Release V2

Release the Electronic Trolley Lock for supermarkets

Overview

• In the UK, over half a million shopping trolleys are abandoned annually.
• It costs British businesses £35m a year, and that doesnt include environmental damages!
• Here is the engineering development to get to the finished prototype.

Methodology

Altium PCB Design

DC to DC power supply

A power supply board was made to test TI DC to DC converters chips for my power requirements. I designed a dev board with two power rails, a 3.3v 200mA high effeciency rail and a higher power 5v 1A for the servo.

• 5v rail
  • TPS61023
  • Boost converter
• 3.3v rail
  • TPS61098 TPS63051
  • Synchronous boost converter with integrated LDO

This was a lovely development for introduction to DC-DC converters and to distinguish buck from boost converters

Main PCB

Here I joined the previous power supply work while including the mcu and pereferals.

• STM32F4
• Embedded rust
• DC-DC converter design
• LED design choices
• Logic Level Converter
• Tag SWG connector
• Manufacturing considerations

MainSchematic

IoT Functionality

• Explain how the IoT system integrates with the hardware.
• Mention key features and benefits, such as remote control and monitoring.

Code & Firmware

Why did you choose rust instead of C?

Good question, I chose rust for the following reasons.

1. Safety. It basically meant that it was extremely hard for me to introduce memory safety bugs, even with my lack of embedded experience!
2. Rust is a new language compared to C. It let me personally find out what all the fuss is about.
3. Rust is backed by a great community. Meaning help was only a few tickets away.
4. Most importantly A driver for the RFID reader I was using already existed MFRC522,

• Hierarchical directory structure for embedded rust

#![no_std]
#![no_main]

use defmt_rtt as _;
use panic_probe as _;
use stm32f3xx_hal as hal;

use mfrc522::{Mfrc522, Uid};

#[cortex_m_rt::entry]
fn main() -> ! {
    // Initialization code for peripherals and configuration
    
    // List of cards with access 
    let uid_card_pass_1 = [74, 152, 138, 25];
    
    // Peripheral initialization and clock configuration

    // SPI configuration for MFRC522
    
    // Main loop for RFID card detection and handling
    
    // Loop for locking and unlocking 

    loop {
        // Main loop content
    }
}

// Function for handling RFID card operations
fn handle_card<E, SPI, NSS>(mfrc522: &mut Mfrc522<SPI, NSS>, uid: &Uid, write: bool)
where
    SPI: spi::Transfer<u8, Error = E> + spi::Write<u8, Error = E>,
    NSS: OutputPin,
{
    // RFID card authentication and read/write operations
}

// Panic handler and application termination function
#[defmt::panic_handler]
fn panic() -> ! {
    cortex_m::asm::udf()
}

Hardware development Phase 4.

Locking Mechanism

For the trolley to be locked and unlocked, I needed to design a mechanism that could be controlled by a microcontroller instead of a 1£ coin. Initial concept was a Slider crank mechanism where we convert the rotational displace from a servo to a forward displacement to push a pin into the key.

Slider crank mechanism required 3 joins, which would wear down overtime and require lubrication. For a mass produced product, this is not idea.

Compliant Mechanism

Exploring compliant mechanisms became the logical next step, especially with the accessibility of a 3D printer, aiming to overcome the setbacks encountered with the slider crank mechanism. The iterative process is vividly captured in the following table:

Mech Pin Iterations

Mech Case Iterations

Comparison of O-Ring and Gasket Designs

• The intended solution from ReleaseV1 was to split the case at the seam to hide the split lines.
• Due to all the organic angles and corves this made designing the gasket complicated

Solution: O-ring Design!

O-Ring Design	Gasket Design

Design of stainless steel key

1

2

3

4

• Initial thoughts were using aluminium but once I started manufacturing I quickly realised how soft and easy to work with aluminium was… not an ideal material to use for a secure lock.

Conclusion & Contact

• IP34 Rating
• Weeks of battery life
• Fully programmed in Rust

testing