[Show abstract][Hide abstract] ABSTRACT: Purpose:
Minimally invasive surgery is becoming the standard treatment of care for a variety of procedures. Surgeons need to display a high level of proficiency to overcome the challenges imposed by the minimal access. Especially when operating on a dynamic organ, it becomes very difficult to align instruments reliably and precisely. In this paper, a hybrid rigid/continuum robotic system and a dedicated robotic control approach are proposed to assist the surgeon performing complex surgical gestures in a dynamic environment.
The proposed robotic system consists of a rigid robot arm on top of which a continuum robot is mounted in series. The continuum robot is locally actuated with McKibben muscles. A control scheme based on quadratic programming framework is adopted. It is shown that the framework allows enforcing a set of constraints on the pose of the tip, as well as of the instrument shaft, which is commanded to slide in and out through the entry point.
Through simulation and experiments, it is shown how the robot tool tip is able to follow sinusoidal trajectories of 0.37 and 2 Hz, while maintaining the instrument shaft pivoting along the entry point. The positioning and tracking accuracy of such system are shown to lie below 4.7 mm in position and [Formula: see text] in angle.
The results suggest a good potential for applying the proposed technology to assist the surgeon during complex robot-assisted interventions. It is also illustrated that even when using flexible hence relatively safe end-effectors, it is possible to reach acceptable tracking behaviour at relatively high frequencies.
Full-text · Article · Dec 2015 · International Journal of Computer Assisted Radiology and Surgery
[Show abstract][Hide abstract] ABSTRACT: This paper introduces a framework for constraint-based force/position control of robots that exhibit large nonlinear structural compliance and that undergo large deformations. Controller synthesis follows hereto the principles of the Task Frame and instantaneous Task Specification using Constraints (iTaSC) formalisms. iTaSC is found particularly suitable due to its ability to express and combine control tasks in a natural way. Control tasks can be formulated as combinations of target positions, velocities, or forces expressed in an arbitrary number and type of coordinate frames. The proposed framework is applied to a mixed mechatronic system composed of a traditional rigid-link robot whose end-effector is a continuum (flexible) link. A selection of different position/force control tasks is prepared to demonstrate the validity and general nature of the proposed framework.
Full-text · Article · Oct 2015 · IEEE Transactions on Robotics
[Show abstract][Hide abstract] ABSTRACT: This paper presents a novel centroiding algorithm for star trackers. The proposed algorithm, which is referred to as the Gaussian Grid algorithm, fits an elliptical Gaussian function to the measured pixel data and derives explicit expressions to determine the centroids of the stars. In tests, the algorithm proved to yield accuracy comparable to that of the most accurate existing algorithms, while being significantly less computationally intensive. Hence, the Gaussian Grid algorithm can deliver high centroiding accuracy to spacecraft with limited computational power. Furthermore, a hybrid algorithm is proposed in which the Gaussian Grid algorithm yields an accurate initial estimate for a least squares fitting method, resulting in a reduced number of iterations and hence reduced computational cost. The low computational cost allows to improve performance by acquiring the attitude estimates at a higher rate or use more stars in the estimation algorithms. It is also a valuable contribution to the expanding field of small satellites, where it could enable low-cost platforms to have highly accurate attitude estimation.
No preview · Article · Jun 2015 · Journal of the Astronautical Sciences
[Show abstract][Hide abstract] ABSTRACT: This paper presents an overview and comparison of minimal and complete rigid body motion trajectory descriptors, usable in applications like motion recognition and programming by demonstration. Motion trajectory descriptors are able to deal with potentially unwanted variations acting on the motion trajectory such as changes in the execution time, the motion's starting position, or the viewpoint from which the motion is observed. A suitable rigid body motion trajectory descriptor retains only the trajectory information relevant to the application. This paper compares different trajectory descriptors for rigid body motion and validates their usefulness for dealing with motion variation in a motion recognition experiment. Furthermore, a new type of invariant trajectory descriptor is introduced based on the Frenet-Serret formulas.
[Show abstract][Hide abstract] ABSTRACT: An increasing number of robotic systems are using compliant actuators in which springs are placed in series with the actuator. The need for identification procedures tailored to these systems is consequently rising. When measurements of both the link side and the motor side of the spring are available the dynamic parameters can be identified independent of the spring model. The excitation of the identification procedure is optimized for the identification of the dynamical parameters to provide a rich data set by maximizing a reduced information matrix. Adopting this reduced matrix leads to a near-optimal excitation. A Fourier series is chosen as parametrization of the excitation, resulting in a periodic movement of the system under identification. This periodicity is exploited to develop an alternative weighting of the parameter estimation. This proposed weighting uses the motor torque variance at each time step of the trajectory, instead of one global torque variance. This identification procedure is demonstrated and validated on one leg of a lower limb exoskeleton. Only the dynamic model in the sagital plane (2D) is identified.
[Show abstract][Hide abstract] ABSTRACT: This paper presents a model that closely approximates the generalized Maxwell-Slip (GMS) model, a multistate friction model known to describe all essential friction characteristics in presliding and sliding motion. In contrast to the GMS model, which consists of a switching structure to accommodate for its hybrid nature, the new model, referred to as the smoothed GMS (S-GMS) model, consists of an analytic set of differential equations. Such a model is well suited for gradient-based state and parameter estimation, as in the extended Kalman filter (EKF) or in moving horizon estimation. Similar to the GMS model, the S-GMS model is a multistate model that also describes all essential friction characteristics. Moreover, the S-GMS model description includes the single-state LuGre model and Elastoplastic model as special cases. This paper also discusses the implementation of the EKF estimator for the S-GMS friction model and compares its performance to the LuGre model in joint state and parameter estimation.
No preview · Article · Oct 2014 · IEEE/ASME Transactions on Mechatronics
[Show abstract][Hide abstract] ABSTRACT: Abstract— This paper presents a new framework for
constraint-based task specification of robot controllers. A task
specification language (eTaSL) is defined as well as a corre-
sponding implementation of a controller (eTC). This new frame-
work is based on feature variables and a new concept referred
to as expression graphs. It avoids some of the common pitfalls
in previous frameworks, and provides a flexible and composable
way to define robot control tasks. An architecture for a robot
controller is proposed, as well as an implementation that
can execute tasks described in the new specification language.
Typical usage patterns for the new framework are explained on
an example consisting of a kinematically redundant, bi-manual
task on a PR2 robot. A comparison with existing frameworks
shows the advantages of the new approach.
[Show abstract][Hide abstract] ABSTRACT: Abstract— For an intelligent dynamic motion interaction
between a human and a lower-limb exoskeleton, it is necessary
to predict the future evolution of the joint gait trajectories
and to detect which phase of the gait pattern is currently
active. A model of the gait trajectories and of the variations
on these trajectories is learned from an example data set. A
gait prediction module, based on a statistical latent variable
model, is able to predict, in real-time, the future evolution
of a joint trajectory, an estimate of the uncertainty on this
prediction, the timing along the trajectory and the consistency
of the measurements with the learned model. The proposed
method is validated using a data set of 54 trials of children
walking at three different velocities.
[Show abstract][Hide abstract] ABSTRACT: Background
Enabling persons with functional weaknesses to perform activities of daily living (ADL) is one of the main challenges for the aging society. Powered orthoses, or exoskeletons, have the potential to support ADL while promoting active participation of the user. For this purpose, assistive devices should be designed and controlled to deliver assistance as needed (AAN). This means that the level of assistance should bridge the capability gap, i.e. the gap between the capabilities of the subjects and the task requirements. However, currently the actuators of exoskeletons are mainly designed using inverse dynamics (ID) based calculations of joint moments. The goal of the present study is to calculate the capability gap for the lower limb during ADL when muscle weakness is present, which is needed for appropriate selection of actuators to be integrated in exoskeletons.
A musculoskeletal model (MM) is used to calculate the joint kinematics, joint kinetics and muscle forces of eight healthy subjects during ADL (gait, sit-to-stand, stand-to-sit, stair ascent, stair descent). Muscle weakness was imposed to the MM by a stepwise decrease in maximal isometric force imposed to all muscles. Muscle forces were calculated using static optimization. In order to compensate for muscle weakness, ideal moment actuators that represent the motors of an exoskeleton in the simulation were added to deliver AAN required to perform the task.
The ID approach overestimates the required assistance since it relies solely on the demands of the task, whereas the AAN approach incorporates the capabilities of the subject. Furthermore, the ID approach delivers continuous support whereas the AAN approach targets the period where a capability gap occurs. The level of muscle weakness for which the external demands imposed by ADL can no longer be met by active muscle force production, is respectively 40%, 70%, 80% and 30%.
The present workflow allows estimating the AAN during ADL for different levels of muscle weakness, which can be used in the mechatronic design and control of powered exoskeletons. The AAN approach is a more physiological approach than the ID approach, since the MM accounts for the subject-specific capabilities of the user.
Full-text · Article · Aug 2014 · BioMedical Engineering OnLine
[Show abstract][Hide abstract] ABSTRACT: Path following deals with the problem of following a geometric path with no predefined timing information and constitutes an important step in solving the motion-planning problem. For differentially flat systems, it has been shown that the projection of the dynamics along the geometric path onto a linear single-input system leads to a small dimensional optimal control problem. Although the projection simplifies the problem to great extent, the resulting problem remains difficult to solve, in particular in the case of nonlinear system dynamics and time-optimal problems. This paper proposes a nonlinear change of variables, using a time transformation, to arrive at a fixed end-time optimal control problem. Numerical simulations on a robotic manipulator and a quadrotor reveal that the proposed problem formulation is solved efficiently without requiring an accurate initial guess.
Full-text · Article · Aug 2014 · IEEE Transactions on Robotics
[Show abstract][Hide abstract] ABSTRACT: The quality of space telescope observations greatly depends on the pointing performance of the spacecraft. In
this paper, recent advances in star tracker algorithms are discussed. This paper discusses efficient star tracker
algorithms that improve the pointing performance of the satellite, resulting in observations of higher quality.
Furthermore, the greatly reduced computational cost of these algorithms facilitates the inclusion of astronomical
payload measurements in the attitude determination and control loop. When the payload is used as an additional
star tracker, the pointing performance of the spacecraft increases drastically, which in its turn improves the quality
of the scientific measurements. Simulations show that with these improvements, the absolute pointing error of
the spacecraft can be reduced considerably.
[Show abstract][Hide abstract] ABSTRACT: We have conceived a novel compound multicopter (helicopter type utilizing multiple different size propellers for separate lift and attitude control) configuration specifically for flight through narrow corridors. Its design combines the contradictory requirements of limited width, high agility and long endurance while carrying a significant payload. This configuration can be scaled for both indoor and outdoor applications. The development is part of a doctoral research in which an autonomous unmanned rotary helicopter is designed, constructed and flight tested for inspecting fruit orchards and vineyards while flying in between the tree rows in outdoor conditions such as wind and gusts. The compound hexacopter configuration combines two large lift propellers, with a constant rotational velocity, with four small control propellers commanded by an autopilot. The autopilot is configured as a quadcopter commanding only the control propellers as only these change the attitude and overall thrust of the hexacopter. The benefit of using large lift propellers is their lower disk loading (thrust divided by disk area) which results in a higher Figure of Merit and lower power consumption compared to the smaller control propellers, while the latter are better suited for outdoor (windy) conditions due to their fast reaction time in spooling up and down. Compared to a standard quadcopter with the same width, payload and battery capacity, the endurance of the compound hexacopter is potentially up to 60% higher. As a concept validator, a small-scale prototype has been designed, constructed and successfully flight tested.
[Show abstract][Hide abstract] ABSTRACT: This work aims to extend the application field of the constraint-based control framework called iTaSC (instantaneous task specification using constraints) toward tasks where physical interaction between the robot and the environment, or a human, is contemplated. iTaSC, in its original formulation, allows for a systematic derivation of control schemes from task descriptions; tasks are defined as constraints enforced on outputs (e.g. distances, angles), and the iTaSC control takes care to fulfil such constraints by computing desired velocities to be commanded to the robot(s) joints. This approach, being based on a velocity resolution scheme, principally addresses tasks where positioning is the main issue. However, tasks that involve contacts with the environment or with the user, either desired or accidental, can be considered as well, taking advantage of impedance control, when position is controlled, or with force control. This paper describes the implementation of force tasks, and, by the combination of conflicting force and position tasks, impedance control, within the iTaSC formalism. This result is achieved by taking advantage of an approximate physical modelling of the robotic system and the environment. The proposed control scheme is tested by means of experiments where constraints on forces and/or positions described in cylindrical coordinates are imposed on a Kuka LWR arm.
[Show abstract][Hide abstract] ABSTRACT: bstract—This work aims to extend the application field ofthe constraint-based control framework called iTaSC (instanta-neous task specification using constraints) toward manipulationtasks. iTaSC offers two advantages with respect to other meth-ods: the ability to specify tasks in different spaces (and not onlyin Cartesian coordinates as for the Task Frame Formalism),and the treatment of geometric uncertainties. These propertiesmay be very useful within a manipulation context, where tasksare executed by robots with many degrees of freedom, whichcalls for some degree of abstraction; by choosing a suitableset of coordinates, it is possible to reduce the complexity andthe number of constraints that fully describe such tasks; inaddition, controlling only the subspace that is needed to fulfila task allows us to use the remaining degrees of freedom ofthe robot system to achieve secondary objectives. This paperdiscusses the instruments and techniques that can be employedin manipulation scenarios; in particular it focuses on aspectslike the specification of a grasp and control of the stance of therobotic arm.iTaSC offers the possibility of specifying a grasp. While thisapproach allows for very fine control of a grasping task, inmost cases a less fine-grain specification suffices to guarantee asuccessful execution of the grasping action. To this end synergy- based grasp specification is formulated within iTaSC.We also show how to take into account secondary objectives for the arm stance. In particular we consider, as an example,the manipulability index along a given direction. Such indexesare maximised by exploring the null space of the other tasks.The proposed approach is demonstrated by means of simu-lations, where a robotic hand grasps a cylindrical object.
[Show abstract][Hide abstract] ABSTRACT: This paper presents experimental results of the full 3-axis force vector and 3-axis moment vector acting on a propeller, commonly used for a Vertical Take Off and Landing Micro Aerial Vehicle (VTOL MAV). Measurements were carried out in a wind tunnel using a high resolution 6-axis force/moment sensor embedded in a customized test rig at several wind speeds, propeller rotational speeds and angles of the propeller shaft with respect to the air stream. Results show strong moments acting on the propeller in forward flight and unstable conditions in descending flight. Power calculations reveal a decrease in power consumption during slow forward flight and how motor efficiency can be maximized.
[Show abstract][Hide abstract] ABSTRACT: Background
Spasticity is an important complication after stroke, especially in the anti-gravity muscles, i.e. lower limb extensors. However the contribution of hyperexcitable muscle spindle reflex loops to gait impairments after stroke is often disputed. In this study a neuro-musculoskeletal model was developed to investigate the contribution of an increased length and velocity feedback and altered reflex modulation patterns to hemiparetic gait deficits.
A musculoskeletal model was extended with a muscle spindle model providing real-time length and velocity feedback of gastrocnemius, soleus, vasti and rectus femoris during a forward dynamic simulation (neural control model). By using a healthy subject’s base muscle excitations, in combination with increased feedback gains and altered reflex modulation patterns, the effect on kinematics was simulated. A foot-ground contact model was added to account for the interaction effect between the changed kinematics and the ground. The qualitative effect i.e. the directional effect and the specific gait phases where the effect is present, on the joint kinematics was then compared with hemiparetic gait deviations reported in the literature.
Our results show that increased feedback in combination with altered reflex modulation patterns of soleus, vasti and rectus femoris muscle can contribute to excessive ankle plantarflexion/inadequate dorsiflexion, knee hyperextension/inadequate flexion and increased hip extension/inadequate flexion during dedicated gait cycle phases. Increased feedback of gastrocnemius can also contribute to excessive plantarflexion/inadequate dorsiflexion, however in combination with excessive knee and hip flexion. Increased length/velocity feedback can therefore contribute to two types of gait deviations, which are both in accordance with previously reported gait deviations in hemiparetic patients. Furthermore altered modulation patterns, in particular the reduced suppression of the muscle spindle feedback during swing, can contribute largely to an increased plantarflexion and knee extension during the swing phase and consequently to hampered toe clearance.
Our results support the idea that hyperexcitability of length and velocity feedback pathways, especially in combination with altered reflex modulation patterns, can contribute to deviations in hemiparetic gait. Surprisingly, our results showed only subtle temporal differences between length and velocity feedback. Therefore, we cannot attribute the effects seen in kinematics to one specific type of feedback.
Full-text · Article · Apr 2014 · Journal of NeuroEngineering and Rehabilitation