Being able to measure and monitor the viscosity of a fluid accurately and in real-time can provide insights and prevent field failures of lubricated mechanical elements. A micro electro mechanical system (MEMS) viscosity sensor that measures the properties of liquids through thermal vibrations of a silicon membrane has been previously developed. The device measures viscosity through three different characteristics: the frequency, amplitude and the quality factor of the vibrating membrane. The membrane is actuated via a short pulse of heat delivered by the heater resistor provided by an external voltage. The pulse width is controlled by a waveform generator and a power MOSFET. The movement of the membrane is measured with an in-situ piezoresistor Wheatstone bridge, which is powered by an external voltage source, and amplified with and instrumentational amplifier before the resulting vibrating signal is analyzed in LabView. The end goal of this work is to characterize the sensitivity and real-time response of a thermally actuated MEMS viscosity sensor. In addition, a process modification to include a deep reactive ion etch instead of a KOH etch, has been developed. As viscosity is dependent on temperature, when the membrane is actuated by heat, the effects of locally changing the fluid temperature will affect the sensitivity of the sensor. Optimized test bias condition results were, Wheatstone bridge bias voltage when increased over 7 V, the natural frequency of vibration of the sensor is modified. Pulse width and heater bias value can be adjusted for optimum sensor response. With these established bias conditions, the real-time response of the system was investigated. Epoxy was used to cover the sensor perimeter, protect the 25 μm aluminum wire bond connections to a copper PCB and to glue the sensor onto the PCB. Test result show a spike in frequency and amplitude when different oils were added. As shown with additional tests, the spike is mainly caused by slight temperature variations that are introduced with new oil and how they affect the sensor packaging. Spikes were reduced by lowering the bridge bias voltage from 7 V to 3 V, which minimized the sensor heating. Furthermore, addition of oil in very small quantities, in the ~ μL range, reduced the changes in temperature. Figure 1 shows frequency and amplitude response with varying viscosities without agitation. During testing, when oil is added, the amplitude shows an immediate overdamped response which takes about 1–2 minutes to stabilize, whereas frequency is characterized by an underdamped response with response time 5–7 minutes. Frequency response time was slower as it is very dependent on intrinsic stresses of both sensor and packaging, whereas amplitude of oscillations seems to be more independent to these properties changing and shows faster response.
|Commitee:||Pearson, Robert E., Hirschman, Karl D., Iglesias Victoria, Patricia, Rommel, Sean L.|
|School:||Rochester Institute of Technology|
|School Location:||United States -- New York|
|Source:||MAI 81/6(E), Masters Abstracts International|
|Subjects:||Nanotechnology, Electrical engineering, Fluid mechanics|
|Keywords:||Fluid viscosity, Measures viscosity, MEMS sensor, MEMS viscosity, Real time response, Viscosity sensor|
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