Re: How to caracterize transfer function of signal chain

Dsp1000 wrote:
Dsp1000 wrote:

...

A laser range finder is a laser distance (range) measuring device.
It will give you distance to an object you shine light on as a binary
or
analog output. There are two main methods to use: Time of flight of
the
laser beam and phase difference detection. The latter is better on
short
distances but has some challenges associated with it. It is a more
advanced
version of the latter I am looking at.

All experts on this list: anybody with laser range finder experience?
Not every distance-measuring device is a range finder. Laser interferometers, while capable of measuring movements significantly smaller than a wavelength, do not make absolute measurements. There is a

phase uncertainty of half a wavelength in distance (a whole wavelength in the round trip.

Every laser has a coherence length. Interferometry ceases to function when the round-trip distance approximated the coherence length.

These two issues make time of flight attractive.

Jerry

Ok regarding tine of flight contra phase. However the distance that is
going to be measured here is down to 10 cm and up to approx 30 meters. I
think the pulse delay will be so short that it might be difficult to
detect?

Yes.

How would you solve this problem?

A laser interferometer can count fringes and so make differential measurements. (It takes careful thought to make this work in the presence of even minute vibration, but that's an implementation detail.) If you have some way to establish a base-line distance (contact, or 20 cm, or whatever) You're good to go.

The laser must be eye safe, low cost
(laser pointer type). The detector should use a photodiode or avalanche
photodiode. There might be a CPU running DSP algorithms in the system. The
range will be 10cm to 30 meters. The height must be measured in 1cm
resolution.

The basic resolution of an interferometer is half a wavelength, but you should be able to halve that and maybe halve it again. Fine enough? :-)

You will need at least two photodiodes to get direction information, just as with a quadrature shaft encoder. Counting fringes is useless without knowing whether the distance is increasing or decreasing. The fringes will be going by much too fast for the processor to count them. You'll need some external hardware.

I'm not at all sure that a laser pointer has enough coherence length for a 60-meter round trip.

Jerry
--
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