Laser Doppler Distance Sensor Research Articles

Miniaturized interferometric 3-D shape sensor using coherent fiber bundles

Mach-Zehnder interferometer based sensors for simultaneous distance and velocity measurement allow for absolute 3-D shape measurements of rotating workpieces for instance in cutting lathes. The achievable shape uncertainty is limited to around one micron due to the speckle effect and temperature drifts, however. In this paper, a laser Doppler distance sensor with phase evaluation (P-LDD sensor) with a camera based scattered light detection is investigated. A novel speckle separation technique and in-situ fringe distance calibration method are realized to reduce the measurement uncertainty. A coherent fiber bundle is employed to forward the scattered light towards the camera. This enables a compact and passive sensor head with keyhole access. Compared with a photo detector based sensor, the camera based setup allows to decrease the measurement uncertainty by the order of one magnitude. As a result, the total shape uncertainty of absolute 3-D shape measurements can be reduced to about 100 nm.

Measurement uncertainty of non-incremental, non-contact, in-situ shape measurements

Abstract Optical measurement systems work fast and non-contact and can achieve sub-micron precision. Thus they appear to be well suited for in-situ shape measurement of fast rotating objects such as cutting processes in metal working lathes. Most optical measurement systems, however, allow an axial position measurement only. In order to retrieve the shape of the object from a distance measurement, the distance between the sensor and the centre of the object has to be known. Otherwise, deviations of this distance, for instance due to temperature effects or vibrations, will result in a measurement deviation. In order to allow an absolute shape measurement, which is independent of the sensor position, the mean radius of the rotating object can be retrieved from the object's circumferential velocity. The laser Doppler distance sensor with phase evaluation (P-LDD sensor) allows a simultaneous velocity and distance measurement with high temporal resolution. Thus, the P-LDD sensor allows to measure the mean radius as well as the spatially resolved deviation of the radius independently of the sensor position. In order to quantify the achievable measurement uncertainty, and especially the influence of the temperature the measurement uncertainty budget is derived and considers random as well as systematic errors. It is shown that the P-LDD sensor allows an absolute, three-dimensional shape measurement of fast rotating objects with sub-micron uncertainty. The systematic measurement uncertainty of the absolute shape due to the temperature amounts to only 200 nm/K. Thus the P-LDD sensor is not dependent on temperature-controlled laboratories but can be employed directly in the production process (in-situ or in-process).

A laser-optical sensor system for blade vibration detection of high-speed compressors

Improved efficiency as well as increased lifetime of turbines and compressors are important goals in turbomachinery development. A significant enhancement to accomplish these aims can be seen in online monitoring of the operating parameters of the machines. During the operation of compressors it is of high interest to predict critical events like flutter or stall which can be achieved by observing blade deformations and vibrations. We have developed a laser Doppler distance sensor (LDDS), which is capable of simultaneously measuring the radial blade expansions, the circumferential blade deflections as well as the circumferential velocities of the rotor blade tips. As a result, an increase of blade vibrations is measured before stall at characteristic frequencies. While the detected vibration frequencies and the vibration increase are in agreement with the measurement results of a commercial capacitive blade tip timing system, the measured values of the vibration amplitudes differ by a factor of three. This difference can be mainly attributed to the different measurement locations and to the different measurement approaches. Since the LDDS is applicable to metal as well as ceramic, carbon-fiber and glass-fiber reinforced composite blades, a universally applicable sensor system for stall prediction and status monitoring is presented.

Optical dynamic deformation measurements at translucent materials.

Due to their high stiffness-to-weight ratio, glass fiber-reinforced polymers are an attractive material for rotors, e.g., in the aerospace industry. A fundamental understanding of the material behavior requires non-contact, in-situ dynamic deformation measurements. The high surface speeds and particularly the translucence of the material limit the usability of conventional optical measurement techniques. We demonstrate that the laser Doppler distance sensor provides a powerful and reliable tool for monitoring radial expansion at fast rotating translucent materials. We find that backscattering in material volume does not lead to secondary signals as surface scattering results in degradation of the measurement volume inside the translucent medium. This ensures that the acquired signal contains information of the rotor surface only, as long as the sample surface is rough enough. Dynamic deformation measurements of fast-rotating fiber-reinforced polymer composite rotors with surface speeds of more than 300 m/s underline the potential of the laser Doppler sensor.

On the speckle number of interferometric velocity and distance measurements of moving rough surfaces.

The minimum achievable systematic uncertainty of interferometric measurements is fundamentally limited due to speckle noise. Numerical and physical experiments, regarding the achievable measurement uncertainty of Mach-Zehnder based velocity and position sensors, are presented at the example of the laser Doppler distance sensor with phase evaluation. The results show that the measurement uncertainty depends on the number of speckles on the photo detectors. However, while the systematic uncertainty due to the speckle effect decreases, the random uncertainty due to noise from the photo detector increases with increasing speckle number. This results in a minimal total measurement uncertainty for an optimal speckle number on the photo detector, which is achieved by adjusting the aperture of the detection optics.

Measurement of radial expansion and tumbling motion of a high-speed rotor using an optical sensor system

In order to investigate the load capacity and the strength properties of high-speed rotors, dynamic deformation and vibration measurements are of importance, in particular at lightweight composite devices which cannot be simulated reliably. This is a challenging task in metrology since non-contact inspection techniques are required which offer micron uncertainties and high temporal resolution simultaneously, also under vacuum conditions. In order to meet these requirements, a non-incremental laser Doppler distance sensor system was developed using fiber and diffractive optics. In this paper we present for the first time high-speed deformation measurements of a cylindrical steel rotor using this novel sensor system. The radial rotor expansion of only some microns was determined despite the presence of an unsteady tumbling motion of the rotor, which was measured simultaneously. Future prospects are discussed including the possibility to measure non-metallic devices such as fiber-reinforced composites.

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.

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.