Development of Pneumatic Network Actuators
DESIGN
Pneumatic Network Actuators (PNA) are used as the gripper fingers. They provide a superior dexterity in gripping owing to their flexible nature. The chambers present longitudinally when inflated due to the pressurized air, repel each other. As a cumulative effect of this, there is a net deflection and force at the tip of the actuator. Thus, we can infer that the chamber geometry, arrangement and no. of chambers are important variables apart from material properties, wall thickness, etc. This section discusses the investigation of these parameters, simulation and design of the pneumatic network actuator. This concept was validated using Finite element analysis of the actuator in ANSYS 18.0.
The geometry was designed in SolidWorks and is as shown in Fig 1
Figure 1 Pneumatic Network Actuator Sectional View
Some assumptions were made along the way, viz:
- The contact behaviour was assumed linear frictionless while the individual parts were assumed bonded. These assumptions are valid because there is only slight sliding along consecutive chamber surfaces upon detection.
- The effect of pressure on surfaces parallel to the length of actuators is neglected.
- The effect of the air mass flow rate is neglected.
- Effects of Gravity are ignored.
The setup of the simulation is shown in Fig. 2. The pressure applied was 0.1 MPa gauge. (2 bar atmospheric). One end is kept fixed while the other end is free (cantilever)
Figure 2 Simulation Setup Pneumatic Network Actuator
The features in the control volume were meshed using quadratic triangular elements as large deformations are expected. The quadratic elements capture curved surface deformations and relatively larger size elements can be used as compared to linear elements for the same computation capacity and accuracy. The material for chambers is HT33. to constraint elongation of the actuator, a thin Teflon film is introduced in base components (HT33) during molding
The results are as shown in Figure 3-4
Figure 3 Total Deformation Figure 4 Maximum Principle Stress
From deformation, contour shows that the operation of the actuator is as desired. The curling of the actuator is attained by differential extension of elastomer chambers and Teflon strip. This kind of actuation is suitable for gripper fingers. The free tip was fixed, and the simulation was rerun. The joint reaction gave the actuator force. It was found out to be 0.39 N.
It can be inferred that as the operating pressure increases, the radius of curvature of each membrane wall will decrease and thus the stroke will increase. It is also a function of no. of chambers. The force has a positive coupling with the chamber wall surface area normal to the longitudinal direction of the actuator and the operating pressure. The simulation needs to be done in Time domain to reduce the significance of the mass flow rate assumption.
Fabrication
Pneumatic Network Actuator is an intricate inflatable actuator that acts as a finger to the soft robot, by exerting gripping force when pneumatically actuated through its step-chambers. It contains a series of inflatable chambers connected through a single channel in series to carry pressurized air.
The chambers have a rectangular cross-section and the lower part of the actuator has a stiffening Teflon tape base because of which bending is experienced in the actuator. When pressurized air is sent into the chamber, expansion is caused in the most compliant part (least stiff part) of the chamber. If there was no stiffening Teflon member, the elongation would have been more linear in nature as compared to the desired bending. Thus, is the need of the Teflon member. And this is the reason why the casting of PNA is in two steps. The first step of casting is of the base of the PNA with embedded Teflon member and the second step is for the casting of the chambers.
Different iterations for different grades of PDMS rubber were made with ZA13, ZA35 Fast and HT45 as shown. It was necessary here to prevent the formation of Air bubbles because of the thin chamber walls. A puncture in even a single chamber would make the actuator of no use. Hence, Vacuum casting was done. Material selection of the PNA was done after analysis as well as the desired force enduring and stiffness of the actuators.
Figure 5 PNA first prototype mold Actuator, channel
Figure 6 Vacuum Casting Setup
Figure 7 PNA Second prototype with changing dimensions Figure 8 PNA made from HT45, ZA35,ZA13
Figure 8 PNA third prototype made from HT 45
Figure 9 PNA actuator lower mold Figure 10 PNA actuator upper mold
Fiber-Reinforced Actuator
A Fiber-reinforced Actuator is an actuator like the PNA but with an external customized reinforcement of inextensible fibre material to get a specific desired amount of bending at different sections of the actuator.
Here, the entire actuator is wrapped with inextensible Kevlar or Nylon polymer thread, which restricts radial expansion. This wrap forms a closed chamber around the actuator and changes its properties of inflation and extension as an actuator. Thus, Reinforced enclosure limits the expansion of the chambers of the actuator and helps to get a desired bending property to the actuator.
As shown in figure 11, two different Iterations of Fiber-reinforced actuator were attempted with Kevlar thread and Nylon thread enclosures. The results obtained were not desirably effective and hence this type of actuator was discarded from the project.
Figure 11 FRA made from ZA4 and ZA30 PDMS
Metacarpal joint
The metacarpal joint here acts as a link to be a mounting for the PNA actuators and also to connect them to the Variable stiffness forearm link.
It does not have any other value added function other than load-bearing. After the design of all the mountings and joints on the part, designing the mold for this complicated part was a challenging task.
The intricacies in the part demanded dimensional-restrictions and thus the eventual mold was a 7-part assembled mold, Very complex in structure as shown in the figure.
The material selection was done and the part was cast using HT45 PDMS soft-rubber. The casting of this part didn’t require vacuum because the upper side of the mold had a large area open to the air and thus there was very less possibility of air entrapment.
Hence, the metacarpal joint was manufactured. Special slots were also given for the pneumatic pipes that supplied pressurized air to the PNAs.
Figure 12 a)Metacarpal joint mold and part b)Geometry of Metacarpal joint
Assembled Hand
Three PNAs were mounted on the designated parts of the metacarpal joint and these collectively formed the hand of the soft-robot. As shown in the figure the three-finger gripper hand was assembled and ready to be mounted on the forearm of the robot.
Figure 13 Assembly of hand from three PNA and Metacarpal joint
Testing
Rigid cantilever Constraints were given to the PNA and the weighing scale was calibrated for initial conditions. The construction of the test setup oriented the actuator so as to transmit the force from a single point tip of the actuator touching the weighing scale when actuated. Different results of exerted gripping force were taken at states of constant pressure ranging from 0 to 3 bar in steps of 0.5 bar.
Figure 15 Graph of Force VS PNA Finger Actuator
Test results for PNA are plotted as shown in the graph (figure 15), on the abscissa pressure is varying from 0 bar to 3 bar and on ordinate force produced by one finger is indicated in Newton. Maximum allowable pressure for the test specimen is 3 bar as actuator is failed at 3 bar pressure. Maximum force obtained at the tip is 6.2 newton when the finger is well constrained in the test setup. Three such pneumatic network actuators are used to make one hand.