**4.1. Vulcanized Actuators**

14 Will-be-set-by-IN-TECH

• **Varying the time of the load cycle:** In addition to previous experiments where the load cycle time was six seconds (3 *s* pressure / 3 *s* no pressure) the cycle times were varied at 2,

• **Evaluate the best pressure rise time:** In this experiment the actuators were filled via a restrictor. So the time to fill is adjustable. These different times were 40, 125 and 250 *ms*. A shorter time than 40 *ms* between the electrical signal to the switching valve and the reach

• **Evaluate the best release time:** Analogous to the determination of the pressure rise time, the pressure reduction period was varied. The restrictors were used with the same settings

• **Limitation of the expansion:** This test was necessary to evaluate the effect on durability when the maximum deformation is limited. The expansion is limited to 100, 66, 50, and

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

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

• **Determination of the lifetime curve:** These tests were conducted with ten actuators for each pressure step. This pressure was applied cyclically to determine the durability of the actuator as a function of pressure. Pressure values of 3, 4, 6 and 8 *bar* were used. All tests

• **Continuous operation of the actuators of PEEK reinforced TPU film:** With ten actuators made of this material an endurance test at six bar and a cycle time of 6 seconds was performed. In this endurance test, the results of some of the previous test runs were considered. The thickness and the initial load were adjusted. The other parameters like pressure rise/release time, expansion ratio, and welding tools have not been altered to

• **Continuous operation of the actuators of aramid reinforced TPU film:** The parameters of

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

of 95% of the desired pressure was technically not possible.

cycle are not yet dissipated and a kind of solidification occurs.

were carried out using a commercial film material.

this endurance test were identical to those of PEEK actuators.

ensure the comparability of results.

4, 6, 10, and 20 seconds.

as the pressure rise test.

33%.

the film properties.

All vulcanized actuators are fabricated using the process described in chapter 3. This process represents a scalable and reproducible fabrication process resulting in the following actuator geometries. Figure 17 gives an overview over all rubberbased FFAs. Each section includes the general properties and the actuator characteristics. The standard operating pressure is 0 − 10 *bar* for all vulcanized FFAs.

**Figure 17.** Series of Vulcanized FFAs: (from left to right) 36mm, 18mm, 11mm, and single chamber FFA
