Ways to build a speedometer for a bike or skateboard

Question

I'm trying to think of different ways to build a device that will allow me to mount it to any moving object so that the moving object will be able to discern it's speed (It DOES NOT have to be very accurate)

Requirements:

• I just need to know a few levels of speed: not moving, slow, medium, fast, etc.

• I want to be able to mount it on a bike, maybe a scooter, maybe a skateboard, etc.

• Cheap - I want to be able to make this thing for

Bike computers work with a magnet mounted on a wheel and a hall effect sensor to pick up each rotation.

A velocity can be calculated by discerning the distance traveled given time. (the bike computer method is great, but it doesn't allow me to use it on a skateboard, for example)

Are there optical sensors that can be pointed at the ground?

An infrared distance sensor won't work (right?) because it is always the same distance from the ground.

What about a sound sensor? Can it measure some sort of doppler shift?

GPS is too expensive

I potentially can use an accelerometer. How would I know the difference between a stand still vs. no acceleration.

Answer 1

Yes, there are optical sensors that can measure speed. I worked at the proving grounds for a major car maker. In the past, anyone testing cars on the oval used "fifth wheels" but these were falling out of favor as of ten to fifteen years ago. Now optical sensors that measure the how the correlation of random surface texture with itself changes over time.

A similar technique is being researched to measure blood speed in blood vessels with tiny probes inserted by catheter. Same math, very different application.

There is a possibility of taking the sensor out of a wireless optical mouse, but it's unlikely to work if the distance between sensor and moving surface is more than a few cm. "Few" might mean only one! But then, you could use a lens and change that.

But anyway, what sensor can you run out and buy today? If I had more time I'd look for specific examples, but try googling for "optical flow speed sensor" and see what comes up.