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Autonomous Beacon Seeking Blimp

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					   Independent Learning Project:

Ultrasonic Distance Measuring Sensor

          Kenton Weigold

             8/9/2006

             Dr. Reddy
                                         Ultrasonic Distance Measuring Sensor



                                            TABLE OF CONTENTS
INTRODUCTION............................................................................................................. 3
BACKGROUND INFORMATION ................................................................................ 4
PARALLAX R93-SRF04.................................................................................................. 5
ALTERNATIVE APPROACHES................................................................................... 8
REFERENCES .................................................................................................................. 9


                                              TABLE OF FIGURES
FIGURE 1 – SETUP OF SENSORS ON BLIMP ............................................................................ 4
FIGURE 2 – ULTRASONIC RANGE FINDER BEAM PATTERN ................................................... 5
FIGURE 3 – TIMING DIAGRAM OF THE SRF04 ...................................................................... 6
FIGURE 4 – SRF04 INTERFACE CONNECTIONS ..................................................................... 7




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Introduction
The project will consist of a helium-supported blimp that will autonomously seek
out an infrared broadcasting beacon. The airship will take off roughly 8 meters
from the beacon, rise to an altitude 2 meters, and proceed to find the beacon
station. Using a set of infrared sensors the blimp should be capable of identifying
the direction of the beacon. Once the direction of the beacon is recognized, it will
propel itself until it reaches the location of the beacon and land. In order to
determine the height of the blimp an ultrasonic range finder will be used. Figure
1 shows the location of the ultrasonic range finder on the blimp. For this project
the Parallax R93-SRF04 has been chosen. The device will send out an
ultrasonic pulse and then measure how long it takes to receive the pulse back.
This will determine exactly how high the blimp is currently in order to make
altitude adjustments. Along with the ultrasonic sensor three inferred sensors will
be used to find the beacon. DC motors will also be used to propel the blimp up,
down, forwards, backwards and to rotate the blimp. The ultrasonic sensor will be
able to detect the current height of the blimp so that when it takes off it can rise to
the desired 2 meter altitude before it begins its search for the target. Once it has
achieved a height of 2 meters it will make vertical corrections in case its altitude
changes in the course of finding the target. Once the target has been found the
blimp will land and the ultrasonic sensor will be able to detect that the blimp has
reached the ground.




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                       Figure 1 – Setup of Sensors on Blimp




Background Information
       An ultrasonic range finder consist of two components a transmitter and a
receiver. It is possible to combine these into 1 device that can do both
transmitting and receiving but they are still separate components. In order to
detect distance the device emits a sound wave above the hearing range of a
human, which is between 300 HZ to 14 KHZ. Ultrasonic is everything higher
than that a human can hear and lower than RF. The unit then calculates how
long it takes for the echo of that wave to return. The longer it takes to return the
farther away the object is. Since the speed of sound at room temperature is
known to be roughly 344.31 meters per second it is possible to determine how far
an object is away by measuring the time between when the device emitted the
sound wave to the time the echo was received. The sound waves will bounce off
of any a surface when they come in contact with it. Since ultrasonic is above the
limit that a human can hear using the ultrasonic range finder creates no noise
pollution. It may be the case that something in the surrounding area is emitting
vibrations at the frequency being used by the ultrasonic range finder in which
case the results of the experiment would be hindered. It is important to conduct
experiment to determine if any conditions like this exist so that time is not wasted


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looking for a problem not with your design but with the area used for testing.
Bats use ultrasound for sight. They send out a pulse and then receive back
reflections as a human would receive the light that bounces off and object in
order to see it. In essence they hear in order to see. Dolphins use ultrasonic
waves to communicate under water. Under water ultrasonic waves of low
wavelength can travel great distances and are the best solution for
communications underwater.




Parallax R93-SRF04
       The Range Finder being used is the Parallax R93-SRF04. This device
runs on a 5V power supply and draws 30mA during normal use. The range of
detectable distance is 3cm to 3m and is sensitive to an object 3cm wide at
distances greater than 2 meters. Since this application is interested in detecting
distance from the floor, these specifications meet the requirements. As
mentioned previously, the projected operational altitude of the blimp will be
roughly two meters. This value is well within the constraints of the sensor. See
Figure 2 to see that at 2 meters (6.5 feet) it is well covered in the beam pattern of
the sensor.




                Figure 2 – Ultrasonic Range Finder Beam Pattern


       In order to find a distance the SRF04 first waits for input from the user.
When it receives a 10uS pulse to the trigger input the SRF04 then sends out an 8


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cycle burst of ultrasound at 40KHZ and raises its output line. The output line is
kept high until the echo is received by the SRF04. If no echo is heard then the
line will be lowered automatically after about 36mS. Figure 3 shows the timing
diagram of the SRF04.




                    Figure 3 – Timing Diagram of the SRF04


      In order to interface with the SRF04 from an HC12 two connections are
needed. The trigger pulse input as seen in figure 4 is needed so that the user
can execute a test for measuring the distance and then the echo pulse output pin
as seen in figure 4 is needed in order to calculate the distance based on how
long the output pin was high. The trigger pulse input will be connected to the A
port of a HC12 which the echo pulse output pin will be connected to the T port.




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                     Figure 4 – SRF04 Interface connections


       The design of the SRF04 is supposed to be low cost. A PIC
microcontroller is used to control the functions of the Ultrasonic range finder
using two 40KHZ transducers. In order to drive the transducer to a higher
voltage, for detection of smaller objects, a MAX232 IC is used to get the voltage
into the transmitting transducer to 16v. The receiver to a two stage op-amp
circuit with each gain set to 24 for a combined gain of around 576. Problems
arise when the operation must get down into the 1-2cm range because the
receiver will pick up direct coupling from the transmitter which is located right
next to it. Some designs will remove this time period to be sure that the echo
they are receiving back is actually the echo and not the coupling from the
transmitter. In order to remove this problem it was seen that the actual echo at
that time period was much higher then the coupling that was seen so for the first


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1-2 ms of timing the threshold was adjusted so that only the real echo was
detectable.


       In order to calculate the distance of the object the pulse is measured by
the microcontroller. Given that this pulse is measured in uS it can then be
translated into both inches and cm. Dividing the number of uS by 58 will give you
the distance in cm or dividing by 148 will give the distance in inches. These
calculations are based on the speed of sound at the time and may need to be
adjusted for temperature or altitude.


Alternative Approaches

       Along with using ultrasound for range finding there are alternatives that
can be used. Inferred can be used for range finding but is much harder to do.
Since the speed of light is much higher than the speed of sound it is almost
impossible with a microcomputer to accurately test the differential length of time
from when a IR was sent out to when it was received. The inaccuracies brought
because of the speed of light make this method unusable. The way in which it is
possible to measure distance with IR is to measure the intensity in which the ir is
being received. Based on the intensity it is received it can be calculated how far
the light traveled. This method well better then measuring the time is still not as
accurate or as effective as ultrasound. Another alternative method with IR is to
measure the angle in which the IR is received. Based on the angle it can be
calculated how far away the object is because it is know how far away the
transmitter is from the receiver. This method is fairly accurate but the possibly of
IR pollution and other factors make this method not as reliable as ultrasound.




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References
http://roboteer.net/weblog/staticpages/index.php?page=20060114193627279
http://info.hobbyengineering.com/specs/devantech-srf04-tech.pdf
http://www.acroname.com/robotics/parts/R93-SRF04.html




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