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”