Continuum Robots for Manipulation Applications: A Survey

1.1. Why Continuum Robots?

In the manufacturing industry, robots have steadily gained importance in assembly-line operations due to their compelling value proposition: reduced cycle-time and increased accuracy, along with a skillset [1]. A typical industrial robot is floor-mounted for safety and consists of discrete rigid links that are actuated for gross movement of the end effector and of a task-appropriate end effector with fine motor control. Sophisticated control software operates individual robots or coordinates multiple robots in order to maximize their value in a specific industrial operation [2].

Against this backdrop, continuum robots are emerging as a novel concept, at least in research, with the potential to be used across a wide range of industrial applications [3]. Continuum robots are hyperflexible electromechanical structures with infinite degrees of freedom which provide them with the ability to maneuver complex curvilinear pathways (a survey on continuum manipulators [4]). A key advantage of continuum robots over those with rigid links is that, due to their considerably lower weight for the same maximum output force, they can be ceiling-mounted as opposed to floor-mounted. This advantage significantly increases their safety when they are used in joint operations with humans on the factory floor [5]. On the flip side, continuum robots are inherently more nonlinear and thereby harder to control than their discrete rigid-link counterparts, thereby presenting a barrier to adoption in the industry [6].

Continuum robots have increased flexibility, and thereby dexterity, compared to their rigid-link counterparts (the importance of continuum robots [7]). Figure 1 from [8] illustrates the essential difference between a discrete, a serpentine, and a continuous-link structure. It is clear from this figure that continuum structures have more degrees of freedom to move, and thereby, they are able to move more precisely along the shape of an object. In addition, their ends can position themselves in many more 3D facing directions compared to rigid-link structures. Researchers are developing continuum robots for a variety of navigation [9] and exploration [10], manufacturing and assembly [11], and medical and surgical applications [12].

1.2. Bioinspired Robots

When it comes to continuous actuated structures, biological systems from nature (Figure 1) have some compelling characteristics worth mimicking: symmetric and optimized design, evolution-enabled uniqueness for the task at hand, and, finally, energy-efficient kinematics and dynamics for survival (survey of bioinspiration [13]). Researchers have been inspired by the animal and plant kingdoms when it comes to designing their continuum robots, and Table 1 presents our first taxonomy, namely, the bioinspiration behind various robots surveyed herein: our survey includes multiple references to Walker’s research group publications—they are not only one of the earliest to investigate biological structures for continuum robots, starting with the elephant’s trunk [27], but they have also had the largest footprint of any research group, the left column of Table 1 [14–31].

Table 1. The robots surveyed herein mapped per their bioinspiration.

Bioinspiration	Reference	Model	Continuum robot (example)
Elephant trunk	[14–31]
Spine of a mammal	[32–55]
Snake	[56–71]
Octopus arm	[72–83]
Tentacle	[84–103], [104–113], [114–131]
Human arm and finger	[132–159]
Biological vine and plant	[160–181]
Tongue and tail of a reptile	[182, 183]

1.3. Survey Organization

Continuously articulated structures in the bio-kingdom are made up of muscles, tendons, fibers, and joints. One of the three focus areas of this paper is a survey and taxonomy of continuum robots from the standpoint of their mechanical structure (Section 2). The influence of the bio-kingdom on continuum robots is so complete that an overwhelming majority of continuum robots are made of soft materials that allow these robots to have the required flexibility. The second focus area of this paper is a survey and taxonomy of continuum (soft) robots in terms of materials (Section 2). Continuous robotics use hydraulic, pneumatic, and electrical actuators. Our final taxonomy of continuum robots is based on a survey of the types of actuators used to achieve their motion (Section 4). The paper concludes with some broad remarks regarding the robots surveyed and some observations on where the field is headed (Section 5). More than two hundred references are cited herein, and the citation details are at the end of this paper (References). We reiterate that the kinematics and dynamics of these robots, including sensors and control, as well as their application in various surgical applications, are beyond the scope of this paper. For those topics, we refer to the following excellent surveys: [12, 184, 185].

2.1. Structure

The structure of the continuum robot used for manipulation is broadly classified into a single or multisegment robot in the early phases of research [186]. In order to enhance the functionality, multiple discs are inputted as a backbone for these robots in order to mimic a continuum structure, examples of which are shown in Table 2. Every continuum robot developed falls into the category of bioinspiration mentioned in Table 1, and the description in Table 2 provides a brief outline of the models developed.

In the early phases of research [14–20], known as first-generation continuum robots, the elephant trunk models [187] were constructed with a maximum of three segments with pneumatic actuation. Jones and Walker, along with researchers at Clemson University, had developed two robots, OctArm [14] and Air-Octor [16], with a single segment mimicking a trunk (Figure 2(a)). The soft gripper developed in the shape of a cone [18] is an example of the single-segmented robot. The soft manipulator [17] is bisegmented, and the granular robots [20] are the additional models with dual and triple segments (Figure 2(b)). As an advancement, the research on the second-generation robots consists of trunks with a backbone and multiple discs, with the increased complexity of actuation and manipulation. These trunks consist of multiple segments with dual actuation, i.e., electric motor and pneumatic [21–28]. In the current research scenario, the third-generation continuum robot, which is known as the bionic handling assistant (BHA) model developed by Festo [29–31], has entered the production environment. This is an advanced prototype that is constructed using the concepts of lightweight design and possesses the capability to operate with increased flexibility. This model consists of three segments with variable curvatures supported by tendons and with a weight of 1.8 kg. The structure and components of the BHA are shown in Figure 2(c), along with the three-fingered end effectors to grasp the objects.

Continuum models inspired by the spine of the mammal [32–55] and snake [56–71] are the most common examples of the multidisc biomodels with single or multiple segments that consist of a structured alignment of discs, as shown in Figure 3(a). Spine structures can possess only bending movements with limited angular constraints and with a fixed base, and snakes belong to the class of reptiles that have the capability to elongate their bodies without limbs to grasp or manipulate objects with precision in a confined space. The single-segment-multidisc models, spine [32–42] and snake [56–62], are generally made up of circular metal structures equidistantly placed along the backbone. The discs are magnetically polarized so that the distance between the discs remains constant and can take up the shape of constant curvature to achieve the hemispherical surface of the end effector. These discs are either made up of steel or hard polyamide material. The multisegmented models of the spine [43–55] are aligned with metal intervertebral discs aligned at a constant distance that provide nonlinear damping characteristics, and snake [63–67] is constructed with cylindrical tubes connected with joints with rotational and translational degrees of freedom and mostly used for inspection through holes, as shown in Figure 3(b). To reach the model of a continuum structure, snake models [68–71] are developed that are made up of braided or shape memory alloy materials and consist of dual actuation, i.e., are cable or pneumatic driven.

Octopus- and tentacle-inspired [84–131] models are mostly single-segmented examples. Octopus is a class of cephalopods whose structure is symmetric along the axis bisecting its two eyes, whereas tentacle is a long-elongated organ (example of tentacle inspired robot [121]) present in many invertebrates that usually occurs in pairs. The legs of the octopus have the unique capability to perform locomotion and manipulation [188], and tentacles are tiny thread-like structures that are used for grasping and feeding along with sensory reception, which proves the easy choice for robotics enthusiasts to develop continuum structures that are single-segmented (Figure 4(a)). Laschi et al. [72–76] are one of the early researchers who developed continuum robot models built with longitudinal and transverse actuators using silicone and braided fiber as materials (Figure 3(b)). To develop a realistic case scenario, Guglielmino et al. [78] had obtained consent with respect to EU regulations to perform morphometric analysis on several anaesthetized octopuses with the support of a marine biologist. This experiment provided detailed information regarding the structure of octopus limbs, which assisted in developing prototypes [79–83] using shape memory alloys (current sensitive) and fluid actuators mimicking the animal behavior.

The human arm is the most complex biomodel, and researchers have developed models that augment safety in manipulation applications with respect to physical human interaction. All such robot structures can be categorized into two models, i.e., multifingered hand robots (Figure 5) and multijoint extendable arms (Table 3). All the multifingered hand robots [132–146] were developed to have a similar structure, with a metal or plastic base and fingers made up of hardened polyamide material. The human arm robots [147–159, 189] are designed with multijoints and a unique end effector and are categorized based on the application perspective, as shown in Table 3.

All the biomodels discussed in the previous sections are animal-inspired robots. In this section, the continuum robots that are vine and plant inspired are explored. The hyperflexible manipulator (HFM) developed by Suzuki et al. [160] is one of the best examples of vine inspired, consisting of multiple underactuated links and nonelastic passive joints resembling a rope used for better casting and winding along the object. The primary application of vine-inspired robots is in the field of space and planetary exploration. Scientists at NASA, Mehling et al. [161] and Tonapi et al. [163], developed a tendril robot (Figure 6(a)) and a robotic manipulator for minimally invasive inspection along with manipulation for space operations [160–170]. The tendril robot consists of three subsystems with nine motors of actuation, body mechanism, and avionics, as shown in Figure 6(a) [161]. In order to utilize the capability of extending their segments longitudinally, the class of continuum robots [175–181] inspired by biological plants was developed. Their structure is pneumatically actuated [181], which controls the longitudinal height and is made up of a polyamide structure that is widely used in deploying antennas [177–179] or in operations that require a periscope [175, 180]. To enhance the application areas of vine robots, researchers found advantages of using these structures [171–174] in further applications, including autonomous refueling, exploration [174], water spraying [171], aircraft fueling [173], body inspection [170, 176], and engine repair [172], as shown in Figure 6(b).

The final categorization of the continuum robots is the structure inspired by the tail and chameleon tongue of an animal. The chameleon tongue-inspired robot (Figure 7(b)) [183] consists of a mechanism that can accommodate the principle of elastic energy storage and release, which can elongate to 1.5 times its body length and can exert a force of 500 m/s². The tail (Figure 7(a)) [182] on the structures has very few engineering applications, which are envisioned for use on board a mobile robot to provide a means separate from the locomotion mechanism (e.g., legs or wheels) to generate external forces and moments to stabilize and/or maneuver the robot.

2.2. Material

The material of choice for bioinspired continuum robots is polyamide—a polymerized molecular chain made of nylon braided blend or carbon amalgamation (e.g., borax and vinyl alcohol) whose strength, elasticity, and flexibility can be customized per the application. Other materials used in construction of continuum robots are silicone, nitinol, aluminum, braided fabrics, and shape memory alloys. Tables 4 and 5 provide a taxonomy of various materials used in the construction of continuum robots.

3.1. Pneumatic Actuator

Pneumatic actuators [46, 190–220] are the most commonly used kind in continuum robots because they are less complex and are low cost. The drive is produced by forced air injected or withdrawn from the body of the continuum structure (Table 6). These were first developed under the name of McKibben actuators arranged symmetrically along the central axis of the robot’s body with flexible pairs on opposite sides pressurized by one solenoid valve, which controls the volume and direction of the airflow using a pressure sensor. The dissection of the artificial pneumatic muscle actuator (PMA), along with the body modeled using fiber [209] or braided [206] material, is shown in Figure 8. PMA provides a good balance of actuation performance and power-to-weight ratio, the description of which is significantly explained in the survey [221]. Further examples of continuum robotic structures developed using the PMA are shown in [46, 190, 191, 212–220].

3.2. Electrical Motors

The tendon-driven continuum manipulators are the first structures developed with bending segments of adjustable length. These structures assist the body to generate curves with variable curvature radii, which augments the efficiency of grasping. Researchers examined the properties of tentacle-like continuum robots and arranged tendons in pairs that are actuated by motors. Almost every structure with a built-in tendon is actuated by electric motors (direct current, servo, or stepper motors) (Table 7).

3.3. Drive Mechanisms

In general, the articles referenced in this paper do not discuss the underlying drive mechanism in detail as they focus on the novelty of their continuum robots with respect to design elements and mobility. The drive mechanism of the continuum robot is generally categorized into either tendon or nontendon driven (Figure 9). These mechanisms have common functionality of manipulation along with defining the body shape and the end-effector position of the robot (Table 8).