TechBot line following robot using PIC16F84

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 promotional

robot 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.

Click for Schematic

 

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 placed

under 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.

 

For more detail: TechBot line following robot using PIC16F84