Polycentric Mechanism Research Articles

A Passive Polycentric Mechanism to Improve Active Mediolateral Balance in Prosthetic Walking.

Prosthetic legs are typically passive systems without active ankle control, restricting mediolateral balancing to a hip strategy. Resulting balance control impairments for persons with a lower extremity amputation may be mitigated by increasing hip strategy effectiveness, in which relatively small hip moments of force are adequate for mediolateral balancing. To increase hip strategy effectiveness we have developed a prosthetic leg prototype based on the Peaucellier mechanism, the Sideways Balance Mechanism (SBM). This polycentric mechanism adds a frontal plane degree of freedom, reducing mediolateral body displacements. Adding a passive joint alone introduces instability, in which mediolateral body rotation leads to CoM height loss, ultimately resulting in a fall. The SBM however provides stability typically absent by lengthening under rotation, thereby compensating for CoM height loss. By allowing for both foot rotation (in-/eversion), and increased mediolateral ground reaction force the SBM increases hip strategy effectiveness. We aimed to provide proof of principle that the SBM can improve active mediolateral balance control in prosthetic walking by increasing hip strategy effectiveness compared to a typical set-up. Comparison between a typical set-up and the SBM showed an increased mediolateral ground reaction force at equal hip moments of force for a 2D forwards dynamics computer simulation, and a reduced hip moment of force at equal mediolateral ground reaction force for a case study. Results validate increased hip strategy effectiveness of the SBM compared to a typical set-up, providing proof of principle that adding an SBM to a prosthetic set-up improves mediolateral balance control in prosthetic walking.

Design, Control, and Validation of a Polycentric Hybrid Knee Prosthesis

This paper presents a novel polycentric hybrid knee prosthesis (PHKP) by enabling a multi-bar linkage to work for high-torque activities in active mode and for low-torque activities in passive mode. The mode transition is implemented by a servo-controlled telescopic slide rails. Distinguished from the single-axis hybrid knee prostheses, the PHKP Knee provides geometric stability in stance and notably increases toe clearance in swing by using an optimally designed instantaneous center of rotation (ICR) trajectory. In active mode, the PHKP Knee provides high transmission ratio and continuous peak torque offering sufficient support for torque-demanding activities. Impedance control was employed to regulate joint torque as a response to knee motion in the active mode. It was found that due to the inherent geometric stability of the polycentric mechanism in stance, the damping coefficient of the controller can be tuned much smoother than the single-axis knees leading to more compliant and comfortable interactions among the wearer, the prosthesis and the ground. The stair ascent tests showed that the joint angle and torque profiles of the PHKP Knee are similar to those of the able-bodied by demonstrating a step-over-step pattern with a peak angle of 73 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\deg$</tex-math></inline-formula> and a maximum torque of 0.92 Nm/Kg.

Design and Validation of a Polycentric Hybrid Knee Prosthesis With Electromagnet-Controlled Mode Transition

A hybrid knee prosthesis is proposed in this letter, which consists of a polycentric structure in passive mode for low-torque activities and a single-axis structure in active mode for high-torque activities. A novel mode transition mechanism controls self-holding electromagnets for switching modes between the four-bar linkage and single-axis structure. Compared with the conventional single-axis hybrid knee, the four-bar polycentric mechanism with varying instantaneous center of rotation (ICR) can enhance the geometric stability and increase the toe clearance in passive mode. For active mode, we developed a custom embedded electric system, employed torque control for stance and position control for swing. The results of bench tests indicated that the bandwidth of the controller was suitable for locomotion. The clinical test of level-ground walking without sudden buckling and stumble was validated by three subjects. Regarding climbing stairs, a typical high-torque activity in daily locomotion, all subjects reach the maximum knee torque around 0.95 Nm/kg comparable to the able-bodied.

Evaluating Knee Mechanisms for Assistive Devices.

State-of-the-art knee braces use a polycentric mechanism with a predefined locus of the instantaneous center of rotation (centrode) and most exoskeleton devices use a knee mechanism with a single axis of rotation. However, human knees do not share a common centrode nor do they have a single axis. This leads to misalignment between the assistive device's joint axis and the user's knee axis, resulting in device migration and interaction forces, which can lead to sores, pain, and abandonment of the device over time. There has been some research into self-aligning knee mechanisms; however, there is a lack of consensus on the benefit of these mechanisms. There is no research that looked purely at the impact of the knee mechanisms, either. In this article, we compare three different knee brace mechanisms: single axis (SA), polycentric with predefined centrode (PPC), and polycentric with a self-aligning center of rotation (PSC). We designed and conducted an experiment to evaluate different joint mechanisms on device migration and interaction forces. Brace material, weight, size, cuff design, fitment location, and tightness were consistent across trials, making the knee joint mechanism the sole variable. The brace mechanisms had no significant effect on walking kinematics or kinetics. However, the PPC brace had greater interaction forces on the top brace strap than the SA and PSC. The PSC and SA had significantly lower interaction forces on the bottom strap compared to the PPC brace. The PSC had significantly less migration than both the SA and PPC braces. These results show that a PPC mechanism may not be beneficial for a wide range of users. This also shows that the PSC mechanisms may improve mechanism alignment and lessen device migration.

A Review Study for Measurement, Analysis and Evaluation Four Bar Polycentric Knee

When a transfemoral amputee missing his knee joint, a polycentric mechanism is used as technical substitute to restore the gait function. The motion ability of transfemoral amputee is dependent on the performance of prosthetic knee. Recent results on the kinematic analysis of four-bar polycentric knee mechanism are reviewed in this article. Different experimental tests on data of above-knee prosthetic limb such as F-socket, gait analysis, ground reaction force, static loading, fatigue life and static prosthetic alignment, etc., were used to analyse the gait cycle and to improve the comfortability of amputee after using prosthetic limb, and to increase the stability of prosthetic knee after using. Finally, the optimization techniques included selection of the optimum dimension of polycentric knee is dependent on the data of ICR trajectory estimated from kinematic analysis of four-bar mechanism. These optimization techniques have significant effect on improving the mechanical properties of polycentric knee and reducing its cost.

Design and Fabrication of an Affordable Transfemoral Prosthetic Leg

An invalid is a person with physical or mental abnormalities which hinders their ability of doing normal human activities. One of the aforementioned abnormalities is the loss or malfunction of a body part or organ, such as the human leg. Problems brought about by this disability could be alleviated by the use of prosthetic limbs to restore normal bodily functions [1]. While there are many needs for leg prosthetics in Indonesia, the price of commercially available prosthetic is prohibitively high for most amputees. The current available affordable prosthetics are very basic and employ monocentric knee joints. Here an attempt is made towards acquiring an optimal design of an affordable prosthetic leg using previously developed four-bar knee joint [2,3]. The design is carried out by implementing a systematic design method to arrive at an affordable, yet ones with better performances. The procedure involves a rubric and analyzing various alternatives to arrive at an optimal design conclusion. The evaluation objective is to maximize the performance of the leg and knee prosthetics through an extensive analysis with considerations from various stakeholders, including the potential users, fabricator, and hospitals in Indonesia [4]. The final design of the leg consists of a composite socket, an aluminum pylon, and a solid-ankle cushioned-heel foot design. Its components are modular and compatible with prosthetic industry standards. The knee is made of a combination of polymers and stainless steel and employs a polycentric mechanism in order to satisfy design requirements while keeping costs low [4,5]. The leg and knee were then fabricated according to the analysis results. The total cost of fabrication is approximately 5.5 million IDR, significantly cheaper than commercial prosthetics. The prototype is then tested and examined using a gait analysis system previously developed at Institut Teknologi Bandung by the Biomechanics Research Team [6]. The results show satisfactory performance, albeit with a host of potential improvements.

Multicriteria Synthesis of a Polycentric Knee Prosthesis For Transfemoral Amputees

Abstract— In one of the prosthesis designs for lower limb transfemoral amputations artificial knee joints with polycentric mechanisms are used. Such prostheses are characterized by high stability during the stance phase of the movement. The existing variety of polycentric mechanisms indicates the possibility of finding the optimal prosthesis design satisfying several quality criteria.In this paper we present a multicriteria method for the synthesis of the artifical polycentric knee mechanism based on the uniform systematic study of the design parameters space and on the analysis of Pareto optimal solutions. Keywords— Optimalcriteria, polycentric knee, prosthesis, synthesis,transfemoral amputee.I. I NTRODUCTION OR a long period of time mankind has attempted to create artificial devices (prostheses) capable to replace as fully as possible natural human organs lost or damaged due to various reasons. First of all such devices were created to restore motor functions lost due to loss of the lower limbs. On one side, this problem seemed the most simple on a technical level, on the other side - the restoration of motor activity allows a person to carry out vital actions even with quite simple devices. The obvious drawbacks of such devices have stimulated the search for new, more sophisticated designs that could improve the quality of life for amputees. However, despite the huge technological progress made to date in this area, lower limb prostheses are still not correspond to biological analogues from different points of view [1]. Lower limb prostheses for transfemoral amputations differ by the kind of thigh and shank joint (single-axis and polycentric) and by the control methods [2].Single-axis prostheses have a fixed center of rotation of the hip relative to shank, are relatively inexpensive and with high accuracy simulate the motion of the knee. However, these prostheses have low functionality and are not sufficiently stable during the stance phase.In the polycentric prostheses position of the instant center of rotation (ICR) continuously changes with changing of the angle of knee flexion.

Dynamic analysis of an artificial hybrid leg

When a system is subjected to external loads like external forces or torques, systems behave dynamically. Besides external effects, three types of torques which arise from the motion of a robot manipulator also act on the system. These dynamic torques include inertial, centripetal and Coriolis effects and they must be revealed correctly, if model-based control of the system is to be performed. In this manuscript, dynamics of an artificial hybrid leg with a polycentric knee joint is going to be derived. Distinctive feature of the artificial leg having polycentric knee is that it is a serial manipulator, one of the joints (the knee joint) of which is designed as a close chain, a four bar mechanism. The model of artificial leg is considered as an artificial hybrid leg having two degrees of freedom; one at the ankle joint and one at the exciting joint of the four bar mechanism. For the knee joint, iterative kinematic analysis is done in order to derive the pose of all joints of polycentric mechanism for the given arbitrary trajectory of hip joint. The ankle joint and hip joint poses are computed by using inverse kinematic analysis techniques for the solution of standard serial manipulator poses. After kinematic analysis is made, dynamic torques can be calculated as the external torques are taken into consideration, since all angular velocities and accelerations of the links are known. By obtaining the system dynamics, torque demands for the motors which actuate the artificial hybrid leg system can be computed. Computation of these torques have a great importance for the system design, since control systems can only be constructed after system dynamics is fully obtained.

Designing polycentric mechanisms by curve matching

This paper presents a new method for the design of a slider-crank mechanism to satisfy specified behavior of the moving centrode. The method is based on the technique of curve matching using a series of design curves. The various families of centrode curves possible from the slider-crank mechanism are discussed. An example of the use of the curve matching method is presented.

Active centres in the VOCl 3Al(C 2H 5) 2Cl catalytic system

In order to examine the type of active centres of a VOCl 3Al(C 2H 5) 2Cl catalyst in polymerization of olefins, a study was made of the composition and catalytic activity of the homogeneous and heterogeneous component of the system; the oxidation state of vanadium in the catalytic centres was determined; the effect was examined of the organic aluminium component on the catalytic activity of the system and properties of polymer products formed. It was shown that catalytic centres of the VOCl 3Al(C 2H 5) 2Cl system are chiefly in the heterogeneous phase of the catalyst. It was assumed that heterogeneous catalytic centres are bimetallic complexes of alkyl derivatives of vanadium trichloride and alkyl aluminium chlorides and the polycentric mechanism of catalysis of olefin polymerization in the presence of a VOCl 3Al(C 2H 5) 2Cl catalyst is due to the existence of two kinds of active vanadium-aluminium complexes differing in the nature of the organic aluminium component.