1. Introduction
With the continuous development of society, pipeline transportation has been widely used in the fields of petroleum, chemical industry, energy, food processing, urban water supply and drainage, agricultural irrigation, nuclear industry, etc. However, due to the influence of chemical corrosion of transmission mediums, force majeure natural disasters, and their defects, serious accidents may occur, such as environmental pollution, flammable explosions, energy waste, and so on [
1]. It is necessary to inspect, maintain and clean the inside of the pipeline regularly. Due to the complexity and large number of pipe networks, the traditional pipeline inspection workload is large and efficiency is low, and some pipeline locations cannot be reached to perform inspection. Therefore, pipeline robots are required to perform pipeline inspection and maintenance tasks.
A pipeline robot is a mechanical, electrical and instrument integrated system that can move along the inside or outside of small pipelines, carry one or more sensors and operating machinery, and carry out a series of pipeline operations under the remote control of workers or computer automatic control [
2]. According to different motion forms, they can be divided into mobile pipeline robots [
3,
4], wheeled pipeline robots [
5,
6,
7], tracked pipeline robots [
8,
9], supported pipeline robots [
10], walking pipeline robots [
11,
12,
13], peristaltic pipeline robots [
14,
15,
16] and spiral pipeline robots [
17,
18]. Traditional pipeline robots are generally made of rigid materials. Due to the poor adaptability of a rigidly structured body to the environment, it is difficult for the robot to work in the rugged and complex pipeline environment, and the rigid materials are in contact with the pipeline, which could easily cause damage to the pipeline contact surface and aggravate the damage to the pipeline [
19].
The emergence of soft-bodied robots provides a new idea to solve these problems. The key of a soft-bodied robot lies in its different driving methods [
20,
21], such as pneumatic driving, shape memory alloy driving and dielectric elastomer driving. Among them, pneumatic is more popular because it has the advantages of a simple driving mode, high driving efficiency and strong pressure resistance. In recent years, scientists have designed a series of peristaltic soft-bodied pipe robots driven by air pressure by studying the motion mechanism of worms in nature and combining with soft-bodied robot technology.
For example, the Calderón team from the University of Southern California designed an earthworm-like soft-bodied body pipe peristaltic robot using two contraction actuators and one extension actuator [
22]; Connolly et al. from Harvard University designed a segmented worm-like soft-bodied pipe robot composed of compression actuator, extension actuator, compression actuator and torsion actuator [
23]; Yamazaki et al. from the Central University of Japan designed an earthworm type 25A pipeline inspection robot using pneumatic actuator [
24]; Mohit S. Verma of Harvard University developed a pneumatic soft pipe robot using buckling pneumatic actuators (vacuum-actuated muscle-inspired pneumatic structures, or VAMPs), with which the tube climber can navigate through a tube with turns, inclines, and varying diameters [
25]; Philip Wai Yan Chiu et al. from the University of Hong Kong proposed a pneumatic soft-bodied robot for gastrointestinal examination, which can move peristaltically in a tubular environment such as a colon [
26]; Wang Xueqian et al. from Tsinghua University developed a pipe crawling robot with parallel soft-bodied body actuators [
27]; Xi Zuoyan et al. from Harbin Institute of Technology designed a soft-bodied robot that can crawl in the pipeline by using the series combination of bending actuator and telescopic actuator [
28]; Jiang Cheng et al. from Donghua University designed a worm-like soft-bodied pipe robot based on fabric paper composites [
29]. The above-mentioned peristaltic soft-bodied pipe robots often have a single motion mode and an insufficient ability to deal with environmental changes, and the research theory and method cannot be applied universally.
Our laboratory has developed a multi-motion mode peristaltic soft-bodied pipe robot [
30], which is composed of a hexagonal prism soft-bodied bionic actuator, elongation soft-bodied actuator, and hexagonal prism soft-bodied bionic actuator in series. By adopting different inflation patterns for the hexagonal prism soft-bodied bionic actuator, the robot can realize different motion modes in pipes of different diameters. The key component to realizing the multi-motion mode of the peristaltic soft-bodied pipe robot is the hexagonal prism soft-bodied bionic actuator. In this paper, structural design, mechanical modeling, numerical simulation algorithm verification, physical model preparation, and experimental test analysis are carried out. Finally, it is determined that the research method, process, and results of the hexagonal prism soft-bodied bionic actuator are reasonable and feasible, which lays a foundation for the further study of peristaltic soft-bodied pipe robots.
4. Verification of Numerical Simulation Algorithm and Preparation of Physical Model
4.1. Numerical Simulation Algorithm Verification
Considering the requirements of the motion mode of the peristaltic soft-bodied pipe robot on the deformation function of the actuator, the rationality of the structure and function of the hexagonal prism soft-bodied bionic actuator is verified by using the numerical simulation algorithm, and the accuracy of its deformation analysis theoretical model is tested [
38,
39,
40,
41].
Because the actuator is made of a hyperelastic material, its material characteristics have a nonlinear relationship. Compared with other simulation software, ABAQUS simulation software has great advantages in solving various nonlinear problems. Therefore, this simulation software is used to simulate and analyze the software driver. When ABAQUS is used, ABAQUS/standard is used for simulation analysis. The main steps in the simulation process are:
The structure of each part of the soft actuator is established by using three-dimensional software, and imported into ABAQUS software to impose constraints for assembly.
- (2)
Soft-bodied actuator material and load setting
Since silicone rubber is a hyperelastic material, when defining the material characteristics, in addition to defining the density of the material, it is also necessary to set its hyperelasticity in the hyperelastics option, in which the strain energy function is Yeoh type, the silicone rubber model parameter is , , and the density is .
When applying a load, first create a plane in the surface where the load is to be applied. Secondly, the load application is realized by creating the analysis step and setting the loads in the analysis step.
- (3)
Soft-bodied actuator meshing
When dividing the mesh, the method of dividing the model is adopted, and the hexahedron is used to sweep the mesh. When setting the mesh, because the silica gel is incompressible and has the characteristics of large deformation under stress, the mesh is set as a secondary hybrid solid element (C3D10H).
- (4)
Create a working file and conduct simulation analysis to obtain the deformation analysis diagram of hexagonal prism soft-bodied actuator, as shown in
Figure 7.
According to
Figure 7, the deformation of the hexagonal prism soft-bodied bionic actuator can meet the needs of the peristaltic soft-bodied pipe robot to realize multiple motion modes, and the deformation change law of the cloud diagram conforms to the actual deformation of the silicone rubber material after ventilation, which can verify the rationality of the structure and function of the actuator. Through numerical simulation calculation, the bending angle of the actuator under different driving pressures from 40 kPa to 280 kPa is obtained, as shown in
Table 2.
The calculation results in
Table 2 are compared with the theoretical model of driver bending deformation, as shown in
Figure 8.
According to
Figure 8, when the driving pressure is not greater than 200 kPa, the calculation result curve of the numerical simulation algorithm approximately coincides with the curve of the theoretical model, which can prove that the theoretical model of deformation analysis has a certain accuracy. The driving pressure continues to increase, and the error between the two curves becomes larger. After analysis, this was found to be because the influence of the rectangular cavity on the passive deformation side of the hexagonal prism soft-bodied bionic actuator on the bending deformation is not considered in the theoretical model analysis.
4.2. Physical Model Preparation
At present, there are two main manufacturing methods for pneumatic soft-bodied actuators: mold pouring type and 3D printing type [
42]. Among them, 3D printing silicone products can only produce products with a relatively simple structure, and the cost is high. Therefore, this paper uses the mold pouring method to make the hexagonal prism soft-bodied bionic actuator. The materials required for the preparation method are shown in
Table 3.
The physical model preparation process of the hexagonal prism soft-bodied bionic actuator is shown in
Figure 9. Firstly, the cavity mold must be designed, and then printed by 3D printer; then, mix the platinum silicone rubber of components A and B to a ratio of 1:1, and the stock solution is obtained after uniform stirring; place the stock solution in a vacuum environment to remove bubbles, then pour the stock solution into the prepared mold, and remove bubbles for a second time. After removing bubbles, put the mold containing the stock solution into a 60 °C incubator and stand for 2 h. After curing and forming, demould to obtain the mold cavity; finally, the silicon rubber adhesive is used to bond each part of the cavity in order to obtain the hexagonal prism soft-bodied bionic actuator.
In view of the dual requirements of a pipeline and soft-bodied robot, it is necessary to ensure that the prepared actuators not only have flexibility, but also have a certain strength and wear resistance. Through a large number of data analyses and experimental tests, Shore A45 platinum silicone rubber was chosen as the preparation material for the hexagonal prism soft-bodied bionic actuator.