Robotic Third Arm Can Smash Through Walls

This waist-mounted supernumerary robotic limb is gentle enough to pick fruit but powerful enough to punch through a wall

5 min read

Evan Ackerman is IEEE Spectrum’s robotics editor.

Researchers at the Université de Sherbrooke in Canada developed a waist-mounted hydraulic arm that can help you with all kinds of tasks.
Researchers at the Université de Sherbrooke in Canada developed a waist-mounted hydraulic arm that can help you with all kinds of tasks.
Photo: Université de Sherbrooke

When we’ve written about adding useful robotic bits to people in the past, whether it’s some extra fingers or an additional arm or two, the functionality has generally been limited to slow moving, lightweight tasks. Holding or carrying things. Stabilizing objects or the user. That sort of thing. But that’s not what we want. What we want are wearable robotic arms that turn us into a superhero, like Marvel Comics’ Doc Ock, who I’m just going to go ahead and assume is a good guy because those robotic arms strapped to his torso look awesome.

At ICRA this week, researchers from Université de Sherbrooke in Canada are finally giving us what we want, in the form of a waist-mounted remote controlled hydraulic arm that can help you with all kinds of tasks while also being able, should you feel the need, to smash through walls.

This type of wearable robotic arm is known as a supernumerary robotic arm. The system created by the Canadian researchers (in partnership with Exonetik) has 3 degrees of freedom and is hydraulic, actuated by magnetorheological clutches and hydrostatic transmissions with the goal of “mimicking the performance of a human arm in a multitude of industrial and domestic applications.” Like wall punching. The hydraulic system provides comparatively high power, but the power system itself is connected to the user through a tether, minimizing how much mass the user has to actually wear (and keeping the inertia of the arm low) while also limiting mobility somewhat. Off-board power does put a bit of a dampener on the superhero potential, but in practical terms, users aren’t likely to be moving around all that much, and if they are, mobile options could include being tethered to an autonomous vehicle that follows you around or perhaps, eventually, a more portable backpack power unit.

The robotic arm itself weighs just over 4 kilograms, about the same as a real human arm. It can lift 5 kg, and has a maximum end effector speed of a brisk 3.4 meters per second, with a workspace that’s restricted to keep it from smashing you in the face. At the moment, there isn’t much in the way of autonomy here, with the arm being controlled by a second human via a miniature handheld arm in a master-slave configuration. The researchers suggest that adding some sensors could allow the arm to do things like pick vegetables next to the user, as well as do more collaborative tasks, like providing tool assistance. You can think of it as being able to act as a co-worker, either directly increasing productivity by performing the same task as the user in parallel, or doing some different tasks in order to free up the user to do stuff that requires creativity or judgement.

What we really wanted to know, of course, it’s what it’s like to strap this thing on, so we reached out to lead author Catherine Véronneau via email. If you want to know whether third-arm beer stabilization is on their roadmap, read on, but if you have other questions, the ICRA page and Slack channel can be found here.

Supernumerary robotic arm developed at the Universit\u00e9 de SherbrookeThe researchers envision a number of applications for their supernumerary arm, including: vegetables picking (a, b), painting a wall (c), washing a window (d), handing tools to a worker (e, f), and playing badminton (g).Photo: Université de Sherbrooke

IEEE Spectrum: Can you describe the experience of wearing the robotic arm, especially when it’s moving dynamically? What does it feel like? How quickly do you get used to it? 

Catherine Véronneau: ​That’s a good question, and it is something that really needs to be explored and studied in the future! But, for now, it is still not too bad having this arm on my hips, since it’s only 4.2 kg (without payload) and it is located near my center of mass (to reduce inertia). I get used to it quickly, and I can compensate for some of the movements (x, y, and z translational movements), but I still have some remaining issues to compensate for torsion movements (like if the arm is hitting a tennis ball with a racket), which is funny! We also noticed that the harness needs to be rigidly connected to the body, because if there is some backlash between the harness and the body, it can be uncomfortable.

Why did you choose this particular actuation technology?

First, we used MR [magnetorheological] clutches because they offer very low-inertia (since the inertia from the geared motor is not reflected to the output, because MR clutches are working in continuous slippage). Because of their low-inertia, they react very fast (with high force bandwidth) which allows them to compensate for human unpredictable motions/perturbations. Also, because of their low-inertia, low-friction, they are also intrinsically backdrivable, which is safe for human-robot interactions. We coupled MR clutches to a hydrostatic transmission because of its stiffness (high bandwidth) and its ease of routing (compared to a cable transmission, for instance).

Is the robotic arm safe to use for the wearer and people around you?

​Because of the MR clutches, the mechanics is intrinsically safe (no inertia from geared motor, low arm inertia, low-friction, etc). Also, because MR clutches react quickly and are backdrivable, we can rapidly detect a contact (pressure increases) and apply a negative torque to not hurt anybody. 

“We used MR (magnetorheological) clutches because they offer very low-inertia and can react very fast (with high force bandwidth), which allows them to compensate for human unpredictable motions/perturbations”

What is the best way to make the arm autonomous while still able to work effectively with the person wearing it?

​Good question. Making a third arm (or any SRL [supernumerary robotic limb]) autonomous involves understanding the human intent behind actions, which is really dependent on the application. For instance, if the job of a supernumerary pair of arms is opening a door while the user is holding something, the controller should detect when is the right moment to open the door. So, for one particular application, it’s feasible. But if we want that SRL to be multifunctional, it requires some AI or intelligent controller to detect what the human wants to do, and how the SRL could be complementary to the user (and act as a coworker). So there are a lot of things to explore in that vast field of “human intent.”

What other fun things have you tried (or would you like to try) with the robotic arm?

​Just before the COVID crisis, we were working on the “beer stunt.” We’ve been inspired by this famous YouTube video. Actually, this video is completely fake, but we wanted to make it for real. The major challenges associated with this stunt is that we need hip positions and orientations (or the base of the arm) in real-time to stabilize the end-effector. And having this measure implies an absolute way to measure it (cameras, or GPS, etc). We are now working on it!

Multifunctional Remotely Actuated 3-DOF Supernumerary Robotic
Arm Based on Magnetorheological Clutches and Hydrostatic Transmission Lines
,” by Catherine Veronneau, Jeff Denis, Louis-philippe Lebel, Marc Denninger, Vincent Blanchard, Alexandre Girard, and Jean-Sebastien Plante from Universite de Sherbrooke, is presented at ICRA 2020.

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