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Sensors. Read Chapter 2 of Textbook. 1. Displacement Sensors. Potentiometer (Already discussed) Strain Gages Inductive Sensors (LVDT) Capacitive Sensors Piezoelectric Sensors. =(A/V)/m = S/m. P and N doped Silicon Strain Gages Gage Factor = G = ( Δ R/R)/( Δ L/L)

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## Sensors

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**Sensors**Read Chapter 2 of Textbook**1. Displacement Sensors**• Potentiometer (Already discussed) • Strain Gages • Inductive Sensors (LVDT) • Capacitive Sensors • Piezoelectric Sensors**P and N doped Silicon Strain GagesGage Factor = G =**(ΔR/R)/(ΔL/L) ---Stretching bar of N-type silicon crystal breaks electrons loose from impurity sites, making resistance decrease, producing large negative G.---Stretching bar of P-type silicon crystal inhibits holes from moving away from their impurity sites, producing large positive G**Unbonded Strain Gage Pressure Sensor**This is a deflection mode instrument, so it is important to choose Ri >> R1=R2=R3=R4 To avoid bridge loading**Inductive Transducers**Coils must be wound in opposite directions so magnetic fluxes oppose <- L <- L + Vo(t) -**µ = magnetic permeability of core material**= µ R4π*10-7F/m**Linear Variable Differential Transformer Displacement**Transducer LVDT used to measure very small displacements in a seismometer that measures movements in the earth’s crust due to earthquakes. It consists of a middle primary coil and two outer secondary coil. The magnetic core moves freely without touching bobbins, and at the null (zero) position, it extends halfway into each secondary coil.**Example Medical Applications of Linear Variable**Displacement Transformer (LVDT)**NOTE: The output voltage is actually the PEAK voltage of an**ac sine wave whose frequency is that of the primary winding excitation sine wave.**Disadvantages of LVDTs**• All these advantages, in addition to their reasonable cost, have made the LVDT an attractive displacement measurement sensor. However, LVDTs for use in medical applications have the following disadvantages: • They require a high frequency, constant-amplitude ac sinusoidal excitation. • They cannot be used in the vicinity of equipment that creates strong magnetic fields. • A somewhat complicated “phase sensitive” ac-to-dc converter (detector) must be used if both positive and negative displacements from the middle (null) position needs to be measured.**Phase INSENSITIVE Detector (Diode Half-wave rectifier with**capacitive filter.)**Commercial Diode Ring Modulator (also called “Double**Balanced Mixer”**LVDT Detector Circuit without need for a Phase-Sensitive**Demodulator

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