Shouhei Shirafuji

Development of a tendon-driven robotic finger for an anthropomorphic robotic hand

Our paper proposes a tendon-driven robotic finger based on an anatomical model of a human finger and a suitable method for its analysis. Our study aims to realize an anthropomorphic robotic hand that has the same characteristics and dexterity as that of a human hand, and it also aims to identify the advantages of the human musculoskeletal structure for application to the design and control of robot manipulators. When designing an anthropomorphic robotic hand, several devices are required to apply the human finger structure to a tendon-driven robotic finger. Reasons for this include that one of the human finger muscles, namely, the lumbrical muscle, is situated between tendons, which is an unfavorable configuration for the tendon-driven mechanism. Second, unlike a standard pulley used in a tendon-driven mechanism, some moment arms of the human finger change nonlinearly according to the joint angle. In our robotic finger design, we address these difficulties by rearranging its tendons and develop a mechanism to change the moment arm. We also propose a method to analyze and control this robotic fingers coordinating joints using non-stretch branching tendons based on the human extensor mechanism with a virtual tendon Jacobian matrix and the advantage is that this constraint virtually reduces the degrees-of-freedom (DOF) of the mechanism. Further, we build a prototype to confirm its motion using this method. In addition, we show that the state with the reduced DOF can be lost by external forces acting on the mechanism, and this condition can be changed manually by adjusting the tendon forces. This makes it possible to control the virtual DOFs to satisfy the requirements of the task. Finally, we discuss the benefits from anthropomorphic structures including the tendon arrangement, which mimic the human lumbrical muscle, and the above mentioned mechanism with non-linear moment arms from the perspective that there are two states of DOFs. These insights may provide new perspectives in the design of robotic hands.

Detection and prevention of slip using sensors with different properties embedded in elastic artificial skin on the basis of previous experience

In dexterous robotic manipulation it is essential to control the force exerted by the robot hands while grasping. This paper describes a method by which robot hands can be controlled on the basis of previous experience of slippage of objects held by the hand. We developed an anthropomorphic human scale robot hand equipped with an elastic skin in which two types of sensors are randomly embedded. One of these sensors is a piezoelectric polyvinylidenefluoride (PVDF) film which can be used for the detection of pressure changes. The other is a strain gauge which can measure static pressure. In our system, PDVF films are used to detect slipping, and strain gauges to measure stresses which are caused by normal and shear forces. The stress measured by the strain gauges are used as input data to the neural network which controls the actuators of the robot. Once a slip is detected, the neural network is updated. We show that this system can control the grasp force of the robot hand and adapt it to the weight of the held object. By using this method, it wa shown that robots can hold grasped objects safely.

Designing Noncircular Pulleys to Realize Target Motion Between Two Joints

Many mechanisms reduce the number of inputs for their target motions by restricting the motions of pairs of joints. This study designs a novel constraint mechanism using a pair of noncircular pulleys and a wire. The wire restricts the motion of the joints to the target motion while enabling a compact structure. We analytically derive the shape of the noncircular pulleys using the desired relation between the joints and the lengths of their moment arms. The usability of the proposed mechanism is evaluated on a robotic leg, which maintains the height and posture of its upper body under the constraints.

Design of An anthropomorphic tendon-driven robotic finger

The flexor digitorum profundus, extensor digitorum communis and lumbrical muscle of the human hand play a significant role in the movement of the finger. The structure consisting of these muscles and tendons is important to consider an anthropomorphic tendon-driven finger. However, there are some problems to apply the structure found in humans to robotic fingers using mechanical elements. One of them is that the origin of the lumbrical muscle is not on any bones but on the tendon of the flexor digitorum profundus. Another is the non-constant length of the moment arm of the lateral band at the proximal interphalangeal (PIP) joint. We propose a design based on the kinematic model proposed by Leijnse et al. [1] considering the equalization of the joint torques. The proposed model can be easily realized by a structure consisting of actuators fixed to a base and a tendon-pulley system that maintains the function of those three muscle and their tendons.

Locking mechanism based on flat, overlapping belt, and ultrasonic vibration

Locking devices play an important role in robots and assist suits in terms of energy management and miniaturization. In this work, we develop a new locking device for a flat belt that can lock the belt at arbitrary positions and has a simple design that requires no driving parts. According to a mechanical analysis of a flat, overlapped-belt structure, the locked and unlocked states of a flat belt that passes through the structure depend only on the friction coefficient and the contact angle between the belt and the circular cylinder. Moreover, conventional research mentions that the friction coefficient between two objects can be reduced by apply ultrasonic vibration to the objects. The proposed locking mechanism thus uses ultrasonic vibration to vary the friction coefficient and force a flat, overlapped-belt structure to transition from the locked state to the unlocked state. Experiments were conducted by using a prototype device. Upon applying ultrasonic vibration to a locked belt, the belt is unlocked and transmits 14.0% of the tensile force applied to one to the opposite end.

Tendon Routing Resolving Inverse Kinematics for Variable Stiffness Joint

Recently, the tendon-driven mechanism with variable joint stiffness has received attention for use in the development of a humanoid robot operated in an uncertain environment with physical contact. In this paper, we propose a mechanism to control the position and joint stiffness of a tendon-driven manipulator independently, using dedicated actuators. This mechanism consists of two parts: a component that transforms the movements of the tendons to activate the actuators, and a component that applies tensile forces to adjust the joint stiffness. We named this mechanism “tendon routing resolving inverse kinematics” (TRIK). The methodology for designing this mechanism for various tendon-driven manipulators is presented with several examples. We designed TRIK for a manipulator with one degree of freedom and nonconstant-moment arms. Finally, experiments of variable joint stiffness with nonlinearly elastic components were conducted to validate the proposed mechanism.

Mechanism allowing large-force application by a mobile robot, and development of ARODA

Abstract This study proposes a mechanism and a methodology for large force input to the environment by a mobile robot. To determine the limits on the force that a mobile robot can apply to a target object, we analyzed the forces between the robot, ground, and object, and the frictional-force limits between any two of these three bodies. To prevent the mobile robot from falling during the large-force application, the manipulator is connected to the robot via a passive rotational joint. This mechanism enables the mobile robot to search the environmental parameters. A new mobile robot fitted with the proposed mechanism, named ARODA, was developed. In a validation experiment, the developed mobile robot successfully tilted a relatively large and heavy object while searching the environmental parameters (the frictional coefficients of the floor and object and the size of the object). Equipped with the proposed mechanism, the mobile robot refrained from falling while applying a large force to the object by trial and error.

Source separation and localization of individual superficial forearm extensor muscles using high-density surface electromyography

The limitations of conventional surface electromyography (sEMG) cause it to be unsuitable for use with the deep and compact muscles of the forearm. However, while source separation and localization techniques have been extensively explored to identify active sources in the brain using electroencephalography (EEG) signals, these techniques have not been adapted for identifying active sources in muscles using sEMG signals, despite being of a similar premise. Here, we perform an experiment to explore the capabilities of conventional EEG single-dipole localization techniques to localize the extensor digitorum and extensor indicis when selectively activated. The localization methodology consists of separating the raw sEMG signals using independent component analysis (ICA), estimating a physics-based forward model, and then correlating the obtained lead-field matrix with the ICA mixing matrix. The results show that single-dipole localization is not suitable for describing the active sources of muscles.

Proposal of a Stance Postural Control Model with Vestibular and Proprioceptive Somatosensory Sensory Input

Maintenance of upright stance is one of the basic requirements in human daily life. Stance postural control is achieved based on multisensory inputs such as visual, vestibular and proprioceptive somatosensory inputs. In this paper, we proposed a stance postural control model including a neural controller with feed-forward inputs (muscle stiffness regulation) and sensory feedback of vestibular and proprioceptive somatosensory sensation. Through the optimization, variables of neural controller were designed to keep a musculoskeletal model standing during a 5 s forward dynamics simulation. From the results, we found that when both vestibular and proprioceptive somatosensory sensory input are available, low muscle stiffness is enough to maintain the balance of a musculoskeletal model in a stance posture. However, when vestibular sensory input get lost, higher muscle stiffness will be desired to keep the musculoskeletal model standing.

Mechanism Allowing a Mobile Robot to Apply a Large Force to the Environment

In this study, we investigated a mechanism that allows a mobile robot to apply a large force to the environment. We first investigated the limits on the force that a mobile robot can apply to a target object by analyzing the forces between the robot, ground, and object and the limits on the frictional forces between them. To prevent the mobile robot from falling when applying a large force, we developed a prototype in which the manipulator was connected via a passive rotational joint. We investigated the pushing capacity of the prototype robot through an experiment in which it tilted a large object. The results confirmed that the mechanism allows a mobile robot to apply a large force to an object without falling by trial and error.