Two-Dimensional Remote Actuation of Centimeter-Scale Objects With a Concave Bottom Using Airborne Ultrasound

We propose a method for two-dimensional remote position control of centimeter-scale objects using airborne ultrasound. To actuate objects 1–2 cm in size, previous studies utilized magnetic methods because they are too small to incorporate motors and batteries. In addition to magnets, airborne ultrasound, which produces a noncontact force, can actuate various objects from the outside. However, when the size of an object is larger than the ultrasound wavelength, methods using standing waves cannot be applied. We introduced an acoustically transparent screen and designed objects with concave bottoms so that the position of objects with a size larger than the wavelength could be controlled on the screen by an ultrasonic focus. Because our method relies only on the concave bottom of the object, its materials and visible upper parts can be designed according to the usage scenario. For example, food and transparent objects, which are difficult to equip with mechanical components, can be remotely actuated. It was confirmed that an object with a concave bottom diameter of 12 mm moved continuously at an average speed of 28.5 mm/s. Actuating multiple objects is also achieved by the time-division control of the focal point without depending on their mutual positional relationships.