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Need help with measuring speed of an object

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Re. using sound, I do not think that will work well. The speed of sound in air varies quite a bit with conditions.

I spent the last little bit drawing a sketch of a baseball (3" diameter) going past sensors (red circles) to show the effect of sensor spacing on maximum error, assuming the path of the baseball is perpendicular to the axis of the sensors. As a first approximation, the timer would start and stop at first break. The location of first break (e.g., from an 8-bit or 16-bit word depending on whether 8 or 16 sensors are used at each gate) could be used to help determine path.

View attachment 68892

It is a little fuzzy because of all the conversions I had to go through from my CAD program to get a jpg.

I am not quite sure how you can get path and time with just 2 sensors. As Pommie suggested, measuring the time the beam is broken at each gate could be very useful information.

John

Oops, "apaced" should be "spaced." It is too hard to change now.
 
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Excellent! Thank you! In looking at the 13% Max Error scenario, wouldn't this be slightly less since the point of origin for the ball does not change? i.e. if ball was say 4" behind the beams at the start = T=0, it seems it would trip the lower sensor (albeit briefly) given the angle of approach. Graph looks to be a parallel shift? Or am I not looking at this correctly which is entirely possible!

Thank you!
 
Thinking about this a little more made me realize that the calculation is simpler than I first thought. You effectively get two pulses from the sensors and you can simply calculate the time between the center of the two pulses.

A pic with a Gated Timer1 and Comparators could do the measurement completely in hardware with a resolution of 200nS using a 20MHz crystal.

Mike.
 
Thank you! This is very helpful. Just to use an example to make sure I understand your point. (Ball hit to left side with "A" sensors on left). If sensor A start=1 and stop = 7 ; B start = 3 and stop = 5. So middle of start = 2 and middle of stop = 6 so time value to use would be 4?

So this would essentially give you average time for an assumed "middle" of the ball? It's a great idea!

Thanks!

oops - think I messed up the numbers as you would be looking at pulses. ..
 
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I don't understand your start and stop times. I'm assuming that the sensors are 6" apart and the ball is 3" diameter.

So, the first sensor (A) might be start = 0, stop = 130 and second (B) might be start = 280, stop = 310.

Centers are 65 and 295 so the flight time is 230.

Edit, I just realized that the flight time should be at least double the longest time. Oh well, that's what happens when you make up numbers.

Mike.
 
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I took some time to draw you this. It is a concept I considered for measuring the speed of a pool break shot. I Hope it gives you ideas:

View attachment 68897

Edit: There is an error with the final speed calculation. The result 21.9 is not the speed. It is the time the ball takes to travel one diameter length.
So, if the ball diameter is 10 cm and the time (21.9) is in milliseconds, then the ball is travelling at 4.6 m/s.
 
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Great, thank you! Very interesting. Definitely addresses the issue of the ball traveling at different angles & speeds.
 
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