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PIXIE
PIP
DASH FREEDASH 1 uses two wheels in a “wheelchair” arrangement. Forwards movement is by driving both wheels forwards. It turns by driving one wheel forwards and one backwards. The wheels are mounted on shafts and these are driven by a worm and gear arrangement from two separate DC motors.The motors are driven from an H-bridge using bipolar upper elements and power FET lower elements. Both bipolars ON gives a free-wheel condition, both FETS ON gives a braking action.
Each motor shaft has a disk with 8 holes in it. An LED and phototransistor work with these to give a notional shaft position.
The mechanical system and drive electronics are taken from Swallow Systems’ PIXIE robot.
The electronics uses an Intel 80C31 microprocessor with memory expanded to 32K RAM and 32K EPROM. Two external interrupts are used to measure motor revolutions. Four I/O bits are used for the motor drive. There are only six bits left for sensors and this has proved to be a serious limitation.
The sensors are all based on Hamamatsu photo I.C.s. These make very simple reflective sensors. The LEDs used with them are all high-brightness visible red types. This makes debugging the sensor positions MUCH easier. The sensors use modulated light and are very tolerant of sunshine or flourescent light.
The software is all written in Microprocessor Engineering's FORTH 83. Without the interactive features given by this, DASH 1 would not have been ready for the competition in time.
Some preliminary work on mazes was done early in the year, but development started properly on June 1st. A PIXIE was modified to remove the keyboard and add 32K RAM. Sockets for sensors were added using a modified top plate. A socket was fitted to bring out the UART connections for connection to a host computer. RS232 level shifters were put in a pod connected to the UART socket. a long lead for connection to the host completed the arrangement.
Software development started using an EPROM emulator until the compiler was running satisfactorily. From then on it was a case of bursts of interactive development on a practice maze followed by burning a new EPROM. The current EPROM is number 19.
The algorithms used fall into four parts. One function is wall following and positioning the mouse. This is where more sensors would have helped a great deal. Positioning is done with one sensor each side. Where there is only one wall, DASH 1 has to “waddle” to correct its position continually from too close to too distant.
Another function is navigation in the sense of keeping track where the mouse is. This can cause problems in pathological cases where the mouse has a long run of continuous walls.
The third function is to find the centre of the maze. DASH 1 uses three weighted rules. It likes to carry straight on, it likes to get closer to the centre and it likes to boldly go where it has not gone before. These rules overlay the basic idea of not hitting walls!
The last function is to optimise the route from start to centre. This is done continuously as DASH 1 moves. By the time it reaches the centre it knows its best route, of the cells visited, for the return path.
These main functions are coordinated to follow a scheme of:- find the centre, return to the start by a good route, go back to the centre by a good route, return to the start fast, get to the middle fast.
After DASH 1 has traversed 80 cells, about 6 minutes worth, it panics and starts to travel fast. Fast travel can cause positioning and navigation problems.
There are some known problems with DASH 1. It cannot turn very repeatably due to backlash in the drive train. This will be solved by adding more sensors so that it can position itself more accurately.
If it becomes too close to a wall, it can sometimes fail to detect the wall. This will cause it to jam against the wall with disastrous results. This can be solved by better sensor design.
The software is not well structured. The next version may use the multi-tasker to run the maze-solver and centre-finder functions as separate tasks. The terminal task will also run separately to allow interactive debug DURING a test run.
Final Outcome After all this effort, DASH 1 had some problems in competition. On its first run, it got "lost" and thought it was in the centre when it was one cell away. The problem was caused by a long run of continuous walls on both sides. A swift modification to the software and it managed to reach the centre in about 41/2 minutes. This was good enough to get into the final of the intermediate competition. In the finals it did no better and was placed fifth. Better luck next year!
To put this into perspective, DASH 1 was the slowest to actually reach the centre out of some 15 entrants. The schools class was won by a mouse that only traversed 3 or 4 cells. The winner of the advanced class, after it had researched the maze, travelled from start to centre in 7.9 seconds. We have a long way to go yet!