For final testing, the beacons (transmitters) were mounted on the ceiling pointing straight towards the ground. The height was fixed at 2,7 m since that is the height of the ceiling in the room that was used for testing. The system that was installed is shown in Figure 4.12, the receiver has been placed in the middle.
Figure 4.12: Transmitters and receiver of beaconing system
A grid was drawn on the floor of the room for testing purposes, as well as to determine the accuracy of the system. The layout of the grid can be seen in Figure 4.13.
Figure 4.13: Grid layout used for testing purposes
The circles in Figure 4.13 show the area covered by each transmitter. The numbers one up to nine indicates the positions that the tests were conducted in and the results can be seen in Table 4.3. When the angle between transmitter and receiver goes beyond 50°, it becomes difficult to see the difference between the noise and the signal. The radius of the circle covered by one transmitter is 3 m. With a radius of 3 m, the angle between transmitter and receiver is approximately 47°, as shown in Figure 4.14.
Figure 4.14: Triangle from transmitter to receiver
The final results can be seen in Table 4.3 and all values are in centimeters, the highest being 290 cm.
Table 4.3: Final test results
Position Physical coordinates Beaconing system coordinates Error in cm x y x y x y 1 42 29 40 25 2 4 2 102 89 101 86 1 3 3 162 149 164 148 2 1 4 222 209 217 217 5 8 5 252 239 202 290 50 51 6 192 239 195 237 3 2 7 132 239 134 240 2 1 8 72 239 72 241 0 2 9 9 239 10 240 1 1 3 m 4.07 m 2.7 m 47° 90° 43° Transmitter Receiver 3 m 47° 43°
The distance to each beacon is calculated using the method explained earlier in the chapter and the coordinates are calculated from there. From these results, it is clear that when the receiver is on the edge, or just beyond the edge, of the circle of beacon 1, the results obtained are still in range and can be used. At position 5, the receiver falls outside the operating range. In other words, only two of the three beacons are detected accurately and this explains the large margin of error. Beacon 1 is still detected but the angle becomes too big. At position 5 the angle between transmitter and receiver is approximately 53° and, as expressed earlier, when the angle becomes bigger than 50° it becomes difficult to distinguish between the signal and noise.
If an error occurs, the receiver calculates its coordinates every 450 ms. If the receiver is kept on the same spot, and position calculations are done, then the differences between these positions will be between 3 and 4 cm. To lower this effect, the receiver software can be modified to calculate an average position every second, or longer, depending on the application.
When reviewing this it is evident that the beaconing system works sufficiently if three beacons are detected. There are a few elements that still produce a number of problems, such as the influence that temperature has on the speed of sound and the environmental noises. The system can, however, be improved by mounting the transmitters at a 45° angle, as suggested by Priyantha et. al. [1]. Other improvements to the system will be discussed in Chapter 6.
4.8
References
1. PRIYANTHA NB, CHAKRABORTY A, BALAKRIHNAN H, The Cricket
Location-Support System, Proceedings of 6th annual international conference MobiCom’00, ACM Press, Boston, 2000, pp 32-43
2. HIGHTOWER J, BORRIELLO G, Location Systems for Ubiquitous Computing, IEEE Computer, August 2001, pp. 57-66.
3. CONVICT EPISCOPAL DE LUXEMBOURG, Infrared and ultrasonic beaconing,
www.restena.lu/convict/Jeunes/beacon.htm, 2001
4. HALAWAY D, HŐLLERER T, FEINER S, Coarse, Inexpensive, Infrared Tracking for Wearable Computing, Proceedings of the 7th IEEE International Symposium on Wearable Computers, White Planes, New York, 2003, pp. 69-78.
5. ELERT G, The Physics Factbook,
http://hypertextbook.com/facts/2000/CheukWong.shtml, 2000
6. SENGPIEL E, http://www.sengpielaudio.com/calculator-speedsound.htm, 2006 7. ALA-PAAVOLA J, Triangulation Location Beacon,
http://vodka.tky.hut.fi/~jap/Eurobot/doc/beacon.html, Helsinki University of
Chapter 5
Object Avoidance
5.1 Introduction
For the AGV to be a viable option for operation in industry, it is essential that it is able to avoid obstacles in the environment. In short, the AGV needs “eyes” to see what is happing around it. There are many solutions to this problem. However, the method that was used in this study is infrared (IR) sensors, due to the fact that it was the most cost effective method for object detection and it was easy to implement.
Figure 5.1: Proposed flowchart for object avoidance
AGV Moving
Object detected
?
AGV Stop and determine distance to object
Move around object and use Beaconing system to get on
route No
5.2 Infrared Sensors
Infrared (IR) sensors (Sharp GP2D12) are used for determining the distance from an object. The sensor is shown in Figure 5.2 [1] and Figure 5.3. This sensor can be connected to a microprocessor using the Analogue-to-Digital Converter (ADC).
Figure 5.2: GP2D12 Infrared sensor
Figure 5.3: GP2D12 Infrared sensor connection
The sensor must be supplied with power and ground, and the third pin is the output voltage (VOUT ) which is proportional to the distance measured [1]. A capacitor must be placed on the
sensor itself between ground and the supply to limit the noise. There must be no capacitor between ground and VOUT or the supply voltage and VOUT, as this would cause the sensor to
The fact that the GP2D12 infrared sensor is low in cost, compact and lightweight makes it one of the most commonly used range finders in robotic applications. The GP2D12 infrared sensor operation is based on the triangulation principle. This principle provides greater reliability and accuracy than the IR sensors, which make use of Time-of-Flight techniques [2]. It uses an IR LED for transmitting a burst of pulses. If there is no object in front of the sensor then the light will never return to the receiver, because there is nothing for it to reflect against. Hence, after 40 ms, it will show that there is no object for 80 cm in front of the sensor. If there is an object in front of the sensor, it will reflect back from the object to the receiver, in this case a Position Sensing Device (PSD), and this will form the shape of a triangle. This is illustrated in Figure 5.4 [3]. It can measure distances between 10 and 80 cm, and the update frequency is 25 Hz or every 40 ms [4].
Figure 5.4: Operation of GP2D12 Infrared sensor for different distances
Using this triangulation principle, as shown in Figure 5.5, it is clear that there will be different angles for different distances [5]. These sensors are also less affected by the ambient
lighting conditions, and the colour of different objects does not have that big an influence on the output [2].
Figure 5.5: Different angles for different distances