Two important features that make bats fly so exceptionally well are their multiple joints and the following multiple degrees of freedom in their wings and their membrane. In order to mimic the flying motion of bats it is essential to capture those aspects in the robot design.
1) Multi-articulated wing
The bat wings may have up to 25 actively actuated joints and 34 degrees of freedom in their wings. But applying this directly to a robot would make the system too heavy and it is also unnecessary for prior means. In order to solve this problem of complexity the natural motion of the Cynopterus may be taken for a guide. During the morphing(folding) motion of the wing the digits at the wrist move synchronously with each other (abduct and adduct) and they also move almost synchronously with elbow flexion/ extension. Thus the morphing motion can be implemented by a single actuator and cable of different radii. The overall mechanism looks as follows:
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| Figure 1. Cables connecting the joints: The red lines represent the cables that actuate the shoulder piece moving the wing in a vertical plane. Orange lines represent the cables that actuate the humerus piece moving the wing in the horizontal plane. Yellow lines represent the cables that actuate the radioulna piece flexing the elbow. Green, blue, and purple lines are cables actuating the pieces of digits V, IV, III, respectively. These cables connect to the humerus at the elbow in such a way that the three wrist joints flex synchronously with the elbow.[1] |
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| Table 1. Different radii of the cables |
"The motion of the wrist and elbow are linked through cables similar to a drive belt. A complete cable circuit wraps around the circular distal end of the humerus and then circular tongue of a digit piece (figure 1, green, blue, purple lines). As the radioulna rotates around the end of the humerus, the cables rotate each digit around the end of the radioulna. Each of the digit pieces can thus move synchronously with the elbow. In C. brachyotis, our model species, the digits ab/adduct synchronously in time, but with different magnitudes. To reproduce this feature, the model wrist employs a gearing system where the radius of each of the circular tongues on the digits is proportional to the magnitude of required rotation of the digit relative to that of the elbow (table 3). For example, the tongue on digit V has a radius of 8.0 mm, compared to the elbow joint radius of 6.0 mm, so it moves 0.75 times as much as the elbow."[1]
The actuation of these cables is implemented by servo motors as shown in figure 2.
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| Figure 2. Servo motor connected through cables to the joint |
Through this system the joint can be rotated both ways without any gears or transmission bars. The servo motor is used because it can control angular velocity, angular position and force delivered to the system.
All in all this multi-articulated wing system efficiently recreates the morphing motion of the bat wings without mimicking the whole joint system of the bat. Thus flapping and morphing can be controlled during flight to make flight as similar as possible to the flight of bats.
2) Membrane
In biological bats, the membrane is a tissue, which is very flexible with a low elastic modulus, very thin and very compliant. The membrane provides the surface for the aerodynamic loads while filling in the space between joints and bones. Thus it should not resist wing motion, but still bear the aerodynamic loads. Also the membranes contain dozens of tiny muscles called plagiopatagiales proprii that are supposed to control tension and strain in the membranes and be responsible for stretch perception. Tiny hairs on the surface of the membrane sense airflow conditions and improve flight control. Out of these many features two are very important for a successful bat robot: i) stretch ability and ii) cambering properties.
Stretch ability can be provided by choosing the right material such as polydimethylsiloxane. The material should be able to stretch several times its size without too much resistance and return after. A weakness to the artificial membranes are that they easily tear when directly attached to the bones and body of the robot. Thus they are attached to elastic bands that can slide along the bones - this feature was also mimicked from the biological bat.
The second cambering property is important since it influences lift production, flight velocity, and flapping frequency. Camber is relatively constant and hovers around 10% of the chord length during downstroke as can be seen in figure 3. The spikes in the graves come from the top of upstrokes where the wings are folded. The camber can be implemented by a pre-arranged camber angle of the wings as in figure 4.
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| Figure 3. Camber in bats and camber during bat flight |
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| Figure 4. Camber implemented in a bat robot |
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| Figure 5. Cambering in experiments |
Figure 5 shows and actual testing with cambering properties.
When implementing both the multi-articulated wing and the membrane the wings of a bat robot could look like this.
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| Figure 6. Batbot with multi-articulated wings and membrane in comparison to real bat |
References:
- J.W. Bahlman, S.M. Swartz, K.S. Breuer, 2013, Design and characterization of a multi-articulated robotic bat wing, IOP Publishing, Bioinspiration & Biomimetics
- J.A. Cheney et al., 2014, Membrane muscle function in the compliant wings of bats, IOP Publishing, Bioinspiration & Biomimetic
- J.D.Colorado, 2012, BaTboT: a biologically inspired flapping and morphing bat robot actuated by SMA-based artificial muscles, Universidad Politecnica de Madrid
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