Tobias Ortmaier

Leibniz Universität Hannover, Hanover, Lower Saxony, Germany

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Publications (104)35.6 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: Introducing computational methods to laser surgery are an emerging field. Focusing on endoscopic laser interventions, a novel approach is presented to enhance intraoperative incision planning and laser focusing by means of tissue surface information obtained by stereoscopic vision.
    International Journal of Computer Assisted Radiology and Surgery 05/2014; · 1.36 Impact Factor
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    ABSTRACT: Minimally invasive cochlear implantation is a surgical technique which requires drilling a canal from the mastoid surface toward the basal turn of the cochlea. The choice of an appropriate drilling strategy is hypothesized to have significant influence on the achievable targeting accuracy. Therefore, a method is presented to analyze the contribution of the drilling process and drilling tool to the targeting error isolated from other error sources. The experimental setup to evaluate the borehole accuracy comprises a drill handpiece attached to a linear slide as well as a highly accurate coordinate measuring machine (CMM). Based on the specific requirements of the minimally invasive cochlear access, three drilling strategies, mainly characterized by different drill tools, are derived. The strategies are evaluated by drilling into synthetic temporal bone substitutes containing air-filled cavities to simulate mastoid cells. Deviations from the desired drill trajectories are determined based on measurements using the CMM. Using the experimental setup, a total of 144 holes were drilled for accuracy evaluation. Errors resulting from the drilling process depend on the specific geometry of the tool as well as the angle at which the drill contacts the bone surface. Furthermore, there is a risk of the drill bit deflecting due to synthetic mastoid cells. A single-flute gun drill combined with a pilot drill of the same diameter provided the best results for simulated minimally invasive cochlear implantation, based on an experimental method that may be used for testing further drilling process improvements.
    International Journal of Computer Assisted Radiology and Surgery 04/2014; · 1.36 Impact Factor
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    ABSTRACT: The choice of a navigation system highly depends on the medical intervention and its accuracy demands. The most commonly used systems for image guided surgery (IGS) are based on optical and magnetic tracking systems. This paper compares two optical systems in terms of accuracy: state of the art triangulation-based optical tracking (OT) and optical coherence tomography (OCT). We use an experimental setup with a combined OCT and cutting laser, and an external OT. We simulate a robotic assisted surgical intervention, including planning, navigation, and processing, and compare the accuracies reached at a specific target with each navigation system.
    02/2014;
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    ABSTRACT: Minimally invasive cochlear implantation and residual hearing preservation require both the surgical approach to the cochlea as well as the implant insertion to be performed in an atraumatic fashion. Considering the geometric limitations of this approach, specialized instrumentation is required to insert the electrode while preserving intracochlear membranes carrying the sensory hair cells. An automated insertion tool for cochlear implants, which is capable of sensing insertion forces with a theoretical resolution of [Formula: see text], is presented. In contrast to previous designs, the custom force sensor is integrated in the insertion mechanism. Moreover, a test bench for insertion studies under constant and reproducible boundary conditions is proposed. It is used to experimentally validate the force sensing insertion tool, which is achieved by comparing the acquired forces to a ground truth measurement. The results of insertion studies on both an acrylic cochlear phantom and temporal bone specimen are given and discussed. Results reveal that friction, occurring between the electrode carrier and the inside of the insertion tool guide tube, is likely to affect the force output of the proposed sensor. An appropriate method to compensate for these disturbances is presented and experimentally validated. Using the proposed approach to friction identification, a mean accuracy of [Formula: see text] is observed. The force information provided by the proposed, automated insertion tool can be used to detect complications during electrode insertion. However, in order to obtain accurate results, an identification of frictional forces prior to insertion is mandatory. The insertion tool is capable of automatically executing the appropriate trajectories.
    International Journal of Computer Assisted Radiology and Surgery 08/2013; · 1.36 Impact Factor
  • Christian Hansen, Jens Kotlarski, Tobias Ortmaier
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    ABSTRACT: A new path planning approach for energy demand minimization of general multi axis robots by model-based trajectory optimization is presented. In the proposed method, the efficient utilization of recuperated energy is induced by amplification of energy exchange via the internal DC bus. The energy-based system model includes the robot dynamics, mechanical and drive losses, as well as the exchange of electrical energy. The nonlinear optimization problem is solved using global methods, considering kinematic and dynamic limitations. Simulations results are presented that prove the performance of the algorithm and demonstrate the beneficial effect of electrical energy exchange. Minimum time criteria can be retained if required and the approach is applicable to different multi axis manipulator types with no need for additional investment or hardware modification.
    2013 IEEE International Conference on Mechatronics and Automation (ICMA), Takamatsu, Kagawa, Japan; 08/2013
  • The European Conference on Lasers and Electro-Optics; 05/2013
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    ABSTRACT: In recent years, optical coherence tomography (OCT) has gained increasing attention not only as an imaging device, but also as a guidance system for surgical interventions. In this contribution, we propose OCT as an external high-accuracy guidance system, and present an experimental setup of an OCT combined with a cutting laser. This setup enables not only in situ monitoring, but also automatic, high-accuracy, three-dimensional navigation and processing. Its applicability is evaluated simulating a robotic assisted surgical intervention, including planning, navigation, and processing. First results demonstrate that OCT is suitable as a guidance system, fulfilling accuracy demands of interventions such as the cochlear implant surgery.
    Medical image computing and computer-assisted intervention : MICCAI ... International Conference on Medical Image Computing and Computer-Assisted Intervention. 01/2013; 16(Pt 3):347-54.
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    ABSTRACT: This paper presents an optimization procedure used to efficiently maximize the potentials of parallel manipulators with kinematic redundancy within real-time. The proposed approach consists of reducing the search space defined by the optimization problem beforehand, through minimizing the amount of optimization points necessary to induce the optimization problem. Furthermore, the computation time of the fitness function is reduced during run-time. Therefore, the relationship between the Cartesian path of the end-effector and the resulting optimization problem is studied and a procedure to minimize the given dependencies is presented. Exemplarily, a kinematically redundant 3(P)RRR parallel robot is considered to quantify the efficiency of the discussed procedure. The results demonstrate that the proposed approach is able to outperform existing procedures by one to two orders of magnitude.
    Advanced Intelligent Mechatronics (AIM), 2013 IEEE/ASME International Conference on; 01/2013
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    ABSTRACT: Laparo-endoscopic single-site surgery (LESS) is a widespread approach towards minor invasiveness in surgery. To enlarge the field of application of LESS suitable medical equipment is required. However, most available products, i.e. instruments as well as access devices, cannot satisfy all the needs of this technology, e.g. an appropriate collision-free workspace. Hence, equipment has to be developed offering intervention optimal kinematic structures and dimensions. Therefore, the performance of LESS systems has to be comparable and rateable quantitatively. In this paper, methods and criteria are proposed in order to objectively quantify the performance of LESS instruments. Besides others, this includes approaches to define and determine, the desired workspace with respect to the requirements of an intervention as well as reachable and executable workspaces of LESS systems, depending on their kinematics and dimensions. Furthermore, an algorithm for collision monitoring in between the instrument shafts is introduced, being used while calculating these workspaces. These approaches are based on a modeling tool allowing for a general representation of the instruments’ and ports’ kinematics. Using the aforementioned methods, exemplarily, the performance of existing equipment is quantified and compared. In this context, the influence of kinematic variations, e.g. shape and degrees of freedom, of instruments and access devices to the criteria is clarified.
    IEEE/ASME International Conference on Advanced Intelligent Mechatronics (AIM); 01/2013
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    ABSTRACT: This article presents a one step geometric calibration for an optical coherence tomography (OCT) which forms part of a medical navigation system. The 3D landmark-based geometric calibration with a self-produced 3D reference structure is based on the identification of a parameterized grey-box OCT model. We show in experimental results by comparing common measurement errors in the field of medical surgery before and after calibration, that the proposed methodology reduces systematic errors by more than one order of magnitude. Due to its simplicity, the calibration can be carried out directly before a surgical intervention enhancing the OCT accuracy.
    Proc SPIE 11/2012;
  • Biomedizinische Technik/Biomedical Engineering 08/2012; · 1.16 Impact Factor
  • Biomedizinische Technik/Biomedical Engineering 08/2012; · 1.16 Impact Factor
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    ABSTRACT: This paper proposes a novel cost function formulation for minimization of the energy consumption of industrial robots by trajectory optimization. Besides the dynamics of the robot including friction losses, the model especially takes into account the losses of servo drives and inverters. Furthermore, the ability of energy exchange between the robot axes via the coupled DC-bus is included, since the servo drives support generator mode during deceleration. The utilized energy-based robot model is applicable to different manipulator types. For the energy-efficient motion planning, point-to-point trajectories are defined by B-spline functions. The given nonlinear optimization problem is solved using gradient-based methods, considering kinematic and dynamic constraints. Several simulation results are presented, demonstrating the intense effect of energy exchange in the robot controller's power electronics. Furthermore, a comparative study is given showing that the proposed method is able to outperform existing torque-based approaches.
    2012 IEEE International Conference on Automation Science and Engineering: Green Automation Toward a Sustainable Society, CASE 2012, Seoul, Korea, Republic of; 08/2012
  • B. Munske, J. Kotlarski, T. Ortmaier
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    ABSTRACT: This paper presents a new university course combining theoretical lectures with a robot competition. The main intention is to offer a practical course including hands-on experiences being close to current research topics in the field of mobile robotics. For this purpose, a commercial mobile robot is equipped with state of the art sensors, allowing autonomous execution of manipulation tasks. The course consists of three thematical sections. At the beginning of each, lectures provide the theoretical background. On this basis, the participants are addressed to develop algorithms and to solve specific tasks, delivered in homework packages, self-reliantly or in a team. Finally, the developed software components need to be merged to solve a predefined scenario, e.g. autonomous part handling. At the end of a thematic section, students demonstrate their developed solutions within a challenge and explain their approaches in a presentation. Starting with teleoperation and object recognition, the RobotChallenge ends up with navigation in unknown terrain. Besides others, the participants acquire soft-skills, such as project and team management. Being carried out for the first time in winter term 2011/2012, the RobotChallenge successfully promotes profound understanding of mobile robotics that is applied during practical experiences. It turns out, that aspiring to win competions lead to a high motivation of the students w.r.t. development of appropriate solutions.
    Intelligent Robots and Systems (IROS), 2012 IEEE/RSJ International Conference on; 01/2012
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    ABSTRACT: In this paper a multi-axial electromagnetically actuated punch is presented. This punch can apply forces up to 60 kN and has a maximum stroke of 4 mm. Especially, the punch is qualified for manufacturing micro-components demanding high accuracy and high stroke rates. Due to its multi-axial drive concept, the punch is able to compensate the tilt of the ram during the cutting process. Thus, abrasion at the die is reduced and the punching quality is improved. Therefore, two multiaxial control concepts are introduced and compared based on experimental results.
    American Control Conference (ACC), 2012; 01/2012
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    ABSTRACT: We introduce a new method of rotational image acquisition for four-dimensional (4D) optical coherence tomography (OCT) of beating embryonic chick hearts. The rotational axis and the central A-scan of the OCT are identical. An out-of-phase image sequence covering multiple heartbeats is acquired at every angle of an incremental rotation of the deflection mirrors of the OCT system. Image acquisition is accomplished after a rotation of 180°. Comparison of a displayed live M-mode of the central A-scan with a reference M-mode allows instant detection of translational movements of the embryo. For calculation of 4D data sets, we apply an image-based retrospective gating algorithm using the phase information of the common central A-scan present in all acquired images. This leads to cylindrical three-dimensional data sets for every time step of the cardiac cycle that can be used for 4D visualization. We demonstrate this approach and provide a video of a beating Hamburger and Hamilton stage 16 embryonic chick heart generated from a 4D OCT data set using rotational image acquisition.
    Journal of Biomedical Optics 09/2011; 16(9):096007. · 2.88 Impact Factor
  • J. Kotlarski, B. Heimann, T. Ortmaier
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    ABSTRACT: In this paper experimental results are presented to compare the performance of kinematically redundant parallel robots with respect to their non-redundant counterparts. The main purpose is to validate existing simulated, i.e. claimed, findings demonstrating the advantages of kinematic redundancy in terms of singularity avoidance and, therefore, accuracy and precision. Exemplarily, the kinematically redundant prototype of the Institute at Mechatronic Systems is introduced. It is based on the well known planar 3RRR mechanism. In order to achieve kinematic redundancy, a prismatic actuator is added to the structure allowing one base joint to move linearly. As a result, the mechanism is able to reconfigure, i.e. optimize, its geometry according to different performance criteria and motion strategies. While performing a geometrical reconfiguration and following several desired (optimized) trajectories the pose of the end-effector is determined using an external measurement device. Hence, in addition to the encoder data of the actuators the performance can be analyzed without using any (uncertain) kinematic models. This allows for a meaningful comparative evaluation of the performance of kinematically redundant mechanisms.
    Robotics and Automation (ICRA), 2011 IEEE International Conference on; 06/2011
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    ABSTRACT: Precision skull surgery requires specialized instrumentation to satisfy demanding requirements in cochlear array implantation, deep brain stimulation electrode placement, and related applications. A miniaturized reconfigurable parallel kinematic mechanism which can be directly mounted on a patient's skull was designed, built, and tested for precision skull surgery. A Stewart-Gough platform is attached to a patient's skull so no optical tracking affecting the overall accuracy in keyhole surgery is required. Six bone anchors comprising the mechanism base joints are implanted at positions with sufficient skull thickness. Since no fixation frame is required, intervention planning flexibility is increased. The centers of the spherical shaped bone anchors can be localized accurately in the image space. An implicit registration to the physical space is performed by assembling the kinematics. Registration error is minimized compared to common optical tracker-based approaches. Due to the reconfigurable mechanism, an optimization of the hexapod's configuration is needed to maximize accuracy and mechanical stability during the incision. Mathematical simulation was conducted to estimate system performance. Simulation results revealed significant improvement in accuracy and stability when exploiting the redundant degrees of freedom and the implemented reconfigurability. Inaccurate localization of base points in the image data set was identified as the main source of error. A first prototype with passive prismatic actuators, i.e. micrometer calipers, was successfully built. A head-mounted parallel kinematic device for high precision skull surgery was developed that provides submillimetric accuracy in straight-line incisions. The system offers enhanced flexibility due to the absence of a rigid fixation frame.
    International Journal of Computer Assisted Radiology and Surgery 05/2011; 7(1):137-49. · 1.36 Impact Factor
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    ABSTRACT: In this paper five different optimization strategies for kinematically redundant mechanisms, i.e. mechanisms having additional actuator(s) in at least one kinematic chain, are presented. They are based on two main approaches, a discrete optimization and a classical continuous optimization. Exemplarily, a planar, kinematically redundant 3RRR-based mechanism is introduced. The position of its redundant actuator, i.e. the robot geometry, is optimized according to an optimization criterion that is denoted as the gain of the maximal homogenized pose error. Several analysis examples demonstrate the effectiveness of kinematic redundancy with respect to the introduced optimization procedures. It is shown that in comparison to discrete approaches, classical continuousbased optimization strategies do not necessarily lead to more appropriate results in terms of performance improvement.
    Robotics and Automation (ICRA), 2010 IEEE International Conference on; 06/2010
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    Jens Kotlarski, Bodo Heimann, Tobias Ortmaier
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    ABSTRACT: In this paper, the kinematically redundant 3(P)RRR and 3(P)RPR mechanisms were presented. In each case, an additional prismatic actuator was applied to the structure allowing one base joint to move linearly. After a description of some fundamentals of the proposed PKM, the effect of the additional DOF on the moving platform pose accuracy was clarified. An optimization of the redundant actuator position in a discrete manner was introduced. It is based on a minimization of a criterion that the authors denoted the gain (xh ) of the maximal homogenized pose error xh . Using several exemplarily chosen trajectories a significant improvement in terms of accuracy of the proposed redundant mechanisms in combination with the developed optimization procedure was demonstrated. It could be seen that the suggested index (xh ) leads to more appropriate switching patterns than the well known condition number of the Jacobian. Additional simulations demonstrated the marginal influence of the redundant actuator joint error on the moving platform pose error x. Furthermore, a comparative study on the usable workspaces, i.e. the singularity-free part of the total workspace providing a certain desired performance, of the mentioned mechanisms and their non-redundant counterparts was performed. The results demonstrate a significant increase of the useable workspace of all considered EE orientations thanks to the applied additional prismatic actuator. To further increase the overall and the operational workspace, future work will deal with the design optimization of the prismatic actuator, e.g. its orientation with respect to the x-axis of the inertial coordinate frame as well as its stroke ('length'). In addition, the simulation will be extended to PKM with higher DOF and an experimental validation of the obtained numerical results will be performed.
    04/2010; , ISBN: 978-953-307-070-4