Water-striding robot to help in monitoring water quality

This walking-on-water story has no religious significance, but it is important for homeland security and environmental missions. Two researchers from Carnegie Mellon University — Yun Seong Song, a Ph.D. student in mechanical engineering and Metin Sitti, assistant professor in mechanical engineering — have built a robot that mimics the water-striding abilities of insects. As is the case with the insetcs it mimicks, their water-striding robot, which even looks a bit like an insect, does not ever break the surface tension of the water and is very maneuverable.

Note that we are not talking here of a floating robot: Their robot’s small mass and long legs allow it to use the surface tension force to stay afloat (macroscale bodies, on the other hand, have to rely on buoyancy, which is based on their large volumes). A water-striding robot, equipped with wireless communication capabilities, could be used for monitoring water quality and purity in lakes, reserovirs, and sprawling water treatemt and delivery facilities. Such monitroing would be useful in both environemtnal and homeland security applications.

The robot is called STRIDE (for Surface Tension Robotic Insect Dynamic Explorer), and Sitti told PhysOrg that “STRIDEs… can walk on water only 3-4 mm deep (shallow water). Their power efficiency and agility (speed and maneuverability) are much superior for relatively small water vehicles since the STRIDE legs have much less drag than any buoyancy based robot.” This advantage is negated when the STRIDE becomes meter-scale, since surface tension force scaling is not favorable at large scales.

The researchers calculated that an optimal robot would have hydrophobic wire legs coated with Teflon, each 5 cm long. Twelve of these legs attached to the 1-gram body of the robot could support a payload of up to 9.3 grams in their experiments. The key to avoiding the breaking of the water surface is in maintaining a water-air interface which is more horizontal than vertical. As is the case with insects, for locomotion, the water strider creates a sculling motion with specialized sculling legs: Three piezoelectric actuators, when attached to the legs in a T shape, create both vertical and horizontal motion to cause the elliptical sculling motion required to move. The piezoelectric actuators provided only a small deflection, so an amplifier was needed to create large strokes, so the researchers used a resonant frequency with a vibration mode favorable to generating the sculling motion to drive the actuators.

Song and Sitti are now working on improving the robot in future prototypes.

In a recent prototype, the researchers designed a STRIDE that used two battery-powered micromotors to walk on water. This set-up had an increased mass of 6 grams, but the robot also achieved an increased speed of 8.7 cm/s. “STRIDE is 10-15 times slower than the insect, since the current prototype is almost 10 times larger than the insect,” Sitti said. “Therefore, we would miniaturize the STRIDE more. Moreover, we are integrating wireless communication, sensors, and teleoperated and autonomous control capability to the new STRIDE prototypes. Thus, we could deploy tens or hundreds of these robots to the water surface for environment monitoring.”

-read more in Yun Seong song and Metin Sitti, “Surface-Tension-Driven Biologically Inspired Water Strider Robots: Theory and Experiments,” IEEE Transactions on Robotics 23, no. 3 (June 2007): 578-89 (sub. req.)