Banner MICRO-AMP Series - Manual (Page 17)

Optical design ensures an exact range limit, regardless
of the surface reflectivity of the objects to be sensed
Powerful infrared light source and modulated amp-
lification provide reliable sensing of objects with low
surface reflectivity
Sensor response to background objects is completely
suppressed
Miniature size fits easily into tight areas of machines
Works with special versions of Banner's MICRO-AMP ®
series of modulated amplifiers
Model SP100FF is a fixed-field convergent mode sensor that detects objects
directly by reflection of light from the object's surface. The light is sensed at two
receive points and compared to define a precise limit to the maximum sensing
range. Objects in the background are ignored, regardless of their surface reflectiv-
ity.
This sensing response feature makes the SP100FF an ideal choice for detecting a
part or a surface that is only a small fraction of an inch in front of another surface.
The SP100FF is highly reliable for semiconductor wafer sensing. Wafers of all
reflectivities are sensed without mechanical or system sensitivity adjustment.
Other applications include cut-to-length control, double-thickness detection, and
precision edgeguiding. The SP100FF is an excellent choice for precise position
control (e.g. as a robotic end effector).
The SP100FF works in conjunction with Banner MICRO-AMP modulated ampli-
fier model MA3A or MPC3A. Model MA3A is powered by +10 to 30V dc and uses
a model RS8 socket. Model MPC3A is powered by +5V dc and is designed for
mounting directly onto PC boards.
With a typical excess gain of 100X at the peak signal point (referenced to a 90%
reflectance white test card), the SP100FF has enough optical energy to reliably
sense material of very low reflectivity such as nitride-coated semiconductor
wafers. The typical peak signal point is 0.12 inch from the sensor face. Excess gain
falls off sharply beyond the peak signal point (see excess gain curve, below).
Conceptual Drawing
The SP100FF uses two photoelements that operate with the modulated amplifier
in a differential mode. The photoelements are mechanically convergent with the
LED light source at two different distances from the sensor face. The inner
photoelement (R1) produces a positive-going pulse which turns the output of the
amplifier "on". The outer photoelement (R2) produces a negative-going pulse that
works to turn the amplifier "off".
A target is sensed whenever the amount of light reaching receiver R1 is equal to or
greater than the amount of light "seen" by R2. The output of the amplifier is cut
off as soon as the amount of light at R2 becomes greater than at R1. The location
of this "crossover point" is dictated by the geometry of the optoelement configu-
ration, and remains the same regardless of the target's reflectivity.
Reflections even from highly-polished mirror-like surfaces are ignored if the
reflections originate from beyond the crossover point. Modulated LED design
offers very high excess gain at the convergent point of R1 (see excess gain curve).
As a result, even objects of very low reflectivity may be sensed.
The rapid fall-off in signal strength over a very short distance (as seen in the excess
gain curve) accounts for the highly repeatable distance-sensing accuracy of the
SP100FF, independent of target surface reflectivity.
Theory of Operation
Excess Gain Curve
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17
MICRO-AMP ® System
SP100FF Fixed-field Sensor