation model for a sonar navigation system is described, in which the reflecting elements consist of walls, corners and edges arranged in a floor plan. When the transducer is operating in the pulse/echo mode, each element that falls in the transducer field-of-view contributes to the reflected signal through its impulse response. The floor plan is represented as a two-dimensional grid of visibels, that indicate which elements are visible from a particular location. A visibel is represented by a word in computer memory, each bit of which is assigned to a particular element (corner, wall or edge) in the floor plan. The advantage of this conceptually simple approach is that it allows the transducer to move through the floor plan efficiently. This efficiency results because the visibel values are set prior to signal generation, thus allowing a variety of transducer types or transducer locations to be examined quickly. Examples of simulated sonar maps using the time-of-flight ranging system are shown for two floor plans, one simple and the second more complicated. The simulations are verified by comparing them with actual sonar maps obtained with the Polaroid sensor.

A computer model is described that combines concepts from the fields of acoustics, linear system theory, and digital signal processing to simulate an acoustic sensor navigation system using time-of-flight ranging. By separating the transmitter/receiver into separate components and assuming mirror-like reflectors, closed-form solutions for the reflections from corners, edges, and walls are determined as a function of transducer size, location, and orientation. A floor plan consisting of corners, walls, and edges is efficiently encoded to indicate which of these elements contribute to a particular pulse-echo response. Sonar maps produced by transducers having different resonant frequencies and transmitted pulse waveforms can then be simulated efficiently. Examples of simulated sonar maps of two floor plans illustrate the performance of the model. Actual sonar maps are presented to verify the simulation results.

This paper describes our approach to determining the shape of the human vocal tract from the acoustic speech signal. A digital filter model of the vocal tract, which is specified by the locations of six articulatory structures, was examined to determine the set of acoustic signals that can be synthesized. The synthetic acoustic signals were classified by the frequency locations of the first three formant peaks. A sensitivity analysis was performed to determine an appropriate quantization of the vocal tract parameters. A structured table was generated which allowed the vocal tract parameter values to be determined from the set of formant values. A human glide utterance, /ai/, was analyzed to determine the formant locations of contiguous data segments. Using these values, possible vocal tract shape candidates were found from the table. From the total set of candidates occuring over the duration of the utterance, the set of vocal tract shapes that resulted in the minimum parameter variance was determined. It is shown that this minimum variance set, corresponding to a minimum movement of the articulators, produces a reasonable vocal tract shape trajectory for the utterance.

The acoustic attenuation coefficient of most soft biological tissues has an approximately linear-with-frequency attenuation characteristic. The slope of the attenuation coefficient with frequency, denoted by β, has been observed to vary with the severity of liver disease. Two approaches for estimating the β value from reflected ultrasound signals are examined: the spectral-difference approach, which estimates β from the slope of the difference between log-spectra from two locations within the liver, and the spectral-shift approach, which estimates β from the downward shift experienced by the propagating pulse spectrum with penetration into the liver. This paper considers signals reflected from a small tissue region, defined by a cell measuring D by D centimeters in the plane of the sonogram, thus determining the feasibility of producing attenuation images. Lower bounds on the variance of the two β estimators are calculated by deriving maximum-likelihood estimators and by locating the tissue cell in the focal plane of the transducer. If W denotes the usable bandwidth in the reflected signals, the bounds are shown to be proportional to (WD)-3for β estimates calculated from individual reflected signals, and (WD)-4for estimates from all the signals reflected from the tissue region. With currently available technology, clinically useful results can be obtained for cell sizes measuring approximately 2.0 cm on a side.