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2.1 Ultrasonic Sensor

To get an idea of the ultrasonic sensor's accuracy and its rounding behavior we examined the sensors behavior towards objects within small distances. Figure 1.1 shows the experimental setup.


Figure 1.1

Figure 1.2 shows the test series results and the mean deviance of the exact value.The Object we used was a small cardboard box (14.5 cm x 9.5 cm x 6 cm).


Figure 1.2

Distances smaller than 3 cm can not be measured. The biggest deviance of 2.6 cm was found at 2.5 cm. The sensors mean deviance of 0.408 cm is less than 0.5 cm which shows a correct rounding behavior.
The second tests purpose was to gain knowledge of the sensors field of vision. The same box as in the first test was used (14.5 cm x 9.5 cm x 6 cm). It was moved to different distances and angles relative to the ultrasonic sensor while noting the resulting distance readouts. The sensor was placed in horizontal as well as in vertical position, Figure 1.3 shows the experimental setup with the sensor in horizontal position.


Figure 1.3

Figure 1.4 shows a graphical representation of the field of vision in the horizontal position.


Figure 1.4

The results show that the ultrasonic sensor should always be placed in horizontal position, other positions decrease as well the field of vision as the sighting distance of the sensor. The sensor seems to be a bit 'blind' on the left eye, which can be explained by the fact that the left eye is actually the receiver of the ultrasonic wave while the right eye is the sender.
After these experiments on statical behavior of the ultrasonic sensor we moved on to some dynamic tests. The diagram in Figure 1.5 shows the distance readouts by the ultrasonic sensor while closing in on a wall. The data for this diagram was gained by writing a program in LEGO software based on LabView that stored the current ultrasonic value along with the current angle of one of the moving motors (we used the TriBot model) into a file on the NXT brick. This file was then downloaded an processed.


Figure 1.5

The dynamic test revealed two weaknesses of the ultrasonic sensor. The first issue is that it showed some areas where the sensor tends to measure 255 cm instead of the actual distance. The second even more important issue is the critical area in between 25 cm and 50 cm where the sensor has a high probability of returning the wrong value of 48 cm.