Musings on the new corner to corner competition

These are some of the thoughts that have occurred to me when contemplating the new competition. Some of the ideas will be used in future offerings either in terms of competition entries or kit materials. One important consideration is that competitions are always easiest to win in the first year. We can expect that next year's mice will be considerably quicker than this year's.

One great difference with the new competition compared to the large maze-solving competition, is that it is possible to design mice without a processor! Wall followers need nothing more than reflexes to keep at the correct distance from the wall. It should be possible to solve the problem with a mouse along the lines of DASH FREE with area coverage sensors. It remains to be seen whether such mice are competitive.


Given that the only time that counts is the first run from the start to the end, there seems little benefit in trying to find the best route unless you have some kind of vision system that allows you to find the best route before you set off. If you cannot find the best route you will need to use a strategy that guarantees a successful run and "second guesses" the maze-setter.

There are two obvious strategies:- Follow the left-hand wall and follow the right hand wall. Either of these will work and, for any particular maze, one will be better than the other. The maze-setter can choose to make one much easier than the other so it may be an advantage to be able to select which wall your mouse will follow. Unfortunately, the ability to follow either wall has a cost in terms of the number of sensors needed and hence the weight of the mouse. It may be worth having two entrants, one for each wall and run both in the competition to guarantee best results!

There may be other strategies. The maze-setters have promised to make it so that it favours "apparantly intelligent" mice. I don't know how they will do this.

You can expect that the maze will contain at least one dead-end. A symetrical mouse that can run both backwards and forwards may be able to detect this and simply reverse direction rather than having to turn round.

Wall followers don't need to know where they are in the maze. More intelligent mice may need to know this. This will need to be, at least partly, by dead reckoning as the mouse can only see a "landmark" when it passes a gap in one of the maze walls. This type of dead reckoning can be done by counting how far the wheels have turned but you can be completely messed up by wheel spin or skidding. Some mice use separate undriven sensing wheels to avoid this problem.

Drive train

Our drive train is small enough for the competition. I have used it in DASH ONE and DASH TWO within the same size mazes. However, it is not small enough to guarantee a U-turn within the width of the maze. What this means in practice is that mice using our drive train will need to be able to run at least one of the motors backwards to cope with dead-ends. The simplest way to implement this is with a reversing relay. H-bridge drivers can also be used.


There are at least three strategies for controlling the speed of the DC motors. I have used all of these in the past. Which you find best will depend on your specific needs and capabilities. The strategies are:-

1) Apply a "stiff" DC voltage to the motor and rely on the internal feedback mechanism from the back emf.

2) Time-share the motor between applying power and using the armature emf as a speed sensor.

3) Apply a PWM signal and use the position sensors on the drive train to derive a speed.

Option 1) is very simple to implement but will have problems when the motors are heavily loaded. It has potentially a rapid response time. This is the system used in the DASH FREE kits and has proved to be capable of good performance. My first attempt at the corner to corner competition will use this technique.

Option 2) is moderately complex. It does compensate for increased load on the motor but may have problems due to having a slow response time. Creag Louttit and Steve Eakin's THRUST SSC uses this technique and it works well.

Option 3) is most complex but potentially has the best performance. In practice it is difficult to exploit this at high speed. This system is used in PIXIE, DASH ONE and DASH TWO. All of these machines run comparatively slowly. With my implementation, this system is only practicable for mouse speeds up to about 20cm/sec. I would need to increase the microprocessor throughput proportionately to achieve higher speeds.

Whichever strategy you use, you should consider whether it will allow you to brake the motors. Good brakes will make a tremendous difference to the performance of the mouse.


The maze can be sensed in many ways including mechanical sensors of various types. Infra-red LEDs and phototransistors are favoured by some of the faster competitors. I currently prefer Hamamatsu's S4282-51 with multiple visible red LEDs to give area coverage. This is probably because of my experiences to date with them. In principle, the infra-red systems could be smaller and lighter but I like the ability to see where the light is going and the lack of interference.

What I would like to be able to do with the sensors is to measure how far away a wall is with a resolution of 5 or 6 bits. This would let me make a proper servo-mechanism for maintaining my distance from the wall. Unfortunately, I cannot achieve this resolution with my sensors so I am limited to a simple "bang-bang" control where the mouse is either too close to the wall, just right, or too far away.


Sensor positions will depend very much on the speeds that your mouse is capable of. However, there are a couple of things that you should know.

Imagine a very crude mouse with just one sensor mounted on the right hand side. If it finds a wall the mouse turns away from the wall by running the right motor, if it cannot find a wall it should turn towards the right by running the left motor. This mouse is intended to "wobble" its way round the course. Such a mouse will work but only if the sensor operates over an area rather than at a point. Provided the area covers a line from just behind the right wheel axle forwards it should be possible to make the mouse follow the wall. The sensor area must extend back this far to avoid the mouse getting hooked-up on wall ends where the mouse has to turn 180 degrees. The sensor must extend far enough forward so that when the mouse turns to the right, part of the sensing area gets closer to the wall!

For more intelligent mice, it is very profitable to know how far away the wall in front is. If you know it is further away than your worst-case braking distance, you can afford to go flat out. Once you detect the wall, you can start to apply the brakes. You might like to use the long-range sensor from our application note for this purpose.

 You can email queries to us at:- sales@swallow.co.uk

Date Last Modified:11/2/99