How to measure distance with IR

Hoffa

Hi

I´m trying to make my robot to be able to measure distance using IR. I know that ie. the robot kephera are able to do this and that it is done by measure the quality of the returning IR-signal. But I don´t know how to do this. Someone who have experience in this and knows what to do?

parts-man73

This subject is discussed in the education kit labs. See this http://forums.parallax.com/showthread.php?p=617192 for a list of all the labs.

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Brian Meade

"They who dream by day are cognizant of many things which escape those who dream only by night" - Edgar Poe

Hoffa

No, the method discussed in the education kit labs is not the same method I´m talking about. That method is about how to measure distance by testing different frequencies. What I want is to measure the quality of the returning IR light. This will firstly just demand one measurment and secondly be more exakt. I know that the robot khepera uses this method, but I can´t find any good HowTos on the internet. So I still wonder, has anyone used this method before?

StefanL38

hy

i think there are minimum two factors for the quality of the returning light:

1.) of course distance
2.) the reflectionfactor of the surface where the IR-light is reflected

So reflected light from a bright white surface will give an other quality
as a surface in dark grey at the same distance


one method to measure distancies with light is called triangulation

i haven't found an english version of it at wikipedia

here is a link to the german version

http://de.wikipedia.org/wiki/Abstandsmessung_(optisch)#Lasertriangulation

i think you can recognize the principle in the little picture
but therefore you would need a CCD-Camera-chip
that's pretty much effort

if the khepera robot works only with the intensitiy of the reflected light i suggest it must be very tricky
ore maybe it uses an array-line of receiving diodes as a "low-cost multipixel-sensor"

what about ultrasonic distance measuring
thats a lot of cheaper

Stefan

Graham Stabler

Hoffa,

The method in the education lab sweeps intensity at a fixed 38khz modulation frequency, see page 29 (IR detection with NCO and duty sweep).

Because of the problem of ambient light you will probably want to use a modulated light source, this means that the detector will tend to have a digital output, in those cases you have to use the point at which the power from the led is enough to trigger it, hence the use of a swept intensity.

You could add a filter to a normal photo diode and transimpedance amplifier set at the modulation frequency, then rectify this and measure it with an ADC but I'm not sure I see the point. For the reasons Stefan highlights you will never get especially good results from intensity due to differing reflectivitys.

Graham

rokicki

For a robot I'm currently building I'm using QD1114 IR emitter/detector pairs as a floor sensor. I'm mounting
them just above the floor. These are the same components as the QTI sensors (as a matter of fact I'm using
the QTI kit but modifying the circuitry). I'm modulating the IR myself, and sensing current in the phototransistor
using a small cap and resistor in an A/D configuration to measure current through the transistor. So essentially,
the QD1114, a 120 ohm resistor to current limit the IR diode, a 1K resistor and a 10nf capacitor in the A/D
configuration, with the photodiode current draining the capacitor to ground.

One cool thing about this configuration is it keeps the C-E voltage on the phototransistor very close to constant,
so the Miller capacitance is less of an issue and the phototransistor responds more quickly.

Using this configuration I'll be able to raise the sensors off the floor a bit more and deal with much more
varied surfaces than in the "normal" QTI configuration.

A downside of using a fixed 1K resistor in the A/D circuit however is that in bright sunlight on a highly
reflective surface the phototransistor gets swamped.

But I would not say this is in any way a "distance" sensor, as usual, because the light reflected is the product
of the reflectivity and the inverse distance squared (with an additional effect due to diffuse versus specular
reflection).

TransistorToaster

I remember looking at this a while ago. There's a module made by Sharp that uses triangulation. I'm not entirely positive, but I think this is it:
www.philohome.com/sensors/gp2d12.htm

I recall checking Wikipedia
en.wikipedia.org/wiki/Autofocus

and also going through the patent references found in
electronics.howstuffworks.com/autofocus.htm

I came across 3 distance measurement main techniques used on cameras which didn't rely on a CCD sensor for sharpness evaluation, which is the primary goal of imaging.
1. Echo method with round trip time measurement. Hard to achieve well at small distances. You need a high bandwidth transmitter, high BW receptor plus a fast chronometer.
2. Amount of light reflected back by a target
3. Triangulation

Graham Stabler

TransistorToaster,

The sharp does indeed seem to use triangulation as the package contains a PSD (position sensitive detector), these rather handy devices have an output that changes with the position of the light on the sensor. You can also get 2-axis ones if you ever want to track a corner cube reflector with a laser or crazy stuff like that.

I have a couple of the 8pin 102pixel linear arrays from Taos, I started writing some code up for it but got distracted by other projects, they are very simple and would make great laser range finders, I think it has been done actually in something like servo magazine but with a PIC.

Graham