Application of the insect wing to flexible robot joints

responsibility for wording of article: Akira Takashima (OIST)

The hard insect exoskeleton has many similarities to the rigid materials that form robots. However, when insect locomotor organs are examined in detail, critical differences between them and robots become apparent. Apart from obvious differences between living tissues and nonliving materials, the mechanisms are very different on a more basic level. To completely control mechanical arms, current industrial robots are driven by mechanical motors. In contrast, the muscles of all animals, including insects, are basically flexible, although the degree of flexibility varies depending on their roles. While they cannot have mechanical motors like robots, insects have developed a unique elastic material connected in series with muscles that allows flexibility, thus it is interesting that they can utilize its materials effectively for their own lives. This elasticity can be further explored for practical applications. The material is a rubberlike protein called resilin. It was discovered in the wing hinges of grasshoppers and dragonflies and later found in almost all insect wing mechanisms. Resilin has the capability of storing elastic potential energy during flight, thereby facilitating the high-speed up-and-down rhythmical motion of the wings. While it is important for all muscles to have elasticity, the flexibility of the motor system based on the unique function of resilin is important for insects as well. The biological systems that benefit from flexibility are mainly those involving energy storage for flying or jumping. On the other hand, all muscles have some elasticity, thus the movement of living organisms universally involves series elastic components to some extent. However, conventional robotic engineering has actively avoided elasticity because elasticity leads to vibrations, which are difficult to control and lead to loss of stability in robot motion. In contrast, insects manage to control movement and at simultaneously avoid vibration without difficulty. In fact, insects can use elasticity-derived vibrations for generating very fast and flexible motion. Therefore, it is important to determine whether a multi-joint manipulator with series elastic components in the actuator can be appropriately controlled.

Recently, various concepts for applying flexibility to robotic engineering have been proposed. One of these is a manipulator model involving series elasticity in the actuation of joints. Although this model is a conventional joint-based manipulator, it uses a tension spring functionally that is equivalent to resilin in the insect wings. This model is superior to conventional hard robots. Elastic actuation has definite advantages surpassing difficulties in precise position control. Among other features, the elastic drive robot has positional tolerance. This means that because the arm is not forcibly knocked out of position when there is any interference from outside the arm, it can immediately stop even when collision with an obstacle has occurred. Furthermore, its collision with obstacles can be detected by the arm sensor and there is sufficient time available to direct the motor for an appropriate response. In collision, the elastic system is much less dangerous than a hard system, making it more desirable for interaction with humans. The flexible arm can also move more smoothly toward its target objects. Flexibility is particularly desirable for assembly tasks. The flexible arm can effectively use guide structures to facilitate precise positioning, such as grooves and conical holes. Manipulation such as this is analogous to the manner in which a human arm moves. Series elasticity is advantageous not only for the operation of robot arms but also for the mechanical structure. The biggest difference between the hard system and the elastic arm is that outside forces such as those from heavy loads can change the shape of the elastic arm. Because the shape of the rigid systems can be hardly changed, the arm structure needs to be much heavier. The elastic system makes it possible to reduce the weight of arms even for lifting heavy loads. This feature results in enhanced safety in addition to being advantageous in terms of the system design. Another difference is that series elasticity can endow the gears and motors with flexibility, which can reverse-transmitted to the gears and motors themselves. Elasticity can serve as a filter for forces that may damage the drive system as well. Therefore, the motors and gears do not need to be as robust as they have to be in a rigid system so that they can be downsized.

Further Reading

昆虫ミメティックス Insect Mimetics(2008),針山孝彦,下澤楯夫,pp798-806

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