Measuring speed when approaching an obstacle using ultrasonic sensor is pretty trivial. Take two distance measurements, divide by time between them, you have the speed.
But a sensor aimed diagonally at the ground, in horizontal movement, will keep giving the same distance readout. The only thing changing with speed will be the reflected frequency – doppler shift of the reflected ultrasound.
Can any inexpensive, common, compact sensors perform such measurements? Or would it require "cooking my own", with frequency analysis and such?
…or maybe I'm trying to force open an open door, and there's a different, simple, compact, inexpensive solution for measuring groundspeed at a distance of a couple meters? (other than GPS; this must be able to work in terrain with poor GPS reception.)
Best Answer
I'm not aware of cheap ultrasound sensors that can measure the Doppler in the return signal. At the risk of answering something that isn't quite what you asked do you need ground speed or is air speed sufficient?
If you place two ultrasound sensors facing each other in line with the direction of travel and measure the range between them then you will get a measurement error proportional to the airspeed in the direction between the sensors. e.g. If measure over 1m then you would expect the ultrasound to take 1/330 = 3.03ms to arrive. (taking the speed of sound as being 330m/s for convenience). If you are traveling at 10m/s then you will get a reading of 2.94ms pining from the front to the back since the sound is traveling at 340m/s relative to your device.
This is how ultrasonic wind speed measurement systems work, the only difference is that you are measuring the airflow caused by your movement rather than the wind.
Pinging in both directions (back to front and front to back) and taking the difference between the two allows you to compensate for temperature effects and separation measurement errors.
Of course this all falls apart if your environment has significant wind in comparison to your velocity or if you place the sensors somewhere where the air flow around them isn't representative of your speed.