Hendrikje Pauer

Simulation tool for 3D shape sensors based on Fiber Bragg gratings and optimization of measurement points

3D shape sensing using Fiber Bragg gratings has attracted the interest of several research groups in recent years, but so far no possibility has been presented to optimize the sensing system by simulation. Almost always, the gratings (the points of strain measurement) are distributed equidistantly, and the reconstruction algorithm has not been verified by simulation. In this paper we present our simulation tool that works on a mathematical basis and includes two parts: the first part describes how to determine the strains of Fiber Bragg gratings mounted along a given shape. The second part describes our algorithm to reconstruct the shape from this strain data using theory of differential geometry. This reconstruction algorithm is evaluated within the simulation environment and can be adapted to different system behaviors like circular bending of the instrument, cubic bending or bending according to the Euler beam theory. Furthermore, the gratings can be optimized with respect to their position and designated Bragg wavelength. To demonstrate the effectiveness of the simulation tool, an optimization has been conducted for the grating positions along a shape with two independent bendable segments. For each configuration of the shape, the reconstruction results using the optimized grating positions were significantly better than when using equidistantly distributed gratings.

Using ORMOCER®s as casting material for a 3D shape sensor based on Fiber Bragg gratings

In robot assisted minimally invasive surgery, flexible instruments are a highly interesting research topic, as they promise more flexibility and new possibilities for surgical interventions. Shape sensors are needed in order to retrieve information about the geometry and especially the tip position of the instrument. Those shape sensors are usually based on Fiber Bragg Gratings. In contrast to other research groups, which e.g. use nitinol wires as a core for their fibers, we follow the approach of casting the fibers in soft materials. For our latest prototype, a special ORMOCER® material has been used, which is an inorganic-organic hybridpolymer with adjustable properties. The fabrication of the sensor is described in detail. The reproducibility of the wavelength measurements has been validated for several shapes, proving the reasonableness of our approach.

Motivation of a new approach for shape reconstruction based on FBG-optical fibers: Considering of the Bragg-gratings composition as a sensornetwork

In various fields of application, the shape and the tip position of flexible, snakelike objects have to be reconstructed. For this, the considered objects are fitted with so-called shape sensors. This shape sensors are e.g. applied in medical technology to support minimally invasive surgical interventions by tracking flexible instruments; this way navigation systems can be considerably supported. The sensors consist of a solid snakelike body out of flexible carrier material, as silicone, with embedded FBG - optical glass fibers along the object-axis. Guided along the observed instruments, the sensor is supposed to detect the instruments shape by detecting its own ones. The fibers measure the strain at discrete points along the sensor body, which is caused by deformation of the sensor. From these values the shape is estimated. This estimation is performed using specific algorithms. Accordingly, certain requirements regarding the position, orientation and exact number of the measurement units are made. As part of the manufacturing process of the sensor, however, exact control of fiber positioning cannot be realized. To compensate this inaccuracy and also further occurring problems, a fundamentally new calculation approach is presented in this paper. The basic idea is, to consider the system of measurement units as a sensor network. The position and orientation of the units are not considered to be static, because they can only be detected after production but cannot be exactly implemented in a controlled way with a planned position and orientation. The idea is realized by initializing a tensor field on a manifold, representing the surface of the object. This allows to apply the algorithm to measurement values, measured at randomly distributed positions along the sensor body. The new approach is promising and more accuracy in shape sensing is expected do be achieved. The approach of surface characterization is developed in a way that it is transferable to other applications. In the future, also areas in general can be analysed by applying to adapted algorithms based on the same idea. Interpolation of e.g. temperature- and radiation fields can be done in an intelligent way by measuring discrete values by efficiently distributed measurement units.

Flexible Polymer Shape Sensor Based on Planar Waveguide Bragg Gratings

A newly developed concept for a miniaturized optical shape sensor is demonstrated. The sensor consists of a flexible polymer bending beam with two integrated optical waveguide Bragg gratings spatially staggered to each other. We report on the basic layout and the fabrication process and characterize the fundamental sensing response by an elaborate testing scheme. The polymer shape sensor reveals a pronounced spectral shift of the Bragg wavelength of several nanometers upon deformation with a distinct directionality. In conjunction with an undiscernible hysteresis, these results highlight the general applicability of the polymer Bragg grating-based bending beam as a shape sensor.

Non-linear compensation of production inaccuracies and material drift by adjusting the sensor data fusion algorithms for shape sensing based on FBG-optical fibers

Shape sensing, where the shape of an object is estimated using fiber optical Fiber Bragg Grating (FBG) sensors, has gained increasing popularity in the last years. While the production process and the applications are different for the various research groups, the basic principle of shape sensing is the same: at certain cross-sections along the observed object, the information of three strain measurements is merged to one curvature information, and the information of these curvatures is then used to estimate the shape with various mathematical theories, depending on the application. Some effort has been made to calibrate and determine the accuracy of the shape sensor, but research on the influence of bad positioning of the Fiber Bragg Gratings seems relatively unattended. In this paper, one aspect of bad FBG positioning, namely inaccurate placement of the FBGs within one cross-section, and its influence on the reconstruction results is investigated. Furthermore, a modified approach for the reconstruction algorithm is presented, improving the reconstruction results for bad FBG placement compared to the conventional approach.

Fiber optical sensor system for shape and haptics for flexible instruments in minimally invasive surgery: overview and status quo

In minimally invasive surgery, exible mechatronic instruments promise to improve the overall performance of surgical interventions. However, those instruments require highly developed sensors in order to provide haptic feedback to the surgeon or to enable (semi-)autonomous tasks. Precisely, haptic sensors and a shape sensor are required. In this paper, we present our ber optical sensor system of Fiber Bragg Gratings, which consists of a shape sensor, a kinesthetic sensor and a tactile sensor. The status quo of each of the three sensors is described, as well as the concept to integrate them into one ber optical sensor system.

Polymer planar waveguide Bragg gratings: fabrication, characterization, and sensing applications

In this contribution, we give a comprehensive overview of the fabrication, characterization, and application of integrated planar waveguide Bragg gratings (PPBGs) in cyclo-olefin copolymers (COC). Starting with the measurement of the refractive index depth profile of integrated UV-written structures in COC by phase shifting Mach-Zehnder- Interferometry, we analyze the light propagation using numerical simulations. Furthermore, we show the rapid fabrication of humidity insensitive polymer waveguide Bragg gratings in cyclo-olefin copolymers and discuss the influence of the UV-dosage onto the spectral characteristics and the transmission behavior of the waveguide. Based on these measurements we exemplify that our Bragg gratings exhibit a reflectivity of over 99 % and are highly suitable for sensing applications. With regard to a negligible affinity to absorb water and in conjunction with high temperature stability these polymer devices are ideal for mechanical deformation sensing. Since planar structures are not limited to tensile but can also be applied for measuring compressive strain, we manufacture different functional devices and corroborate their applicability as optical sensors. Exemplarily, we highlight a temperature referenced PPBG sensor written into a femtosecond-laser cut tensile test geometry for tensile and compressive strain sensing. Furthermore, a flexible polymer planar shape sensor is presented.

Partial view geometric reconstruction of objects with rotational and planar symmetries for grasping tasks

In this paper we present a method based on two steps for reconstruction rotational and planar symmetric objects from a single partial view for grasping tasks. In the first step the orientation of the object is determined and in the second step the symmetry axis position is found. Then shape parameters are recovered and the completed geometry is used to plan and execute grasps. We leverage the symmetry information from the reconstruction to plan grasps effectively, resulting in high ratio of good grasps vs. tested grasps. We evaluate our methods on a robot platform that can reliably perform the grasps. Also, a shape characterization is provided for which the first step of the method (finding an objects orientation) will work, giving a starting point for more general shape reconstruction methods.