Radiant energy – Photocells; circuits and apparatus – Optical or pre-photocell system
Reexamination Certificate
2001-01-30
2002-08-13
Le, Que T. (Department: 2878)
Radiant energy
Photocells; circuits and apparatus
Optical or pre-photocell system
C250S559100
Reexamination Certificate
active
06433329
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to methods and apparatus for sensing the position of a moving part. In particular, the invention relates to a dynamically tracking threshold for maintaining the reliability of an optical sensor.
2. Description of the Related Art
Position sensors are typically used in machines to monitor the physical state of a moving mechanical component of an automated system. For example, the exact position of a moving part may need to be determined to establish an “on” or “off” control signal for mechanical applications, such as an end stop or travel limit for an X- or Y-motor to move a cartridge gripper of a robotic system loading and unloading magnetic tapes into and from a tape drive or cartridge cell. In such applications, the position sensor determines when the tape cartridge has reached the desired physical location within the tape drive (or the cartridge cell) and the sensor's output is used both to stop the gripper motor and to trigger the next operation step within the functional logic of the system. For instance, the tape cartridge's advance is stopped and data stored in the tape are accessed by a computer.
Motion detectors used in the prior art to monitor mechanical motion typically consist of mechanical switches and/or optical sensors. Both types of devices require periodic maintenance or replacement to preserve the desired level of reliability. As one skilled in the art would readily appreciate, mechanical switches include moving parts and are prone to contact bounce and malfunction due to early life failure of any moving part in the switch.
Optical sensors, which consist of light sources and detectors, are often utilized to overcome these problems, but they are also susceptible to failures caused by problems inherent in the nature of their components. For example, optical sensors tend to become unreliable as a result of large changes in ambient light, misalignments between the light source and the detector, reduced light levels caused by dirt or debris accumulation, reduced light levels caused by the aging of the internal light sources, and manufacturing differences in sensitivity between devices. Thus, even though optical sensors are more immune than mechanical switches to mechanical noise and failure, their reliability remains uncertain under normal operating conditions.
The operation of an optical sensor is based on detecting the intensity of a light beam emitted by a light source (such as a light emitting diode, “LED”) with a light detector (such as a phototransistor, “PTR”) aligned with the optical path of the beam. One detection approach, often referred to in the art as “through-beam,” involves a first normal state wherein the light is received by the detector at a relatively high intensity level directly from the source. A change of state is established when the light beam is blocked in its optical path toward the detector by a moving part, thereby causing the intensity measured at the detector to vary to a relatively lower value. Another approach, often referred to as “reflective,” involves a first normal state wherein the light is directed away from the detector, which correspondingly measures a relatively low intensity level. A change of state is established when the light beam is reflected toward the detector by the moving part, thereby causing the intensity measured at the detector to vary to a relatively higher value. In either system, the accuracy of the operation of the detector is predicated upon its ability to correctly determine when a change of state has occurred as a result of the present location of the moving part.
A typical through-beam embodiment
10
of optical-sensor apparatus is illustrated in
FIGS. 1A and 1B
(prior art). An LED
12
, appropriately grounded through a system ground G, is energized by a source voltage V to produce a light beam B. The beam is aimed, either directly or by reflection, at a PTR
14
that produces an analog output
16
which is a function of the intensity of the light beam B, as illustrated in FIG.
2
. When a moving part crosses the path of the light beam B, it interrupts its normal path toward the detector
14
and correspondingly causes a significant drop in its output. Thus, the peak
18
of the analog amplitude curve
16
illustrated in
FIG. 2
(prior art) corresponds to a minimum amount of light being blocked by the moving part and a maximum amount of light being received by the detector
14
. Conversely, the low value of the amplitude curve corresponds to a maximum amount of light being blocked by the moving part and a minimum amount of light being received by the detector
14
.
The output
16
of the detector
14
is typically used as the input to a comparator
20
(
FIG. 1A
) or a logic gate, such as a Schmitt trigger
20
′ (FIG.
1
B). As illustrated in
FIG. 3A
(prior art), an arbitrarily fixed detection threshold
22
is used to create a digital logic signal
24
that corresponds to the analog output
16
of the optical sensor. The resulting digital logic signal
24
changes state when the sensor's analog output
16
crosses the threshold level
22
, as shown in
FIGS. 3A and 3B
(prior art).
When a sufficiently large decrease occurs in the ability of the detector
14
to sense the light emitted by the LED, a total loss of detection may result if the peak
18
of the output
16
remains below the detection threshold
22
, as illustrated in
FIG. 4A
(prior art). Correspondingly, the digital logic signal
24
becomes inoperably fixed at a single “low” or “0” logic state, as shown in FIG.
4
B. This condition can result, for example, from partial blockage of the light source
12
or the detector
14
caused by debris accumulation, from a decrease in the output characteristics of the light source, or from a change in the alignment of the detector
14
with respect to the light source
12
. In a reflective embodiment of optical-sensor apparatus (not illustrated in the figures), this problem can similarly result from a decrease in the reflectivity of the moving target.
Similar problems can arise when an increase in the light sensed by the detector
14
occurs to the point where the minimum amplitude
26
of the detector output
16
is always higher than the threshold
22
, as illustrated in
FIG. 5A
, This can happen, for example, when the ambient or background light is too high, or when the light source
12
is supplied with too much current that yields a greater than rated light beam B. In a reflective embodiment, this problem can result from an increase in ambient reflectivity. In any of these cases, the digital logic signal
24
becomes inoperably fixed at a single “high” or “1” logic state, as shown in FIG.
5
B.
In view of the foregoing, it is clear that the conventional fixed detection threshold used with prior-art optical sensors is inadequate to provide maintenance-free, reliable, long-term service under variable operating conditions. Some approaches have been disclosed in U.S. Pat. Nos. 5,898,170 and 5,739,524 to improve similar problems, but they are limited to specific optical-sensor applications. Accordingly, there is still a need for an improved approach of general application to setting the detection-threshold level of an optical sensor such that it reliably determines the logical state of the sensors under variable operating conditions.
BRIEF SUMMARY OF THE INVENTION
The primary, general objective of this invention is a method and apparatus for reducing failures associated with optical sensors in automated systems, thereby reducing downtime, maintenance and repair costs.
Another objective of the invention is a method and apparatus that provide dynamically a detection threshold that is always bound by the maximum and minimum levels of the sensor output signal, so as to produce a correspondingly consistent digital logic signal.
Another goal is an invention that is suitable for relatively simple incorporation within existing robotic equipment.
Still another goal is a method and apparatus that
Butka David
Goodman Brian Gerard
McIntosh Michael Philip
Yardy Raymond
Durando Antonio R.
Durando Birdwell & Janke P.L.C.
Le Que T.
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