Fang Cheng

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 15u2009mm travel length, the repeatability is within 15u2009nm.

A scanning contact probe for a micro-coordinate measuring machine (CMM)

A new high precision contact scanning probe able to measure miniature components on a micro/nano-coordinate measuring machine (CMM) is proposed. This contact probe is composed of a fiber stylus with a ball tip, a floating plate and focus sensors. The stylus is attached to a floating plate, which is connected to the probe housing via four elastic wires. When the probe tip is touched and then deflected by the workpiece, the wires experience elastic deformations and the four mirrors mounted on the plate will be displaced. These displacements can be detected by four corresponding laser focus probes. To calibrate this touch trigger probe, a double-trigger method is developed for a high-speed approach and a low-speed touch. Experimental results show that the probe has a symmetric contact property in the horizontal XY plane. The contact force is found to be about 109 μN. The standard deviation of the unidirectional touch is less than 10 nm and the pre-travel distance is around 10 nm with a standard deviation of less than 3 nm.

The system and the mechatronics of a pagoda type micro-CMM

This paper presents design considerations of a precision micro-CMM system and its mechatronic modules. The basic design concept is to meet the requirements of high stiffness, force balance, thermal balance, Abbe principle, metrology frame and vibration-free. Based on these criteria, a novel bridge of pagoda shape is designed and analysed by optimisation to verify its superior stiffness with force balance and thermal balance structure due to its symmetrical geometry. A high precision Z-ram design with co-axial counterweight and vibration suppression is presented. The design of a novel symmetrical coplanar XY-stage that observes the Abbe principle is explained. This micro-CMM has a measurement range of X: 20 mm, Y: 20 mm and Z: 10 mm. Driven by a commercial ultrasonic motor and fed back by a designed diffraction interference scale, each axis can achieve long stroke and nano-positioning motion. Adding a designed contact scanning measuring probe to the spindle, this system is able to measure any geometry of a...

A linear diffraction grating interferometer with high accuracy

A new miniature nanometer interferometer using grating Doppler effect is developed. The principle of this interferometer can be attributed to the phase information encoded by the ±1st order diffractive light beams. Properly interfering these two light beams leads to modulation similar to Doppler frequency shift, which can be translated to displacement measurement via phase decoding. Because of the measurement standard of grating interferometer system is the grating pitch, compared to the commonly used laser interferometer, the diffractive grating system reduces the environment influences on measurement accuracy. The calibration experiment between grating and HP5529A has been implemented. The measurement results show this grating interferometer measurement system is applicable for higher accuracy in long stroke.

Design of an analogue contact probe for nano-coordinate measurement machines (CMM)

A new high precision analogue contact probe with long measurement range that is able to measure miniature components on a micro/nano-coordinate measuring machine (CMM) is proposed. This analogue probe is composed of a fiber stylus with a ball tip, a mechanism with a wire-suspended floating plate, a two-dimensional angle sensor and a miniature Michelson linear interferometer. The stylus is attached to the floating plate. The wires experience elastic deformation when a contact force is applied and then the mirrors mounted on the plate will be displaced, which displacements can be detected by two corresponding sensors. Each component of the probe is designed, fabricated and assembled in this research. Base on the design requirements and stiffness analysis of the probe, several constrained conditions are established, and optimal structure parameters of the probe are worked out. Simulation and experimental results show that the probe can achieve uniform stiffness, ±20μm measurement range and 1nm resolution in X, Y and Z directions. The contact force is less than 50μN when the ball tip is displaced by 20μm. It can be used as a contact and scanning probe on a Micro/Nano-CMM.

A Robust Control Scheme of Nanopositioning Driven by Ultrasonic Motor

An innovative nanopositioning control system is proposed in this paper. A commercial ultrasonic motor (Nanomotion Co. model HR4) is employed to generate 3-mode motions of different scales. A multi-scale positioning control scheme can thus been developed by integrating the 3 driving modes. A new displacement sensor LDGI (Linear Diffraction Grating Interferometer) is developed and served as the displacement feedback. The uncertainty of LDGI system has been proved less than 10nm in 15mm. By phase subdivision technique the resolution of LDGI can be interpolated to 0.25nm. With this hardware system a software-based controller is developed. A self-tuning module, called Back Propagation Neural Network (BPNN), is added to a PID control loop. This self-tuning PID controller shows more robust than conventional ones, especially when some unpredictable disturbance occurs. Experiments show that this system is able to reach the steady state in 2 seconds without notable overshoot or vibration and hold the position for a long time with the positioning error less than 3nm. When some disturbances occur the system can build a new steady state in 2 seconds.

Error compensation of grating interferometer due to angular error of linear stage

Linear Diffraction Grating Interferometer (LDGI) is a long stroke and nanometer resolution displacement sensor for any linear stages. The principle of this diffractive interferometer is based on the Littrow configuration and phase information encoded by the ±1st order beams diffracted by a holographic grating. Because the emitted beams are separated on the grating plane, the yaw error of the stage during motion will cause the optical path difference of the two beams resulting in phase shift. This study investigates the influence of the yaw error. Experiments show that after yaw error compensation, the LDGI readings can be significantly improved.

New method on real-time signal correction and subdivision for grating-based nanometrology

In this paper a real-time signal process method is proposed for a new grating-based sensor LDGI (Liner Diffraction Grating Interferometer). The LDGI signal shows much higher frequency than conventional optical encoders. When the grating moves 416nm the LDGI system generates one wave cycle. The waveforms have some typical distortions: DC offsets, amplitude variation and phase error. For real-time measurement, in every millisecond the waveforms are normalized to eliminate DC offsets and amplitude variation. Then the phase error is corrected with an operation of coordinate rotation. After that, with zero-pass counting and phase subdivision the displacement can be worked out. If the displacement is too short to generate a whole wave cycle, which means there are not enough data to work out the signal distortions, an optimization method for sine curve fitting is used to calculate the displacement. If the displacement is shorter than 20nm, a group of empirical values are used in signal process. Experiments show that with the proposed method, the measurement repeatability of LDGI is within 5nm. Especially when this system is used for nanoscale measurement the uncertainty can hardly be detected with a laser interferometer. Besides, the proposed method helps to get higher resolution. Experiments show that the minimum displacement that the system can detect is 1nm.

High precision measurement system based on coplanar XY-stage

A coplanar XY-stage, together with a high precise measurement system, is presented in this paper. The proposed coplanar XY-stage fully conforms to the Abbe principle. The symmetric structural design is considered to eliminate the structure deformation due to force and temperature changes. For consisting of a high precise measurement system, a linear diffraction grating interferometer(LDGI) is employed as the position feedback sensor with the resolution to 1 nm after the waveform interpolation, an ultrasonic motor HR4 is used to generate both the long stroke motion and the nano positioning on the same stage. Three modes of HR4 are used for positioning control: the AC mode in continuous motion control for the long stroke; the gate mode to drive the motor in low velocity for the short stroke; and the DC mode in which the motor works as a piezo actuator, enabling accurate positioning of a few nanometers. The stage calibration is carried out by comparing the readings of LDGI with a Renishaw laser interferometer and repeated 5 times. Experimental results show the XY-stage has achieved positioning accuracy in less than 20nm after the compensation of systematic errors, and standard deviation is within 20 nm for travels up to 20 mm.

LDGI Signal Subdivision by Soft Computing for Nanomeasurement

A high-precision optical scale called LDGI (Linear Diffraction Grating Interferometer) has been developed for long stroke measurement to nanometer accuracy. This study presents the principle of newly developed LDGI and its possible waveform errors of output signals. Mathematical methods for waveform error correction, pulse counting, signal subdivision and displacement calculation are proposed. All the digital processes are carried out by software. Experimental results show that the system has high tolerance to geometrical errors of the moving stage. The standard deviation of measured values is within 20nm even up to 18mm long stroke.