
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 Authors: (1) Jorge Francisco Garcia-Samartın, Centro de Automatica y Robotica (UPM-CSIC), Universidad Politecnica de Madrid — Consejo Superior de Investigaciones Cientıficas, Jose Gutierrez Abascal 2, 28006 Madrid, Spain (jorge.gsamartin@upm.es); (2) Adrian Rieker, Centro de Automatica y Robotica (UPM-CSIC), Universidad Politecnica de Madrid — Consejo Superior de Investigaciones Cientıficas, Jose Gutierrez Abascal 2, 28006 Madrid, Spain; (3) Antonio Barrientos, Centro de Automatica y Robotica (UPM-CSIC), Universidad Politecnica de Madrid — Consejo Superior de Investigaciones Cientıficas, Jose Gutierrez Abascal 2, 28006 Madrid, Spain. Table of Links Abstract and 1 Introduction 2 Related Works 2.1 Pneumatic Actuation 2.2 Pneumatic Arms 2.3 Control of Soft Robots 3 PAUL: Design and Manufacturing 3.1 Robot Design 3.2 Material Selection 3.3 Manufacturing 3.4 Actuation Bank 4 Data Acquisition and Open-Loop Control 4.1 Hardware Setup 4.2 Vision Capture System 4.3 Dataset Generation: Table-Based Models 4.4 Open-Loop Control 5 Results 5.1 Final PAUL version 5.2 Workspace Analysis 5.3 Performance of the Table-Based Models 5.4 Bending Experiments 5.5 Weight Carrying Experiments 6 Conclusions Funding Information A. Conducted Experiments and References 2 Related Works 2.1 Pneumatic Actuation There are numerous possibilities for designing and constructing pneumatic actuators. As is common in soft robotics, bio-inspired actuators exist. In [21], an actuator is presented that combines both pneumatics and tendons to mimic and attempts to mimic the behaviour of an octopus arm. Similar in final operation, although inspired by the human finger, can be considered the work of [22]. This consists of three one-dimensional valves that swell, thus mimicking the movement of the phalanges. Nevertheless, the most common bio-inspired pneumatic actuators are Pneumatic Artificial Muscles (PAMs). They are based on achieving extension or contraction of a pneumatic chamber, similar to that of a muscle. The walls of the chamber are usually made of a very thin and flexible membrane, which facilitates large deformations with little air flow introduced [23]. Although some actuators lengthen longitudinally upon inflation [24], the prevalent approach involves utilising McKibben muscles, consisting of a bladder inserted into a braided mesh that constrains the movement of the bladder as it inflates, producing a contraction of the whole [25]. An arm of two PAM segments is designed and modelled in [26]. Another alternative that has gained particular importance in recent years is the Pneunet-type actuators, firstly introduced in [27]. Manipulators that use this type of action consist of a finned beam type structure and, in some cases, some material of varying stiffness. It has been widely used for grippers [28, 29], soft gloves [30] and for modelling human body parts [31]. Different authors have proposed evolutions to this structure. An optimisation of the geometry is carried out at [32], whereas [33] presents PneuFlex actuator, which has evolved the Pneunet concept by making the beam have a variable cross-section. On the other hand, the works of [34] and [35] show how it is possible to design pneumatic segments based on this actuator with bidirectional bending. Jamming, initially used for soft grippers [36] can also be used to create manipulators in the form of beams [37]. Their function is the reverse of that of Pneunets: in their natural state they are bent and, when negative pressure is applied, they stiffen. In [38], a TPU-printed segment at which pressure or vacuum can be applied, is introduced. 2.2 Pneumatic Arms In addition to the design of actuators, pneumatic soft robotics has to face the challenge of their integration for the construction of arms with various degrees of freedom. While some of the previous work, such as [26], does form small arms, several alternatives have been developed for the construction of manipulators. A first option are hybrid approaches, in which both rigid and purely soft elements are combined, which makes it possible to obtain a relatively stable mechanism in a simple way. An example of this can be found in the work [39], in which antagonistically actuated PAM pairs are used to move a rigid beam arm with seven degrees of freedom. A very similar procedure is followed in [40]. To prevent the robot from causing harm to humans, inflatable sleeves are added to the arm. In [41], a pneumatic segment with rigid bases is developed. This consists of six tubes which, due to their geometry, when inflated in groups of three, allow the assembly to rotate on the axis perpendicular to the bases. Although a whole robotic arm is not developed as such, but only a segment of one degree of freedom, it is integrated into an entirely soft robotic arm. The same authors had previously developed a six-degree-of-freedom robotic arm from 85 cm-long segments, capable of lifting loads of up to 3 kg [42]. The work of [43], on the other hand, presents an origami robotic arm made of TPU. This is inflated and deflated by pneumatic actuation, but its position is controlled by tendons which, while slowing down the system, make it much more precise. Among 3D printed robots, [44] presents a three-degree-of-freedom segment, whose movement is achieved by inflating or deflating one or more of its three pneumatic tubes. It also has cables that play the role of antagonist and stiffen the movements of the manipulator. Its working principle is very similar to that of the work of [1]. Also based on the philosophy of printable and deployable robots, [45, 46] present a Honeycomb Pneumatic Network (HPN) arm. It has been constructed by concatenating TPU honeycomb structures, each with an airbag inside. Several prototypes are presented in the paper, one of which can reach a length of 600 mm after joining four segments together. While several of its advantages are discussed, it presents the problem that its weight, without being exaggerated, is high: the arm, considering all the tubes, weighs 4.4 kg. The robot of [47] consists of two segments, each with three pneumatic tubes. Although this theoretically gives it six degrees of freedom, the controllers are only responsible for the upper module, which does not allow all positions and orientations to be freely fixed. The STIFF-FLOP manipulator was introduced in [48]. This consists of an elastomeric cylinder with a series of pneumatic chambers inside, the inflation and deflation of which causes deformation of the cylinder and therefore movement of the robot. Various iterations of this design exist, such as the STIFF-FLOP segment with stiffening tendons demonstrated in [31]. In this line, SoPrA, presented in [49], is made combining three fibre-reinforced silicone segments, each shaped like a conical trunk, so that the end of the robot is much narrower than the base. Although the truncated cone shape is advantageous because, as the authors point out, the upper segments require more torque and hold more tubes inside them, the manufacturing process used to achieve the taper prevents new segments from being easily added to the robot. In [50], a 3-segment arm is built using silicone rubber. Each segment is 110 mm long, has a diameter of 45 mm and, contrary to traditional STIFF-FLOP structure, it is equipped with four inflatable cavities. This implies an increase of weight and difficulty of control, as redundancy is increased compared to having only three degrees of freedom per segment. 
 
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