Gangfeng Liu

A miniature surface tension-driven robot using spatially elliptical moving legs to mimic a water strider’s locomotion

The highly agile and efficient water-surface locomotion of the water strider has stimulated substantial interest in biomimetic research. In this paper, we propose a new miniature surface tension-driven robot inspired by the water strider. A key feature of this robot is that its actuating leg possesses an ellipse-like spatial trajectory similar to that of a water strider by using a cam-link mechanism. Simplified models are presented to discuss the leg-water interactions as well as critical conditions for a leg penetrating the water surface, and simulations are performed on the robots dynamic properties. The final fabricated robot weighs about 3.9 g, and can freely and stably walk on water at different gaits. The maximum forward and turning speeds of the robot are measured as 16 cm s(-1) and 23°/s, respectively. Furthermore, a similarity analysis with Bond number and Weber number demonstrates that the locomotion of this robot is quite analogous to that of a real water strider: the surface tension force dominates the lifting force and plays a major role in the propulsion force. This miniature surface tension-driven robot might have potential applications in many areas such as water quality monitoring and aquatic search and rescue.

A water-walking robot mimicking the jumping abilities of water striders

The highly efficient and agile water-surface locomotion of water striders has attracted substantial research attention. Compared with imitating the horizontal rowing motion, imitating the jumping capability of water striders is much more challenging because the strong interaction in the jumping process easily causes the robot to sink. This study focuses on designing a miniature robot capable of continuously jumping on the water surface. A spring-based actuating mechanism is proposed to produce a large jumping force. The center of gravity of the robot is carefully designed to allow the robot to jump on the surface continuously and smoothly. The influences of several critical factors, including the area of the supporting legs, the spring stiffness, the jumping angle, etc on jumping ability are analyzed by means of dynamic simulation and experiments. The jumping performance under different jumping angles is tested. The fabricated robot weighs approximately 10.2 g and can continuously jump on water with a maximum leap height and length of 120 and 410 mm, respectively. This study helps researchers understand the jumping mechanism of water striders and provides a reference for developing water-jumping robots that can perform various aquatic tasks in the future.

A miniature surface tension-driven robot mimicking the water-surface locomotion of water strider

Aiming at mimicking water striders water-surface locomotion, this study proposes a new miniature surface tension-driven robot. A key feature of this robot is that its actuating legs possess ellipse-like spatial trajectories like water strider by using a cam-link mechanism, and never pierces water surface when rowing. A set of simple models and equations are proposed to analyze the interaction forces between leg and water as well as the critical condition for a leg penetrating a water surface. The final fabricated robot weights about 3.9 g with a load capacity of 5.6 g. By controlling the motions of actuating legs, the robot can freely and stably walk on water with different gaits. The maximum forward and turning speeds of the robot are measured as 16 cm/s and 23 °/s, respectively. Moreover, a similarity analysis with Bond Number and Weber Number reveals that the locomotion of this robot is quite analogous to that of a water strider: surface tension force dominates the lifting force and plays a major role in the propulsion.

On the utility of leg distal compliance for buffering landing impact of legged robots

Many legged robots have compliant mechanisms in the distal segments of their legs called distal compliance. One important function of such characteristic is to buffer landing impact at touchdown. However, there is still no general design strategy for it. In particular, nonlinear compliance behaviors are supposed to be more beneficial than linear ones, yet it is open what type of nonlinearity is a good fit. From this perspective, we used a simple spring–mass model performing free drop to investigate the design principles of distal compliance. The model includes damping and preload in spring and realistic limitations on spring compression, therefore gives a straightforward correspondence with actual hardware systems. We confirmed the benefits of using distal compliance over purely stiff structures, in terms of landing impact buffering. By assessing the relative influences of a variety of compliance configurations through numerical simulations, we found that for compliance behaviors of the same average stiffness, nonlinearities had little effect on the impact magnitude (<1 N), but stiffening compliance behaviors were able to provide better buffering performance by extending the impact time. It was also noticed that stiffening compliance behaviors were inevitably accompanied by a larger amplitude of spring compression, indicating that necessary trade-off has to be made for those systems concerning torso stationarity. The experimental data with our hexapod robotic platform agreed well with the results found with the proposed model, confirming that the spring–mass model could be a template to provide a useful guide for the design of distal compliance in legged robots.

Space calibration of the cranial and maxillofacial robotic system in surgery

Abstract Space registration in cranial and maxillofacial surgery is intended to map the image space to the robot space. This requires calibration of multiple coordinate systems. In this process, the calibration accuracy between the robot coordinate system and the NDI vision coordinate system directly determines the precision of the surgical navigation system, which is the key to success. In this paper, the relationship between robot space and visual space is studied according to the requirements of surgery, and with reference to the characteristics of the vision system itself. Based on this analysis and traditional methods, a new linear rotation calibration method is presented. Calibration can be automated to decrease human error and increase the reliability and stability. Finally, an experiment is conducted in order to evaluate the effectiveness of the calibration algorithm. The results show that the minimum position error was less than 0.87 mm and the minimum posture deviation was about 0.83 degrees, indicating that the calibration precision can meet the operation requirements. There are good prospects for this method using in surgical calibration application.

A Wheeling-Hopping Combination Scout Robot

A hopping robot can jump over the barrier several times higher than its own height. The combination of the hopping movement and the wheeling movement can greatly enhance the scope of robots activities. In this paper, a novel five-shank hopping mechanism was employed to build the wheeling-hopping combination scout robot. The nonlinear character of the five-shank hopping mechanism was analyzed and then used in the proposed nonlinear spring-mass model for the robot. The rules of robots movement were deduced, influencing factors of the jumping height were analyzed and the countermeasure was adopted. Finally, a simulation analysis and an experiment of the robots movement were carried out. The results showed that the robot has strong locomotivity and survival ability.

Optimal design of a Stewart platform using the global transmission index under determinate constraint of workspace

The Stewart platform is a typical parallel manipulator. It is used as a space docking mechanism whose requirement for movement scope is determinate. This article addresses the dimension synthesis of the space docking mechanism. First, this article compares the common indexes used to evaluate the performance of parallel manipulator. These indexes can mainly be divided into two types. The evaluation indexes based on the Jacobian matrix, including the singular value index, the manipulability index, and condition index, are derived from the Jacobian matrix of the manipulator, and the transmissibility indexes, such as pressure angle, transmission angle, and motion/force transmission index, lay emphasis on the power transmission ability of manipulator. This article proposes the global transmission index under determinate constraint of workspace on the basis of the previous transmissibility indexes to evaluate the performance of parallel manipulator whose workspace is determinate and movement is the combination of translation and rotation. And the optimization design of the manipulator is carried out by taking the global transmission index under determinate constraint of workspace into account. The detailed process of determining the optimal configuration for the Stewart manipulator used as space docking mechanism is presented. The optimal result is gotten, which has a large value of global transmission index under determinate constraint of workspace in the required workspace, and a simulation is carried out to verify the validity of the optimal result.

Modeling the fractal development of modular robots

Modeling and controlling self-reconfiguration of modular robots is still a challenging problem in the field of distributed control. The two main constrains are the design of target shapes and the absence of global state for decentralized modules. We present a new way for those two problems inspired from the developmental process of plant growth. As a mathematical theory of plant development, L-systems capture the essence of growth process. We extend L-systems to the self-reconfiguration process of modules robots. Target configurations will be described in a string of symbols, and robotic structures capture fractal characters through the rewriting function. Extended graphical interpretation of L-system symbols can generate module-level predictions about robotic global states. Simulations of different self-reconfiguration processes illustrate the proposed method.

Adaptive Impedance Control for Docking of Space Robotic Arm Based on Its End Force/Torque Sensor

Aiming at space transposition using Space Robotic Arm (SRA), flexible docking between SRA’s end effecter (EE) and grapple fixture (GF) is the most important for space tasks. To avoid position errors leading to large contact force between EE and GF in the docking process, an adaptive impedance control method is proposed in this paper. PID feedforward with adaptive parameters is added into the impedance controller, and the force error function is used to deduce the adaptive parameters according to Lyapunov stability theory, which makes the force error decrease automatically during the connection process. Simulation proves that the adaptive impedance strategy gets better force control effect than the traditional impedance algorithm. Finally the SRA EE/GF connection experiments were conducted respectively based on traditional and adaptive impedance control strategy. The results showed that the adaptive impedance control strategy can achieve better control effect than the traditional strategy.