Project aims to give manufacturing and construction industries a pair of extra hands

The Institute of Electrical and Electronics Engineers' (IEEE) International Conference on Robotics and Automation (ICRA) in Hong Kong saw a number of interesting presentations, but perhaps the most exciting and attention grabbing was the Massachusetts Institute of Technology's (MIT) Supernumerary Robotic Limbs (SRLs) project.  MIT presented not one, but three papers on upcoming robotics limbs that could potentially revolutionize the construction and manufacturing industries.
I. A New Kind of Limb
The robotic limbs project is led by Professor H. Harry Asada, the principle investigator of MIT's d'Arbeloff Lab. Rather than seeking to replace or augment standard human arms or legs, the goal the SRL effort is to give a human a second set of arms.
Ph.D candidate Federico Parietti is handling much of the design duties of the mechanical system.  Baldin Llorens-Bonilla, another Ph.D candidate with the lab is among the students working on the algorithms that drive the limbs.
IEEE Spectrum eloquently summarizes the value of the project versus traditional exoskeletons, writing:
The researchers say that the constraint of an exoskeleton is that by definition it's bound to the body of the user: no matter what the most advantageous orientation for your limbs might be, the exoskeleton is putting all of its force wherever you decide to put your arms and legs. Having limbs that are powered yet completely separate gives the system many more option for helping you out.
The prototype shown at the ICRA shows off the substantial advances that have been made in prosthetics in recent years.  The system weighs only 4.5 kilograms (10 pounds) and can be comfortably worn via a backpack-like harness, which has straps over the shoulders and a strap around the user's waist.

Robotic Limbs
MIT's Supernumary Robotic Limbs can act as an extra pair of arms or as an extra set of legs.
[Image Source: d'Arbeloff Lab]

The limbs are mounted to the central back-like machinery just over the iliac crest, the thick edge of the hipbone.  This positioning prevents the system from interfering with the user's natural motions.  It also allows the system to act both as a pair of extra legs or as a pair of extra arms.
The joint positioning is driven by flat, brushless DC motors, which feed into circular hollow-section polyurethane torsion bars that modulate the transfer of force into Series Viscoelastic Actuators (SVAs).  The system is designed to deliver a high amount of torque in order to make it a practical tool for the construction and manufacturing industries.
The limbs feature five degrees of freedom and customizable "end effectors" ranging from simple caps (for pushing or propping up objects) to more complex design such as (eventually) prosthetic hands.
II. Mankind's Little Helper
The limbs are aimed to act like a human-like helper to the user.  They take cues from the user's limb movements, position, and muscle exertion.  To detect those cues the system use exotic algorithms -- colored Petri nets and partial least squares predictions.  While they may eventually employ more exotic hybrid forms of control that would allow the user to control the limbs mentally, for now the limbs are autonomous.  

They mirror the user's motions waiting for the opportunity to assist.  For example if the user is holding a heavy object – such as a panel to mount to the ceiling or a door frame -- they helpfully swing into place, propping up the object so the user can free at least one of their hands to do mounting work.
Mr. Llorens-Bonilla has developed so-called "behavioral modes", which detect and assist in common motions.  He comments in an interview with IEEE's Spectrum magazine:
Once we combine the most significant behavioral modes we are able to control the robot such that, from the wearer's perspective, it behaves like an extension of his own body.
The result is an uncanny odd couple of human and robot that resembles the exoskeletons seen in movies like Elysium or The Edge of Tomorrow.

Another key innovation on the project, which was presented at last year's ICRA conference, was the use of sensors to counteract shaky users.  Writes the lab:

Although the SRL has the potential to provide the wearer with greater strength, higher accuracy, flexibility, and dexterity, its control performance is hindered by unpredictable disturbances due to involuntary motions of the wearer, which include postural sway and physiological tremor.
Robotic Limb sensors
[Image Source: d'Arbeloff Lab]

The solution involves a sensor at the effector and multiple encoders at the joints.  The system uses advanced digital filters to correct for operator shakiness.
III. Commercial Partners Salivate at the Possibilities
In the near term the limbs could play a crucial role in labor intensive manufacturing duties.  It's no coincidence that much of the group's research is funded by some of America's largest high-tech manufacturers, including the Boeing Comp. (BA).
Boeing has a keen interest in finding a way to reduce stress on its workforce.  Much of assembling a commercial jumbo jet consists of fastening large, bulky parts.  Workers must squat, bend over, carry heavy objects or brace up frames during the construction process.  Many of the workers are aging and having careers cut short by injuries.  Robotic limbs could reduce injuries, which would both cut costs for Boeing and improve the quality of life for its workers.

Boeing Example
A worker uses the limbs to prop up an airframe during aircraft construction.
[Image Source: d'Arbeloff Lab]

These robots are even quite the gentlemen, opening the door for their human companion if it notices them carrying a heavy object.

Robotic Limbs
The robotic limbs can even open a door for you. [Image Source: d'Arbeloff Lab]

The project is reaching a very mature state.   While the researchers did not comment on how long before it was deployed in a commercial capacity, given the usefulness of it, it seems likely to be sooner rather than later.  

And why stop at two?  Eventually the system could offer four, six, or even eight limbs.  Looks like the humble human is evolving into new forms with the help of modern robotics.

Source: IEEE Spectrum

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