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  • Dror Margalit

The Invisible Interaction Between Shoppers and Carts

By Teresa L Wang and Dror Margalit

Parked your car too far from the grocery store? Wanted to take the shopping cart for a walk? How about stealing some groceries and running as far as you can? Well… In many grocery stores, you will realize very quickly that you are out of luck. As you push the shopping carts down the store’s aisles, you may not have known that you are participating in an intangible interaction. If you pass a certain point at the store, the cart’s wheels will lock, preventing you from taking carts outside their designated operation space.


The interaction between people to the shopping cart is fascinating because, in most cases, it is not apparent: the cart’s wheels roll around the store – as long as they are within the constraints of invisible space. At that stage, the system is proactive in the background. Through the implicit behavior of unknowingly walking outside the designated area, a person can trigger the radio frequency (RF) sensor – making the system proactive in the foreground as the cart stops abruptly.

While this mechanism does its job of preventing theft, it creates a very weird, unpleasant interaction. And until the system is activated, the user does not know they are participating in this interaction. Furthermore, even after the system is activated, people might not be aware of it. We do not think of shopping carts as “smart” and do not anticipate any form of interaction that is not merely shopping. Because of that, when the system is activated, people still try to use the cart normally, creating an unwanted behavior. For example, people might still try to push the cart or drag it with locked wheels across parking lots, as seen in this video:

How it works:

We focused on Trader Joe’s shopping carts and saw that they use wheels developed by Gatekeeper. In this video, they explain the different components that make it work.

We drew a diagram to show how the different components work together. The central transmitter is connected to a perimeter to measure its distance from the cart and send locking signals. Once the cart reaches the antenna, the transmitter sends the locking signal to the wheel, that locks and send a locking signal to the other wheel. Once the shopper takes the cart back, the transmitter sends a signal to release the wheels.

As we learned about how the system works, we thought about ways we could recreate it. We could use an RF transmitter and receiver with Arduino to create a system that measures distance and sends information. The challenge would be to create a unique RF ID for every cart to ensure that only one cart is activated at a time.


When thinking about how to improve this project, one of the first thing that came to our mind was creating a warning system. As I mentioned earlier, users are not aware of the system, making them surprised by the interaction. If, however, we add a speaker to the cart or a light indication that it is about to be locked, the interaction would be less frustrating.

We also thought of making the system worse in multiple ways:

  1. Making the cart tip over when it exits the store.

  2. Once the wheels lock, it does not allow them to unlock.

  3. Making the cart accelerate when it is close to the store’s exit.

  4. Making the cart follow shoppers and shaming them for trying to steal it.

  5. Making the cart criticize users on their food choices.

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