Tobias Ortmaier

Leibniz Universität Hannover, Hanover, Lower Saxony, Germany

Are you Tobias Ortmaier?

Claim your profile

Publications (110)38.82 Total impact

  • Automated Systems and Technologies, Gabsen, Germany; 10/2014
  • [Show abstract] [Hide abstract]
    ABSTRACT: Die Entwicklung und Erprobung von Elektrodenträgern, die für eine modiolusnahe Endlage mit einer auf dem Formgedächtniseffekt basierenden Aktorik funktionalisiert sind, stellt besondere Anforderungen an die experimentelle Erprobung unter reproduzierbaren Laborbedingungen. Elementar ist dabei die einstellbar temperierte Lagerung von künstlichen Modellen der Cochlea oder Felsenbeinpräparaten, da der Formgedächtniseffekt temperaturgesteuert ausgelöst wird. Vorgestellt wird ein eigens dafür entwickelter Versuchsstand bestehend aus einem thermostatgesteuerten Wasserbecken, eine eigens entwickelte Halterung für Felsenbeinpräparate und Cochleamodelle sowie ein Insertionstool zur automatisierten und damit reproduzierbaren Insertion der Elektrodenträger. Der Implantatvorschub wird dabei reproduzierbar mittels Linearaktoren realisiert. Eine stereooptische Navigationskamera erlaubt die Positionierung des Insertionstools relativ zum Cochleamodell bzw. Humanpräparat basierend auf bildgestützten Planungsdaten für einen optimierten Insertionswinkel. Das Insertionstool ist dazu über einen Ausleger an einem Hexapod befestigt, dessen einzustellende Beinlängen für jede Insertion individuell über ein Matlabprogramm berechnet werden können. Eine modifizierte Felsenbeinhalterung erlaubt zusätzlich die Ausrichtung des Felsenbeinpräparats bei gleichzeitig verlässlicher Umströmung des Präparats mit dem Temperiermedium. Eine wasserfest gekapselte USB-Mikroskopkamera dient der Dokumentation der Insertionsversuche. Der beschriebene Versuchsaufbau konnte unter Laborbedingungen erfolgreich bei Insertionsversuchen erprobt werden und ist aktuell bei der Entwicklung neuartiger, temperaturgeregelt formverändernder Elektroden im Einsatz. Weitere Insertionsversuche in Felsenbeinpräparate sind geplant.
    85. Jahresversammlung der Deutschen Gesellschaft für Hals-Nasen-Ohren-Heilkunde, Kopf- und Hals-Chirurgie e. V., Dortmund; 05/2014
  • [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; · 1.66 Impact Factor
  • [Show abstract] [Hide abstract]
    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
  • [Show abstract] [Hide abstract]
    ABSTRACT: In recent years, optical coherence tomography (OCT) has gained increasing attention not only as an imaging device, but also as a navigation system for surgical interventions. This approach demands to register intraoperative OCT to pre-operative computed tomography (CT) data. In this study, we evaluate algorithms for multi-modal image registration of OCT and CT data of a human temporal bone specimen. We focus on similarity measures that are common in this field, e.g., normalized mutual information, normalized cross correlation, and iterative closest point. We evaluate and compare their accuracies to the relatively new normal distribution transform (NDT), that is very common in simultaneous localization and mapping applications, but is not widely used in image registration. Matching is realized considering appropriate image pre-processing, the aforementioned similarity measures, and local optimization algorithms, as well as line search optimization. For evaluation purpose, the results of a point-based registration with fiducial landmarks are regarded as ground truth. First results indicate that state of the art similarity functions do not perform with the desired accuracy, when applied to unprocessed image data. In contrast, NDT seems to achieve higher registration accuracy.
    SPIE Medical Imaging; 03/2014
  • SPIE Medical Imaging; 03/2014
  • [Show abstract] [Hide abstract]
    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;
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Due to continuously rising electricity prices, the energy efficiency of production systems is an increasingly important factor in industrial manufacturing. One goal is therefore to reuse the braking energy of manipulator axes. The acceleration and deceleration phases of axes must be synchronized such that, ideally, the braking energy can be reused immediately within the system. In our current technique for the energy-efficient motion planning of concurrent movements of axes, we do not yet consider changing the discrete control logic to reorder movement sequences. This is difficult, since this reordering must not violate critical system requirements. In this paper, we outline a new technique for automatically synthesizing energy-efficient discrete controllers from a scenario-based specification of a production system, which we enrich with information about consumed and generated energy. Last, we provide an outlook on challenging open research problems.
    Procedia Technology. 01/2014; 15:388–397.
  • [Show abstract] [Hide abstract]
    ABSTRACT: Designing a Kalman filter requires knowledge about the stochastic part of the system. Thus, disturbances affecting states and measurements should be known. However, in practical application these disturbances are usually unknown. In this contribution a modification of the autocovariance least-square method is presented. This method converts the measurement and process noise covariance estimation problem into a least squares functional, which can be solved with a Landweber iteration to regularize the ill-posed problem. Then, a tuned Kalman filter gain can be calculated. A simulative evaluation is introduced to prove the method regarding robustness against modeling error and variance of the estimates.
    12/2013;
  • Christian Hansen, Jens Kotlarski, Tobias Ortmaier
    [Show abstract] [Hide abstract]
    ABSTRACT: The improvement of energy efficiency in automation and robotic systems still has high significance in engineering research. This paper presents experimental results of minimum energy trajectory optimization for multi-axis manipulators using electrical energy exchange via the shared inverter DC link. Thus, existing simulation findings demonstrating the potential of the proposed minimum energy optimization approach are validated. In this context, a global optimization method is applied utilizing an advanced cost function formulation that is based on an energy flow model including all servo drive components. After the validation of the utilized system model, increased efficiency improvements are proven and compared to established methods. The presented approach as well as the experimental results are fully transferable to any kind of multi-axis system comprising DC link energy supply.
    16th International Conference on Advanced Robotics, Montevideo, Uruguay; 11/2013
  • [Show abstract] [Hide abstract]
    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
    [Show abstract] [Hide abstract]
    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
  • [Show abstract] [Hide abstract]
    ABSTRACT: form only given. Optical techniques are utilized in robot aided surgery as navigation tools to guide surgical instruments. The advantages are a reduction of severe damage during surgery and of post-surgical trauma. A common approach is to place spherical markers inside the operating field, which can be detected by a 3D imaging device, in order to determine the position of the patient with respect to the instrument. Optical coherence tomography (OCT) is not only capable of detecting these kind of artificial landmarks, but also natural features within the operating field and the bone. For example, during insertion of artificial cochlear implants channels have to be drilled inside the temporal bone [1]. Air inclusions in the surrounding bone, known as mastoid cells, can be used as natural landmarks and their position has to determined with high precision (see figure 1 (left)). However, when using optical devices as navigation tool, the optical properties of biological tissue distort the three dimensional data set due to refractive index changes which have to be corrected for navigation. This has been performed in ophthalmology but has not been done so far for bone and similar materials [2].In this contribution, we present a strategy to correct OCT data for refractive index changes in bone. The correction was carried out with in vitro measurements of porcine temporal bone which were obtained by using a swept source OCT (OCS1300SS) of Thorlabs, Inc. To gain information on the optical properties of the specimen, we prepared a 1 mm thick bone sample with planar and parallel surfaces. The refractive index was determined from OCT scans normal to the bone boundaries according to [3] and was found to be 1.51 ± 0.02. In this work, it was assumed that the bone is a homogeneous material which is a valid assumption in a statistical average. Before refractive index correction, we carried out a geometrical calibration of the OCT, as described in [4], which provides - nformation about the optical path of the OCT's scanning beam in free space. For correction, we determined the incidence angle of the scanning beam with respect to the air-bone boundary and performed the refractive correction by applying Snell's law and linear scaling of the optical path with the refractive index. The performance of the correction was tested first at OCT scans of a water droplet on a microscope cover slide (see figure 1 (middle)) and afterwards at a porcine bone sample with marker holes on the upper and lower surface. We found that the image distortion which accounts for more than 100 μm can be significantly reduced by our correction algorithm.
    The European Conference on Lasers and Electro-Optics; 05/2013
  • [Show abstract] [Hide abstract]
    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.
  • [Show abstract] [Hide abstract]
    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
  • [Show abstract] [Hide abstract]
    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
  • [Show abstract] [Hide abstract]
    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.
    Proceedings of SPIE - The International Society for Optical Engineering 11/2012;
  • Source
    Mechanism and Machine Theory 09/2012; 55:103. · 1.31 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: This paper addresses both modeling and dynamics identification of kinematically redundant parallel robots. Based on the Lagrangian equations of the first kind and using the coordinate partitioning method the dynamic equations of the regarded mechanism are derived analytically in a reduced symbolic form. The set of minimal dynamic parameters is automatically obtained thanks to the Lagrange function and the virtual work. The direct pattern search technique is applied to calculate optimal excitation trajectories to obtain reliable dynamic parameters. The direct pattern search technique is further used to identify the dynamic parameters. The proposed algorithms can be applied on both serial and parallel mechanisms in order to solve the parameter identification problem.Exemplarily, the redundant 3-(P)RRR mechanism of the Institute of Mechatronic Systems is introduced and described in detail. 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 can be able to reconfigure its geometry according to different optimization criteria and strategies. Several experimental results demonstrate the effectiveness and, therefore, the capability of the introduced identification procedure.
    Mechanism and Machine Theory 09/2012; 55:104–121. · 1.31 Impact Factor
  • Biomedizinische Technik/Biomedical Engineering 08/2012; · 1.16 Impact Factor