Robótica Médica
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Instrument movement control

Positioning problems of surgical instrumentsPositioning problems of surgical instruments. The endoscope is inserted through the abdomen at the so called point of fulcrum. It consists of positioning the instrument with a steering angle α, a rising angle β and positioning the camera at a distance ρ of the fulcrum. These spherical coordinates are defined by a coordinates system expressed in the point of fulcrum. These aspects are detailed in the right picture.

Passive wrist



Passive wrist. It is a wrist mechanism used in the surgical assistant consisting of two joints perpendicular to each other (see figure on the left). Thanks to the joint encoders, α and β-positions can be calculated, however ρ (defined as the distance from the passive wrist to the fulcrum) is impossible to be calculated. This latter increases the accuracy in placing the endoscope.




Control schemeControl scheme. . The control schemes of surgical tools are based on the estimation of the distance ρ (from the camera to the point of fulcrum) through geometrical methods. Likewise, they do not avoid the orientation errors resulting from the endoscope due to a bad estimation of ρ. The suggested scheme (see figure on the right) proposes a solution to this. It is based on the control theory, which does not just estimate ρ, but it also avoids the orientation errors mentioned above. The controller has two nested loops with two different functionalities. The inner loop imposes the robot a specific dynamic behaviour when following a Cartesian trajectory. The outer loop carries out two actions: it avoids the orientation errors resulting from the endoscope by means of spherical control, and at the same time, the geometric estimator calculates the ρ value in order to reduce minimally the estimation errors. This coupling effect provides strength and accuracy to the control scheme. This strategy has been implemented in the ERM robot.




Three-Layer Control by Passive Wrist EmulationThree-Layer Control by Passive Wrist Emulation. Robots with active wrists can apply undesired forces to the abdominal wall of the patient if any uncertainty concerning the fulcrum location is produced. In order to solve this issue, it has been proposed a methodology that allows the emulation of a passive wrist with an actuated one. To achieve this, the measure of the force applied to the abdomen permits the fulcrum estimation in order to control the endoscope’s orientation. The control scheme, which is divided into three layers to facilitate the system stability study, consists of an inner layer based on the Cartesian control for the robot, an intermediate layer devoted to control the estimated fulcrum position, and an outer layer devoted to control the spherical positioning of the endoscope.




See publication: Instrument movement control




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