eug_titles/BallBot/Part3

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This website contains an archive of files for the Acorn Electron, BBC Micro, Acorn Archimedes, Commodore 16 and Commodore 64 computers, which Dominic Ford has rescued from his private collection of floppy disks and cassettes.

Some of these files were originally commercial releases in the 1980s and 1990s, but they are now widely available online. I assume that copyright over them is no longer being asserted. If you own the copyright and would like files to be removed, please contact me.

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Filename:	eug_titles/BallBot/Part3
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File size:	1DC6 bytes
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Exec address:	0000

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Personal collection » Acorn ADFS disks » Electron » EUG_submission.ADF » BallBot/Part3
Personal collection » Acorn hard disk » misc » misc3 » eug_titles/BallBot/Part3

Ultrasonic Sonar System

By far the largest area of development in BallBot has been the work in
developing the sonar system which the robot uses to navigate. This sonar
system must be able to determine what objects are present around the
robot, how far away they are, and whether they are interesting or not.

The system essentially consists of two units: a transmitter, which is
essentially a loudspeaker, and a reciever, which is a microphone. Both
have a very fine resonant frequency of 40kHz. The system works as follows:

On a regular basis (eight times a second), BallBot emits a loud ultrasonic
pulse from the loudspeaker. As this pulse moves away at the speed of sound
(330m/s), some of this energy is reflected back at the robot in the form
of echoes. If there is an object such as a tennis ball present, the echoes
are much stronger, as shown by the diagram. These strong echoes are
recieved by very sensative microphones on board the robot. The distance of
the object from the robot can be calculated because we know that the time
taken for the sound wave to travel out to the object and then return back
to BallBot is equal to the time delay between the signal being
transmitted, and its echo being recieved. Therefore the following formula
can be applied:

The type of object present can also be determined. Different objects
produce different amplitudes of echo depending upon their size and how
absorbant they are to sound waves. As soon as the first signal from the
object is recieved, BallBot's computer begins sampling the sound being
recieved. The hardware converts the peaks in amplitude of the ultrasound
being recieved into binary 1s, and the troffs into binary 0s. Thus a form
of code is formed which is passed to the computer's input circuitry. The
computer then processes the binary code to search for patterns, which it
recognises as objects such as tennis balls.

Systems such as this are only effective, however, when the echoes are
being reflected off objects which are thicker than the wavelength of the
waves being transmitted. In our case, this can be found to be
approximately 8mm using the formula V=fl. Therefore some objects such as
the net and the fence surrounding the court may not be reliably detected.
Othere objects such as the balls and players will, however, still be
visable when this is allowed for. The pattern recognition system must,
therefore, be programmed to ignore any traces which are recieved from
these items if reliable consistent results are to be obtained, and the AI
code must be programmed to work without traces from these surfaces. It is
important that such traces are either picked up reliably every time, or
ignored completely. If they are necessary for the efficient running of the
AI, infra-red sensory systems may be added to detect the net, the fence,
and anything else of interest.

The Transmitter

The circuitry for this sonar system is still very much under development.
The transmitting system has now been designed, built and tested, and seems
to be working satisfactoraly. This part of the circuitry takes one output
line from the computer, and transmits 40kHz from the speakers when it is
high, and turns off the speaker when it is low. Essentially it consists of
a wave generator circuit, producing 40kHz sine waves, and a transistor to
switch the output to the speaker on and off. The circuit diagram for this
is shown below:

It is at first surprising that this circuit works. The collector ->
emitter connection of a transistor acts as a diode, and yet the voltage
across this negative in the lower half of the 40kHz sine wave. Therefore
we would expect half-wave rectified output from the speaker. I suspect
that the reason why this circuit works is that induction or resonance in
the transmitter fills in the lower-half of the wave.

Having built this circuit, and shown that it worked by monitoring the
output using a microphone and an oscilloscope, I decided to test the
response rate of the circuit to see how long it took the transmitter to
reach maximum amplitude after the output line from the computer goes high,
switching on the transmitter. By programming the computer to switch the
transmitter on and off at high frequency, and monitoring the sound output
on an oscilloscope, I found that it takes about 1ms to reach maximum
amplitude from the "blip" control signal being sent, and 1ms to return to
complete silence after being turned off. Therefore I decided that the
computer should send the "blip" signal to this circuit for a minimum of
1ms for it to be sure to have the effect of producing a sound of maximal
amplitude, but not too long that the recieved waveforms from different
objects become blurred in any way.

The Reciever

The ultrasonic reciever is rather more complicated than the transmitter,
and work is still at a very early stage. Our first objective was to find
out exactly work kind of signal the ultrasonic microphone produces when
exposed to ultrasound. Using an oscilloscope, I found that the signal
produced is an a.c. signal which oscillates at 40kHz, and has an amplitude
which varies with the intensity of the ultrasound to which it is exposed.
I expected the voltage of this signal to be proportional to the inverse
square of the distance. The sound spreads out from the source in the shape
of a sphere (or part of a sphere). Therefore at any one time the sound
energy is spread over the surface area of a sphere of distance d around
the source, where d is the distance between the source and the
transmitter. This would indicate the following formula:

The data actually collected through tests with the ultrasonic equipment is
shown below:

Both tests strongly suggest that the voltage of the signal from the
microphone is inversely proportional to the distance. Therefore, this
suggests that:

As you can see, the product of the voltage recieved and the distance of
the source from the reciever is constant. This could be very useful.
Graphs for Voltage x Distance should, therefore, be a straight horizontal
line. Such graphs of voltage x distance are plotted below:

So far, all studies have been of a system involving transmission in a
straight line, with the reciever facing the transmitter. In BallBot,
however, the transmitter and the reciever will be together, and facing in
the same direction. Therefore some tests have been performed for the input
recieved when an echo is bounced off a surface. Data for such tests are
shown below:

Once again, it appears that voltage is inversely proportional to distance.
This suggests that surfaces reflect a constant proportion of the sound
energy back at the robot. Once again, the product of distance and voltage
should be a constant value. But because we know that the distance of the
object is proportional to the time taken for the echo to be recieved, we
can convert this to saying that the product of the time elapsed since the
ultrasonic blip was transmitted and the input voltage is a constant value
for particular surfaces. This is more useful than distance with regard to
designing the BallBot hardware. Graphs for voltage x distance for echo
tests are plotted below:

The constant value of the product of voltage and distance can be found for
different surfaces for the given speakers and output amplitude. The unit
of this value is the volt-metre (Vm). Work is currently in progress to try
to find actual values of this for the kind of objects which BallBot will
come across (e.g. tennis balls). The value is equal to the voltage
produced across the reciever when an echo is recieved off that type of
surface at a distance of one metre.

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00001950  74 20 61 70 70 65 61 72  73 20 74 68 61 74 20 76  |t appears that v|
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00001990  69 73 20 73 75 67 67 65  73 74 73 20 74 68 61 74  |is suggests that|
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00001a90  20 66 6f 72 20 74 68 65  20 65 63 68 6f 20 74 6f  | for the echo to|
00001aa0  20 62 65 20 72 65 63 69  65 76 65 64 2c 20 77 65  | be recieved, we|
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00001b00  73 6f 6e 69 63 20 62 6c  69 70 20 77 61 73 20 74  |sonic blip was t|
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00001b90  65 73 69 67 6e 69 6e 67  20 74 68 65 20 42 61 6c  |esigning the Bal|
00001ba0  6c 42 6f 74 20 68 61 72  64 77 61 72 65 2e 20 47  |lBot hardware. G|
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00001bd0  20 65 63 68 6f 0d 74 65  73 74 73 20 61 72 65 20  | echo.tests are |
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00001bf0  54 68 65 20 63 6f 6e 73  74 61 6e 74 20 76 61 6c  |The constant val|
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00001c60  6b 65 72 73 20 61 6e 64  20 6f 75 74 70 75 74 20  |kers and output |
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00001c90  65 20 69 73 20 74 68 65  20 76 6f 6c 74 2d 6d 65  |e is the volt-me|
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00001cf0  20 74 68 65 20 6b 69 6e  64 20 6f 66 20 6f 62 6a  | the kind of obj|
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00001d20  6f 73 73 20 28 65 2e 67  2e 20 74 65 6e 6e 69 73  |oss (e.g. tennis|
00001d30  20 62 61 6c 6c 73 29 2e  20 54 68 65 20 76 61 6c  | balls). The val|
00001d40  75 65 20 69 73 20 65 71  75 61 6c 20 74 6f 20 74  |ue is equal to t|
00001d50  68 65 20 76 6f 6c 74 61  67 65 0d 70 72 6f 64 75  |he voltage.produ|
00001d60  63 65 64 20 61 63 72 6f  73 73 20 74 68 65 20 72  |ced across the r|
00001d70  65 63 69 65 76 65 72 20  77 68 65 6e 20 61 6e 20  |eciever when an |
00001d80  65 63 68 6f 20 69 73 20  72 65 63 69 65 76 65 64  |echo is recieved|
00001d90  20 6f 66 66 20 74 68 61  74 20 74 79 70 65 20 6f  | off that type o|
00001da0  66 0d 73 75 72 66 61 63  65 20 61 74 20 61 20 64  |f.surface at a d|
00001db0  69 73 74 61 6e 63 65 20  6f 66 20 6f 6e 65 20 6d  |istance of one m|
00001dc0  65 74 72 65 2e 0d                                 |etre..|
00001dc6

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