Literature review of Soft Shoulder Joints

In an exploration of the human arm, my aim with this project is to generate an interactive sculpture which will mimic and motorize  the shoulder and elbow joints with as much accuracy as possible. The joints of the wrist and hand will not be motorized for this project and will only imitate simplified ranges of motion.  I   selected  several articles and sites whose research has provided me with information on how best to complete this project.

The articles in this review have different points about the replication of humanoid arm rotation:  They span bio-mechanical subjects specifically applied to kinematics and robotics.

R. Debski, I. M. Parsons III, J. Fenwick, and A. Vangura, “Ligament mechanics during three degree-of-freedom motion at the acromioclavicular joint,” Annals of Biomedical Engineering,  vol. 28, pp. 612-618, April 2000.

Kinematically mapped the shoulder joint of cadavers using a robotic clavicle and scapula to simulate the force and range of living humans.  They mapped the range of motion with pseudo ligaments with varied loads.

J. Lenarcic and N. Klopcar, “Positional kinematics of humanoid arms,”  Robotica, vol. 24, pp 105-112, April 2005.

Kinematically mapped the joints of the human arm (reach and posture specifically) and improved the standard shoulder joint replicator (a single universal joint) and applied these improved mechanisms to study the mechanism itself.

This article spells out just how the human shoulder and elbow  joint mechanism works in conjunction with actuators of the other joints.  They then delve into mathematical equations which define the roles of each joint of the arm as it realtes to the reach of the arm based on kinematic studies of human subjects’ reach.  I found this article to be the most useful as the authors clearly define the ranges of motion and how best to create a pseudo-humanoid joint for both the elbow and shoulder.  They define specific points and articulate how to execute these points within the ranges of motion.  The fig. below is one of several :

M. Okada, Y. Nakamura, and S. Hoshino, “Development of the cybernetic shoulder – a three DOF mechanism that imitates biological shoulder motion,” International Conference on Intelligent Robots and Systems, Vol. 2, pp. 543-548, 1999.

This paper discusses the concept of “mechanical softness” in humanoid robotics.  This paper defines  human-like mobility as an important means of effective human mimicry.

Below are two diagrams of the cybernetic shoulder this team designed.

It outlines the problems with using a similar joint I plan on implementing (theirs being much more precise than my own).  Their design incorporates a two DOF universal joint, a two DOF gimbal joint, and a three DOF ball joint.  (I have a pan and tilt servo mechanism coming which will account for the 2 DOF gimbal, I have a universal joint in my design, and now I can insert one of the ball joints I have and I can “easily” recreate the range of motion of the cybernetic shoulder).  They discuss the coding involving in controlling the mechanism.  This, like the previous article, involves a lot of math which I don’t understand).

Then, of course, I refer to the link that I mentioned last week:  http://www.designboom.com/technology/eccerobot-mimics-human-skeleton-and-muscles/

Here’s a journal article about people discussing the joints of the robot:
eccerobot
V. Potkonjak, K. Jovanovic, P. Milosavlijevic, N. Bascarevic, O. Holland, “The puller-follower control concept in the multi-jointed robot body with antagonistically coupled compliant drives,”  Proceedings of the IASTED International Conference, Nov. 2011.

This paper discusses the anthropomimetic realm of robotics and the subsequent problems of a successful version.  The authors detail how the eccerobot’s design was to directly mimic human mechanics as opposed to other robot designs, despite an anthropomimetic nature, did not follow the human mechanism as detailed as eccerobot.  This group looked to eccerobot as a standard and used it as a model to discuss what they call “antagonistically coupled compliant drives”.  I took this to mean their attempt to eliminate the “backlash” of the the robots joints (I must note here that all previous papers mentioned in this review also noted the backlash effect when detailing their shoulder joints and all define it as a slackening of the tendons resulting in a-human-like movements).  Their design incorporates a motor which initiates the action and the other motor works against that initial motion to create something they call the “puller-follower” motion.  With tweaking of the “tendons”, the torque, and the force they were able to mitigate the “backlash” effect of (almost all of these which I have researched) common shoulder-joint mechanisms.

G. Yang, S. K. Mustfafa, S. H. Yeo, W. Lin, W. B. Lim, “Kinematic design of an anthromimetic 7-DOF cable-driven robotic arm,” Frontiers of Mechanical Engineering, vol. 6.1, pp. 45-60, Oct. 2011.

This paper discusses the existing robotic arms and their heavy mechanical structures and payload restrictions.  This group designed a different arm joint entirely.  All of the previous papers outlined specific kinematic studies and mathematical equations to generate true anthromimetic motions (all involved with the human arm). These engineers developed a simple robotic arm comprised of a single ball joint for the shoulder, a revolute joint for the elbow (I had to look that one up- and they simply  mean a hinge) and a ball joint for the wrist.  Their design has more complicated platform/base modules and utilises a single cable for skeletal function.

As for tendon function, they do tons of math to discuss the placement of cables, as well as lengths.  Their model’s cabling of tendons define the effectiveness of the arm.  Through diagrams, they easily explain how they managed to get a seven degree-of-freedom arm in using only the placement and lengths of cables.

Every article I have discussed has discussed the topic of anthromimetic robotic arms.  Each delves into the mechanisms themselves and explains (sometimes over my head in – in numbers) how the specific degrees of freedom were attained and with what sort of fluidity.  Each researcher aimed for human motion, each had differing standards for fluidity.  In my own design, I believe I may attempt to incorporate both another joint ( see the cybernetic shoulder). I may also attempt to incorporate the platforms of the “cable-driven robotic arm” and will consider their points of the cable (as I use a rigid rubber band in a similar way, I may now be able to look at their placements and lengths to glean how best to use them).  I will also consider an “antagonistically compliant drive” in powering my motors, and will consider with more emphasis the use of springs.

All in all, these papers codified many of the strategies I had already begun to implement, and have only served to instruct me on my next steps.