Surfaces made by Additive Manufacturing (AM) processes normally show higher roughness and more complicated microstructures than conventional machined surfaces. In this study, AM surface roughness measurements using both tactile and optical techniques are analyzed, theoretically and experimentally. Analytical results showed both techniques have comparable performance when measuring AM samples with good surface integrity. For surfaces with steep features, coherence scanning interferometry showed more reliable performance especially when peak-to-valley value was required. In addition of the benchmarking study, development of a low-cost measurement system, using laser confocal technology, is also presented in this paper. By comparing the measurement results with those from a coherent scanning interferometer, accuracy levels of the proposed system can be evaluated. It was concluded that with comparable accuracy, the proposed low-cost optical system was able to achieve much faster measurements, which would make it possible for in-situ surface quality checking.
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 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.
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.
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.
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.