Adam Hurst

An Experimental Frequency Response Characterization of MEMS Piezoresistive Pressure Transducers

Silicon micro-machined piezoresistive based pressure transducers are often used to make high frequency dynamic pressure measurements. The spectral or frequency response of these microelectromechanical systems (MEMS) is a function of the natural resonance of the sensor structure, sensor size, sensor packaging, signal conditioning and transducer mounting in the desired measurement location. The advancement of MEMS micro-fabrication, which has reduced sensor size dramatically, and the high elastic modulus of silicon have allowed the natural resonance of these devices to range from 100kHz to several MHz [1]. As a result, packaging and mounting at the point of measurement are the major factors that determine the flat (0dB) frequency response envelope of the transducer, which is typically quantified by a transfer function. The transfer function quantifies the difference both in magnitude and phase between an input signal and a measured signal in the frequency domain. The dynamic response of pressure transducers has historically been estimated via a unit step input in pressure created through a shock tube test that excites the high natural resonance of the chip. Unfortunately, these tests are less effective at accurately quantifying the frequency response of the transducer in the domain of greatest interest (DC-20kHz), specifically the bandwidth over which the response is flat (0dB). In this work, we present a test methodology using a speaker-driven dynamic pressure calibration setup for experimentally determining the transfer function of a pressure transducer from 1–50kHz. The test setup is validated using capacitive-based microphones with claimed flat spectral characteristics well beyond 50kHz. Using this test setup, we present experimental spectral response results for low-pressure miniature MEMS piezoresistive pressure transducers over the frequency range of 1–50kHz and qualitatively compare these results to traditional shock tube tests. The transducers characterized have been manufactured with several different standard sizes and front-end configurations.Copyright

Enhanced Static-Dynamic Pressure Transducer for the Detection of Acoustic Level Flow Instabilities in Gas Turbine Engines

The push to advance the performance and longevity of gas turbine engines requires better characterization of flow instabilities within the compressor and most importantly the combustor. Detecting the earliest onset of these flow instabilities can help engineers either manipulate the flow to restabilize it or make informed design changes to the engine. The pressures within gas turbine engines are typically composed of an undesired, low-level oscillatory pressure of less than 1kPa to several kPa superimposed on top of a large, relatively constant pressure of several thousand kPa [1–7]. The high-pressure transducers used to measure the pressures within these environments are often unable to resolve these low-level oscillatory pressures that characterize the flow instabilities because the signal output for such pressures is often the same level as the noise within the sensor-data acquisition system. This paper presents an engine test ready, high temperature, combined static and dynamic pressure transducer that uses static pressure compensation in order to measure these low-level dynamic pressures with an excellent signal to noise ratio and, at the same time, captures the overall static pressure within a gas turbine [8–10]. Test bench experiments demonstrate the static-dynamic transducer’s unique ability to capture both large static or quasi-static pressures of 1,380kPa or greater and simultaneously measure the acoustic-level dynamic pressures superimposed on top of these pressures. The static-dynamic transducer achieves this advanced sensitivity through the use of a low-pass acoustic filter that passes the large static pressure to the reference port of a high sensitivity dynamic pressure sensor within the transducer such that the overall static pressures cancel out and the sensor measures all acoustic-level dynamic pressures. These bench tests additionally demonstrate the transducer’s ability to operate reliably when exposed to the harsh, high temperature environment (up to 500°C) within a gas turbine [8–10].Copyright

A molybdenum disulfide piezoelectric strain gauge

This work experimentally probes and demonstrates the piezoelectric properties of single-atomic-layer MoS2. MoS2 devices were fabricated on a thin amorphous fused silica substrate, which was bonded to the high-stress location of a tuning fork to maximize the strain and resulting piezoelectric output. The dynamic strain was simultaneously measured in-situ with a commercial semiconductor strain gauge. This strain characterization technique allows us to dynamically strain MoS2 at a set frequency with maximum peak-to-peak strain levels up to 500 με (0.05%). We thus correlate piezoelectric output from the MoS2 sensor with the applied dynamic strain.

A graphene accelerometer

This work presents an SU-8 clamped graphene nano-electro-mechanical-system (GNEMS) accelerometer. A suspended graphene membrane is circularly clamped by SU-8, with an additional proof mass made of either SU-8 or gold, located at the center of the membrane. This GNEMS accelerometer is approximately three orders of magnitude smaller than state of the art micro-electromechanical (MEMS) accelerometers with the diameter of the suspended graphene membrane being 3-10 μm and the proof mass diameter being 1-5 μm. Here, we present the fabrication, simulation, and experimental aperiodic calibration results of the GNEMS accelerometer, demonstrating a repeatable response to an input acceleration levels of ~1000-3000 g.

Real-Time, Advanced Electrical Filtering for Pressure Transducer Frequency Response Correction

The frequency response of a pressure transducer is influenced by the natural resonance of the sensor structure, the spatial resolution of the sensor due to its diaphragm size, the sensor packaging, signal conditioning and mounting at the measurement location. The resonance of the sensor and aerodynamically-driven resonances related to the sensor packaging and/or mounting, specifically, can distort dynamic pressure measurements within the range of greatest interest (10Hz–20kHz), typically resulting in erroneous amplification. Historically, correcting for such errors within the frequency response of a pressure transducer or measurement system has been challenging, because such errors are hard to quantify with unknown resonant frequencies and damping factors (quality factors). However, with the ability to fully characterize resonant frequencies that lie within 10Hz–50kHz using a previously demonstrated dynamic pressure characterization methodology, it is possible to apply electrical filtering to substantially extend the flat (0±2dB) frequency response of a transducer before any digital signal conversion. In this work, we present a real-time frequency response compensation scheme that uses electrical filtering to correct for aerodynamically driven packaging or mounting related resonances while at the same time preventing signal distortion caused by the sensor resonances. The compensation extends the useable, flat amplitude bandwidth of the transducer while also correcting the phase response to maintain constant time delay over the extended bandwidth. This real-time frequency response correction scheme can be similarly used to compensate for chip resonances, which can limit the frequency response in applications such as shock and blast testing. A theoretical model of the frequency response correction methodology is presented. We additionally present temperature dependent experimental results that compare the frequency response with and without the correction scheme. These results demonstrate that the usable bandwidth of pressure transducers can be increased when real time, analog frequency response correction is applied. This work shows that if the frequency response of a transducer is well characterized, advanced signal conditioning can be implemented to substantially extend the flat bandwidth of the transducer without changes to the sensor, packaging or mounting.Copyright

An Experimental and Theoretical Investigation of Wave Propagation in Teflon and Nylon Tubing With Methods to Prevent Aliasing in Pressure Scanners

Accurate static and dynamic pressure measurements provide the feedback needed to advance gas turbine efficiency and reliability as well as improve aircraft design and flight control. During turbine testing and aircraft flight testing, flush mounting pressure transducers at the desired pressure measurement location is not always feasible and recess mounting with connective tubing is often used as an alternative. Resonances in the connective tubing can result in aliasing within pressure scanners even within a narrow bandwidth and especially when higher frequency content DC to ∼125Hz is desired. We present experimental results that investigate tube resonances and attenuation in 1.35mm inner diameter (ID) (used on 0.063in tubulations) and 2.69mm ID (used on 0.125in tubulations) Teflon and Nylon tubing at various lengths. We utilize a novel dynamic pressure generator, capable of creating large changes in air pressure (<1psi to 10psi, <6.8kPa to 68.9kPa), to determine the frequency response of such tubing from ∼1Hz to 2,800Hz. We further compare these experimental results to established analytical models for propagation of pressure disturbances in narrow tubes. While significant theoretical and experimental work relating to the frequency response of connective tubing or transmission lines has been published, there is limited literature presenting experimental frequency response data with air as the media in elastic tubing. In addition, little progress has been made in addressing the issue of tubing-related aliasing within pressure scanners, as the low sampling rate in scanners often makes post-processing antialiasing filters ineffective.The experimental results and analytical models presented herein can be used as a guideline to prevent aliasing and signal distortion by guiding the proper design of pressure transmission systems, resulting in accurate static and dynamic pressure measurements with pressure scanners. The data presented here should serve as a reference to instrumentation engineers so that they can make higher frequency measurements (up to ∼125Hz, currently) and are able to quantify the expected pressure transmission line (tube) attenuation and know if aliasing will be a concern. This information will give engineers greater measurement capability when using pressure scanners to make static and dynamic pressure measurements.Copyright

High Temperature Static and Dynamic Pressure Transducer for Combustion Instability Control Using Acoustic Low-Pass Filter Structures

There is a need to measure static and dynamic conditions in many gas turbine applications, in particular for combustion instabilities, such as those in the afterburner. The DC and low frequency components are typically used for conventional engine control, while the high frequency data is essential for acoustic screech and rumble diagnostics and control. This paper presents a static-dynamic piezoresistive pressure transducer that measures low amplitude, dynamic pressure perturbations superimposed on top of a high pressure through the implementation of low pass mechanical structures. The transducer, which is capable of operating at ultra-high temperatures and in harsh environments, consists of a static piezoresistive pressure transducer, which measures the large pressures on the order of 200psi and greater, and an ultrasensitive, dynamic piezoresistive pressure transducer which captures small, high frequency pressure oscillations on the order of a few psi. The heightened sensitivity in high pressure environments is achieved by filtering the measured pressure of high frequency content through an innovative low pass mechanical filter structure. The large static pressures passed by the low-pass mechanical filter structures are routed to the backside of the dynamic pressure sensor, which results in both the front and the back of the dynamic sensor being exposed to the large pressures within the environment. Therefore, the large static pressures cancel out, and the dynamic sensor only senses the low magnitude, high frequency pressure perturbations. This dual sensor, static-dynamic pressure transducer reproduces pressure signals with sensitivity far higher than any single high pressure transducer available today. The dual sensor, static-dynamic transducer meets the pressure sensing specification of numerous applications including, but not limited to, the following: the optimization of turbine operation, turbine design and testing, the detection of the onset of rotating stall and surge in turbine compressors, and combustion instabilities. This paper describes a six element model of the static-dynamic transducer’s low-pass mechanical filtering structures. The paradigm is derived from first-principles of fluid motion in acoustic ducts with viscous dissipation. A dynamic pressure source is used to verify the model and its operation. Finally, a transfer function characterization of a fully operational static-dynamic pressure transducer over a wide bandwidth is presented. Based upon the analytical and experimental results, the static-dynamic pressure transducer will make it possible for turbine users and manufacturers to implement ultra-sensitive pressure monitoring to reduce compressor and combustion instabilities [1] [2].Copyright