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I am trying to create a small robot toy car with the ability to follow a person around, with a range of about 50 m.

Ideally the person will be provided with a small transmitter of some kind, and the car will have a receiver which will allow it to determine the direction to the transmitter so that the car can follow the person holding it.

However, I understand that systems such as infrared require line of sight to operate, while others like WiFi are too omnidirectional to pinpoint the transmitter direction.

Is there any recommended method or system that would meet my requirement?


Added:

Notes:

To elaborate, it is more for remote photo taking.
A person would hold a remote tag and trigger a command; after which the robot would seek out the person, and position itself in the same direction to take a shot.
Unfortunately GPS not not a solution as for this application is mostly for indoors. I understand RFID tags may work although i am not sure if it has sufficient directionality

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An ultrasonic ping mechanism using 3 ultrasonic receiver transducers on the vehicle, and a single omnidirectional ultrasonic TX transducer (a piezo speaker or metal-cone ultra-tweeter would work) on the device to be followed, would yield direction and distance information, so long as something approximating line of sight (sound) could be maintained. A simplistic illustration:

schematic

simulate this circuit – Schematic created using CircuitLab

The follow-me unit generates an ultrasonic pulse at a calibrated time interval. The difference in time-of-flight of the pulse on the three RX units allows determination of relative distance from each RX, and thus spatial location via triangulation.

If there is a wall blocking one of the sound paths but not the others, this would add some uncertainty to the process, but so long as the follow-me is moving, this too can be worked around using a bit of signal processing and interpolation between known-good pulses.

If highly directional RX units are used, direction can be determined with just two units sufficiently separated from each other, but this will not allow estimation of distance, only angular direction.

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I see no reason why high-frequency direction finding wouldn't work. You could use the 433MHz band for which little tx/rx modules are widely available. The important thing would be to find one which gave reliable RSSI (recieved signal strength). You'd end up with three or four antennas on the robot pointing in different directions; the robot angles itself towards the highest signal strength, like a line-following robot.

Obviously this only works for driving directly towards the target. If you're on the other side of a wall or want the robot to drive round a corner it gets much more complicated. The other problem is stopping the robot when it gets close enough to run over your foot.

On the other hand, if you've got a camera and a substantial processor on board, you could have it use face tracking for direction with instructions to maintain your face in the center of its field of view and at a particular percieved size by moving towards and away from you.

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  • \$\begingroup\$ To avoid running over the target's foot, one can add a ultrasound sensor to the vehicle, or microswitches with levers, if touching is ok. \$\endgroup\$ – Ricardo Jan 8 '14 at 12:02
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Line of sight does not mean that the tx is directional - BUT it does help a RX determine direction.

You want directionality in general.

You say this is for a toy,but even then more detail will help. Telling us what you are actually trying to do in more detail will help improve answer quality. eg if this is for something akin to a "follow me" golf cart for use on any golf course you may value a standalone system. If it is for something more like eg a medicine trolley in a rest home or hospital then the ability to provide position beacons or a guidance grid may be attractive.


The technically easiest solution if you have 1 x TX and 1 x RX is a scanning system in either the RX (preferred) or TX. This will probably be mechanical - it can be achieved electronically but probably adds cost and complexity to a basically simple system. A mechanical scanner canbe extremely simple and would probably be effective in this application.

You could use IR (Infrared) with an approximately omnidirectional TX and use a rotating mechanical scanner to either rotate a photo sensor or a mirror or an aperture such that the direction that the sensor "sees" is progressively swept through 360 degrees. If you know the angle that the swept input is pointing then you can detect the direction where sighnal input is maximum.

The mirror and aperture arrangements have the advantage that the sensor proper can be non rotating.

IR tends to be reflected quite well from surfaces that are not obviously reflective optically and you may find that maximum signal comes from a reflection. Generally I'd expect that as sender and receiver moved that the tx would tend to be the strongest source on average. If it is possible to have the direct path blocked then reflections may cause some interesting results.

GPS: A potentially useful 'high tech' system is to use GPS receivers at TX and RX with the TX sending its coordinates and the RX working out which way top go from the calculated difference in positions. While raw GPS is not liable to be accurate enough for precision homing, as long as TX and RX use the same satellite constellation they are liable to produce a differential accuracy far better than the absolute accuracy. [Using a standard GPS I can plot a path as I drive along a city street, do a U turn at an intersection and return. The plotted path on a map may suggest that I drove on the footpath or wrong side of the street on the outward leg, but the return path will typically be plotted within a metre of the true path relative to the outward path if the same satellites are used. Drive the same course an hour later and the go and return paths will again be close to correct relative to each other - but the path pair may be 5+ metres different than the earlier path pair. The cost of GPS receivers is such that this may be a viable solution in many applications. TX method for passing signals then becomes unconstrained by the need to determine direction from it.

Even eg WiFi can be rendered directional by use of mechanical or electrical aerial systems.

Other methods are possible but knowing more will help avoid long discourses which turn out not to be relevant.


Added:

You now mention a camera.
If camera is sensitive in IR (some are more so than others) then a pulsed IR LED would provide a visual beacon. A separate camera could do likewise - either by scanning as above with fixed sensor plus a rotating mirror or by rotating a mast with camera on or rotating the whole robot. As the device is meant to seek and destro... er photograph, then initial rotational seeking should be acceptable and potentially expected. Once the "beacon" is seen the robot can move to keep it central in its view field. With a suitably wide range zoom lens on the main camera it would often be possible to zoom in on and photograph the target from a substantial percent of your target range, depending on your photographic framing and other requirements.

As above, telling us as much as you are able about what you are trying to achieve greatly helps the answering process. Death by a thousand questions and information dribbles is a common but unproductive way to home on a solution.

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  • \$\begingroup\$ To elaborate, it is more for a remote photo taking. A person would hold a remote tag and trigger a command; after which the robot would seek out the person, and position itself in the same direction to take a shot. Unfortunately GPS not not a solution as for this application is mostly for indoors. I understand RFID tags may work although i am not sure if it has sufficient directionality. \$\endgroup\$ – John Tan Jan 8 '14 at 9:32
  • \$\begingroup\$ @JohnTan It would be more helpful if you added this type of information to your question. Please also consider adding more details like potential obstacles/barriers, height differences, building construction, potential interferers etc.. \$\endgroup\$ – Andy aka Jan 8 '14 at 10:05

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