Best methods for long term position tracking?

Question

I am in the beginning stages of development for a project where I need to continuously track the position of an object for long amounts of time. The object will not move very much, but when it does I want to track it to see when it moves 10 meters from its initial position.

I looked into IMU tracking as I have some past experience with IMUs but they seem to have huge problems with long term position tracking due to bias buildup.

The object will be in an area where people can walk, so something like ultrasound won't really work because someone can get in the way of the sensor and make it look like the object is closer to the wall than it used to be when in reality someone just stepped in front of it. The object in question is a large chair that will be stationary most of the time and can only be moved by humans but it can be lifted up. I am mainly concerned in tracking its location on a 2D surface which is the floor its on, but it CAN be lifted off the surface.

Triangulation has been mentioned but I do not know what technology is involved. Do I need to mount reflectors on the object? There will be other things present and moving in the room like people and furniture so how do I account for that and only focus on the object I am interested in n?

The response to the object moving must be somewhat short, preferably less than 30 seconds but closer to 5-10 if possible (and of course lower is always better).

What are the most common techniques used for long term position tracking? When I say "long term" I mean months preferably but right now I'm just trying to figure out what's possible with current technology.

Answer 1

Since your object seems not to be moving most of the time, you may still use an IMU using the following assumption :

If the object is not accelerating and approximately not moving during a long time, then it's probably not moving at all.

Then, use an IMU and check for this condition ; as soon as this condition is detected, consider the position to be that at the beginning of the static phase until acceleration is detected. This effectively zeroes out the drift bias during all immobility phases.

That's simple programming, rather power-efficient, and very robust to interference if the above assumption is valid.

Answer 2

Triangulation MAYBE could be done by placing three low power emitters (check the allowable frequencies for indoor transmission) on the perimeter of the area, each one of them emitting at a different frequency. Place three receivers (or one tunable receiver) on the moving device.

On the mobile device rectify and smoothen the desired signal, amplify it by a fixed amount, measure amplitude, and you have it. I think RFID do a similar thing to power themselves, you should be able to use the same principle to measure the strength of the received signal. Since signals get weaker with the distance, you may be able to get a good estimate of the position.

Obviously transmission can be intermittent, according to your power/movement requirements. If you synchronise them, you may need only a single frequency.

Answer 3

For a range of upto 3 meters, you may investigate the PKE solution. The systems involves a low frequency transmitter which sends a beacon and a receiver which receives and sends back the RSSI to the LF transmitter over RF (has to be battery operated, low power). The accuracy will be accurate upto a sub ten cm's for ideal tests. Just one idea I have seen working..

Answer 4

Presumably a chair has room for some electronics. This leads me to suggest you use ultrasonic/radio triangulation methods. By having a signal ping from the chair at 0.5s intervals then receive it with 2 or more devices on the edge of the area.

An old electronic whiteboard system called "eBeam" used that to turn any smooth surface into a USB whiteboard of up to 5m x 5m. The pens lasted for weeks of daily use on a CR2032, so on a bunch of AA or even D batteries...

You can get a differential distance from two known locations and do some trigonometry to expand that information into a quite accurate position. If you get a differential distance from three points you can do that even easier.

By keeping a counter in the message that you pulse out you can determine if the signal differences you measure are on the same package and you could even add anti-echo schemes if the room is very echo-y, so you don't get skewing. By pulsing every 0.5s or 1s, as long as there's not a party going on with people packed into the room, some of the packets should arrive on all sensors within 20 ~ 30 seconds. Probably much sooner. If you want you can do it the GPS way and use many more receivers and just use the ones that get the same package to do the maths, ignoring the ones that are obscured at that time.

Of course, since the speed of sound is still pretty high, your counter probably only has to be 1 or 2 bit to be able to fully exclude wrong packages and echoes and such. May not even need a counter, actually.

You can also do this at such low average power that some crazy power harvesting scheme could keep some rechargeable battery under the chair full enough to use for very long times, but... couple of AA would then also cut it.

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Another option is RFID-type triangulation, where the chair has a RF-powered chip that communicates back by attenuating the signal. By receiving the attenuated signal at various points the timing difference can again be used for triangulation and you can do this with simple RFID tags or similar objects. Given sufficient intelligence in the implementation you can then also use the actual response part to compare, allowing you to use the same RFID system to track several tags.

This is much more complicated to build and get right, however, but it is already being done out there in the world.