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Introduction
The micromouse competition has been running since the late 1970s around the world. As far as I know, the modern form of the competition originates in 1980 or so. Essentially, you have a wooden maze made up of a 16 by 16 grid of cells. Mice must find their way from a predetermined starting position to the central area of the maze unaided. The mouse will need to keep track of where it is, discover walls as it explores, map out the maze and detect when it has reached the goal. As if that was not enough, the winner is the mouse that manages this the fastest. There are many versions of the full rules on-line and there are a number of minor variations on how the score of the mouse is determined. Although modern micromice are relatively sophisticated beasts, this is an extremely challenging undertaking. One of the earliest mice, now about 20 years old is still regularly entered in competitions and puts up a very respectable show. The micromouse competition appears to have waned a little in popularity in recent years. The UK events haven’t attract very large entries. There still seems to be plenty of interest in…
History
The micromouse competition has been running since the late 1970s around the world. As far as I know, the modern form of the competition originates in 1980 or so. 1977 IEEE Spectrum magazine introduced the concept of the micromouse. In May 1977, Spectrum announced the ‘Amazing Micromouse Competition’ which would be held in 1979 in New York. There were 15 competitors running out of around 6000 initial entries. This competition involved mice finding their way out of a 10′ by 10′ maze. When the competition was held, the winner was a high-speed, dumb wall follower. 1980 Professor John Billinsley, of Portsmouth Polytechnic, modified the rules and introduced the first European competition – held in London at Euromicro. the rule changes required the mice to find a goal in the centre of the maze and wall followers could be prevented from finding the goal. There were 200 enquiries and 100 entries, but only 9 mice at the finals. Nick Smith’s Sterling Mouse became the first ever (and that year the only) micromouse to find the centre and know it had done so. Although performance was less than stunning at about 0.18m/s, it was and still is a remarkable feat. 1981 At…
Rules
There are minor regional variations of the competition rules. I have put the rules relating to the maze in the maze pages. The rest of the rules relate to the micromouse itself or to the running of the competition. These rules have been shamelessly cribbed from http://micromouse.cs.rhul.ac.uk/mtech/rules_main.shtml where the definitive UK rules are to be found. The Mice Although the superstructure of the mice may ‘bulge’ above the top of the maze walls, mice must be subject to the following size constraints – width 25cm, length 25cm. There is no height limit. Mice must be completely self-contained and must receive no outside assistance. All mice should be fitted with a suitable hook or loop, suitable for lifting the mouse out from the centre of the maze, should this prove necessary. The method of wall sensing is at the discretion of the builder; however, the mouse must not exert a force on any wall likely to cause damage. The method of propulsion is at the discretion of the builder, provided that the power source is non-polluting – internal combustion engines would probably be disqualified on this count. If the judges consider that a mouse has a high risk of damaging or…
Batteries
Batteries are going to be your energy source for all practical purposes. There are a number of variables for you to consider when looking at the battery choice. For economy, you will almost certainly want to choose rechargeable cells. These are more expensive in terms of initial outlay but will soon pay for themselves after a few charging cycles. Size is another important consideration. in general, larger cells last longer. Higher voltages require more cells. While your processor probably only needs a 5 volt supply at a few tens of milliamps, the motors and sensors may require high voltages and large capacities. Take, for example, a stepper motor driven mouse. For best dynamic performance, you may want to use a dozen or more cells to give you 15 or so volts. The steppers may draw 2 amps when fully energised and you mouse will need to stay running for at least 15 minutes. For this example, 600mAhr will just about do the trick but with no margin for error. Power management will become important under these circumstances. Motors must be turned off when not needed, sensors will need to be pulsed for minimal periods and you probably want to avoid…
Command and Control
Any project like micromouse is a tradeoff between hardware and software. One thing seems to be universally true however: No matter how easy you think the software design is going to be, you won’t have enough time to do it right. From reading around, it would seem that most constructors leave too little time for software development. No amount of work in a simulation environment can guarantee success in the finished platform. From the outset, you will probably have some ideas about what will be the most difficult part of the control task. Chances are, it will be the basic motor control. Perhaps the most difficult task that your micromouse faces is just running down a corridor in a straight line. The faster you want to do it, the harder it is. Oh, and you had better be able to stop where you want to as well. The maze solving and navigation may seem more difficult but turns out to have some well-known solutions which can be tested on a simulation. I have dealt with maze solving elsewhere. Bart Provo suggested a development sequence in a newsleter article in 1990: Motor control and speed sensing Control of distance moved Turning…
Mazes and Maze Solving
The maze used for micromouse is described in the rules. For (my) convenience, I have reproduced the maze description from the IEE rules. I have seen no significant local variation from this description for other micromouse competitions. The maze shall be a 16 x 16 square array of 180 mm x 180 mm unit squares. The walls constituting the maze shall be 50 mm high and 12 mm thick. Passageways between the walls shall be 168 mm wide. The outside wall shall enclose the entire maze. The sides of the maze shall be white, and the top of the walls shall be red. The floor of the maze shall be made of wood and finished with a non-gloss black paint. The coating on the top and sides of the walls shall be selected to reflect infrared light and the coating on the floor shall absorb it. The start of the maze shall be located at one of the four corners. The starting square shall have walls on three sides. The starting square orientation shall be such that when the open wall is to the “North”, outside maze walls shall be on the “West” and “South”. At the centre of the…
Robot Dynamics
By dynamics, I mean things like, how fast can you make it go. How much acceleration is enough and how fast can you negotiate bends. Tempting though it might be to think that speed is of the essence, there is no point tearing up the coutryside if you don’t know where you are going, or you get lost or you end up in a ditch. As you cannot win without getting to the maze centre, solve all those problems first then work on doing it quickly. Having said that, it might be as well to ensure that your design is capable of adequate performance when (if) you have solve the more critical task of just getting there. There are many factors to consider in relation to the dynamic performance of your micromouse. Friction, particularly between wheel and maze, is pretty critical to your ability to accelerate, brake and turn reliably. the maze surface will probably be something like plywood or particle board, painted black. In spite of the existence or rules, you cannot depend upon the surface to be smooth, clean or particularly grippy for your tyres. You can’t even depend on it to be the same everywhere. Apart from…
Navigation
The competition is a race against the clock. you have to explore the maze, find the optimum route to the target cells and then run from start to target as fast as you can. these phases present different challenges to the mouse. During exploration you will want to be relatively careful, you can not afford to lose your location at any time. Extra care may allow you to detect walls in cells you have not yet visited. Mouse speeds are likely to be lower and additional wall checking activity will help ensure that you don’t miss wall data because of variations in the reflectivity of the walls. Additional, real-time calibration of the sensors may be possible. After you have found the centre, you have to decide whether to carry on exploring in the hope of finding a better route or just run with the one you have. A cunning maze design may penalise you for not exploring further. Now you have to perform a speed run. Assuming the rules reward you for your fastest run, many mice will perform several runs. A fast phase from start to centre should be followed by a relatively safe run back to the start.…
Motors
You have to make it go somehow. I don’t think there are any serious alternatives to a mouse that uses batteries for stored energy and some kind of electric motor for the driving force. There are some basic (ie simple enough for me) physics to look at and then the choice between two common types of electric motor. I have only bothered with stuff on steppers and DC motors as these will do the job and present enough complexity to keep anyone happy. If you want to try synchronous, AC or brushless motors then you carry on. If you want steam or clockwork then you are already in trouble. Remember that there is a rule about non-polluting drive systems. Essentially, you want a driving unit which you can easily and accurately control in terms of acceleration, speed and position. It must be light, small and use as little energy as possible. A minimum of ancillary electronics would be good and it should preferably be easy to mount and cheap. If you find a motor that genuinely meets these criteria, tell lots of people, they all want to know. In the real world, which of these are you going to forfeit?…
Sensors
Your mouse is going to need sensors to tell it about itself and its environment. Odometry is about how far and how fast your mouse has travelled. Open loop control is used with stepper motors. DC motors can take advantage of more sophisticated closed-loop control systems. Wall sensors are used to detect the presence or absence of walls and to verify your position in the maze. They will also be important in ensuring that the mouse maintains an appropriate path without hitting any walls. For wall sensors, it may be more important to have good repeatability that absolute accuracy. The key is to avoid hitting anything. Thus it does not matter as much if you run with a small error as long it does not grow. It is important to make sure that you place wall sensors well in front of the driving walls, or at least, the centre of rotation of your mouse. The greater this distance, the better your ability to maintain a straight course down the centre of the maze cells. You will want to sample the sensors constantly while the pose is moving for good positional control. There are some key places/time for sampling. Detecting the…
Actual Mice
As you can imagine, there have been quite a few mice built over the years. Here are details of a very few:
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Robot Dynamics
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