Trying to measure speed

Hello there!

I am looking for a method to measure speed.  My current thoughts are to get two ultrasonic sensors (A and B) set at a defined distance and measure the time between an object tripping sensor A and subsequently tripping sensor B.  Time and distance will give me speed.

What would be the best way for me to accomplish this?  As a Software Engineer I feel comfortable with the software side and learning OSC or other API as needed.  That said, I have not worked with hardware at this depth before.

Thanks,
Jason

I think the first question to ask is:

What is the object you are measuring speed of? That would give some insight into what kind of sensors you'll need.

You have the right idea, though, you'll just use two points and measure the time between their triggers. You could even make the distance programmable. Using an LCD screen for readouts, you could have everything inside the board with no PC necessary.

Thus no OSC, only C programming and firmware API. Quite easy actually

Previously Aaron Tunell wrote:

I think the first question to ask is:

What is the object you are measuring speed of? That would give some insight into what kind of sensors you'll need.

You have the right idea, though, you'll just use two points and measure the time between their triggers. You could even make the distance programmable. Using an LCD screen for readouts, you could have everything inside the board with no PC necessary

Thanks for the response.

The final project is to measure a car traveling between 25 and 55mph on a standard road (30mph ~ 30ft/sec).  Im not sure how much distance I will need in-between the sensors.  I also do not know how accurate the sensors can be.  Im sure its a function of money in that if I had infinite money, I could get the perfect sensor.

If its only some C coding, it should be an issue.  Ive been over the high level of the build process, I dont see any major issues there except the standard learning curve.

My current plan is to acquire the MC and 2 sensors (if the price is right).  Prototype the system inside and if all looks ok, rig everything up to work outside.  I will have other obstacles outside, but I believe if I can get the sensors to read within 2mph or actual, I should be OK.

Where is a good place to look for these sensors?  How can I tell which sensors will meet my needs?  How can I determine what sort of distance I will need between the sensors to accurately read the speed?  My biggest concern is the reaction time of the sensors back to the MC.

Thanks all,

Jason

If its just speed your trying to measure i would try another way. In my experience I find that range finders can be erratic and require some buffering. the range is limited also. so you will only be able to tell speed as long as the object is within so many yards or meters from you. Another option is an optical encoder or something similar. I would try putting reflective tape(or something stronger) inside the wheel and using IR led and sensors to watch how many times its turning per second then crunch the numbers to get a value for velocity.

Previously Jon Gervais wrote:

If its just speed your trying to measure i would try another way. In my experience I find that range finders can be erratic and require some buffering. the range is limited also. so you will only be able to tell speed as long as the object is within so many yards or meters from you. Another option is an optical encoder or something similar. I would try putting reflective tape(or something stronger) inside the wheel and using IR led and sensors to watch how many times its turning per second then crunch the numbers to get a value for velocity.

Thanks for the reply.

The cars travelling through the sensors are not part of a controlled environment.  I am having trouble finding information on the precision of available sensors.  What sort of experiences has everyone had?

i'd point 2 lasers across the road and point them at light sensors, when the beam breaks, your object has gone through the "light gate." Take the time between gates 1 and 2 and calculate speed using the distance between the gates.

You could also use one of those rubber hose thingies that the city governments use when they're trying to count traffic on a road.

Previously Aaron Tunell wrote:

i'd point 2 lasers across the road and point them at light sensors, when the beam breaks, your object has gone through the "light gate." Take the time between gates 1 and 2 and calculate speed using the distance between the gates.

You could also use one of those rubber hose thingies that the city governments use when they're trying to count traffic on a road.

 

Which ones would you recommend to work well with the MC?

there are light sensors in the store on this site, and sparkfun.com has some compact lasers you can use.