Re: Help with PSD (range finding system)

You haven't given us much to go on here. Note that any effect that
spatially modulates the laser (ie changes its transverse power
distribution) will affect the centroid location as detected by your
Position Sensitive Detector. Are you sure that you are not detecting a
contrast edge or some other change in the spatial function (over the
spatial coordinates of your target) for the BRDF? Also note that laser
light scatter and other stray light sources may be affecting your
calulated position and this will contribute offset errors in different
levels for strong returns and weak ones. Laser light scatter is the
worst since it can't be sampled by disabling or modulating the laser.

Note: I have not see any evidence of light level induced variance in
the normalized signal ratio from a PSD. My guess is that either the
optical return varies (is spatially modulated differently) between your
"black surface or a white surface" or that you have a non-linearity in
your receiver electronics.

Couple of things:

Why are you modulating the laser if you're not using the dark state
(laser-off) PSD signals? That's the only reason that I know for using a
modulated source.

Why are you modulating so fast? If you PSD has any size at all, it may
have enough capacitance to give a first order (RC) time constant that
is comparable to your modulation period, especially if you're using a
large valued load resistance. If this is the case and you're digitizing
before the signal presented to the ADC has "settled," then position
calculation may be affected especially for cases when the X1 and X2
have very different values. I have design PSD receivers and have never
exceded 20kHz for this reason. It is also important to sample the
signal (on or off state) only after the detector and amplifiers respond
(usually no sooner than 2 "RC" time constants after the laser
switcfhes? Note that the X1 and X2 channels use diferent discrete
components; thus, their behavior may vary a few percent (gain and
especially time constants). A few percent is a large variance when
you're trying to achieve accuracies of 5 to 50 parts per million.

Calibration should only be used to get the last bit of performance when
the factors affecting performance are well known and understood. If you
have a fundamental design flaw (trans-impedance amps, pre-amps, ADC,
sample timing, poor beam quality etc.), calibration will only impress
your boss. It won't make your product better or impress your customers.

James A Carter III
http://www.jacarter3.com

skolpojken72@xxxxxxxx wrote:
Hello

I'm trying to make a range finding system with one PSD and a laser
that is modulated with 70kHz 50% duty cycle.

The trouble is that I don't get the same position when the laser is
reflected of a black surface or a white surface. The position shifts a
little bit.

Does PSD-elements have nonlinear regions when the reflected laser light
is low (like from a black surface)?

When I calculate the position I do like this:

x1 = x1_raw - x1_dark_value;
x2 = x2_raw - x2_dark_value;
pos = k*(x1-x2)/(x1+x2);

The x1_raw and x2_raw are the values I get from the AD-converter. The
x1_dark_value and x2_dark_value are the values I get from the ADC when
I cover the PSD-sensor.

Or how should I calibrate my system?

.

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