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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

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

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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