Measurement Principle
The NanoGage is a folded optical triangulation sensor based on a
bi-cell detector and precisely controlled focal spot size. The optical
layout is illustrated in the figure below. High resolution results from
the use of a small focal spot size in conjunction with an
AGC-controlled laser diode source and a bi-cell detector with a 10
micron gap. Individual sensors can be corrected to achieve 0.1% or
better linearity over their specified measurement range. Automatic Gain
Control (AGC) provides uniform response for a wide range of surface
reflectivities. Unlike triangulation sensors based on position-sensing
detectors, in which the linear range is set by the detector size, the
NanoGage sensor has a linear range that is defined by its focal spot
size. This allows for high resolution in a compact and robust package.
Good linearity is achieved by tailoring of the
lens aberrations to give a uniform circular focal spot; final linearity
is accomplished based on individual sensor calibration and correction.
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NanoGage optical layout
Because of the oblique illuminating and viewing angles, a vertical
displacement of the target surface causes a lateral displacement of the
laser image in the plane of the bi-cell detector. The signals from the
bi-cell are processed to generate a voltage that is proportional to the
vertical displacement. The fold mirrors allow the sensor to be
configured as a compact cylinder.
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| Technical Background - Proximity Technology |
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| Technical Background - Proximity Technology |
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| Signal Detection
The figure at left shows how the
(A–B) signal from the bi-cell detector is generated from the lateral
displacement of the laser image on the detector. Each of the
bi-cell elements generates a separate signal due to light from the
laser spot image. As shown, the (A-B) plot is approximately
linear with the spot location of the detector; the exact shape of the
plot depends on the distribution of light within the laser spot.
When the spot is centered on the dividing line between the two detector
elements, the signals are balanced and (A-B) is equal to zero.
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The NanoGage’s high resolution is
due to the large signals made possible by the use of a laser diode
light source. The target surface may be a mirrored surface, an
un-coated glass surface, or a polished or finely machined metal
surface. Unlike a capacitance gage, this sensor does not require a
conductive surface, and is largely immune to electrical or magnetic
interference. The structure of the sensor head is made of Invar to
minimize the effects of temperature on the sensor performance.
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