JavaBot1...........
A line following robot. by James Vroman
Design Goal:
The JavaBot1 is a small line following robot designed to follow a black line drawn on a dry erase board. It is designed to follow very tight curves. The software still has lot's of room for improvement but works well as is.
Motive Power:
The JavaBot1 uses 2 Cirrus CS-70 servos that have been modified
for full rotation and have had their controller boards removed to
convert them from servos to gear motors.
Servos are a common motive power for small robots due to their
low cost, ready availability, standardized sizes and the fact that it
only requires 1 bit on your processor to control the motor.
We initially tried this approach but found that the speed control was
very minimal with a finer control needed for this application. The
servo controller boards were then removed and the wires soldered
to the motor terminals and case ground. The motors were then
controlled by an H-bridge circuit to allow direction and speed control
with only 2 processor bits per motor. This is implemented as one bit for
direction and another bit for power/speed control per motor.
Sensors:
In order to follow the line I/R reflective sensors were used to
detect if a line was present or not. The sensors chosen are the
QRB1114 from QT Optoelectronics and have a focal point of
about 1/4 inch. They are available from DIGI-KEY.
Most line followers use 2 or 3 sensors of this type to do their
detection. This works but does not give the ability to follow
lines with very tight turns. An array of seven sensors arranged
in an "inverted V " patternare bolted under the front of the robot
The sensors are wired with all the receivers connected in parallel and
fed to an LM311 comparator to set the threshold trigger level with it's
output fed to a processor bit. The transmitter LEDs are connected
to a 74HCT138 with a current limiting resistor to VCC. This allows
the entire array to use 4 bits for the sensors.
Processor:
The PIC16F84 was chosen for it's small size, easy reprogramability
and interrupts ( the fact that we manufacture a PIC processor
emulator also helped in this decision). It is clocked at 4 MHZ by
a ceramic resonator and is powered by 4 AA rechargeable batteries.
These same batteries power the motors. This is usually not recommended
since surges in motor current can affect the processors operation, but with
decoupling caps in place and the watchdog timer being used in the
software no problems were experienced. The watchdog could reset the
processor if it went stupid before you could ever see it act up.
Mechanical:
The servos were modified for full rotation by disassembling the servo
to gain access to the gear compartment. The main gear is then removed
and the stop that keeps it from rotating removed with an hobby knife.
The plastic key that keeps the feedback pot hooked to the main gear is
removed to allow full rotation without moving the feedback pot. The
servo controller was removed and the feed back pot removed as well.
The wires were removed from the control board and resoldered to the
motor terminals. The servos were reassembled and taped together.
This assembly was then attached to the bottom with the standoffs that
held the line follower board in place. For this application the circuitry
was split into a sensor board and a processor/h-bridge board. The two
boards were connected by a ribbon cable. The entire assembly could
be built on one circuit board with the same board being used as the chassis.
The sensor board is mounted under the front of the chassis with
the processor/motor control board above. A skid (plastic knob )
is attached to the rear of the robot.
The battery holder is mounted over the skid to keep the weight
to the back and counterbalance the line sensors .
The drive wheels are model airplane aluminum, racing wheels and are bolted to the
servo horns with #2 bolts. Both the Dallas Personal Robotics Group
and the Seattle Robotics Society have more information on
modifying servos for use in robotic applications.
Firmware:
The firmware for JavaBot1 is based on the code written by
Jerry Merrill for the TechBot1.
The program is divided into 3 sections - The main program loop,
The pwm isr ( pulse width modulation interrupt sub routine) and
The action routines. The program functions to turn on the
transmitter portion of the sensors in order of priority and see
if a line is seen by the receiver section. The outer sensors are
tested first. If a line is seen then one motor is reversed while
the other continues forward. If no line is seen the next set in
are checked. If a line is seen here then one motor is stopped
while the other continues forward. If no line is seen here then
the next set in is checked. If a line is seen here then the speed
of one motor is reduced while the others speed is maintained.
If no line is seen by these sensors then the centermost sensor
is checked. If a line is seen here then the motors are set for
both full forward (we reduced this from full speed for better
reliability). If no line is seen by this last sensor than the last
motor setting is maintained in hopes of finding the line again.
Once a sensor detects a line and sets the motor settings then
the sensor routine is started again. In this manor priority is
given to the outermost sensors for the biggest corrections
with the more minor corrections being serviced last. More
details on this can be seen in the code listing. The firmware
was developed using the ClearView Mathias PIC emulator
and assembler which can be downloaded for evaluation
from HTTP://www.tech-tools.com
What's next?
The firmware could be updated to give more decision
capability and a memory of the corrections. If the robot
gets off course and intersects a line with both outside
sensors it can sit there and oscillate till a sensor clears
the line. A search capability that if the line is not seen
in a certain period of time it can run a search pattern
to locate the line would be a good addition.
Add a speaker or sound effects chip. Robots
that make noise get far more attention than ones that do
not. The recovered I/O lines could be used for more sensors
or as a buss to talk to other processors.
Only your imagination will limit you.
James Vroman is a Technical sales and support representative
for TechTools and an active member of the Dallas Personal
Robotics Group. James has been involved in Electronics for
over 15 years and can be reached at james@vroman.com.
Sources:
DigiKey-HTTP://www.digikey.com
TechTools - HTTP://www.tech-tools.com
Dallas Personal Robotics Group - HTTP://www.dprg.org
Seattle Robotics Society - HTTP://www.seattlerobotics.org
James Vroman -HTTP://www.james.vroman.com
