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Measuring RPM via Photo reflector
Computer
Interface
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TechBot1-A line following robot. by James Vroman Design Goal: The TechBot1 is a small line following robot designed for for the 1998 Embedded Systems conference in San Jose California. It was built by Jerry Merrill and myself and was designed as a promotionalrobot that would follow a black line drawn on a dry erase board. Motive Power: The TechBot1 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. Initially a 4 transistor Bridge was used but during power up the transistors would turn partially on and start conducting. This was corrected by adding a 5th transistor to the top of the H-bridge to control motor power. This also simplified the software into having 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. We used an array of seven sensors arranged in an "inverted V " arrangement placedunder the front of the robot behind the skid wheel. 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. Each transmitter LED is connected through a current limiting resistor to a processor bit. This allows the entire array to use 8 bits for the sensors.
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. Onceit was determined that the servo controller board did not provide enough control, it 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 of a plastic case with double sided tape. 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 wheel (closet roller wheel with a music wire frame) is attached to the front of the robot. The battery holder is mounted over the motors to keep the weight to the back and over the drive wheels.The drive wheels were salvaged from a toy car and bolted to the servo horns with #2 bolts. Hot glue will work for a while but eventually will come loose. 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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