Although the community has faced an unprecedented year, I am pleased to report that JMR is thriving and remains on a growth trajectory. While the lives of many were seriously impacted by the pandemic, our community has been resilient. Last year, in 2020, JMR had a record number of submissions (over 600) and is on track to meet this mark again this year. Thanks to the hard work of our Editorial Board, reviewers, and ASME staff, paper review and publication has also remained on track. As we approach the mid-point of this challenging year, in addition to expressing thanks for everyone's hard work, I would like to take this opportunity to share a few recent initiatives aimed at making JMR one of the top journals in mechanisms and robotics (MR).

Soft robots join body and actuation, forming their structure from the same elements that induce motion. Soft actuators are commonly modeled or characterized as primary movers, but their second role as support structure introduces strain-pressure combinations outside of normal actuation. This manuscript examines a more complete set of possible strain-pressure combinations for McKibben actuators, including passive, or unpressurized, deformation, pressurized extension and compression of a pressurized actuator beyond the maximum actuation strain. Each region is investigated experimentally, and empirical force-displacement-pressure relationships are identified. Particular focus is placed on ensuring empirical relationships are consistent at boundaries between an actuator's strain-pressure regions. The presented methodology is applied to seven McKibben actuator designs, which span variations in wall thickness, enclosure material and actuator diameter. Empirical results demonstrate a trade-off between maximum contraction strain and force required to passively extend. The results also show that stiffer elastomers require an extreme increase in pressure to contract without a compensatory increase in maximum achieved force. Empirical force-displacement-pressure models were developed for each variant across all the studied strain-pressure regions, enabling future design variation studies for soft robots that use actuators as structures.

The conventional scissor mechanism is used in modern engineering and robotic applications due to its metamorphic ability. The folding configuration provides space-saving and unfolding provides longer linear expansion capability. However, a conventional scissor suffers unexpected and uncontrolled large bending deformation due to low bending stiffness while unfolding configuration, which may damage its structure. It also has a sudden bending singularity during unfolding, which may also damage the actuator. These limitations impose a significant constraint on real-life applications such as foldable robot arms, space robot arms, and reconfigurable robots. In this paper, we proposed a multi-strands parallel twisted-scissor mechanism (PTSM) to enhance its usability. The PTSM is inspired by a rope structure and designed by introducing a metamorphic segment (MS) using the S-shaped linkage design approach to improve its bending stiffness without affecting conventional scissors’ fundamentals. The PTSM has a unique feature of several automatic-link locking mechanisms to avoid singularity without using additional sensors, mechanism, or control. We experimentally checked the proposed design’s functionality and its feasibility. We formulated a cantilever bending model for foldable PTSM with N metamorphic segments considering revolute joint clearance for bending estimation, experimentally verified, and analyzed the bending deformation in the X–Y and Y–Z planes. Also, it is compared with a conventional scissor. Finally, we found that PTSM is stronger than conventional scissor and can fold/unfold smoothly using a single linear actuator. PTSM can provide large linear displacement with small bending deformation without bending singularity.

This paper presents a novel Schönflies-motion Parallel Manipulator with Rotational Pitch motion (SPM-RP) based on a single-platform fully parallel mechanism. An analysis of the position, workspace, velocity, and singularity of the SPM-RP is carried out in detail, and a dimensionless Jacobian is proposed to evaluate the manipulability of the SPM-RP. The analysis shows that the SPM-RP is with position-decoupled kinematics, a large singularity-free workspace, and excellent manipulability. The SPM-RP is actuated by four parallel prismatic actuators, enabling the manipulator to provide the identical kinematic performance at all generic cross sections perpendicular to the prismatic joint axes within its workspace. This paper thus proposes a reduced design optimization formulation, where the traditional optimization over the entire workspace is reduced to the optimization on a representative workspace cross section of the SPM-RP. According to this approach, the design optimization of the SPM-RP is carried out by maximizing its manipulability over the total orientation workspace, which is crucial for precision assembly. Based on the achieved optimal design, an SPM-RP prototype is developed. The mobility, orientation capability, total orientation workspace, and repeatability of the optimal design are tested and verified by the developed SPM-RP prototype. Experiments show that the SPM-RP can achieve a large total orientation workspace with excellent precision performance.

In this article, we present the design of a novel ankle rehabilitation robot (ARR), called the Flex-ARR, that employs a compliant parallel kinematic mechanism (PKM) with decoupled degrees-of-freedom. While multiple ARRs have been developed and commercialized, their clinical adoption has been limited primarily because they do not emulate the natural motion of the ankle. Based on a review of existing ARRs and their limitations, this article defines functional requirements and design specifications for an optimal ARR. These are then used to develop a design strategy followed by conceptual and detailed design of a novel ARR. The proposed Flex-ARR is designed to collocate the biological center of rotation of the ankle with that of the robot's center of rotation to allow natural ankle motion. The strategic use of a compliant PKM in the Flex-ARR not only absorbs any residual misalignment between these two centers but also helps inherently accommodate variations in user foot sizes with minimal adjustments. Detailed design includes the ARR structure with adjustable features, compliant PKM optimization, sensor and actuator selection, and an alignment tool.

Direct position analysis (DPA) of parallel manipulators (PMs) is in general difficult to solve. Over on PMs' topology, DPA complexity depends on the choice of the actuated joints. From an analytic point of view, the system of algebraic equations that one must solve to implement PMs' DPA is usually expressible in an apparently simple form, but such a form does not allow an analytic solution and even the problem formalization is relevant in PMs' DPAs. The ample literature on the DPA of Stewart platforms well document this point. This paper addresses the DPA of a particular translational PM of 3-URU type, which has the actuators on the frame while the actuated joints are not adjacent to the frame. The problem formulation brings to a closure-equation system consisting of three irrational equations in three unknowns. Such a system is transformed into an algebraic system of four quadratic equations in four unknowns that yields a univariate irrational equation in one of the four unknowns and three explicit expressions of the remaining three unknowns. Then, an algorithm is proposed which is able to find only the real solutions of the DPA. The proposed solution technique can be applied to other DPAs reducible to a similar system of irrational equations and, as far as this author is aware, is novel. Keywords: Kinematics, Position Analysis, Parallel Manipulators, Lower Mobility, Translational Manipulators

Antagonistically actuated variable stiffness actuators (VSAs) take inspiration from biological muscle structures to control both the stiffness and positioning of a joint. This paper presents the design of an elastic mechanism that utilizes a cable running through a set of three pulleys to displace a linear spring, yielding quadratic spring behavior in each actuator. A joint antagonistically actuated by two such mechanisms yields a linear relationship between force and deflection from a selectable equilibrium position. A quasi-static model is used to optimize the mechanism. Testing of the fabricated prototype yielded a good match to the desired elastic behavior.

The importance of an untethered microrobotic platform that can operate on high flow rate microfluidic channels for in-vitro applications is increasing rapidly. This paper presents a method to manipulate a microrobot in a fluidic chip when high flow rates (4 mL/min, 82.304 mm/s) are applied. This method is based on a novel permanent magnet-based diamagnetic levitation configuration. This configuration includes a thin layer of pyrolytic graphite, which is placed just below the microrobot. In this way, microrobot stability and manipulation capability are increased. Also, we aim to increase the longitudinal forces imposed on the microrobot to withstand the drag force proportional to the flow rate. Hence, magnetic field lines are generated more linearly around the microrobot by a different combination of permanent magnets. The proposed magnetic configuration, named “Kerkan configuration,” significantly improves the microrobot's longitudinal forces. In this configuration, two different ring-shaped ferromagnetic magnets are used. One of the magnets has a smaller diameter than the other magnet. A combination of one smaller and one bigger magnet is placed above and below the microrobot. In order to validate the advantages of this configuration, analytical and simulation studies are conducted. Their results are then compared with experimental results. Experimental results are on par with analytical and simulation studies. Kerkan configuration has a lower displacement than the next best configuration at the highest flow rate we applied (relatively 3301 µm, %21.8).

In this paper, a complete morphing system consisting of a variable geometry truss manipulator (VGTM) is presented that is fully covered by a flexible panel skin. Two approaches are studied for the morphing control. The first one is to have the VGTM act as a driving mechanism and the flexible panels as a passive system. In this case, the VGTM is composed of active members and passive lockable members. It is shown that the morphing system can reach the desired shapes through intermediate steps. The second method is to have the flexible panels act as drivers and the VGTM as a passive supporting structure. In this case, the VGTM is only composed of passive lockable members. The morphing system can also achieve the desired poses through several steps. The control strategies of the two methods are discussed along with kinematic analysis, a comparison study is conducted to show their pros and cons, two prototypes are fabricated, and experiments are carried out to verify the feasibility of two actuation methods.

Geckos can climb freely on various types of surfaces using their flexible and adhesive toes. Gecko-inspired robots are capable of climbing on different surface conditions and have shown many important applications. Nonetheless, due to poor flexibility of toes the movements of gecko-inspired robots are restricted to flat surfaces. To improve the flexibility, by utilizing design technique of soft actuator and incorporating the characteristics of a real gecko's toe, the design of new bionic soft toes is proposed. The abilities of this bionic toe are verified using modelling and two soft toes are manufactured. One is Type A toe having varied semi-circle cross-sections as the feature of real gecko toe and the other is Type B toe with a constant semi-circle cross-section. The bending behaviors of the bionic toes subjected to a range of hydraulic pressure are also experimentally studied. It demonstrated that both toes can perform similarly large bending angles for the adduction (attachment) and abduction (detachment) motions. In comparisons, Type B toe exhibits larger output force, which is ascribed to the fact that at proximal section of Type B corresponds to larger volume for bearing fluid. Both toes can not only provide sufficient adhesion, but can be quickly detached with low peeling forces. Finally, different curved surfaces are used to further justify the applicability of these bionic toes. In particular, the flexible toes developed also have the advantages of low cost, lightweight, and simple control, which is desirable for wall-climbing robots.

Following recent work on Stephenson-type mechanisms, the synthesis equations of Watt six-bar mechanisms that act as timed curve generators are formulated and systematically solved. Four variations of the problem arise by assigning the actuator and end effector onto different links. The approach produces exact synthesis of mechanisms up to eight precision points. Polynomial systems are formulated and their maximum number of solutions is estimated using the algorithm of random monodromy loops. Certain variations of Watt timed curve generators possess free parameters that do not affect the output motion, indicating a continuous space of cognate mechanisms. Packaging compactness, clearance, and dimensional sensitivity are characterized across the cognate space to illustrate trade-offs and aid in selection of a final mechanism.

Variable stiffness actuators (VSAs) are widely explored as they could improve the safe performance for human-robot interaction and make the system torque controllable based on the internal compliance. This article presents a novel VSA based on the cam-based relocation mechanism (CRM-VSA), which is utilized to change the locations of pivot and spring of a lever mechanism simultaneously. Consequently, such structure makes the actuator compacted and the stiffness regulation designable, which could help engineers to pursue different demands of stiffness regulation. The simultaneous relocations of the pivot and spring also permit a wide range of adjustable stiffness. By introducing linear guide pairs, the internal friction of the relocations of pivot and spring could be greatly reduced, thus enhancing the energy efficiency. To evaluate the performance of the proposed CRM-VSA, the point-to-point control strategy is developed, which contributes to a higher tracking accuracy and oscillation attenuation at both the start and end points of the trajectory. In addition, the performance of torque controllability is also verified through experiments. These excellent capabilities enable the proposed CRM-VSA to be qualified for constructing a robotic arm toward service applications.

Presented in this paper is a method for modeling and simulation of a complete morphing mechanism. The said mechanism has a rigid panel morphing skin that morphs along with a driving mechanism. The said skin is made of segmented panels, inspired by fish scales. Since the gaps between these panels are undesirable, a gapless design is introduced in this paper by using shape-memory polymer (SMP) joints. This paper aims to solve two fundamental problems for the entire system: (1) motion control and (2) force control. The motion control is addressed through the kinematic modeling of two equations including (a) the passive rigid panels and (b) the passive rigid panels to the active mechanism. Force control is achieved through force modeling. This is to develop a relationship of the SMP deformations to the required actuator forces. The experiment is carried out to determine the SMP forces versus deformation, and simulations are conducted to investigate how a complete morphing mechanism behaves. It also reveals that the workspace and singularity of the original mechanism will change after covered by a morphing skin. The developed method sheds light on the design of a complete morphing mechanism.

In this article, the inverted honeycomb cell, known to exhibit an auxetic behavior, is considered to design two pneumatic linear actuators. The actuators are built using a combination of soft and rigid structures. They present complementary performances in terms of displacement, force, and stiffness. Experimental evaluations are conducted using prototypes produced using multimaterial additive manufacturing to combine soft and rigid materials with freedom of shape. The first actuator is inspired by origami structures. The possibility to obtain large deformations under low pressure is observed. The second actuator is based on a cylindrical auxetic structure based on the inverted honeycomb cell. Smaller deformation is reached but the design favors the off-axis stiffness, so the component can be integrated without any additional mechanical joint for translation. A discussion on the relative performances of these two actuators and their possible uses conclude the paper.

This paper presents a novel solution for the posture control and ride comfort between the proposed wheel-legged robot (four wheel-legged robot (FWLR)) and the unstructured terrain by means of an actively passively transformable suspension system. Unlike most traditional robots, each leg of FWLR is independent of each other with a spring-damping system (passive system) is connected in series with an actuator (active system), so the posture control and ride comfort in complex terrain can be realized by the combination between active and passive systems. To verify the performance of posture control in complex terrain, a prototype and complex terrain are established first, then a posture control model, algorithm, and controller considering the suspension system are proposed and verified by the comparison between co-simulation and experiment, the results showed that the pitch angle and roll angles in complex terrain can be controlled. To show the impact of the actively passively transformable suspension system on ride comfort (vibration isolation performance), different dynamic models with different degree-of–freedom (DOF) are established, the co-simulation results showed that the passive system and active posture control system can also effectively improve the ride comfort of FWLR in complex terrain. The research results of this paper have important reference significance and practical value for enriching and developing the mechanism design and theoretical research of wheel-legged robot and promoting the engineering application of all-terrain robot.

This paper extends the concept of underactuated redundancy for physical human–robot interaction (pHRI) in a context of industrial assembly by introducing a novel 1-dof gravity balanced rotational manipulator. The proposed architecture consists of a rotational active counterweight with a passive joint equipped with an encoder. The proposed architecture is first described, and the static equilibrium conditions are used to describe the operation of the mechanism. Then, alternative architectures are briefly introduced. Finally, an experimental validation is provided to demonstrate the viability of the concept for rotational low impedance pHRI.

Soft continuum arms (SCAs) have a large workspace, dexterity, and adaptability, but at the cost of complex design construction highlighted by concatenating several serial segments. In this paper, we propose a new design architecture for SCAs composed of a parallel combination of pneumatic actuators. The BR 2 SCA featured in this work is asymmetric as it combines one soft bending (B) actuator and two soft rotating (R 2 ) actuators as opposed to state of art symmetric architectures that adopt bending segments. Spatial deformation is obtained by combining the bending and rotating feature of the individual actuators. This paper also formulates an approximate forward analysis method based on Kirchhoff rod equations to predict the spatial deformation under external loads with an accuracy less than 9% of the SCAs length. In addition, the model also takes into account the “coupling effect” inherent to the asymmetric parallel combination, where actuating rotating actuator attenuates the bending performance and vice versa. Consequently, this work also refines the design of the SCA that minimizes the coupling effect. A detailed performance study of the refined BR 2 manipulator on a swiveling base demonstrates larger workspace and higher dexterity when compared with state of art single section SCAs. The performance of the design is validated through different tasks like obstacle avoidance, pick and place task, and whole arm grasping. These performance attributes surpass any other single segment soft module and is a potential building block for constructing customized SCAs.

Previously, we reported the conceptual design of a novel parallel-kinematic flexure mechanism that provides large and decoupled motions in the X, Y, and Z directions, along with good actuator isolation, and small parasitic error motions (Awtar, S., Ustick, J., and Sen, S., 2012, “An XYZ Parallel-Kinematic Flexure Mechanism With Geometrically Decoupled Degrees of Freedom,” ASME J. Mech. Rob., 5(1), p. 015001). This paper presents the detailed design and fabrication of a high-precision experimental setup to characterize and validate the motion attributes of this proposed flexure design via comprehensive measurements. The unique aspects of this experimental setup include a novel modular construction and exact-constraint assembly of the flexure mechanism from 12 identical parallelogram flexure modules. The flexure mechanism along with the sensing and actuation setup in the experiment is designed to enable large range (10 mm) in each direction. Experimental measurements and finite-elements analysis demonstrate <3% variation in motion direction stiffness, 20.4% lost motion, <11.6% cross-axis error, <3.3% actuator isolation, and <9.5 mrad motion stage rotation over the entire 10 mm × 10 mm × 10 mm range of motion.

This paper illustrates the development and experimental validation of a robotic ankle–foot orthosis (AFO) with a series elastic actuator (SEA) and a magneto-rheological (MR) brake. First, the biomechanics of a human ankle joint during walking is explained. Next, the hardware design of the robotic AFO is introduced, including its mechanical structure, actuator design and configuration, and electronic system. The SEA is primarily composed of an electric motor, a planetary gearbox, a torsion spring, and a pair of bevel gears. The MR brake can modulate the viscosity of the robotic AFO and generate a large braking torque of 21.8 Nm with a low power of 8.8 W. Additionally, the modeling of the robotic AFO is presented, followed by an introduction to its control; several gait evaluation indices are proposed as well. Finally, a pilot study is conducted to verify the effectiveness of the developed robotic AFO. The experimental results demonstrate that the robotic AFO has the potential to provide dorsiflexion assistance, thus preventing foot slap and toe drag, in addition to plantarflexion assistance for the forward propulsion of the body. During a gait cycle, an average power of 0.23 W is harvested, and an 8% improvement in the system energy efficiency is achieved.

Soft pneumatic actuators (SPAs) that can twist dominantly provide a promising solution for the design of soft robots due to their flexibility, compliance, and easy fabrication. However, the torsional SPAs also face challenges such as overpressure risk and small torsion angle. To tackle those challenges, a new type of torsional SPAs is designed based on the reversible, cooperative buckling of elastomer, which yields a torsion angle of 1.94 deg/mm and an output torque of 26 N .mm with a secure operation pressure. Moreover, this actuator can achieve a wide range of torsional motion by varying the structure parameters, including the height of actuators and the pitch of helical chambers. The relationship between structure parameters and actuator performance is investigated experimentally, and experimental results show that the torsion angle and output torque increase with the height growing from 20 to 44 mm, while decrease with the pitch rising from 75 to 150 mm. The effect of different materials used for constructing the actuator is also studied numerically, and results show that the output torque can be improved by changing the materials. Additionally, several soft machines constructed by utilizing the actuators as a torsional joint are able to perform different manipulating tasks, such as screwing the light bulb, grasping and rotating objects. The actuator developed in this paper is capable of extending the researches on SPAs and offering an alternative for the actuation of soft machines.