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Cheap Azimuth Indicator ??

We have a mobile truck equipped with a pneumatic push-up pole. On top of this pole, we have a camera and yagi antenna mounted to a simple pan-tilt-zoom controller. We would like to know which way the contraption is pointed, regardless of the truck's orientation.

What is the easiest way to do this? (Surely this requirement has been beat to death, right??)

The PTZ controller is very simple: left, right, up, down. No azimuth info. No known starting posiition regardless of azimuth (though we might be able to cobble something up?).

While the camera could possibly "look at" something on the mount (like a compass?) in addition to it's normal intended field of view, our preference would be something other than "visual-only", maybe a 3- digit RS-232 feed, or something we can at least have the option to capture via computer. But hey, we'd take visual-only if that's the only viable way to do it without upgrading to a more capable PTZ controller. I don't have the camera lens specs handy, but it is an auto-focus, auto-iris design.

Weight is a factor (it's a 30-foot pole) w/ 8-pounds dead weight already. Simplicity is a factor, as is low-cost.

Another possibility is to have the contraption superimpose the azimuth data on the NTSC 1-volt video feed coming off the camera itself (somehow). That would be ideal, actually.

Worst case, we'll stick a colored ball on a stick, and use that as a reference, but then we'll need to know which way the truck is pointing all the time, and calculate from there. I should mention the mount is metal, so that could affect a compass. It would have to be mounted in a way to minimize errors.

Thanks!!

-mpm

Reply to
mpm
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something like this?:

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add a small micro if you want rs232

-Lasse

Reply to
langwadt

By that, I assume your reference to azimuth is wrt magnetic north (or some other geographic reference). Is the truck stationary at the times you want this information?

I imagine you "stow" the antenna in some configuration while traveling? (or, *could* do so?). If the procedure you use to deploy the antenna knows that it will start in this orientation (wrt the truck), you can arrange for it to pass through a reference point, rotationally. "Top dead center" (i.e., "front dead center" :> ).

Any sort of incremental encoder could then be used to track the movement relative to that reference -- reestablishing the reference any time it passes through it (though I assume you can't go 'round and 'round and 'round indefinitely -- unless you have a slip-ring-ish arrangement).

You could also use an absolute position encoder depending on the resolution required. Have any synchro resolvers lying around?? :>

RVDT's, optical encoders, etc.

I've seen "low resolution" optical encoders "cascaded" (mechanical gain on the shafts) to make low cost high(er) resolution encoders. And/or used to augment an "index/reference" position indicator.

Flux gate magnetometer?

The mechanism is *probably* powered by DC servos -- "open loop". So, you can't even reliably determine how much the mechanism

*should* have moved (stepping motors -- operated closed loop -- could provide that information if you can "count steps").

A prism/mirror mounted in the lower half of the field of view reflecting an image of a compass into the video feed?

I've seen compasses made from "magnetized" (floating) transarent plates having an "opacity gradient". A light passes through the plate and the sensed signal indicates "how dark" the portion of the disk in the light path is. (Often, a pair of such detectors operating in a sort of bridge configuration)

How accurate do you need to be? How fast does the antenna's orientation change (i.e. via the PTZ controller *and* any motion that the truck imparts to it)?

*Several* collored balls located (unobtrusively) in the camera's field of vision. E.g., like points on a compass referenced to the front of the *truck*.
Reply to
Don Y 

--
With respect to what?
Reply to
John Fields

GPS? ...Jim Thompson

--
| James E.Thompson, CTO                            |    mens     |
| Analog Innovations, Inc.                         |     et      |
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    |
| Phoenix, Arizona  85048    Skype: Contacts Only  |             |
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  |
| E-mail Icon at http://www.analog-innovations.com |    1962     |
             
I love to cook with wine.     Sometimes I even put it in the food.
Reply to
Jim Thompson

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A gps will give position. you only get heading if you are moving.

Reply to
brent 

--
Possibly useful for "Where am I?" and maybe, even, for "How high is my
camera off the ground" but, I think, probably problematical for "In
which way is my camera pointing?"
Reply to
John Fields

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I guess two GPS recievers with sufficient distance could give you heading

-Lasse

Reply to
langwadt

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Thanks, I also found this online, but don't know anything about them (yet), or if they're even available. I will check out the SparkFun product as well.

-mpm

Reply to
mpm

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As a follow up to what you said:

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states:

There are other devices which are not conventionally called compasses but which do allow the true cardinal directions to be determined. They are said to work "like a compass", or "as a compass", without actually being a compass. For example, a Global Positioning System (GPS) satellite receiver determines its own position on the ground, as true latitude and true longitude. If the receiver is being moved, even at walking pace, it can follow the change of its position, and hence determine the compass bearing of its direction of movement, and thence the directions of the cardinal points relative to its direction of movement. Some GPS receivers have two antennas, fixed some distance apart to the structure of a vehicle, usually an aircraft. The exact latitudes and longitudes of the antennas can be determined simultaneously, which allows the directions of the cardinal points to be calculated relative to the heading of the aircraft ........

------------------------------------------

Reply to
brent

Yes - position relative to true north is preferred, but magnetic north will do. The truck will be stationary. I can almost picture the insurance adjuster's face when we explain how 30-foot extended mast meets highway overpass. :)

Yes - travel is not round and round. It's actually only about 300 degrees. More than enough for what we do with it, which is interference abatement. And we don't need it every job. Maybe 5%-10% of the tough ones, out in the boonies usually with no natural elevation opportunities, and by "natural" I mean multi-story parking garages of course.

Resolution - Realistically, if we can get 3 to 5 degrees I think we'd be happy. Within that range, repeatability is probably more important that absolute accuracy.

As to rotation speed, the mast is straight-up/down. The PTZ is fairly slow, though I've not measured it. My guess is it takes 15-20 seconds (at least) to travel 300 degrees.

You gave me an idea. (..and I'm not sure it's a good one yet, but it qualifies for low-tech..?) Since we always have the mast full-up when deployed, maybe we could use a wire, cobble up some kind of tension gear, and just measure the length of wire travel with something "widget" mounted back at the truck? It doesn't necessarily solve the initial "which way is North", but I haven't flushed it out yet either. Maybe a quick pre-calibration with a Boy Scout compass before raising the mast? Then again, 30-feet of wire flapping in the breeze might be problematic to measure (unless we route it inside the mast?)

The camera prism thing sounds messy, though I agree it would likely work. Would just have to review the camera focus specs, etc...

Reply to
mpm

That won't work. In my experience GPS can be of by 150m / 450feet (yes, it really is that bad in real life). Useless for determining the heading. An electronics compass is probably the best solution. But expect some error. The compass I worked with had an error of about 16 degrees. But there may be better ones available.

--
Failure does not prove something is impossible, failure simply
indicates you are not using the right tools...
nico@nctdevpuntnl (punt=.)
--------------------------------------------------------------
Reply to
Nico Coesel

food.

If two are used and compared the absolute error should be the same for both. I think you can get heading with two receivers. However, it may fail the requirement for "cheap".

Reply to
brent

food.

I read something somewhere that accuracy improves dramatically (or it may even be a requirement) that differential GPS setups like we're describing here only work when both GPS units are using the exact same constellation of satellites in calculating their position fixes. If one GPS uses a different set of satellites, it can really throw off the calculated results - even if the sets differ by as little as a single satellite.

That could be complete BS. I'm not an expert on differential GPS. I know my Garmin Montana 650t often shows a position error estimate nearly as long as the truck. Great GPS by the way - terribly pricey, but it does everything I need it to.

16 degrees is too much error, or too limited a resolution.
Reply to
mpm

So, anything that doesn't report orientation against this reference would require having something in the truck that does so.

We had a van that we used to test marine radars with. You *always* remembered to secure the antenna (narrow edge to the wind). But, you didn't always remember to turn off the transmitter! :-/

So, a camera mounted on the antenna POINTED DOWN (or, on the truck BELOW the antenna) POINTED UP. Feed this to a PC and superimpose a set of compass bearings on it -- from knowledge of the direction the truck is facing. The antenna would always be a fixed distance away from the camera (or, in a very narrow range). Paint a big red line on the bottom of the antenna. A piece of software superimposes a *rotated* N-S-E-W (marked in 5 degree increments) on the image, centered at the mast. Big red line looks like the needle of the "compass" on the screen. (i.e., use your eyes to resolve the position)

So, you could just watch the "big red line" spin through those 300 degrees to get an idea as to which general direction it is facing. "OK, Bob, looks like we just spotted something at around 245 degrees. Why don't you stop and back up a bit... easy... easy... *there*!"

Any linear transducer can serve this role. A string that winds around a mast-mounted bobbin, an LVDT driven by a rack-and-pinion, etc.

You could also just fasten a big red *pointer* (broomstick?) on the bottom of the mast INSIDE the truck (if the whole mast were to rotate).

Or, a large NSEW ring mounted on the roof of the truck that you "spin" to face North (or wherever) when you set up.

Why is the camera on the mast, again? Just so you can see what the antenna is looking at? (so you know whose door to kick down?)

Reply to
Don Y

The way I saw it done years ago was the azimuth was manual, and there was a compass rose built into the ceiling of the station wagon. The mast was a telescoping air cylinder with a key in the sections, so the top stayed aligned with the bottom. Once on site, they used a big surveyor's magnetic compass a distance from the vehicle, and sighted through the sight slots on it through the mast, and somebody inside the vehicle turned the compass rose to align to the compass sights. The compass rose was locked, and now you had your north reference. This was used for FCC field strength surveys, so it had to be pretty accurate.

The mast was elevated by 12 V DC compressor, and if the air relief valve was closed or the power to the compressor was turned on, a HUGE red light started flashing inside the car and a buzzer went off. That pretty much made sure the mast stayed down when it was in motion. (When I say huge, it was an after-market truck taillight.)

Jon

Reply to
Jon Elson

Before buying an expensive heading reference system, please check with a magnetic compass how much disturbance comes from the auto structure. To get absolute directions, you'll need some kind of magnetic reference system (i.e. a fancy compass).

--

Tauno Voipio
Reply to
Tauno Voipio

Actually, there are "satellite compasses" that use two (or more) GPS antennas. They essentially track the carrier phase difference (similar to a conventional RDF antenna but done with DSP). The ones I'm familiar with spec a heading accuracy of 1.0 to 0.5 degrees; cost goes up with accuracy, of course.

They'll run from 12 or 24 VDC; should be a decent fit for a truck/van. I'm most familiar with the Furuno products but there are probably others out there.

--
Rich Webb     Norfolk, VA
Reply to
Rich Webb

[snip]

You mean the truck go there without moving ?:-) ...Jim Thompson

--
| James E.Thompson, CTO                            |    mens     |
| Analog Innovations, Inc.                         |     et      |
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    |
| Phoenix, Arizona  85048    Skype: Contacts Only  |             |
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  |
| E-mail Icon at http://www.analog-innovations.com |    1962     |
             
I love to cook with wine.     Sometimes I even put it in the food.
Reply to
Jim Thompson

My car (Infiniti Q45) has some kind of flux-gate compass that works quite well. ...Jim Thompson

--
| James E.Thompson, CTO                            |    mens     |
| Analog Innovations, Inc.                         |     et      |
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    |
| Phoenix, Arizona  85048    Skype: Contacts Only  |             |
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  |
| E-mail Icon at http://www.analog-innovations.com |    1962     |
             
I love to cook with wine.     Sometimes I even put it in the food.
Reply to
Jim Thompson

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