Transmitter receiver unit that can know distance HELP

[atseng3]

Hey guys,
do any of you know of a transmitter/receiver device that can let you know the distance between them? I really need some help fast and would really appreciate it!

 

[berkeman]

Hey guys,
do any of you know of a transmitter/receiver device that can let you know the distance between them? I really need some help fast and would really appreciate it!

Welcome to the PF.

What is the application? Most methods have limitations, so the application and environment are important considerations.

If you can use line-of-sight, then RADAR or LIDAR are good for distance determination. If not line-of-sight, and you don't need super accuracy, you can just use GPS at both ends to get an approximate separation.

Because of the variability in attenuation and issues with multi-path, you generally cannot use receive signal strength as a distance indicator.

 

[atseng3]

I need it to be able to tell me the distance between two specific points, one may be a mobile location. The general range would be from 1 meter to 100 meters. I looked into LIDAR and RADAR but I'm not sure about RADAR because it seems like you the two objects need to be very far apart for it to work. Also could you tell me more about LIDAR? is there a receiving end for the laser or is it just point the laser anywhere and it tells you the distance?

What I need is something that like this: If I stand 1 meter away from the receiver, then it will tell me 1 meter, if I stand 17 meters away, then it'll tell me 17 meters.

Thanks!

 

[atseng3]

Also it the signal doesn't need to penetrate through anything, air is the only thing in between these devices.

 

[berkeman]

I need it to be able to tell me the distance between two specific points, one may be a mobile location. The general range would be from 1 meter to 100 meters. I looked into LIDAR and RADAR but I'm not sure about RADAR because it seems like you the two objects need to be very far apart for it to work. Also could you tell me more about LIDAR? is there a receiving end for the laser or is it just point the laser anywhere and it tells you the distance?

What I need is something that like this: If I stand 1 meter away from the receiver, then it will tell me 1 meter, if I stand 17 meters away, then it'll tell me 17 meters.

Thanks!

Also it the signal doesn't need to penetrate through anything, air is the only thing in between these devices.

Ah, line of sight makes it much easier. LIDAR info:

http://en.wikipedia.org/wiki/Lidar

You would put a cube corner reflector on the thing that you want to measure the distance to (to get a good reflection straight back to you).

For short distance range finding, ultrasound is pretty useful:

http://www.google.com/search?source...T4GGLL_enUS301US302&q=ultrasonic+range+finder

But the 100 meter distance is probably too far for ultrasound.

You could also make something pretty inexpensively (and it would be fun to build). Combine sound and light -- you have a circuit that sends out a set of claps/snaps in a particular cadence, the receiver at the other end hears them and flashes a light when it hears the final clap. Your detector circuit is watching for the light flash, and measures the time difference between sending the last clap and seeing the light flash. You'd probably use something like an optical scope to aim at the far device, and maybe use a camera flash as the optical indicator. Would be fun to build...

 

[atseng3]

Thanks! That was really helpful, do you know of something like this that would sense a particular item or a spot? Because I have a fixed point and then a point in motion so if I were to have the sensor on the point in motion I would need to find a way to point it in that direction, with this device, could I pin point it to one particular spot or thing?

 

[atseng3]

I may be a little unclear but what I mean is, is there maybe something that has a receiver on the other end that would do distance sensing?

 

[berkeman]

Thanks! That was really helpful, do you know of something like this that would sense a particular item or a spot? Because I have a fixed point and then a point in motion so if I were to have the sensor on the point in motion I would need to find a way to point it in that direction, with this device, could I pin point it to one particular spot or thing?

I may be a little unclear but what I mean is, is there maybe something that has a receiver on the other end that would do distance sensing?

Okay. If the target is going to be moving, and you won't have the ability to track it from the fixed point, then there is a variation on my idea that uses RF and sound. Can you figure out what it is?

 

[atseng3]

I was thinking along the lines of a laser beam emitting from the moving object, it can point to the receiving fixed point where there is a receiver, and that receiver can figure out the distance...is this something you're thinking about?

 

[berkeman]

I was thinking along the lines of a laser beam emitting from the moving object, it can point to the receiving fixed point where there is a receiver, and that receiver can figure out the distance...is this something you're thinking about?

No, I was thinking of a system where you don't have to point actively. So the mobile station can aim at the fixed station? Then you can still use a combination of light and sound...

 

[skeptic2]

There are a couple of approaches you could use. If you want to use a digital approach you're probably limited to some kind of timer. In order for a timer to work you need be timing the round trip of the signal such as with radar. Your timer will have to count pulses as short as 6.67 nS. It is pretty difficult to transmit a pulse, turn off the transmitter, let all the residual oscillations die out and turn on the receiver in 6 nS.

There is another radar technique that involves sweeping the transmit frequency and mixing the received signal with the transmitted to get a difference frequency. The difference frequency represents the range. However to measure ranges as short as 1 meter you'd have to sweep the carrier so fast it wouldn't be practical.

That leaves an analog approach and about the only thing you can do is measure signal strength. In order to avoid multipath, your wavelength should be longer than your maximum distance. That puts you in the LF band where there are restrictions on transmitted power. The best would probably be between 160 and 190 kHz where you can transmit 1 watt. You'll have to keep your transmit and receive antennas vertical and parallel.

I suggest using an FM receiver chip that works down to DC and has a logarithmic RSSI output. They typically have a range of 60 - 70 dB of range which is what you'll need because you'll be in the near field region. Even if you do everything right your error may be as much as 10% or higher.

 

[sophiecentaur]

It strikes me that you need to use Radar or an optical equivalent, which works by looking at reflections from the 'target at the 'base station - because the timing /distance information is easy to find out that way. Then send the distance (and bearing?) measurement to the remote end by a separate radio link. A more sophisticated active frequency-shifting repeater on the target would eliminate the problem of clutter - possibly worth considering.
There is the option of using a hyperbolic system with three synchronised fixed base stations and have the 'intelligence' in the mobile receiver unit. (This could be like the Decca Navigator system, of old, or could use a digital modulation equivalent). This sort of approach can be very accurate as it relies on timing measurement rather than amplitude measurement (much less reliable). Several independent mobile units could use the system at once.

 

[Studiot]

Reading through this thread what you haven't told us is the required aquisition time.
That is how long are you allowed to take to make the measurement?
Can you, for instance, stand still and mount an instrument on a tripod and take to readings and do a resection?

As an aside there is an article in this month's Elektor to use an ordinary mobile phone to do just this locally. But the syustem reuqires precalibration.

 

[atseng3]

The required acquisition time is probably a couple seconds. ideally should reach 10 seconds.
There is time for precalibration, could you tell me what article in the Elektor you saw?

 

[atseng3]

I'm just looking for something that can tell me the difference between two things. There is time for precalibration, and time is not that big of a deal, as long as it works.

Does anyone know if I put two ultralsonic range finders pointed at each other, would that work? I thought it might not, because the two are transmitting waves at different frequencies, but I might be wrong. Does anyone know?

 

[Studiot]

I only noted it in passing in the newsagents. I'm thought it was in Elektor, not EPE. If that is wrong I apologise.
However I can't see the article listed on either website. I will look again when I next pass the newsagents.

Meanwhile a good surveyor should be able to resect a plane table within 10 seconds or so, which brings us back to the techniques I mentioned before. A theodolite/ tacheometer/ distomat/ could all be employed to perform a resection from remote fixed points.

 

[N.Tesla]

Would Sonar work ?

 

[berkeman]

Would Sonar work ?

Welcome to the PF. As I mentioned in post #5, S/N is a problem for ultrasound/sonar.

 

[hharrison]

Hi! I am trying to design an experiment where we need a similar device to what you are asking about. We need 4 devices, all mobile, each one is measuring and recording the distance between itself and each of the 3 others. Ideally at least 1 measurement per second, more is better. Distances are <= 10 m roughly. Is this possible?

 

[sophiecentaur]

This is an easier problem to solve. All you need is for each of the devices to have a receiver and a transmitter. When it receives an appropriate signal from one of the others, it re-transmits the received signal (a transponder) plus its own ident signal. There is an overall delay in each loop but that can be calibrated out and any excess represents the transit time -giving distance.

The units could all work on the same frequency, dividing up the time co operatively. A bit of intelligence needed inside the boxes but nothing that a small processor couldn't cope with.

 

[skeptic2]

Hi! I am trying to design an experiment where we need a similar device to what you are asking about. We need 4 devices, all mobile, each one is measuring and recording the distance between itself and each of the 3 others. Ideally at least 1 measurement per second, more is better. Distances are <= 10 m roughly. Is this possible?

In addition to the signal strength approach I mentioned in post #11, I'd like to add another one referenced here.

http://en.wikipedia.org/wiki/Near-field_electromagnetic_ranging

Because in the near field the E & H fields are 90 deg. out of phase and in the far field they are in phase, in the range where the propagation is transitioning from near field to far field, the distance can be determined by measuring the phase difference between the E & H fields.

 

[hharrison]

Thanks for the suggestions! We are cognitive scientists and not engineers so these solutions aren't so obvious to us. But we will get working on it. I'll let you know how it goes.

 

[sophiecentaur]

In addition to the signal strength approach I mentioned in post #11, I'd like to add another one referenced here.

http://en.wikipedia.org/wiki/Near-field_electromagnetic_ranging

Because in the near field the E & H fields are 90 deg. out of phase and in the far field they are in phase, in the range where the propagation is transitioning from near field to far field, the distance can be determined by measuring the phase difference between the E & H fields.

I imagine this could be seriously affected by local structures - both at the transmitter and receiver. But, for interest, how many wavelengths away can you use this effect, under free space conditions?

 

[skeptic2]

I imagine this could be seriously affected by local structures - both at the transmitter and receiver.

From the Wiki entry:
The low-frequency, multipath-resistant characteristics of NFER make it well suited for tracking in dense metallic locations, such as typical office and industrial environments.[citation needed] Low frequencies also readily diffract around the human body, which makes tracking people possible without the body blockage experienced by microwave systems like Ultra-wideband (UWB).[citation needed] Systems deployed in complicated indoor propagation environments reportedly achieve 60 cm (2 ft) accuracy or better at ranges of 46 m (150 ft) or more.[5] There is also an indication that multiple frequency implementations may yield increased accuracy.[6]

The only thing I would change is the phase "Low frequencies also readily diffract around the human body". Low frequencies go right through the human body with no noticeable loss.

But, for interest, how many wavelengths away can you use this effect, under free space conditions?

I would like to know that too. I suppose it depends upon how sensitive and accurate the receiver is at discerning phase differences. Even if it were only 1 wavelength, one can choose the frequency appropriate to the distance being measured.

I once used a loran receiver with both a whip and ferrite rod antenna. It was intended to determine the direction of the loran station. I'm guessing it had to do with the signal from the ferrite rod being out of phase with the signal from the whip. The combination resulted in a null when the ferrite rod was perpendicular to the direction of the station.

 

[sophiecentaur]

That Wiki article surprises me. If you are near a structure that happens to be 'at all' resonant, the phases of the local fields are going to have a quadrature component -surely? Perhaps you just don't get many resonant structures at 1MHz.
The article seems to imply that the alternative of using timing measurements would require around 1ns accuracy (position precision in the region of 300mm) . Would that really be a problem?
There are many ways of killing a cat, though.