Mission Preparation and Accident Prevention
You can prevent accidents through awareness and preparation, and we encourage you to follow the practices we lay out in the pre-mission preparation card we provide with your Spiri. This essay discusses the approach to safety we have built into Spiri, including automated emergency recovery and crashworthiness by design. But the best approach to accidents is to prevent them. For that to work, you have a role to play.
First of all, you need to know the proximity of people, animals, vehicles, ground and air traffic corridors, airports and other no-fly zones, and the height of the tree canopy, power lines, and structures in the area. You also need to know the ground — which areas are boggy or covered in water, inaccessible by foot, steep, or covered in brush. As we discuss the safety features built in to Spiri and how you can optimize them for any given autonomous mission, it will become clear why we have singled out these aspects of pre-mission awareness.
There are numerous settings on Spiri that govern what actions it takes when its battery is depleted, when it finds itself outside a set distance from its launch position, when it loses internal or network communications, when it loses the ability to navigate visually or through global navigation satellite systems, when it bumps into something, when it calculates that it cannot complete a mission, and so on. You may always optimize them for a particular mission, one by one, through a graphic user interface such as QGroundControl. However, we have scripts you can edit and save which allow you to assign all the safety settings at once. This is faster and less prone to error, and it enables you to build up a repertoire of settings for different missions. This way, you may maintain settings that are appropriate for all the different locations and conditions in which you operate.
The overarching principle to Spiri’s automated safety system is to give Spiri the best possible chance for a safe recovery under the worst possible operating conditions. This is done through advance planning for emergencies.
Three important settings are: (1) the mission “home” location and the locations of any safe “rally points” where Spiri may safely land if needed, (2) the limit to how far from “home” any Spiri will be allowed to wander, and (3) the minimum altitude at which Spiri will be above the tree line, power lines, and structures covering the entire mission area. Here we mean “altitude” in the sense of the tallest tree on the tallest hill, not the height of Spiri over the ground, as the ground is not necessarily flat.
The home and rally points come in to play most often when Spiri’s battery depletes faster than anticipated. A day with high winds can cause this. So can the age of the battery or an extra load Spiri is carrying. As the battery drops below a first threshold, Spiri will warn you. At that point you may take action or let the matter ride out. Below the next threshold, Spiri will seek the nearest home or rally point and land there. In such an instance, Spiri will rise above the minimum safety altitude, fly in a straight line to the home or rally point, and then land. We cannot assume there is enough light or time to perform obstacle avoidance, or follow any course but a straight line. For these reasons, home and rally points should be chosen that have no overhead interference. Rally points, moreover, need to be on acceptably flat and dry ground for a landing, and accessible to you by foot without putting yourself in danger.
Redundant Position Estimates
Spiri fuses hundreds of variables in its extended Kalman filter (EKF) to figure out where it is, how it is angled, and how fast it is moving. It allows up to three direct measurements of its position. The first two, GNSS (global navigational satellite system) with real-time kinetic correction, and SLAM (simultaneous location and mapping) with stereo vision, are built in and operate by default whenever possible. The third, external position measurement, is optional. You can set it up if you have motion capture, radar, infrared beacons, or similar equipment separate from Spiri that can judge its position and communicate with it.
There are three safety advantages to having both GNSS and SLAM: first, positioning is more accurate when both systems are operating; second, if one system fails the other might still work; third, you may set Spiri to behave more conservatively when using only one direct position measurement, giving it a better chance of a safe recovery if all direct position measurements become unavailable.
Recovery from Tumbles, Bumps, and Brush
If Spiri is put into a tumble, for example if a wicked human throws it spinning or if something bumps into it hard enough to flip it, more than ninety percent of the time it will recover automatically. The flight control system will detect the tumble and work through a sequence of high speed attitude corrections that re-establish normal flight. Spiri can punch through thin foliage. Sometimes it can recover from a collision with a thin tree trunk. Spiri’s default behavior after recovering from these sorts of situations is to proceed to a home or rally point.
Spiri is designed to be quickly repairable after a hard landing. First of all, the most expensive electronics are protected by numerous small and replaceable plastic parts. Secondly, the major electronic and electro-mechanical systems are isolated onto their own boards, so if any one is damaged, it can be replaced alone. We provide instructions on do-it-yourself repairs. We also provide a service agreement under which you may send us a broken Spiri and we will send you a replacement.
We have designed as safe and smart a battery as we can. A detailed discussion of battery safety is out of the scope of this essay, but here are a few considerations. First of all, we have programmed Spiri to tell you when a battery is too old to keep using, and we have a service agreement under which you can send your depleted battery to us, and we will send you a fresh one. Second, if a battery is visibly damaged, swollen, hissing, or you can smell it, do not plug it in to anything, treat it like it could combust at any moment. We provide a card with instructions on dealing with this kind of battery situation with your Spiri, and being ready for a battery fire, as rare as it is, should be an integral part of your safety practice.