Homework 3 solutions

Homework 3 solutions - Homework 3 12 1. How might a...

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Homework 3 1. How might a transducer convert the following changes in a sensor to an electrical signal? a) Humidity sensor – absorption of water. b) Mass sensor – change in frequency of oscillation. c) Chemical sensor – change in color. a) If the humidity sensor is a polymer that can absorb water, the polymer can be loaded with conductive particles. As the polymer swells due to water absorption, the particles will increase in distance from each other, increasing the resistance of the material as percolation pathways through the material are lost. b) A piezoelectric crystal can be used. The charge on the surface of a piezoelectric changes with stress, and this can be measured as a current as the piezoelectric material oscillates mechanically. As the mass on its surface changes, and its resonant frequency shifts, the frequency of the current signal changes. c) One can shine light through the material onto a photodetector. As the color changes, the intensity of the light falling on the photodetector changes, altering its conductivity. This can be detected by applying a constant voltage (current) and recording the resulting current (voltage). 2. a) What physical effects might cause a sensor system to display memory effects or hysteresis? b) Give an example. In general terms, systems that change physically/chemically/electronically in a quasi-reversible (slow) or irreversible manner. 1. Plastic deformation (e.g. due to uncoiling of polymer chains, breaking of physical crosslinks). 2. A material with 2 stable states under the same conditions, such as a shape memory alloy, which transitions from one to another under stress or temperature, but will then remain in that new state until the pressure or temperature is significantly reduced from that which induced the change. 3. A system that changes chemically in response to the input, and the change is in some way irreversible.
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Homework 3 solutions - Homework 3 12 1. How might a...

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