Optical contour detector and methods for making and using

An optical contour detector (11) having a source of electromagnetic radiation (12) aimed at a contoured surface is provided. A first detector (13) and a second detector (14) are positioned near the contoured surface for detecting electromagnetic radiation reflected from a first portion of the surface. Outputs of the first detector (13) and the second detector (14) are differentially amplified to produce an output signal indicting when a change in contour or discontinuity in the surface is illuminated by the source of electromagnetic radiation (12).

BACKGROUND OF THE INVENTION 
The present invention relates, in general, to solid state sensors, and more 
particularly, to solid state sensors for detecting contours in a traveling 
surface using reflected electromagnetic radiation. 
Optical sensors, including combinations of light emitting diodes (LEDs) and 
photodiode detectors are used to monitor motion of moving objects. 
Currently available systems use a single LED aimed at a portion of a 
surface of an object being monitored, and usually use a single detector to 
detect a change in reflected light from the surface. The detector diode is 
positioned in the reflection path of the light reflected from the surface 
of the object being monitored. An example of such a system is shown in 
U.S. Pat. No. 4,850,712 issued to James B. Abshire on Jul. 25, 1989. 
Since the single detector only responds to a change in reflectivity of the 
surface, the surface must be treated or marked to provide a clear change 
in reflectivity. For example, rotation of a wheel can be optically 
monitored by placing one or more "dark" or low reflectivity marks on a 
normally reflective surface of the wheel. As the wheel turns, the quantity 
of light reflected from the surface varies. This variation is then 
detected by the single photodiode in the reflection path. 
One problem with current surface monitors is that it is that the 
reflectivity of common machine parts varies significantly as a result of 
surface roughness, cleanliness, and wear over time. Moreover, surface 
texture and finish must be carefully controlled for the surface monitor to 
work. This natural variation in reflectivity decreases the signal to noise 
ratio of the surface monitor, making it difficult to reliably detect the 
low reflectivity marks placed on the surface to be monitored. 
What is needed is a surface monitor that is less sensitive to surface 
finish and texture, and can detect small changes in surface contour. 
SUMMARY OF THE INVENTION 
Briefly stated, the present invention is achieved by an optical contour 
detector including a source of electromagnetic radiation aimed at a 
contoured surface and first and second detectors positioned near the 
contoured surface for detecting electromagnetic radiation reflected from a 
first portion of the surface. 
In a method for making an optical contour detector in accordance with the 
present invention, a source of electromagnetic radiation is positioned at 
a first predetermined angle sufficient to allow reflection of some of the 
electromagnetic radiation from a portion of the surface being monitored. 
First and second detectors for detecting the reflected electromagnetic 
radiation are positioned adjacent to each other and at a second 
predetermined angle sufficient to allow substantially equal portions of 
the reflected light to fall on the first and second detectors. 
In a method for using an optical contour detector in accordance with the 
present invention a moving surface having contours at predetermined 
locations is monitored. A portion of the surface is illuminated with a 
source of electromagnetic radiation having a wavelength and angle of the 
radiation chosen to at least partially reflect from the moving surface. 
First and second signals are generated that are a function of a quantity 
of radiation reflected from a first portion of the surface. The first and 
second signals are differentially amplified to generate a difference 
signal when a surface contour is illuminated by the source of 
electromagnetic radiation.

DETAILED DESCRIPTION OF THE DRAWINGS 
FIG. 1 illustrates a highly simplified cross-section view of a portion of a 
system using an optical contour detector in accordance with the present 
invention. The system shown in FIG. 1 is useful in particular for 
monitoring the speed of wheel 16, but it will be apparent that any number 
of surfaces having a variety of shapes can be monitored using the optical 
contour detector in accordance with the present invention. In the 
embodiment shown in FIG. 1, wheel 16 includes a number of low regions 17 
and high regions 18 positioned at predetermined locations. Thus, the 
circumference of wheel 16 provides a contoured surface having a number of 
edges where low regions 17 meet high regions 18. 
An optical contour detector is shown housed in a single housing 11, 
although discrete components may be used. A source of electromagnetic 
radiation 12 is positioned at an angle to emit energy in the direction of 
the surface of wheel 16. The wavelength of radiation produced by source 12 
is chosen to at least partially reflect from the surface of wheel 16. In a 
preferred embodiment source 12 comprises a light emitting diode (LED) 
emitting energy in the infrared portion of the electromagnetic spectrum. 
As shown by the dashed lines in FIG. 1, a transmission path exists between 
source 12 and the surface being monitored of wheel 16. 
The emitted energy can pass through optional lens 19 which is used to aim, 
focus, or collimate the electromagnetic energy. When used, lens 19 is 
located in the transmission path between source 12 and wheel 16. In 
applications where source 12 can be located close enough to the surface to 
be monitored, lens 19 is usually not required. 
The emitted energy illuminates a portion of the surface of wheel 16, and 
reflects towards detectors 13 and 14. As shown by the dashed lines in FIG. 
1, a reflection path exists between the surface being monitored of wheel 
16 and detectors 13 and 14. Detectors 13 and 14 are chosen to be sensitive 
to the wavelength of radiation emitted from source 12, and are positioned 
adjacent to each other. In the preferred embodiment, detectors 13 and 14 
are positioned at an angle to maximize the quantity of reflected light 
that reaches them, and are located so that substantially the same quantity 
of light falls on each detector except when a contour discontinuity is 
illuminated by source 12. While detectors 13 and 14 can be provided by 
discrete components such as photodiodes, in the preferred embodiment they 
comprise two photodiodes formed on a single monolithic semiconductor 
substrate to ensure matched performance. 
The reflected energy can pass through optional lens 21 which is used to 
aim, focus, or collimate the reflected electromagnetic energy. Lens 21 is 
positioned in the reflection path between wheel 16 and detectors 13 and 
14. In applications where detectors 13 and 14 can be located close enough 
to the surface to be monitored, lens 21 is usually not required. 
In operation, detectors 13 and 14 are constantly bathed in electromagnetic 
energy reflected from the surface of wheel 16. As wheel 16 turns, high 
regions 18 and low regions 17 are alternately illuminated by source 12. 
While only high regions 18 are illuminated, both detector 13 and detector 
14 receive approximately equal quantities of energy, and produce a closely 
matched output. Similarly, when only low regions 17 are illuminated, both 
detector 13 and detector 14 receive approximately equal quantities of 
energy, and produce a closely matched output. 
However, whenever a contour, such as the step between low regions 17 and 
high regions 18 is illuminated, one of detectors 13 or 14 will momentarily 
receive a greater quantity of light than the other. As wheel 16 continues 
to rotate, this imbalance in reflected energy travels across the surface 
of detectors 13 and 14 creating an equal and opposite response at a moment 
later in time. 
As shown in FIG. 2, each of detectors 13 and 14 create an output. In a 
preferred embodiment, detector 13 is coupled to one input 24 of a 
differential amplifier 22, while detector 14 is coupled to another input 
24 of differential amplifier 22. Differential amplifier 22 may comprise 
any of a number of well known circuits that differentially amplify or 
compare two input signals and produce an output 23 that is a function of 
the difference between the two inputs. Differential amplifier 22 is 
optionally formed on the same monolithic semiconductor substrate as 
detectors 13 and 14. 
As long as detectors 13 and 14 receive substantially the same quantity of 
reflected energy, their outputs will be substantially equal and output 23 
produces a constant signal. As described above, when a contour 
discontinuity in the surface of wheel 16 (shown in FIG. 1) is illuminated 
by source 12 (shown in FIG. 1) a brief pulse is produced at output 23 
because of the imbalance in light received by detectors 13 and 14. A 
moment later, as wheel 16 rotates, a second pulse of opposite polarity 
from the first pulse is generated at output 23. 
Although a number of useful functions are possible for the optical contour 
monitor in accordance with the present invention, in the preferred 
embodiment the speed of wheel 16 is monitored. Given the two pulse signal 
from output 23, speed can be calculated by counting pulses, or by 
measuring the time delay from the first pulse to the second pulse. Another 
useful function includes monitoring the amplitude of the pulses at output 
23 to determine wear on a machine part such as a gear or saw blade. 
Because two detectors are differentially amplified, the magnitude of the 
output is more reliable an indicator of the surface contour that is 
possible with a single detector. 
By now it should be appreciated that an optical contour detector and method 
for making and using it are provided. The structure in accordance with the 
present invention provides two detector outputs that are differentially 
amplified to produce an output that is sensitive to contours in the 
surface being monitored. Because the system in accordance with the present 
invention actually detects a change in contour as opposed to a change in 
reflectivity, the signal generated is much more immune to variations 
caused by surface reflectivity, texture, and finish as well as ambient 
lighting conditions. Moreover, differential amplification of two matched 
detectors produces a high signal to noise ratio output that is sensitive 
to small contour changes in a surface being monitored.