This week I’m attending the 2017 International Symposium on Robotics Research (ISRR) in Puerto Varas, Chile. Yesterday, I presented some work on motion planning for systems that make and break contact with their environment. This is a really pervasive problem in robotics that shows up in both walking and manipulation. Fundamentally, it’s challenging because impacts and friction lead to trajectories with discontinuities (large jumps in velocity). These discontinuities break a lot of traditional algorithms that rely on derivatives and assume some level of smoothness.
We developed a new, more accurate, mathematical formulation of rigid body contact physics that works well for trajectory optimization. Instead of explicitly reasoning about impact and friction forces, our formulation treats the physics as a constrained optimization problem, where forces are generated implicitly by inequality constraints that, for example, say that objects are not allowed to fall through the floor. Using this approach, we can generate walking trajectories for legged robots like Spring Flamingo without having to pre-specify a contact sequence. You can check out the slides from my talk, download the full paper and get the code on GitHub.
On June 23 the PSLV-C38 launch carried 31 satellites into low-Earth orbit. Among them were Max Valier and Venta-1, built by OHB of Germany, which were carrying six of my tiny Sprite spacecraft as secondary payloads. One each is attached to the outside of Max Valier and Venta-1, and four more are currently housed inside Max Valier waiting to be deployed as free-flying spacecraft at some point in the future. We can now confirm that at least one of the Sprites (probably the one mounted to Venta-1) is transmitting and has been successfully decoded by several ground stations around the world.
This is the first time we’ve successfully demonstrated a Sprite on orbit and, as far as I know, they are the smallest spacecraft ever flown by at least one order of magnitude. I’d like to extend a huge thanks to the Breakthrough Foundation and OHB for making this ride share happen and for helping to advance the state of the art in small spacecraft. Check out the Sprite mounted on Max Valier before launch:
This week I attended the Robotics: Science and Systems (RSS) conference at MIT. I presented some work on robust trajectory optimization, where we explicitly take into account disturbances and modeling errors at the trajectory planning stage to make sure our systems can handle these things in the real world. What sets our new algorithm, DIRTREL (short for “Direct Trajectory optimization with Ellipsoidal disturbances and LQR feedback”), appart from existing methods is that it’s fast and scales up to complex real-life systems. For example, we can plan motions for things like quadrotors with unknown wind gusts and robot arms carrying containers with difficult-to-model fluid slosh. A short video of my talk from the conference is below. You can also download the full paper and check out the code on GitHub.
I just got back from the 2017 Breakthrough Discuss Conference at Stanford. It was a fascinating couple of days hearing about the latest exoplanet discoveries from the astronomers themselves, as well as recent developments in SETI and the search for life outside our solar system. Many videos from the conference are being uploaded to YouTube and I highly recommend checking them out. I gave a talk on the stability of laser-propelled sails for Starshot, and a photographer caught this great picture of me doing a little experiment:
A paper I wrote together with Avi Loeb on the stability of laser-propelled light sails just appeared in the Astrophysical Journal Letters today. The paper takes a look at how to make sure a sail being pushed by a laser beam, like the one proposed for Breakthrough Starshot, actually stays on the beam without getting pushed off. We found that a cone-shaped sail riding on a Gaussian beam, which has been suggested as a possible architecture for Starshot, is unstable without active feedback control or other modifications. Instead, we suggested a hollow spherical sail — like a shiny balloon — riding on a special hollowed-out beam shape (shown in the picture below). This architecture has passive stability while also allowing the payload to be sheilded from the laser beam inside the sphere. If you’re interested in the details, a preprint of the paper is available on the ArXiv.
Today I gave a lunch talk at the Harvard-Smithsonian Center for Astrophysics on some work I’ve been doing for the Breakthrough Starshot project. In particular, I’ve been trying to figure out how to make sure the lightsail stays on the laser beam as it is accelerated. In the video below, I talk about some analysis on different beam and sail configurations and thier stability properties. You can also check out a draft of the paper on ArXiv for all of the gory mathematical details.