Supernumerary Limbs: Challenges and Solutions

Dr. Octopus or Spiderman, these fiction work give us some peek into the possible capabilities of extra-numerary limbs. Not just into the fight against an alien invasion, such extra limbs find many applications like to help assembly workers on the shop floor, also. While it might seem so easy to develop such super-limbs, it requires a lot of research and in-depth studies. Particularly it is challenging to develop intuitive or autonomous control systems for such supernumerary robotic limbs. Prof. Harry Asada at MIT d'Arbeloff lab is working on the development of such super-limbs,

The following image shows some applications of such supernumerary robotic limbs (SRL/Superlimbs). Assembly of In addition to increasing the payload capacity and decreasing fatigue of human muscles. such super limbs can be highly useful in providing support while working in an unsuitable posture, or even by hand-holding some tools during operations.

​A) Assisting a drilling task: the worker controls the drill, while the robot holds a part in place and vacuums the carbon chips. B) Assisting an assembly task: the worker tightens a bolt on a beam held in place by the robot. The SRL also supplies new bolts to the user. C) Workload support: the user works in an uncomfortable (overhead) position, while the robot supports part of his weight by bracing his body against the aircraft structure. D) Increasing worker precision: the SRL stabilizes the body of the user by grasping the environment, and guides the power drill.

A) and B) Workload support: the robot supports the user’s weight and follows his motions. C) Balance augmentation: the robotic limbs coordinate with natural human walking, augmenting the user’s balance.

Challenges in Control System Design

The design and development of such super-limbs involve some challenges regarding the ergonomics & control of such super-limbs. Fully manual control of such robotic limbs is not totally viable. It needs to adapt to the human subject's intentions and goals and balance itself accordingly.

A variety of control methods have been attempted on these super limbs, starting from Leader-Follower-based control methods, and going up to advanced level control methods like training the human brain to control the super limbs in addition to its own limbs. Professor Asada explains the outcomes of this study in a Seminar he delivered at the Robotics Institute, CMU.

Can the brain support Superlimbs? and can a human control both the natural limbs and superlimbs simultaneously? Researchers have tried to prove this capability through experiments with 1DOF superlimbs, where the human subject had to track four objects using both of his hands as well as two superlimbs. But due to attention allocation and attention distraction, he was not able to control all four limbs simultaneously. So, humans will have to give special attention while controlling the SRL.

An attempt was made to overcome this by providing proprioceptive feedback to the human. This was achieved by giving the haptic feedback of the force exerted by the superlimbs to the muscles of the operator. So he can feel and be sure that superlimbs are working properly and holding objects as desired. Also, they have investigated if this additional haptic feedback reduces the performance of performing visual tasks, and they found that there is no statistically meaningful difference between them.

Challenges in Actuator Design

In addition to challenges in realizing effective control of the superlimbs, they require special actuators to perform effectively and efficiently. Superlimbs have been imagined to support a human while changing his posture (e.g. from sitting to standing posture). This operation requires the actuators to have low speed but high torque output. Whereas in other cases when superlimbs are required to catch a human if he goes off-balance, requires its actuator to have high speed and low torque. This issue has been intelligently solved by developing Dual-Motor, Dual-Speed actuators. These actuators are made up of two motors: (1) High Torque Low-Speed Motor (2) Low Torque High-Speed Motor. Through torque superposition, these motors can give desired characteristics. These motors are selectively engaged using differentials or using electrical relays to selectively use one of them to actuate the limb.

References:

1. H. Song and H. H. Asada, “Integrated Voluntary-Reactive Control of a Human-SuperLimb Hybrid System for Hemiplegic Patient Support,” in IEEE Robotics and Automation Letters, vol. 6, no. 2, pp. 1646-1653, April 2021, doi: 10.1109/LRA.2021.3058926. The contents of this paper were also selected by ICRA2020 Program Committee for presentation at the conference.

2. P. Daniel and H. H. Asada, “Stable Walking Policy for Wearable SuperLimbs Attached to a Human with Tuned Impedance,” IEEE/RSJ International Conference on Intelligent Robots and Systems, 2020. Las Vegas, NV. 25-29 Oct 2020

3. J. W. Guggenheim, R. Hoffman, H. Song, and H. H. Asada, “Leveraging the Human Operator in the Design and Control of Supernumerary Robotic Limbs”, IEEE Robotics and Automation Letters (RA-L), Vol. 5, Issue 2, pp. 2177-2184, April 2020. The contents of this paper were also selected by ICRA2020 Program Committee for presentation at the conference.

4. Jacob W. Guggenheim, Federico Parietti, Tamar Flash, and H. Harry Asada. 2020. Laying the Groundwork for Intra-Robotic-Natural Limb Coordination: Is Fully Manual Control Viable? J. Hum.-Robot Interact. 9, 3, Article 18 (July 2020), 12 pages.

5. J. W. Guggenheim and H. H. Asada, “Inherent Haptic Feedback from Supernumerary Robotic Limbs,” in IEEE Transactions on Haptics, 2020

6. D. J. Gonzalez and H. H. Asada, “Hybrid Open-Loop Closed-Loop Control of Coupled Human-Robot Balance During Assisted Stance Transition with Extra Robotic Legs,” IEEE Robotics and Automation Letters (RA-L), July 2019. The contents of this paper are also selected by the ICRA’19 Program Committee for presentation at the Conference.

7. F. Parietti and H. H. Asada, “Independent, voluntary control of extra robotic limbs,” 2017 IEEE International Conference on Robotics and Automation (ICRA), Singapore, 2017, pp. 5954-5961.

8. F. Parietti and H. Asada, “Supernumerary Robotic Limbs for Human Body Support“, IEEE Transactions on Robotics, Vol. 32, No. 2, pp.301-311, April 2016.

9. L. Treers, R. Lo, M. Cheung, A. Guy, J. W. Guggenheim, F. Parietti, and H. H. Asada, “Design and Control of Lightweight Supernumerary Robotic Limbs for Sitting/Standing Assistance,” Paper presented at the International Symposium on Experimental Robotics (2016). Tokyo, Japan.