The vision of humans with extra limbs is taking shape at MIT with researchers adding “supernumerary robotic arms” to assist with tasks that ordinary two-armed humans would find difficult.
The extra robotic arm concept was demonstrated with installing ceiling panels in an airplane, a task that must be duplicated dozens of times in the construction of an airliner. A single person installing a large panel overhead must struggle between holding the panel to the ceiling, inserting screws into holes, and using a powered screwdriver to attach the panel. The juggling and dropping of screws would drive any of us to frustration and profanity.
The Supernumerary Robot Limbs (SRL) team looked at this task, and added two lightweight robot arms to a frame attached to a backpack. The arms are attached directly over the spine so that the body can carry the extra weight without strain. This alone would be noteworthy, except that the user now has no way to command the robot arms to move – a conventional joystick or gamepad would take up hands already busy with panels.
So the amazing part of this research is having the arms decide for themselves when and where to help. Sensors on the human’s wrists and on the robot mount determine where the human is on the task, and assigns the robot arms to help.
The robot arms assist the user by pushing up the panel against the ceiling, allowing the human operator to put in screws to attach the panel. As the panel is able to support itself, first one arm goes down, and then the other as the robot senses that the force required to maintain the pose has diminished.
The arms are programmed by the “teach by demonstration” method. A second human first demonstrates to the robot how to help by moving the robot arms manually with the the robot remembering this lesson so it is then able to make the moves on its own.
The system is worn through a backpack-like harness with padded straps and hip belt. The mechatronic structure, connected to the harness with a compliant element, is composed of a backpack unit and two robotic arms. The unit is placed behind the lower back of the user, and hosts the system actuators.
The robotic arms are attached to the main structure in correspondence of the iliac crest, the thick edge of the hip bone. This minimizes the interference with human motion and maximizes the robotic limbs’ workspace, allowing them to act both as arms and as legs.
The custom actuators developed for the SRL are compact, lightweight, and satisfy the demanding torque requirements of the target manufacturing applications. All joints are controlled by DC brushless motors of the flat type. Circular hollow-section polyurethane torsion bars are placed between the gearhead output and the actuator output, realizing Series Viscoelastic Actuators (SVAs).
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