Design of an ultrasound-based sensory system for environment inspection robots Emir Sokic, Mina Ferizbegovic, Jasmina Zubaca, Kenan Softic, Melita Ahic-Djokic Department of Automatic Control and Electronics Faculty of Electrical Engineering, University of Sarajevo Sarajevo, Bosnia and Herzegovina esokic@etf.unsa.ba Abstract—Many robotic systems rely on infrared sensors, lasers, cameras and/or ultrasonic transducers for perceiving their environment. Most of these sensors can easily determine the distances to the surrounding objects, and even their shape. However, they are often unable to discriminate among dif- ferent nearby-placed objects, obstacles, materials or surfaces. This paper presents the design and development of a low-cost ultrasonic-based sensory system, which is able to exploit the information contained in the magnitudes of the reflected sound waves. Therefore, the common ultrasonic distance measurement is complemented with the value of the acoustic reflection coeffi- cient of the observed object. The estimated reflection coefficient facilitates the classification of different materials. Experiments are conducted to demonstrate solid performance of the proposed sensory system. Keywords—Ultrasound; Sensor; Distance; Material; Reflec- tion; Robotics I. I NTRODUCTION Nowadays, significant effort is put into the development of the robotic systems that are able to autonomously navigate in unknown environments, perceive their surroundings, and deter- mine their location [1]. Autonomous navigation and planning is equally important in the field of mobile robotics (unmanned underwater, ground and aerial vehicles) as in industrial en- vironments [2]. In all autonomous robotics applications, the correct and precise sensing of the environment is of paramount importance. However, accurate measurements often come with great expense, especially when the number of needed sensors increases (e.g. in swarm robotics [3]). Most of the systems used for non-invasive environment sensing in robotics are based on one of the following types of sensors: optical/photo-sensors (usually infrared), lasers, cam- eras and ultrasonic transducers. Optical sensors (see e.g. [4]) are very reliable, simple to use, economic, but consequently very sensitive to environmental conditions (rain, fog, dust, etc.). Laser based systems are extremely accurate, but they are often very expensive [5], bulky and power demanding, which is not suitable for larger scale distributed mobile robotics systems. Perceiving the environment using the camera and image processing is very popular [6]. However, in cases of low visibility or a monochromatic environment, the usefulness of a camera is significantly reduced. Ultrasonic transducers are widely used in mobile robotics for distance measurements [7]. Moreover, they are characterized by solid performance and satisfactory accuracy. Ultrasonic transducers can also be used for flow measurements [8], material inspection [9], medical imaging [10], etc., ranging from very inexpensive to extremely expensive off-the-shelf solutions. A robotic system often needs to discriminate among differ- ent types of materials and objects. Low-cost variants of the previously introduced sensors are generally unable to perform these classification tasks. To this end, we design a sensory system which exploits the magnitude of the reflected ultra- sonic wave received by the inexpensive transducer to obtain additional information about the environment. The proposed ultrasound-based sensory system is able to discriminate among different observed materials. Experimental results indicate that the system can be used in practical applications. The paper is organized as follows. Section 2 explains the theoretical foundations upon which the system is designed. The implementation details are given in Section 3. Main experimental results are presented in Section 4, while the conclusion and guidelines for future work are given in the last section. II. PRINCIPLES OF OPERATION The main structure of the proposed measurement system is depicted in Fig. 1. The sensory system is based on two piezoelectric ultrasonic transceivers located in close vicinity. They are used to transmit and receive ultrasonic sound waves. The time t elapsed while the sound wave has traveled from the transmitter, reflected from the obstacle, and was picked up by the receiver is used to compute the distance d to the obstacle: d = 1 2 ct, (1) where c is the speed of sound in air. The speed c depends on many factors, such as temperature, humidity, density, pressure etc. The relation c = 331.4+0.6T [m/s] was used to compute sound speed [11], where T is the ambient temperature. The magnitude of the reflected sound wave will be different for various obstacles on the same distance. Two parameters