Line Robot

 

Line Robot

Recently
many kind of robot contests have being opened and some interesting reports of
the challenge are found on the web. The Line Following is a kind of the robot
contests to vie running speed on the line. I build a tiny line following robot
which can run on the desk, moving the key board aside will do. It is for only
a personal toy reduced its size less than one fifth compared to typical line
following robots, not in formura. But I believe that it is suitable for home
use in the small Japanese houses said that rabbit burrow…(^_^;. Of course
I have also no time to take part in the robot contests

About Line Follower

The line follower is one of the self operating robot that follows a line
that drawn on the floor. The basic operations of the line following are as follows:

1. Capture line position with optical sensors mounted at front end of the
   robot. Most are using several number of photo-reflectors, and some leading
   contestants are using an image sensor for image processing. The line sensing
   procss requires high resolution and high robustness.
2. 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.
3. Control speed according to the lane condition. Running speed is limited
   during passing a curve due to friction of the tire and the floor.

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.

Hardware

Mechanics

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 chasis.

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
drive driving wheels, two tiny
vibration motors that used
for cellular phone, pager or any mobile equipment are used. Its shaft is pressed
onto the tire with a spring plate, the output torque is transferred to the wheels.

The steearing mechanism is realized in differential drive that steear
the robot by difference in rotation speed between the left wheel and the right
wheel. It does not require any additional actuator, only controling the wheel
speed will do.

Electronics

Controller	ATmega8 (Atmel)
Line sensor	Six photo-reflectors
Power supply	Two CR2032 lithium cells (One is for controller, the other is for motors)
Motor	Two micromotors for left wheel and right wheel
Dimensions	45(L), 33(W), 12.5(H) [mm]
Weight	15 grams (Body:8g, Cells:7g)
Performance	53 centimeter per second at oval course

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 here.

Software

Using photo-reflectors

To detect a line to be followed, most contestants are using two or more number
of poto-reflectors. Its output current that proportional to reflection rate
of the floor is converted to voltage with a resister and tested it if the line
is detected or not. However the threshold voltage cannot be fixed to any level
because optical current by ambent light is added to the output current like
the image shown right.

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, this is effective when the detector
is working in linear region.

In this project, pulsed light is used to cancel ambient light. This is suitable
for arraied sensors that scanned in sequence to avoid interference from next
sensor. The microcontroller starts to scan the sensor status, sample an output
voltage, turn on LED and sample again the output voltage. The difference between
the two samples is the optical current by LED, output voltage by the ambient
light is canceled. The other sensors are also scanned the same avobe in sequence.

Signal processing of line detection

Right image shows the actual line posisiton vs detected line position in
center value of 640. The microcontroller scans six sensors and calcurates 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 value that proportioning to reflection ratio, and the sensitivity
have variety between each one of them. In this system, to remove the variations
from the outputs, calibration parameters for each sensor can be held into 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.

Tracking control

The line position is compeared to the center value to be tracked, the position
error is processed with Proportional/Integral/Diffence filters to generate steering
command. The line folloing robot tracks the line in PID control that the most
popular argolithm for servo control.

The proportional term is the commom process in the servo system. It is only
a gain amplifire without time dependent process. The differencial term is applied
in order to improve the responce to disturbance, and it also compensate phase
lag at the controled 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 resons.
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.

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.

Notes

• Development diary [Ja]
• Circuit diagram
• Firmware May 23, 2004
• Following motion
  with only P control
  This is a video file of line following motion with only P control. The servo
  system oscllated.
• Following motion
  with P and D controls
  Adding D control could improve the servo stability. The robot follows the
  line correctly. Therefore the servo parameter must be optimized for mechanical
  characterristics to improve the tracking stability.

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