Designing PAUL: A Soft Robot with Pneumatic Precision

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

3 PAUL: Design and Manufacturing

3.1 Robot Design

Before starting the design and subsequent manufacture, some dimensional and geometrical specifications imposed by the functionality sought have been set. These requirements can be summarised in the following points:

• The resulting robot must consist of three independently actuated segments, each with three degrees of freedom.

• The actuation of the segments that make up the robot must be pneumatic.

• The segments must be made of flexible silicone.

• These segments must allow easy assembly and disassembly as well as a modular design.

• The pneumatic tubes must be completely embedded in the body of the robot to avoid breakage and to allow more complex movements.

It was decided that each segment should be cylindrical, 100 mm high and 45 mm in diameter, with three pneumatic bladders evenly distributed along the cylinder. In order to fix the height of the cylinder, other existing robots in the literature were used as references [12, 50], in order to achieve comparable working spaces. The diameter, on the other hand, was to be as small as possible –in order to make the robot as light as possible– but at the same time to allow all the tubes to pass through the inside of the segment. In addition, it had to have a sufficient wall thickness between the bladder and the outside to prevent ruptures during inflation and deflation. The final size was decided after several design iterations and three manufacturing tests.

Bladders, which are depicted in Figure 2, have a Pneunet structure. As was the case when designing the rest of the segment, some CAD iterations were carried out before a final geometry was fixed. In particular, we chose to use as rounded edges as possible to reduce the stresses, and therefore the probability of punctures, during the numerous inflations and deflations. In addition, different values for the number of protrusions in each bladder were considered, finally using nine, as it was seen that a higher number made the deformation of the segment more curved when it was inflated, which can facilitate its modelling using PCC.

As there are three bladders per segment, each segment should have three degrees of freedom, however, it was resolved to inflate only one or two simultaneously to reduce the redundancy of the system, making control easier.

Final design of PAUL’s segments is shown in Figure 3. Each segment has three U-shaped grooves on its surface, designed for the subsequent insertion of elastomeric sensors to measure the deformation experienced by the robot when it inflates and thus predict the position of the robot and/or the times that each bladder has been actuated.

It was decided that the connection between segments would be made using 3D printed devices, as the ones presented in Figure 4, each 200 mm high. While PLA is not a completely soft material, some soft robots incorporate parts with a degree of stiffness within their overall soft structure [50, 55, 65]. This choice enhances PAUL’s modularity

as the connections can be easily established. It is enough to attach a new segment and insert the tubes –and the cables, when sensors will be implemented– through the central hole of the previous ones, eliminating the need for adhesives or sealants.

In order to achieve optimum adhesion of the segments to the printed parts, a larger diameter section has been incorporated at the end, increasing in consequence the adhesion surface. Likewise, in order to achieve a better seal, the pneumatic tubes are not inserted directly on the bladders, but on some previous projections on which plastic flanges will be fitted to reinforce this union and prevent accidental leaks.

3.2 Material Selection

Due to the pneumatic operation of the robot, it was essential to find a material that combined flexibility to deform with air and the ability to maintain a reliable seal to prevent air leaks. Therefore, silicone was chosen for the segments, and 3D printed connectors were employed to link them, as explained in the preceding section.

The silicone segments were fabricated through a moulding process, with the moulds being 3D printed on an Artillery Genius Pro printer. The printer was configured using the parameters detailed in Table 1. The same table provides the manufacturing parameters for the connectors.

Three different silicones were tested throughout the manufacturing process: PlatSil FS10, EasyPlat 0030 and TinSil 8015, whose characteristics can be compared in Table 2. The first silicone, PlatSil FS10, was rejected as its low curing time results in the appearance of bubbles in the final segment. These acted as stress concentrators, causing damage and air leakages after a few working cycles. The second silicone, EasyPlat 0030, was finally discarded as its low hardness required very thick-walled segments if leakage was to be avoided, which inevitably entailed a high weight.

TinSil 8015, a tin-cured silicone, was selected as it produced segments with an optimal balance between durability and weight. However, it has two drawbacks. First, it is highly toxic, which necessitates the use of special protective measures when working with it, and makes it impossible for the segments to cure in a human traffic area. On the other hand, it shows contractions during the curing of 1% due to the production of alcohols, necessitating a complete filling of the module to counteract this effect.