ARTIGO TÉCNICO Daniel Albuquerque, José Vieira and Carlos Bastos Department of Electronics, Telecommunications and Informatics University of Aveiro, Portugal dfa@ua.pt, jnvieira@ua.pt, cbastos@ua.pt
ROOM ACOUSTICS SIMULATOR FOR ULTRASONIC ROBOT LOCATION ABSTRACT In this paper we present a room acoustics simulator based on an hybrid method. This method considers all the wave reflections as specular which is a good approximation to the way the sound waves propagate in a closed space, when the wave length is much smaller than the obstacles. Although other acoustic simulators are available, they are difficult to modify. Building this simulator from the beginning, will allow complete control and understanding of its behaviour. It has a modular structure, to facilitate future expansion of the physical model. The model was implemented in Matlab code, so that it can be easily modified. This simulator was developed in order to have a controlled environment to test the performance of ultrasonic location systems. The present version of the simulator is simple and includes wall reflections, sound attenuation and the transducer beam characteristics.
I. INTRODUCTION Location systems is an active research area with many applications. The Global Positioning System (GPS) is presently, the most successful location system and it is widely used in civilian and military applications. However, the performance of the GPS degrades inside buildings and the accuracy is not enough for indoor location. Ultrasonic based location systems can be a viable low cost solution for most applications. In certain environments, such as hospitals, ultrasonic location systems present several advantages and have become successful commercial products. However, ultrasound propagation in closed spaces, presents problems similar to those encountered in electromagnetic waves propagation, such as multipath, fading, etc. A good model of the acoustic channel is needed to allow the test and comparison of ultrasonic indoor location systems. It is very hard and expensive to make all the tests in real environments, even a simple simulator of the acoustic channel can be a great help to reduce the development time of an ultrasonic system. The system needs to pass all the tests in the simulator before being implemented and tested with real hardware. In this paper we present an acoustic simulator that uses an hybrid method. For a certain configuration of the transducers, the simulator evaluates the impulse response of the room acoustics. This way, it is possible to test the influence of the room acoustics on a ultrasonic location system. This paper is organised as follows, in the next section we present a brief description of a previously developed location system using ultrasounds. Then, we describe of the acoustic channel simulator and the characteristics included in the model. Finally, we present, in section IV, the results of the model validation tests.
II. LOCATION SYSTEMS The GPS system allows the mobile receiver units to determine their position by measuring time-of-arrival of the radio signals transmitted by a satellite constellation. The GPS performs well outdoor, but for indoor applications, due to strong multipath and signal attenuation, the GPS does not work properly most of the time or when it works, it is not accurate enough. To solve this problem a number of dedicated indoor location systems have
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been developed [1]. These systems usually use static references (beacons) to compute their location. Some of them use infrared transmitters [2] with coded pulses with one beacon per room. The receivers detect the light code to determine in which room they are. There are, location systems that use radio signals [3] to compute the location. They use wireless networks and signal strength measurements to compute the location. These systems require a training phase and have an accuracy worst than the ultrasonic systems. Ultrasonic location systems can be a viable alternative [1]. These systems have proved to have good accuracy, and they are very simple and cheap.
A. Location System Using Ultrasounds There are many location systems based on ultrasounds with a lot of different configurations. Almost all these systems use an auxiliary radio channel as a reference trigger to measure the time of flight of the ultrasound signal. The Bat system [4] uses an ultrasound transmitter on the mobile station. The ultrasonic transmitter sends an ultrasonic pulse immediately after receiving a radiotrigger signal. A network of ultrasonic receivers on the ceiling detects the ultrasound pulse and sends the time information to a central controller. This controller computes the location by the time-of-flight with an average accuracy of 3 cm. The Cricket system [5] uses beacons distributed through a room. Each beacon sends two signals simultaneously, an ultrasonic signal and a radio signal. To minimize signal collisions the beacons send the signals randomly in time. When the mobile device receives the radio signal from the transmitter, it waits for the ultrasonic signal and calculates the distance to the beacon by the time difference between the radio signal and the ultrasonic signal. The first implementation of this system could only be operated using a high density of beacons and presented an accuracy of 5 to 25 cm.
B. Locus Location System We have developed an ultrasonic bi-dimensional location system, called Locus [6], that uses ultrasonic beacons and an auxiliary RF channel. When
new experiments with instrumentation transducers. Nevertheless, the actual version of the simulator will be very useful to test the behaviour of ultrasonic location systems in closed spaces. The proposed simulator is also simple and easy to use and modify.
B. Future Work
Figure 8 . Envelope outputs of the match-ďŹ lter for test one.
We intend to improve the accuracy of the simulator (to get the best approximation to reality) while trying to maintain the simplicity that the simulator has now. We want to include in the moddel various factors like: i]Z 9deeaZg Z[[ZXi YjZ id i]Z gdWdi Y^heaVXZbZci0 i]Z gddb \gVY^Zci iZbeZgVijgZ0 i]Z igVch[Zg [jcXi^dc d[ i]Z igVchYjXZgh0 i]Z VXdjhi^X cd^hZ YjZ id V^g ijgWjaZcXZ0 VcY i]Z ZaZXig^X VcY fjVci^oVi^dc cd^hZ d[ i]Z 698 (Analog-to-Digital Converter) and DAC (Digital-to-Analog Converter).
B. Test two
VI. ACKNOWLEDGMENTS
In this test the transmitter was placed at (1.610, 0.915, 0.625) with direction (0.682, 0.731, 0.000). The receiver was placed at (0.780, 1.645, 0.647) with direction (0.000, 1.000, 0.000). Figure 9 shows a top view of this test. The simulated and practical envelope outputs of the match-ďŹ lter are shown in ďŹ gure 10.
We would like to thank Professor Rocha Pereira from the Department of Electronics, Telecommunications and Informatics, University of Aveiro for gently allowing the access to the Faraday case (cubic room) where the validation tests of the model were carried out.
This simulator is available at the URL: (2.450, 2.775)
http://www.ieeta.pt/locus/locusim/
Receiver (0.780, 1.645) z = 0.647 Transmitter (1.161, 0.915) z = 0.625
REFERENCES
(0, 0) Figure 9 . Top view of test two.
Figure 10 . Envelope outputs of the match-ďŹ lter for test two.
V. CONCLUSIONS AND FUTURE WORK A. Conclusions From the results of the test described in the previous section we can conclude that the simulator can be used as an approximation of a real room acoustic channel. As we can see in ďŹ gures 8 and 10, the practical and simulated main peaks of the envelope response are almost the same. We can also see that after 20ms, the real and simulated signals start to diverge. This shows that the simulator requires further improvements and
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ROOM ACOUSTICS SIMULATOR FOR ULTRASONIC ROBOT LOCATION