The technologies about the mechanical design and motion control of traditional robots are relatively mature. However, traditional robots for heavy industrial applications usually utilize rigid materials, inelastic articulation components, and stiffness actuators, which have severe limitations in other circumstances, such as collaboration and manipulation in confined space [
]. In the complex and unstructured environment, traditional robots are limited by their own rigid structures, which also restrict their obstacle avoidance and manipulation capabilities [
]. Those traditional robots have relatively poor flexibility and adaptability and therefore cannot efficiently complete some tasks in a safe manner, especially involving the physical human-robot interaction. In contrast, looking back on the animals in the biological world, they are characterized by soft and compliant substance [
], such as their skin, limbs, muscles, even including muscular hydrostatic skeletons [
]. In order to deal with the challenges of traditional robots, some flexible or soft robots have been developed with the characteristics of infinite degrees of freedom (DoF), high flexibility, environmental adaptability, and extended manipulation capability in recent years. There are three roadmaps to achieve the compliance characterized by the nature animals, including soft robot with intrinsic actuation [
], continuum robots with extrinsic actuation, and traditional robots with elastic/compliant actuation [
Soft robots are a class of robots using soft materials including gases, liquids, polymers, foams, gels, colloids, granular materials, as well as most soft biological materials [
]. Inherent compliance of gas and liquid make soft pneumatic actuator and hydraulic actuator as good candidates for many challenging tasks [
]. Some work pointed out that soft pneumatic actuator have some considerable characteristics, such as multiple degree of freedom (Multi-DoF) actuation [
], high force, and reliability [
]. Shiva et al. [
] proposed a pneumatically continuum manipulator with embedded chambers in the soft silicone structure. However, the overall size and weight of peripheral device such as compressor unit, on-board electronics, confines the possibility of using these actuators. Sometimes, the term of soft robot also can be especially described a class of robots using some advanced functional materials with intrinsic actuation as the soft-body material, such as shape memory polymer (SMP) [
], shape memory alloy (SMA) [
], electroactive polymer (EAP) [
], and so on, therefore soft robots are featured with physical self-driven characteristics [
]. Soft material have great prospects in the small-scale robots because of their inherent advantages, such as lightweight, compliance, high strain with continuum deformation. However, full soft material robots are usually not able to achieve some functional requirements including large force, fast motion and controllability from the current research status.
Continuum robot is used here to especially refer to an actuatable structure with extrinsic actuation. Continuum robot usually have discrete structure, multi-backbone, and driven by tendon/cables [
]. Many continuum manipulators are usually proposed. An elephant’s trunk robot described in Ref. [
], with very few driving constraints, consists of a series of novel underactuated linkage units and a single motor to drive, which can achieve stable grasping of objects with different shapes and sizes. A multi-section soft manipulator dynamics was presented based on a discrete Cosserat approach [
]. The model can accommodate continuum robots having any number of sections with varying physical dimensions [
]. Yeshmukhametov et al. [
] presented a novel approach of kinetic, kinematic modeling and design solutions of the wire-driven universal joint backboned continuum robot. However, existing continuum robots with multiple segments require the separate power source to actuate different segments [
], therefore, the dependent control and planning process is very complicated.
Traditional robot combined with elastic/compliant actuator [
] is another recipes to obtain structural compliance. The advantages of elastic/compliant actuator is that series elastic actuators [
] and variable stiffness actuators [
] combine the elastic element or compliant mechanism with conventional stiffness actuators to regulate energy transmission [
], to improve energy efficiency, and to achieve safety physical interaction. Ham and Sugar et al. surveyed the various elastic actuators with passive adjustable compliance and controllable stiffness for robotic applications [
]. However, the typical elastic/compliant actuators comply with compliance only along their axes of rotation. Structural compliance is hard to achieve motion control of Multi-DoF. Therefore, this drawback restricts the range of motion space.
Comprehensive research on soft and continuum robots has accomplished significant progress in recent years, but the following deficiencies generally remain: a) Environmental adaptability in extreme and unstructured environment. b) Complex control of multiple segments. Existing continuum robots require the separate power source to actuate different segments [
Inspired by the biological feature and mechanism of the octopus tentacles, a kind of novel flexible manipulator based on the bio-tensegrity structure is proposed in this paper. The underlying beauty of the proposed flexible bio-tensegrity manipulator is manifold. Firstly, the conventional materials, including springs and cables, are used in the proposed method to constitute a flexible bio-tensegrity manipulator, enabling to mimic the behaviors of transversal and longitudinal muscles. This tensegrity mechanism is not only lighter and easier to handle for actuation [
], but also has the main advantage of high load capacity. Secondly, each segment with Multi-DoF can be independently controlled to achieve the shrinking and bending motions through the variable structure mechanism, while there is no need for a separate actuation source. The variable structure mechanism innovatively contains seven lock-release states to control the variable motion pattern of the flexible bio-tensegrity manipulator. This flexible bio-tensegrity manipulator not only has continuum deformation and Multi-DOF as soft materials but also has better stability against various decays and is easier for modeling and control.
The main contents of this paper include the following aspects. Firstly, inspired by the biological principle of the octopus tentacles, the novel manipulator based on the bio-tensegrity structure is proposed in Section
. Then, the simplified modeling and kinematic analysis of the flexible manipulators are carried out in Section
. After that, one manipulator prototype is constructed according to the above design principles. After integrating the mechanism and controlling modules, the performance and effectiveness of the bionic flexible tensegrity manipulator are validated by some proof-of-concept experiments on this platform in Section
. Finally, Section
discusses the conclusions and future work to be conducted.
In this paper, inspired by the variable motion patterns and muscle of the octopus tentacles, a flexible bio-tensegrity manipulator was proposed to mimic the shrinking behavior and achieve the variable motion patterns of each segment. Some proof-of-concept experiments were conducted to validate the feasibility and validity of our proposed prototype. The following conclusions are drawn:
The flexible bio-tensegrity manipulator combines the elastic element spring and cable-driven to mimic soft structure. Compared with other cable-driven continuum robots, this flexible bio-tensegrity manipulator not only shows the bending motion, but also can demonstrate the shrinking motion that not featured in the other cable-driven continuum robots. Meanwhile, the proposed manipulator has the characteristics of Multi-DOF and high flexibility, which extended their environmental adaptability and manipulation capability.
The main variable structure mechanism innovatively contains seven kinds of lock-release states to achieve the lock and release of different cables. In this way, we control the length of each cable and adjust the motion patterns of each segment. Compared with other multi-section continuum robots [
], the independent control of each segment is realized under the conditions of without adding more actuation power source.
We then designed and fabricated a mechanical prototype. One open-loop control method was utilized to adjust the lock-release states and control the cable length. By this way, the flexible bio-tensegrity manipulator can achieve the considerable motion and the variable motion patterns. We also perform some relative experiments, such as bending motion, shrinking motion, and variable mechanisms motion, for demonstrating its high flexibility, environmental adaptability, manipulation capability.
In the future, we need to integrate the system more tightly with sensor devices and considerable control scheme to fully demonstrate its abilities. We are also considering to perform closed-loop feedback control to realize more precise position and stiffness control. Meanwhile, those influence factors in applications which are not analyzed here, such as slack cables, transmission clearance, and vibrating spring, should be taken into account. Moreover, the range of its speed and stiffness within its workspace should be carefully evaluated and directly compared with other existing research results. In order to improve reliability and expand its application area, modular design can be adopted. If each module can be conveniently arranged in series and replaced, the reliability of the whole manipulator will be greatly improved, and this modularization is easy to be popularized.