precise speedometer

[donkey]

so I got to thinking about a project I have in mind but to actually get it to work I need a speedometer and odometer. the idea is pretty easy plug a rotary encoder on the axle code a little bit and use some awesome calculations to get the end result...

however there are a few issues with this, for example if a wheel is only 1mm smaller then the circumference changes enough that the measurements are way out.
if at any point the axle is in the air then the wheels will turn but the device will not actually move
lastly if for whatever reason the device slides then it will not detect his

so what alternatives do I Have? as this is a small item I relay don't want to use gps as its kinda overkill I was thinking along the lines of a few NRF24L01s with the same principal but am unsure how big of a task this would be, also do you reckon I could get a height from using this too?

lastly I thought about lasers, like a mouse, but there are heaps of surfaces where mice just don't work.

the device is free roaming (not on tracks or anything and no set path) so any help would be appreciated

 

[(*steve*)]

you could use the Apollo lunar rover method. Measure the speed of each wheel and use the third fastest (or the second slowest).

you may be able to use g force sensors to compare expected acceleration with actual acceleration in order to tune rotational speed with velocity., although GPS would probably be easier. I.e. It would probably be further from your target.

 

[Mongrel Shark]

Are there walls or other objects around?
Could you use Sonar/acoustic distance metering?
What about a laser or sonar to measure tire wear? You could get an ideal location etc so it would be fairly consistent.
If its a small thing traveling distances less than kilometers. gps is not going to be real accurate either.

If high accuracy is required I'd be looking at multiple data collection methods. ie all of the above... And check them against each other.

With the wheel method, if its a dedicated measuring wheel with no other purpose it should be possible to remove 98% of the issues you are concerned about. With good mechanical setup. ie a passive Castor on reasonable suspension wont get airborne or slip/slide in any way. And you could get turn radius info to compare against wheel rotations etc.