Longqi Xu

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.

Magnetically actuated resonant piezoresistive microcantilever operating in fluid for dc current measurement

A novel dc current sensor based on magnetically actuated resonant piezoresistive microcantilever is proposed. The sensor measures the dc current by detecting the shift of resonant frequency of the microcantilever. A detailed theoretical study on the relationship between the current and the cantilevers resonant frequency is also given. To reduce force and displacement noise, the microcantilever works in the fluid with good thermal conductivity. The experimental results show that the sensor can achieve a sub-milliampere resolution and the accuracy is found to be 3.93% for the current range of 0.5 to 5 mA.