Friedrich M. Wahl

Technische Universität Braunschweig, Brunswyck, Lower Saxony, Germany

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Publications (196)68.27 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: Intraoperative fracture of the lateral cortex fractures of the tibia is a potential complication of high tibial osteotomy (HTO), which may result in inadequate rotational alignment of the distal tibia. Our aim was to determine how rotational malalignment of the distal tibial segment distal would affect intraarticular contact pressure distribution in the knee and ankle joints. A medial, L-shaped opening-wedge HTO was performed on seven human lower body specimens. A stainless steel device with integrated load cell was used to axially load the leg. Pressure-sensitive sensors were used to measure intraarticular contact pressures. Intraoperative changes in alignment were monitored in real time using computer navigation. Measurements were performed in the native knee alignment, after 10° and 15° of alignment correction and with the distal tibia fixed at 15° of external rotation. Moderate-to-large alignment changes after medial opening-wedge HTO resulted in a shift in intraarticular contact pressures from the medial compartment of the knee towards the lateral compartment. However, fixation of the distal tibial segment at 15° of external rotation neutralized this intended beneficial effect. In the ankle, external rotation of the distal tibia also caused a reduction in contact pressures and tibiotalar contact area. Malrotation of the distal tibial fragment negates the intended effect of offloading the diseased compartment of the knee, with the contact pressures remaining similar to those of the native knee. Furthermore, malrotation leads to abnormal ankle contact pressures. Care should be taken to ensure appropriate rotational alignment of the distal tibial segment during intraoperative fixation of HTO procedures.
    Knee Surgery Sports Traumatology Arthroscopy 03/2015; DOI:10.1007/s00167-015-3553-4 · 2.84 Impact Factor
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    ABSTRACT: Background For the further development of robot-assisted endoscope guidance in functional endoscopic sinus surgery (FESS), ground data about the workspaces and endoscope movements in conventional FESS are needed. Methods Applying a self-developed marker-based tracking system, we collected the pose data (position and orientation) of the endoscope and all other instruments used in five real sinus surgeries. Results The automated segmentation of the endoscope poses shows the shape of a hourglass, with a pivot region or pivot point at the 'waistline' of the hourglass, close to the nasal entrance in the nasal dome. Conclusion We were able to identify a pivot point at the waistline of the segmented endoscope poses. The size of the pivot point corresponds with the diameter of the 4 mm endoscope. Because of the reduction to four degrees of freedom for endoscope motions (three rotations and one translation), easier and safer robot-assisted endoscope guidance becomes feasible.
    International Journal of Medical Robotics and Computer Assisted Surgery 03/2015; 11(1). DOI:10.1002/rcs.1599 · 1.53 Impact Factor
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    ABSTRACT: High tibial osteotomy (HTO) is a commonly used surgical technique for treating moderate osteoarthritis (OA) of the medial compartment of the knee by shifting the center of force towards the lateral compartment. Previous studies have documented the effects of HTO on the biomechanics of the knee. However, the effects of the procedure on the contact pressures within the ankle joint have not been as well described. Seven cadavers underwent an HTO procedure with sequential 5° valgus realignment of the leg up to 15° of correction. An axial force of up to 550 N was applied and the intraarticular pressure was recorded. Minor valgus realignment of the proximal tibia does not significantly alter the biomechanics of the ankle. However, moderate-to-large changes in proximal tibial alignment result in significantly decreased tibiotalar contact surface area and in changes in intraarticular ankle pressures. These findings are clinically relevant, as they provide a biomechanical rationale for the diagnosis and treatment of ankle symptoms in the setting of lower limb malalignment or after alignment correction procedures. This article is protected by copyright. All rights reserved
    Journal of Orthopaedic Research 11/2014; 33(4). DOI:10.1002/jor.22793 · 2.97 Impact Factor
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    Carsten Last, Simon Winkelbach, Friedrich M. Wahl
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    ABSTRACT: One major problem, when using statistical shape information in image segmentation problems, is that many training samples are needed in order to obtain a satisfactory shape prior for a particular class, especially when the intra-class variability of the object shapes is high. To cope with this problem, we present an elegant variational formulation that allows local adaptations of the parameters associated with a trained shape prior. This enables us to obtain accurate segmentation results with a limited amount of training shapes. We provide a sound mathematical foundation for our approach and embed it into the well-known level set segmentation framework, which makes our approach applicable to a large class of problems. Moreover, we show how a smooth transition from global to local adaptations of the shape parameters can be achieved. We point out the advantages of our new variational global-to-local approach by comparing it with another level set segmentation approach that includes a global shape prior.
    Image Processing (ICIP), 21st IEEE International Conference on, Paris, France; 10/2014
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    ABSTRACT: BACKGROUND: Revision joint replacements are challenging surgical tasks. Knowing the exact type of primary prosthesis is essential to avoid long preoperative organisation, long operation times, and especially loss of bone and soft-tissue during operation. In daily routine there is often no information about the primary prosthesis. OBJECTIVE: We are developing methods for identifying implanted prostheses from x-ray images by means of matching template images generated from prosthesis CAD data. METHODS: The application is separated into three major components: The "Template Image Generation" adds 3d models of endoprostheses to a database. The "X-ray Image Segmentation" extracts endoprostheses from provided sets of x-ray images. The "Template Matching" finds the best matching prosthesis types in the data base. At the current stage, one prosthesis model (Corin, Knee ProthesisUniglide) was used for evaluating these algorithms. RESULTS: Very accurate identifications with accuracies of about 90% for lateral and over 70% for frontal images could be achieved. CONCLUSIONS: The current results of this feasibility study are very promising. A reliable and fast prosthesis identification process seems realistic to support the surgeon when planning and performing revision arthroplasty. Further improvements of segmentation accuracies and extending the prosthesis data base are intended next steps towards this goal.
    Technology and health care: official journal of the European Society for Engineering and Medicine 10/2014; 22(6). DOI:10.3233/THC-140858 · 0.64 Impact Factor
  • Volker Schomerus, Dennis Rosebrock, Friedrich M. Wahl
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    ABSTRACT: In this paper we present a new approach for monocular image based lane border detection in situations where the characteristics of the pavement and the lane border are unknown. To achieve real time performance on standard hardware we analyze statistical characteristics of 1D signals on certain profile lines to find different types of features which belong to the lane border. These features are used for fitting a border model in. The proposed method shows good results in complex situations. It was evaluated in different scenarios with cobblestone pavement, lowered curbs, lane markers, parking cars defining the lane border as well as disturbances on the road like shadows, dirt and asphalt damages. The method was successfully used within the Volkswagen research vehicle 'eT' (electronic Transporter).
    2014 IEEE Intelligent Vehicles Symposium (IV); 06/2014
  • Yosef Dalbah, Dirk Koltermann, Friedrich M. Wahl
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    ABSTRACT: We present a method that extracts plane segments from images of a time-of-flight camera. Future driver assistance systems rely on an accurate description of the vehicle’s environment. Time-of-flight cameras can be used for environment perception and for the reconstruction of the environment. Since most structures in urban environments are planar, extracted plane segments from single camera images can be used for the creation of a global map. We present a method for real time detection of planar surface structures from time-of-flight camera data. The concept is based on a planar surface segmentation that serves as the fundament for a subsequent global planar surface extraction. The evaluation demonstrates the ability of the described algorithm to detect planar surfaces form depth data of complex scenarios in real time. We compare our methods to state of the art planar surface extraction algorithms.
    Sixth International Conference on Digital Image Processing; 04/2014
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    ABSTRACT: Grasping objects is a well-known problem in robotics. If the objects to be grasped are known, usually they are to be placed at a desired position in a desired orientation. Therefore, the object pose w.r.t the gripper has to be known before placing the object. In this paper we propose a simple and efficient, yet robust approach to this challenge, which can (nearly) eliminate dead times of the employed manipulator — hence speeding up the process significantly. Our approach is based on the observation that the problem of finding a pose at which the object can be grasped and the problem of computing the pose of the object w.r.t. the gripper can be solved separately at different stages. Special attention is paid to the popular bin-picking problem where this strategy shows its full potential. To reduce the overall cycle time, we estimate the grasp pose after the object has been grasped. Our estimation technique relies on the inertial parameters of the object — instead of visual features — which enables us to easily incorporate pose changes due to grasping. Experiments show that our approach is fast and accurate. Furthermore, it can be implemented easily and adapted to diverse pick and place tasks with arbitrary objects.
    Robotics and Automation; 01/2014
  • Yosef Dalbah, Nicolas Dingeldey, Friedrich M. Wahl
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    ABSTRACT: We present a method that estimates the ego motion of a vehicle based on camera data of high resolution (207x204 pixels) Time-of-Flight cameras using visual odometry techniques. Translation and rotation of camera motion in six degrees of freedom are calculated. Point correspondences in consecutive amplitude image pairs are built. By consideration of the depth image of the camera 3D point correspondences are derived from the two dimensional point correspondences. Camera motion between the two images is then computed by registration of the two resulting point clouds. The process is optimized by incorporation of outlier removal and a multi sensor setup. The presented optimizations raise the precision and robustness of the method and enable visual odometry by Time-of-Flight camera data as an alternative to common odometry systems in low speed scenarios.
    Proceedings of SPIE - The International Society for Optical Engineering 11/2013; DOI:10.1117/12.2050903 · 0.20 Impact Factor
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    Carsten Last, Simon Winkelbach, Friedrich M Wahl
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    ABSTRACT: Statistical shape models provide an important means in many applications in computer vision and computer graphics. However, the major problems are that the majority of these shape models require dense point-correspondences along all training shapes and that a large number of training shapes is needed in order to capture the full amount of intra-class shape variation. In this contribution, we focus on a statistical shape model that can be constructed from a set of training shapes without defining any point-correspondences. Additionally, we show how a local statistical shape model can make better use of the available shape information, greatly reducing the number of required training shapes. Finally, we present a new framework to fit this local statistical shape model without correspondences to range scans that represent incomplete parts of the trained shape class. The fitted model is then used to reproduce a natural-looking approximation of the complete shape.
    VMV 2013: Vision, Modeling & Visualization; 09/2013
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    ABSTRACT: The problem of object localization is a well-known problem in industrial robotics. Manufactured parts arrive at factories as bulk goods in boxes. Single parts need to be picked out of the boxes and have to be fed to a machine. The task of automatically isolating single objects is known as the bin-picking problem. Even in modern factories the task of bin-picking is not automated widely yet. The automatization of this task is expensive since state-of-the-art solutions require object-class specific algorithms. In this paper we present an applicable solution for the bin-picking problem which is based on a standard 3d-sensor and is able to handle arbitrary objects. Furthermore, it is robust against noise and object occlusions. Additionally, we propose an approach for optimal grasp pose estimation with collision avoidance that effectively reduces system cycle times.
    Robotics and Automation (ICRA), 2013 IEEE International Conference on; 01/2013
  • Dennis Rosebrock, Friedrich M. Wahl
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    ABSTRACT: The generic camera model considered in this work can be regarded as a mapping between image pixels and viewing rays. These rays are independent of each other which prohibits a standard parametric approach for calibration and modeling of these cameras. Spline surfaces are used here to calibrate and model generic imaging devices. This allows the utilization of sparse planar calibration boards and facilitates general forward projection as well as subpixel back projection. In contrast to other works the complete image area is to be calibrated, not only a part of it. This is done by adding further views of calibration patterns after an initial calibration step, which expands the calibrated region of the camera image. Results with two different imaging devices prove the general applicability of the proposed method and the comparison to an established parametric calibration procedure shows its superiority.
    Asian Conference on Computer Vision (ACCV); 11/2012
  • Ulrike Thomas, Friedrich Wahl
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    ABSTRACT: In this paper, it is shown how robust execution of assembly skills can be planned by using sensor state space graphs. The here proposed method is evaluated by some assembly skills in which force feedback is applied. Assembly skills are implemented by manipulation primitive nets which constitute an interface between planning and execution of robotic systems. The sensor state space graph is introduced, which is an extension of the contact formation graph in a more general way, when various sensors might be used simultaneously for assembly execution. It is shown, how contact formation graphs can be generated by simulation of rigid body motions. The known contact formation graphs are enhanced by the definition of contact types between higher order surfaces. Additionally, a more general view is given by introducing sensor state space graphs. It is shown how contact formation graphs can be mapped to manipulation primitive nets, which allow the robust execution of assembly skills, despite the appearance of uncertainties. The approach is demonstrated successfully on some assembly tasks. Here the task of plugging a power socket on a top hat rail is illustrated due to its complex sequence. The shown assembly task is characterized by small fitting tolerances, where the application of force feedback is indispensable.
    International Conference on Intelligent Robotics and Applications; 10/2012
  • 06/2012; 73(S 02). DOI:10.1055/s-0032-1314331
  • Jens Spehr, Simon Winkelbach, Friedrich M Wahl
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    ABSTRACT: Articulated structures like the human body have many degrees of freedom. This makes an evaluation of the configuration's likelihood very challenging. In this work we propose new linked hierarchical graphical models which are able to efficiently evaluate likelihoods of articulated structures by sharing visual primitives. Instead of evaluating all configurations of the human body separately we take advantage of the fact that different configurations of the human body share body parts, and body parts, in turn, share visual primitives. A hierarchical Markov random field is used to integrate the sharing of visual primitives in a probabilistic framework. We propose a scalable hierarchical representation of the human body and show that this representation is especially well suited for human gait analysis from a frontal camera perspective. Furthermore, the results of the evaluation on a gait dataset show that sharing primitives substantially accelerates the evaluation and that our hierarchical probabilistic framework is a robust method for scalable detection of the human body.
    Computer methods and programs in biomedicine 02/2012; 106(2):104-13. DOI:10.1016/j.cmpb.2011.12.010 · 1.09 Impact Factor
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    ABSTRACT: In this paper we propose a generic 6d object localization approach based on surface normal images and CAD model data. Normal images or “normal maps” can be obtained using only one single camera shot of a simple camera-projector system. The advantages of this sensor setup are very short acquisition times and the exclusive use of consumer hardware, namely a projector and a grey value camera, making it easy and cheap to build up. All computations are analytically tractable and can be computed in the Fourier domain allowing a very effective and fast computation.
    Pattern Recognition (ICPR), 2012 21st International Conference on; 01/2012
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    ABSTRACT: Rotary encoders and resolvers are by far the most common sensors to measure joint angles in articulated robots. Since various control approaches require angular rates as well, resolvers and encoders are also employed to derive angular rate signals. Due to the involved differentiation operation, however, quantization noise may be augmented significantly. Advanced filtering approaches can only partially overcome this drawback. Therefore, direct measurement of angular rates is desirable. Due to advancements in manufacturing technology and pushed by applications in entertainment devices, MEMS gyroscopes have become an attractive alternative for angular rate measurement. Unfortunately, they are affected by bias and other non-negligible disturbances, which may be a serious problem if consistent joint angle and angular rate measurements are required. The proposed approach fuses encoder and angular rate signals to mitigate their major drawbacks - bias and quantization errors - by exploiting their individual strong points. Bias in the angular rate signal is eliminated by analyzing the deviation between the integrated angular rate signal and the encoder signal. Quantization errors in the encoder signal are reduced by a so-called complementary filter which blends the integrated angular rate output with the encoder signal. Apart from a description of the approach and a theoretical analysis of its characteristics, experimental results demonstrating the effectiveness of the approach in closed-loop control of articulated robots are presented.
    Intelligent Robots and Systems (IROS), 2012 IEEE/RSJ International Conference on; 01/2012
  • D Rosebrock, F M Wahl
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    ABSTRACT: Cameras are a commonly used sensor in advanced driver assistance systems (ADAS). They serve to get vast amounts of information about a vehicle's environment. To accurately localize the measured data in relation to the own car, exact camera calibration is a prerequisite. This includes extrinsic as well as intrinsic parameters. While many works in the area of ADAS focus on extrinsic calibration, this work covers the intrinsic calibration. We use a generic camera model which regards the viewing ray of every pixel separately and can therefore be used to describe arbitrary imaging devices even with massive lens distortions. As the calibration procedure works for any camera, only one method has to be implemented, which simplifies the sensor calibration process. Former works have shown the applicability of generic camera models but do not cover important practical aspects which are subpixel ray determination and forward projection of arbitrary 3d points to the image plane. Furthermore, the calibration processes described so far are cumbersome and prone to inaccuracies. We propose to use spline surfaces to simplify the calibration procedure and implement general back and forward projection. The applicability of our approach is proved by showing calibration results for various real cameras.
    Intelligent Vehicles Symposium (IV), 2012 IEEE; 01/2012
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    ABSTRACT: Statistical shape models are one of the most powerful methods in medical image segmentation problems. However, if the task is to segment complex structures, they are often too constrained to capture the full amount of anatomical variation. This is due to the fact that the number of training samples is limited in general, because generating hand-segmented reference data is a tedious and time-consuming task. To circumvent this problem, we present a Locally Deformable Statistical Shape Model that is able to segment complex structures with only a few training samples at hand. This is achieved by allowing a unique solution in each contour point. Unlike previous approaches, trying to tackle this problem by partitioning the statistical model, we do not need predefined segments. Smoothness constraints ensure that the local solution is restricted to the space of feasible shapes. Very promising results are obtained when we compare our new approach to a global fitting approach.
    Machine Learning in Medical Imaging - Second International Workshop, MLMI 2011, Held in Conjunction with MICCAI 2011, Toronto, Canada, September 18, 2011. Proceedings; 09/2011
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    ABSTRACT: Electric motors clearly constitute the most common drive principle in robotics and mechatronics. Smart materials, however, offer considerably higher power-to-mass ratios than electric motors. If mechanical energy instead of electrical energy can be distributed through a system, highly dynamic and efficient torque transmission elements based on smart materials, e.g. piezoceramics, can be used to transmit torque from an input to an output element. Just like electric motors, they can thus provide position, velocity, and force-torque control of the output element. This paper introduces machine components, called adaptronic couplers, which can transmit variable torques highly dynamically from an input element to an output element employing static and/or dynamic friction. In the long run, systems (e.g. robots) based on these machine components are envisaged to compete with systems based on classic drive principles - especially electric motors - w.r.t. dynamics and power-to-mass-ratio. Apart from the concept itself, this paper addresses different control approaches and discusses their influence on energy consumption and wear. Moreover, various experimental results proving the basic concept are presented.
    Robotics and Automation (ICRA), 2011 IEEE International Conference on; 06/2011

Publication Stats

1k Citations
68.27 Total Impact Points

Institutions

  • 1970–2015
    • Technische Universität Braunschweig
      • Institut für Robotik und Prozessinformatik
      Brunswyck, Lower Saxony, Germany
  • 2008
    • Klinikum Braunschweig
      Brunswyck, Lower Saxony, Germany
    • Deutsche Gesellschaft für Hals-Nasen-Ohren-Heilkunde, Kopf- und Hals-Chirurgie e.V.
      Bonn, North Rhine-Westphalia, Germany
  • 2007
    • University of Bonn
      Bonn, North Rhine-Westphalia, Germany
  • 2006
    • Szczecin University of Technology
      • Institute of Control Engineering
      Stettin, West Pomeranian Voivodeship, Poland
    • Hannover Medical School
      • Trauma Department
      Hannover, Lower Saxony, Germany
  • 2002
    • California State University, Bakersfield
      • Computer Science
      Bakersfield, California, United States