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 s pace, 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 solu tion asap. The real goal is to learn more about this stuff.
Any pointers/suggestions as to where to get started would be much appreciat ed!
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.
Thank you Uwe. The ultimate goal is to be able to align the devices when th ere is some type of obstruction between the devices. In my particular examp le I'd like to put one device *under* my table and be able to align the oth er device which is on *top* of the table.
Thus, I believe I need to use something other than light.
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
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Dr Philip C D Hobbs
Principal Consultant
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hobbs at electrooptical dot net
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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).
Any books, articles, online lecture, suggestions etc. would be much appreciated!
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.
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.
I'm not sure this is what you want, but it sounds very much like a wireless game controller to me. I'm not sure if this one actually exists, but it looks like a good start. If not, one of the other wireless game controllers should suffice (e.g. Wii).
There's also a 3D mouse, which might work:
At the receiving end, setup a few RC servos to mimic the motion of the game controller. Note that this will give you relative motion, not absolute motion. To get absolute, place both the controller and the servos in a known position as a starting reference. See "inertial guidance" for gyro or integration drift which is similar to the problems you'll have syncing the relative motion of the accelerometer, with the absolute position of the servos.
Another way to read your description is that you're trying to sync two servos via wireless. That would be 3 shaft encoders, one for each axis, a 3 channel multiplexed RF link, and three stepper motors to provide the output motion. Again, it will be necessary to provide a starting position.
Yet another way to look at your description is a radio control airplane. The traditional joystick can be replaced by a 3 axis encoder. Transmission is via PWM (pulse width modulation), which drives the servos.
Still another way to decode your description is to just add a 3rd axis to a common digitizer pad, which is an absolute positioner. The output could be an RC servo or a stepper motor.
Perhaps it would be helpful if you would describe what you're actually trying to accomplish, and what you have to work with, rather than how you propose to accomplish it.
--
Jeff Liebermann jeffl@cruzio.com
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558
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.
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.
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
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"it's the network..." "The Journey is the reward"
speff@interlog.com Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com
Such a device is called a "Waldo" from the Robert Heinlein story: The modern version can be found in various flying drones (UAV). They're fun to fly, especially if there's a noticeable delay in response time.
--
Jeff Liebermann jeffl@cruzio.com
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558
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.
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.
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