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Friday, 21 May 2010

Line Follower

The buggy features two main wheels positioned opposite each other, and independently driven by
stepper motors. The chassis is balanced with a simple peg that skids along the ground.
The motors and sensors plug into two circuit boards mounted in the buggy chassis, and this in turn
is linked by means of umbilical ribbon cable, to an input/output port used in conjunction with a
Sinclair ZX81.
The ZX81 provides the intelligence to make the buggy follow a black line (electrical black
insulation tape). It could be argued that a basic line follower does not really require the use of a
computer, with the buggy being made to operate properly by getting the sensors to control the motors
through more direct electronic means. However, using a computer allows easy behaviour refinement by
software changes. For example after the basic line following was implemented the buggy was
programmed to be able to negotiate branches in the line.

The chassis is built from a combination of Meccano® and Perspex®. The Meccano enabled a chassis to
be quickly constructed, and the Perspex facilitated the non Meccano parts (stepper motors and
wheels) to be easily incorporated into the design.
The robot electronics comprised two circuit boards - the driver board and the sensor board. These
boards are stacked one over the other.
The step resolution of the stepper motors is 1.8 degrees. To turn this step size into a smaller
wheel travel, a reduction gearing comprising a small cog on the motor shaft and a much larger cog
connected on the wheel is utilised on each motor drive.

Driver board
Two SAA1027 stepper motor drive ICs are employed on the driver board, each one to control a four
phase stepper motor. The ICs simplify control of the stepper motors by requiring just a digital
direction signal (clockwise/anti clockwise) and digital clock signal (advance step) for each motor.
The SAA1027 ICs require a 12v power supply, and 12v control signals. LM324 quad operational
amplifiers are used to level shift the 5v TTL levels from the ZX81 up to the 12v control signals.

Sensor Board
To enable the buggy to follow a black line, two optical sensors (TIL81) are used. They are
positioned at the front underside of the buggy. The sensors are separated by a distance of 1cm.
Additionally an infra-red LED (TLN1 10) is placed between the sensors, so that they are less
effected by the surrounding ambient light. Depending on the surface either black or white the
infrared beam is either absorbed or reflected respectively.

The sensor board comprises two identical circuits each connected to a corresponding optical sensor.
Each circuit converts the optical sensor output from an analogue value to a 5v TTL signal that can
be read by the ZX81 via the input port.


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