spatial coordinates

[Vincent BERRIER]

I am working on a project trying to locate a
robot moving on a the hull of a ship.
The aim is having the spatial coordinates of the
robot

The robot will test the huge during a dry-docking
and will move on with a max speed of 5 feet/sec.
The ship will be arond 65 feets in length.



I think about using a 3-axis accelerometer
coupled with an other accelerometer for the
gravity.

What do you think about this solution?
Have you other solutions?

 

[Roger Hamlett]

I am working on a project trying to locate a
robot moving on a the hull of a ship.
The aim is having the spatial coordinates of the
robot

The robot will test the huge during a dry-docking
and will move on with a max speed of 5 feet/sec.
The ship will be arond 65 feets in length.



I think about using a 3-axis accelerometer
coupled with an other accelerometer for the
gravity.

What do you think about this solution?
Have you other solutions?

You won't get the accuracy you need, without spending a massive amount on
the accelerometers, and even then the accuracy will degrade with time and
distance moved. Consider using an 'absolute' positioning system, either
counting wheel rotations on the robot, or (possibly the simplest/best),
having a number of ultrasonic receivers, mounted at fixed locations in the
dock. The robot sends a radio 'marker', and at the same time an ultrasonic
'pulse'. Each receiver, responds with the time delay it measured between the
marker and the pulse. You can then work out the distances to the markers. If
you look at commercial positioning systems using accelerometers, they
generally use these to give 'instantaneous' motion detection, and combine
the values with an 'absolute' signal at intervals, to avoid the errors
building too far. This then reduces the overall accuracy needed from the
accelerometers, and prevents the errors from building too far.

Best Wishes

 

[John Woodgate]

I read in sci.electronics.design that Vincent BERRIER

I think about using a 3-axis accelerometer coupled with an other
accelerometer for the gravity.

What do you think about this solution?
Have you other solutions?

I don't think dead reckoning will work. You will get slip in your
traction against the hull, which will introduce large errors. I suggest
you put a radio beacon, emitting short pulses of carrier, on the robot
and arrange three receivers in a triangular layout. By noting the
differences of time-of-arrival of the pulses at the receivers, you can
determine the position of the robot quite accurately. But even over 65
feet you will need high-speed electronics. Maybe looking at the phase
differences of a CW carrier (as in Decca Navigator) would be better. But
you may not be allowed to transmit on a most suitable frequency.

I suggest radio rather than ultrasonics because the environment will be
very noisy, even above the human audio band.

 

[Tim Wescott]

I read in sci.electronics.design that Vincent BERRIER


I don't think dead reckoning will work. You will get slip in your
traction against the hull, which will introduce large errors. I suggest
you put a radio beacon, emitting short pulses of carrier, on the robot
and arrange three receivers in a triangular layout. By noting the
differences of time-of-arrival of the pulses at the receivers, you can
determine the position of the robot quite accurately. But even over 65
feet you will need high-speed electronics. Maybe looking at the phase
differences of a CW carrier (as in Decca Navigator) would be better. But
you may not be allowed to transmit on a most suitable frequency.

I suggest radio rather than ultrasonics because the environment will be
very noisy, even above the human audio band.
--
Regards, John Woodgate, OOO - Own Opinions Only.
The good news is that nothing is compulsory.
The bad news is that everything is prohibited.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk

And if you do use accelerometers to augment your positioning you'll also
need gyros. As slow as your robot is I suspect that the accelerations are
also slow, and you should just do it all by echo location if you can.

 

[Don Pearce]

I am working on a project trying to locate a
robot moving on a the hull of a ship.
The aim is having the spatial coordinates of the
robot

The robot will test the huge during a dry-docking
and will move on with a max speed of 5 feet/sec.
The ship will be arond 65 feets in length.



I think about using a 3-axis accelerometer
coupled with an other accelerometer for the
gravity.

What do you think about this solution?
Have you other solutions?

A differential GPS module should do everything you need for this job,
provided the hull doesn't shroud the signal too much. Experiment with
a hand-held to see if it is good enough.

d

_____________________________

http://www.pearce.uk.com

 

[Vincent BERRIER]

I didn't think that by using accelerometers, it
would be so complicated.

On the other hand, if I use three receivers
(ultrasonics or radio waves) in a triangular
layout, could I get a accuracy of a few inches
maximum?

 

[John Woodgate]

I read in sci.electronics.design that Vincent BERRIER

I didn't think that by using accelerometers, it
would be so complicated.

On the other hand, if I use three receivers
(ultrasonics or radio waves) in a triangular
layout, could I get a accuracy of a few inches
maximum?

Yes, using radio wave phase-difference detection, but you will need
careful design.

 

[Vincent BERRIER]

John Woodgate a utilisé son clavier pour écrire :

I read in sci.electronics.design that Vincent BERRIER

Yes, using radio wave phase-difference detection, but
you will need careful design.

Do you know where I can find information about
this on the internet?

 

[John Woodgate]

I read in sci.electronics.design that Vincent BERRIER

Do you know where I can find information about this on the internet?

I tried a Google search for 'Decca Navigator', which gave over 500 hits,
but I did not see the sort of technical detail you need, and the
accuracy figures would not apply to your application. But I feel sure
that the information you need is available. Some people here have
probably worked on such systems and know exactly where to find
information.

 

[Norm Dresner]

I read in sci.electronics.design that Vincent BERRIER

Yes, using radio wave phase-difference detection, but you will need
careful design.

I helped design and used a "tri-lateration" system in the '70s that
worked by timing the signal path between an object and three ground
stations. The object carried a radio repeater with a calibrated delay and
the system worked by having each of the three ground stations in turn ping
and receive the response from the object. Since the object motion was
leisurely, successive measurements were adequate. But remember that in
"free space" (and the atmosphere pretty well qualifies), the speed of
"light" is roughly a nano-second per foot. That means that you'll have to
measure the time-of-flight of the pulses to within a single nanosecond to
get 1 foot resolution.

Norm

 

[John Woodgate]

But
remember that in "free space" (and the atmosphere pretty well
qualifies), the speed of "light" is roughly a nano-second per foot.
That means that you'll have to measure the time-of-flight of the pulses
to within a single nanosecond to get 1 foot resolution.

That's why I recommend investigating a phase-detection system, using CW
transmission. It doesn't necessarily need such fast circuitry.

 

[Fred Bartoli]

That's why I recommend investigating a phase-detection system, using CW
transmission. It doesn't necessarily need such fast circuitry.
--

Sure, I've built a sync detector with almost junkbox parts and I can see 1cm
(yes) of coax at 100kHz.


Fred.

 

[Guy Macon]

Do you know where I can find information about
this on the internet?

Look here:

_"Where am I?" Sensors and Methods for Mobile Robot Positioning_

http://www-personal.umich.edu/~johannb/shared/pos96rep.pdf

 

[Vincent BERRIER]

Don Pearce a pensé très fort :

A differential GPS module should do everything you need
for this job, provided the hull doesn't shroud the
signal too much. Experiment with a hand-held to see if
it is good enough.

d

_____________________________

http://www.pearce.uk.com

Yes, it's really interesting, but what about the
prices?

 

[Don Pearce]

Don Pearce a pensé très fort :



Yes, it's really interesting, but what about the
prices?

You should be able to buy a GPS with a Bluetooth radio to get data
back to a remote PC for around $120.

d

_____________________________

http://www.pearce.uk.com

 

[Vincent BERRIER]

Don Pearce a exprimé avec précision :

You should be able to buy a GPS with a Bluetooth radio
to get data back to a remote PC for around $120.

d

_____________________________

http://www.pearce.uk.com

Great !!!!
Do you know where I could find it?

 

[Don Pearce]

Don Pearce a exprimé avec précision :


Great !!!!
Do you know where I could find it?

That will be a normal handheld, you can buy it in stores - or on the
web.

d

_____________________________

http://www.pearce.uk.com