Abstract— This paper presents an overview of Sonar and how it can be used in mobile robots. The use of sonar in mobile robots is compared to ultrasonic sensing capabilities of a bat. The concepts are similar, but the use of sonar is much simpler. Even though sonar is simple to use, there are still several disadvantages. These disadvantages include: poor directional resolution due to the wide beam used; slow sound speed reduces sensing rate; obstacles with smooth surfaces appear invisible to the mobile robot; non-existing objects are detected due to reflections produced by the sonar; the environment can cause fluctuations in the travel time of the echo.
Sonar equations are used in the prediction of performance and sonar design. These equations are used in a number of different applications. Sonar has many different uses in mobile robotics including: distance measurement, localization, positioning, and mapping and navigation.
Index Terms— mobile robots, sonar, sound detection, mapping, localization, navigation, bats, obstacle avoidance
I.
I NTRODUCTION
One of the earliest references to the fact that sound exists beneath the surface of the sea occurred in one of Leonardo da
Vinci’s notebooks. In 1490, Leonardo, an archetypal engineer, wrote: “If you cause your ship to stop, and place the head of a long tube in the water and place the outer extremity to your ear, you will hear ships at a great distance from you.” This example has all the essential elements of a passive sonar system [1, 2].
The first practical application of sonar was the submarine bell, which was used for offshore navigation, in the early
1900s. This system led to the first commercial manufacturer of sonar equipment in the United States, the Submarine Signal
Company [1]. Sonar is a form of echo location, the detection and location of objects by the echoes they give off. There are many practical devices that have been created using this technology- navigation, weather forecasting, geophysical research, military weaponry, and even fishing [3]. The term
Sonar stands for Sound Navigation and Ranging, and was initially developed for under water applications [1].
There are two different types of sonar sets: active and passive. Typically, active and passive sonar systems have been used for military intelligence applications in marine environments. An active sonar set sends out sound pulses, or pings, and then receives the returning echoed sound. Active sonar systems are used for communications, navigation, detection, and tracking. Passive sonar sets receive pings without transmitting their own sound signals and are used in surveillance [1, 10]. Sonar has a rate of data acquisition of
343.2 meters per second at 20 degrees Celsius, or the speed of sound [4].
II.
With the decreasing costs of optical rangefinders is sonar still a reliable and useful source for mobile robotics? One motivation for using sonar in mobile robot navigation comes from the impressive ultrasonic sensing capabilities of bats, which rely on echolocation to determine their position and to hunt their prey [6]. Bats use frequency modulation, or
Doppler, and other ultrasonic ranging techniques that are far more sophisticated than the simple time-of-flight (TOF) ranging. However time-of-flight sonar can fulfill the perception role for autonomous navigation, in a large class of application environments. Time-of-flight sonar has great potential in typical indoor environments because they contain surprisingly few acoustic targets. Therefore, the task of achieving and maintaining correspondence between observations and map targets will be considerably easier in acoustic sensing. Also Mobile Robots tend to exploit simpler strategies in sonar sensors versus the more sophisticated systems that bats use for navigation. The simplest and most common of these is to use the same physical transducer as both the emitter and transmitter for a short sound pulse, and to measure the time delay until the echo is received [5]. Another selling point for sonar can use the extended Kalman Filter
(EKF) to directly observe the corners and edges that populate many environments and estimate their locations directly [4].
III.
D
ADVANTAGES OF SONAR
ISADVANTAGES OF SONAR
Despite having its advantages, sonar also have its limitations that would not be useful in all applications of mobile robots.
The major limitations of sonar are: that the wide beam used in sonar causes poor directional resolution, which can lead to limitation in mobile robot navigation; the slow sound speed, in comparison to optical sensors, reduces the sonar sensing rate; detectable echoes are not produced smooth surfaces at oblique incidences, making the obstacle invisible to the mobile robot using sonar for obstacle avoidance; non-existing objects can be improperly detected by the robot because of multiple reflections being produced and beam side-lobes; lastly random fluctuations can be detected in the echo travel time, depending on the robots environment, which can cause minor changes made to the sonar map [15].
1

2
The sonar equations were first formulated during World
War II as the logical basis for calculations of the maximum range of sonar equipments [7]. The simple sonar equations serve two important practical functions in sonar systems: prediction of performance and sonar design. In the prediction of performance function the design characteristics of the sonar set are known or assumed, and what is desired is an estimate of performance [1]. In the sonar design case, a pre-established range is required for the operation of the equipment being design. The sonar equations can be used to solve a number of different applications in sonar including, but not limited to: active and passive submarine detection, minesweeping and mine hunting, depth sounding, fish finding, echo ranging, and communication [1, 8].
V.
IV.
S ONAR EQUATIONS
U SES IN MOBILE ROBOTS
With advancements of sonar since the beginning of their existence, sonar systems have developed in to sophisticated systems that can be use in mobile robots for various tasks.
Some specific applications of sonar sensors in robotics include simple distance measurements [11], localizing object surfaces
[12], determining the position of a robot in a given environment, some ad hoc navigation schemes [13], and mapping and navigation for autonomous mobile robots [14].
One common use of sonar in mobile robots is for the
“stolen” robot problem – a situation where the robot must determine its position and orientation independent of any previous movements or location. The ability for a mobile robot to do this is called Localization. Localization is the direct measurement of vehicle position and is useful in overcoming the errors encountered from dead reckoning, which deduces the vehicle’s position from its velocity history. Therefore using localization can be used for safe and reliable mobile robot navigation [9].
Mapping and navigation is typically done in unknown and unstructured environments – the system has no a priori map of its surroundings [14]. Sonar mapping is a collection of echoes acquired by performing a rotational scan or from a sonar array used to construct a map of the environment [15].To do this a set of sonar sensors are used to build sonar maps of the robot’s operating environment. From these maps several things can be achieved; path planning, obstacle avoidance, motion solving, and landmark recognition [14].
VI.
C ONCLUSION
Sonar is the location of objects by using sound echoes. The first types of sonar were developed in the early 1900s and have grown to become very sophisticated systems. As noted there are many practical uses for sonar in mobile robots: the “stolen” robot, mapping, obstacle avoidance, and navigation; but it still has its limitations. Sonar equations can be very valuable in solving different application types of sonar systems. While this paper presents some uses of sonar in mobile robots, there are many other practical uses of sonar in commercial, industrial, and military environments.
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Blake Bright (M’08) was born on September 12, 1987. Blake will be receiving his bachelor degree in electrical engineering from Auburn
University in December 2011.
He will begin his career as an Engineer III with Alabama Power Company,
Transmission Maintenance department in Birmingham, Alabama. He has worked for Southern Company Services has as Co-Op in the Energy
Management and Bulk Power Operations departments; along with Praestare
Engineering as a Co-Op.
Mr. Bright is a member of Eta Kappa Nu (HKN) and has been a recipient of the Auburn University Board of Trustees Scholarship twice.
Matt Curl (M'10) was born on November 9, 1987. Matt will be receiving his bachelor degree in electrical engineering from Auburn University in
December 2011.
He will begin his career as a Systems Engineer with Tennessee Valley
Authority, Combustion Turbine Engineering department in Muscle Shoals,
AL. He has also worked as an Intern in this department.
Michael Elswick was born on April 4, 1989. Michael will be receiving his bachelor degree in electrical engineering from Auburn University in
December 2011.
He previously worked in the Design Field for Southern Natural Gas as a
Electrical Co-Op.