High Precision Dimensional Measurement Research Articles

Sub-pixel high precision dimensional measurement method for aero-engine hollow turbine blade based on industrial computed tomography image

Aero-engine hollow turbine blades are work under prolonged high temperature, requiring high dimensional accuracy. Blade profile and wall thickness are important parameters to ensure the comprehensive performance of blades, which need to be measured accurately during manufacturing process. In this study, a high accuracy industrial computed tomography (ICT) measuring method was developed based on standard cylindrical pin and ring workpieces of different sizes. Combining ICT with cubic spline interpolation, a sub-pixel accuracy was achieved in measuring the dimension of component. Compared with the traditional and whole-pixel level image measurement method, the cubic spline interpolation algorithm has the advantages of high accuracy in image edge detection and thus high accuracy of dimensional measurement. Further, the technique was employed to measure the profile and wall thickness of a typical aerospace engine turbine blade, and an accuracy higher than 0.015 mm was obtained.

Multi-sensor registration method based on a composite standard artefact

Registration is a critical and essential procedure in any multi-sensor dimensional measuring systems. Most of the traditional registration methods are achieved by measuring a common standard by multiple sensors; however, it is difficult to ensure that heterogeneous sensors measure the same feature points. In this study, a new multi-sensor registration method based on a composite standard artefact is proposed. The base of the standard artefact is equipped with a standard sphere and a standard chrome-circle for registering the visual sensor (VS) and the tactile probe sensor (TPS) on a multi-sensor dimensional measuring machine. The VS and TPS measure the standard chrome-circle and the standard sphere, respectively. The coordinate system conversion parameters between the two sensors were solved based on the constant distance between the standard chrome-circle centre and the standard sphere centre. Both the simulations and experiments demonstrate higher precision and stability of the proposed method for the registration of multiple heterogeneous sensors in high-precision dimensional measurement systems. The root mean square errors of the registration experiments in X and Y -directions are 6μm and 4μm, respectively. The combined standard uncertainties are uc(tx)=1.6μm and uc(ty)=1.2μm.

High Precision Dimensional Measurement with Convolutional Neural Network and Bi-Directional Long Short-Term Memory (LSTM).

In modern industries, high precision dimensional measurement plays a pivotal role in product inspection and sub-pixel edge detection is the core algorithm. Traditional interpolation and moment methods have achieved some success. However, those methods still have shortcomings. For example, the accuracy is still insufficient with the resolution limitation of the image sensor. Moreover, prediction results can be affected by image noise. With the recent success of deep learning technology, we propose a sub-pixel edge detection method based on convolution neural network (CNN) and bi-directional long short-term memory (LSTM). First, one-dimensional visual geometry group-16 (VGG-16) is employed to extract edge features. Then, a transformation operation is developed to generate sequence information. Lastly, bi-directional LSTM with fully-connected layers is introduced to output edge positions. Experimental results on our steel plate dataset demonstrate that our method achieves superior accuracy and anti-noise ability than traditional methods.

Multi-Sensor Registration in High-Precision CMM Based on a Composite Standard.

Registration is a critical step in multi-sensor dimensional measurement. As the accuracy of registration directly impacts the quality of final results, a reference sphere as a common standard is problematic in high-precision registration. In this paper, a novel method based on a composite standard is proposed to fuse the multiple heterogeneous sensors in high-precision coordinate measuring machines (CMMs), which will void the drawbacks of a reference sphere. The composite standard consists of a cone and cylinder, which share a same central axis. To ensure high precision in the submicron range, or better, the standard is manufactured by an ultra-precision machine. Three features of the composite standard are inspected by three sensors: a video camera (VC), a tactile probe (TP), and a chromatic confocal displacement sensor (CC). All features will concentrate on a common point through which the relation between the three sensors will be obtained. The errors of each measurement were analyzed theoretically, and simulations and real experiments were carried out to verify the composite standard. This study demonstrates that the proposed registration method is stable and that the standard has potential use for the registration of multiple sensors in high-precision dimensional measurement.

A Method for Measurement of Nonlinearity of Laser Interferometer Based on Optical Frequency Tuning.

A method for measuring the nonlinearity of laser interferometer using optical frequency tuning technique is presented in this paper. The basic principle of this method is to make the fractional part of an interference fringe change by tuning the laser frequency and determining the nonlinearity of interferometer by comparing the fractional fringe change measured by the interferometer to that calculated from the laser frequency change. An experimental interferometric system with a wavelength tunable laser source is set up and the nonlinearity of the interferometer is measured. Since it does not require the precise displacement mechanism to produce the optical path difference change, this method is more convenient to use and may achieve a higher accuracy than the conventional measurement methods. The nonlinearity of the arbitrary interferometric phase can be measured by changing the laser frequency with this method. Experiments results have shown that the repeatability of nonlinearity measurement is less than 0.2 nm. This method can be applied to interferometry-based high precision dimensional measurements, such as coordinate measurement and displacement sensor calibration.

Establishing Traceability to the International System of Units for Scattering Parameter Measurements From 750 GHz to 1.1 THz

In this paper, we describe a new measurement capability which provides fully calibrated, traceable scattering parameter measurements in rectangular metallic waveguide in the frequency range 750 GHz-1.1 THz. The instrumentation consists of a vector network analyzer (VNA) with waveguide extender heads, situated at the University of Leeds, Leeds, U.K., and primary measurement standards characterized by the National Physical Laboratory, Teddington, U.K.. The measurement standards consist of lengths of precision waveguide that are used during the calibration of the instrumentation. Traceability to the International System of Units (SI) is established by performing high-precision dimensional measurements on the waveguide sections. A preliminary uncertainty budget is presented, indicating the expected sizes of the main sources of error in both reflection and transmission measurements.

Parametric investigation of Linear Quadratic Gaussian and Model Predictive Control approaches for thermal regulation of a high precision geometric measurement machine

A numerical comparison between Linear Quadratic Gaussian (LQG) Compensator and Model Predictive Control (MPC) is investigated in this paper for thermal regulation of a high precision dimensional measurement machine subject to four thermal disturbances (laser interferometers). The aim is to control temperature at four locations thanks to four surface actuators, whatever the shape of perturbation signals in the range of magnitude of a few Watt considered for the actual device. Since the complex 3D thermal model of the machine cannot be used for real-time control, a modal reduced model is built and then used for state feedback control.Two configurations (points to be controlled close to or far from actuators and perturbations) are studied. A parametric analysis is carried out and shows the main differences between both control techniques. For MPC, the lower the penalization coefficient is, the better the results are, provided that the number of time steps for prediction horizon is sufficiently large. On the contrary, for LQG, it has been observed that decreasing the cost parameter down to its critical value for which the LQR Riccati equation cannot be solved anymore, leads to the control failure.

Probes for high precision dimensional measurement

Looks at work carried out by The Centre for Manufacturing Metrology at Brunel University into developing new probes for high precision dimensional measurement. Describes the conventional “touch‐trigger” probe and the errors in its performance. Also describes three prototype probes all based on fibre optic principles: a two dimensional Triggering Probe; a three dimensional Analogue Probe and a non‐contact probe. Concludes that the 2‐D triggering probe is a simple construction but limited to 2‐D operation; the analogue 3‐D is smaller in comparison with existing types, comparable in accuracy and cheaper to manufacture, and the non‐contact probe has potential for future development with some form of “intelligent” data processing.