IR distance detection speed

[BioniC187]

Hey all,
I just wanted to know how fast is IR? Say if i want it to detect distance, and then if it is at 38KHz, get the rate of change to get velocity and acceleration.
I would like to now its limits, coz if i want to detect an acceleration that occurs of a moving part of say a distance of 300mm in 15ms - Would it be capable of doing that? What other possible options do i have for monitoring such a scenario?

 

[MikeMl]

IR propagates at 3*10^8 m/s, just like visible light.

 

[crutschow]

IR propagates at 3*10^8 m/s, just like visible light.

For the few anachronists of us still using English units, that translates to about 1ns/ft, so you need very fast circuits to detect short distances using light.

 

[Mr RB]

Maybe he could use some IR light beams (lasers would be better as the beams are small and don't spread out) and count the time between different beam breaks?

They make cheap laser beam-break devices to measure bullet speed at 2000 metres per second so 300mm in 15mS should be easy enough even at a hobby level.

 

[BioniC187]

So the problem lies in the speed of the detection of the waves. Alright, so does the 38KHz mean that thats the rate that the Rx will sample at?

 

[BioniC187]

I could look in to lasers. In what medium do you get the fastest Rx devices? I can start from that component and build outward. I am aiming to be able to get about 250+ sample points within a 200mm movement in 15ms

 

[MikeMl]

So the problem lies in the speed of the detection of the waves. Alright, so does the 38KHz mean that thats the rate that the Rx will sample at?

38kHz is the carrier frequency at which the IR beam is modulated. The data stream carried by the carrier is a series of carrier bursts of 38kHz. It takes the IR receiver many cycles of 38kHz to detect the beginning of a new burst.

I think that the IR remote control receivers are way too slow for your application.

 

[Sceadwian]

You would need a hyper abrupt pulse, I think MikeMI is right there's no way a 38khz xmit/rcvr could ever provide a sharp enough on/off, the components used in those types of circuits will drift quiet a bit with time/temperature making calibration virtually impossible.

 

[languer]

Can you provide more information on what you're trying to achieve? Measuring speed in 200mm is doable (space sensors about 100mm and measure time between sensor incursions); but measuring acceleration (which is what I think you're after) looking at something like "250+ sample points within a 200mm movement in 15ms" is not trivial (imho).

 

[BioniC187]

I currently have a mechanical method to achieve this. But i wanted to investigate useing optical methods as i thought they would be more accurate and easier to install. Thanks all

 

[Sceadwian]

They might be more accurate, not sure about easier to install, but they're also quiet complicated to construct.

 

[Mr RB]

If acceleration is constant, you could measure it with three laser "break beams" in a row. The difference in time from the first two beams to the last two would give acceleration after a calculation, and also give final speed (assuming like I said that acceleration is constant).

 

[ronsimpson]

Hey all,
I just wanted to know how fast is IR? Say if i want it to detect distance, and then if it is at 38KHz, get the rate of change to get velocity and acceleration.
I would like to now its limits, coz if i want to detect an acceleration that occurs of a moving part of say a distance of 300mm in 15ms - Would it be capable of doing that? What other possible options do i have for monitoring such a scenario?

Have you thought about doing this with sound? There are range finders that work with 40khz sound.

 

[picbits]

Could you not use an accelerometer attached to the item to be measured ?

 

[dr pepper]

This isnt really my area, however I think newer police speed traps use an infra red laser, instead of the old microwave system, they can book you from a long way off.
I think these speed traps use a phase discriminator, the phase of the transmitted wave is compared to the received wave and the difference is the speed of the target, sort of an optical doppler shift.