A novel micro-robotic platform is designed and developed. The micro-robot motion is induced by centrifugal forces generated by two DC vibration motors, installed inside the platform body. When the micro-motors are driven in a controlled manner, then the resulting vibrations cause the platform to perform controlled x, y, θ planar motion with micrometer resolution and speeds greater than 1 mm/s. This is a novel motion principle, and radically different form all previous techniques used for micromotion. The great advantage of this type of actuation is the low power consumption, the simple driving electronics and the low cost and readily available mechanical parts used for the construction.
Three fixed small steel balls located at the vertices of an equilateral triangle provide the contact points between the micro-robot and the ground. The actuation of the platform employs 2 miniature- vibration motors. Each vibration motor is axially coupled to an eccentric load, while the control input is the rotation speed ω of the motor. During motor rotation, the eccentric mass of the load generates periodic forces, depending on the rotation speed, which are transferred to the contact points and interact with the friction forces. When the actuation forces exceed the Coulomb friction level, slip of the platform occurs, and motion (sliding) is induced. When the actuators are operating at the same rotation speed, then their operation is called synchronous, otherwise is called asynchronous. In the synchronous mode, if the motors rotate at the same sense, the platform performs pure linear motion. If the motors rotate at an opposite sense, then the platform performs pure rotational motion. On the other hand, the asynchronous mode produces a two-dof motion, depending on the difference and the mean value of the rotation speed of the motors.
Two closed-loop motion controllers have been designed to generate controlled planar motion of the micro-robotic platform. Both drive the end-effector (the tip of a needle mounted on the micro-robot) along a desired path strip towards a desired goal position. The first control algorithm in based on a set of simple rules, where three pairs (A, B, C) of motor rotation speeds (2 vibration motors) have been experimentally predefined. The A and B denote rotation speed pairs that result in a platform displacement with a positive or negative instantaneous curvature respectively. The C results in a straight-line translation. The controller, using an image-processing algorithm, tracks the tip of the needle under the microscope, and compares the actual trajectory of the needle tip with the desired path strip. If it translates inside the desired path strip, then the C rotation speed pair that results in a straight-line translation is commanded. Otherwise, the A or B pairs are commanded, depending on the location of the needle tip, forcing the micro-robotic platform to enter and translate inside the desired path strip. When the needle tip reaches the target location, both motors are stopped. The second control algorithm is a PI controller with the same goal. The main difference is that the controlling commands are not anymore predefined and static motor rotation speed pairs, but dynamic changing rotation speed values that depend on the difference between the actual location of the needle-tip and the target.
In both algorithms, the input to the controller is the location of the needle tip. To this end, and since the micro-robot operates under a microscope, an image-processing algorithm has been developed that reads the microscope image and outputs the x, y, θ coordinates of the needle-tip in an adequate frequency. The algorithm, running on a PC, reads the image from a video camera attached on the microscope, and then saves a suitable Region-Of-Interest (ROI). The acquired image is converted into a binary image, and the outline of the needle is traced. Next, the needle-tip position and orientation are calculated from the coordinates of the points that comprise the outline of the needle.
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