Accuracy improvement of linear stages using on-machine geometric error measurement system and error transformation model.

In order to improve the accuracy of linear stages, a compact, portable and easy installation of a six-degree-of-freedom (6DOF) geometric error measurement system, in which two mirrors with special position and orientation are innovatively regarded as the sensitive elements of the roll error, is proposed. A set of combined focus lenses is integrated into the 6DOF measurement system to improve the resolution of the roll error. The accuracy of a linear stage is evaluated by the positional errors at the functional point, which is located at the working volume of a linear stage. An error transformation model based on the Abbe principle and the Bryan principle is established to estimate the positional errors at the functional point according to those at the measurement point. A series of experiments are carried out to verify the capable of the designed system and the effectiveness of the established model.

Self-compensation method for dual-beam roll angle measurement of linear stages.

In this paper, a self-compensation method for improving the accuracy of roll angle measurement of a linear stage caused by the non-parallelism of dual-beam due to time-dependent mechanical deformation of the support is proposed and integrated into a 5-DOF sensor to verify the feasibility. The non-parallelism between two laser beams is online real-time monitored by a pair of small autocollimator units. Through the ray-tracing analysis, the method to separate the roll angle of the moving stage and non-parallelism induced roll error is determined. A series of experiments under different supporting forces and ambient conditions have been carried out. The compensated P-V values of the roll angles are all within ±4 arc-sec, no matter how bad the originally measured value of the linear stage is. The average improvement of about 95% is significant. The effectiveness and robustness of the proposed measurement system in the changing environment are verified.

A five degrees-of-freedom errors measurement system for rotary axis with reference laser for reference axis alignment.

This paper proposes a five degrees-of-freedom measurement system for measuring geometric errors of the rotary axis. To align the measured rotary axis with the reference axis, a diode laser is used to represent the rotary axis of the measured rotation stage. Based on the proposed measurement system, a model for separating the position independent geometric errors and position dependent geometric errors of the measured rotary axis from the measured value is established and verified by measurement experiments. The results of measurement experiments repeated for five times show that the measurement uncertainty of the proposed measurement system is less than ±1.6 µm for radial motion, the measurement uncertainty is less than ±1.7 arc sec for tilt motion, and the measurement uncertainty is less than ±1.3 arc sec for angle position.

Design of a compact four degree-of-freedom active compensation system to restrain laser's angular drift and parallel drift.

The laser collimation technique is widely used in science research and industrial applications. The pointing stability will be affected by the common problem of beam drift. A compact active compensation system is presented in this paper. The angular drift of the diode laser and parallel shift caused by angular drift compensation can be measured and actively compensated through a 4 degree-of-freedom active compensation module. The design of the whole system is compact, which makes it easy to be integrated into a measurement system. Experimental results indicate that the approach proposed can enhance the point stability to 88% for only angular drift compensation and further to 96.1% if both angular drift and parallel shift are compensated. This compensation module for point stability control can be used in any laser applications.

Modeling and Optimal Design for a High Stability 2D Optoelectronic Angle Sensor.

The structural deformations caused by environmental changes in temperature, vibration, and other factors are harmful to the stability of high precision measurement equipment. The stability and optimal design method of a 2D optoelectronic angle sensor have been investigated in this study. The drift caused by structural deformations of the angle sensor has been studied and a drift error model has been achieved. Key components sensitive to thermal and vibrational effects were identified by error sensitivity analysis and simulation. The mounts of key components were analyzed using finite element analysis software and optimized based on the concept of symmetric structures. Stability experiments for the original and optimized angle sensors have been carried out for contrast. As a result, the stability of the optimized angle sensor has been improved by more than 63%. It is verified that the modeling and optimal design method is effective and low-cost, which can also be applied to improve the stability of other sensors with much more complex principles and structures.

Real-Time Correction and Stabilization of Laser Diode Wavelength in Miniature Homodyne Interferometer for Long-Stroke Micro/Nano Positioning Stage Metrology.

A low-cost miniature homodyne interferometer (MHI) with self-wavelength correction and self-wavelength stabilization is proposed for long-stroke micro/nano positioning stage metrology. In this interferometer, the displacement measurement is based on the analysis of homodyne interferometer fringe pattern. In order to miniaturize the interferometer size, a low-cost and small-sized laser diode is adopted as the laser source. The accuracy of the laser diode wavelength is real-time corrected by the proposed wavelength corrector using a modified wavelength calculation equation. The variation of the laser diode wavelength is suppressed by a real-time wavelength stabilizer, which is based on the principle of laser beam drift compensation and the principle of automatic temperature control. The optical configuration of the proposed MHI is proposed. The methods of displacement measurement, wavelength correction, and wavelength stabilization are depicted in detail. A laboratory-built prototype of the MHI is constructed, and experiments are carried out to demonstrate the feasibility of the proposed wavelength correction and stabilization methods.

Error Analysis and Compensation of a Laser Measurement System for Simultaneously Measuring Five-Degree-of-Freedom Error Motions of Linear Stages.

A robust laser measurement system (LMS), consisting of a sensor head and a detecting part, for simultaneously measuring five-degree-of-freedom (five-DOF) error motions of linear stages, is proposed and characterized. For the purpose of long-travel measurement, all possible error sources that would affect the measurement accuracy are considered. This LMS not only integrates the merits of error compensations for the laser beam drift, beam spot variation, detector sensitivity variation, and non-parallelism of dual-beam that have been resolved by the author's group before, but also eliminates the crosstalk errors among five-DOF error motions in this study. The feasibility and effectiveness of the designed LMS and modified measurement model are experimentally verified using a laboratory-built prototype. The experimental results show that the designed LSM has the capability of simultaneously measuring the five-DOF error motions of a linear stage up to one-meter travel with a linear error accuracy in sub-micrometer and an angular error accuracy in sub-arcsecond after compensation.

An Embedded Sensor System for Real-Time Detecting 5-DOF Error Motions of Rotary Stages.

The geometric error motions of rotary stages greatly affect the accuracy of constructed machines such as machine tools, measuring instruments, and robots. In this paper, an embedded sensor system for real-time measurement of two radial and three angular error motions of a rotary stage is proposed, which makes use of a rotary encoder with multiple scanning heads to measure the rotational angle and two radial error motions and a miniature autocollimator to measure two tilt angular errors of the axis of rotation. The assembly errors of the grid disc of the encoder and the mirror for autocollimator are also evaluated and compensated. The developed measuring device can be fixed inside the rotary stage. In the experiments, radial error motions of two points on the axis (h = 5 mm and 60 mm) were measured and calibrated with LVDTs, and the data showed that the radial error motions of the axis were less than 20 µm, and the calibration residual errors were less than 2 µm. When intermittent external forces were applied to the stage, the change of the stage's error motion could also be monitored accurately.

Robust roll angular error measurement system for precision machines.

This paper presents a robust and low-cost roll angle measurement system (RAMS) on the basis of two parallel beams in association with two position detectors. The commonly occurring influences of beam drift, beam diameter, and intensity variations, and non-parallelism of dual-beam on the roll angular error measurement of precision linear stages are thoroughly considered and reduced. The effectiveness of the designed system and proposed methods were demonstrated by a series of experiments. It has been verified that the designed system's measurement accuracy is within ± 1.2 arcsec over a measurement range of 1 m. The designed system is easy to construct both in the laboratory environment and factory field.

Development of a Micro/Nano Probing System Using Double Elastic Mechanisms.

To meet the requirement of high precision measurement of coordinate measurement machine system, a compact microprobe has been designed for 3D measurement in this paper. Aiming to reduce the influences of signal coupling during the probing process, the probe has been designed by adopting two elastic mechanisms, in which the horizontal and vertical motions of the probe tip can be separated by differential signals of quadrant photodetectors in each elastic mechanism. A connecting rod has been designed to transfer the displacement of the probe tip in vertical direction from lower to upper elastic mechanisms. The sensitivity models in horizontal and vertical directions have been established, and the sensor sensitivity has been verified through experiments. Furthermore, the signal coupling of three axes has been analyzed, and mathematical models have been proposed for decoupling. The probing performance has been verified experimentally.

Development of a High-Sensitivity Optical Accelerometer for Low-Frequency Vibration Measurement.

Low-frequency vibration is a harmful factor that affects the accuracy of micro/nano-measuring machines. Low-frequency vibration cannot be completely eliminated by passive control methods, such as the use of air-floating platforms. Therefore, low-frequency vibrations must be measured before being actively suppressed. In this study, the design of a low-cost high-sensitivity optical accelerometer is proposed. This optical accelerometer mainly comprises three components: a seismic mass, a leaf spring, and a sensing component based on a four-quadrant photodetector (QPD). When a vibration is detected, the seismic mass moves up and down due to the effect of inertia, and the leaf spring exhibits a corresponding elastic deformation, which is amplified by using an optical lever and measured by the QPD. Then, the acceleration can be calculated. The resonant frequencies and elastic coefficients of various seismic structures are simulated to attain the optimal detection of low-frequency, low-amplitude vibration. The accelerometer is calibrated using a homemade vibration calibration system, and the calibration experimental results demonstrate that the sensitivity of the optical accelerometer is 1.74 V (m·s-2)-1, the measurement range of the accelerometer is 0.003⁻7.29 m·s-2, and the operating frequencies range of 0.4⁻12 Hz. The standard deviation from ten measurements is under 7.9 × 10-4 m·s-2. The efficacy of the optical accelerometer in measuring low-frequency, low-amplitude dynamic responses is verified.

Low cost, compact 4-DOF measurement system with active compensation of beam angular drift error.

In this paper, a compact four-degree-of-freedom (4-DOF) measurement system is presented. With a special optical configuration, the pitch error, yaw error, and two straightness errors of the moving target are able to be detected by only a single laser beam from a collimated laser diode. A 2D hybrid mirror angle steering mount is designed to perform the large angle turning for the axis alignment and very fine angle tuning by PZT actuators for real-time beam drift compensation. A series of calibration and comparison experiments have been carried out to verify the performance of the proposed system. The developed active compensation system could effectively suppress the beam's angular drift to within ± 0.01 arc-sec in both of yaw and pitch directions. The developed 4-DOF measuring system is compact, low cost, and suitable for long distance geometric error measurement of linear stages.

Linear diffraction grating interferometer with high alignment tolerance and high accuracy.

We present an innovative structure of a linear diffraction grating interferometer as a long stroke and nanometer resolution displacement sensor for any linear stage. The principle of this diffractive interferometer is based on the phase information encoded by the ±1st order beams diffracted by a holographic grating. Properly interfering these two beams leads to modulation similar to a Doppler frequency shift that can be translated to displacement measurements via phase decoding. A self-compensation structure is developed to improve the alignment tolerance. LightTool analysis shows that this new structure is completely immune to alignment errors of offset, standoff, yaw, and roll. The tolerance of the pitch is also acceptable for most installation conditions. In order to compact the structure and improve the signal quality, a new optical bonding technology by mechanical fixture is presented so that the miniature optics can be permanently bonded together without an air gap in between. For the output waveform signals, a software module is developed for fast real-time pulse counting and phase subdivision. A laser interferometer HP5529A is employed to test the repeatability of the whole system. Experimental data show that within 15 mm travel length, the repeatability is within 15 nm.

Flexible temperature sensor array based on a graphite-polydimethylsiloxane composite.

This paper presents a novel method to fabricate temperature sensor arrays by dispensing a graphite-polydimethylsiloxane composite on flexible polyimide films. The fabricated temperature sensor array has 64 sensing cells in a 4×4 cm2 area. The sensor array can be used as humanoid artificial skin for sensation system of robots. Interdigitated copper electrodes were patterned on the flexible polyimide substrate for determining the resistivity change of the composites subjected to ambient temperature variations. Polydimethylsiloxane was used as the matrix. Composites of different graphite volume fractions for large dynamic range from 30 °C to 110 °C have been investigated. Our experiments showed that graphite powder provided the composite high temperature sensitivity. The fabricated temperature sensor array has been tested. The detected temperature contours are in good agreement with the shapes and magnitudes of different heat sources.