Philipp Günther

Laser Doppler distance sensor using phase evaluation

This paper presents a novel optical sensor which allows simultaneous measurements of axial position and tangential velocity of moving solid state objects. An extended laser Doppler velocimeter setup is used with two slightly tilted interference fringe systems. The distance to a solid state surface can be determined via a phase evaluation. The phase laser Doppler distance sensor offers a distance resolution of 150 nm and a total position uncertainty below 1 microm. Compared to conventional measurement techniques, such as triangulation, the distance resolution is independent of the lateral surface velocity. This advantage enables precise distance and shape measurements of fast rotating surfaces.

Interferometric Sensor System for Blade Vibration Measurements in Turbomachine Applications

In order to improve the safety, lifetime, and energy efficiency of turbomachines, the dynamic behavior of the rotor has to be analyzed. Blade vibrations have to be monitored during operation to optimize the rotor design and to validate numerical models. However, measuring the vibration amplitude and frequency of the blades is a challenging task for metrology, since the blades to be measured are rotating quickly, and noncontact measurements are demanded. To solve this problem, we present a measurement system consisting of four laser Doppler sensors that have been mounted around the circumference of the rotor. These sensors measure simultaneously and contactlessly the in-plane velocity and the out-of-plane position of laterally moving objects. By analyzing the variation of the blade tip velocities, the vibration amplitude and frequency of the blades were estimated. Blade vibration measurements down to amplitudes of only 20 μm in tangential direction have been carried out. We achieved a standard uncertainty of approximately 400 nm for these experiments.

Distance measurement technique using tilted interference fringe systems and receiving optic matching.

The precise measurement of the distance of fast laterally moving rough surfaces is important in several applications such as lathe monitoring. A nonincremental interferometer based on two tilted interference fringe systems and a precise phase-difference estimation has been realized for this task. However, due to the speckle effect, the two scattered light signals exhibit different phase jumps and random envelopes causing small correlation coefficients and high uncertainties of the phase difference as well as the distance. In this Letter we present for the first time a method to enhance the signal correlation coefficient significantly. The interference signals are generated by scattered light of a rough surface from two different directions. A matching of illumination and receiving optic is performed. By this novel method, distance measurements with an uncertainty down to 1.2 μm at about 10 m/s lateral moving velocity have been achieved. Together with the simultaneously measured lateral velocity, the shape of rotating objects can be precisely determined.

Displacement, distance, and shape measurements of fast-rotating rough objects by two mutually tilted interference fringe systems

The precise distance measurement of fast-moving rough surfaces is important in several applications such as lathe monitoring. A nonincremental interferometer based on two mutually tilted interference fringe systems has been realized for this task. The distance is coded in the phase difference between the generated interference signals corresponding to the fringe systems. Large tilting angles between the interference fringe systems are necessary for a high sensitivity. However, due to the speckle effect at rough surfaces, different envelopes and phase jumps of the interference signals occur. At large tilting angles, these signals become dissimilar, resulting in a small correlation coefficient and a high measurement uncertainty. Based on a matching of illumination and receiving optics, the correlation coefficient and the phase difference estimation have been improved significantly. For axial displacement measurements of recurring rough surfaces, laterally moving with velocities of 5 m/s, an uncertainty of 110 nm has been attained. For nonrecurring surfaces, a distance measurement uncertainty of 830 nm has been achieved. Incorporating the additionally measured lateral velocity and the rotational speed, the two-dimensional shape of rotating objects results. Since the measurement uncertainty of the displacement, distance, and shape is nearly independent of the lateral surface velocity, this technique is predestined for fast-rotating objects, such as crankshafts, camshafts, vacuum pump shafts, or turning parts of lathes.

Shape and vibration measurement of fast rotating objects employing novel laser Doppler techniques

This contribution presents novel laser Doppler techniques, which allow simultaneous measurement of radial position and tangential velocity and, thus, determination of the shape of rotating objects with one single sensor. Conventional laser Doppler velocimeters measure only velocities. A concurrent position measurement can be realized by generating two fan-like interference fringe systems with contrary fringe spacing gradients and evaluating the quotient of the two resulting Doppler frequencies. Alternatively, two tilted fringe systems in combination with phase evaluation can be employed. It is shown that the position uncertainty of this sensor is not only independent of the surface roughness but, most notably, that it is in principle independent of the object velocity. Thus, in contrast to conventional distance sensors, the novel laser Doppler position sensor offers high temporal resolution below 3 &mgr;s and high position resolution in the micrometer range simultaneously. The sensor was applied to automatic 3D shape measurements of turning parts and to monitoring rotor unbalance and dynamic deformations. Furthermore, in situ measurements of tip clearance and rotor vibrations at turbo machines for up to 600 m/s blade tip velocity are reported. The results are in excellent agreement with those of triangulation and capacitive probes, respectively.

Heterodyne laser Doppler distance sensor with phase coding measuring stationary as well as laterally and axially moving objects

Both in production engineering and process control, multidirectional displacements, deformations and vibrations of moving or rotating components have to be measured dynamically, contactlessly and with high precision. Optical sensors would be predestined for this task, but their measurement rate is often fundamentally limited. Furthermore, almost all conventional sensors measure only one measurand, i.e. either out-of-plane or in-plane distance or velocity. To solve this problem, we present a novel phase coded heterodyne laser Doppler distance sensor (PH-LDDS), which is able to determine out-of-plane (axial) position and in-plane (lateral) velocity of rough solid-state objects simultaneously and independently with a single sensor. Due to the applied heterodyne technique, stationary or purely axially moving objects can also be measured. In addition, it is shown theoretically as well as experimentally that this sensor offers concurrently high temporal resolution and high position resolution since its position uncertainty is in principle independent of the lateral object velocity in contrast to conventional distance sensors. This is a unique feature of the PH-LDDS enabling precise and dynamic position and shape measurements also of fast moving objects. With an optimized sensor setup, an average position resolution of 240 nm was obtained.

Absolute and dynamic position and shape measurement of fast moving objects employing novel laser Doppler techniques

In this contribution a novel laser Doppler distance (LDD) sensor is presented, which allows simultaneous measurement of axial position and tangential velocity and, thus, determination of the shape of moving and especially rotating objects with one single sensor. Conventional laser Doppler velocimeters measure only velocities. A concurrent position measurement can be realized by generating two fan-shaped interference fringe systems with contrary fringe spacing gradients and evaluating the quotient of the two resulting Doppler frequencies. Alternatively, two tilted fringe systems in combination with phase evaluation can be employed. It will be shown that, in contrast to conventional distance sensors, high temporal resolution below 3 μs and high position resolution of about 1 μm can be achieved simultaneously, because the position uncertainty of the LDD sensor is in principle independent of the object velocity. This is advantageous especially for monitoring highly dynamic processes e.g. at turbo machines, where in-process measurements of tip clearance and rotor vibrations are reported for up to 600 m/s blade tip velocity.

Miniaturized nonincremental interferometric fiber-optic distance sensor for turning process monitoring

For in-process shape monitoring of rotating objects such as workpieces in a turning machine, contactless and compact sensors with high temporal resolution are necessary. For this challenging task, we developed a miniaturized and robust nonincremental interferometric fiber-optical distance sensor with dimensions of only 30×40×90 mm 3 , which enables attaching the sensor head directly to the mount of a turning tool bit. We present the results of in-process 3-D shape measurements of turning parts at a metal working lathe. To proof the accuracy of the measurement results, comparative measurements with tactile and optical sensors were performed. A maximal deviation between the different measurement methods of 2.2 μm was achieved for the determination of the mean height of a radial step.

Abstands- und Formvermessung schnell bewegter Festkörperoberflächen mit dem optischen Doppler-Effekt (Distance and Shape Measurement of Fast Moving Solid-State Surfaces Using the Optical Doppler Effect)

In diesem Beitrag werden auf der Laser-Doppler-Velozimetrie basierende neuartige Sensoren vorgestellt, die eine gleichzeitige Messung von Geschwindigkeit und Position bewegter Festkörper ermöglichen. Dadurch ist es möglich, die absolute Form eines rotierenden Objektes zu berechnen. Da die Messunsicherheit dieser Sensoren unabhängig von der Geschwindigkeit des Messobjektes ist, sind sie besonders für die Vermessung schnell bewegter Objekte geeignet. In this paper we present novel laser Doppler sensors based on the laser Doppler velocimetry for simultaneous measurement of velocity and position of moving objects. Because of the simultaneous measurement it is possible to determine the absolute shape of a rotating object. Due to the fact that the measuring uncertainty is independent of the object velocity the sensor is especially dedicated for measuring fast moving objects.

Novel Dynamic Rotor and Blade Deformation and Vibration Monitoring Technique

Monitoring rotor deformations and vibrations dynamically is an important task for improving both the safety and the lifetime as well as the energy efficiency of motors and turbo machines. However, due to the high rotor speed encountered in particular at turbo machines, this requires concurrently high measurement rate and high accuracy, which is hardly possible to achieve with currently available measurement techniques. To solve this problem, in this paper, we present a novel nonincremental interferometric optical sensor that measures simultaneously the in-plane velocity and the out-of-plane position of laterally moving objects with micrometer precision and concurrently with microsecond temporal resolution. It will be shown that this sensor exhibits the outstanding feature that its measurement uncertainty is generally independent of the object velocity, which enables precise deformation and vibration measurements also at high rotor speed. Moreover, this sensor does not require an in situ calibration and it allows a direct measurement of blade velocity variations in contrast to blade tip timing systems. For application under harsh environmental conditions such as high temperatures, a robust and miniaturized fiber-optic sensor setup was developed. To demonstrate the capability of this sensor, measurements of tip clearance changes and rotor blade vibrations at varying operating conditions of a transonic centrifugal compressor test rig at blade tip velocities up to 600 m/s are presented among others.