Soft robotic fingers can safely grasp fragile or variable form objects, but their force capacity is limited, especially with less contact area: precision grasps and when objects are smaller or not spherical. Although most research focuses on improving force capacity through mechanical design modifications, optimizing grasping parameters for precision tasks remains crucial. To address this problem, this paper proposes an analytical rotational stability model for soft fingers’ precision grasping, considering grip failure involving slip and dynamic rotational stability. Comprehensive experiments across various objects, grip condition and types of fingers (PneuNet and commercial fingers) are conducted by examining the relationship between grasp parameters and model variables including coulomb friction, bulk stiffness, and dynamic stability. The findings demonstrate the model’s utility in identifying optimal grip parameters that enhance the force capacity of soft fingers without causing dynamic instability. This research contributes to the development of more effective and stable soft robotic fingers for precision grasping tasks.

Recognizing dynamic gestures from a continuous data stream has been treated as a difficult task. One challenge is the gesture spotting problem, or segmentation of a gesture pattern from a data stream containing consecutive gestures. Although various types of gesture spotting strategies have been introduced so far, all methods has its own limitations to be applied to real world situations such as sign language conversation. One of the conditions that must be satisfied for practical applications is to detect repetitive gestures for emphasizing a certain meaning.In the view of gesture spotting, repetitive gestures are hard to be detected because similar starting and ending moments are repeated without constraints of the number of repetitions. We introduce a data glove-based real-time dynamic gesture recognition system that covers both repetitive and nonrepetitive gestures. The proposed system consists of three steps: Gesture spotting, gesture sequence compression, and gesture recognition. For gesture spotting, we define a quantity termed as a gesture progress sequence, which quantifies a progress of a gesture using numbers from 0 to 1. Gesture sequence compression removes variations including the number of repetitions that seek to impart the same message. At the gesture recognition step, the compressed gesture patterns are classified. The proposed system was evaluated using 17 American sign language (ASL) gestures and some ASL sentences.

Most of the wearable hand kinesthetic (force) feedback systems have only been evaluated from a physical perspective, i.e., motion measurement, force transmission, system stability, etc. However, by taking this approach, we cannot gather sufficient data to evaluate the performance of wearable kinesthetic feedback systems as haptic devices. In this paper, we evaluated the wearable hand kinesthetic feedback system developed in our laboratory from psychophysical and user experience (UX) perspectives. We designed the experiments considering the characteristics of our wearable hand system. For psychophysical evaluation, ten subjects participated in force detection, discrimination, and identification experiments. The force detection threshold of our system was better than those of the commercialized haptic systems, and discrimination threshold was similar with the human perception threshold. In the identification experiment, the subjects could distinguish the size and shape of virtual objects over 2.5 levels. For UX evaluation, 15 subjects answered a questionnaire after conducting a task using our wearable hand system in laboratory environment. The UX results verified that the subjects were satisfied with our system by giving a score of 5.87 out of 7 points. It also concluded that our system can provide a good VR experience to users by achieving over 5.5 points in ease of use, learnability, and utilitarian value.

A sensing glove is an attractive application of wearable devices. Soft sensors are emerging to replace rigid sensing units, especially for wearable sensor systems, due to its inherent softness, flexibility, and stretchability. However, the fabrication process for the soft sensors is usually complex, time-consuming, labor-intensive, and has low production rate. To integrate a sensor system, an assembly process is essential, which may make the system bulky. Moreover, a solution for the electrode parts has rarely been suggested, although a bulky electrode part may obstruct the user's movement and degrade performance of the sensor. Thus, in this study, a novel fabrication process is suggested based on direct ink writing (DIW) of eutectic gallium-indium (EGaIn), which forms all the items in the sensor system from the sensing units, wiring, and the electrode part. A sensing glove for 2D finger motions was fabricated, and its performance was verified in terms of linearity, dynamic response, and accuracy. The sensing glove can be used as an easily-wearable and an intuitive interface to the virtual reality environment.

Sensed information greatly helps a robot to adjust its motion or modulate the locomotory behavior. While many sensing components have been developed for macroscale robots, such off-the-shelf sensors are hardly integrated with a mesoscale (i.e., 0.1 mm to 10 mm) robot due to the size limitation. In this work, we propose a Compliant Mechanosensory Composite (CMC) to fabricate a small compliant mechanism with embedded sensing ability. As the first demonstration of CMC, we directly print a conductive polymer PEDOT:PSS onto the flexible joint of a compliant mechanism to sense the motion of the flexible joint itself. Owing to the variation of electric contact resistance (ECR) upon bending, the CMC could estimate its bending angle. The performance of the CMC was verified by analyzing the cyclic bending, transient and stationary response. Overall, a sparsely printed serpentine pattern with thicker line exhibited consistent response without a noticeable hysteresis. To demonstrate the applicability of the CMC process, a small gripper actuated by a SMA (shape memory alloy) coil was fabricated, and its motion was successfully measured using the embedded sensors. We expect the proposed CMC will enable a small robot to become sensible at its self motion, external load, and physical contacts in future.

For decades, various robotic grippers have been developed due to its necessity for the robotic manipulators. In case of the conventional robotic grippers with rigid components, an underactuated mechanism was required to satisfy gripping motion. Recently, soft grippers have been studied actively, which have realize bending motion with a simple morphological structure itself and inherent compliance to the environment. In this field of study, it has been rarely investigated to improve the fingertip force and actuation speed with specified design parameters. Thus, in this study, a hybrid gripper, which consists of both soft and rigid components, was suggested based on the key design principles: 1) the ratio of rigid parts against the soft chamber, 2) the cross-sectional shape of the chamber. The suggested principles were verified using the finite element methods (FEMs). As a result, the improved performance of the hybrid gripper was verified in terms of the fingertip force and the actuation speed, compared with the performance of the previously developed soft pneumatic actuators (SPAs). As an application, the three-fingered gripper was manufactured and tested by grasping different types of objects.

Locomotion of water beetles have been widely studied in biology owing to their remarkable swimming skills. Here, we investigated the coordination between the two pairs of legs to achieve steady swimming with novel hair-like appendages. Some design considerations and a fabrication of the hair-like appendages, which can passively adjust their projected area to obtain net thrust, are proposed. As do water beetles in nature, the coordination between the two pairs of legs were considered to achieve steady swimming without jerky motion by varying the beating frequency and phase of the legs. To verify the functionality of the hair-like appendages and their coordinations, six different types of appendages were fabricated, and two robots (one with single pair and the other with two pairs of legs) were built. Locomotion of the robots were compared through experiments, and steady swimming was achievable by properly coordinating the two-pairs of legs without sacrificing the swmming speed.

A Marangoni propulsion is exhibited by some aquatic insects such as rove beetles as a means of rapid escaping by secreting surfactant that lowers the surface tension behind them. Inspired by their locomotion, we proposed a non-tethered miniature robot that can skim and steer on the water surface without employing oscillatory movements of limbs. To make a robot afloat on water surface using the surface tension while attaining higher support force, a circular footpad with an equilateral triangular cross section was used. Other types of cross section shapes such as a circle and a square were also considered; however, their maximal support force obtained from numerical solution was less than an equilateral triangle. The robot had four circular footpads, which coated with superhydrophobic spray, and weighed 18.5 gram including on-board electronics, a battery, and two servo motors. During the experiment, the robot could successfully skim over the water surface by dripping methanol behind and change its moving direction by lowering a small oar either left or right side.

In this paper, a wearable hand system for virtual reality is proposed. The proposed system can deliver accurate force to three fingertips (thumb, index, and middle fingers), which play dominant roles in manipulating and interacting with the external environment, and also measure fingertip positions of the three fingers. The proposed system has five degrees of freedom (DOFs) for each finger structure to permit natural finger motion, can be easily worn by clicking the silicon-coated fingertip structures and buckling the wearable structure around the palm. The forces delivered to the fingertips are controlled by the motor current with the robust control algorithm, and the fingertip position is calculated by forward kinematics. The performance of the proposed system was verified experimentally; the calculated fingertip position was compared to a camera-based motion capture system and interactions with objects of different stiffness were tested.

In this paper, an asymmetric cable-driven mechanism is proposed for accurate force control of exoskeleton systems with a compact structure. Inspired by the fact that the required forces in human motions are not symmetric in many cases, a spring-actuator type cable-drive mechanism is adopted, which enables a compact cable routing structure. The joint is connected with the exoskeleton frame through a rotary series elastic mechanism to transmit the desired force to the human user. High performance in force control is achieved by advanced control algorithms, which combine a proportional and differential (PD) controller optimized with a linear quadratic (LQ) method with a disturbance observer (DOB) and a zero phase error tracking (ZPET) feedforward filter. The proposed system was tested for the elbow joint. Experimental results confirmed that the proposed system was able to generate and deliver accurate force to the human user even with external disturbances and modeling uncertainties introduced by human motions.

For physical human-robot interaction (pHRI), it has been an important issue to control the output force of actuators. The aim of this study was to apply a new control strategy, named model-inverse time delay control (MiTDC), to series elastic actuators (SEA) in a lower extremity exoskeleton, even in the presence of uncertainties from pHRI. The law for time delay control (TDC) is derived and implementation issues are discussed, including the design of the state observer and the selection of the nominal value of the control distribution coefficient. Additionally, a new concept, a new reference position using the inverse of model dynamics, is introduced to realize satisfactory tracking performance without delay. Experimental results showed that the suggested controller achieved satisfactory performance without accurate information on system parameters, requiring only a nominal value of the control distribution coefficient.

A water beetle is skillful at drag-powered swimming by using its oar-like legs. Inspired by this mechanism, a miniature robot, whose mobility is obtained by a pair of legs attached with swimming appendages, was studied in this work. The robot in this paper has optimally designed linkage structures to maximize the sweep angle, which is actuated by a single DC motor with series of gears and a spring. A simplified swimming appendage model was proposed to calculate deflection by drag force applied. Also, each of swimming appendage was optimized individually by considering the attached locations on the legs using the two fitness functions, and three different configurations were selected. In the experiments, the performance of the proposed robots was verified using a high-speed camera, and motion capture cameras. The robot with the proposed configuration showed the fastest movement comparing with other robots.

In this paper, a hand exoskeleton system for virtual reality is proposed. As a virtual reality interface for the hand, a wearable system should be able to measure finger joint angles and apply force feedback to the fingers with a simple and light design. In the proposed system, the finger joint angles are measured by a tendon-inspired cable mechanism. Also, another cable is used for force feedback to the finger. Using the measured finger joint angles and motor currents, the cable-driven actuation system applies desired force to the fingers. That is, when the desired force is zero, the motor position is controlled to track the changed cable length; when the desired force needs to be applied, the motor current is controlled to generate the desired force. For a smooth transition between two control strategies, linearly changing proportions of each control strategy is applied in the transition range. A prototype of the proposed system was manufactured, and the proposed control algorithms were verified by experiments.

In this paper, a wearable hand exoskeleton with force-controllable and compact actuator modules is proposed. In order to apply force feedback accurately while allowing natural finger motions, the exoskeleton linkage structure with three degrees of freedom (DOFs) was designed, which was inspired by the muscular skeletal structure of the finger. As an actuating system, a series elastic actuator (SEA) mechanism, which consisted of a small linear motor, a manually designed motor driver, a spring and potentiometers, was applied. The friction of the motor was identified and compensated for obtaining a linearized model of the actuating system. Using a LQ (linear quadratic) tuned PD (proportional and derivative) controller and a disturbance observer (DOB), the proposed actuator module could generate the desired force accurately with actual finger movements. By integrating together the proposed exoskeleton structure, actuator modules and control algorithms, a wearable hand exoskeleton with force-controllable and compact actuator modules was developed to deliver accurate force to the fingertips for flexion/extension motions.

In this paper, a rotational leg-type miniature robot with an actuated middle joint and tail, called RoMiRAMT, is proposed for stable locomotion and improved climbing ability. The robot has four independently actuated rotational legs, giving it advantages of both wheel-type and leg-type locomotion. The design parameters of the rotational legs were determined by 3D simulation within the properly selected candidates. It also has unique characteristics inspired by biological structures: a spine and a tail. An actuated middle joint allows the frontal body to be lifted or lowered, which was inspired by a spine, to climb higher obstacles. Additionally, a multi-functional two degrees of freedom (2-DOF) tail was added; the tail provides instant deceleration, and minimized the likelihood of flipping, while allowing the robot to climb higher obstacles. A microcontroller was embedded in the robot, along with a micro-camera and an inertia measurement unit (IMU) sensor. By controlling the robot using the yaw angle signal measured by the IMU sensor, straight movement was enhanced. The body size of the RoMiRAMT is 155 × 80 mm without the rotational legs, and the weight is 593 g including batteries. The maximum velocity of the robot was 2.58 m/s (16.65 body lengths per second) and the maximum height of an obstacle that the robot can climb was 95 mm, which all were verified by experiments.

In this paper, a wearable sensing glove for measuring the motion of the fingers is proposed. The system consists of linear potentiometers, flexible wires, and linear springs, which makes it compact and lightweight so that it does not interfere with the natural motion of the fingers. Inspired by the way wrinkles on finger joints are smoothed out when the finger is flexed, a flexible wire is attached to the back of each finger. As the flexible wire moves due to the motion of the finger, the joint angles are calculated by measuring the change in length of wire. Linear potentiometers with linear springs were used to maintain the tension of the wires in order to measure the wire length change consistently. Because the motion of the proximal interphalangeal (PIP) joint is dependent on that of the distal interphalangeal (DIP) joint, only two linear potentiometers were used for each finger. A compact sensing module including 10 linear potentiometers and springs was attached to a glove. The proposed system can widely be applied for the systems, which require to measure finger motions accurately, e.g., virtual reality or teleoperation systems. Such feasible applications were actually implemented and introduced in this paper.

In this paper, a network-based rehabilitation system is proposed to increase mobility of a rehabilitation system and to enable tele-rehabilitation. Control algorithms and rehabilitation strategies distributed at the central location (physical therapist) and the local site (patient) communicate over wireless network to realize a network-based rehabilitation system. To deal with possible packet losses over wireless network, a modified linear quadratic Gaussian (LQG) controller and a disturbance observer (DOB) are applied. The performance of the proposed system and control algorithms is verified by simulation and experiment with an actual knee rehabilitation system. The simulation and experiment results show that the network-based rehabilitation system with the proposed control schemes can generate the desired assistive torque accurately in presence of packet losses.

Precise and large torque generation, back-drivability, low output impedance, and compactness of hardware are important requirements for human assistive robots. In this paper, a compact rotary series elastic actuator (cRSEA) is designed considering these requirements. To magnify the torque generated by an electric motor in the limited space of the compact device, a worm gear is utilized. However, the actual torque amplification ratio provided by the worm gear is different from the nominal speed reduction ratio due to friction, which makes the controller design challenging. In this paper, the friction effect is considered in the model of cRSEA, and a robust control algorithm is designed to precisely control the torque output in the presence of nonlinearities such as the friction. The mechanical design and dynamic model of the proposed device and the design of a robust control algorithm are discussed, and actuation performance is verified by experiments.