Displacement Measurement System Research Articles

Passive compliant module with the displacement measurement sensor and its application for automatic assembly

Abstract Robot manipulators have been applied in the various automated manufacturing for handling, transferring, welding, polishing, painting, deburring, simple assembly and so on. Robotic assembly of the precision parts is difficult because the accuracy of the robot manipulator is not enough and the teaching procedure is complex. There have been various technologies about robotic assembly such as the position based assembly strategy, the active torque feedback control algorithm with FT sensor or joint torque sensors and the remote center compliance which gives the passive compliance to the end-effector. In the paper, we proposed a new type of the passive compliant device which can measure its deformation. The proposed device, which is called “Magic Gripper”, is composed of the passive compliance module, the gripping module, the displacement measurement module and the controller module. Also, a new assembly strategy using the passive compliant device with the displacement measurement system is proposed. The proposed device and the proposed assembly strategy are proved to be very useful for automatic assembly of the precision parts.

基于位置敏感探测器的六自由度精密位姿测量系统

基于位置敏感探测器的六自由度精密位姿测量系统

An underground three-dimensional displacement measurement system

With the development of society, the damage of geological disaster to people's life and property is more serious, so the improvement of monitoring means of geological disaster is particularly urgent, there are some shortcomings in traditional monitoring methods, and it is impossible to measure the three-dimensional displacement of underground. The authors have designed an integrated three-dimensional displacement sensor to measure underground horizontal displacement and vertical displacement and tilt angle, and realize the real-time on-line monitoring of underground three-dimensional displacement visually through multi-group structure co-measurement.

Interference signal processing for dynamic displacement measurement with 1 ns time resolution

We propose a novel signal processing method for a laser-interferometer-based displacement measurement system with significantly improved time resolution that enables us to measure transient state vibration with an instantaneous frequency of 100 MHz. In this method, a reference light is modulated by a sinusoidal waveform, and dynamic displacement is derived from the envelope curves of the interference signal. Using a phase modulator to emulate dynamic displacement with an instantaneous frequency of 100 MHz and a maximum peak-to-peak amplitude of over 100 nm, we confirmed that the proposed method can be used to measure fast dynamic displacement with 1 ns time resolution within 2.6 nm standard deviation.

Self-Aligning and Self-Calibrating Capacitive Sensor System for Displacement Measurement in Inaccessible Industrial Environments

High-precision positioning often requires high speed and high resolution displacement measurements in order to compensate for the small vibrations of critical components. The displacement sensor must be precise and stable over a long period of time to avoid expensive recalibration. This requires tight mounting tolerances, which are especially difficult to meet in inaccessible environments. The proposed sensor system is based on a capacitive sensor and consists of three subsystems: 1) a mechanical “zoom-in” system that performs self-alignment of the capacitive sensor electrode in order to reduce the mounting tolerances of the sensor; 2) a real-time capacitance-to-digital converter that employs an internal reference and electrical zoom-in technique to effectively reduce the dynamic range of the measured capacitance, thus improving the power efficiency; and 3) a self-calibration circuit that periodically calibrates the internal references to eliminate their drift. In previous publications, the three subsystems have been introduced. This paper shows how the different subsystems can be integrated to achieve optimal performance and presents new repeatability and stability measurement results. The overall system demonstrates a displacement measurement resolution of 65 pm (in terms of capacitance 65 aF) for a measurement time of 20 $\mu \text{s}$ . Furthermore, the thermal drift of the sensor is within 6 ppm/K, owing to the self-calibration circuit. In measurement mode, the system consumes less than 16 mW.

Micro-displacement reconstruction using a laser self-mixing grating interferometer with multiple-diffraction.

In this paper, we demonstrated an improved laser self-mixing grating interferometer (SMGI) with auto-collimation design which can avoid the disturbance from the light feedback of the zero-order diffraction beam. In order to obtain higher optical subdivision, SMGI with multiple-diffraction is implemented. Both theoretical analysis and experimental work show that the proposed system for displacement measurement can achieve high sensitivity and low measurement uncertainty. Using the proposed system, different forms of micro-displacement signals applied on the target (grating) have been reconstructed with accuracy of a few nanometers. The work presented in this paper provides a good way to achieve robust and high precision measurement with compact system configuration.

Development of a High Precision Displacement Measurement System by Fusing a Low Cost RTK-GPS Sensor and a Force Feedback Accelerometer for Infrastructure Monitoring.

A displacement measurement system fusing a low cost real-time kinematic global positioning system (RTK-GPS) receiver and a force feedback accelerometer is proposed for infrastructure monitoring. The proposed system is composed of a sensor module, a base module and a computation module. The sensor module consists of a RTK-GPS rover and a force feedback accelerometer, and is installed on a target structure like conventional RTK-GPS sensors. The base module is placed on a rigid ground away from the target structure similar to conventional RTK-GPS bases, and transmits observation messages to the sensor module. Then, the initial acceleration, velocity and displacement responses measured by the sensor module are transmitted to the computation module located at a central monitoring facility. Finally, high precision and high sampling rate displacement, velocity, and acceleration are estimated by fusing the acceleration from the accelerometer, the velocity from the GPS rover, and the displacement from RTK-GPS. Note that the proposed displacement measurement system can measure 3-axis acceleration, velocity as well as displacement in real time. In terms of displacement, the proposed measurement system can estimate dynamic and pseudo-static displacement with a root-mean-square error of 2 mm and a sampling rate of up to 100 Hz. The performance of the proposed system is validated under sinusoidal, random and steady-state vibrations. Field tests were performed on the Yeongjong Grand Bridge and Yi Sun-sin Bridge in Korea, and the Xihoumen Bridge in China to compare the performance of the proposed system with a commercial RTK-GPS sensor and other data fusion techniques.

Displacement Measurement Using Two-Photon Absorption Process in Si-Avalanche Photodiode and Fiber Bragg Gratings

This paper proposes a displacement measurement system using two-photon absorption (TPA) process in a Si-avalanche photodiode and fiber Bragg gratings (FBGs). The use of TPA process eliminates high-frequency and complicated RF circuits usually required for intensity correlation measurements. By controlling the phase difference between the modulation signals for probe and reference light, the displacements of FBGs are precisely measured. Small displacement of an FBG by 0.4 mm was successfully measured in the proof-of-concept experiment.

On-machine multi-directional laser displacement sensor using scanning exposure method for high-precision measurement of metal-works

We propose a multi-directional triangulation optical system and scanning exposure method for precise height displacement measurement of metal-works. It is difficult to measure metal surfaces precisely with a conventional triangulation-laser displacement sensor due to machining traces. Because reflection characteristics vary by position on the workpiece, measurement errors occur even with positions of the same height. Our multi-directional triangulation optical system performs triangulation using a lens array consisting of four lenses located in a cross. Since our optical system can detect reflected light from four directions, we can achieve high-precision measurement even if the reflection characteristics vary. In addition, our scanning exposure method accumulates reflected light from many positions by scanning a projection beam onto a workpiece during the image sensor’s exposure time. Because the shape of a spot image on an image sensor is the sum of the reflected light from many positions, measurement error due to measurement position is reduced. Our proposed methods can achieve on-machine deflection measurement of metal-works with a target accuracy of ±1μm.

Vision-based hybrid 6-DOF displacement estimation for precast concrete member assembly

Precast concrete (PC) members are currently being employed for general construction or partial replacement to reduce construction period. As assembly work in PC construction requires connecting PC members accurately, measuring the 6-DOF (degree of freedom) relative displacement is essential. Multiple planar markers and camera-based displacement measurement systems can monitor the 6-DOF relative displacement of PC members. Conventional methods, such as direct linear transformation (DLT) for homography estimation, which are applied to calculate the 6-DOF relative displacement between the camera and marker, have several major problems. One of the problems is that when the marker is partially hidden, the DLT method cannot be applied to calculate the 6-DOF relative displacement. In addition, when the images of markers are blurred, error increases with the DLT method which is employed for its estimation. To solve these problems, a hybrid method, which combines the advantages of the DLT and MCL (Monte Carlo localization) methods, is proposed. The method evaluates the 6-DOF relative displacement more accurately compared to when either the DLT or MCL is used alone. Each subsystem captures an image of a marker and extracts its subpixel coordinates, and then the data are transferred to a main system via a wireless communication network. In the main system, the data from each subsystem are used for 3D visualization. Thereafter, the real-time movements of the PC members are displayed on a tablet PC. To prove the feasibility, the hybrid method is compared with the DLT method and MCL in real experiments.

One-way ViSP (Visually Servoed Paired structured light system) for structural displacement monitoring

Nowadays, the evaluation of structural safety and serviceability is becoming more essential due to the deterioration of civil infrastructures. In particular, the structural displacement which provides important information on structural conditions is considered one of the important indicators for the health monitoring of structures. To estimate the structural displacement, this paper proposes a one-way projection-type six degree-of-freedom (DOF) displacement measurement system based on vision and laser sensors, named one-way ViSP (Visually Servoed Paired structured light system). The system is composed of a transmitter and a receiver, which are facing to each other. The transmitter consists of a 1D laser range finder, two lasers, and a 2-DOF manipulator, while the receiver consists of a screen and a camera. The one-way ViSP estimates relative displacements between a transmitter and receiver by measuring the coordinates of three projected beam spots and a distance. Besides the advantages such as high accuracy in 6-DOF displacement measurement, low cost, and real-time monitoring possibilities, the one-way ViSP can cover large areas such as dams and inclined planes by using multiple receivers. Through various simulations and experiments, it is shown that the one-way ViSP has high accuracy in 6-DOF displacement estimation.

Towards a traceable probe calibration method for white light interference based AFM

Probe calibration is the basis of accurate atomic force microscope (AFM) measurement. In white light interference based AFM (WLI-AFM), the relationship between the vertical displacement of probe and the position of the interference fringes on probe cantilever should be calibrated for accurate measurement. In this paper, a traceable probe calibration method for WLI-AFM is proposed. A high resolution displacement measurement system based on laser interference is constructed to obtain the vertical displacement of probe with high accuracy. This system is used for standard displacement input for calibration and makes the measurement result traceable to optical wavelength. A zero-order fringe positioning algorithm is presented to determine the position of the interference fringes on probe cantilever. A probe deflection model is established for the calibration relationship which is continuous and can be used for measurement. While calibration is conducted, series of zero-order fringe positions are obtained corresponding to the probe displacement inputs. The data are processed by the probe deflection model to obtain the calibration relationship. The proposed method is applied in a self-developed WLI-AFM. It is verified that the accuracy of the method is higher than the previous methods by certain experiments on this WLI-AFM.

Computer Vision-based Displacement Measurement Method With Arbitrarily Positioned Camera

Displacement is broadly used in structural health monitoring as a meaningful indicator of structural status. Measuring structural displacement using conventional devices such as the linear variable differential transformer is challenging due to practical issues such as equipment inaccessibility, insufficient measurement accuracy, or expensive system cost. Recently, computer vision-based displacement measurement methods have been developed to build high field applicable measurement system. However, the existing computer vision-based displacement measurementtechniques still have practical issues such as absence of camera installation point, marker attachment, or lightinduced error. This paper presents a computer vision-based displacement measurement system focusing on solving camera positioning issue. The proposed method adopts homography transform to allow cameras to be positioned at arbitrary point. A laboratory scale experiment was conducted to verify accurate displacement measurement of the proposed method with the arbitrarily positioned camera.

Micro-displacement measurement based on multiple self-mixing interference technology

Multiple self-mixing (MSM) interferometry, a high-precision and effective method for measuring micro-displacement, is proposed in this paper. First, a triangular wave is used to drive an electro-optic modulator for the phase modulation of MSM interference signals. Then, the five-step phase-shifting algorithm is used to demodulate the modulated MSM interference signals. Finally, the displacement curves of the vibrating target are reconstructed. MSM interferometry is used to measure the displacement of sinusoidal motion with a peak-to-peak value of 2000 nm and a frequency of 10 Hz. Simulation results show that the maximum reconstruction error of MSM interferometry is 0.074 nm. Experimental results show that the maximum reconstruction errors of MSM interferometry are less than 4 nm. Therefore, MSM interferometry satisfies the high precision required by displacement measurement systems.

Traceable technique for an in situ full field in-plane measurement validation of digital image correlation

For digital image correlation to be firmly accepted as a validated displacement measurement system in the industrial arena, a measurement must be captured by the analysis system at time of test which confirms that the image correlation hardware and software system is performing as expected. To this end, a method for validating stereo digital image correlation optical test setups is presented, which is traceable to the length standard. The method employs a screen, on which is displayed a randomized speckle pattern of appropriate pitch for the test in question. This speckle pattern is then artificially translated by a known number of pixels on the screen, and image pairs captured of the original and translated speckles. Processing of these data image pairs with image correlation, and calibration of the pixel pitch of the display screen using a traceable measurement system, allows the image correlation test setup to be traceably calibrated in terms of in-plane displacement. The method is shown to be sufficiently sensitive and repeatable to provide a reasonably accurate, traceable validation in a practical environment.

PTZ Camera-Based Displacement Sensor System with Perspective Distortion Correction Unit for Early Detection of Building Destruction.

This paper presents a pan-tilt-zoom (PTZ) camera-based displacement measurement system, specially based on the perspective distortion correction technique for the early detection of building destruction. The proposed PTZ-based vision system rotates the camera to monitor the specific targets from various distances and controls the zoom level of the lens for a constant field of view (FOV). The proposed approach adopts perspective distortion correction to expand the measurable range in monitoring the displacement of the target structure. The implemented system successfully obtains the displacement information in structures, which is not easily accessible on the remote site. We manually measured the displacement acquired from markers which is attached on a sample of structures covering a wide geographic region. Our approach using a PTZ-based camera reduces the perspective distortion, so that the improved system could overcome limitations of previous works related to displacement measurement. Evaluation results show that a PTZ-based displacement sensor system with the proposed distortion correction unit is possibly a cost effective and easy-to-install solution for commercialization.

영상처리기법을 이용한 실시간 변위 측정 및 붕괴감지 시스템

The collapse of concrete forms or structures often occur at places with many risks like construction sites. One of the most important factors to assess stability of structure can be the velocity ratio of horizontal direction respect to the vertical direction. In this study, the accuracy of measurement was verified through the steel beam test with the real-time displacement measurement system using the self-developed image processing, and the satisfactory results with the error of 0.1mm were reached when comparing test results using a LVDT. Additionally, through the small-scale model test and the field experiment, the change of displacement with the collapse of structures was measured and the results showed that the horizontal displacement at the time of the collapse increased sharply compared to the vertical displacement. Thus, after measuring the displacement, the collapse can be predicted using the ratio of the vertical to the horizontal direction for the velocity or acceleration.

Vision Based Navigation for Omni-directional Mobile Industrial Robot

In this paper, an Omni-directional mobile industrial robot drilling system for aerospace manufacture is introduced. Mecanum wheels are used for the robot's maneuverability in congested workspace. An industrial robot is applied to complete the drilling work for a rocket shell. A vision system is applied to enhance the precision of mobile drilling. Additional sensor systems such as laser measurement system and displacement measurement system are equipped to do the autonomous navigation and anti-collision job. To increase the flexibility and working volume of the mobile industrial robot, the autonomous mobile drilling scheme is presented. In order to fulfil the requirement for drilling precision in aerospace industry, a vision-based deviation rectification solution is developed. Some experiments are carried out to compare the influence of different calibration targets on the robot system. Numerical tests show that the rectification system is able to satisfy the accuracy of the positioning in the autonomous drilling work.

High accuracy displacement measuring system using strain gauge based displacement sensor and direct sequence spread spectrum techniques in data acquisition system

The present article portrays a design and implementation of microcontroller based system for displacement measurement using strain gauge based displacement sensor. A displacement transducer may be capacitive or resistive, is an uncontrollable device. Improvement in filtering increases the speed of measurement and enhances the measurement accuracy. In this paper a new algorithm and architecture is proposed as filter for the displacement measuring system with strain gauge based displacement sensor. We verify and implement the novel algorithm with this displacement transducer in the measuring of displacement successfully.

基于衍射光栅的干涉式精密位移测量系统

基于衍射光栅的干涉式精密位移测量系统