Tag Archives: gravity
#439495 Legged Robots Do Surprisingly Well in ...
Here on Earth, we’re getting good enough at legged robots that we’re starting to see a transition from wheels to legs for challenging environments, especially environments with some uncertainty as to exactly what kind of terrain your robot might encounter. Beyond Earth, we’re still heavily reliant on wheeled vehicles, but even that might be starting to change. While wheels do pretty well on the Moon and on Mars, there are lots of other places to explore, like smaller moons and asteroids. And there, it’s not just terrain that’s a challenge: it’s gravity.
In low gravity environments, any robot moving over rough terrain risks entering a flight phase. Perhaps an extended flight phase, depending on how low the gravity is, which can be dangerous to robots that aren’t prepared for it. Researchers at the Robotic Systems Lab at ETH Zurich have been doing some experiments with the SpaceBok quadruped, and they’ve published a paper in IEEE T-RO showing that it’s possible to teach SpaceBok to effectively bok around in low gravity environments while using its legs to reorient itself during flight, exhibiting “cat-like jumping and landing” behaviors through vigorous leg-wiggling.
Also, while I’m fairly certain that “bok” is not a verb that means “to move dynamically in low gravity using legs,” I feel like that’s what it should mean. Sort of like pronk, except in space. Let’s make it so!
Just look at that robot bok!
This reorientation technique was developed using deep reinforcement learning, and then transferred from simulation to a real SpaceBok robot, albeit in two degrees of freedom rather than three. The real challenge with this method is just how complicated things get when you start wiggling multiple limbs in the air trying to get to a specific configuration, since the dynamics here are (as the paper puts it) “highly non-linear,” and it proved somewhat difficult to even simulate everything well enough. What you see in the simulation, incidentally, is an environment similar to Ceres, the largest asteroid in the asteroid belt, which has a surface gravity of 0.03g.
Although SpaceBok has “space” right in the name, it’s not especially optimized for this particular kind of motion. As the video shows, having an actuated hip joint could make the difference between a reliable soft landing and, uh, not. Not landing softly is a big deal, because an uncontrolled bounce could send the robot flying huge distances, which is what happened to the Philae lander on comet 67P/Churyumov–Gerasimenko back in 2014.
For more details on SpaceBok’s space booking, we spoke with the paper’s first author, Nikita Rudin, via email.
IEEE Spectrum: Why are legs ideal for mobility in low gravity environments?
Rudin: In low gravity environments, rolling on wheels becomes more difficult because of reduced traction. However, legs can exploit the low gravity and use high jumps to move efficiently. With high jumps, you can also clear large obstacles along the way, which is harder to do in higher gravity.
Were there unique challenges to training your controller in 2D and 3D relative to training controllers for terrestrial legged robot motion?
The main challenge is the long flight phase, which is not present in terrestrial locomotion. In earth gravity, robots (and animals) use reaction forces from the ground to balance. During a jump, they don't usually need to re-orient themselves. In the case of low gravity, we have extended flight phases (multiple seconds) and only short contacts with the ground. The robot needs to be able to re-orient / balance in the air. Otherwise, a small disturbance at the moment of the jump will slowly flip the robot. In short, in low gravity, there is a new control problem that can be neglected on Earth.
Besides the addition of a hip joint, what other modifications would you like to make to the robot to enhance its capabilities? Would a tail be useful, for example? Or very heavy shoes?
A tail is a very interesting idea and heavy shoes would definitely help, however, they increase the total weight, which is costly in space. We actually add some minor weight to feet already (in the paper we analyze the effect of these weights). Another interesting addition would be a joint in the center of the robot allowing it to do cat-like backbone torsion.
How does the difficulty of this problem change as the gravity changes?
With changing gravity you change the importance of mid-air re-orientation compared to ground contacts. For locomotion, low-gravity is harder from the reasoning above. However, if the robot is dropped and needs to perform a flip before landing, higher gravity is harder because you have less time for the whole process.
What are you working on next?
We have a few ideas for the next projects including a legged robot specifically designed and certified for space and exploring cat-like re-orientation on earth with smaller/faster robots. We would also like to simulate a zero-g environment on earth by dropping the robot from a few dozens of meters into a safety net, and of course, a parabolic flight is still very much one of our objectives. However, we will probably need a smaller robot there as well.
Cat-Like Jumping and Landing of Legged Robots in Low Gravity Using Deep Reinforcement Learning, by Nikita Rudin, Hendrik Kolvenbach, Vassilios Tsounis, and Marco Hutter from ETH Zurich, is published in IEEE Transactions on Robotics. Continue reading
#439372 Legged Robots Do Surprisingly Well in ...
Here on Earth, we’re getting good enough at legged robots that we’re starting to see a transition from wheels to legs for challenging environments, especially environments with some uncertainty as to exactly what kind of terrain your robot might encounter. Beyond Earth, we’re still heavily reliant on wheeled vehicles, but even that might be starting to change. While wheels do pretty well on the Moon and on Mars, there are lots of other places to explore, like smaller moons and asteroids. And there, it’s not just terrain that’s a challenge: it’s gravity.
In low gravity environments, any robot moving over rough terrain risks entering a flight phase. Perhaps an extended flight phase, depending on how low the gravity is, which can be dangerous to robots that aren’t prepared for it. Researchers at the Robotic Systems Lab at ETH Zurich have been doing some experiments with the SpaceBok quadruped, and they’ve published a paper in IEEE T-RO showing that it’s possible to teach SpaceBok to effectively bok around in low gravity environments while using its legs to reorient itself during flight, exhibiting “cat-like jumping and landing” behaviors through vigorous leg-wiggling.
Also, while I’m fairly certain that “bok” is not a verb that means “to move dynamically in low gravity using legs,” I feel like that’s what it should mean. Sort of like pronk, except in space. Let’s make it so!
Just look at that robot bok!
This reorientation technique was developed using deep reinforcement learning, and then transferred from simulation to a real SpaceBok robot, albeit in two degrees of freedom rather than three. The real challenge with this method is just how complicated things get when you start wiggling multiple limbs in the air trying to get to a specific configuration, since the dynamics here are (as the paper puts it) “highly non-linear,” and it proved somewhat difficult to even simulate everything well enough. What you see in the simulation, incidentally, is an environment similar to Ceres, the largest asteroid in the asteroid belt, which has a surface gravity of 0.03g.
Although SpaceBok has “space” right in the name, it’s not especially optimized for this particular kind of motion. As the video shows, having an actuated hip joint could make the difference between a reliable soft landing and, uh, not. Not landing softly is a big deal, because an uncontrolled bounce could send the robot flying huge distances, which is what happened to the Philae lander on comet 67P/Churyumov–Gerasimenko back in 2014.
For more details on SpaceBok’s space booking, we spoke with the paper’s first author, Nikita Rudin, via email.
IEEE Spectrum: Why are legs ideal for mobility in low gravity environments?
Rudin: In low gravity environments, rolling on wheels becomes more difficult because of reduced traction. However, legs can exploit the low gravity and use high jumps to move efficiently. With high jumps, you can also clear large obstacles along the way, which is harder to do in higher gravity.
Were there unique challenges to training your controller in 2D and 3D relative to training controllers for terrestrial legged robot motion?
The main challenge is the long flight phase, which is not present in terrestrial locomotion. In earth gravity, robots (and animals) use reaction forces from the ground to balance. During a jump, they don't usually need to re-orient themselves. In the case of low gravity, we have extended flight phases (multiple seconds) and only short contacts with the ground. The robot needs to be able to re-orient / balance in the air. Otherwise, a small disturbance at the moment of the jump will slowly flip the robot. In short, in low gravity, there is a new control problem that can be neglected on Earth.
Besides the addition of a hip joint, what other modifications would you like to make to the robot to enhance its capabilities? Would a tail be useful, for example? Or very heavy shoes?
A tail is a very interesting idea and heavy shoes would definitely help, however, they increase the total weight, which is costly in space. We actually add some minor weight to feet already (in the paper we analyze the effect of these weights). Another interesting addition would be a joint in the center of the robot allowing it to do cat-like backbone torsion.
How does the difficulty of this problem change as the gravity changes?
With changing gravity you change the importance of mid-air re-orientation compared to ground contacts. For locomotion, low-gravity is harder from the reasoning above. However, if the robot is dropped and needs to perform a flip before landing, higher gravity is harder because you have less time for the whole process.
What are you working on next?
We have a few ideas for the next projects including a legged robot specifically designed and certified for space and exploring cat-like re-orientation on earth with smaller/faster robots. We would also like to simulate a zero-g environment on earth by dropping the robot from a few dozens of meters into a safety net, and of course, a parabolic flight is still very much one of our objectives. However, we will probably need a smaller robot there as well.
Cat-Like Jumping and Landing of Legged Robots in Low Gravity Using Deep Reinforcement Learning, by Nikita Rudin, Hendrik Kolvenbach, Vassilios Tsounis, and Marco Hutter from ETH Zurich, is published in IEEE Transactions on Robotics. Continue reading
#438553 New Drone Software Handles Motor ...
Good as some drones are becoming at obstacle avoidance, accidents do still happen. And as far as robots go, drones are very much on the fragile side of things. Any sort of significant contact between a drone and almost anything else usually results in a catastrophic, out-of-control spin followed by a death plunge to the ground. Bad times. Bad, expensive times.
A few years ago, we saw some interesting research into software that can keep the most common drone form factor, the quadrotor, aloft and controllable even after the failure of one motor. The big caveat to that software was that it relied on GPS for state estimation, meaning that without a GPS signal, the drone is unable to get the information it needs to keep itself under control. In a paper recently accepted to RA-L, researchers at the University of Zurich report that they have developed a vision-based system that brings state estimation completely on-board. The upshot: potentially any drone with some software and a camera can keep itself safe even under the most challenging conditions.
A few years ago, we wrote about first author Sihao Sun’s work on high speed controlled flight of a quadrotor with a non-functional motor. But that innovation relied on an external motion capture system. Since then, Sun has moved from Tu Delft to Davide Scaramuzza’s lab at UZH, and it looks like he’s been able to combine his work on controlled spinning flight with the Robotics and Perception Group’s expertise in vision. Now, a downward-facing camera is all it takes for a spinning drone to remain stable and controllable:
Remember, this software isn’t just about guarding against motor failure. Drone motors themselves don’t just up and fail all that often, either with respect to their software or hardware. But they do represent the most likely point of failure for any drone, usually because when you run into something, what ultimately causes your drone to crash is damage to a motor or a propeller that causes loss of control.
The reason that earlier solutions relied on GPS was because the spinning drone needs a method of state estimation—that is, in order to be closed-loop controllable, the drone needs to have a reasonable understanding of what its position is and how that position is changing over time. GPS is an easy way to take care of this, but GPS is also an external system that doesn’t work everywhere. Having a state estimation system that’s completely internal to the drone itself is much more fail safe, and Sun got his onboard system to work through visual feature tracking with a downward-facing camera, even as the drone is spinning at over 20 rad/s.
While the system works well enough with a regular downward-facing camera—something that many consumer drones are equipped with for stabilization purposes—replacing it with an event camera (you remember event cameras, right?) makes the performance even better, especially in low light.
For more details on this, including what you’re supposed to do with a rapidly spinning partially disabled quadrotor (as well as what it’ll take to make this a standard feature on consumer hardware), we spoke with Sihao Sun via email.
IEEE Spectrum: what usually happens when a drone spinning this fast lands? Is there any way to do it safely?
Sihao Sun: Our experience shows that we can safely land the drone while it is spinning. When the range sensor measurements are lower than a threshold (around 10 cm, indicating that the drone is close to the ground), we switch off the rotors. During the landing procedure, despite the fast spinning motion, the thrust direction oscillates around the gravity vector, thus the drone touches the ground with its legs without damaging other components.
Can your system handle more than one motor failure?
Yes, the system can also handle the failure of two opposing rotors. However, if two adjacent rotors or more than two rotors fail, our method cannot save the quadrotor. Some research has shown that it is possible to control a quadrotor with only one remaining rotor. But the drone requires a very special inertial property, which is hard to satisfy in real applications.
How different is your system's performance from a similar system that relies on GPS, in a favorable environment?
In a favorable environment, our system outperforms those relying on GPS signals because it obtains better position estimates. Since a damaged quadrotor spins fast, the accelerometer readings are largely affected by centrifugal forces. When the GPS signal is lost or degraded, a drone relying on GPS needs to integrate these biased accelerometer measurements for position estimation, leading to large position estimation errors. Feeding these erroneous estimates to the flight controller can easily crash the drone.
When you say that your solution requires “only onboard sensors and computation,” are those requirements specialized, or would they be generally compatible with the current generation of recreational and commercial quadrotors?
We use an NVIDIA Jetson TX2 to run our solution, which includes two parts: the control algorithm and the vision-based state estimation algorithm. The control algorithm is lightweight; thus, we believe that it is compatible with the current generation of quadrotors. On the other hand, the vision-based state estimation requires relatively more computational resources, which may not be affordable for cheap recreational platforms. But this is not an issue for commercial quadrotors because many of them have more powerful processors than a TX2.
What else can event cameras be used for, in recreational or commercial applications?
Many drone applications can benefit from event cameras, especially those in high-speed or low-light conditions, such as autonomous drone racing, cave exploration, drone delivery during night time, etc. Event cameras also consume very little power, which is a significant advantage for energy-critical missions, such as planetary aerial vehicles for Mars explorations. Regarding space applications, we are currently collaborating with JPL to explore the use of event cameras to address the key limitations of standard cameras for the next Mars helicopter.
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