Lars Hoffmann

Fiber Bragg Grating-Based Force-Torque Sensor with Six Degrees of Freedom

Robotic control and force-feedback applications require multi-axial force and torque sensing. One possible implementation of future sensors is seen in fiber optic force torque sensors, since the signal demodulation may be located in some distance to the actual sensor and they also do not have to include any magnetic material. This article presents a fiber Bragg grating-based force and torque sensor with six degrees of freedom. The general setup resembles a Stewart platform. Its connecting beams are formed by the fiber used to measure the deformation of the transducer. The element creating stiffness may be of arbitrary form. We demonstrate how the sensor is realized and show results of all six force and torque measurements. We present a theoretical model of the sensor. The results in this work demonstrate the feasibility of a fiber-optic force-torque sensor.

Fiber-Optic Sensor Interrogation Based on a Widely Tunable Monolithic Laser Diode

Fiber-optic sensors have experienced great interest in research and development since their invention. Dedicated measurement systems are essential for utilization and development of these sensors. Instruments based on tunable lasers are established devices for the demodulation of such sensors. We introduce and demonstrate a tunable laser measurement system based on a widely tunable monolithic laser diode (TML). It is capable of demodulating different types of fiber-optic sensors, although we focus on the interrogation of fiber Bragg grating (FBG) sensors. The rapid tuning of the lasers wavelength is critical for achieving high measurement rates. However, the high tuning rate is demanding and requires careful characterization. We present a method for this and show, to our knowledge for the first time, data on the rapid tuning through the whole spectrum of this type of laser diode. We propose a modified centroid algorithm that can cope with nonequidistantly sampled spectra caused by the lasers functional principle. Finally, we demonstrate the dynamic performance of the new measurement system in an application: An FBG-based acceleration sensor is demodulated at a 5-kHz sample rate during a vibrational test showing the potential of the approach.

Full Strain Tensor Treatment of Fiber Bragg Grating Sensors

Embedded fiber Bragg gratings can be subjected to arbitrary states of strain including shear strain. Such perturbations can cause coupling between polarization modes. Coupled-mode theory in Bragg gratings so far neglected this effect and only considered forward-backward coupling. Polarization mode coupling within a Bragg grating leads to interdependencies between Bragg reflection peaks which have so far been unaddressed. We formulate a full strain tensor treatment of fiber Bragg gratings, considering the coupling of the polarization modes within the grating. We give an approximation for the coupling coefficients affecting the polarization mode coupling and numerically solve the coupled-mode equations for representative states of strain. We show in which way shear strain affects the optical response of a grating and demonstrate how the fibers beat length influences this characteristic.

Realization of a fiber-optic force-torque sensor with six degrees of freedom

Multi-axial force and torque sensing is of importance for robot control and many force-feedback applications. Minimal invasive robotic surgery (MIRS) is a possible field of application of force and torque sensors with up to six degrees of freedom. Although these sensors are not yet employed in current commercial MIRS systems, extensive work has been carried out on the development of these sensors. Some of their issues are related to their electric working principle: they are limited in performance by thermal noise, need electric power inside the patient and are not usable under influence of strong magnet fields (e. g. in MRI machines). One possible alternative is seen in fiber optic force torque sensors, since the signal demodulation may be located in some distance to the actual sensor and they also do not have to include any magnetic material. This article presents a fiber optic force and torque sensor with six degrees of freedom. The general setup resembles a Stewart Platform, whereas its connecting beams are formed by the fiber itself, and the element creating stiffness may be of arbitrary form. Only a single fiber is needed to extract all six parameters since they are measured on six multiplexed fiber Bragg grating sensors. We demonstrate how the sensor is realized and show results of torque measurements with variable load.

Soldering fiber Bragg grating sensors for strain measurement

When measuring strain with a FBG sensor fixed to a structure by an adhesive, one will notice, that only axial strain is monitored. This is due to the low Youngs modulus of both standard coating materials and adhesives. Metal coated fibers with high Youngs modulus have become available recently, which may be surface bonded by soldering. Motivated by this, finite element simulations with different coating materials and adhesives were carried out. Increasing the Youngs modulus of coating and adhesive in the simulation shows a difference in transverse strain of around 20 % for the two cross sectional fiber axes. On this basis a process to substitute standard fiber coatings with a copper coating is described. Satisfying results were obtained using a copper electroplating process. Especially its simple experimental access is noticeable. A sensor modified in the electroplating process is soldered to a cantilever beam and a quasi-static strain measurement is performed. Soldering the modified fiber sensors changes the spectral response of the FBG. Towards shorter wavelength which is a result of the strains induced by cooling.

Fiber optic strain measurement for machine monitoring

Monitoring machines during operation is an important issue in measurement engineering. The usual approach to monitoring specific machine components is using strain gauges. Strain gauges, however, may sometimes not be used if conditions are harsh or installation space is limited. Fiber optic sensors seem to be an alternative here, but dynamic health monitoring has been dificult so far. The focus of this field study is to measure vibration characteristics of machine parts during operation using fiber optic sensors with the objective of early damage detection. If that was possible, downtime and maintenance costs could be minimized. Therefore a field test for dynamic fiber optic strain measurement on a roller bearing was carried out. The test setup consisted of the bearing built into a gear test stand and equipped with an array of fiber Bragg grating sensors. Fifteen fiber sensors were interrogated with a sample rate of 1 kHz and the vibration pattern was extracted. The radial load distribution was measured with high spatial resolution and a high degree of compliance with simulation data was found. The findings suggest that fiber optic health monitoring for machine components is feasible and reasonable. Especially with the help of distributed sensing on various components extensive health monitoring on complex technical systems is possible.

Fiber optic sensing for telecommunication satellites

Modern telecommunication satellites can benefit from the features of fiber optic sensing wrt to mass savings, improved performance and lower costs. Within the course of a technology study, launched by the European Space Agency, a fiber optic sensing system has been designed and is to be tested on representative mockups of satellite sectors and environment.

Wälzlagerüberwachung mit faseroptischer Sensorik (Monitoring Roller Bearings with Fiber Optic Sensors)

Funktionskritische Maschinenkomponenten wie Wälzlager während des Betriebes zu überwachen, ist eine wichtige messtechnische Aufgabe. Es ist Gegenstand der Untersuchung, inwieweit sich faseroptische Sensorik dafür eignet. Anhand eines Demonstrators zur dynamischen Dehnungsmessung an einem Wälzlager wird der Stand der faseroptischen Messtechnik sowie deren Potenzial für die Maschinendiagnose beurteilt. Monitoring of critical machine components during operation is an important task in measurement engineering. It is examined whether fiber optic sensors can be employed. By setting up a demonstrator for dynamic strain measurement on a roller bearing the state of the art in fiber optic strain sensing and its potential for machine monitoring is evaluated.

Fiber-optic Sensor Demonstrator (FSD) integration with PROBA-2

Modern telecommunication satellites can benefit from the features of fiber optic sensing wrt to mass savings, improved performance and lower costs. Within the course of a technology study, launched by the European Space Agency, a fiber optic sensing system has been designed and is to be tested on representative mockups of satellite sectors and environment.

Nonlinear-programming optimized fiber Bragg grating based force-torque-sensor with six degrees of freedom

Force-torque sensors are key elements in modern force feedback and robotic control applications. For special applications resistance against electromagnetic interference, high amount of load cycles or chemical resistance are important. For these applications, fiber-Bragg-grating based force torque sensors have been seen as possible solutions. Yet the implementation of a force-torque-sensor with six degrees of freedom and well conditioned sensitivities is still lacking demonstration. In this work, we demonstrate the design of a miniaturized fiber-Bragg-grating based force-torque sensor with six degrees of freedom using a numerical nonlinear programming technique. We implement the optimized structure and show its feasibility and sensitivity.