**3.1. FFA - Materials**

As the name implies FFAs are made of flexible shell materials. This fact limits the number of relevant materials naturally. Normally laminated foils [112], vulcanized elastomers [36], coated fabrics [97], layered set-ups 1, or various combinations are used. A reasonable differentiation can be made between primary shaping processes like rubber molding and

<sup>1</sup> http://www.otherlab.com/

processes that use semifinished products like foils, in order to form the actuator's cavity. Both processes have their pros and cons.

#### *3.1.1. Semifinished materials*

8 Will-be-set-by-IN-TECH

Flexible fluidic structural elements complete the biomimetic approach. There are many

Inflatable structures are well known in fields of crisis intervention [26] and exhibition stand construction [94]. Other applications include space structures like antennas [33, 79, 82, 132]

Most robotic designs with inflatable structures aim at space applications since they have a small shipping volume when deflated. Koren et al. [72] proposed a design for zero gravity applications and operating pressures of about 3.5 *bar* (figure 9(c)). Shoham et al. [104, 105, 118] developed a inflatable robot as shown in figures 9(a) and 9(b). They also characterized the

The works of Sanan et al. [106], Maruyama et al. [83], and Voisemebert et al. [135] focus more on service and inspection robotics. Operating pressures are in the range of 0.4 − 0.6 *bar*. The

As the name implies FFAs are made of flexible shell materials. This fact limits the number of relevant materials naturally. Normally laminated foils [112], vulcanized elastomers [36], coated fabrics [97], layered set-ups 1, or various combinations are used. A reasonable differentiation can be made between primary shaping processes like rubber molding and

(b) deflated [118] (c) [72] (d) [106]

(b) Whole Skin Gripper by Brown et al. [14]

(a) Whole Skin Motion by Kimura et

al. [69]

**Figure 8.** Actuators based on Whole Skin Effects

**2.2. Flexible fluidic structural elements**

and rovers [28, 48, 63, 72].

(a) inflated [118]

**3.1. FFA - Materials**

<sup>1</sup> http://www.otherlab.com/

examples of hydrostatic skeletons in nature [130].

**Figure 9.** Different Robots with Inflatable Structures

1-DOF arm developed by Sanan is shown in figure 9(d).

**3. Flexible fluidic actuators - fabrication**

robot regarding its stiffness at internal pressures up to 2 *bar*.

Material selection cannot be considered without looking at the fabrication process. Of all welding technologies high-frequency (HF) welding can produce the most resilient seams [2]. However, HF-welding can only be achieved with materials that contain sufficiently strong dipoles. Thermoplastic Urethanes (TPU) have compared to PVC and PA the best material properties [144]. The latest material developments regarding composite sheet materials will be described in the following sections. The HF-welding technology is mostly used for more complex actuator technologies since the technology does not require very complicated molds.

## *3.1.2. Vulcanized elastomers*

Vulcanized elastomers are often processed in compression molding processes. The uncured rubber monomer is put in the heated cavity of a compression mold. The press is closed and under the influence of heat and pressure the rubber is cross-linked. The variety of rubber compounds is infinite. The final material properties can be influenced by fillers, reinforcements and a large variety of chemical additives. How to achieve materials with tailored properties for flexible fluidic actuators will be presented. Compression molding requires complex molds, which limits variations in shape but provides actuators for high operating pressures.

## **3.2. FFA - fabrication processes**

The two main processes for FFA fabrication are compression molding and HF-welding. A detailed description of both process developments is given here.

### *3.2.1. Compression molding*

The requirements for vulcanized flexible fluidic actuators are simple:


In order to achieve these properties the actuator shell is divided into two layers and fabricated in a two-step compression molding process. In the **first step** the inner shell is made. The inner shell (figure 10(b)) is responsible for tightness and fatigue strength of the actuator. It is vulcanized in a mold as shown in figure 10(a).

The **second step** needs some preparation. First the inner shell is covered with a braided aramid fiber sleeve, which is fixed around the rubber with a Vectran® yarn. This fiber reinforcement determines the pressure resistance of the actuator. After that the metal connectors are inserted and the mold adapters are added. Finally a thin layer of rubber is applied to the surface (figure 11(a)). The set-up is now ready for the second vulcanization

10 Will-be-set-by-IN-TECH 576 Smart Actuation and Sensing Systems – Recent Advances and Future Challenges

**Figure 10.** First Fabrication Step

step. The prepared actuator is inserted in the second mold and pressure is applied as depicted in figure 11(b). Fiber reinforcement and metal connectors are vulcanized to the inner shell.

Inner Shell

(a) Partial Section of the Actuator Prepared for the Second Fabrication Step (b) Cross Section View of the second Mold

#### **Figure 11.** Second Fabrication Step

Operation shows that fatigue crack growth is the main failure mode of vulcanized flexible fluidic actuators. There are three main mechanisms to enhance crack growth resistance: Use of stress and strain crystallizing rubbers, use of fillers (mainly carbon black), and dispersion of pulp fibers. The first two mechanisms are implemented by the proper choice of the basis material ( in our case a chloroprene rubber mixture (CR)). However, the particle morphology of carbon blacks limits their contribution to fatigue resistance [142]. Crack bridging effects can only be achieved by dispersing micro fibers in the basis elastomer. Aramid fiber pulps are well-suited for this purpose [3, 70, 131]. With a specific surface are of 5 <sup>−</sup> <sup>15</sup> *<sup>m</sup>*<sup>2</sup> *<sup>g</sup>* and a minimal fiber diameter of 10 *μm* they can enhance the overall material properties significantly (figure 14).

(a) Single Pulp Fiber (b) Aramid Fiber Pulp

**Figure 12.** Morphology of Fiber Pulps

In order to determine the best concentration masterbatches with 0,5%, 1%, 2%, and 5%-mass have been tested according to DIN 53 504. Figure 13(a) shows how the pulp fibers get oriented in flow direction. Hence the samples with parallel and orthogonal pulp fiber orientation have been tested. The orientation has a significant influence on the force-displacement characteristics. The figures 13(c)-13(d) shows crack surfaces with different fiber orientations.

Geometry and Pulp Fiber Orientation

10 Will-be-set-by-IN-TECH

Upper Mold

(a) Cross-section View of the first Mold (b) Partial Section of the demolded

step. The prepared actuator is inserted in the second mold and pressure is applied as depicted in figure 11(b). Fiber reinforcement and metal connectors are vulcanized to the inner shell.

> Pressure Connector

Operation shows that fatigue crack growth is the main failure mode of vulcanized flexible fluidic actuators. There are three main mechanisms to enhance crack growth resistance: Use of stress and strain crystallizing rubbers, use of fillers (mainly carbon black), and dispersion of pulp fibers. The first two mechanisms are implemented by the proper choice of the basis material ( in our case a chloroprene rubber mixture (CR)). However, the particle morphology of carbon blacks limits their contribution to fatigue resistance [142]. Crack bridging effects can only be achieved by dispersing micro fibers in the basis elastomer. Aramid fiber pulps are

fiber diameter of 10 *μm* they can enhance the overall material properties significantly (figure

(a) Single Pulp Fiber (b) Aramid Fiber Pulp

well-suited for this purpose [3, 70, 131]. With a specific surface are of 5 <sup>−</sup> <sup>15</sup> *<sup>m</sup>*<sup>2</sup>

Metal Connector

Inner Shell

Druckluft Compressed Air

Pi

(b) Cross Section View of the second Mold

*<sup>g</sup>* and a minimal

Core Lower Mold

Outer Ferrule Inner Yarn Braided Sleeve + Elastomer

Inner Shelll of

**Figure 10.** First Fabrication Step

Inner Shell Mold Adaptors

Fabrication Step

14).

**Figure 11.** Second Fabrication Step

**Figure 12.** Morphology of Fiber Pulps

(a) Partial Section of the Actuator Prepared for the Second

the Actuator Mold Cavity

(b) Sample Plate (c) Pulp Fiber Orientation Parallel to Crack Surface

(d) Pulp Fiber Orientation Orthogonal to Crack Surface

**Figure 13.** Material Characterization

Generally the goal is to define the maximum fiber concentration that does not lower the tensile strength significantly. The results show that a concentration of 1%-mass leads to the best material properties. The force-displacement plots show a clear influence of the fiber orientation, but in parallel configuration there is no significant difference compared to the pure rubber material (figure 14).

**Figure 14.** Force-Displacement Plots for different Pulp Fiber Concentrations

#### 12 Will-be-set-by-IN-TECH 578 Smart Actuation and Sensing Systems – Recent Advances and Future Challenges

#### *3.2.2. High frequency welding*

Commercially available fiber reinforced TPU foils show several drawbacks, such as low tensile strength, delamination between fibers and TPU-matrix, and axial fiber porosity. The general requirements for HF-welded FFAs are:


Two material systems meet these requirements. **PEEK-monofilament reinforced TPU films** are composite sheets with two layers of monofilament PEEK-mesh between three layers of TPU-films. The overall material properties are: tensile strength 33 *<sup>N</sup> mm*<sup>2</sup> , Young's modulus of 168 *<sup>N</sup> mm*<sup>2</sup> , specific weight 269 *<sup>g</sup> <sup>m</sup>*<sup>2</sup> , thickness 450 *μm*.

**Aramid-fiber reinforced TPU films** are processed slightly different. Since the aramid fibers are spun to a yarn and are not available as monofilament material with sufficient strength, the yarn must be sealed prior to the laminating process. This is done by applying a TPU solution on the aramid fabric. After the evaporation of the solvent all yarn surfaces are covered with a thin layer of TPU. The pretreated aramid fabric is then laminated between two layers of TPU film. The resulting material properties are: thickness 580 *μm*, specific weight 365 *<sup>g</sup> <sup>m</sup>*<sup>2</sup> , tensile strength 60 *<sup>N</sup> mm*<sup>2</sup> .

A schematic view of the general production process is illustrated in figure 15. The inter-chamber connections of the pre-cut foil pieces are welded first. These pre-assembled pieces are then welded together around the outer contour in as many layers as desired. This second step closes the actual actuator chamber.

**Figure 15.** (A) upper welding electrode, (B) lower welding electrode, (C) centering coil, (D) two foil pieces, (E) hole for air passage, (F) lower welding electrode with undercut, (G) centering coil, (H) upper welding electrode with undercut, (I) foil segments from first step

#### **3.3. FFA - evaluation**

All dynamic testing was accomplished on a fatigue test rig consisting of a pressure-tight container, in which the actuators are mounted. The pressure in the container is monitored which allows detection of leaks in the actuators.

#### *3.3.1. Vulcanized actuators*

12 Will-be-set-by-IN-TECH

Commercially available fiber reinforced TPU foils show several drawbacks, such as low tensile strength, delamination between fibers and TPU-matrix, and axial fiber porosity. The general

Two material systems meet these requirements. **PEEK-monofilament reinforced TPU films** are composite sheets with two layers of monofilament PEEK-mesh between three layers of

**Aramid-fiber reinforced TPU films** are processed slightly different. Since the aramid fibers are spun to a yarn and are not available as monofilament material with sufficient strength, the yarn must be sealed prior to the laminating process. This is done by applying a TPU solution on the aramid fabric. After the evaporation of the solvent all yarn surfaces are covered with a thin layer of TPU. The pretreated aramid fabric is then laminated between two layers of TPU

A schematic view of the general production process is illustrated in figure 15. The inter-chamber connections of the pre-cut foil pieces are welded first. These pre-assembled pieces are then welded together around the outer contour in as many layers as desired. This

**Figure 15.** (A) upper welding electrode, (B) lower welding electrode, (C) centering coil, (D) two foil pieces, (E) hole for air passage, (F) lower welding electrode with undercut, (G) centering coil, (H) upper

All dynamic testing was accomplished on a fatigue test rig consisting of a pressure-tight container, in which the actuators are mounted. The pressure in the container is monitored

film. The resulting material properties are: thickness 580 *μm*, specific weight 365 *<sup>g</sup>*

*mm*<sup>2</sup> , Young's modulus of

*<sup>m</sup>*<sup>2</sup> , tensile

*mm*<sup>2</sup>

*3.2.2. High frequency welding*

requirements for HF-welded FFAs are:

• A tensile strength between 30 and 100 *<sup>N</sup>*

• Gas-tight including no axial fiber porosity

second step closes the actual actuator chamber.

welding electrode with undercut, (I) foil segments from first step

which allows detection of leaks in the actuators.

*mm*<sup>2</sup>

TPU-films. The overall material properties are: tensile strength 33 *<sup>N</sup>*

*<sup>m</sup>*<sup>2</sup> , thickness 450 *μm*.

• A maximum thickness of 700 *μm*

• Processable with HF technology

*mm*<sup>2</sup> , specific weight 269 *<sup>g</sup>*

• A modulus of 100 to 150 *<sup>N</sup>*

• Odourless

strength 60 *<sup>N</sup>*

*mm*<sup>2</sup> .

**3.3. FFA - evaluation**

168 *<sup>N</sup>*

The relationship between fatigue resistance, operating pressure, and material combination is shown in figure 16. The choice of the base rubber compound influences the fatigue resistance significantly. The best fatigue resistance can be achieved with CR-rubber containing 1%-mass of aramid pulp fibers. At 6 *bar* the actuator withstands over 1, 200, 000 load cycles. In order to keep testing simple all fatigue evaluation was carried out with actuators 18 *mm* in diameter. All actuators failed due to fatigue cracks in the inner rubber shell. This results in excellent fail-safe characteristics, since the actuator never bursts and can be changed without any danger to the whole system.

**Figure 16.** Fatigue Resistance of 18 *mm*-Actuators

#### *3.3.2. HF-welded actuators*

To evaluate the influence of the various production parameters during HF-welding on lifespan, several endurance tests were performed. In these tests, one parameter is varied and all others kept constant. Each experiment was performed with ten actuators. This was necessary because the deviation in lifespan was about 30% of the average lifespan. The influence of the following parameters on the lifespan has been determined:


It has been found that the lifespan increases when the deformation speed is low. When the pressure rise time is six times higher (40 → 250ms), the lifespan increases by a factor of 2.5. The pressure release time influences the lifespan similarly. The limitation of the maximum expansion to 50% of the nominal stroke increases the lifespan by a factor of 4. The use of two electrodes with the same mass and same shape also creates an increase by a factor of 2. The production of a seam with a higher initial welding force also has a positive effect on the lifespan of the actuators. This increase is only about 10%. The optimal seam thickness varies depending on the structure of the material used. The increases on the lifespan which can be achieved with this optimization are by a factor of two. Weld seams with a thickness less than 20% or greater than 80% of the nominal thickness of the two films are not useful. The variation of the load cycle time shows that the actuators fail very quickly at an operating frequency of 6 seconds. For longer load cycles, the composite material has more time to relax and reduce internal stresses. With shorter cycle times the internal stresses caused by the previous load cycle are not yet dissipated and a kind of solidification occurs.

Endurance tests of actuators with different film materials were conducted in order to compare the film properties.


Determining the lifespan trajectory has shown that the operating pressure has a big effect on the durability. If the average lifetime at 6 *bar* of about 500 cycles is set to one, the actuators achieve at 8 *bar* only the value of 0.2. At a pressure of 4 *bar* they reach a 8 and at 3 *bar* they reach a value of 66. In comparison, the average lifetime of PEEK actuators at 6 *bar* is around 112 and 205 for the aramid-reinforced actuators. The failure mode mostly was breaking of the weld between the tube and the TPU.
