Open Source Cave Surveying Tool Aids in Mapping and Navigation
Affectionately called the "Shetland Attack Pony", this compact device combines a laser range finder, multi-axis IMU, and BLE connectivity.
A brief history of the Shetland Attack Pony
As an avid caver, Phil Underwood depends on his equipment for effectively mapping the narrow passageways and sharp turns of the caves he explores, as even a relatively minor amount of inaccuracy can lead to the resulting maps being very incorrect.. Back in the 1990s, he would bring along a mechanical compass, clinometer (for measuring angles relative to the ground), and reel of tape to get distances. However, this was not an ideal setup due to the cramped conditions and the need for more light than his headlamp could provide, so he decided to invent a digital device to provide this functionality.
After coming up with the memorable name of "Shetland Attack Pony" whilst in a pub, Underwood's initial prototype consisted of a laser pointer, LED readout, and a user-facing button on the side. Shortly after, the next iteration contained an OLED module for displaying the current time, distance, and battery as well as a visible-spectrum laser rangefinder that eliminated the need for a tape measure, although some functionality was still lacking.
Plans for improvement
For Underwood's most recent iteration of the Shetland Attack Pony, he opted to integrate as many sensors as he could in order to build the ultimate cave surveying tool. The handheld device would maintain its compact shape, Bluetooth connectivity, and laser rangefinder, but also have a built-in magnetometer and IMU for finding angles deep below the surface.
The circuit design and PCB
The circuit is based around a Seeed Studio XIAO nRF52840 Sense owing to its very compact footprint, BLE radio, and 6-axis IMU that can find the downwards direction with the accelerometer. For the magnetometer, Underwood went with an RM3100 since it can measure magnetic fields in three separate axes and maintain accuracy once calibrated. Power is delivered through a LiPo battery which is regulated by a 3.3V LDO and can be quickly recharged with an external charging circuit. Finally, communication pins are exposed for both the UART-driven laser rangefinder and the I2C OLED module along with a small buzzer and buttons for interacting with the user.
Building a device
After fabricating the PCB, Underwood assembled the electronics and battery into a single unit before placing it within the 3D printed enclosure. The front of the device has a clear window for the laser rangefinder to see through and the top cutout has an additional piece of clear acrylic for the screen. Everything else was sealed with a layer of silicone grease to aid in waterproofing since caves tend to be quite damp and muddy.
Mapping caves
One round of calibrating later and Underwood was ready to start surveying. Thanks to the "SexyTopo" mobile app, he could easily connect the Shetland Attack Pony to his phone and begin importing measurements for the distance, inclination, and heading. The group of points could then be joined into a single path and leveraged to build a virtual map of the cave's interior features and dimensions. To see more about how Underwood built this compact device, you can watch his build log video here on the element14 Presents YouTube channel.
