With state-of-the-art 3D measurement systems, short-wave structures such as tool marks cannot be resolved directly inside a machine tool chamber. Up to now, measurements had to be performed outside the machine tool. We present an interferometric sensor that carries out such measurements inside the machine tool, which saves time-consuming and expensive setup procedures. Our sensor HoloCut uses digital holography as measurement principle. By the use of multiple wavelengths, we get a large unambiguous axial measurement range of up to 2 mm and achieve micron repeatability, even in the presence of laser speckles. With a lateral resolution of 7 μm across the entire 20 x 20 mm2 field of view, both macro- and microstructures (such as tool marks) are measured with an axial resolution of 1 μm. Consequently, this qualifies HoloCut for in-situ measurements and integration in a machine tool. In this paper, the boundary conditions of integrating interferometers inside a machine tool are evaluated. Occurring vibrations and limited available space are particularly challenging constraints: The optical and mechanical design of HoloCut is introduced along with numerical correction algorithms: A piezo-stage setup is used to induce known displacements. Using these algorithms, measurements even with a closed-loop control of the machine tool head activated are demonstrated on a coin measurement. The use of HoloCut is motivated on the base of the daily operation of a 5-axis machine tool: We present an evaluation of an exemplary ISO 25178 parameter Sq using HoloCut measurements and compare those with reference, yet not inline-capable systems.
Multiwavelength digital holography enables precise and fast 3D height measurements of rough surfaces. To inspect objects during motion would enlarge the range of applications enormously. In this work the limits of this technique with respect to velocity and inclination angles are studied for linearly moving as well as for rotating objects. We demonstrate measurements on surfaces with inclination angles of up to 40° , moving linearly with a velocity of 2 mm/s, providing 2 μm accuracy, and on a rotating cylinder with circumferential speed of 10 mm/s, we achieve 1.1 μm precision. All measurements are conducted with less than 1 mW of continuous-wave laser light, so the object moves several micrometers during exposure time.
In this paper we present a miniaturized digital holographic sensor (HoloCut) for operation inside a machine tool.
With state-of-the-art 3D measurement systems, short-range structures such as tool marks cannot be resolved inside a machine tool chamber. Up to now, measurements had to be conducted outside the machine tool and thus processing data are generated offline.
The sensor presented here uses digital multiwavelength holography to get 3D-shape-information of the machined sample. By using three wavelengths, we get a large artificial wavelength with a large unambiguous measurement range of 0.5mm and achieve micron repeatability even in the presence of laser speckles on rough surfaces. In addition, a digital refocusing algorithm based on phase noise is implemented to extend the measurement range beyond the limits of the artificial wavelength and geometrical depth-of-focus. With complex wave field propagation, the focus plane can be shifted after the camera images have been taken and a sharp image with extended depth of focus is constructed consequently.
With 20mm x 20mm field of view the sensor enables measurement of both macro- and micro-structure (such as tool marks) with an axial resolution of 1 µm, lateral resolution of 7 µm and consequently allows processing data to be generated online which in turn qualifies it as a machine tool control.
To make HoloCut compact enough for operation inside a machining center, the beams are arranged in two planes: The beams are split into reference beam and object beam in the bottom plane and combined onto the camera in the top plane later on. Using a mechanical standard interface according to DIN 69893 and having a very compact size of 235mm x 140mm x 215mm (WxHxD) and a weight of 7.5 kg, HoloCut can be easily integrated into different machine tools and extends no more in height than a typical processing tool.
Multiwavelength digital holography on moving objects enables fast and precise inline-measurements of surface pro files. Due to the use of multiple wavelengths, optically rough surfaces with structure heights in the micrometer range can be mapped unambiguously. In this work we explore the influence of the object velocity on height measurements on inclined surfaces. We show measurements using spatial-phase-shifting holography employing two wavelengths and object velocities of up to 90 mm/s with eye-safe cw-lasers with less than 1 mW of laser light. Despite motion blur exceeding the mean speckle size, reliable height measurements can be conducted at these velocities. The height map of a metal cone with two different slope angles (1° , 10° ) is measured at an exposure time of 2 ms. Using line shaped illumination, each frame yields a height map of approximately 2 x 17 mm2. The overlap between the frames allows averaging as the image is put together, improving data quality. The mean repeatability of the height information in the investigated setup is better than 4.5 µm at a synthetic wavelength of 214 µm.
We present a novel optical system for distance measurement based on the combination of optical time-of-flight metrology and digital holography. In addition absolute calibration of the measurement results is performed by a sideband modulation technique. For the time-of-flight technique a diode laser (1470 nm) is modulated sinusoidally (128 MHz). The light reflected and scattered by an object is detected by an avalanche-photo-diode. The phase difference between the sent and detected modulation is a measure for the distance between the sensor and the object. This allows for distance measurements up to 1.17 m with resolutions of ~2 mm. The interferometric setup uses 4 whispering-gallery-mode lasers to perform multiwavelengths-holographic distance measurements. The four wavelengths span the range from 1547 nm to 1554 nm. The unambiguous measurement measurement-range of the interferometric setup is approx. 7 mm while resolutions of 0.6 μm are observed. Both setups are integrated into one setup and perform measurements synchronously. Exact knowledge of the frequency differences of hundreds of GHz between the four lasers is crucial for the interferometric fine scale measurement. For this aim the light of the lasers is phase-modulated with frequencies of 36 GHz and 40 GHz to produce optical sidebands of higher order, thus generating beat signals in the hundreds-of-MHz regime, which can be measured electronically. The setup shows a way to measure distances in the meter range with sub-micron resolution.
Electronic speckle pattern interferometry (ESPI) is a powerful technique for differential shape measurement with submicron resolution. Using spatial phase-shifting (SPS), no moving parts are required, allowing frame acquisition rates limited by camera hardware. We present ESPI images of 1 megapixel resolution at 500 fps. Analysis of SPS data involves complex, time-consuming calculations. The graphics processing units found in state-of-the-art personal computers have exceptional parallel processing capabilities, allowing real-time SPS measurements at video frame rates. Deformation analysis at this frame rate can be used to analyze transient phenomena such as transient temperature effects in integrated circuit chips or during material processing.
Using a digital holographic microscope setup, it is possible to measure dynamic volume changes in living cells. The cells were investigated time-dependently in transmission mode for different kinds of stimuli affecting their morphology. The measured phase shift was correlated to the cellular optical thickness, and then of the cell volume as well as the refractive index were calculated and interpreted. For the characterization of the digital holographic microscope setup, we have developed a transparent three-dimensional (3-D) reference chart that can be used as a lateral resolution chart and step-height resolution chart included in one substrate. For the monitoring of living cells, a biocompatible and autoclavable flow chamber was designed, which allows us to add, exchange, or dilute the fluid within the flow chamber. An integrated changeable coverslip enables inverse microscopic applications. Trypsinization, cell swelling and shrinking induced by osmolarity changes, and apoptosis served as model processes to elucidate the potential of the digital holographic microscopy (DHM).
We present a phase-shifting holographic set-up for the microscopic imaging of adherent cells. The superposition of an object wave field and a reference wave is recorded on a digital sensor with three reference wave phases. The reference phases are then recovered by statistical analysis of the recorded intensities. Subsequently, the object wave phase is calculated by the generalized phase shifting algorithm. After phase unwrapping and background subtraction, the phase shift introduced by the adherent cell culture is reconstructed. As the interferograms are recorded in the image plane of the microsope objective, the full lateral resolution is achieved in contrast to off-axis holography where the reconstruction requires numerical propagation for the separation of 0th and 1st order. Our approach uses three arbitrary unknown reference phases and poses thus minimum requirements on the mechanical and thermal stability of the set-up. We give preliminary results of images from a Vero cell line and pollen grains.