25-04-2012, 05:17 PM
Displacement Measurement
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Differential Transformer (LVDT or RVDT)
AC voltage output corresponding to position of a magnetically
permeable, movable core
❖ Primary coil is excited sinusoidally (typically 0.5V-5V
amplitude, 1kHz-30kHz frequency)
❖ Secondary coils are wired in opposition, so induced signal has
zero amplitude when core is centered
❖ As core moves, magnetic field induces a larger voltage in one
of the coils resulting in an ac voltage signal.
❖ The phase of the output voltage can tell the direction of the
core movement
Features of LVDT’s
Non-contact
❖ No friction, no wear
“Infinite resolution” (limited by…?)
Overtravel damage resistant design
Single-axis sensitivty
Coils and cores are physically separated
❖ Good if measurements need to be made in a pressurized fluid
for example
Environmentally robust
❖ Coils are potted in epoxy
❖ Unit is relatively shock and vibration resistant
Excellent null-point repeatability
Absolute output
Encoder Performance (off-the-shelf)
Resolution
❖ Linear: 10 micron – 1 nm
Repeatability
❖ Approximately ½ the resolution
Accuracy
❖ Linear: ±1 micron over 1 m - ±0.1 micron over 70 mm
Speed
❖ Linear: up to 6 m/s
❖ Rotary: up to 12 kRPM (incremental, less for absolute)
Travel
❖ 30 mm - >1 km (magnetic)
Limited to about 100 deg C
Non-contact
Cleanliness is important for optical encoders