📄 seattle robotics society encoder.htm
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####################################################### --><HTML><HEAD><TITLE>Seattle Robotics Society Encoder</TITLE>
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<H1 align=center>Desktop Line Following Robot</H1>
<H2 align=center>By ChaN</H2>
<HR class=hds>
<DIV align=right><IMG height=352
src="Seattle Robotics Society Encoder.files/track.jpg" width=505
align=right></DIV></DIV>
<P align=left>Recently many kind of robot contests have being opened and some
interesting reports of the challenge are found on the web. Line Following is a
kind of robot contests which measures running speed on the line. I built a tiny
line following robot which can run on the desk-only moving the key board aside
will do. It is for only a personal toy as it is reduced in size less than one
fifth compared to typical line following robots. I believe that it is suitable
for home use in small houses and apartments.</P>
<DIV></DIV>
<H3>About Line Follower</H3>
<P>The line follower is a self operating robot that follows a line drawn on the
floor. The basic operations of line following are as follows:</P>
<OL>
<LI>Capture line position with optical sensors mounted at the front end of the
robot. Most competitors are using several photo-reflectors, and some leading
contestants are using an image sensor for image processing. The line sensing
procss requires high resolution and high robustness.
<LI>Steear robot to track the line with any steearing mechanism. This is just
a servo operation, any phase compensation will be required to stabilize
tracking motion by applying digital PID filter or any other servo argolithm.
<LI>Control speed according to the lane condition. Running speed is limited
during passing a curve due to friction of the tire and the floor. </LI></OL>
<P>There are two line styles, white line on the black floor and black line on
the white floor. Most contest are adopting the first one in line width of
between 15 and 25 millimeters.</P>
<DIV align=right><IMG hspace=10
src="Seattle Robotics Society Encoder.files/physicallayout.jpg" width=300
align=right> </DIV>
<H3>Hardware</H3>
<H4>Mechanics</H4>
<DIV class=para>
<P>Right image shows bottom view and side view of the built line following
robot. All mechanical and electrical parts are mounted on a proto board, and it
also constitutes the chassis.</P>
<P>The line following robot is upheld in three points of two driving wheels and
a free wheel. The driving wheels are made with a 7 mm dia ball bearing and a
rubber tire. The free wheel is a 5 mm dia ball bearing attached loosely. To
power driving wheels, two tiny vibration motors that are used for a cellular
phone, pager or any mobile equipment. Each shaft is pressed onto the tire with a
spring plate, and the output torque is transferred to the wheels.</P>
<P>The steering mechanism is realized in <EM>differential drive</EM> that steers
the robot by difference in rotation speed between the left wheel and the right
wheel. It does not require any additional actuator, only controlling the wheel
speed will do.</P></DIV>
<DIV class=para>
<H4>Electronics</H4>
<TABLE class=lst id=spec align=right border=10>
<TBODY>
<TR>
<TD>Controller</TD>
<TD>ATmega8 (Atmel)</TD></TR>
<TR>
<TD>Line sensor</TD>
<TD>Six photo-reflectors</TD></TR>
<TR>
<TD>
<DIV align=right>Power supply</DIV></TD>
<TD>Two CR2032 lithium cells<BR><SMALL>(One is for controller, the other
is for motors)</SMALL></TD></TR>
<TR>
<TD>Motor</TD>
<TD>Two micromotors for left wheel and right wheel</TD></TR>
<TR>
<TD>Dimensions</TD>
<TD>45(L), 33(W), 12.5(H) [mm] </TD></TR>
<TR>
<TD>Weight</TD>
<TD>15 grams <SMALL>(Body:8g, Cells:7g)</SMALL></TD></TR>
<TR>
<TD>Performance</TD>
<TD>53 centimeters per second at oval course</TD></TR></TBODY></TABLE>
<P>An Atmel ATmega8 is used for the controller and it is powered by a lithium
coin cell. The other lithium coin cell is for only motors. Separating the power
supply into two cells is to avoid accidental reset of the microcontroller due to
voltage dip by motor start current. Six photo-reflectors are mounted at front
end of the chasis. They sense reflection rate of the floor under them. Motors
are driven in PWM to control rotation speed lineary. The latest circuit diagram
is shown below:</P>
<P> </P>
<P> </P>
<P><IMG height=750 src="Seattle Robotics Society Encoder.files/ltc.gif"
width=1000 vspace=20></P></DIV>
<H3>Software</H3>
<H4>Using photo-reflectors</H4><IMG
src="Seattle Robotics Society Encoder.files/SensorDiagram.png" align=right>
<P>To detect a line to be followed, most contestants are using two or more
photo-reflectors. The output current-proportional to reflection rate of the
floor-is converted to voltage with a resister and tested to determine if the
line is detected or not. However the threshold voltage cannot be fixed to any
level because optical current created by ambent light is added to the output
current like the image shown right.</P>
<P>Most photo-detecting modules for industrial use are using modurated light to
avoid interference by the ambient light. The detected signal is filtered with a
band pass filter and disused signals are filtered out. Therefore only the
modurated signal from the light emitter can be detected. Of course the detector
must not be saturated by ambient light because it is effective when the detector
is working in its' linear region.</P>
<P>In this project, pulsed light is used to cancel ambient light. This is
suitable for arrayed sensors that are scanned in sequence to avoid interference
from the next sensor. The microcontroller starts to scan the sensor status,
sample the output voltage, turns on the LED and samples again the output
voltage. The difference between the two samples is the optical current created
by the LED, as the output voltage produced by the ambient light is canceled. The
other sensors are also scanned the same way in sequence.</P>
<P> </P>
<H4>Signal processing of line detection</H4>
<DIV align=right>
<P align=left><IMG hspace=10
src="Seattle Robotics Society Encoder.files/SensorDetail.gif" align=right>The
image at right shows the actual line posisiton vs detected line position with a
center value of 640. The microcontroller scans six sensors and calculates the
line position by output ratio of two sensors near the line. Thus the line
position can be detected lineary with only six sensors. All the sensor outputs
are captured as analog values that are proportionate to each sensors' reflection
ratio, and the sensitivity varys between each one of them. In this system, to
remove the variations from the outputs, calibration parameters for each sensor
can be held in non-volatile memory. This can be done with online mode. The
microcontroler enters the online mode when an ISP cable is attached, and it can
be controlled with a terminal program in serial format of N81 38.4kbps. S1
command monitors sensor values, and S2 command calibrates variation of sensor
gain on the reference surface (white paper). The ATmega8 must be set to 8MHz
internal osc. </P>
<H4 align=left>Tracking control</H4><IMG hspace=15
src="Seattle Robotics Society Encoder.files/steeringDetail.gif" align=right>
<P align=left>The line position is compared to the center value to be tracked,
and the position error is processed with Proportional/Integral/Diffential
filters to generate steering command. The line folloing robot tracks the line in
PID control that is the most popular argolithm for servo control.</P>
<P align=left>The proportional term is the common process in the servo system.
It is only a gain amplifier without time dependent process. The differential
term is applied in order to improve the responce to disturbance, and it also
compensate phase lag at the controlled object. The D term will be required in
most case to stabilize tracking motion. The I term is not used in this project
from following reasons. The I term that boosts DC gain is applied in order to
remove left offset error, however, it often decrease servo stability due to its
phase lag. The line following operation can ignore such tracking offset so that
the I term is not required.</P>
<P align=left>When any line sensing error has occured for a time due to getting
out of line or end of line, the motors are stopped and the microcontroller
enters sleep state of zero power consumption.</P>
<H2 align=left>NOTES:</H2><IMG
src="Seattle Robotics Society Encoder.files/closeup.jpg" align=right>
<P align=left>The Firmware can be found <A
href="http://www.seattlerobotics.org/encoder/200610/article2/ltc.txt">here</A>
and the include file is <A
href="http://www.seattlerobotics.org/encoder/200610/article2/avr.inc">here</A>.</P>
<P align=left><A
href="http://www.seattlerobotics.org/encoder/200610/article2/Ponly.mpeg">Line
following motion with only proportional correction.</A> This is a video of the
robot following the line with only proportional control. Notice that it leaves
the line entirely at the end.</P>
<P align=left><A
href="http://www.seattlerobotics.org/encoder/200610/article2/PandD.mpeg">Line
following motion with both proportional and differential contro</A>l. This video
shows the smooth operation attained with both proportional and differential
control.</P></DIV>
<P> </P>
<P> </P>
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