hello. I’m a absolute novice at electronics(but can code .NET) and wanted to know if the following would be possible. I want to measure the speed of a small object which could be moving at 100mph. I was thinking about using 2 IR reflective sensors at a set distance appart and just measure the time between sensor triggers. would IR sensors be acurate/fast enough at detecting such quick movement.
@ i*windows - Welcome 
What distance / accuracy are you looking at?
Being that .netmf is not real time it wont be perfect but depending on your needs might be fine with the IR triggers
hi @ Justin. So the distance of the object from the sensors will be <5cm, and the distance to measure will be <20cm. I am trying to measure the speed of a golfclub as it travels over 2 points. I would like it to be acurrate to 2 or 3mph - but the most important thing is that the results are consistent. I dont mind if the calculated speed is a little out, but I want it to be consistent. Once I have managed to get 2 points to work I would like to create an array of sensors that can also measure the path of the club.
By my maths this speed equates to 44.704 m/s and therefore I think the real question is how far apart will your two sensors be? If they were 44.704 metres apart then 1 second would elapse between the first and second sensor firing. As they get closer the time between the two timing events shorter. At 1 metre the time between the first and second timing event would be approx. 22ms. When you know how close the sensors are you should be able to determine the time between the two events. Then you will need to determine if H/W running a RTOS will be accurate enough for the accuracy you need in your measurements.
It might be worth considering some external hardware that accumulates ticks between the first and second timing events and these ticks could then be used to determine speed.
Hope that helps a little.
Not sure how accurate that will be.
20mph is 894.08cm per second…
Edit…James bet me to it… 
So the rest is …
@ Jason, thanks for the input. your suggestion is pretty much what I summised. Ideally I would like to store the ticks then just calculate the speed. I guess I can just store the ticks in memory or a filesystem. I guess I need to buy a board and some sensors and play around with them.
Normal .NET interrupts provide the time at which the interrupt was fired, so even though it’s not real-time, you should have good numbers to work from. I think it could probably work, even though I’m normally a glass-half-empty sort of guy
another issue is how would I connect an IR reflector? I dont see a module for this. there seems to be a grove component that I could maybe connect.
I’d use a simple phototransistor and IR LED setup. You can detect the output of the phototransistor going low when the “eye” is blocked by the ball. Phototransistors are fairly quick, in the microseconds to tens of microseconds range, which ought to be fast enough for reasonable accuracy.
thanks for the feedback
By the way, as you mentioned you’re measuring golf balls, we have one of these: http://www.trugolf.com/golf_simulator/indoor_resort.php
It actually uses strategically placed microphones to determine the speed of the ball (using timing) and the location of the impact on the “screen” (using triangulation), so it can derive trajectory from that.
@ godefroi. To start with I’m just interested in the club head and how quickly that travels over 2 points. For trajectory I was planning on measuring the impact location on a screen in relation to the start position… but I am a long way from that.
To get me started I’ve decided to purchase a FEZ Cerbuino Bee and some reflective optical sensors. I’ve found a very nice resource here .NET Gadgeteer | Integral Design which shows how to connect non gadgeteer components.
I’d like the trugolf below, but I dont have $10k+ and its more fun to try and figure out how these things work 