Minimum Detectable Acceleration Research Articles

Lightweight fiber optic microphones and accelerometers

We have designed, fabricated, and tested two lightweight fiber optic sensors for the dynamic measurement of acoustic pressure and acceleration. These sensors, one a microphone and the other an accelerometer, are required for active blanket sound control technology under development in our laboratory. The sensors were designed to perform to certain specifications dictated by our active sound control application and to do so without exhibiting sensitivity to the high electrical voltages expected to be present. Furthermore, the devices had to be small (volumes less than 1.5 cm3) and light (less than 2 g). To achieve these design criteria, we modified and extended fiber optic reflection microphone and fiber microbend displacement device designs reported in the literature. After fabrication, the performances of each sensor type were determined from measurements made in a dynamic pressure calibrator and on a shaker table. The fiber optic microbend accelerometer, which weighs less than 1.8 g, was found to meet all performance goals including 1% linearity, 90 dB dynamic range, and a minimum detectable acceleration of 0.2 mg/Hz. The fiber optic microphone, which weighs less than 1.3 g, also met all goals including 1% linearity, 85 dB dynamic range, and a minimum detectable acoustic pressure level of 0.016 Pa/Hz. In addition to our specific use in active sound control, these sensors appear to have application in a variety of other areas.

Design and fabrication of submicrometer, single crystal Si accelerometer

A lateral accelerometer has been designed, simulated, and fabricated using a 3-mask high-aspect ratio technology. Electron beam lithography and high-density plasma etching in an inductively coupled plasma source enabled aspect ratios >30 to be achieved. This makes possible beams with very small spring constants. Combining the ability to measure very small displacement of a proof mass due to narrow capacitive gaps between comb fingers, a highly sensitive accelerometer can be obtained. The fabricated accelerometer with 1 /spl mu/m beams and 0.2 /spl mu/m comb gaps had a spring constant of 0.127 N/m, which is close to the calculated values of 0.146 N/m. Based on the capacitance measurements, the accelerometer sensitivity is calculated to be 6.3 fF/g. Reducing the beam width to 0.4 /spl mu/m lowered the spring constant to 0.03 N/m, and an improved equivalent sensitivity of 79.2 fF/g is calculated. The minimum detectable acceleration is on the order of a few microgravity over a range of hundreds of gravities.

High-responsivity fiber-optic flexural disk accelerometers

This paper presents performance measurements of fiber-optic flexural disk accelerometers. The flexural disk acts as a mass-spring element to which the fiber is bonded, such that an acceleration causes a strain to be imposed on the fiber which is measured interferometrically. Simple analytical models have been written to calculate the responsivity and resonant frequency of disks under various boundary conditions and the results of the models have been shown to be in good agreement with the measured responsivity for the case of moderately thick disks. Six optical fiber accelerometers based on flexural disks of different thickness and supports have been demonstrated to exhibit a responsivity in the range from 28 to 39 dB re 1 rad/g with a resonant frequency between 2.4 kHz to greater than 5 kHz, respectively. Of the designs considered, the centrally supported disk is shown to give the highest combination of responsivity and bandwidth. A centrally supported disk has been demonstrated to exhibit a flat response up to 2 kHz and a responsivity of 37 dB re 1 rad/g which when combined with an interferometric phase resolution of 6 /spl mu/rad//spl radic/Hz, would give a minimum detectable acceleration of 84 ng//spl radic/Hz. We have attempted to cover all aspects of the sensor design including responsivity, bandwidth, cross-responsivity, phase response and size and find that a complicated compromise between all of these design parameters is required to achieve the optimum performance.

High-resolution micromachined interferometric accelerometer

We demonstrate a promising type of microfabricated accelerometer that is based on the optical interferometer. The interferometer consists of surface-micromachined interdigital fingers that are alternately attached to a proof mass and support substrate. Illuminating the fingers with coherent light generates a series of diffracted optical beams. Subangstrom displacements between the proof mass and frame are detected by measuring the intensity of a diffracted beam. The structure is fabricated with a two-mask silicon process and detected with a standard laser diode and photodetector. We estimate that the minimum detectable acceleration is six orders of magnitude below the acceleration of gravity, i.e., 2 μg/Hz in a 1 Hz bandwidth centered at 650 Hz.

CMOS interface circuitry for a low-voltage micromachined tunneling accelerometer

This paper describes the design and development of three types of complementary metal-oxide-semiconductor (CMOS) interface circuitry for low-voltage micromachined tunneling accelerometers. Using a simple inductance-capacitance-resistance (LCR) circuit as well as two nonlinear current/voltage-controlled current sources, an electromechanical model enables simulating and predicting the performance of the accelerometer with CMOS interface circuitry. Of the three types of interface circuitry, the one utilizing a pn-junction diode as a logarithmic current-to-voltage converter has the best performance. The hybrid sensor-circuit module can be incorporated into a portable battery-operated multisensor instrumentation microsystem. Only one 12-V power supply is required for device operation with a power dissipation of 2.5 mW. The accelerometer has a sensitivity of 125 mV/g and bandwidth of 2.5 kHz with a measurement range of 30 g. The noise spectral density with a 1/f behavior drops from 4 mg//spl radic/(Hz) (at 0.5 Hz) to 0.1 mg//spl radic/(Hz) (at 2.5 kHz). The accelerometer in turn provides a dynamic range over 71 dB with a minimum detectable acceleration of 8 mg in a bandwidth of 2.5 kHz. In continuous operation over 720 h, the long-term variations of the output offset voltage and device sensitivity are /spl plusmn/10 mV (0.12%) and /spl plusmn/0.2 mV/g (0.15%), respectively.

Simple low-frequency optical fiber accelerometer with large rotating machine monitoring applications

A simple fiber optic accelerometer with large rotating equipment monitoring applications is presented. The sensor is optimized for detection of mechanical vibrations in the frequency range 0.2-140 Hz. The minimum detectable acceleration is 0.025 m/s/sup 2/ root mean square (rms), and it has low transverse sensitivity. The transducer operating principle, based on a fiber optic cantilever beam without any selamic mass on it, is simple and easy to construct, and the intensity-modulation and compensation techniques developed require simple electronic processing and provide very good stability and accuracy. Other advantages of the system are its robustness, compact size, and cost-effectiveness, which make it appropriate for industrial applications, specially for those where harsh environments are involved. The complete process of research and development of the sensor has been carried out, from the theoretical study of its operation principles, through the design, fabrication, experimental characterization and enhancement, to its starting up in a high-power electric station for hydrogenerator monitoring.

A low-voltage tunneling-based silicon microaccelerometer

This paper describes the design, fabrication, and testing of a low-voltage tunneling-based silicon microaccelerometer. The device has been successfully batch-fabricated by the boron etch-stop dissolved wafer process. A 4 h, 1100/spl deg/C oxygen, post-diffusion annealing process has been developed to eliminate the stress gradient in and warpage of thin (/spl ap/3 /spl mu/m) heavily-boron-doped silicon microstructures. Using a simple discrete readout circuit, the device with an active area of 400/spl times/400 /spl mu/m/sup 2/ provides a measured sensitivity of 1.66/spl times/10/sup 4/ ppm/g (133 mV/g), bandwidth of 2 kHz in air, and a full scale range of 30 g with a nonlinearity of 0.6%. The measured noise spectrum exhibits a typical 1/f behavior and drops from 1.75 mg//spl radic/Hz (at 50 Hz) to 0.25 mg//spl radic/Hz (at 2 kHz), corresponding to a minimum detectable acceleration of 22.8 mg. The variations of the offset output voltage and device sensitivity are /spl plusmn/40 mV (/spl ap/0.5%) and /spl plusmn/0.65 mV/g (/spl ap/0.49%) in continuous operation over thirty days. The temperature coefficient of offset (TCO) and temperature coefficient of sensitivity (TCS) are -600 ppm//spl deg/C and 1200 ppm//spl deg/C, respectively.

DC fibre-optic accelerometer with sub-μg sensitivity

We describe a fibre-optic accelerometer, based on the technique of nonlinear displacement-to-strain conversion (NDSC), suitable for use at DC and low frequencies. The sensor exhibits a minimum detectable acceleration of less than 1 μg RMS (g = 9.8 m/s 2 ) at 0.5 Hz and a linear dynamic range greater than 10 6 .

Tilting-mirror fiber-optic accelerometer

Experimental and theoretical results are presented on a simple compact fiber-optic sensor that uses a mirror placed on the end of a mass-loaded cantilevered elastic beam. Deflections of the beam due to acceleration are read out with multimode optical fibers and with a quarter-pitch GRIN rod lens. Two output fibers are used in a balanced-detection scheme. In the audio-frequency range, the minimum detectable acceleration is 2.4 × 10−6 Grms, and the maximum measurable acceleration is 39 Grms. The device is also sensitive to dc accelerations. However, during dc sensing, a fiber bending noise is found. This noise decreases the dynamic range of the accelerometer.