Editorial Feature

Design of Surface-Tension-Driven Robots Inspired by Water Striders

Unlike the Illusionist David Blane, Gerridae or otherwise known as water striders (a family of freshwater bugs) do have the unique ability to walk on the surface of water. Take a look at the following video which demonstrates a water strider copulating. Copulating aside, what you see in this video is the ability of the strider to maintain a balance on the bed of water. Just how are these insects capable of gliding on water?

Well, unlike the majority of us who would avoid tension like the plague, the water strider appears to live off this instinctive behaviour as a survival technique, and so it should as it would otherwise be found dead floating on the bed of the water it inhabits. Gerridae species have a layer of waterproof hair that resides on the base of their hind legs. During contact with water, the water molecules remain as a lattice and avoid attraction to the waterproof layer on the gerrid’s legs, which create a clean barrier allowing this insect to glide across the water. Another unique feature to this insect is its claws. Claw-like projections are found on the upper part of the legs of the strider, which will prevent the top layer of the water being punctured by the insect. It is quite obvious that the ability to remain on the surface of the water is a survival technique for the gerridae by adapting a degree of surface tension and this is exactly what has inspired researchers to design and engineer surface-tension-driven legged robots.

Inspiration

The adaption of suboptimal biological systems into robotic technology is a big challenge among design engineering. Designing a robotic water strider involves playing close attention to the physical characteristics of Gerridae. Two main physical characteristics to the gerridae are particularly important:

  • The ability of this insect to stay on water
  • The sculling motions created by the middle legs that force the insect to move across the bed of water.

Suhr SH et al (2005) presented a model of a robotic water strider, which includes 1) the supporting legs, 2) the actuating legs that support the steering of the robotic body, 3) the central component to the body (made up of carbon fibers in an effort to reduce the body weight of the strider robot), and 4) the main body of the robot, which is structurally made up of a power source to drive the on-board electronics, a central control unit, a controller, and sensors.

It is important that the actuators to such a small light-weight structure are capable of steering the body of the robot with precision and speed with enough force output and little energy consumption. Though this is a difficult obstacle to overcome when working on such miniature robotic systems, Suhr SH et al, did manage to explore this to find that piezoelectric actuators were ideal based on the following advantages:

  • They allow a motor to move at high speed
  • Lightweight
  • Very low power consumption making them energy efficient.

However, the fact that they require a high voltage also means that they adapt a small motion, which will take a strider robot much longer to get from point A to point B in a controlled space. To overcome this disadvantage, Suhr SH et al designed their strider robot to have multiple long actuator legs to allow for better complexity of motion based on creating more degrees of freedom for the robot structure. So we now know a little about the structure of a strider robot based on the research mentioned, but how exactly does this structure that is made up of actuators and electronic components keep its head above water?

We’ve already established that water striders can balance their body on the bed of water without puncturing the surface and plunging straight down into the deep end by adapting to the surface tension force of water. The actuator legs to a strider robot work in a similar fashion - the legs to the robot deform the water surface. The video below illustrates a buoyant force, the same concept adapted to design and engineer a strider robot that can stay above water.

The lift force is characterized as pressure applied by the strider robot as it rests on the bed of water, which causes the weight of the water volume to push down. This effect has to be balanced out by a lifting force keeping the object resting on the water. Mathematically, this phenomenon is based on the amount of water pushed down by an external force (surface tension force [fr]) that is equal to the amount of water creating an upward force (buoyancy force [fb]) will give the effective lifting force (FL). Researchers at The NanoRobotics Laboratory, Carnegie Mellon University have engineered a microbot that demonstrates use of the surface tension of water in order to manoeuvre (see video below).

Design and Analysis

A study by Song YS et al (2007), which focused on the theory and applications for biologically inspired water strider robots suggests that the length and number of the actuator legs need to be proportional to the lift force. This research involved designing the strider robot with legs that were approximately 12.6 mm apart to maximise the lift force. This research team also designed the tip of the legs to position in an upward direction to minimize penetrating the surface of the water. The team also found that it was important not to have an actuator leg that was too long as this could compromise the motion of the central actuator leg.

The research mentioned in this article clearly opens new possibilities for studying robot locomotion on water and will allow for better judgement on what allows for better stability of a robot to manoeuvre on the surface of water.

Sources and Further Reading

  • O. Ozcan, H. Wang, J. D. Taylor, and M. Sitti, ''Surface Tension Driven Miniature Legged Robot Inspired by Water Strider Insects,'' Journal of Research of NIST, in press.
  • Y. S. Song and M. Sitti, ''Surface Tension Driven Biologically Inspired Water Strider Robots: Theory and Experiments,'' IEEE Trans. on Robotics, vol. 23, no. 3, pp. 578-589, June 2007.
  • Y. S. Song, S. Suhr, and M. Sitti, ''Modeling of the Supporting Legs for Designing Biomimetic Water Strider Robots,'' Proc. of the IEEE Robotics and Automation Conference, pp. 2303-2310, Orlando, FL, May 2006.
  • Suhr SH, et al. (2005). Biologically Inspired Miniature Water Strider Robot. IEEE Robotics and Automation Conference. Department of Mechanical Engineering, Carnegie Mellon University. USA.
  • Brenbaum, M. (1989). Ninety-Nine Gnats, Nits, and Nibblers. USA: Board of Trustees of the University of Illinois.

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