Yong-Jai Park

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.

Design of a variable-stiffness flapping mechanism for maximizing the thrust of a bio-inspired underwater robot

Compliance can increase the thrust generated by the fin of a bio-inspired underwater vehicle. To improve the performance of a compliant fin, the compliance should change with the operating conditions; a fin should become stiffer as the oscillating frequency increases. This paper presents a novel variable-stiffness flapping (VaSF) mechanism that can change its stiffness to maximize the thrust of a bio-inspired underwater robot. The mechanism is designed on the basis of an endoskeleton structure, composed of compliant and rigid segments alternately connected in series. To determine the attachment point of tendons, the anatomy of a dolphins fluke is considered. Two tendons run through the mechanism to adjust the stiffness. The fluke becomes stiffer when the tendons are pulled to compress the structure. The thrust generated by a prototype mechanism is measured under different conditions to show that the thrust can be maximized by changing the stiffness. The thrust of the VaSF device can approximately triple at a certain frequency just by changing the stiffness. This VaSF mechanism can be used to improve the efficiency of a bio-inspired underwater robot that uses compliance.

Design and Manufacturing a Bio-inspired Variable Stiffness Mechanism in a Robotic Dolphin

To maximize the dynamic performance; especially swimming; of a robotic fish or an underwater vehicle, lots of research of mechanisms and actuators have been conducted. Among them, the compliance of the structure can help the robotic fish or the underwater robots to increase the thrust. Also, when oscillating frequency of a propulsion system increases, the stiffness of the caudal fin should increase to increase the thrust of the robotic fish. Therefore, the variable stiffness mechanism is needed to maximize the thrust of the robotic fish. In this paper, we present a design and manufacturing procedure using our bio-inspired variable stiffness mechanism which was developed before. We find the appropriate design of the bio-inspired variable stiffness mechanism for applying to a robotic dolphin. The novel variable stiffness mechanism was designed using the inspiration of anatomy of a fluke. Tendons which are used for changing the stiffness are arranged from dolphins anatomy. This design and manufacturing procedure can be helpful to some researchers who try to apply the variable stiffness mechanism using flexible material.

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.

Stabilizing the head motion of a robotic dolphin with varying the stiffness of a caudal fin

Various oscillating parameters have been used to increase the velocity of a robotic dolphin. Among them, an efficient and easy way to increase the thrust is to increase an oscillating frequency of the robotic dolphin. However, there is a trade-off. If the oscillating frequency increases, the stability of the robotic dolphin decreases. Especially, a reactive motion is occurred in a head of the robotic dolphin due to the fluctuating motion of a caudal fin. The unsteady motion in the head position can be controlled using an additional actuator. However, the best solution is to reduce an undesired motion without adding the actuator. In this paper, to reduce the fluctuating problem, we consider the flexibility of the caudal fin. An oscillating amplitude of the caudal fin and head motion is changed with varying the stiffness of the caudal fin. Therefore, if we find the appropriate stiffness of the caudal fin, the head motion can be stabilized.

Dual-stiffness structures with reconfiguring mechanism: Design and investigation

To improve the multi-functionality of a structure, a foldable or deployable structure with variable stiffness is needed. This article presents dual-stiffness structures with two stiffness states: a stiff state and a flexible state for a multi-mission capability. This dual-stiffness structure is based on a hybrid structure that combines rigid and flexible segments; when the rigid segments are rearranged, the bending motion of the compliant material is constrained by the rigid segments, which varies the stiffness of the structure. Instead of continuously changing the stiffness, the dual-stiffness structure abruptly changes the stiffness state with a simple reconfiguring mechanism. We developed two reconfiguring mechanisms: a sliding mechanism and a folding mechanism. Using a layering process, the dual-stiffness structure with a two-dimensional multi-layer design was manufactured. To verify the behavior of the structure, a simplified structure with no sliding mechanism was designed and simulated using a finite element method. The ratio of the length of a rigid segment to the length of a compliant segment determined the stiffness of the structure. This dual-stiffness structure with the reconfiguring mechanism can be effective for applications that require a big change in stiffness, such as for a deployable solar panel, or flexible display.

Design Concept of Hybrid Instrument for Laparoscopic Surgery and Its Verification Using Scale Model Test

This paper proposes a new design concept of hybrid instrument for single-port laparoscopic surgery (SPLS) and a new method of verification using a scaled-up prototype based on the principle of elastic similarity. The proposed concept is a hand-held instrument that uses a tendon-gear mechanism for dexterous movement of its end-effector and servomotors with flexible tendon-sheath transmission to maintain the dexterity by compensating the loss of output angle from tendon elongation during the manual operation. The kinematic relationship of the tendon-gear mechanism was derived mathematically, and the ratio of external moment to resistive flexural stiffness of the articulating joint was matched between the real-sized model and the large-scale prototype. Our scale model tests have shown good agreement between their input-output relationships under the equivalent loading conditions, and thus verified the validity of similarity analysis. Also, the proof-of-concept experiments have demonstrated the functionality of output loss compensation of the hybrid instrument. Our methodology can be used to simplify and speed up the prototype development process for SPLS by avoiding miniaturization challenges such as high precision manufacturing, which is costly and time-consuming.

Design and manufacturing a robotic dolphin to increase dynamic performance

Robotic fish and dolphin have been studied to increase their dynamic performance, especially the velocity of a robotic dolphin. There are lots of parameters to increase the velocity of the robotic dolphin such as increasing an oscillating frequency, the area of a caudal fin, or the oscillatory amplitude and so on. The efficient and easy way to increase the thrust is to use a compliant caudal fin. Using the flexible caudal fin, the velocity of the robotic dolphin can increase. Furthermore, using a novel variable stiffness mechanism, the stiffness of the caudal fin can be varied depending on an oscillating frequency to maximize the thrust. We introduce the design and manufacturing of a robotic dolphin which has a variable stiffness mechanism.

Maximum Thrust Condition by Compliant Joint of a Caudal Fin for Developing a Robotic Fish

Fish generates large thrust through an oscillating motion with a compliant joint of caudal fin. The compliance of caudal fin affects the thrust generated by the fish. Due to the flexibility of the fish, the fish can generate a travelling wave motion which is known to increase the efficiency of the fish. However, a detailed research on the relationship between the flexible joint and the thrust generation is needed. In this paper, the compliant joint of a caudal fin is implemented in the driving mechanism of a robotic fish. By varying the driving frequency and stiffness of the compliant joint, the relationship between the thrust generation and the stiffness of the flexible joint is investigated. In general, as the frequency increases, the thrust increases. When higher driving frequency is applied, higher stiffness of the flexible joint is needed to maximize the thrust. The bending angles between the compliant joint and the caudal fin are compared with the changes of the thrust in one cycle. This result can be used to design the robotic fish which can be operated at the maximum thrust condition using the appropriate stiffness of the compliant joint.