Fundamentals of high-speed, piezo-actuated, three-legged motion for miniature robots designed for nanometer-scale operations

摘要

The fundamental principles of a locomotion system being part of the development of a miniature wireless autonomous robot capable of sophisticated tasks at the molecular and atomic scales is briefly described. The robot can perform several thousands of steps per second with step sizes as small as a few tenths of nanometers. The mechanical structure allows a scanning tunneling microscope (STM) tip to be installed. Since the main purpose is to bring an instrument in the form of a miniature robot to the samples, very high precision in movement and positioning is needed in order to move the robot within the limited range of an integrated instrument such as the STM. The mechanical structure of the microrobot is built upon a tripod design. The base is made up of three piezo-ceramic tubes arranged in a conical shape with the apex pointing upward. The end of each tube closest to the surface is capped with a conducting ball for power delivery to the robot. The legs are equidistant to each other, and are at an angle of 45 degrees to the surface. Each tube has four axial segmenting electrodes arranged as four 90 degrees quadrants along the tube or leg. Resting on top of the three legs is a platform on which the electronics and an instrument are attached. Motion of the robot is achieved through bending and stretching of the three legs through applied voltages to the different quadrants of each tube. The friction and the properties of the walking floor are very important parameters. Since 15 to 20 Watts of power are provided to the high-performance embedded electronics of each microrobot through the legs when in contact with the floor, arcing that causes fast erosion of the floor and the legs becomes a real concern. A special circuit embedded onto each robot has been developed to compensate for this problem, but this causes constraints on the time available to perform each motion step. Preliminary results indicate successful motion of the microrobot using 4000 steps per second with step sizes varying from a few tenths of nanometers to a few micrometers. To achieve a higher level of miniaturization, the typical method of modulation of piezo-actuators has been replaced by a series of high-voltage spikes, similar to the method of activation used in insects. It was found also that friction plays a great role and that the properties of the surface of the legs in contact with the floor and the floor itself, must be chosen accordingly to several factors. Furthermore, it was found that the effect and the theory of friction at that scale is not very well defined and this brings uncertainties in the prediction of motion, especially at the nanometer-scale.