AutomatonRoboticsRobot Sensors & Actuators Ghost Robotics' Minitaur Quadruped Conquers Stairs, Doors, and Fences and Is Somehow Affordable

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In Professor Dan Koditschek’s lab at the University of Pennsylvania, Avik De and Gavin Kenneally put a lot of work into developing their own quadruped robot, called Minitaur.

Bipedal and quadrupedal locomotion has been an ongoing challenge for robots. There’s been a lot of progress over the last few years, though, especially when it comes to dynamic motions: not just walking without falling over but also climbing, running, jumping, and more. This is the real value of legs: They enable robots to deal with the kinds of obstacles and terrain and situations that wheels and tracks can’t.

Getting quadrupeds to do these kinds of useful and fun things requires that a) you know what you’re doing and b) you have a robot that can do what you want it to do. Unfortunately, building legged quadrupeds is difficult, expensive, and time consuming. There is a small handful of bespoke research quadrupeds doing some very good work, but for the rest of us, having to actually do all of the hardware stuff is a major obstacle that makes it difficult to focus on the software, which is where the potential for real-world applications comes in.

Minitaur is a small but very capable platform that uses innovative direct-drive electric motors for a lot of power, virtual compliance, and integrated sensing.

In Professor Dan Koditschek’s lab at the University of Pennsylvania, Avik De and Gavin Kenneally put a lot of work into developing their own quadruped robot, called Minitaur. It’s a small but very capable platform that uses innovative direct-drive electric motors for a lot of power, virtual compliance, and integrated sensing. Minitaur was introduced this past July in an article in IEEE Robotics and Automation Letters, and there’s been enough interest in this little guy that Kenneally and De have started a company called Ghost Robotics to make sure that Minitaur is affordably available to anyone who wants one.

To understand why there’s enough demand for a small quadrupedal research platform like Minitaur to justify starting a company to build and sell them, just watch this video:

Well, that’s one way for a little robot to get through a human-size door. Wow.

What sets Minitaur apart from other legged robots are its direct-drive legs. Direct-drive means that there’s no gearing in between the electric motors and the legs and the ground, and there’s no elastic element either: It’s one rigidly connected system. This sounds bad, because compliance has been a big thing lately that everyone seems to want to put into their robots, and they usually do it with springs, or by relying on force or current sensors. Minitaur doesn’t use any of those, but the direct-drive motors can act compliant anyway, so you get the same benefits. You also get the advantage of being able to adjust every detail about the springy-ness in software, which is one of the reasons that Minitaur is such a useful research platform, as well as other advantages, like increased robustness, lower cost, and the ability to sense the ground through the motors.

For all of the details (all of them!), we spoke with Ghost Robotics cofounders De and Kenneally (who are also Ph.D. candidates at the University of Pennsylvania), and the company’s CEO, Jiren Parikh.

IEEE Spectrum: Can you tell me about Minitaur’s direct-drive legs?

Gavin Kenneally: In the 1980s, Harry Asada and Kamal Youcef-Toumi did a lot of research on direct-drive robot arms. For a robot arm, you can put heavy motors near the base so you don’t have to support their mass, and you can minimize the inertia of the arm. For a legged machine, it’s much more challenging because there’s no supported mass: You have to move your whole body weight whenever you walk around.

The problem with direct-drive legs is that you need to develop a lot of torque in actuators that need to be light and not heat up too much. By being very careful, limiting the number of degrees of freedom in the machine, and with an innovative leg design, we think we have the first direct-drive, legged robot. There’s also no gearbox, and that offers a number of advantages in terms of the robot being able to feel forces on the legs very quickly, and then also react very quickly. Also we’re able to do these really dynamic things without having to worry about destroying your gearboxes every time you have a hard landing.