The comanipulation paradigm, in which a user and a robot simultaneously hold a tool, allows for gesture guidance. In particular, virtual fixtures, which are geometrical constraints imposed to the tool by the robot, have received a great interest in the domain of surgical applications. So far, this concept has been implemented in the context of open surgery. This paper explores the application of virtual fixtures for minimally invasive surgery, where the tool is inserted in the patient through a fulcrum. Here, a key issue is to return to the surgeon forces that are virtually applied at the instrument distal tip, while the robot is physically attached to the instrument proximal handle. To this aim, two approaches are investigated. A first approach consists in applying a full wrench at the proximal end of the instrument that is equal to the wrench constituted by a pure force applied to the instrument's distal tip. A second approach consists in applying a pure force to the instrument proximal end, thanks to a lever model about the fulcrum. The two approaches are compared through experiments during which naive subjects blindly perform virtual object palpation and robot guided movements. During experiments, indicators involving motion and force analysis are computed. The user capacity to distinguish between several virtual objects is evaluated as well. Although drastically different, the two approaches provide assistance with a similar level of efficiency.
Abstract-A robotic device, aimed at assisting a urologist in positioning an endorectal ultrasound probe to perform prostate biopsies, is presented. The proposed system is a comanipulator that holds the probe simultaneously with the urologist. This robot supports two modes of operation: the free mode, where the entire movement control is left to the urologist when he/she positions the probe with respect to the prostate thanks to the feedback provided by the ultrasound images; the locked mode, where the robot's role is to precisely maintain the targeted biopsy site at a given location, while the urologist can insert a needle through a guide mounted on the probe and proceed to biopsy. The device combines three brakes and three motors. This allows both transparent comanipulation in the free mode with six degrees of freedom liberated and stabilization of the probe in the locked mode. At the control level, a main challenge in the locked mode raises from antagonistic constraints: the needle placement shall be precise in spite of unknown external forces due to the contact between the probe and the rectum; the robot apparent impedance shall be low due to security constraints. This is solved by an inner low stiffness controller and an outer slow integration for canceling steady-state errors. Both in vitro and in cadavero experimental results show the efficiency of the system in the two modes of operation.
A novel robotic device, aimed at assisting a urologist in performing prostate biopsies guided by an endorectal ultrasound probe, is described. The paper describes the robot kinematics and the actuation system. The actuation system combines electromagnetic brakes, balancing springs, and electrical motors with cable transmissions. The robot supports two modes of operation: the free mode, where the entire control of the probe movements is left to the urologist, and the blocked mode, where the robot precisely maintains the probe at a given position and orientation with respect to the prostate. For the blocked mode, the set of specifications is antagonistic: firstly, a security constraint requires a low robot stiffness to allow to compliantly adapt to potential movements from the patient; secondly, a precision constraint requires a high robot stiffness in order to maintain the position and orientation of the probe in the presence of unmodeled external forces, when the robot is switched from the free mode to the blocked mode. A control strategy is developed to obtain this behavior. It combines an inner impedance controller with a relatively low stiffness and an outer intelligent position integrator that operates only during a limited period of time, when switching from the free mode to the blocked mode. Both in vitro and in cadavero experimental results show the efficiency of this approach.
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