Monday 20 October 2014

Student Jamie's scaled down John Deere is a small wonder

Derek Casey

Published 19/12/2012 | 06:00

In their final year of studying at the University of Limerick to become metalwork and tech graphics teachers, students are asked to complete a project that will demonstrate the skills and knowledge they have acquired during the course.

Young Waterford student Jamie Casey – by his own admission a sufferer of "heavy metal disease" – set out to achieve this through the design and construction of a fully automated scale model John Deere 7930 tractor, complete with a 530 John Deere trailed silage mower.

The tractor had to be fully autonomous, so the challenge for Jamie was to ensure it was driven by its own sensors and relayed information to a programmable microchip, which in turn controlled the tractor and mower.

The tractor was built in the scale of 1:16, a scale which is popular among collectors who traditionally add to their sets at this time of year.

"The model was designed to be built within a metalwork classroom, so I machined all the parts using a manual milling machine, a lathe, a pillar drill and a welder," explained Jamie.

"First, I made the chassis which would carry the drive motor, the differential, the rear half-shaft bearing housings, the pivoting front axle and the weight block.

"The drive system consists of a geared down DC motor coupled to a differential that I ripped out from a remote control car. The differential is feeding out to the two rear half-shafts, which are mounted in their own bearings. The wheels were then mounted directly onto the half-shafts."

According to Jamie, the front axle was one of the most difficult parts of the build. It consisted of an axle with a steerable hub at either side.

The tricky part was coming up with some sort of steering mechanism and linkage.

"I replicated the action of hydraulic steering rams on a real tractor by using an electrical servo motor on the model," he explained.

"The difference between a standard DC motor and a servo motor is that the output shaft of a DC motor revolves continuously in one direction, while the output shaft of a servo motor can only rotate through 140°, or just under half of one revolution.

"The steering linkage was coupled to the output shaft of this motor, so when the shaft turned either direction the steering linkages caused the tractor to steer.

"The axle and hubs were machined from aluminium and mild steel respectively on a manual milling machine."

Jamie designed the cab so that it could be easily removed to access the electronic components inside. The frame of the cab is made from 5mm square-section mild steel, and the top was milled from 5mm thick aluminium and then painted. The cab was glazed with black acrylic.

Originally, Jamie planned to make the model remote control, but he then decided to take it a step further and make it fully autonomous. The model is designed to follow a line on the ground by itself, and a circuit was designed for the mower to sense and identify any foreign object, like stones, and to lift it clear of the obstruction. The mower returns to the cutting position once the obstruction had been passed. The mower itself was bought in, but substantial changes were made to fit a lifting/lowering mechanism and the sensor circuit.

To make the tractor follow a line, Jamie placed two sensors underneath the chassis, one at either side of the line the tractor is following. If one of the sensors senses that it is straying on to the line, the servo motor for the steering changes position and steers the tractor back on to the right course. If the opposite sensor then senses the line, it will steer in the opposite direction to right itself again. A circuit board and microchip were needed to carry out these functions. A programme was written on the computer and downloaded onto the chip, which then controls the tractor as necessary.

Not surprisingly, Jamie got a top grade for his project.

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