Ferdinando Rodriguez y Baena

Active Constraints/Virtual Fixtures: A Survey

Active constraints, also known as virtual fixtures, are high-level control algorithms which can be used to assist a human in man-machine collaborative manipulation tasks. The active constraint controller monitors the robotic manipulator with respect to the environment and task, and anisotropically regulates the motion to provide assistance. The type of assistance offered by active constraints can vary, but they are typically used to either guide the user along a task-specific pathway or limit the user to within a “safe” region. There are several diverse methods described within the literature for applying active constraints, and these are surveyed within this paper. The active constraint research is described and compared using a simple generalized framework, which consists of three primary processes: 1) constraint definition, 2) constraint evaluation, and 3) constraint enforcement. All relevant research approaches for each of these processes, found using search terms associated to “virtual fixture,” “active constraint” and “motion constraint,” are presented.

Toward a Miniaturized Needle Steering System With Path Planning for Obstacle Avoidance

Percutaneous intervention is among the preferred diagnostic and treatment options in surgery today. Recently, a biologically inspired needle steering system was proposed, where a novel “programmable bevel” is employed to control the tip angle as a function of the offset between interlocked needle segments. The new device, codenamed soft tissue intervention and neurosurgical guide (STING), can steer along arbitrary curvilinear trajectories within a compliant medium, and be controlled by means of an embedded position sensor. In this study, we provide details of our latest attempt to miniaturize the STING, with the design and manufacture of a 4-mm outer diameter (OD) two-part prototype that includes unique features, such as a bespoke trocar and insertion mechanism, which ensure that the segments do not come apart or buckle during the insertion process. It is shown that this prototype can steer around tight bends (down to a radius of curvature of ~70 mm), a performance which is comparable to the best systems in this class. With the need to comply with the specific mechanical constraints of STING, this paper also introduces a novel path planner with obstacle avoidance, which can produce a differentiable trajectory that satisfies constraints on both the maximum curvature of the final trajectory and its derivative. In vitro results in gelatin for the integrated prototype and path planner demonstrate accurate 2-D trajectory following (0.1 mm tracking error, with 0.64 mm standard deviation), with significant scope for future improvements.

Patellar thickness and lateral retinacular release affects patellofemoral kinematics in total knee arthroplasty

PurposeTo study the effect of increasing patellar thickness (overstuffing) on patellofemoral kinematics in total knee arthroplasty and whether subsequent lateral retinacular release would restore the change in kinematics.MethodsThe quadriceps of eight fresh-frozen knees were loaded on a custom-made jig. Kinematic data were recorded using an optical tracking device for the native knee, following total knee arthroplasty (TKA), then with patellar thicknesses from −2 to +4xa0mm, during knee extension motion. Staged lateral retinacular releases were performed to examine the restoration of normal patellar kinematics.ResultsCompared to the native knee, TKA led to significant changes in patellofemoral kinematics, with significant increases in lateral shift, tilt and rotation. When patellar composite thickness was increased, the patella tilted further laterally. Lateral release partly corrected this lateral tilt but caused abnormal tibial external rotation. With complete release of the lateral retinaculum and capsule, the patella with an increased thickness of 4xa0mm remained more laterally tilted compared to the TKA with normal patellar thickness between 45° and 55° knee flexion and from 75° onwards. This was on average by 2.4°xa0±xa02.9° (pxa0<xa00.05) and 2.°9xa0±xa03.0° (pxa0<xa00.01), respectively. Before the release, for those flexion ranges, the patella was tilted laterally by 4.7°xa0±xa03.2° and 5.4°xa0±xa02.7° more than in the TKA with matched patellar thickness.ConclusionPatellar thickness affects patellofemoral kinematics after TKA. Although lateral tilt was partly corrected by lateral retinacular release, this affected the tibiofemoral kinematics.Level of evidenceIV.

Experimental characterisation of a biologically inspired 3D steering needle

Percutaneous intervention is a popular minimally invasive surgical technique, as it offers many potential advantages for the patient. Research efforts to date have focussed on improving the accuracy and applicability of this procedure through robotic control, in particular with the application of needle steering systems. Previously, we demonstrated two-dimensional (2D) steering within gelatine, with a prototype of a novel biologically inspired multi-segment needle, the STING. Then, a novel `programmable bevel concept, where the steering angle of the needle is a function of the offset between segments, was used to control the trajectory taken within the steering plane. This paper presents our first attempt to demonstrate controllable three-dimensional (3D) steering with a new four-segment prototype of the STING. We show that an approximately linear relationship exists between segment offset and curvature of the tip path for a single leading segment, as well as for two segments which are moved forward of the others by an equal amount. This characterisation is then demonstrated with 3D open loop experiments, which show that the established behaviour is applicable for controlled 3D steering along eight principal directions.

Soft Tissue Phantoms for Realistic Needle Insertion: A Comparative Study

Phantoms are common substitutes for soft tissues in biomechanical research and are usually tuned to match tissue properties using standard testing protocols at small strains. However, the response due to complex tool-tissue interactions can differ depending on the phantom and no comprehensive comparative study has been published to date, which could aid researchers to select suitable materials. In this work, gelatin, a common phantom in literature, and a composite hydrogel developed at Imperial College, were matched for mechanical stiffness to porcine brain, and the interactions during needle insertions within them were analyzed. Specifically, we examined insertion forces for brain and the phantoms; we also measured displacements and strains within the phantoms via a laser-based image correlation technique in combination with fluorescent beads. It is shown that the insertion forces for gelatin and brain agree closely, but that the composite hydrogel better mimics the viscous nature of soft tissue. Both materials match different characteristics of brain, but neither of them is a perfect substitute. Thus, when selecting a phantom material, both the soft tissue properties and the complex tool-tissue interactions arising during tissue manipulation should be taken into consideration. These conclusions are presented in tabular form to aid future selection.

Adaptive Hands-On Control for Reaching and Targeting Tasks in Surgery

Cooperatively controlled robotic assistants can be used in surgery for the repetitive execution of targeting/reaching tasks, which require smooth motions and accurate placement of a tool inside a working area. A variable damping controller, based on a priori knowledge of the location of the surgical site, is proposed to enhance the physical human-robot interaction experience. The performance of this and of typical constant damping controllers is comparatively assessed using a redundant light-weight robot. Results show that it combines the positive features of both null (acceleration capabilities > 0.8m/s2) and optimal (mean pointing error < 1.5mm) constant damping controllers, coupled with smooth and intuitive convergence to the target (direction changes reduced by 30%), which ensures that assisted tool trajectories feel natural to the user. An application scenario is proposed for brain cortex stimulation procedures, where the surgeons intentions of motion are explicitly defined intra-operatively through an image-guided navigational system.

Dynamic frictional constraints in translation and rotation

Active constraints and virtual fixtures are popular control strategies used within human-robot collaborative manipulation tasks, particularly in the field of robot-assisted surgery. Recent research has shown how active constraints, which robotically regulate the motion of a tool that is primarily manipulated by a human, can be implemented in dynamic environments which change and deform throughout a procedure. In a dynamic environment, movement of the constraint boundary can cause active forcing of the surgical tools, potentially reducing the surgeons control and jeopardising patient safety. Dynamic frictional constraints have been proposed as a method for enforcing dynamic active constraints which do not generate energy of their own, and simply dissipate or redirect the energy of the surgeon to provide assistance. In this paper, dynamic frictional constraints are reformulated to allow formal proof that they are indeed dissipative, and hence also passive. This new formulation is then extended such that dynamic frictional constraints can simultaneously constrain the position and orientation of a tool. Experimental results show that the method is of significant benefit in performing a dynamic task when compared to cases without any assistance; with position and orientation constraints individually and with a conventional frictional constraint without energy redirection.

Dissipative Control for Physical Human–Robot Interaction

Physical human-robot interaction is fundamental to exploiting the capabilities of robots in tasks and environments where robots have limited cognition or comprehension and is virtually ubiquitous for robotic manipulation in highly unstructured environments, as are found in surgery. A critical aspect of physical human-robot interaction in these cases is controlling the robot so that the individual human and robot competencies are maximized, while guaranteeing user, task, and environment safety. Dissipative control precludes dangerous forcing of a shared tool by the robot, ensuring safety; however, it typically suffers from poor control fidelity, resulting in reduced task accuracy. In this study, a novel, rigorously formalized, n-dimensional dissipative control strategy is proposed that employs a new technique called “energy redirection” to generate control forces with increased fidelity while remaining dissipative and safe. Experimental validation of the method, for complete pose control, shows that it achieves a 90% reduction in task error compared with the current state of the art in dissipative control for the tested applications. The findings clearly demonstrate that the method significantly increases the fidelity and efficacy of dissipative control during physical human-robot interaction. This advancement expands the number of tasks and environments into which safe physical human-robot interaction can be employed effectively.

Dynamic frictional constraints for robot assisted surgery

Collaborative, as opposed to autonomous, control strategies are used within the majority of commercially available, surgical robotic systems. Amongst these, active constraints and virtual fixtures, where assistance is in the form of regulation applied to the motion of surgical tools, offer an effective means to maximise both user and robot capabilities. Conventional active constraint approaches, however, are likely to result in active forcing of the tools when used within a dynamically changing surgical environment. It is posited that such behaviour inherently reduces a surgeons control over the procedure, and therefore compromises patient safety and clinical acceptance. Utilising a friction model to enforce constraints ensures that energy is never introduced into the system; however frictional constraints suffer from problems once penetration of a constrained region has occurred. A frictional constraint formulation is proposed which eliminates this by redirecting a users motion, guiding him towards the surface. Experimental validation shows that the proposed constraint significantly improves a users path-following performance over unassisted cases, while approaching the performance benchmark of a viscoelastic active constraint.

Mass and Friction Optimization for Natural Motion in Hands-On Robotic Surgery

In hands-on robotic surgery, the surgical tool is mounted on the end-effector of a robot and is directly manipulated by the surgeon. This simultaneously exploits the strengths of both humans and robots, such that the surgeon directly feels tool-tissue interactions and remains in control of the procedure, while taking advantage of the robots higher precision and accuracy. A crucial challenge in hands-on robotics for delicate manipulation tasks, such as surgery, is that the user must interact with the dynamics of the robot at the end-effector, which can reduce dexterity and increase fatigue. This paper presents a null-space-based optimization technique for simultaneously minimizing the mass and friction of the robot that is experienced by the surgeon. By defining a novel optimization technique for minimizing the projection of the joint friction onto the end-effector, and integrating this with our previous techniques for minimizing the belted mass/inertia as perceived by the hand, a significant reduction in dynamics felt by the user is achieved. Experimental analyses in both simulation and human user trials demonstrate that the presented method can reduce the user-experienced dynamic mass and friction by, on average, 44% and 41%, respectively. The results presented robustly demonstrate that optimizing a robots pose can result in a more natural tool motion, potentially allowing future surgical robots to operate with increased usability, improved surgical outcomes, and wider clinical uptake.