Chih-Liang Chu

Development of a low-cost autofocusing probe for profile measurement

In this research, a low-cost and precision autofocus laser probe system was developed. Modified from the pickup head of a commercially available DVD as a basis, it can detect the focus error signal (FES) of the object with a built-in four-quadrant photodiode. The FES will feed back to the developed controller, through which the objective lens can be adjusted in position automatically to remain in focus, driven by a voice coil motor (VCM). The driving current (converted from servo-FES) of the VCM and the objective lens displacement are one to one linearly related within several hundred micrometres. Because of this relationship, the surface profile and the surface roughness of the tested object can be realized. Calibrated results showed that the designed probe has 200 µm linearity range, 0.2 µm accuracy, 0.1 µm resolution and standard deviation of 0.2 µm on average. Some examples are given to show the applicability of this probe system.

Development of a low-cost nanoscale touch trigger probe based on two commercial DVD pick-up heads

This study develops a low-cost three-dimensional nanoscale touch trigger probe based on two commercially available DVD optical pick-up heads. The probe is attached to a cross-form structure suspended from a symmetrical arrangement of thin steel strips. The displacements of the probe tip in response to a contact force are initially investigated using a theoretical approach and are then confirmed using ANSYS finite element software. A touch trigger probe system is then constructed in accordance with the finite element simulation results, and a series of experimental investigations is performed to evaluate its performance characteristics. In general, the results show that the symmetrical suspension mechanism restricts the probe to three degrees of motional freedom, achieves an equal stiffness condition in every direction and reduces the pre-travel error. It is shown that the touch trigger probe has a pre-travel variation of less than 96 nm in all measurement directions and a unidirectional repeatability of 46 nm. Finally, the maximum triggering force is found to be less than 0.1 mN for any approach angle.

Development of a high-precision straightness measuring system with DVD pick-up head

Al ow-cost and high-precision straightness measurement system using a DVD pick-up head has been developed in this research. By removing its objective lens the pick-up head of a commercial DVD player possesses excellent properties of stable laser power, collimated laser beam, circular Gaussian distribution and sensitive photodetectors. A moving knife-edge piece is placed between the pick-up head and a plane mirror. According to the knife-edge principle, the non-shaded part of the laser beam will be reflected back and focused onto the four-quadrant photodetector of the pick-up head. The four-quadrant signals are added to measure the returned laser power. Theoretical analysis based on the circular Gaussian distribution theory and regression method can attain the straightness error of the knife-edge while it moves along the beam direction. This low-cost system can apply to the straightness measurement of precision coordinate measuring machines and NC machine tools. Experimental results showed the achieved accuracy is better than ±0.2 µ mw ithin the measuring range of 200 µm.

Two-dimensional optical accelerometer based on commercial DVD pick-up head

This study develops a low-cost, highly sensitive two-dimensional optical accelerometer based on a commercially available DVD optical pick-up head. Vibrations of the structure of interest cause a change in the angle of the seismic mass within the accelerometer. The relative movement between the seismic mass and the base produces a change in the distribution of a focused light spot on the surface of a four-quadrant photodetector. The resulting change in the output of the voltage signals by the photodetector is then used to calculate the corresponding acceleration of the base. The experimental results indicate that the resonant frequencies of the accelerometer in the X- and Y-axis directions are 92.75 Hz and 92.87 Hz, respectively. Furthermore, the useful frequency range of the accelerometer is found to be approximately 20% of its resonant frequency. The sensitivities of the accelerometer in the X- and Y-axis directions are 22.9 V/g and 21.3 V/g, respectively. Finally, the noise equivalent acceleration (NEA) of the accelerometer is found to be less than 30 µg Hz−1/2 over the frequency range 0.5–50 Hz.

Development of an optical accelerometer with a DVD pick-up head

A low-cost highly sensitive optical accelerometer is developed based on the pick-up head of a commercially available DVD. The proposed accelerometer detects the focus error signal (FES) generated by a moving object using a built-in four-quadrant photodiode. The linear range of the FES curve is employed to indicate the displacement of the seismic mass relative to the base of the accelerometer. The acceleration acting on the base is then determined on the basis of the measured displacement.

Development of an optical accelerometer for low-frequency vibration using the voice coil on a DVD pickup head

In this paper, an economical and highly sensitive optical accelerometer using a commercial optical pickup head is proposed. In the proposed design, the moving part of a voice coil motor (VCM) built in the DVD pickup head is used as the seismic mass which combines with the optical system of the DVD pickup to form an accelerometer. This system not only reduces the design complexity and the manufacturing process but also obtains good measurement effect. Experimental results have shown that the accelerometer at low frequency has a sensitivity of 24.362 V/g.

A compensation method for the hysteresis error of DVD VCM

The present study considers an autofocusing laser probe system used for the measurement of the surface profile and roughness of an object. The system is based upon a modified pickup head of a commercially available DVD player which uses a voice coil motor (VCM) to drive an objective lens during the autofocusing process. It is known that hysteresis of the VCM during the autofocusing process reduces the precision of the measurement results. Consequently, the present investigation adopts a hysteresis model to develop a compensation method which can improve the accuracy of the optical pickup head within the measurement system.

Optimal modal vibration suppression of a fluid-conveying pipe with a divergent mode

This study deals with the divergence characteristics of pipes conveying fluid and explores the applicability of active modal vibration control for suppressing the associated excessive structural vibration. The Timoshenko beam theory is used to establish the system equation of motion. The analysis is based on the finite element method. Active modal control technique is developed in this work for pipes conveying fluid with a flow speed exceeding the critical one. Optimal independent modal space control (IMSC) is applied for the design. For pipes conveying super-critical flow speed, as considered in this work, the systems eigenvalues have both real and complex roots, which must be dealt with in a different way from what has been established in the literature. A weighting matrix with finite weights is applied for the control of complex modes, whereas a weighting matrix with an infinite weight is used for controlling the divergent mode, with roots being real. From this study, it is demonstrated that the control approach proposed in this work can ensure closed loop stability. The mode switching scheme of directing control to the mode which has higher modal response is found to be beneficial in reducing the overall structural vibration of the fluid-conveying pipe.

Finite Element Analysis of Fluid-Conveying Timoshenko Pipes

A general finite element formulation using cubic Hermitian interpolation for dynamic analysis of pipes conveying fluid is presented. Both the effects of shearing deformations and rotary inertia are considered. The development retains the use of the classical four degrees-of-freedom for a two-node element. The effect of moving fluid is treated as external distributed forces on the support pipe and the fluid finite element matrices are derived from the virtual work done due to the fluid inertia forces. Finite element matrices for both the support pipe and moving fluid are derived and given explicitly. A numerical example is given to demonstrate the validity of the model.

Active modal control of timoshenko pipes conveying fluid

Abstract This paper deals with vibration control of Timoshenko pipes conveying fluid. Excessive vibration in this flow induced vibration problem is suppressed via an active feedback control scheme. The strategy of the active vibration suppression technique will be presented and system dynamics formulated. Optimal independent modal space control technique is applied for best obtainable system performance. The computation consideration, implementation of control strategy, and the effectiveness of the control scheme are addressed. A finite element method is used for both the vibration and control analyses of the problem considered. Classical analysis by solving the partial differential equation is only applicable to systems with simple boundary conditions, such as simply supported pipes. When a more complex situation occurs, a closed form expression becomes very difficult, if not impossible, to obtain. The use of a finite element approach alleviates such deficiencies and is readily applicable to pipes with intermediate supports, nonuniform cross sections, and flexible supports. The problem of a large model size for the control system due to the use of the finite element approach is avoided by using the control technique presented. It is demonstrated in this work that a critical damping design can be achieved for the targeted mode controlled disregarding how large the model size is.