*2.1.1. Flexible fluidic "Expansion" actuators*

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These requirements are obvious in the field of service robotics since any interaction with animals, technology, or humans will be safer by implementing them. Operating industrial robots usually requires a strict division between the working area of the robot and the working area of the human, since industrial robots normally do not fulfill these requirements. However, there are many tasks that could be accomplished resulting in higher quality or more

A lower inertia of the robot allows faster operating speed. At the same time lower inertia reduces the impact in case of a collision. The application of composite materials can achieve this while maintaining stiffness and precision of the robot. An impressive example is the DLR "Lightweight Robotic Arm III" (LWR III) [50]. Another example presenting a robot for the

When looking at compliant actuation systems for robots it becomes clear that there is currently much effort to add compliance to conventional drives by adding elastic elements to the drive chain. This concept is referred to as series elastic actuation and has been carried out in many various forms [24, 121, 138, 140]. Other drives with more or less inherent compliance are piezo-drives, shape-memory-actuators (SMA), electrorheological drives, and polymeric actuators. Fluidic actuators are a well-suited actuation principle for compliant actuation. Whereas pneumatic actuators are already compliant because of the compressibility of gases, hydraulic actuators need the integration of compliant membrane structures in order to achieve

In addition to its drive elements a robot consists of structural elements connecting the drive elements. So independently from the drive system a robot can exhibit compliant

A Flexible Fluidic Actuator generally consists of a flexible shell that transmits potential energy, delivered by the pressurized fluid, into a mechanical force, which then can be used to create a

The flexible fluidic operating principle has a strong background in biomimetics. Gutmann [41, 42] established the "Hydroskelett-Theorie" as an approach to explain evolutionary biology via the concepts of constructional morphology. He understood that the design-principles of hydrosceletts are responsible for the general designs of organisms. Gudo et al. [39, 40] developed this idea further by introducing the term "Engineering Morphology which describes how Gutmann's ideas can be applied to technical design. The biomimetic background of flexible fluidic actuation was specifically discussed in [11, 130]. These works discuss several examples on weevils and spiders and describe how crucial the membrane properties are regarding efficiency and durability of the whole actuation system. The transfer of the evolutionary optimized flexible fluidic drives developed by nature into powerful

compliance. This group of actuators is referred to as "Flexible Fluidic Actuators".

characteristics via the integration of compliant structural elements.

1. Lightweight

2. Inherent Compliance

efficiency, with closer human-robot interaction.

manipulation of small masses is described in [75].

**2. Historical background**

**2.1. Flexible fluidic actuators**

technical systems is one focus of this work.

motion.

The history of this group of actuators starts with simple designs for lifting applications [32, 90, 133, 136]. Current "lifting bags" are mainly used for rescue operations [134]. However, the biggest group of expansion actuators is air-springs or damping elements [17]. These systems are either bellow-type actuators or rolling lobe-type actuators. The first rolling lobe systems were developed in the 19th century [77] but the principle is still popular today. Some differences between types of air springs are depicted in figure 1.

Bellow-type actuators are suitable drives for application in harsh environments since there is no friction and they can compensate or create tilt motions up to 30◦ without any additional transmission elements. The smallest commercially available bellow-type actuators have a diameter of 160 *mm*. The following patents [10, 31, 37, 80, 96] give an idea as to how these air actuators have developed throughout the last century. However, these patents only describe the general design. Newer patents discuss problems concerning fatigue of the elastomer as well as the connectors [107].

**Figure 1.** Comparison of Different Kinds of Expansion Actuators [17]

Another big group is expansion actuators that work as rotary drive elements. [119] describes a solution where the structural integrity is created by the housing and the torque is created by an internal bellow-type actuator (figure 2(a))

The design proposed in [62] describes a bellow-type actuator suitable for linear or rotary motions as well as single chamber actuators (figure 2(b)-2(d)). The proposed materials for the bellow include rubbers as well as metals.

A very interesting design is introduced in [13]. The patent refers to fabrication techniques used in the tire industry and describes a layered membrane design with several reinforcement layers (figure 2(e), 2(f)).

[23] discusses the stress distribution in the membrane of a rotary actuator and requirements for achieving low bending stiffness and high tensile strength. Another approach is described in [34]. Here the drive element consists of a mono-material system. The whole drive is fabricated in one step (figure 2(g)).

4 Will-be-set-by-IN-TECH 570 Smart Actuation and Sensing Systems – Recent Advances and Future Challenges

**Figure 2.** Radial Cross-Section View of Different Fluidic "Expansion" Drives

While the presented concepts are designed to operate with pressures in the range of 0 − 20 *bar* the development in [74] is operated with pressures up to 200 *bar*. The actuator set-up allows both, linear and rotary actuation (figure 2(h), 2(i)). The focus in these works lays on heavy duty machinery but rotary drives or trunk-like structures are discussed for robotic applications as well. Detailed concepts regarding layered fiber reinforcements in the shell are introduced.

While the previous example requires complex knowledge and technology to produce an individually shaped membrane, other examples implement standard materials for flexible fluidic actuators. In [61, 81, 110] a design is proposed that uses bulky materials such as ordinary water hoses to form the actuator. Figure 2(j) shows one set-up of this FLEXATOR muscle. Subsequent developments applied the FLEXATOR technology to the fields of rehabilitation [103] and horticultural robotics [129]. The works of Prior et al. [103] introduced a unique approach that came to be known as "hybrid actuation" [117]. Here the powerful fluidic actuators are combined with precisely controllable electrical actuators in a parallel configuration.

#### *2.1.2. Flexible fluidic "Contraction" actuators*

This type of actuators generates a tensile force when pressure is applied. There is large variety of "contraction" actuators. They must be sorted into two groups. The first group includes actuators that generate a tensile force due to **"Anisotropic Membrane Stiffness"**. Daerden and Lefeber described some of those actuators in their review article [20]. These actuators increase in surface area when pressurized. The axial contraction is coupled to a radial expansion in which some of the energy is used for membrane deformation. Generally Joseph L. McKibben is said to be the inventor of the most popular design, often referred to as "McKibben Muscle". However, earlier patents describe the same design. In 1929 Dimitri Sensaud de Lavaud [22] introduced a fluidic muscle as shown in figure 3(a). This early work was later followed up by the patents of Morin [89] in 1947 and Woods [141] in 1953, where the design and characteristics of the fluidic muscle were described in detail. The actuators consist of a highly elastic inner membrane that is covered with a helically wound fiber reinforcement like a braided fiber hose (figure 3(b)). When pressurized the fiber angles change until the critical fiber angle of *θ* = 54, 4◦ is reached (figure 3(c)) [141].

**Figure 3.** Different Fluidic Muscles

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(a) [119] (b) [62] (c) [62] (d) [62] (e) [13]

(f) [13] (g) [119] (h) [74] (i) [74] (j) [47]

While the presented concepts are designed to operate with pressures in the range of 0 − 20 *bar* the development in [74] is operated with pressures up to 200 *bar*. The actuator set-up allows both, linear and rotary actuation (figure 2(h), 2(i)). The focus in these works lays on heavy duty machinery but rotary drives or trunk-like structures are discussed for robotic applications as well. Detailed concepts regarding layered fiber reinforcements in the shell are introduced. While the previous example requires complex knowledge and technology to produce an individually shaped membrane, other examples implement standard materials for flexible fluidic actuators. In [61, 81, 110] a design is proposed that uses bulky materials such as ordinary water hoses to form the actuator. Figure 2(j) shows one set-up of this FLEXATOR muscle. Subsequent developments applied the FLEXATOR technology to the fields of rehabilitation [103] and horticultural robotics [129]. The works of Prior et al. [103] introduced a unique approach that came to be known as "hybrid actuation" [117]. Here the powerful fluidic actuators are combined with precisely controllable electrical actuators in a parallel

This type of actuators generates a tensile force when pressure is applied. There is large variety of "contraction" actuators. They must be sorted into two groups. The first group includes actuators that generate a tensile force due to **"Anisotropic Membrane Stiffness"**. Daerden and Lefeber described some of those actuators in their review article [20]. These actuators increase in surface area when pressurized. The axial contraction is coupled to a radial expansion in which some of the energy is used for membrane deformation. Generally Joseph L. McKibben is said to be the inventor of the most popular design, often referred to as "McKibben Muscle". However, earlier patents describe the same design. In 1929 Dimitri Sensaud de Lavaud [22] introduced a fluidic muscle as shown in figure 3(a). This early work was later followed up by the patents of Morin [89] in 1947 and Woods [141] in 1953, where the design and characteristics of the fluidic muscle were described in detail. The actuators consist of a highly elastic inner membrane that is covered with a helically wound fiber reinforcement like a braided fiber hose (figure 3(b)). When pressurized the fiber angles change until the

**Figure 2.** Radial Cross-Section View of Different Fluidic "Expansion" Drives

configuration.

*2.1.2. Flexible fluidic "Contraction" actuators*

critical fiber angle of *θ* = 54, 4◦ is reached (figure 3(c)) [141].

In [101] Paynter describes a variation of this type of fluidic actuator. The "hyperboloid muscle" is equivalent to a prestretched fluidic muscle, which extends the range of motion (figure 3(d)). Other set-ups from Paynter are discussed in [98] and [100].

Commercially available fluidic actuators were introduced by Bridgestone Corporation, Japan, FESTO AG&Co. KG, Germany, and Shadow Robot Company, UK. Bridgestone introduced a single-acting [128] and a antagonistic [92] actuator design (figure 3(e)) but soon stopped their activities in the field. With operating pressures up to 2 *bar* and a fatigue life of 67, 000 load cycles these actuators weren't really competitive.

Nowadays FESTO offers the biggest portfolio of fluidic muscles [29, 30]. Operating pressures are 0 − 8 *bar* in connection with a fatigue life of 10, 000 − 1 *Mio* load cycles depending on the load case.

Lewis [76] and Monroe [88] proposed a design with only axially fiber reinforcements. Thus actuation is connected with a radial stretch of the pure rubber sections between the axial fiber strands (figure 3(f), 3(g)).

The second group of "Contraction" actuators generates the force due to **"Vectored Structural Degrees of Freedom"**. These actuator designs try to raise efficiency and to minimize the hysteresis compared to the first group of actuators. Ideally there is no strain of the membrane and almost no internal friction. When pressurized these actuators increase in volume while maintaining the same surface area. Yarlott [143] proposed a folded structure that unfolds when pressurized and thus contracts (figure 4(a))

**Figure 4.** Some FFAs based on "Vectored Structural Degrees of Freedom"

#### 6 Will-be-set-by-IN-TECH 572 Smart Actuation and Sensing Systems – Recent Advances and Future Challenges

The work of Kukolj [73] shows an actuator with a net as the fiber reinforcement. This eliminates the friction between the fiber strands, but the friction between membrane and fiber reinforcement remains (figure 4(b)). Immega [58] enhanced this idea by implementing a stiff, folded membrane in between the fiber mesh (figure 4(c)).

A newer design known as "Pleated Pneumatic Artificial Muscles (PPAM)" was introduced by Daerden and Lefeber [19, 21]. The design is similar to the Yarlott muscle. Figure 4(d) and 4(e) show the design and the force-displacement characteristics of these artificial muscles.

Erickson [25] described a contraction actuator that can be considered an inverse rolling-lobe cylinder. This set-up has a large working range of 40-60% of the initial length (figure 4(f)).
