Research Papers

J. Mechanisms Robotics. 2018;10(6):061001-061001-8. doi:10.1115/1.4041213.

In this paper, we introduce a mechanism consisting of a pair of noncircular pulleys with a constant-length cable. While a single noncircular pulley is generally limited to continuously winding or unwinding, the differential cable routing proposed here allows to generate nonmonotonic motions at the output of the arrangement, i.e., the location of the idler pulley redirecting the cable. The equations relating its motion to rotation angles of the noncircular pulleys and to the cable length are presented in the first part of this paper. Next, we introduce a graphical method allowing us to obtain the required pulley profiles for a given output function. Our approach is finally demonstrated with two application examples: the guiding of a cable-suspended robot along a complex trajectory using a single actuator, and the static balancing of a pendulum with a 360 deg rotational range of motion.

Commentary by Dr. Valentin Fuster
J. Mechanisms Robotics. 2018;10(6):061002-061002-11. doi:10.1115/1.4041055.

Various slim robots, such as surgical robots or humanoid robot fingers, are remotely actuated using transmission cables. If pull–pull drive is applied to actuate them using circular driving spools regardless of the shape of the joints, the tension of the driving cable becomes difficult to be maintained properly. Fortunately, it is possible to solve such a cable slack problem by providing an appropriate cable actuation length to the joint structure of the robot from the cable driving unit. Therefore, we propose a harmonious nonlinear cable actuation mechanism suitable for driving noncircular shaped joints. The proposed cable driver can mechanically provide the required cable actuation length to suit the angle change of the target joint using a pair of noncircular pulleys without increasing the number of actuators. In this paper, a design methodology of a noncircular pulley that can be applied to pulleyless rolling joints (PR joints) as well as pulleyless hinge joints is shown. Moreover, a practical cable driver is designed for actuating a hyper-redundant discrete bending joint composed of PR joints, and its effectiveness is verified through experiments. This novel cable actuation mechanism using noncircular pulleys or gears is expected to be applicable to various miniature robots such as surgical robots and animal robots of continuum structure in the future.

Topics: Cables , Pulleys , Tension , Design
Commentary by Dr. Valentin Fuster
J. Mechanisms Robotics. 2018;10(6):061003-061003-17. doi:10.1115/1.4040961.

In this paper, an approach to physical collaboration between a patient and a therapist is proposed using a bilateral impedance control strategy developed for delayed tele-robotic systems. The patient performs a tele-rehabilitation task in a resistive virtual environment with the help of online assistive forces from the therapist being provided through teleoperation. Using this strategy, the patient's involuntary hand tremors can be filtered out and the effort of severely impaired patients can be amplified in order to facilitate their early engagement in physical tasks. The response of the first desired impedance model is tracked by the master robot (interacting with the patient), and the master trajectory plus a deviation as the response of the second impedance model is tracked by the slave robot (interacting with the therapist). Note that the first impedance model is a virtual mass-damper-spring system that has a response trajectory to the combination of patient and therapist forces. Similarly, the second impedance model is a virtual mass-damper-spring system that generates the desired slave–master deviation trajectory as its response to the therapist force. Transmitted signals through the communication channels are subjected to time delays, which exist in home-based rehabilitation (i.e., tele-rehabilitation). Tracking of the impedance models responses in the presence of modeling uncertainties is achieved by employing a nonlinear bilateral adaptive controller and proven using a Lyapunov analysis. The stability of delayed teleoperation system is also proven using the absolute stability criterion. The proposed control method is experimentally evaluated for patient–therapist collaboration in resistive/assistive tasks. In these experiments, a healthy human operator simulates a poststroke patient behavior during the interaction with the master robot.

Commentary by Dr. Valentin Fuster
J. Mechanisms Robotics. 2018;10(6):061004-061004-9. doi:10.1115/1.4041262.

There is a growing interest in assistive wearable devices for laden walking, with applications to civil hiking or military soldiers carrying heavy loads in outdoor rough terrains. While the solution of powered exoskeleton is known to be heavy and energy consuming, recent works presented wearable light-weight (semi-)passive elements based on elastic springs engaged by timed clutches. In this work, we theoretically study the dynamics of a five-link model of a human walker with point feet, using numerical simulations. We propose a novel mechanism of a spring and two triggered clutches, which enables locking the spring with stored energy while the device's length can change freely. For a given gait of joint angles trajectories, we numerically optimize the spring parameters and clutch timing for minimizing the metabolic energy cost. We show that a cleverly designed device can, in theory, lead to a drastic reduction in metabolic energy expenditure.

Topics: Optimization , Springs
Commentary by Dr. Valentin Fuster
J. Mechanisms Robotics. 2018;10(6):061005-061005-7. doi:10.1115/1.4041256.

This paper presents a new type of flexible screw mechanism (FSM), which is composed of a nut, flexible axle, and roller. It can be used in a worm robot to achieve flexible peristaltic motion, as well as curvilinear motion and deformation. This type of FSM uses a roller to decrease the friction. We investigated the transmission principle and the kinematic characteristics of this FSM, established the model of the velocity, acceleration of the roller, characterized the feed motion characteristics of the flexible shaft, and achieved an analytical solution of the flexible shaft's velocity. Furthermore, by considering the position of the pure rolling section of the roller, the spin slide model is proposed based on Hertz theory. To investigate the friction loss between the roller and the flexible axle, we established a friction work model of the entire FSM system. Finally, the motion characteristics of the FSM are evaluated through experiments.

Commentary by Dr. Valentin Fuster
J. Mechanisms Robotics. 2018;10(6):061006-061006-15. doi:10.1115/1.4041209.

A new model for mechanical computing is demonstrated that requires only two basic parts, links, and rotary joints. These basic parts are combined into two main higher level structures, locks, and balances, and suffice to create all necessary combinatorial and sequential logic required for a Turing-complete computational system. While working systems have yet to be implemented using this new approach, the mechanical simplicity of the systems described may lend themselves better to, e.g., microfabrication, than previous mechanical computing designs. Additionally, simulations indicate that if molecular-scale implementations could be realized, they would be far more energy-efficient than conventional electronic computers.

Commentary by Dr. Valentin Fuster
J. Mechanisms Robotics. 2018;10(6):061007-061007-9. doi:10.1115/1.4041199.

Mechanical advantage is traditionally defined for single-input and single-output rigid-body mechanisms. A generalized approach for identifying single-output mechanical advantage for a multiple-input compliant mechanism, such as many origami-based mechanisms, would prove useful in predicting complex mechanism behavior. While origami-based mechanisms are capable of offering unique solutions to engineering problems, the design process of such mechanisms is complicated by the interaction of motion and forces. This paper presents a model of the mechanical advantage for multi-input compliant mechanisms and explores how modifying the parameters of a model affects their behavior. The model is used to predict the force-deflection behavior of an origami-based mechanism (Oriceps) and is verified with experimental data from magnetic actuation of the mechanism.

Commentary by Dr. Valentin Fuster
J. Mechanisms Robotics. 2018;10(6):061008-061008-8. doi:10.1115/1.4041331.

The assembly task is of major difficulty for manufacturing automation. Wherein the peg-in-hole problem represents a group of manipulation tasks that feature continuous motion control in both unconstrained and constrained environments, so that it requires extremely careful consideration to perform with robots. In this work, we adapt the ideas underlying the success of human to manipulation tasks, variable compliance and learning, for robotic assembly. Based on sensing the interaction between the peg and the hole, the proposed controller can switch the operation strategy between passive compliance and active regulation in continuous spaces, which outperforms the fixed compliance controllers. Experimental results show that the robot is able to learn a proper stiffness strategy along with the trajectory policy through trial and error. Further, this variable compliance policy proves robust to different initial states and it is able to generalize to more complex situation.

Commentary by Dr. Valentin Fuster
J. Mechanisms Robotics. 2018;10(6):061009-061009-8. doi:10.1115/1.4040634.

In this paper, we present the redesign and analysis of a century old walking toy. Historically, the toy is made up of two wooden pieces including a rear leg and a front leg and body (as a single piece) that are attached to each other by means of a pin joint. When the toy is placed on a ramp and given a slight perturbation, it ambles downhill powered only by gravity. Before the toy can walk successfully, it needs careful tuning of its geometry and mass distribution. The traditional technique of manual wood carving offers very limited flexibility to tune the mass distribution and geometry. We have re-engineered the toy to be three-dimensional (3D) printed as a single integrated assembly that includes a pin joint and the two legs. After 3D printing, we have to manually break-off the weakly held support material to allow movement of the pin joint. It took us 6 iterations to progressively tune the leg geometry, mass distribution, and hinge joint tolerances to create our most successful working prototype. The final 3D printed toy needs minimal postprocessing and walks reliably on a 7.87 deg downhill ramp. Next, we created a computer model of the toy to explain its motion and stability. Parameter studies reveal that the toy exhibits stable walking motion for a fairly wide range of mass distributions. Although 3D printing has been used to create nonassembly articulated kinematic mechanisms, this is the first study that shows that it is possible to create dynamics-based nonassembly mechanisms such as walking toys.

Commentary by Dr. Valentin Fuster
J. Mechanisms Robotics. 2018;10(6):061010-061010-7. doi:10.1115/1.4041212.

A general accuracy analysis method of redundantly actuated and overconstrained parallel mechanisms is proposed. Coupled effects of actuation errors and internal elastic forces arose from the elastic deformation are both considered. The accuracy analysis approach is based on the Lie-group theory and screw theory, and it includes three steps. First, stiffness matrices of serial legs are obtained. Second, the movement of each leg is modeled based on group theory and the elastic forces arose from the deformation are represented using the stiffness matrices, following which the multiclosed-loop structure constraint and self-balanced force constraint are modeled. Finally, the error pose is estimated. The proposed method is illustrated by the accuracy study of a redundantly actuated and overconstrained Stewart platform. The error modeling is easy as the use of stiffness matrix can model the passive joint motions and deformation together. Moreover, the proposed method is computationally cheap as all computations are linear.

Commentary by Dr. Valentin Fuster
J. Mechanisms Robotics. 2018;10(6):061011-061011-10. doi:10.1115/1.4040962.

Jacobian matrix plays a key role in the analysis, design, and control of robots. For example, it can be used for the performance analysis and evaluation of parallel mechanisms (PMs). However, the Jacobian matrix of a PM generally varies with the poses of the moving platform in the workspace. This leads to a nonconstant performance index of the PM. PMs with a constant Jacobian matrix have simple kinematics and are easy to design and control. This paper proposes a method for obtaining PMs with a constant Jacobian matrix. First, the criteria for detecting invariance of a Jacobian matrix are obtained based on the screw theory. An approach to the synthesis of PMs with a constant Jacobian matrix is then proposed. Using this approach, PMs with a constant Jacobian matrix are synthesized in two steps: the limb design and the combination of the limbs. Several PMs with a constant Jacobian matrix are obtained. In addition to the translational parallel mechanisms (TPMs) with a constant Jacobian matrix in the literature, the mixed-motion PMs whose moving platform can both translate and rotate with a constant Jacobian matrix are newly identified. The input/output velocity analysis of several PMs is presented to verify that Jacobian matrix of these PMs is constant.

Commentary by Dr. Valentin Fuster
J. Mechanisms Robotics. 2018;10(6):061012-061012-7. doi:10.1115/1.4041484.

When designing and optimizing spatial flexure mechanisms, it is hard to predict collision due to 3D motion and large deformations, which compromises the utilization of spatial freedom. A computationally efficient collision test is desirable to assure that feasible mechanism designs are found when algorithmically optimizing the shape of elastic mechanisms, which are prone to collision. In this paper, a method is presented to test for collision specifically suited for flexure mechanisms by taking advantage of the typical slender aspect ratio and shape of the elastic members. Hereby, an efficient collision test is obtained that allows for the computation of a quantitative value for the severeness of collision. This value can then be used to efficiently converge to collision free solutions without excluding good mechanism designs leading to improved mechanisms, which utilize the maximum spatial design freedom.

Commentary by Dr. Valentin Fuster
J. Mechanisms Robotics. 2018;10(6):061013-061013-7. doi:10.1115/1.4041261.

A mechanism consisting of twisting and tilting joints is introduced to provide omnidirectional thrust-vectoring capabilities to a quadrotor system. This mechanism eliminates mechanical constraints and kinematic singularities to provide full directional authority to all four individual thrust vectors. The presented system fully decouples position and attitude dynamics to overcome the intrinsic maneuverability limitations of traditional multirotors while maximizing thrust efficiency over its entire configuration space. This paper presents a mathematical model of the system, introduces a control method for position and attitude tracking, and presents numerical simulation results that demonstrate the system benefits.

Commentary by Dr. Valentin Fuster
J. Mechanisms Robotics. 2018;10(6):061014-061014-12. doi:10.1115/1.4041430.

Elastic storage has been reported to help flying insects save inertial power when flapping their wings. This motivates recent research and development of elastic storage for flapping-wing micro air vehicles (fwMAVs) and their ground (tethered) flight tests. The previous designs of spring-loaded transmissions are relatively heavy or bulky; they have not yet been adopted by freely hovering prototypes of fwMAVs, especially those with four flapping wings. It is not clear if partial elastic storage can still help save power for flapping flight while not overloading the motorized transmission. Here, we developed ultralight and compact film hinges as elastic storage for four flapping wings. This spring-assisted transmission was motor driven such that the wing beat frequency was higher than the natural frequency of elastically hinged wings. Our experiments show that spring recoil helps accelerate wing closing thus generating more thrust. When powered by a 3.18 g brushless motor, this 13.4 g fwMAV prototype with spring-assisted transmission can take off by beating four flexible wings (of 240 mm span) with up to 21–22 g thrust generation at 22–23 Hz. Due to lower disk loading and high-speed reduction, indirect drive of the four elastically hinged wings can produce a thrust per unit of electrical power of up to 4.6 g/W. This electrical-power-specific thrust is comparable to that generated by direct drive of a propeller, which was recommended by the motor (AP-03 7000kv) manufacturer.

Topics: Wings , Hinges , Motors , Springs , Storage
Commentary by Dr. Valentin Fuster
J. Mechanisms Robotics. 2018;10(6):061015-061015-12. doi:10.1115/1.4041333.

This paper presents a novel assembly sequence planning (ASP) procedure utilizing a subassembly based search algorithm (SABLS) for micro-assembly applications involving geometric and other assembly constraints. The breakout local search (BLS) algorithm is adapted to provide sequencing solutions in assemblies with no coherent solutions by converting the final assembly into subassemblies which can be assembled together. This is implemented using custom-made microparts which fit together only in a predefined fashion. Once the ASP is done, the parts are manipulated from a cluttered space to their final positions in the subassemblies using a path-planning algorithm based on rapidly exploring random tree (RRT*), a random-sampling based execution, and micromanipulation motion primitives. The entire system is demonstrated by assembling LEGO® inspired microparts into various configurations which involve subassemblies, showing the versatility of the system.

Commentary by Dr. Valentin Fuster
J. Mechanisms Robotics. 2018;10(6):061016-061016-8. doi:10.1115/1.4041258.

Insects are one of the most diverse group of animals on the planet and are almost ubiquitous. Their walking locomotion has inspired engineers and provided effective solutions for designing transport methods for legged robots. In this paper, we introduce a hexapod walking robot that mimics the design and walking motions of insects. The robot is characterized by small size, light weight, simple structure, and considerably fast walking speed. Three pairs of its legs are driven by three five-degrees-of-freedom (5DOF) soft actuators based on dielectric elastomer (DE) actuators which can provide up to five movements (including three translations and two rotations) within a compact structure. The robot imitates the crawling motion of an insect using the alternating tripod gait. The experiments show that the robot can achieve an average walking speed of 5.2 cm/s (approximately 21 body-lengths per minute) at 7 Hz of actuation frequency on flat rigid surfaces. Furthermore, the robot also demonstrates the omnidirectional capabilities of walking sideways and rotating its body direction, which enhance the potential of applying the proposed robot in practical uses.

Commentary by Dr. Valentin Fuster
J. Mechanisms Robotics. 2018;10(6):061017-061017-14. doi:10.1115/1.4040884.

During extraterrestrial planetary exploration programs, autonomous robots are deployed using a separate immovable lander and rover. This mode has some limitations. In this paper, a concept of a novel legged robot with decoupled functions was introduced that has inbuilt features of a lander and rover. Currently, studies have focused mainly on performance analysis of the lander without a walking function. However, a systematic type synthesis of the legged mobile lander has not been studied. In this paper, a new approach to the type synthesis used for the robot was proposed based on the Lie group theory. The overall concept and design procedure were proposed and described. The motion requirements of the robot and its legs were extracted and described intuitively. The layouts of the subgroups or submanifolds of the limbs were determined. A family of particular joints with one rotation and one translation was proposed for the first time. The structures of the limbs were synthesized. Numerous structures of the legs were produced and listed corresponding to the desired displacement manifolds. Numerous novel structures of the legs for legged mobile lander were evaluated and listed. Then, four qualitative criteria were introduced. Based on the proposed criteria, a particular case of legs' configuration with a rhombus joint was selected as the best one among them. A typical structure of the legged mobile lander was obtained by assembling the structures of the proposed legs with a rhombus joint. Finally, the typical robot was used as an example to verify the capabilities of the novel robot using a software simulation (adams).

Commentary by Dr. Valentin Fuster
J. Mechanisms Robotics. 2018;10(6):061018-061018-13. doi:10.1115/1.4041431.

This paper presents a nonlinear vision-based observer to estimate 3D translational position and velocity of a quadrotor aerial robot for closed-loop, position-based, visual-servo control in global positioning system (GPS)-denied environments. The method allows for motion control in areas where GPS signals are weak or absent, for example, inside of a building. Herein, the robot uses a low-cost on-board camera to observe at least two feature points fixed in the world frame to self-localize for feedback control, without constraints on the altitude of the robot. The nonlinear observer described takes advantage of the geometry of the perspective projection and is designed to update the translational position and velocity in real-time by exploiting visual information and information from an inertial measurement unit. One key advantage of the algorithm is it does not require constraints or assumptions on the altitude and initial estimation errors. Two new controllers based on the backstepping technique that take advantage of the estimator's output are described and implemented for trajectory tracking. The Lyapunov method is used to show asymptotic stability of the closed-loop system. Simulation and experimental results from an indoor environment where GPS localization is not available are presented to demonstrate feasibility and validate the performance of the observer and control system for hovering and tracking a circular trajectory defined in the world frame.

Commentary by Dr. Valentin Fuster

Technical Brief

J. Mechanisms Robotics. 2018;10(6):064501-064501-5. doi:10.1115/1.4041200.

Origami-based paper folding is being used in robotics community to provide stiffness and flexibility simultaneously while designing smart structures. In this paper, we propose a novel design inspired by origami pattern service robot, which transforms its shape in the axial direction and introduce peristaltic motion therein. Here, servo motor is being used for translational actuation and springs maneuver self-deployable structure when necessary. Self-deployable springs are compressed by the application of axial force as the string gets wound around the servo motor programed to rotate with a particular speed for specified time duration. Specially coated photopolymer resin structures have been used to provide external rigidity to the springs so to avoid buckling while operation. In future, this friction coated origami service robot is envisioned to be used in an unstructured environment as the scope of applications increases at the nexus of surgical robotic navigation, houses to disaster areas.

Commentary by Dr. Valentin Fuster
J. Mechanisms Robotics. 2018;10(6):064502-064502-7. doi:10.1115/1.4041332.

This paper presents a cooperative robot exploration (CRE) strategy that is based on the sensor-based random tree (SRT) star method. The CRE strategy is utilized for a team of mobile robots equipped with range finding sensors. Existing backtracking techniques for frontier-based (FB) exploration involve moving back thorough the previous position where the robot has passed before. However, in some cases, the robot generates inefficient detours to move back to the position that contains frontier areas. In an effort to improve upon movement and energy efficiencies, this paper proposes the use of a hub node that has a frontier arc; thereby, the robots backtrack more directly to hub nodes by using the objective function. Furthermore, each robot cooperatively explores the workspace utilizing the data structure from the entire team of robots, which consists of configuration data and frontier data. Comparative simulations of the proposed algorithm and the existing SRT-star algorithm are implemented and described. The experiment is presented to demonstrate the application of the proposed strategy in real-time. Utilizing the proposed algorithm and exploration strategy, the results indicate that a team of robots can work more efficiently by reducing the distance of exploration and the number of node visited.

Topics: Robots , Algorithms , Sensors
Commentary by Dr. Valentin Fuster
J. Mechanisms Robotics. 2018;10(6):064503-064503-8. doi:10.1115/1.4041334.

In this paper, a new hybrid parallel robot (HPR) manipulator is introduced. First three kinematic limbs of six-legged general Stewart platform (6DOF GSP) manipulator are disconnected. Afterward, each passive universal joint of remaining three-legged parallel manipulator (three-UPS) is mounted at the center of each second passive revolute joint of RPR planar parallel manipulator (3DOF PPM) where underlined letters present active joints. Active actuators of PPM mounted between base platform of GSP and ground perform translations along x and y-axes, and rotation about z-axis. Remaining three limbs of GSP mechanism provide translation z-axis, and rotation about x- and y-axes only. Thus HPR can perform motion with full dimensions (translation and rotation about x-, y-, and z-axes). Optimizations are performed by using particle swarm optimization algorithm. Optimization results demonstrated that HPR provides better dexterity and singularity-free workspace characteristics than GSP.

Commentary by Dr. Valentin Fuster

Design Innovation Paper

J. Mechanisms Robotics. 2018;10(6):065001-065001-9. doi:10.1115/1.4041259.

Needles are widely used in medicine for minimally invasive procedures. A steerable flexible needle was first introduced about 15 years ago, which was a new type of needle and could follow three-dimensional curved trajectory during medical procedures. The flexible needle has the limit of a single and low curvature. In this paper, we overcome this limit by designing mechanisms for tube-wire type flexible needles. We also provide a systematic planning method for an automated operation of the needle insertion using the mechanisms. Using the new system, we can achieve high and multiple curvatures from needle trajectories. The proposed design consists of an inner prebent wire and an outer tube, which are connected to two special mechanisms, an extension switch and a friction cart. It allows the trajectory of the needle to have high and multiple curvatures, which will enable the needle to easily reach target positions while efficiently avoiding obstacles. Users can efficiently control the needle device with simple inputs (insertion and rotation) using the special operation mechanism, which achieves three system functions (insertion/retraction, rotation, curvature changes) using only two actuation motors. Compared to prebent needles or duty-cycled spinning, this needle design causes less tissue damage. We build an automatic system to operate the new design of the steerable needle and test it. The performance of the new needle is verified by experiments with ballistic gelatin and animal tissue samples.

Commentary by Dr. Valentin Fuster

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