Bat morphology: Shape of a bat robot

 In bat-inspired robot design two major factors are taken from the biological model:
1. shape and 2. working mechanism. This section reviews shape of the bat robot and compares it to the one of its biological counterpart.
 The first thing we need to know is the basic anatomy of a bat. Bats are the only mammals that are able to fly and what makes their flying special are their wings. As can be seen in figure 1, a wing of a bat has a shoulder, an elbow, a wrist and five digits, which help the bat control its flight. These many joints provide the bat with much more freedom in flight maneuvers than birds or insects. The second thing that makes bat wings special is the membrane that connects the joints and bones. The high flexibility of the membrane combined with a low elastic modulus enables the various contractions and extensions of the wings without rupture and big resistance to the motion. The membrane also changes camber passively according to the aerodynamic load and it is suggested that muscles in the membrane increase stiffness or prestrain the membrane to adjust to different airflow conditions. Thus modeling the wing structure and the functions of the membrane are key points to a successful bat robot.

Figure 1. Basic anatomy of a bat

   For the bat robot the morphology of the bat, especially its wings, becomes the biological framework of robot design and construction. Geometric parameters as well as quantification of morphology, kinematics and aerodynamics parameters are taken from the bat and defined as guide lines of design.

Table 1. Weight and geometric parameters of model bat

Table 2. Quantified parameters of morphology, kinematics and aerodynamics

Table 1 and 2 are examples of design targets that are used for robot design. The data in the tables are taken from the species Rousettus aegyptiacus, which was selected based on four criteria: i) efficiency in lift production at ii) lowest flapping frequency with iii) about half-meter wingspan and iv) minimum weight. An important point with those design targets is that rather than the exact values the ratios of those aspects should be aimed at. Also these parameters are orientated to the total mass, which has to be taken into account for a successful design. 

 As mentioned above the wings of the bat are most important and thus morphology of these have to be well understood. In order to model the wings, simulate their movements and calculate flying mechanisms from camera observation the wings can be represented in three geometric parameters: i) wing chord, ii) leading edge position and iii) position of the center of mass. These are shown in figure 2 and table 3.

Figure 2. Wing segments

Table 3. Geometric parameters of wing segments

 Using this biological design frame a bat robot can be designed. An example of a bat robot to simulate wing motion is shown in figure 4.

Figure 4. Multi-articulated robotic bat wing

References: 

1. J.D.Colorado, 2012, BaTboT: a biologically inspired flapping and morphing bat robot actuated by SMA-based artificial muscles, Universidad Politecnica de Madrid
2. J.W. Bahlman, S.M. Swartz, K.S. Breuer, 2013, Design and characterization of a multi-articulated robotic bat wing, IOP Publishing, Bioinspiration & Biomimetics
3. J.A. Cheney et al., 2014, Membrane muscle function in the compliant wings of bats, IOP Publishing, Bioinspiration & Biomimetics