Useok Jeong

Kinematic Condition for Maximizing the Thrust of a Robotic Fish Using a Compliant Caudal Fin

The compliance of a fin affects the thrust of underwater vehicles mimicking the undulatory motion of fish. Determining the optimal compliance of a fin to maximize thrust is an important issue in designing robotic fish using a compliant fin. We present a simple method to identify the condition for maximizing the thrust generated by a compliant fin propulsion system. When a fin oscillates in a sinusoidal manner, it also bends in a sinusoidal manner. We focus on a particular kinematic parameter of this motion: the phase difference between the sinusoidal motion of the driving angle and the fin-bending angle. By observing the relationship between the thrust and phase difference, we conclude that while satisfying the zero velocity condition, the maximum thrust is obtained when a compliance creates a phase difference of approximately π/2 at a certain undulation frequency. This half-pi phase delay condition is supported by thrust measurements from different compliant fins (four caudal-shaped fins with different aspect ratios) and a beam bending model of the compliant fin. This condition can be used as a guideline to select the proper compliance of a fin when designing a robotic fish.

Development of a polymer-based tendon-driven wearable robotic hand

This paper presents the development of a polymer-based tendon-driven wearable robotic hand, Exo-Glove Poly. Unlike the previously developed Exo-Glove, a fabric-based tendon-driven wearable robotic hand, Exo-Glove Poly was developed using silicone to allow for sanitization between users in multiple-user environments such as hospitals. Exo-Glove Poly was developed to use two motors, one for the thumb and the other for the index/middle finger, and an under-actuation mechanism to grasp various objects. In order to realize Exo-Glove Poly, design features and fabrication processes were developed to permit adjustment to different hand sizes, to protect users from injury, to enable ventilation, and to embed Teflon tubes for the wire paths. The mechanical properties of Exo-Glove Poly were verified with a healthy subject through a wrap grasp experiment using a mat-type pressure sensor and an under-actuation performance experiment with a specialized test set-up. Finally, performance of the Exo-Glove Poly for grasping various shapes of object was verified, including objects needing under-actuation.

Implementation of various control algorithms for hand rehabilitation exercise using wearable robotic hand

In this paper, the control algorithms for strength exercise using wearable robotic hand are reviewed and the experimental results are analyzed and discussed. The SNU Exo-Glove is a soft exoskeleton that actuates motor function in disabled hands. This new type of device comprises a jointless simple mechanical structure and is actuated with wires. The strength exercise algorithms include isotonic, isokinetic, and impedance control exercises. An electromyography (EMG) regulation algorithm is proposed to limit the maximum level of activation of the muscles to prevent injury of the muscles and joints. The tension of the wire and the sEMG signal are analyzed to validate the effectiveness of rehabilitation with SNU Exo-Glove.

Feasibility study of a slack enabling actuator for actuating tendon-driven soft wearable robot without pretension

A soft wearable robot with a tendon drive is a promising technology that enables a wearable robot to be compact and lightweight. A soft tendon routing system was previously proposed to apply a tendon drive to a soft wearable robot. In this study, a slack enabling mechanism was proposed to increase the efficiency and guarantee the safety of the soft tendon routing system. The proposed mechanism eliminates the pre-tension of the tendons and minimizes the friction induced by the pre-tension, which causes inefficiency and a lack of safety. Furthermore, the slack enabling mechanism mechanically prevents the derailing of the tendon from the spool. In order to verify the benefits of the proposed mechanism, a prototype was built and tested on the Exo-Glove, which is a soft wearable robot for the hand. The experiment results showed that the prototype could completely remove the pre-tension, whichproposed to apply a tendon drive to a soft wearable robot. In this study, a slack enabling mechanism was proposed to increase the efficiency and guarantee the safety of the soft tendon routing system. The proposed mechanism eliminates the pre-tension of the tendons and minimizes the friction induced by the pre-tension, which causes inefficiency and a lack of safety. Furthermore, the slack enabling mechanism mechanically prevents the derailing of the tendon from the spool. In order to verify the benefits of the proposed mechanism, a prototype was built and tested on the Exo-Glove, which is a soft wearable robot for the hand. The experiment results showed that the prototype could completely remove the pre-tension, which allowed the Exo-Glove to function well with the prototype.

Investigation on the control strategy of soft wearable robotic hand with slack enabling tendon actuator

A soft wearable robot, which is an emerging type of wearable robot, can take advantage of tendon-driven mechanisms with a Bowden cable. These tendon-driven mechanisms benefits soft wearable robots because the actuator can be remotely placed and the transmission is very compact. However, it is difficult to compensate the friction along the Bowden cable which makes it hard to control. This study proposes the use of a position-based impedance controller, which is robust to the nonlinear dynamics of the system and provides compliant interaction between robot, human, and environment. Additionally, to eliminate disturbances from unexpected tension of the antagonistic wire arising from friction, this study proposes a new type of slack enabling tendon actuator. It can eliminate friction force along the antagonistic wire by actively pushing the wire while preventing derailment of the wire from the spool.

The effect of compliant joint and caudal fin in thrust generation for robotic fish

Fish generates large thrust through an oscillating motion with a fin. It is assumed that the flexibility of a fin affects the thrust generated by the fish. However, detailed investigation on the relationship between the flexibility of the fin and thrust generation is lacking. In this paper, the driving mechanism of a robotic fish is implemented using a compliant joint and caudal fin that is adapted from fish. The effect of the passive mechanism on the thrust generation with changes in the stiffness and frequency is investigated with this driving mechanism. The present research is carried out to understand the relationship among the compliance of the joint and caudal fin, the frequency of oscillating motion and the thrust which is generated by oscillating motion. The thrust is measured using a force transducer while varying the frequency and compliance. The bending angles between the compliant joint and caudal fin are also compared with the changes of the thrust in one cycle. The results show the appropriate stiffness of the compliant mechanism can be found in order to generate maximum thrust under the condition that the other parameters are not varied.

Feedforward friction compensation of Bowden-cable transmission via loop routing

Friction along the Bowden-cable transmission degenerates control performance unless it is properly compensated. Friction is produced when the bending angle of the Bowden-cable changes as the relative position of the actuator and the end-effector changes. This study proposes a method, termed loop routing, to compensate friction along the Bowden-cable. Loop routing involves making a one-round loop along the sheath that continuously maintains the sheaths bending angle at 2π regardless of the end-effectors position in 2-D space. This minimizes the bending angle change of the sheath as the end-effector translates in a 3-D workspace, which minimizes the friction change and enables feedforward friction compensation of the Bowden-cable without employing a sensor. An experiment in open-loop tension control of the Bowden-cable was conducted to evaluate the performance of the proposed method. Results show that the output tension follows the reference tension well, with an RMS error of 4.3% and a peak error of 13.3% of maximum reference.

A Novel Low-Cost, Large Curvature Bend Sensor Based on a Bowden-Cable

Bend sensors have been developed based on conductive ink, optical fiber, and electronic textiles. Each type has advantages and disadvantages in terms of performance, ease of use, and cost. This study proposes a new and low-cost bend sensor that can measure a wide range of accumulated bend angles with large curvatures. This bend sensor utilizes a Bowden-cable, which consists of a coil sheath and an inner wire. Displacement changes of the Bowden-cable’s inner wire, when the shape of the sheath changes, have been considered to be a position error in previous studies. However, this study takes advantage of this position error to detect the bend angle of the sheath. The bend angle of the sensor can be calculated from the displacement measurement of the sensing wire using a Hall-effect sensor or a potentiometer. Simulations and experiments have shown that the accumulated bend angle of the sensor is linearly related to the sensor signal, with an R-square value up to 0.9969 and a root mean square error of 2% of the full sensing range. The proposed sensor is not affected by a bend curvature of up to 80.0 m−1, unlike previous bend sensors. The proposed sensor is expected to be useful for various applications, including motion capture devices, wearable robots, surgical devices, or generally any device that requires an affordable and low-cost bend sensor.

A Novel Slack-Enabling Tendon Drive That Improves Efficiency, Size, and Safety in Soft Wearable Robots

Tendon drives are widely used in robotics. The compliance of the tendon in such drives suits them for soft robots, including soft wearable robots, but several issues impede their use. Generally, the tendon should always maintain tension to prevent derailment from the spool. However, in soft robots, tendon tension induces high friction forces owing to the absence of ball bearings. Because the kinematics of the soft robot is basically nonlinear and changed by the deformation of the structure, the kinematic difference between the soft structure and the spool causes derailment of the tendon. Moreover, continuously maintained tension in soft wearable robots causes safety issues. The linear actuator can be an option. However, the need to increase the length of the linear actuator to accommodate the excursion length of its tendon is a barrier to its use in small-scale applications. To preclude this issue, a slack-enabling actuator that employs a spool is proposed. The space efficiency of the spool enables the mechanism to be small, and a one-way clutch applies unidirectional friction force to the tendon to tighten the tendon around the spool. This paper describes the design concept for the slack-enabling mechanism, its design optimization, and system identification for force control.

An application of user-friendly control for a Respiratory Rehabilitation and Assistance Robot

To treat patients suffering from dysfunction of the abdominal muscles, positive pressure devices that can inject air directly into a patients mouth have been developed. But injection of air requires blocking of a patients mouth with an air mask or air hose, which causes the patient additional inconvenience in talking and in breathing with their own mouth. The Respiratory Rehabilitation and Assistance Robot was developed to provide ventilation assistance without hindering patients from talking and breathing with their own mouth. The basic principle of the robot is a negative pressure assistance, using an abdomen pressuring part to push the abdomen of the user and help the user to exhale. The robot is mainly manipulated with an attached joystick. But without an additional control routine to govern the amount of force applied when the joystick is moved, usability and safety issues occur. In this research, linear, logarithmic, and exponential profiles that modify the joystick trajectory were tested to advance maneuverability of the robot conditioning the joystick signal. Through experiments, we found that logarithmic profiles with certain shape factors improve both usability and safety.