Reliability analysis of a tendon-driven actuation for soft robots

Abstract

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.

Remote Actuation Systems for Fully Wearable Assistive Devices: Requirements, Selection, and Optimization for Out-of-the-Lab Application of a Hand Exoskeleton

“…The developed RAS features high durability of > 20,000 cycles before the first failure occurs, which allowed its application without technical issues for several hours in user studies in out-of-the-lab scenarios. While many research projects target the translation to out-of-the-lab applications (e.g., home use), the durability of RAS is rarely evaluated, allowing limited insight into applicability and usability of developed wearable assistive devices (Jeong et al, 2020 ). Often, durability is only reported for specific components, e.g., a battery runtime of 3.8 h powering a soft robotic glove in Polygerinos et al ( 2015 ), or 5,000 cycles for fluidic fabric muscle sheets intended to be integrated as an output in wearable soft robotics in Zhu et al ( 2020 ).…”

“…However, an additional step to connect the output of the RAS to the assistive device is required. Regarding technical maintenance, this modularity of cable-based RAS is beneficial, allowing quick repair or replacement of specific components of the RAS since structural failures are very common along the transmission path, e.g., especially breaking of the cable (Jeong et al, 2020 ). The increased wear and tear of soft pneumatic and hydraulic actuators, artificial muscles, bellows, and cylinders leads to an increased need for replacement compared to cable-based RAS.…”

“…To avoid the need for these additional structures and allow the use of lighter, small-scale actuators, transmission components, and power sources, the bending angle-dependent power losses along the transmission need to be minimized (Popov et al, 2017 ). Selecting axially stiff (against compression due to cable traction) sheaths with a longitudinal construction of flat-band steel rather than spiral-spring type constructions (Letier et al, 2006 ; Chen D. et al, 2014 ) and coated cables can reduce the friction losses and wear (Xiloyannis et al, 2016 ; Jeong et al, 2020 ). Stiff sheaths further prevent bending of the cable transmission at very small deflection radii, which would increase the wear on the cable and, consequently, decrease the transmission efficiency over time (Letier et al, 2006 ).…”

“…In particular, a major challenge in the transfer of wearable assistive technologies to such out-of-the-lab applications is durability. This aspect is rarely evaluated and reported in research publications, although highly significant for longitudinal applications and testing (Jeong et al, 2020 ). Furthermore, properties influencing wearability, e.g., overall mass and volume, and device ergonomics (e.g., wearing comfort, and heat and noise emission) become even more important.…”

Editorial: Soft Robotic Modeling and Control: Bringing Together Articulated Soft Robots and Soft-Bodied Robots

“…Jeong et al developed a lifetime model for tendon-driven soft actuators. The model was validated extensively through experiments under various levels of actuation stresses.…”