proximity-sensorrfidsensor
I am carrying out a project in which a trolley or carrier follows a person and follows him continuously wherever he moves inside a shopping complex or at airport.
I am confused whether to use RFID or some other technology which provides me identification of person and measuring of distance as I have mentioned. If I am able to get all requirements using RFID alone or by alternative method welcome. I am a completely newbie to RF area.
There are a few ways you might get feedback from the kite, without using vision.
1) GPS. Don't rule this out simply because the kite's base might be in a different location each time you use it. To counter this, you simply need a GPS receiver on both the kite and the base. Take both readings, and convert them into Cartesian coordinates. The difference between these coordinates is the offset of the kite from the base. Note that while GPS offers fairly low accuracy, its resolution is better than its accuracy. The kite and the base will both have the same position error, and so the kite's location relative to the base can be (reasonably) accurately calculated.
2) IMU. A 9-axis Inertial Measurement Unit, will help. There are some pretty small, lightweight parts available, like the MPU-9150 from Invensense. If you use a Kalman Filter to combine the GPS reading with the IMU reading, you can hugely increase your position resolution.
(Sorry, that's a similar part, but the same size)
3) Load cells. The reason I asked how easily you could fly a kite with your eyes shut, is because this will give you some idea of how useful the information from Load Cells will be. Perhaps you wouldn't be able to fly the kite brilliantly, but I bet you could keep it in the air. Each kite string will need a 3-axis force measurement so that you know the exact force and angle of each string. Arrange the three load cells like this:
Each one has a string coming perpendicularly out of the top, in the direction of the load cell's maximum sensitivity. Tie the three strings together, and to the main kite string. The sum of the three measured force vectors will be the tension and direction of that kite string. Make sure that the angle between the strings is greater than the maximum angle of the kite string, otherwise one of the three strings may go slack, making for a false measurement.
4) Pressure. I'm sure you could learn a lot by measuring the air pressure at several points on the kite's inside surface. There are some tiny lightweight barometric sensors available, like the BMP085.
It's even available from Sparkfun on a breakout board. These sensors will also help you to measure the kite's altitude (if they're out of the wind), and you can even use them to measure the windspeed if you place two inside a pitot tube.
Four sensors and three Pitot tubes, plus one sensor on the ground, will give you wind speed and direction, and altitude.
I can only relate my experiences: -
If you want to detect a normally-not-powered passive type tag at extreme distances you have to power to that tag from a significantly bigger magnetic field. Making your magnetic field stronger is the only way I can know (and can recommend). Making your tag more efficient in recovering a fraction of this power is also part of the deal. Making the energy needed by the tag smaller is also part of the deal.
Once the "passive" tag is receiving sufficient energy from that magnetic field, it can transmit an RF signal to announce its presence - because it is only very weakly powered it may not be able to transmit more than a few hundred microwatts. This transmission should not have to do-battle with the prevailing magnetic field that powers it - it should be on a carrier frequency that is unconnected with the power magnetic field for this to work most effectively. This will require that the stationary object that generates the power magnetic field is capable of receiving this RF signal.
So now you have two transmissions - the transmission that powers the tag and the transmission from the tag containing ID data - neither are at the same frequency if you want maximum distance.
At about 4 inches (maybe 5 inches if I pushed it), a system I developed could detect the presence of a normally unpowered device. However, I needed to transmit about 1 watt across the gap because the device was doing other things that needed the power - it was rotating on a shaft and wires wouldn't work. The FM transmitter it used was at 80MHz and transmitted at about 1mW. The receiver could detect this at about 1m but it wasn't particularly designed to detect it more than 4 inches. The magnetic field it generated was quite large and the coil it used was wound from Litz wire - I reckon it was about 3 uH and had about 400 volts peak to peak across it at 600kHz (work out the current for yourself!!). Operating the magnetic field at 13MHz could be better but it starts to become a trade-off because, in your situation you want the "detection area" to be large - this means a large diameter coil and you want maximum current through it to produce the bigger and more far-reaching field you are fighting against the inductance of the coil. You need current in that coil to produce a magnetic field and the more the better.
To get that current, I used 250 strand Litz wire and parallel tuning to make the circulating current in the coil much much bigger than the drive current from the generator. This makes it easier to design the generator of course.
In short, if you want to power the tag at distance, think big coil and think litz wire and think parallel tuning for maximum efficency. The power receive coil was also very low loss and highly tuned to get as much voltage as possible when set at the maximum distance. This is what you should focus on in my opinion.
Best Answer
I haven't done this, so this is just thinking out loud, but I'd try coded IR first. The person would wear a belt or something with IR LEDs are various angles.
The cart would have two sets of receivers on it arranged so that each receiver can determine a rough idea of the direction it received the coded IR transmission from. This could be accomplished, for example, with a few IR detectors in each receiver, with each detector having a narrow reception angle. The two receivers would be spaced far enough apart so that when the cart got close enough to the person the parallax would be great enough to reliably discern it from inifinite distance.
If the same IR message with the right code isn't received from both receivers at the same time, or there is significant intensity difference between the two, the reception is ignored like it never happened. The transmitter would send often enough so that missing a few individual receptions causes no real harm.