A smart shopping cart

I'm building a smart shopping cart for my final year project, it makes use of four ultrasonic sensors and one rfid reader/tag combo. The user walks into a store, picks up an rfid card and swipes it in front of the cart, the cart then begins to move and follow the user with the help of the ultrasonics. I quickly realised how much error this design was prone to, as it did not accurately follow the user for very long. I thought about using rfid readers in place of the ultrasonic sensors in order to track specific tags, but the extremely short rfid range is an issue. Is there some other sensor or transceiver that transmits unique IDs so the cart can accurately follow a specific shopper and not the other shoppers in the store. Or do i need to think of indoor positioning systems(i wud rather not becos they seem really complex, triangulation and what not). I really need guidance on this

I'm curious about how you prevent the ultrasonics of multiple carts interfering with each other.

The four ultrasonic sensors on the smart cart are faced front, back, left and right, a lot of space between them. Actually, I know this isn't an accurate design because the shopping cart would follow pretty much anyone or anything.

Please How can i solve this problem, RFIDs have too short a range, what else can i use?

The four ultrasonic sensors on the smart cart are faced front, back, left and right, a lot of space between them.

No, I meant how would your cart know that the echoes it received were not from another shopper's cart's "pings"?

The four ultrasonic sensors on the smart cart are faced front, back, left and right, a lot of space between them.

That keeps the sensors on your cart from interfering with each other. That's fine if there is only one shopper allowed in the store at a time.

Most stores would refuse to buy such a system, though.

The question was what keeps the sensors on one cart from interfering with sensors on another cart. And, the answer is nothing.

Have you googled self following suitcase?

I'm not entirely sure. I don't know many sensors relating to movement, so the ultrasonic sensor was my first thought.
I have googled the self following suitcase and it makes use of ultrasonic sensors and a radio technology i don't really understand

You definitely need some sort of unique ID for the shopper (rfid type) or they will get confused (just one case of the cart with someone's purse in it following someone else that stepped between the shopper and their cart to grab something off the shelf (which happens all the time) and everybody will be mad. There are rfid tags that they use with cattle etc. that can be read from a greater distance which could be a consideration. You still are faced with the question another person asked about how the cart knows if the "echos" it is getting are from its own transducers or another cart's transmitters. Keeping the transmit rate low and only looking at echos within a short window (close to the user) would help. If it required multiple returns to determine a valid user distance AND used an idea from Ethernet where re-transmit (for collisions) is at some (pseudo)random time would help prevent the problem of multiple echos from the wrong transmitter (another cart).

Another possibility could be a "transponder" the user wears that gives a coded echo back when it is pinged from the cart so the cart knows which one it is talking to. (similar to what they use in aircraft for radar - you dial in a 4 digit number which then shows up on the ground radar screen as part of the location information). As you are discovering, it is not quite as simple a task as it first seems.

proposed solution 1
i have recently seen some RF/ultrasonic positioning systems that make use of one ultrasound transmitter and one RF receiver carried by the person, and two ultrasound receivers and one RF transmitter carried by the robot. The robots RF transmitter sends out a signal, which when received by the persons RF receiver triggers the persons ultrasound transmitter to transmit a signal. This signal is received by the robots two ultrasound receivers(with a distance d between them), and the distance and angle to the target person can be calculated.

Sounds very feasible, but my question is, if the robot is in a crowded place, where the other people are also carrying ultrasound transmitters and RF receivers:

  1. Will the RF signal meant for the owner of the robot be accidentally received by the RF receiver on another person, or is there some kind of unique ID shared between RF receiver/transmitter pairs that could prevent this problem, or maybe some kind of frequency hopping spectrum, so there would be no mistaken identity.
  2. How will the ultrasound receivers on the robot be sure that the received signal is from its matching ultrasound transmitter. Again, is there some kind of special ID systems employed by some ultrasonic transmitter/receivers.

solution 2
i have also read about this new device called Trackr, it makes use of bluetooth low energy to track objects based on their proximity to bluetooth enabled devices like smart phones.
Considering a scenario where various bluetooth devices(beacons) are placed at specific points in a room, and the target to be tracked is equipped with a bluetooth emitter/bluetooth chip/dongle. When the target passes each of those beacons, the signal strength intensity is received and distance from the target is calculated.
I understand this concept, but have some questions:

  1. When distance is calculated between beacon and target, what about the angle, how would the robot follow the target based on distance alone?
  2. If this method is not accurate enough to be used in a crowed space, what other method can be used, i have checked out some indoor GPS devices and they are way to expensive(above $200). Please i need help, i cant sleep, just thinking about this. Would a camera based tracking system be better and how easy would it be for a non expert like me to implement. Please, anyone...

I think you must see this video. It uses some computer vision algorithm for tracking a specific person without rfid.