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Research Papers

J. Mechanisms Robotics. 2019;11(4):041001-041001-10. doi:10.1115/1.4043456.

Active exoskeletons have capacity to provide biologically equivalent levels of joint mechanical power, but high mass of actuation units may lead to uncoordinated walking and extra metabolic consumption. Active exoskeletons normally supply assistance directly during push-off and have a power burst during push-off. Thus, the requirements on power of motors are high, which is the main reason for the high mass. However, in a muscle-tendon system, the strategy of injecting energy slowly and releasing quickly is utilized to obtain a higher peak power than that of muscle alone. Application of this strategy of peak power amplification in exoskeleton actuation might lead to reductions of input power and device mass. This paper presents an ankle exoskeleton which can accumulate the energy injected by a motor during the swing phase and mostly the stance phase and then release it quickly during push-off. An energy storage and release system was developed using a four-bar linkage clutch. In addition, evaluation experiments on the exoskeleton were carried out. Results show that the exoskeleton could provide a high power assistance with a low power motor and reduced the requirement on motor power by 4.73 times. Besides, when walking with the exoskeleton, the ankle peak power was reduced by 25.8% compared to the normal condition. The strategy which imitates the working pattern of the muscle-tendon system leads to a lightweight and effective exoskeleton actuation, and it also supplies ideas for the designs of lightweight actuators that work discontinuously in other conditions.

Commentary by Dr. Valentin Fuster
J. Mechanisms Robotics. 2019;11(4):041002-041002-11. doi:10.1115/1.4043045.

The dynamics model of 4-SPS/PS parallel mechanism with a flexible moving platform is formulated based on the equation of motion. Firstly, the dynamics model of flexible moving platform is formulated based on the floating frame of reference formulation. In order to avoid the wrong solutions caused by an inappropriate set of reference conditions, the fixed-fixed reference conditions are carefully selected according to the structure of parallel mechanism. Secondly, considering that the original Craig–Bampton (CB) method only represents the free-free modes. In order to use CB method to obtain fixed-fixed modes, the original CB method is improved by imposing the reference conditions prior to obtaining the static correction modes and fixed interface modes. In addition, the dynamics analysis of 4-SPS/PS parallel mechanism with flexible moving platform based on both free-free modes and fixed-fixed modes are implemented, respectively. Finally, the simulations show that the dynamic responses obtained using fixed-fixed modes are close to the ideal dynamic response, which proves the correctness of improved CB method. Moreover, the maximum percentage error of simulation results between using free-free modes and using fixed-fixed modes exceeds 100%, it is clear that the solutions based on free-free modes are not reasonable. Eventually, the conclusions prove that the deformation caused by high-speed and heavy-load should not be neglected in the parallel mechanism.

Commentary by Dr. Valentin Fuster
J. Mechanisms Robotics. 2019;11(4):041003-041003-12. doi:10.1115/1.4043291.

Driving system parameter optimization (DSPO) is an important approach to improve robots' dynamic performances such as acceleration capacity, load carrying capacity, and operation stability. To achieve better dynamic performance, motors with high power and high cost are generally used. But this leads to a waste of resources. It is difficult to simultaneously make the robots satisfy the prescribed requirements and avoid over conservative design. This issue is much more challenging for parallel machining robots due to the coupling characteristics of the closed kinematic chains. In this paper, a 5 degrees-of-freedom (DoF) parallel machining robot with planar kinematic chains is presented, and its dynamic model is established based on the virtual work principle. Then, a DSPO method for 5-DoF machining robots is proposed by considering the classical machining trajectories that can reflect the robots' performance requirements. The motor output under these trajectories and candidate motor parameters are presented in a comprehensive graph. Combined with motor selection criteria, the feasible motors and usable reduction ratio range are derived. To optimize the reduction ratio, a dynamic index is proposed based on the variance degree of the motor output torque to evaluate driving system's operational stability. On this basis, the optimal reduction ratio is obtained by minimizing this index to improve the stability of machining robots. Based on the proposed method, the DSPO for the 5-DoF parallel machining robot is implemented, and the optimal driving units are generated. The proposed method can be used for the DSPO of other 5-DoF parallel machining robots.

Commentary by Dr. Valentin Fuster
J. Mechanisms Robotics. 2019;11(4):041004-041004-8. doi:10.1115/1.4043317.

A force-limiting buckling flexure has been created which can be used in a wide range of applications where excessive force from an implement can cause harm or damage. The buckling flexure is monolithic, contains no electronics, and can be manufactured using a single shot in an injection molding machine, making it cost effective. In this paper, the design of the flexure is applied to a force-limiting toothbrush as a design study to show its application in a real-world technology. An overview of the buckling flexure is presented, and a structural model is presented to predict when the flexure will elastically buckle. Flexures of different geometries were tested and buckled. The data show that the model can predict buckling of the flexure with an error of 20.84%. A finite element model was also performed which predicts buckling of the flexure within an error of 25.35%. Furthermore, a preliminary model is presented which enables the design of the buckling beam’s displacement, such that the total breakaway deformation can be maximized, making sensing the sudden deformation easier to detect. As part of the application of the buckling flexure, an ergonomic, injection moldable toothbrush was created with the flexure built into the neck of the brush. When the user applies too much force while brushing, the flexure gives way and alerts the user when they have applied too much force; when the user lets off the force, the brush snaps back to its original shape. This design methodology is generalized and can be utilized in other force limited applications where an injection-moldable, pre-set force, and purely mechanical breakaway device is desired.

Commentary by Dr. Valentin Fuster
J. Mechanisms Robotics. 2019;11(4):041005-041005-12. doi:10.1115/1.4043458.

Knowing the set of allowable motions of a convex body moving inside a slightly larger one is useful in applications such as automated assembly mechanisms, robot motion planning, etc. The theory behind this is called the “kinematics of containment (KC).” In this article, we show that when the convex bodies are ellipsoids, lower bounds of the KC volume can be constructed using simple convex constraint equations. In particular, we study a subset of the allowable motions for an n-dimensional ellipsoid being fully contained in another. The problem is addressed in both algebraic and geometric ways, and two lower bounds of the allowable motions are proposed. Containment checking processes for a specific configuration of the moving ellipsoid and the calculations of the volume of the proposed lower bounds in the configuration space (C-space) are introduced. Examples for the proposed lower bounds in the 2D and 3D Euclidean space are implemented, and the corresponding volumes in C-space are compared with different shapes of the ellipsoids. Practical applications using the proposed theories in motion planning problems and parts-handling mechanisms are then discussed.

Commentary by Dr. Valentin Fuster
J. Mechanisms Robotics. 2019;11(4):041006-041006-8. doi:10.1115/1.4043459.

When designing linkage mechanisms for motion synthesis, many examples have shown that the optimal kinematic constraint on the task motion contains too large deviation to be approximately viewed as a single rotational or translational pair. In this paper, we seek to adopt our previously established motion synthesis framework for the design of cam-linkages for a more accurate realization, while still maintaining a 1-degree-of-freedom (DOF) mechanism. To determine a feasible cam to lead through the task motion, first a kinematic constraint is identified such that a moving point on the given motion traces a curve that is algebraically closest to a circle or a line. This leads to a cam with low-harmonic contour curve that is simple and smooth to avoid the drawbacks of cam mechanisms. Additional constraints could also be imposed to specify the location and/or size of the cam linkages. An example of the design of a lower-limb rehabilitation device has been presented at the end of this paper to illustrate the feasibility of our approach. It is shown that our design could lead the user through a normal walking motion.

Topics: Kinematics , Linkages , Design
Commentary by Dr. Valentin Fuster
J. Mechanisms Robotics. 2019;11(4):041007-041007-11. doi:10.1115/1.4043687.

This paper applies geometric design principles to planar mechanical systems, attempting to explain complex motion in mechanism–object/environment interaction for realizing multiple kinematic tasks in the vicinity of a contact location. The latter is a critical feature not fully captured in existing design methodologies. This is achieved through the development of a general planar geometric model that allows the derivation of multiple velocity and acceleration specifications compatible with mechanism–object curvature constraints in the vicinity of the contact. By incorporating these higher-order kinematic specifications into the design task formulation, contemporary planar kinematic synthesis is generalized allowing robust designs. The results are illustrated by the kinematic synthesis of two planar revolute–revolute (RR) linkages that are able to push and roll-slide along the object’s curvature in the vicinity of a contact location, a hybrid hand that incorporates a four-bar prosthetic finger that is able to grasp objects in two different modes, and a six-bar orthotic wheelchair for disabled canines that integrates body tilt and climbing motions.

Commentary by Dr. Valentin Fuster
J. Mechanisms Robotics. 2019;11(4):041008-041008-10. doi:10.1115/1.4043599.

This paper presents the novel design and integration of a mobile robot with multi-directional mobility capabilities enabled via a hybrid combination of tracks and wheels. Tracked and wheeled locomotion modes are independent from one another, and are cascaded along two orthogonal axes to provide multi-directional mobility. An actuated mechanism toggles between these two modes for optimal mobility under different surface-traction conditions, and further adds an additional translational axis of mobility. That is, the robot can move in the longitudinal direction via the tracks on rugged terrain for high traction, in the lateral direction via the wheels on smooth terrain for high-speed locomotion, and along the vertical axis via the translational joint. Additionally, the robot is capable of yaw axis mobility using differential drives in both tracked and wheeled modes of operation. The paper presents design and analysis of the proposed robot along with a dynamic stabilization algorithm to prevent the robot from tipping over while carrying an external payload on inclined surfaces. Experimental results using an integrated prototype demonstrate multi-directional capabilities of the mobile platform and the dynamic stability algorithm to stabilize the robot while carrying various external payloads on inclined surfaces measuring up to 2.5 kg and 10 deg, respectively.

Commentary by Dr. Valentin Fuster
J. Mechanisms Robotics. 2019;11(4):041009-041009-14. doi:10.1115/1.4043600.

A micropump sucker employs a gas film micropump to produce a negative pressure adhesion in a suction cup. In this study, a piezo-driven flexible actuator was developed based on a bridge-type mechanism as a vibrator for such a micropump film. The model of the flexible actuator under an external load is built based on an elastic model, and the displacement, driving force, and work efficiency are formulated in terms of the external loads, materials, and geometric parameters. The finite element method was used to verify this analytical model. An increase in the compliance of flexure hinges was found to improve the performances of the flexible actuator. The Young’s modulus of materials decides force performances and the effects of external loads. Based on the elastic analysis, the proposed flexible mechanism, made of silicon, was optimized to realize optimal output displacement in a compact size and employed in the prototype of a micropump sucker with a weight of 1.3 g that produced a maximum negative pressure of 2.45 kPa. It can hold on a weight of 1.4 g. When the inlet of the proposed sucker is open, it has the maximum flow rate of 4 ml/min.

Commentary by Dr. Valentin Fuster
J. Mechanisms Robotics. 2019;11(4):041010-041010-10. doi:10.1115/1.4043689.

This paper studies a novel fluid actuated system for a spherical mobile robot. The robot’s mechanism consists of two essential parts: circular pipes to lead spherical moving masses (cores) and an internal driving unit to propel the cores. The spherical shell of the robot is rolled by displacing the cores in the pipes filled with fluid. First, we describe the structure of the robot and derive its nonlinear dynamics using the D’Alembert principle. Next, we model the internal driving unit that actuates the core inside the pipe. The simulated driving unit is studied with respect to three important parameters—the input motor torque, the actuator size, and the fluid properties. The overall model of the robot is then used for analyzing motion patterns in the forward direction. Analytical studies show that the modeled robot can be implemented under the given design specifications.

Commentary by Dr. Valentin Fuster
J. Mechanisms Robotics. 2019;11(4):041011-041011-9. doi:10.1115/1.4043602.

Dielectric elastomer (DE), as a group of electro-active polymers, has been widely used in soft robotics due to its inherent flexibility and large induced deformation. As sustained high voltage is needed to maintain the deformation of DE, it may result in electric breakdown for a long-period actuation. Inspired by the bistable mechanism which has two stable equilibrium positions and can stay at one of them without energy consumption, two bistable dielectric elastomer actuators (DEAs) including a translational actuator and a rotational actuator are proposed. Both the bistable actuators consist of a double conical DEA and a buckling beam and can switch between two stable positions with voltage. In this paper, the analytical models of the bulking beam and the conical DEA are presented first, and then the design method is demonstrated in terms of force equilibrium and moment equilibrium principle. The experiments of the translational bistable DEA and the rotational bistable DEA are conducted, which show that the design method of the bistable DEA is effective.

Commentary by Dr. Valentin Fuster
J. Mechanisms Robotics. 2019;11(4):041012-041012-15. doi:10.1115/1.4043603.

Recent advancements in powered lower limb prostheses have appeased several difficulties faced by lower limb amputees by using a series-elastic actuator (SEA) to provide powered sagittal plane flexion. Unfortunately, these devices are currently unable to provide both powered sagittal plane flexion and two degrees of freedom (2-DOF) at the ankle, removing the ankle’s capacity to invert/evert, thus severely limiting terrain adaption capabilities and user comfort. The developed 2-DOF ankle system in this paper allows both powered flexion in the sagittal plane and passive rotation in the frontal plane; an SEA emulates the biomechanics of the gastrocnemius and Achilles tendon for flexion while a novel universal-joint system provides the 2-DOF. Several studies were undertaken to thoroughly characterize the capabilities of the device. Under both level- and sloped-ground conditions, ankle torque and kinematic data were obtained by using force-plates and a motion capture system. The device was found to be fully capable of providing powered sagittal plane motion and torque very close to that of a biological ankle while simultaneously being able to adapt to sloped terrain by undergoing frontal plane motion, thus providing 2-DOF at the ankle. These findings demonstrate that the device presented in this paper poses radical improvements to powered prosthetic ankle-foot device (PAFD) design.

Commentary by Dr. Valentin Fuster
J. Mechanisms Robotics. 2019;11(4):041013-041013-11. doi:10.1115/1.4043683.

The narrow and redundant body of the snake robot makes it suitable for the inspection of complex bar structures, such as truss or tree structures. One of the key issues affecting the efficient motion of snake robots in complex bar structures is the development of mechanical models of snake robots on cylinders. In other words, the relationship between the payload and structural and performance parameters of the snake robot is still difficult to clarify. In this paper, the problem is approached with the Newton–Euler equations and the convex optimal method. Firstly, from the kinematic point of view, the optimal attitude of the snake robot wrapped around the cylinder is found. Next, the snake robot is modeled on the cylinder and transformed into a convex optimization problem. Then, the relationship between the payload of the snake robot on the cylinder and the geometric and attitude parameters of the body of snake robots is analyzed. Finally, the discussion for the optimal winding attitude and some advices for the design of the snake robot are proposed. This study is helpful toward the optimal design of snake robots, including geometry parameters and motor determination.

Commentary by Dr. Valentin Fuster
J. Mechanisms Robotics. 2019;11(4):041014-041014-20. doi:10.1115/1.4043457.

The looped-synchronous mechanism (LSM) is a special one degree-of-freedom (DOF) closed chain of transmission with a large number of duplicated units that synchronizes the motion of many output links. This kind of mechanism can be found in many applications such as stator blade adjusting mechanisms for various aero-engines. The LSMs are composed of a large number of links and joints and must be designed by specific means. Spatial Assur-group, which is a concept extended from traditional Assur-group(in planar scope), and usually with a little number of parts and joints, is used in this work to design LSM. First, based on the formula of DOF of spatial Assur-group, all possible combinations are listed and two feasible combinations are chosen as the main body of each unit of LSM, combining with a prime mover to meet the requirement to be inexpandable and adjustable. Second, the condition for transmission ratio of the used Assur-group to be 1 is distilled for being synchronous and looped under the situation that all units of LSM have the same topology. To meet the condition, the needed dimensional conditions are researched and mathematical deduction is used to figure out the possibilities. Third, after confirming that it is impossible to meet the condition strictly, an optimization method in the environment of Simulink is used to approach the condition as close as possible. Finally, numerical and dynamic simulations are carried out to verify the effectiveness of the mentioned methods.

Commentary by Dr. Valentin Fuster

Technical Brief

J. Mechanisms Robotics. 2019;11(4):044501-044501-7. doi:10.1115/1.4043330.

This article presents a new variable actuation mechanism based on the 3-RPR parallel robot. This mechanism is an evolution of the NaVARo robot, a 3-RRR parallel robot, for which the second revolute joint of the three legs is replaced by a scissor to obtain a larger working space and avoid the use of parallelograms to operate the second revolute joint. To obtain better spatial rigidity, the leg mechanism is constructed by placing the scissors in an orthogonal plane to the plane of the manipulator displacement (3-RRR or even the 3-RPR). This geometric property brings the significant consequence of allowing the scissors to directly substitute the prismatic chains in the 3-RPR and enjoy the same kinematics advantages for the overall robots as only one solution to the inverse kinematic model. From the Jacobian expression, surfaces of singularity can be calculated and presented in a compact form. The singularity equations are presented for a robot with a similar base and mobile platform. The properties of the scissors are then determined to have a prescribed regular workspace.

Commentary by Dr. Valentin Fuster
J. Mechanisms Robotics. 2019;11(4):044502-044502-8. doi:10.1115/1.4043601.

This paper presents the construction method of a family of reconfigurable deployable polyhedral mechanisms (RDPMs) based on straight elements. First, reconfigurable straight element (RSE) is designed from two aspects: two prismatic-revolute-revolute-prismatic mechanisms aspect and reconfigurable angulated element aspect, and the kinematics and multifurcation of RSE are investigated. Then reconfigurable multiple straight elements (RMSEs) with n pairs of straight elements are proposed, RMSEs can reach two different transition configurations, the constraint conditions of RMSEs at transition configuration I and transition configuration II are analyzed with all link lengths identified. Finally, two typical polyhedra are used as a basis to construct RDPMs to verify the feasibility of the proposed construction method. A combination of half platforms and whole platforms for the first time is used in the construction of RDPMs, and the obtained mechanisms can switch between two kinds of conventional deployment configurations (the Hoberman sphere motion configuration and radially reciprocating motion configuration) and their compound configurations.

Commentary by Dr. Valentin Fuster

Design Innovation Paper

J. Mechanisms Robotics. 2019;11(4):045001-045001-8. doi:10.1115/1.4043688.

To solve the difficult parking problem, developing a mechanical parking device is a practical approach. Aiming at longitudinal parking, a novel compact double-stack parking system is put forward based on a 1-DOF (degree of freedom) cam-linkage double-parallelogram mechanism. Due to the unique structure, the whole device can be driven by a single motor to realize three motion periods, including lifting, translation, and fillet transition. Meanwhile, all parts of this compact mechanism can be well contained in the filleted rectangular trajectory. This rectangular trajectory is essential that we no longer need to take out the ground vehicles so as to realize stack parking. Furthermore, to overcome the singularity collinear problem of the parallelogram which may lead to the polymorphic state, the double-parallelogram mechanism is proposed to maintain the orientation of the parking platform. The digital simulation and kinetostatic analysis results demonstrate the feasibility that this novel cam-linkage double-parallelogram mechanism can improve the space utilization of the residential area, alleviate the parking problem, and can be quickly put into application on campuses or streets in a short period.

Commentary by Dr. Valentin Fuster
J. Mechanisms Robotics. 2019;11(4):045002-045002-10. doi:10.1115/1.4043604.

Automating the double-hull block welding improves the shipbuilding efficiency and mitigates health risk to the welders. Due to the unique challenge posed by the internal structures, existing technologies are at most semi-autonomous. Believing that mobility is key to full autonomy, we employ mobile robot technology to transport the welding manipulator, treating the internal structures as obstacles. Though many robot designs for obstacle scaling exist, there is no selection guideline. To this end, we surveyed existing robots and came out with a taxonomy to explain the design philosophies behind them. From the survey, it is ascertained that there are two suitable philosophies: bridging and conforming. Bridging mechanisms create links between points on obstacles while conforming mechanisms have the robot’s body attuned to the surface contour of obstacles. Understanding the pros and cons, we conclude that having a hybrid mechanism with tracked arms and articulated body would be ideal for the structured environment. Subsequently, we studied the feasibility of the design in terms of configuration, geometry, kinematics, and stability. Lastly, the proposed design was tested by building a 1/3 scale prototype robot. It was made to perform the expected motions in a mock double-hull block setup. The experiment proved that the design achieves the mobility objectives of the robot. With this mobility design, we solved the most challenging issue in enabling fully autonomous welding in double-hull blocks. The taxonomy is instrumental in our design selection and could be helpful for other robot designers too.

Commentary by Dr. Valentin Fuster

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