Challenges in using compliant ligaments for position estimation within robotic joints

Abstract

Abstract-The mechanical advantages of bio-inspired condylar robotic knee joints for use in prosthetics or rehabilitation has been argued extensively in literature. A common limitation of these designs is the difficulty of estimating joint angle and therefore accurately controlling the joint. Furthermore, the potential role of ligament-like structures in robotic knees is not very well established. In this work, we investigate the role of compliant stretch sensing ligaments and their integration into a condylar robotic knee. Simulations and experiments are executed out in order to establish whether measurement of stretch in these structures can be used to produce a new feedback controller for joint position. We report results from a computer model, as well as the design and construction of a robotic knee that show, for a chosen condyle shape, ligament stretch is a function of muscle force and joint velocity as well as joint angle. We have developed a genetic algorithm optimised controller incorporating ligament feedback that demonstrates improved performance for a desired joint angle in response to step inputs. The controller showed marginal improvement in response to a cyclic command signal and further investigation is required in order to use these measurements in robust control, nevertheless we believe these results demonstrate the that ligament-like structures have the potential to improve the performance of robotic knees for prosthetics and rehabilitation devices.

A Kinematic Model for the Design of a Bicondylar Mechanical Knee

“…For this reason the joint was manipulated by hand. Although previous work on an earlier version of the joint suggests that these forces are likely to have an effect on the ligament stretch [24] the effect of these forces on the centre of rotation and moment arm is not known. A full investigation of how these loads move the centres of rotation and how this effects joint performance will be a matter for future work.…”

“…Specifically, we add elastic stretch sensing ligaments and a biologically derived condyle profile. Initial data from a prototype version of the joint has shown that stretch in compliant ligaments is a function of joint forces and angular velocity as well as joint position and could provide some sensory benefit [24]. The method presented here employs a computer model developed in MATLAB to generate a tibia joint surface profile given a number of inputs taken from cadaver studies.…”

“…To maintain the two halves of the joint centred on one another a 25 mm radius was added to each condyle in the coronal plane. The design of the ligaments is the same as that used in Russell et al [24] with a spring for elasticity and an LVDT for stretch measurement. The free length of the ligaments (i.e.…”

The Impact of ACL Laxity on a Bicondylar Robotic Knee and Implications in Human Joint Biomechanics

“…More physiologically driven studies [37], [38] implemented a robotic system based on condylar knee design with demonstrated mechanical benefits, though tendons or compliance were not directly investigated. Our own work on human-like knee joints [39]- [41] explored similar issues for the purpose of designing new joints for walking robots or exoskeletons. The patella was simplified as a bracket attached to the tibia or pulley attached to the femur in those studies.…”

A biomimicking design for mechanical knee joints

“…The design presented here goes further by including elastic stretch sensing ligaments and a biologically derived condyle shape. In our previous work [33] a design for stretch sensing compliant ligaments and a condylar knee was presented. Using this first iteration of the joint we show that stretch in these ligaments is a function of joint forces and angular velocity as well as joint position.…”