Design of Spiri Mu
The principle around which we focused development of the Spiri Mu is autonomous action. When we began conceptual work, there were many quadcopter drones on the market. Ours, however, would fly itself, read signs, respond to changing conditions, and require minimal human supervision. It would be a programmable, aerial, robotic deep learning platform, and it would rest comfortably in a human hand.
This meant the Spiri Mu needed the best possible sensors and the best possible "brain." Given the small size we intended, we came to choose the Nvidia TX2 as our main computer (sometimes called a "companion computer") with the Mayan Robotics PixRacer as our flight controller. The TX2 has a spectacular GPU for its size, which the Spiri Mu needs for vision and learning. Meanwhile, the PixRacer is fully capable of handling flight, allowing us to separate higher order "thought" on the Spiri Mu from its flight "instincts." We sacrificed pixels in favor of a global shutter in selecting our built-in camera. In terms of providing vision for our robot, the need for accurate images undistorted by fast movement was overwhelmingly more important than maximizing pixels on still images. With its enormous capacity to handle video, we provided the Spiri Mu with two cameras, like most animals, to enable stereo vision.
Further in support of machine autonomy, as well as the creativity of developers and inventors, we provided Spiri with a generous array of free input/output feeds, plus a great deal of spare physical power: it has up to three times its weight in available thrust. It can carry a gimbal and 8k camera and still have more spare lift than most drones in its class.
We see the Spiri Mu, able to fly while thinking, to make sense of what it sees in real time, and to run sophisticated programming, as the superlative platform for the development and deployment of machine intelligence in drones.
An autonomous system should be adaptable to the widest possible variety of circumstances, and this concept of versatility was central to our design. The Spiri Mu can switch between indoor and outdoor modes. It can be modified for upside-down flight with a change of propellers and firmware. It can be an individual or a member of a formation. In formation, it can coordinate either client-server or peer-to-peer. The choice to use 3D printing for the mechanical parts was also in support of versatility – parts can be customized for adding extra gear.
We provide Spiri with four kinds of power: computing power, sensing power, propulsion power, and teamwork power. The first three set requirements and constraints on the physical design of the robot. This is why the Spiri Mu is so compact – extra material is simply a drain on the thrust-to-weight ratio. This is also why we favored computational efficiency (parallel processing with lots of optimization) over processor size. The Spiri Mu is unequalled in terms of practical operational "thinking" per non-battery gram of robot and it fits into carry-on luggage. As for teamwork, this comes into our collaborative software. We designed around the possibility that within a large ensemble of Spiri Mu there could be lots of specialization, robots with different jobs and equipment working together.
Spiri is a premium product and it should last a long time. At the same time, rapid improvements in battery, sensing, computational, and materials technology will make many of the components in a Spiri obsolete over time. Our approach to this issue is to modularize the build – any given component is made to be interchangeable with its replacement without affecting the rest of the robot (except to improve the function of the given component). When you open a Spiri you will see how the functional areas are isolated for this purpose.
We intend for Spiri Mu to be able to interact closely with us, in spaces designed for humans to live and work. For this reason, we wanted Spiri to behave and sense in intuitive ways, for example having cameras like eyes pointed forward, to be left-right but not forward-back symmetrical like an animal, to be able to read signs and gestures. This makes its movements more predictable for us. We gave the Spiri Mu a more aqueous than birdlike or insectoid form, to match with the sense of it floating in air rather than gliding or flapping in it. We also worked to make it safe around us, with rotor interference detection and the option for propeller guards, autonomous recovery after bumping into things, and a size that makes collision with a Spiri, while still potentially dangerous for the eyes, otherwise not a danger to life or limb.
Multiple Independent Position Estimators
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 and Bumps
If some wicked human throws a Spiri Mu into a wild tumble, more than ninety percent of the time it will recover and right itself. Our flight control system is designed to detect a rapid tumble and work through a sequence of high speed attitude corrections that re-establish normal flight. Spiri can punch through thin foliage. It can usually recover from a collision with a tree trunk or even with another Spiri in flight. The world is full of dangers for an autonomous robot. Being able to recover from collisions is a major factor in the viability of such a robot.
The Spiri Mu is designed to be quickly repairable after a hard landing. First of all, the most expensive electronics are protected by numerous small and replaceable 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.
Our intent in designing the Spiri Mu was certainly that it would inspire hardware and software innovation, but also that it would inspire a new problem solving approach to the many challenges we face around the world and into our future. Everything outlined above is a part of that. We made the Spiri Mu entirely to put it in your hands so that you and your peers, all friends of the robots, can use it to make a better world.