Using ultrasonic range finder to detect speed

Projects General Guidance
1

Hi, I'm new here and pretty new to Arduino and electronics(but eager to learn), and since my ultrasonic range finder hasn't arrived yet I'm limited to just thinking about it...

Could a person use one of these sensors to detect an object's speed? Say the sensor was positioned on the ground, facing up and a ball was thrown through the beam at around 50mph. I'm thinking that if I measure the amount of time the ball is "seen" by the sensor, then use range of the ball and knowledge of the sensor's beam angle to judge the width of the beam at that particular distance, I can find the object's speed.

  • distance between sensor and ball will vary from 3' to 6' (91-183cm) to accommodate different pitchers' heights
  • the object will always be a standard softball, 3.8" (9.7cm) in diameter with a relatively flat trajetory
  • ball will be thrown directly over the sensor and always at the same angle to the sensor
  • if the sensor beam is not completely conical but wider in one direction, sensor will be placed so that the ball travels along the widest dimension
  • multiple sensors always an option

I've never played with one of these sensors, so I have yet to see the output and how it behaves. The possible problems I see are:

  • the sensor can not take readings quickly enough to get a good enough resolution on speed
  • the "detection boundary" is not precise enough to just use trigonometry to accommodate various pitchers who pitch at different heights

So, is this possible or should I not expect this much from this type of sensor? Multiple range finders are completely doable if that is what this task requires. Sorry for the long post, but it's the first time I've tried to explain this idea in detail. Any ideas and experiences are much appreciated.

Thanks,
Stan

2

Most rangefinders have a limited repetition rate, which makes them less than ideal for break-the-beam type applications.
Typically, you will be able to make 20 to 50 readings per second, which means you could easily miss a ball.

Managing multiple sensors could reduce the update rate still further, because it will be impossible to determine which echo belongs to which transmitter, so you would need to fire one transmitter and wait for all possible echoes to die away before before firing the next.

3

Speed measurements by Sound or Radio waves usually use the doppler effect, the apparent change in frequency due to changing position of some part of the system.

Lots of stuff: Google "ultrasonic doppler speed measurement"

4

...and don't expect hobbyist-grade ultrasonic rangers to be able to make Doppler measurements.

5

I see, might end up with a back-up sensor after all.

Thanks for the help.