Cong Dung Pham

Analysis of a moving remote center of motion for robotics-assisted minimally invasive surgery

This paper presents a novel control architecture for controlling a moving remote center of motion in addition to the end-effector motion during robotic surgery. In minimally invasive surgery, it is common to require that the point at which the robot enters the body, called the incision point or the trocar, does not allow for any lateral motion. It is generally considered that no motion should be applied to this point in order to avoid inflicting damage to the patients skin. However, in surgery, the patients body may be moving, for example due to breathing or the beating of the heart. In order to compensate for this motion-or if we for some other reason want to leverage the possible motion of the incision point to improve performance in any other way-we derive a new framework which allows us to actively control the motion both at the incision point and the end effector. The novelty of the approach lies in the possibility of controlling both the incision point and the end effector to follow a trajectory, and that we find a Jacobian matrix that satisfies the velocity constraints in both the end-effector and the incision point frames. This allows us to formulate a framework that is not only suited for control, but also for analyzing the condition number of the Jacobian and avoid any singular configurations that may arise either as a result of the constrained motion or the manipulator geometry. The approach is verified experimentally on a redundant industrial manipulator.

Control allocation for mobile manipulators with on-board cameras

This paper presents a new set of approaches for teleoperation of mobile manipulators with on-board cameras. Mobile manipulators consist of a robotic arm which provides for interaction and manipulation, and a mobile base which extends the workspace of the arm. While the position of the onboard camera is determined by the base motion, the principal control objective is the motion of the manipulator arm. This calls for intelligent control allocation between the base and the manipulator arm in order to obtain intuitive control of both the camera and the arm. We implement virtual mass-spring-damper forces between the end-effector and the camera so that the camera follows the end-effector with an overdamped characteristics. The operator therefore only needs to control the end-effector motion, while the vehicle with the camera will follow naturally. The operator is thus able to control the more than six degrees of freedom of the vehicle and manipulator through a standard haptic device. The control allocation problem, i.e., whether the vehicle or manipulator arm actuation is applied, is then performed automatically so that the operator can concentrate on the manipulator motion.

On the design of a low-cost, light-weight, and highly versatile agricultural robot

This paper describes the development and the main considerations for designing the Thorvald platform, a versatile robot for operation in agricultural fields. The main objective is to develop a robot that can perform all operations, from seeding, to weeding and harvesting. This requires the robot to be able to carry a wide variety of tools. In addition, we require the robot to be lightweight so that it can operate during wet periods without getting stuck or damaging the soil structure. We focus on keeping the overall costs of the robot at a level which makes it economically viable compared to conventional solutions. We obtain this by constructing the frame so that it flexes, which reduces complexity and makes the frame cheap to build, but at the same time guarantees that all wheels are in contact with the ground. We also describe the development of tools to be attached to the platform, and discuss the implications of the flexible design on the robot and tool control.

An Analytical Approach to Operational Space Control of Robotic Manipulators with Kinematic Constraints

Abstract This paper presents a novel control architecture for operational space control when the end effector or the robotic chain is kinematically constrained. Particularly, we address kinematic control of robots operating in the presence of obstacles such as point, plane, or barrier constraints imposed on a point on the manipulator. The main advantage of the proposed approach is that we are able to control the end-effector motion in the normal way using conventional operational space control schemes, and by re-writing the Jacobian matrix we also guarantee that the constraints are satisfied. The most challenging problem of obstacle avoidance of robotic manipulators is the extremely complex structure that arises when the obstacles are mapped from the operational space to joint space. We solve this by first finding a new set of velocity variables for a point on the robot in the vicinity of the obstacle, and on these new variables we impose a structure which guarantees that the robot does not hit the obstacle. We then find a mapping denoted the Constrained Jacobian Matrix from the joint variables to these new velocity variables and use this mapping to find a trajectory in joint space for which the constraints are not violated. We present for the first time the Constrained Jacobian Matrix which imposes a kinematic constraint on the manipulator chain and show the efficiency of the approach through experiments on a real robot.

A control allocation approach to haptic control of underwater robots

This paper presents a new set of approaches for teleoperation of remotely operated vehicles (ROVs), or autonomous underwater vehicles (AUVs) with on-board cameras. We address the control allocation problem for underwater vehicles, i.e., how to best distribute the control forces so that the robot behaves as intuitively as possible based on the operators commands. This calls for intelligent control allocation in order to obtain intuitive control of the camera as we do not control the camera directly, but rather the underwater robot, with slow and inaccurate dynamics, in addition to normally just a few degrees of freedom to the camera, which has a lot faster dynamics. For the presented approach, the operator only needs to control the camera motion, while the vehicle will follow the camera naturally. The operator is thus able to control a system that is kinematically very different from the haptic device very intuitively. The control allocation problem, i.e., how the vehicle and camera actuation is applied, is then performed automatically so that the operator can concentrate on the camera motion.

Singularity analysis of robotic manipulators with velocity-constraints for minimally invasive surgery

This paper presents a novel framework for analyzing the mobility of constrained serial manipulators. We focus on the type of constraints that arise in minimally invasive surgery, where a long shaft is inserted into the human body through a incision point, or trocar. The trocar constraint will in this case change the mobility of the manipulator and the location and nature of the singularities. For minimally invasive surgery both the mobility and the singularities of the system need to be known to obtain safe and reliable operation. Velocity constraints on the chain will in general complicate the mobility analysis of the manipulator as conventional methods such as manipulability and other methods that require the manipulator Jacobian cannot be applied because these methods do not take the constraints into account. The main contribution of the paper is to find the end-effector velocity by adding the velocity at the constraint and the velocity of the joints after the constraint (often called the wrist) and observing that these velocities need to span the whole end-effector velocity space. We then use this new representation of the end-effector velocity to find the mobility of the constrained manipulators. The framework can be used both to determine the optimal manipulator geometry in the presence of chain constraints and, once the manipulator geometry is chosen, to control the robot such that the mobility is maintained high and singularities avoided. The first property of the presented framework can for example be used to find the optimal geometry of the wrist of a surgical robot subjects to incision point constraints, and the latter can be used to control the robot so that singularities are avoided. The singularities typically take a very different form than for the unconstrained case.

Dynamic manipulability of velocity-constrained serial robotic manipulators

This paper presents a new performance metric for serial manipulators with velocity constraints on the chain. These systems arise in several different applications such as serial manipulators in the presence of obstacles and in robotics-assisted minimally invasive surgery. It is important to know how constraints on the chain affect the mobility of the end effector and we therefore present a reformulation of the dynamic manipulability of the end effector for serial manipulators with velocity constraints on the chain. In this paper we propose to use the Constrained Jacobian Matrix, i.e., an analytical mapping between the end-effector and joint velocities that also takes the chain constraints into account. The approach allows us to compare the dynamic manipulability of serial manipulators with and without constraints and we show through a simple planar example how the dynamic manipulability is reduced as a result of the trocar constraint in minimally invasive surgery.

Comparison of Mental and Theoretical Evaluations of Remotely Controlled Mobile Manipulators

Abstract The focus of this research is to compare the mental and theoretical evaluations of remotely controlled mobile manipulators. Evaluating the performance of control methods for mobile manipulation is challenging because both the user experience and the actual performance of the completed task need to be taken into account. How the user perceives the control law is of course very subjective and in general hard do quantify numerically. Theoretical evaluations of the performance are easier to find, but do not tell us anything about the stress, frustration, and mental demand that the operator experiences. Several studies have been performed to evaluate the performance of teleoperation schemes, but the literature lacks a comparison between objective and subjective performance metrics for evaluating these. In this paper we evaluate the mental and theoretical performance of three relatively simple approaches for controlling a mobile manipulator with a haptic device. We study to what extent objective performance metrics such as execution time, number of failures, and manipulator mobility can be used to distinguish the approaches, and compare this to subjective measures like the NASA-TLX test.