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.

The reliability of soft robotic devices will be the bottleneck that slows their commercialization. In particular, fatigue failure issues are a major concern. Thus, reliability should be taken into account from the earliest stages of development. However, to date, there have been no attempts to analyze the reliability of soft robotic devices in a systematic manner. When soft robots are employed to force transmission applications, reliability is typically a dominant issue, since soft robotic structures are constructed with soft material components; these materials have highly nonlinear properties that arise due to the large distribution in the material properties. Furthermore, reliability should be analyzed from the robot’s system down to the components using domain knowledge about the system; this requires a systematic approach. This study presents a framework for reliability analysis of soft robotic devices taking into account a probability distribution that has not been considered before and examines a case study of a tendon-driven soft robot. This study focuses specifically on the (a) concept design process, (b) lifetime analysis process, and (c) design and optimization process. A life model that considers distribution is proposed using accelerated life testing based on analysis of the failure mechanism of the tendon-driven system. The tensile stress of the wire was varied during the experiment with different bend angles and output tension. The result was validated with different stress levels using a testbed to simulate an actual application. The proposed reliability analysis methodology could be applied to other soft robotic systems, such as pneumatic actuators, to improve the reliability-related properties during the robot design stage, and the life model can be used to estimate the device lifetime under various stress conditions.

Robots that share activity spaces or physically interact with humans typically benefit from appropriate payload capacity, extensible workspace, low weight, safety, and space efficiency. The soft origami design and mechanism can meet many of these beneficial factors; however, achieving a high payload capacity remains challenging. In this letter, we developed a soft origami arm module with high variable stiffness (x300) and spatial efficiency (compressed x3.1). The buckling of facets into a cylindrical tube followed by its pressurization enables the arm to be highly stiffened. High-pressure capacity was obtained via the sewing-heat press fabrication process. We used a pneumatic pressure–tendon pair and utilized the frictional force between origami and tendon to prevent unintentional gravity-induced deformation while deploying. An analytical model was developed and compared to the experimental results. With our modular design, we could easily build functional robotic structures. Two robotic demonstrations were performed to examine the expandability of the modules. A variable-length robotic arm that mimics a human arm was built to manipulate typical objects. Additionally, a soft rover, which could carry 14 kg of weight and change its volume 29 times for improved spatial efficiency, was developed. This research suggests a new design methodology for practical soft robotic systems.

Control of a Bowden-cable transmission requires dealing with varying frictional nonlinearity of the cable, which varies as the bend angle of the cable changes and degrades the performance of the output tension control if not compensated for. This paper proposes a novel method of compensating for the changing nonlinearity of the Bowden-cable. The method enables controlling the tension of a Bowden-cable without directly measuring the output tension. The output tension differs from the input tension because of friction along the cable, which changes with the cable's bend angle. The bend angle can be estimated by using a Bowden-cable Angle (BoA) Sensor and the input tension of the actuation wire to compensate for the friction change. Friction compensation allows for feedforward control of the output tension of the Bowden-cable transmission despite the varying shape of the cable. The results show that the control error of the output tension decreased from 5.21 N to 0.50 N when the bend angle of the Bowden-cable changed from 0° to 400°. The proposed control method can be fully embedded into a Bowden-cable system without redesigning the original device, minimizing the complexity and size of the actuation system. A Bowden-cable implementing the proposed control method was demonstrated with a soft wearable robotic hand.

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.