Zhigang Liu

Surface stress-induced deflection of a microcantilever with various widths and overall microcantilever sensitivity enhancement via geometry modification

The issues of surface stress-induced deflection of a microcantilever with various widths and overall microcantilever sensitivity enhancement of microcantilever-based biosensors are investigated in this paper. A remarkably precise and simple analytical formula for calculating surface stress-induced deflection of a microcantilever with various widths is deduced. Particularly, the effect of surface stress on the location of the microcantilevers neutral axis is considered. This explicit analytical formula is validated by the finite element method simulation. An analytical equation for computing the fundamental resonant frequency of a microcantilever with various widths is also derived. This paper explores the deflections and resonant frequencies of the microcantilevers having basic and modified shapes. It is found that minimizing the effective mass near the microcantilevers free end and the clamping width at the fixed end significantly enhances the overall microcantilever sensitivity. A novel microcantilever, which is expected to have much more excellent performance and overall sensitivity than the simple rectangular-shaped microcantilever, is proposed as sensor element in biological detection.

The Influence of Axial Magnetic Field Distribution on High-Current Vacuum Arc

The influence of the distribution of axial magnetic fields (AMFs) was investigated experimentally with three pairs of specially designed testing electrodes generating a conventional bell-shaped AMF profile and different saddle-shaped AMF distributions. The characteristics of high-current vacuum arcs (HCVAs) up to 20 kA (rms), e.g., the shape of the arc column and the distribution of cathode spots, were investigated with the aid of a high-speed digital camera with an exposure time of 2 mus. Experimental results further manifested the distinct influence of AMF distribution on the characteristics of HCVAs. Saddle-shaped AMFs could resist the constriction of HCVAs more efficiently than traditional bell-shaped AMFs. Furthermore, cathode spots in saddle-shaped AMFs distributed more uniformly than that in bell-shaped AMFs. It was found that AMFs in the central region of saddle-shaped AMFs should not be too weak. Otherwise, cathode spots would be out of control in the central region as observed in experiments. The ldquooptimalrdquo minimal AMF in the central region was estimated to be about 3-4 mT/kA.

The Influence of Axial-Magnetic-Field Distribution on the Initial Expansion Process in Triggered Vacuum Arc

In this paper, the initial expansion process after trigger of vacuum arc was investigated experimentally with two typical axial-magnetic-field (AMF) distributions, i.e., bell-shaped AMF generated by traditional cup-shaped AMF electrodes and saddle-shaped AMF generated by specially designed coil-type AMF electrodes. The motion of cathode spots (CSs) in the initial expansion process was investigated by high-speed digital camera with exposure time of 2 mus. Experimental results indicated that CSs expanded faster under saddle-shaped AMF than under bell-shaped AMF at 5 (rms), 10, 15, and 20 kA. Furthermore, the motion of CSs slowed down between 15 and 20 kA in the case of bell-shaped AMF, whereas CSs tended to quicken up with the increase of arc current under saddle-shaped AMF. Based on the images of CSs, it was proposed that, besides direct influence of AMF on the retrograde motion of CSs, the concentration of CSs at relatively high current and strong central AMF could also inhibit the outward motion in the initial expansion process.

The influence of gap distance on the random walk of cathode spot in vacuum arc

Experiments were conducted in a detachable vacuum chamber for Cu vacuum arc with arc current in the range 19?24?A. Experimental results indicated that the gap distance had a distinct influence on the characteristics of the random walk of the cathode spot (CS) for the gap distance adopted, i.e. d = 4.8?mm and d = 6.8?mm. It was found that the increase in the gap distance could lead to a larger diffusion parameter. Based on the dynamics of fragments constituting the CS, it was proposed that with a longer gap distance, the magnetic interaction between fragments would be strengthened. It would result in the increase of the mean step length of the CS and the decrease of the mean step time, which would lead to a larger diffusion parameter as observed. The plasma density in the region of the CS was also found to decrease with the increase in the gap distance. It would result in the CS having a higher probability of jumping to the contaminated region but not to the vicinity of the existing crater.

Active Frequency Tuning for Magnetically Actuated and Piezoresistively Sensed MEMS Resonators

This letter, for the first time, reports an active frequency tuning for a magnetically actuated and piezoresistively sensed MicroElectroMechanical Systems resonator. Magnetic transduction is used to drive the device into the resonance. Piezoresistive transduction is used as resonance sensing technique and to tune the devices frequency by the Joule heating generated in the Wheatstone bridge without fabricating additional electrothermal heaters. The experiments are performed in air at room temperature and atmospheric pressure. The measurements shows that, by increasing the DC bias current of Wheatstone bridge from 0.5 to 5 mA, a maximum tuning range of -3403 ppm is achieved with a device resonating at 25.77 kHz.

In-Situ Measurement of Fluid Density Rapidly Using a Vibrating Piezoresistive Microcantilever Sensor Without Resonance Occurring

The Micro Electro Mechanical Systems (MEMS) density sensor is developed to achieve in-situ measurement of fluid density rapidly. The sensors sensitive chip with a rectangular microcantilever is fabricated using MEMS technology. In the sensitive chip, an Au coil and four piezoresistors are fabricated on the rectangular microcantilever. When the sensitive chip is placed in the uniform magnetic field and the Au coil is powered with alternating input-voltage, the alternating Lorentz force is generated to drive the microcantilever to vibrate. Then, the powered Wheatstone full bridge consisting of four piezoresistors generates alternating output-voltage based on the piezoresistive effect. The sensor is tested when it is immerged in the silicone oil and in air under local atmospheric pressure. According to the experiment results, it is found that the data of alternating input-voltage of Au coil and alternating output-voltage of the Wheatstone bridge have a linear relationship. The linear function can be fitted easily through least-squares fitting to obtain slope and intercept. We also discover that every slope is inversely proportional to the corresponding fluid density. Hence, the fluid density can be measured rapidly by the calculated slope. The experimental results show that the absolute deviations of the measured densities from the reference densities are .

A fluid viscosity sensor with resonant trapezoidal micro cantilever

A MEMS (Micro-electro-mechanical systems) fluid viscosity sensor with resonant trapezoidal micro cantilever is introduced in this paper. The fluid viscosity can be measured through the relationship between the fluid viscosity and the quality factor, the resonant frequency of the micro cantilever immersed in the fluid. In order to improve the sensitivity of the viscosity sensor, the geometry of the sensor chip should be carefully designed. Rectangle and trapezoidal micro cantilever chip with different lengths are proposed. The first resonant frequencies and harmonic response analyses of two sensor chips are carried out with ANSYS software. And the quality factors of the sensor chips are calculated through Matlab software. The simulation results show that the resonant frequency and quality factor of trapezoidal cantilever are higher than that of rectangular cantilever with the same length. Therefore, the viscosity sensor with trapezoidal micro cantilever has a better performance. The experimental results show that the viscosity sensor with trapezoidal micro cantilever has a fine accuracy to measure the viscosity of toluene.

DC current measurement utilizing a resonant magnetically actuated piezoresistive microcantilever

This paper presents a novel current sensor by using a resonant piezoresistive microcantilever combined with magnetic actuation for the first time. An actuation power of several microwatts was necessary for stable self-oscillation. The sensor measured the DC current by obtaining the electrothermally induced resonant frequency shift of the microcantilever as a result of the Joule heating dissipated when the DC current flowed through the Wheatstone bridge on the microcantilever. Two theoretical models were established between the microcantilevers resonant frequency and the square of the DC current. The experimental results showed that the accuracy in the range of 0.5?5?mA (apart from 0.5?mA) using the two models was 2.60% and 1.00% with correlation coefficient value R > 0.999 in both cases, respectively. The current sensitivity of the sensor was about??3.983?Hz mA?2?in the range of 0.5?5?mA. To maintain stable results, the sensor chip should be sealed in vacuum and integrated into an oven-control system.

A trapezoidal cantilever density sensor based on MEMS technology

A trapezoidal cantilever density sensor is developed based on micro-electro-mechanical systems (MEMS) technology. The sensor measures fluid density through the relationship between the density and the resonant frequency of the cantilever immersed in the fluid. To improve the sensitivity of the sensor, the modal and harmonic response analyses of trapezoidal and rectangular cantilevers are simulated by ANSYS software. The higher the resonant frequency of the cantilever immersed in the fluid, the higher the sensitivity of the sensor; the higher the resonant strain value, the easier the detection of the output signal of the sensor. Based on the results of simulation, the trapezoidal cantilever is selected to measure the densities of dimethyl silicone and toluene at the temperature ranges of 30 to 55 °C and 26 to 34 °C, respectively. Experimental results show that the trapezoidal cantilever density sensor has a good performance.

Sensitivity enhancement of a microcantilever based DC current sensor by using its torsional modes

This paper investigated the current sensitivity of a resonant sensor based on a magnetically actuated piezoresistive microcantilever in different resonant modes, which were the first flexural mode and the first and second torsional modes. The sensor was based on the idea of measuring the electrothermally induced resonance frequency shift as a result of the Joule heating dissipated when the DC current flowed through the Wheatstone bridge on the microcantilever. Two theoretical models between the microcantilever?s resonance frequency and the square of the applied DC current for the sensor operating under the flexural and torsional modes were established. From the experimental results, it can be seen that the current sensitivity of the first torsional mode is an order of magnitude greater than the first flexural mode, but less than that of the second torsional mode. In addition, the effect of the DC current?s direction on the measured results should be taken into account before detecting the DC current.