Accurately aligning two wireless devices across short distances

[Latency]

Hi,

I am interested in learning how to align two devices (separated by several feet) using radio waves/antennas. By "align" I meant physically align, in space, along all axes.

For example, if one device is sitting on a table, I would like to hold the other device several feet above it and know when the devices are verticallyaligned (one device is directly over the other device).

I'd like the alignment to be as accurate as possible (< 1 cm).

I am a software engineer (20 years designing commercial software) but I don't have any real experience on the hardware side of things, DSP, etc. Also,this is just a personal project - so I'm not concerned with finding a solution asap. The real goal is to learn more about this stuff.

Any pointers/suggestions as to where to get started would be much appreciated!

-LK

 

[Uwe Hercksen]

I am interested in learning how to align two devices (separated by several feet) using radio waves/antennas. By "align" I meant physically align, in space, along all axes.

For example, if one device is sitting on a table, I would like to hold the other device several feet above it and know when the devices are vertically aligned (one device is directly over the other device).

I'd like the alignment to be as accurate as possible (< 1 cm).

Hello,

if I had to align with a precision better than 1 cm, I would not use
radio waves and antennas, I would use light waves and optical sensors.

Bye

 

[RobertMacy]

Hi,

I am interested in learning how to align two devices (separated by
several feet) using radio waves/antennas. By "align" I meant physically
align, in space, along all axes.

For example, if one device is sitting on a table, I would like to hold
the other device several feet above it and know when the devices are
vertically aligned (one device is directly over the other device).

I'd like the alignment to be as accurate as possible (< 1 cm).

I am a software engineer (20 years designing commercial software) but I
don't have any real experience on the hardware side of things, DSP, etc.
Also, this is just a personal project - so I'm not concerned with
finding a solution asap. The real goal is to learn more about this stuff.

Any pointers/suggestions as to where to get started would be much
appreciated!

-LK

You didn't say whether system was 'tethered', had 'line of sight', or was
trying to do this, say through a table, or such.

For either, in the base unit you can use some simple air core coils, in
multiple quadratures, drive each one with a different tone, from a $1 uP,
then at the portable receive unit use quadrature coils and similar uP, but
use PLL and synchronously detect. Such a system will easily tell you where
you are in space up to 3 foot cube, using less than 20mA at 3V. Accuracy?
at 8 inch separation, I've MEASURED noise of around 1 milrms. Also will
work in most environments, even near old style monitors.

 

[Latency]

if I had to align with a precision better than 1 cm, I would not use

radio waves and antennas, I would use light waves and optical sensors.

Thank you Uwe. The ultimate goal is to be able to align the devices when there is some type of obstruction between the devices. In my particular example I'd like to put one device *under* my table and be able to align the other device which is on *top* of the table.

Thus, I believe I need to use something other than light.

 

[Phil Hobbs]

Hi,

I am interested in learning how to align two devices (separated by
several feet) using radio waves/antennas. By "align" I meant
physically align, in space, along all axes.

For example, if one device is sitting on a table, I would like to
hold the other device several feet above it and know when the devices
are vertically aligned (one device is directly over the other
device).

I'd like the alignment to be as accurate as possible (< 1 cm).

I am a software engineer (20 years designing commercial software) but
I don't have any real experience on the hardware side of things, DSP,
etc. Also, this is just a personal project - so I'm not concerned
with finding a solution asap. The real goal is to learn more about
this stuff.

Any pointers/suggestions as to where to get started would be much
appreciated!

-LK

That's a hard problem using radio. It would be very much easier to use
infrared. For the sensor end, use a nice bright LED and an array of 3
photodiodes half-hidden behind a triangular shadow mask, and a small
piece of high-efficiency retroreflecting tape on the other object.
You'd get two axes per sensor, so in general you'd need three sensors in
a non-collinear arrangement.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics

160 North State Road #203
Briarcliff Manor NY 10510

hobbs at electrooptical dot net
http://electrooptical.net

 

[Latency]

You didn't say whether system was 'tethered', had 'line of sight', or was
trying to do this, say through a table, or such.

Hi Robert,

No tether, no line of sight. You are exactly correct in that I want to be about to align the devices when something like my tabletop is between them.

For either, in the base unit you can use some simple air core coils, in
multiple quadratures, drive each one with a different tone, from a $1 uP,
then at the portable receive unit use quadrature coils and similar uP, but
use PLL and synchronously detect. Such a system will easily tell you where
you are in space up to 3 foot cube, using less than 20mA at 3V. Accuracy?
at 8 inch separation, I've MEASURED noise of around 1 milrms. Also will
work in most environments, even near old style monitors.

Given my "intervening" table scenario, does your suggestion (above) still hold? I believe that 3 ft of separation would be a good maximum distance for my purposes.

If you believe that this would be a good way to go for a prototype I will start trying to learn about the hardware and how it works (n00b here at the moment).

Any books, articles, online lecture, suggestions etc. would be much appreciated!

-LK

 

[RobertMacy]

..snip...
Given my "intervening" table scenario, does your suggestion (above)
still hold? I believe that 3 ft of separation would be a good maximum
distance for my purposes.

If you believe that this would be a good way to go for a prototype I
will start trying to learn about the hardware and how it works (n00b
here at the moment).

Any books, articles, online lecture, suggestions etc. would be much
appreciated!

-LK

Yes, works through objects. Magnetic fields are a lot like sound, once
made, difficult to get rid of.

Break system into small pieces, then explore EACH piece, learning its
characteristics. Once you have developed a 'vocabulary' of little gadget
thingies, you can then think about how to put them together going from
what you have to what you want.

Let's see. Learn:
magnetic fields, vector fields
micro processor topologies
communication theories
phase lock loop performances
advantages of synchronous detection
breadboarding and debugging

and you pretty much have it

OR, you could buy a 'virtual reality' glove as an input device for your
computer and be done with it.

 

[Bill Sloman]

No tether, no line of sight. You are exactly correct in that I want to be
about to align the devices when something like my tabletop is between them.


Given my "intervening" table scenario, does your suggestion (above) still
hold? I believe that 3 ft of separation would be a good maximum distance for > my purposes.

If you believe that this would be a good way to go for a prototype I will
start trying to learn about the hardware and how it works (n00b here at the
moment).

The "table" had better not contain anything conductive. "Old Style" monitors tend to be wrapped in electrically conductive shielding, and air-cored coils tend to generate eddy currents in such shielding which distort the radiated field a bit.

If the intervening surface is just a table, it might make more sense to optically align the two units of interest with a third device which has a direct line of sight to both of the units of interest.

 

[miso]

I would bet once there is an obstruction, all bets are off on the
accuracy. Obstruction implies diffusion and reflection.

The other problem I see here is you will probably have to know the near
field radiation of your antenna accurately for this project. I'd
research RF applications of hyperthermia. That is probably where the
near field gurus hang out.

 

[Jasen Betts]

Hi,

I am interested in learning how to align two devices (separated by
several feet) using radio waves/antennas. By "align" I meant
physically align, in space, along all axes.
....

I am a software engineer (20 years designing commercial software)
but I don't have any real experience on the hardware side of things,
DSP, etc. Also, this is just a personal project - so I'm not concerned
with finding a solution asap. The real goal is to learn more about
this stuff.

Differential GPS. overkill, but all off-the shelf parts, so should be
fairly easy for a software guy to set up.

 

[RobertMacy]

Differential GPS. overkill, but all off-the shelf parts, so should be
fairly easy for a software guy to set up.

From reading the description above, the OP wants to align "...along all
axes." Sounds like he wants to have two objects 'track' each other,
matching EVERY positional characteristic except for the single dimensional
offset along the z axis. [envision the two objects tied together like
controller and puppet, but with 'invisible' strings that can go through
wooden table.]

GPS only tells you 'where' an object is on an x-y plane, right? leaving
out the height [z position] *and*, of course, tilt.


To OP, good thing you're software, you'll need it. I'm still sold on a
simple matrix of magnetic field sensors. You can actually rig a tethered
prototype up using your soundcard and a lot of software to experiment with
the position sensor I described earlier. For example, Envision two coils,
one transmits, one receives.Not a lot of information. Now four coils, two
transmit each a different frequency, and two receive BUT each receiving
coil can identify and mesure the fields from TxA and TxB, a lot more
information. Now envision six coils, with three transmitting each a
different frequency, andnow the recivers get TxA, TxB, and TxC -- a LOT
more information per receive coil. Now envision 8 coils....

As software guy, you get the idea. A soundcard is capable of supplying
each unique tone and receiving up to something like 8 channels of audio at
44.1kS/s that is MORE than enough to get you started. Uh, sound cards do
put out several channels of sound, right?

Don't mess with ferrite or iron cores, no need. Just take a nonconductive
2-3 inch diameter coil form, plastic lid?, and wrap 100 turns of 36 Awg
wire on it to make a coil. With even as low as 1-2 mA of current you'll
get a very large field for quite a distance. Make all the coils
identically for simplicity. You might need to superglue the wires down to
keep them from flipping off the form. But this tethered prototype will get
you started, and teach you a lot about the principles involved.

 

[Spehro Pefhany]

Differential GPS. overkill, but all off-the shelf parts, so should be
fairly easy for a software guy to set up.

From reading the description above, the OP wants to align "...along all
axes." Sounds like he wants to have two objects 'track' each other,
matching EVERY positional characteristic except for the single dimensional
offset along the z axis. [envision the two objects tied together like
controller and puppet, but with 'invisible' strings that can go through
wooden table.]

With a strong magnet you could track the magnetic field gradient. With
a strong enough magnet, a ferromagnetic puppet would follow on its
own.



Best regards,
Spehro Pefhany

 

[[email protected]]

Differential GPS. overkill, but all off-the shelf parts, so should be
fairly easy for a software guy to set up.

From reading the description above, the OP wants to align "...along all
axes." Sounds like he wants to have two objects 'track' each other,
matching EVERY positional characteristic except for the single dimensional
offset along the z axis. [envision the two objects tied together like
controller and puppet, but with 'invisible' strings that can go through
wooden table.]

GPS only tells you 'where' an object is on an x-y plane, right? leaving
out the height [z position] *and*, of course, tilt.

No, GPS also gives altitude (if you can see enough satellites to solve
the equations). Basically you know the position of and distance to
the satellites, solve for your position. Of course it can't give
orientation.

 

[[email protected]]

GPS only tells you 'where' an object is on an x-y plane, right? leaving
out the height [z position] *and*, of course, tilt.

A GPS definitively calculates the position in xyz coordinates relative
to the center of the Earth and some stellar directions.

It then tries to convert this to latitude, longitude and height
above/below earths surface. Unfortunately, the Earth is not a perfect
sphere, so calculating the height requires some gimmickry.

Using some variation of differential-GPS, you can eliminate most of
this ambiguity.

 

[RobertMacy]

...snip....

With a strong magnet you could track the magnetic field gradient. With
a strong enough magnet, a ferromagnetic puppet would follow on its
own.



Best regards,
Spehro Pefhany

problem is that that is a single field, no identification possible *and*
1/f noise eats you alive at the fields you'll get out there.