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J. Mechanisms Robotics. 2017;9(4):041001-041001-10. doi:10.1115/1.4036221.

Unlike a traditional yeaechanism, where typically only the pose of the moving platform is of significance, a shape-morphing mechanism requires additional provisions. Mainly, any covers or skin panels that enclose the mechanism have to conform to additional constraints to avoid interference and clashing of said covers and achieve certain shapes during morphing. This paper presents a new method for kinematic modeling and analysis of such six degree-of-freedom (DOF) shape-morphing mechanisms enclosed by a number of rigid sliding panels. This type of mechanism has applications in aircraft morphing, where the shape of the enclosing skin is of significant importance in the design. Based on traditional parallel robot kinematics, the proposed method is developed to model the motions of multisegmented telescopic rigid panels that are attached via additional links to the base and platform of a driving mechanism. When the robot actuators are locked, each panel will have 3DOFs. The DOFs are utilized to satisfy constraints among adjacent panels, such as maintaining parallelism and minimal gap. Through this modeling and analysis, nonlinear formulations are adopted to optimize orientations of adjacent sliding panels during motion over the workspace of the mechanism. This method will help design a set of permissible panels used to enclose the mechanism while remaining free of collision. A number of cases are simulated to show the effectiveness of the proposed method. The effect of increased mobility is analyzed and validated as a potential solution to reduce panel collisions.

Commentary by Dr. Valentin Fuster
J. Mechanisms Robotics. 2017;9(4):041002-041002-9. doi:10.1115/1.4036218.

In lower-limb rehabilitation, there is a specific group of patients that can perform voluntary muscle contraction and visible limb movement provided that the weight of his/her leg is fully supported by a physical therapist. In addition, this therapist is necessary in guiding the patient to switch between hip-only and knee-only motions for training specific muscles effectively. These clinic needs have motivated us to devise a novel reconfigurable gravity-balanced mechanism for tackling with the clinical demands without the help from therapists. The proposed mechanism has two working configurations, each leading the patient to do either hip-only or knee-only exercise. Based on the principle of static balancing, two tensile springs are attached to the mechanism to eliminate the gravitational effect of the mechanism and its payload (i.e., the weight of the patient's leg) in both configurations. The gravity balancing design is verified by a numerical example and adams software simulation. A mechanical prototype of the design was built up and was tested on a healthy subject. By using electromyography (EMG), the myoelectric signals of two major muscles for the subject with/without wearing the device were measured and analyzed. The results show that the myoelectric voltages of the stimulated muscles in both hip-only and knee-only motion modes are reduced when the subject is wearing the device. In summary, the paper for the first time demonstrates the design philosophy and applications by integrating the reconfigurability and static balancing into mechanisms.

Commentary by Dr. Valentin Fuster
J. Mechanisms Robotics. 2017;9(4):041003-041003-10. doi:10.1115/1.4036220.

In this paper, the performance augmentation of underactuated fingers through additional actuators is presented and discussed. Underactuated, also known as self-adaptive, fingers typically only rely on a single actuator for a given number of output degrees of freedom (DOF), generally equal to the number of phalanges. Therefore, once the finger is mechanically designed and built, little can be done using control algorithms to change the behavior of this finger, both during the closing motion and the grasp. We propose to use more than one actuator to drive underactuated fingers to improve the typical metrics used to measure their grasp performances (such as stiffness and stability). In order to quantify these improvements, two different scenarios are presented and discussed. The first one analyzes the impact of adding actuators along the transmission linkage of a classical architecture while the second focuses on a finger with a dual-drive actuation system for which both actuators are located inside the palm. A general kinetostatic analysis is first carried out and adapted to cover the case of underactuated fingers using more than one actuator. Typical performance indices are subsequently presented and optimizations are performed to compare the best designs achievable with respect to stiffness and grasp stability, depending on the number of actuators.

Commentary by Dr. Valentin Fuster
J. Mechanisms Robotics. 2017;9(4):041004-041004-7. doi:10.1115/1.4036222.

This paper examines the problem of geometric constraints acquisition of planar motion through a line-geometric approach. In previous work, we have investigated the problem of identifying point-geometric constraints associated with a motion task which is given in a parametric or discrete form. In this paper, we seek to extend the point-centric approach to the line-centric approach. The extracted geometric constraints can be used directly for determining the type and dimensions of a physical device such as mechanical linkage that generates this constrained motion task.

Commentary by Dr. Valentin Fuster
J. Mechanisms Robotics. 2017;9(4):041005-041005-11. doi:10.1115/1.4036425.

Thin foldable origami mechanisms allow reconfiguration of complex structures with large volumetric change, versatility, and at low cost; however, there is rarely a systematic way to make them autonomously actuated due to the lack of low profile actuators. Actuation should satisfy the design requirements of wide actuation range, high actuation speed, and backdrivability. This paper presents a novel approach toward fast and controllable folding mechanisms by embedding an electromagnetic actuation system into a nominally flat platform. The design, fabrication, and modeling of the electromagnetic actuation system are reported, and a 1.7 mm-thick single-degree-of-freedom (DoF) foldable parallel structure reaching an elevation of 13 mm is used as a proof of concept for the proposed methodology. We also report on the extensive test results that validate the mechanical model in terms of the loaded and unloaded speed, the blocked force, and the range of actuation.

Commentary by Dr. Valentin Fuster
J. Mechanisms Robotics. 2017;9(4):041006-041006-9. doi:10.1115/1.4036223.

Large-stroke flexure mechanisms inherently lose stiffness in supporting directions when deflected. A systematic approach to synthesize such hinges is currently lacking. In this paper, a new building block-based spatial topology synthesis method is presented for optimizing large-stroke flexure hinges. This method consists of a layout variation strategy based on a building block approach combined with a shape optimization to obtain the optimal design tuned for a specific application. A derivative-free shape optimization method is adapted to include multiple system boundaries and constraints to optimize high complexity flexure mechanisms in a broad solution space. To obtain the optimal layout, three predefined three-dimensional (3D) “building blocks” are proposed, which are consecutively combined to find the best layout with respect to specific design criteria. More specifically, this new method is used to optimize a flexure hinge aimed at maximizing the frequency of the first unwanted vibration mode. The optimized topology shows an increase in frequency of a factor ten with respect to the customary three flexure cross hinge (TFCH), which represents a huge improvement in performance. The numerically predicted natural frequencies and mode shapes have been verified experimentally.

Commentary by Dr. Valentin Fuster
J. Mechanisms Robotics. 2017;9(4):041007-041007-9. doi:10.1115/1.4035992.

This paper seeks to speed up the topology optimization using a pseudorigid-body (PRB) model, which allows the kinetostatic equations to be explicitly represented in the form of nonlinear algebraic equations. PRB models can not only accommodate large deformations but more importantly reduce the number of variables compared to beam theory or finite element methods. A symmetric 3R model is developed and used to represent the beams in a compliant mechanism. The design space is divided into rectangular segments, while kinematic and static equations are derived using kinematic loops. The use of the gradient and hessian of the system equations leads to a faster solution process. Integer variables are used for developing the adjacency matrix, which is optimized by a genetic algorithm. Dynamic penalty functions describe the general and case-specific constraints. The effectiveness of the approach is demonstrated with the examples of a displacement inverter and a crimping mechanism. The approach outlined here is also capable of estimating the stress in the mechanism which was validated by comparing against finite element analysis. Future implementations of this method will incorporate other pseudorigid-body models for various types of compliant elements and also try to develop multimaterial designs.

Commentary by Dr. Valentin Fuster
J. Mechanisms Robotics. 2017;9(4):041008-041008-12. doi:10.1115/1.4036517.

Parallel manipulators (PMs) with redundant actuation are attracting increasing research interest because they have demonstrated improved stiffness and fewer singularities. This paper proposes a new redundantly actuated parallel manipulator that has three degrees-of-freedom (DOFs) and four limbs. The proposed manipulator is a 2UPR-2PRU parallel manipulator (where P represents an actuated prismatic joint, R represents a revolute joint, and U represents a universal joint) that is actuated using four prismatic joints; two of these joints are mounted on the base to reduce the movable mass. Mobility analysis shows that the moving platform has two rotational DOFs and one translational DOF. First, the inverse displacement solution, velocity, and singularity analyses are discussed. Next, the local transmission index (LTI) and the good transmission workspace are used to evaluate the motion/force transmissibility of the 2UPR-2PRU parallel manipulator. Finally, the parameter-finiteness normalization method (PFNM) is used to produce an optimal design that considers the good transmission workspace. It is thus shown that the motion/force transmission of the proposed manipulator is improved by optimizing the link parameters.

Commentary by Dr. Valentin Fuster
J. Mechanisms Robotics. 2017;9(4):041009-041009-11. doi:10.1115/1.4036569.

Kinematic control of manipulators with joint physical constraints, such as joint limits and joint velocity limits, has received extensive studies. Many studies resolved this problem at the second-order kinematic level, which may suffer from the self-motion instability in the presence of persistent self-motion or unboundedness of joint velocity. In this paper, a unified approach is proposed to control a manipulator with both joint limits and joint velocity limits at the second-order kinematic level. By combining the weighted least-norm (WLN) solution in the revised joint space and the clamping weighted least-norm (CWLN) solution in the real joint space, the unified approach ensures the joint limits and joint velocity limits at the same time. A time-variant clamping factor is incorporated into the unified approach to suppress the self-motion when the joint velocity diverges, or the end-effector stops, which improves the stability of self-motion. The simulations in contrast to the traditional dynamic feedback control scheme and the new minimum-acceleration-norm (MAN) scheme have been made to demonstrate the advantages of the unified approach.

Commentary by Dr. Valentin Fuster
J. Mechanisms Robotics. 2017;9(4):041010-041010-9. doi:10.1115/1.4036571.

Hyper-redundant snakelike serial robots are of great interest due to their application in search and rescue during disaster relief in highly cluttered environments and recently in the field of medical robotics. A key feature of these robots is the presence of a large number of redundant actuated joints and the associated well-known challenge of motion planning. This problem is even more acute in the presence of obstacles. Obstacle avoidance for point bodies, nonredundant serial robots with a few links and joints, and wheeled mobile robots has been extensively studied, and several mature implementations are available. However, obstacle avoidance for hyper-redundant snakelike robots and other extended articulated bodies is less studied and is still evolving. This paper presents a novel optimization algorithm, derived using calculus of variation, for the motion planning of a hyper-redundant robot where the motion of one end (head) is an arbitrary desired path. The algorithm computes the motion of all the joints in the hyper-redundant robot in a way such that all its links avoid all obstacles present in the environment. The algorithm is purely geometric in nature, and it is shown that the motion in free space and in the vicinity of obstacles appears to be more natural. The paper presents the general theoretical development and numerical simulations results. It also presents validating results from experiments with a 12-degree-of-freedom (DOF) planar hyper-redundant robot moving in a known obstacle field.

Commentary by Dr. Valentin Fuster
J. Mechanisms Robotics. 2017;9(4):041011-041011-9. doi:10.1115/1.4036611.

This paper describes a dimensional synthesis method used in the design of a passive finger exoskeleton that takes into account the user limb anthropometric dimensions and contact requirements for grasping objects. The paper is the first step in our current efforts on the design of wearable devices that use a common slider at the hand to passively drive each exofinger. The finger exoskeleton is comprised of a 3R serial limb and is constrained to multiloop eight-bar slider mechanism using two RR constraints. To design the exolimb, the pose of the limb was captured using an optical motion capture and its dimensions were determined using a constrained least square optimization, which takes into account human skin movement. To illustrate the generality of our approach, an example of the design of an index and middle finger exolimb is described.

Commentary by Dr. Valentin Fuster
J. Mechanisms Robotics. 2017;9(4):041012-041012-8. doi:10.1115/1.4036579.

A screw-based formulation of the kinematics, differential kinematics, and statics of soft manipulators is presented, which introduces the soft robotics counterpart to the fundamental geometric theory of robotics developed since Brockett's original work on the subject. As far as the actuation is concerned, the embedded tendon and fluidic actuation are modeled within the same screw-based framework, and the screw-system to which they belong is shown. Furthermore, the active and passive motion subspaces are clearly differentiated, and guidelines for the manipulable and force-closure conditions are developed. Finally, the model is validated through experiments using the soft manipulator for minimally invasive surgery STIFF-FLOP.

Commentary by Dr. Valentin Fuster
J. Mechanisms Robotics. 2017;9(4):041013-041013-8. doi:10.1115/1.4036610.

To measure unbalanced moments, the knife-edge is used as a support module in traditional platforms, but performances rapidly deteriorate as the edge is worn down. In this paper, considering the requirements of measurements, a two degree-of-freedom (DOF) flexure mechanism is, thus, presented to overcome this drawback. First, off-axis stiffness and manufacturability are improved qualitatively by means of configuration analysis. Then, four generalized cross-spring pivots are exploited in the 2DOF flexure mechanism, and the geometric parameters are analyzed to achieve approximately constant rotational stiffness and reduced center shift simultaneously, which benefits calibration procedure and measurement precision. Models are further developed to determine the shape parameters of leaf-springs and transducer performances. Therefore, a low rotational stiffness is obtained to ensure a high resolution for measurements, and a high load-carrying capacity is achieved via strength checking. Finally, finite element analysis (FEA) is carried out to validate the proposed design, and experimental results demonstrate that the developed platform is capable of unbalance measurements with a high precision and resolution.

Commentary by Dr. Valentin Fuster
J. Mechanisms Robotics. 2017;9(4):041014-041014-9. doi:10.1115/1.4035879.

Voice coil actuators (VCAs) are simple electro-mechanical devices, which are capable of generating linear motion in response to an electrical input. The generic cylindrical design of commercially available actuators imposes a large variety of limitations on the end user. The most prominent is the requirement to design and fit extra components to the actuator in order to increase functionality. To solve this issue, a novel voice coil actuator was created, which reconfigures the standard cylindrical design with one of a rectangular structure. The novel actuator incorporates planar magnets in a modified Halbach array configuration to ensure compactness and an exceptionally intense, uniform magnetic field. The moving coil is substituted with a printed circuit board (PCB) encompassing numerous current conducting traces. The board contains a miniature linear rail and bearing system, unified drive electronics, and highly adaptive position feedback circuitry resulting in a compact, highly dynamic and accurate device. In pursuit of optomechatronic applications, two distinct parallel kinematic mechanisms (PKMs) were developed to utilize the high dynamics and accuracy of the novel actuator. These devices were configured to function in only rotational degrees-of-freedom (DOF) and because of their underlying kinematic structures can be referred to as parallel orientation manipulators (POMs). In particular, two structures were defined, 2-PSS/U and 3-PSS/S, in order to constrain their payloads to two and three degrees of rotational freedom, respectively. The resultant manipulators are highly dynamic, precise and fulfill size, weight, and power requirements for many applications such as sense and avoidance and visual tracking.

Commentary by Dr. Valentin Fuster
J. Mechanisms Robotics. 2017;9(4):041015-041015-13. doi:10.1115/1.4036570.

Retractable plate structure (RPS) is a family of structures that is a set of cover plates connected by revolute joints. There exists wide range of possibilities related with these structures in architecture. Configuring the suitable shape of rigid plates that are able to be enclosed without any gaps or overlaps in both closed and open configurations and eliminating the possibility of contact between the plates during the deployment have been the most important issues in RPS design process. Many researchers have tried to find the most suitable shape by using kinematical or empirical analysis so far. This study presents a novel approach to find the suitable shape of the plates and their assembly order without any kinematical or empirical analysis. This approach is benefited from the one-uniform mathematical tessellation technique that gives the possibilities of tiling a plate using regular polygons without any gaps or overlaps. In the light of this technique, the shape of the plates is determined as regular polygons and two conditions are introduced to form RPS in which regular polygonal plates are connected by only revolute joints. It should be noted that these plates are not allowed to become overlapped during deployment and form gaps in closed configuration. Additionally, this study aims to reach a single degree-of-freedom (DoF) RPS. It presents a systematic method to convert multi-DoF RPS into single DoF RPS by using the similarity between graph theory and the duality of tessellation.

Commentary by Dr. Valentin Fuster
J. Mechanisms Robotics. 2017;9(4):041016-041016-9. doi:10.1115/1.4036739.

Kinematic redundancy may be an efficient way to improve the performance of parallel manipulators. Nevertheless, the inverse kinematic problem of this kind of manipulator presents infinite solutions. The selection of a single kinematic configuration among a set of many possible ones is denoted as redundancy resolution. While several redundancy resolution strategies have been proposed for planning the motion of redundant serial manipulators, suitable proposals for parallel manipulators are seldom. Redundancy resolution can be treated as an optimization problem that can be solved locally or globally. Gradient projection methods have been successfully employed to solve it locally. For global strategies, these methods may be computationally demanding and mathematically complex. The main objective of this work is to exploit the use of differential dynamic programing (DDP) for decreasing the computational demand and mathematical complexity of a global optimization based on the gradient projection method for redundancy resolution. The outcome of the proposed method is the optimal inputs for the active joints for a given trajectory of the end-effector considering the input limitations and different cost functions. Using the proposed method, the performance of a redundant 3PRRR manipulator is investigated numerically and experimentally. The results demonstrate the capability and versatility of the strategy.

Commentary by Dr. Valentin Fuster

Technical Brief

J. Mechanisms Robotics. 2017;9(4):044501-044501-5. doi:10.1115/1.4036219.

This paper presents a solution region synthesis methodology to perform the dimensional synthesis of spatial 5-spherical–spherical (SS) linkages for six specified positions of the end-effector. Dimensional synthesis equations for an SS link are formulated. After solving the synthesis equations, the curves of moving and fixed joints can be obtained, and they are called moving and fixed solution curves, respectively. Each point on the curves represents an SS link. Considering the limited range of joints at the first position, we can obtain the feasible solution curves. The link length curves can be obtained based on the feasible solution curves. We determine three SS links by selecting three points meeting the requirements on link length curves. Then, the solution region is built by sorting and adding feasible solution curves and projecting the feasible solution curves on the line. The feasible solution region can be obtained by eliminating defective linkages and linkages that fail to meet the other requirements from the solution region. The validity of the formulas and applicability of the proposed approach is illustrated by example.

Commentary by Dr. Valentin Fuster
J. Mechanisms Robotics. 2017;9(4):044502-044502-7. doi:10.1115/1.4036515.

A methodology to develop springs with zero-free-length (ZFL) characteristics is presented, and the configurations for placing the springs precisely on the manipulators are introduced. A spring–string arrangement installed between two separate links of a serial-type manipulator is employed and is divided into three regions for mounting, tensioning, and placing the string. The springs can develop ZFL characteristics if adequate length is ensured for mounting the springs. To shorten the length for placing strings, a reference length acquired from the link configurations is utilized. The minimization of the placing length can therefore be described clearly. Because the overextended springs and links occupied the workspace of the other links as a result of the long mounting length, the springs are reorganized using pulleys and wire winding configurations to shorten the mounting length. The springs can then be arranged in alignment on the links. As achieved with this additional arrangement, comprehensive spring configurations on the manipulators can be shown. Two examples are presented after deriving the spring configurations for ZFL characteristics and the configurations with wire winding, respectively.

Commentary by Dr. Valentin Fuster
J. Mechanisms Robotics. 2017;9(4):044503-044503-7. doi:10.1115/1.4036609.

This paper proposes a series of double C-type flexure hinges for lamina emergent mechanisms (LEMs), designs the structure, and deduces the formula of the equivalent stiffness of the double C-type flexure hinge. Theoretical calculation and finite element simulation analyses of the design examples are used to verify the correctness of the equivalent stiffness calculation formula. In order to improve the bending performance of the flexure hinges, we propose a method to remove some materials of the semicircle of the flexure hinges according to certain rules. Then, the structure of the double C-type flexure hinge is further improved. Finally, the performance of the improved and unimproved double C-type flexure hinges is compared through the finite element simulation analysis, and the results show that the bending performance of the improved double C-type flexure hinge is better than the unimproved double C-type flexure hinge, while the antitensile properties undergo no significant decline.

Commentary by Dr. Valentin Fuster

Errata

J. Mechanisms Robotics. 2017;9(4):047001-047001-1. doi:10.1115/1.4036457.

The incorrect version of Fig. 15 was published with text missing in control flow boxes. The corrected diagram is given below.

Commentary by Dr. Valentin Fuster

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