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[DPRG] Phase Shift

Subject: [DPRG] Phase Shift
From: Kenneth Maxon kmaxon at qwest.net
Date: Tue Jul 22 21:46:28 CDT 2008


Hi Tom & Ray,

I built one of the laser range finders as described in the article from the
Circuit Cellar Magazine that you are referring to.

http://nikita.argia.net/kmaxon/ranprot.jpg

The CCD is clearly visible in the left-fwd portion of the photo.  The other
windowed IC in the background is an eeprom erasable pic chip.   The first
version of this that I built (in the photo) is an exact copy from the
article.  I later laid out a pcb and made one of these for the WSU robotics
club back in my college days.

A later incarnation of this technology I developed on my own uses a CMOS
camera which (if you think about it) is just an array of linear CCDs stacked
side by side.   Using a state machine in a CPLD and a dual ported ram I am
able to build up 2-d distance range maps in front of the robot.  The dual
ported RAM interface makes it very simple to interface to many embedded
processor systems.  I dropped complete details on this work into an article
for the Seattle robotics society some years back.

http://www.users.qwest.net/~kmaxon/page/side/art9_137.htm

>From my experimentation this type of range finding (parallax based with
optical filtering) is definitely *NOT* for outdoor / extreme lighting
conditions, but for an indoor close range robot, (desktop or small room) it
can and does work very well...  The issues largely stem from separating the
spectral content of sunlight from the signal of interest with a laser and
sensor (video camera) operating in the visible spectrum.  Possibly others
can share their results as well...

Jeff Sampson of the Twin Cities Robot Club and a few others have built
similar range finders as well...  I had a chance to see Jeff's here in
Chicago just a few weeks ago...  Jeff monitors this list from time to time
so if you have questions you can probably ask him here as well.

By the way, the circuit cellar magazine and the preceding circuit cellar
column in byte magazine for all those years are largely responsible for the
choices that lead me to the career path I have chosen today.   Somewhere in
there exists a direct causal or at least decision path linkage.  :)

-Kenneth
www.users.qwest.net/~kmaxon


-----Original Message-----
From: dprglist-bounces at dprg.org [mailto:dprglist-bounces at dprg.org]On
Behalf Of Tom Brusehaver
Sent: Tuesday, July 22, 2008 3:33 PM
To: ray xu
Cc: dprglist at dprg.org
Subject: Re: [DPRG] Phase Shift


There was a very clever trick that one guy used. The article was in
circuit cellar
about 10 years ago.

Using a linear CCD (like out of a scanner), it measures the angle offset
from the laser and the CCD.

don't point the laser straight ahead, but at a slight angle. The reflection
will traverse the CCD. Some quick trig, and you can get an angle.



On Tue, Jul 22, 2008 at 12:40 PM, ray xu <rayxu at tx.rr.com> wrote:
> Then if phase shift wont work, what methods are there to find the absolute
> distance (besides triangulation and time of flight)?  What are those
> "tricks" (or any other details)?
>
> ___________________
> Ray Xu
> rayxu at tx.rr.com
> DPRG member
> OOPic group member
> Seattle Robotics group member
> My Blog
>
>
> -----Original Message-----
> From: Dean Hall [mailto:dwhall256 at gmail.com]
> Sent: Tuesday, July 22, 2008 12:31 PM
> To: ray xu
> Subject: Re: [DPRG] Phase Shift
>
> Sorry, I assumed you meant frequency shift when you said phase
> shift.  My statement (corrected): a single frequency shift reading
> doesn't give range.
>
> Measuring phase shift alone can give you a change-of-distance, but
> not absolute distance.
>
> The hokuyo sensors do some clever combination of laser readings.  The
> most telling paragraph that I can find says, "Since the speed of
> light would require Ghz or better timing for the ranges used by the
> rangers, there are tricks employed with pulsed laser light to view
> phase change of the pulses which greatly reduces the absolute timing
> requirements of the internal sensor/laser interfacing circuitry."
>
>  From http://www.acroname.com/robotics/info/articles/laser/laser.html
>
> !!Dean
>
>
> On Jul 22, 2008, at 09:32 , ray xu wrote:
>
>> But doesn't sensors like the Hokuyo sensors work with phase shift?
>> Are
>> there any other ways?
>>
>> ___________________
>> Ray Xu
>> rayxu at tx.rr.com
>> DPRG member
>> OOPic group member
>> Seattle Robotics group member
>> My Blog
>>
>>
>> -----Original Message-----
>> From: dprglist-bounces at dprg.org [mailto:dprglist-bounces at dprg.org]
>> On Behalf
>> Of Dean Hall
>> Sent: Tuesday, July 22, 2008 9:15 AM
>> To: dprglist at dprg.org
>> Subject: Re: [DPRG] Phase Shift
>>
>> A phase shift won't give you the range to an object, only the speed
>> of that object with respect to the sensor.
>>
>> !!Dean
>>
>>
>> On Jul 22, 2008, at 08:55 , ray xu wrote:
>>
>>> Hi, I'm moving on from using complex high speed circuits for my
>>> laser range finder to using other methods like phase shift; I
>>> understand little about this, can someone explain it to me?  Also,
>>> is there any way of measuring phases that are off by as little as 1
>>> ns?
>>>
>>>
>>> Thanks.
>>>
>>>
>>> ___________________
>>> Ray Xu
>>> rayxu at tx.rr.com
>>> DPRG member
>>> OOPic group member
>>> Seattle Robotics group member
>>> My Blog
>>
>
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> DPRGlist at dprg.org
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