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Measuring RPM via Photo reflector
Computer
Interface
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JavaBot1- A line following robot.
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 itonly requires 1 bit on your processor to control the motor.We initially tried this approach but found that the speed control wasvery 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 fordirection 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 theQRB1114 from QT Optoelectronics and have a focal point ofabout 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 followlines with very tight turns. An array of seven sensors arrangedin an "inverted V " patternare bolted under the front of the robotThe sensors are wired with all the receivers connected in parallel andfed 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 isremoved 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 circuitrywas split into a sensor board and a processor/h-bridge board. The twoboards 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 theservo 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.
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