Sensing

Primary location tracking capability is provided by the MTK3339 Chipset GPS (~$22, DigiKey). This GPS interfaces directly with the AMTC’s onboard, WiFi-equipped computer to enable real time location tracking from anywhere in the world. This GPS was selected due to its low power draw (.066 W), high position accuracy (less than 3 meters), and ease of sourcing. Airport operators are able to view locations of all AMTCs in a fleet by simply logging in to the web-based Chairiot dashboard.

Chairiot is equipped with 4 half-bridge strain gauges specially designed for steel structures ($26, UXCell). The strain gauges are mounted on the cart chassis in the location of maximum strain – the chassis is designed in such a way that the front and back rails act like beams, with the wheels acting as support and the body of the cart applying force in controlled locations. Readings from the strain gauges pass to a Load Combinator ($2, SparkFun) and an HX711 amplifier ($3, eBay) before being reported to the onboard, WiFi-equipped computer and the web-based dashboard. Since vibrations can also affect readings in the strain gauge, small minute fluctuations in weight can tell us when the cart is moving or standing still, data which can be used for tamper-reporting.

Our AMTC is equipped with 2 TMP36 sensors ($0.73ea, DigiKey), one monitoring the temperature of the storage cavity and the other monitoring the temperature of the battery. The TMP36 was selected due to a wide operating range (-40C to 150C), low power draw (<15 uW), and simplicity (no calibration). Airport operators are able to view temperatures through the web-based Chairiot dashboard or onboard LCD, and are alerted when temperatures in either location fall outside of a specified range.

A single waterproof HC-SR04 ultrasonic ranging module ($8, Alibaba) is mounted on the front of the cart, designed to notify the operator whether or not there is an obstruction in front of the cart. Chairiot’s full width can be covered within 53 inches, but the module itself has a range of 13 feet, and a field-of-view of 30 degrees. The module will start aurally alerting the operator once it starts to detect an object within 3 feet of the cart, and more modules can easily be added to other walls of the cart.

A 5MP, 1080P camera module ($13, Robotshop) located on the front of the cart allows the operator to see the other side; the camera acts in conjunction with the ultrasonic module to allow the operator to see all around the cart. Much like a car backup camera, this can be incredibly useful for safety and added fairly easily. Video feed is shown at will to the operator.

Chairiot features additional access and inventory management capabilities. Through use of the onboard PN532 RFID reader ($7, DigiKey), operators are able to remotely view storage cavity contents of any RFID tagged items. RFID tagging is an increasingly popular practice in goods management. The RFID reader is also used for access to the cart with RFID cards.

Good battery management and health is vital to the long-term success of Chairiot because it drives all sensors and security on the cart. Our AMTC includes a voltage sensor ($4.22, RobotShop), Pololu current sensor ($7.16, RobotShop), TMP102 temperature sensor ($0.63, DigiKey), and MCP3008 ADC ($1.76, DigiKey) to provide a complete picture of the battery health. The AMTC reports voltage, predicted remaining life, time since last charge, and time to full charge on the AMTC’s LCD screen and to the web-based Chairiot dashboard. Anomalies in battery health trigger an onboard alarm, an on screen notice, and a web notice.

Chairiot is equipped with four 12V, 22Ah rechargeable lead-acid batteries ($12.30, Alibaba). At Chairiot’s expected power usage, the batteries and included solar panels can energize the cart for 30 days. Batteries are mounted in their own compartment at the very bottom of the cart and covered with a removable panel that allows for easy battery replacement. A female power plug is also included on the side of the cart for battery charging from a wall outlet. The battery’s typical charge time is 5 hours.

Chairiot also comes with two 12V, 20W solar panels ($41.25, DHGate). The solar panels give Chairiot a bit of added flexibility by increasing it’s expected battery life. These specific panels are waterproof and have a 20 year lifespan, with a drop-off of 10% in 10 years and 20% within 25. They are mounted flat on top of the cart.

Chairiot’s suite of sensors allow it to be more than just a smart, connected device. These sensors can also be largely used for security. For example, the AMTC’s weight sensing strain gauges can verify whether or not the cart is moving when it’s not supposed to. A remote operator can turn on the camera at any time and view the cart’s surroundings. The cart’s included GPS reports it’s location at all times. Chairiot’s sensors capture all data and store it in an Activity Log which is uploaded to a server and ideally backed up.

Arguably the most important part of tamper-proofing the cart is determining when a malicious person is attempting to gain access to the cart’s contents. To this end, Chairiot is equipped with several sensors (strain gauges) on the door lock that detect when someone attempts to gain access to the cart without first unlocking it via any of the three unlock methods. In addition, if any unlock methods fail after a set number of tries, an alarm is tripped and a notification sent to a remote operator and logged. The cart itself will buzz loudly.

Our AMTC is capable of unlocking/locking in three different ways – remotely via Chairiot web-based dashboard, on-cart via RFID card (e.g., badge), or on-cart via LCD keypad code. The frame mounted Futaba servo ($14.49, Alibaba) rotates a latch onto/off of a door-mounted pin. When the servo is unlocked, the door is manually opened by a tamper resistant cam latch ($17.81, McMasterCarr). After closing, the cam latch automatically falls over a frame-mounted pin, ensuring the door is completely shut.