Optical sensor safety system for monitoring laser crystals and optical components

The present monitoring system for monitoring operation of a laser system includes, in one embodiment, respective optical sensors coupled to respective, selected optical components. Each optical sensor, in operation, generates signals indicative of optical signal radial leakage of a respective optical component. When the radial leakage of a respective optical component. When the radial leakage-indicative signal from an optical sensor exceeds a predetermined threshold, operation of the laser system is interrupted.

The present invention is related to laser materials processing systems, and 
more particularly, to an optical sensor safety system for monitoring 
operation of laser systems and associated optical components. 
RELATED APPLICATIONS 
The present application is related to copending commonly assigned U.S. Pat. 
No. 4,960,970 and U.S. patent application Ser. Nos. 07/487,092 and 
07/489,306, respectively, entitled "Method and Apparatus For Acoustic 
Breakthrough Detection", "Method and Apparatus For Optically/Acoustically 
Monitoring Laser Materials Processing" and "Method and Apparatus For 
Optically Monitoring Laser Materials Processing", respectively, issued 
Oct. 2, 1990, and filed Mar. 2, 1990 and Mar. 5, 1990, both now allowed. 
BACKGROUND OF THE INVENTION 
Laser materials processing as known in the art and used herein refers to 
performance of materials processes, such as cutting, welding, drilling and 
soldering, using a continuous wave or pulsed laser beam. The average power 
of such a laser beam may range from as little as approximately one watt to 
hundreds of watts, the specific power being selected on the basis of the 
particular process being performed. Laser beam power required for 
materials processing generally is much greater than laser beam power 
required for other laser-based systems such as communication systems. 
A laser beam source, i.e., a laser resonator, typically includes a laser 
head having a crystal, such as a face-pumped laser as described in 
commonly assigned U.S. Pat. No. 3,633,126, "Multiple Internal Reflection 
Face-Pumped Laser", disposed therein. The crystal may, for example, have a 
rectangular cross-sectional shape and have six surfaces including 
respective pumping and cooling surfaces. In operation, energy is injected, 
i.e. pumped, into the crystal through the pumping surfaces. Laser crystal 
flashlamps, sometimes referred to herein as laser flashlamps, disposed 
within the laser head and along axes parallel to the pumping surfaces 
usually are utilized as pumping means. The laser flashlamps are coupled to 
a high energy power supply. The crystal is cooled, for example, by flowing 
coolant along the crystal surfaces. As is known in the art, the slab 
crystal has two crystal surfaces which are finished to brewster's angle. 
When operating as a laser resonator, a beam to be utilized for processing 
is emitted from one of the finished crystal surfaces. 
Optical components such as lenses and mirrors form part of the laser 
resonator and are disposed for extracting a high power laser beam from the 
crystal volume. A beam expanding lens combination and a focusing lens may 
be aligned with the laser resonator for shaping an emitted beam to be 
utilized in processing. 
A laser head may operate in a pulsed mode or in a continuous mode. A pulsed 
mode means that pulses of beams are emitted from the laser resonator. Such 
pulses of beams are obtained by exciting, i.e., energizing, the crystal 
with pulses of energy, e.g., pulsing the laser flashlamps. A continuous 
mode means that a continuous beam is emitted from the laser resonator. 
Such a continuous beam is obtained by providing continuous energy to the 
crystal, e.g., by leaving the laser flashlamps on. 
A laser head may be configured to operate as a laser oscillator or as a 
laser amplifier. When operating as an oscillator, the crystal is excited 
to a state wherein the crystal emits electromagnetic energy. When 
operating as an amplifier, the crystal is excited and, simultaneous with 
crystal excitation, a beam of electromagnetic energy from a separate 
source is injected into the crystal. As the beam travels through the 
crystal, it is amplified due to the excited state of the crystal. An 
amplified beam is then emitted from the finished surface of the crystal. 
In operation of the crystal in either mode, energy emitted from the laser 
flashlamps is injected into the crystal, through the pump surfaces, and 
excites, or optically pumps, the crystal. The laser beams generated are 
very narrow beams of radiation and the intensity within the beams is 
exceptionally high. 
Fast pulse repetition rates or long continuous mode operation of the 
crystal causes heat to be generated within the crystal. The crystal, in 
normal operation, may be cooled by flowing a coolant along the crystal 
cooling surfaces. If an optical component, e.g., a mirror, within the 
laser resonator becomes damaged or if some other abnormality occurs within 
the laser source during an operation, the crystal could discontinue 
lasing, i.e., discontinue emitting a laser beam. The laser flashlamps, 
however, will continue pumping the crystal. More specifically, if the 
laser flashlamps are pumping the crystal above the lasing threshold, and 
if the crystal is not emitting a laser beam, then parasitics i.e., 
irregular lasing paths, may develop within the crystal. The appropriate 
action in these circumstances usually is to stop energizing the crystal, 
such as by turning off the laser flashlamp power supply. 
Damaging optical components, and especially the crystal, is undesirable 
because, among other things, laser crystals are expensive and replacements 
may not be readily available. Also, if a component becomes damaged, the 
laser source usually must be shut down to make repairs. Shutting down 
operation of the laser source for a long period of time may be very 
costly, especially if the laser source is part of an assembly line. The 
whole line may have to be shut down as a result of laser source failure. 
It would be beneficial, therefore, to provide a means for detecting 
abnormal operations within a laser source so that timely appropriate 
actions may be taken to prevent damage, or further damage, to the crystal 
and other optical components. 
It would also be beneficial to provide means for detecting abnormal 
operations throughout an entire laser processing system. For example, a 
laser system may include, in addition to a laser source, an optical fiber 
and an output coupler. Transmission of laser beams through optical fibers, 
at power levels suitable for performing materials processing, greatly 
enhanced the flexibility of laser-based materials processing systems. 
Various techniques for the efficient injection of a high power laser beam 
from a laser source into an optical fiber for transmission therethrough 
are disclosed, for example, in commonly assigned U.S. Pat. Nos. 4,564,736; 
4,676,586; and 4,681,396 respectively entitled "Industrial Hand Held Laser 
Tool and Laser System", "Apparatus and Method for Performing Laser 
Material Processing Through a Fiber Optic", and "High Power Laser Energy 
Delivery System". Generally, lenses adjacent a laser source are utilized 
to focus a beam onto an input end of an optical fiber, and these lenses 
may be referred to herein, collectively, as a fiber injection unit. 
An output end of the optical fiber is disposed in an output coupling 
device, sometimes referred to herein as an output coupler, which includes 
means to collimate and focus the beam emitted from the fiber output end. 
The output coupling device is moved relative to a workpiece by, for 
example, a computer-controlled robotic arm. With optical fiber 
transmission, a system user must monitor, during the processing and in 
addition to the laser source, a fiber injection unit, an output coupler, 
and an optical fiber. Failure of any one component may result in failure 
of the entire system. 
Also available to enhance laser materials processing are systems for time 
sharing of a materials processing laser beam among a plurality of optical 
fibers. Such systems are described in commonly assigned U.S. Pat. Nos. 
4,739,162 and 4,838,631 entitled "Laser Beam Injecting System" and "Laser 
Beam Directing System", respectively. Manufacturers of beam time sharing 
systems include Robolase Systems, Inc. of Costa Mesa, Calif. and Lumonics 
Corporation of Livonia, Mich. By the use of such beam time sharing 
systems, a beam generated by one laser source can be shared among multiple 
optical fibers. The respective output ends of each optical fiber may be 
positioned proximate respective process points on one or more workpieces. 
Laser beam time sharing systems, sometimes referred to herein as 
multiplexers, have further increased the flexibility and efficiency of 
laser materials processing. With a multiplexer-based laser system, the 
system user must monitor a laser source, a multiplexer, multiple beam 
injecting systems, multiple couplers, and multiple optical fibers. The 
sequence of optical components in such systems is sometimes referred to 
herein as an optical train. 
A monitoring system for monitoring laser system components preferably 
facilitates obtaining desired processing results and aids in preventing 
damage to the components. The monitoring system, however, should not slow 
down laser materials processing operations. Otherwise, advantages of 
utilizing optical fiber/laser technology, such as a reduction in 
processing time, may be lost. Further, it is preferred that the monitoring 
system operate in substantially real-time. The monitoring system 
preferably should be able to obtain data simultaneous with materials 
processing so that if adjustments to components are needed, such 
adjustments can be made before further damaging processing components. 
It is therefore an object of the present invention to provide a system for 
monitoring and detecting an onset of abnormal operation of laser 
processing components so that appropriate action, such as laser flashlamp 
power supply turn-off, may be taken in a timely manner. 
Another object of the present invention is to provide a system for 
monitoring laser source operation including the performance of optical 
components disposed within the laser source. 
Still another object of the present invention is to provide a system for 
monitoring laser beam transmission through an optical fiber and through 
output coupler optical components. 
Still yet another object of the present invention is to provide a system 
which monitors, in substantially real time, laser materials processing 
components in a manner that does not slow laser materials processing. 
SUMMARY OF THE INVENTION 
The present optical sensor safety system for monitoring laser materials 
processing components includes optical sensors, such as photodiodes, 
mounted or adhesively attached to optical components of a laser system. 
The optical sensors are mounted so as to be able to detect radial leakage 
of optical signals transmitted/reflected by the optical components. As 
radial leakage in an optical component increases, a signal generated by 
the respective optical sensor increases in magnitude. Each optical sensor 
is coupled to a means for determining whether a signal generated by an 
optical sensor has exceeded a predetermined threshold. The determining 
means is coupled to a power source interrupt means, such as a laser safety 
interlock which turns off the laser flashlamp power source. 
In normal operation, signals received from each optical sensor mounted to 
an optical component should have a small magnitude. That is, in normal 
operation, optical component radial leakage is very small. If an optical 
component becomes damaged, however, the optical component will begin 
exhibiting less efficient transmission characteristics. One such 
characteristic is increased radial leakage of optical signals. The optical 
sensor mounted to the component will generate a signal of greater 
magnitude indicative of the increased radial leakage. 
If the sensor signal exceeds the predetermined threshold, the power source 
interrupt means will be triggered, thereby cutting off power to the laser 
flashlamps. Lasing will terminate and any further damage to processing 
components due to component malfunction will be prevented. 
The present system detects the onset of abnormal laser processing component 
operation and facilitates timely action to prevent further damage to the 
laser source and other laser system components. The present system can be 
utilized for monitoring components within a laser source as well as 
components throughout an entire laser system. Further, the present 
invention monitors, in substantially real time, laser materials processing 
components in a manner that does not slow laser materials processing.

DETAILED DESCRIPTION OF THE DRAWINGS 
Referring now more particularly to the drawings, FIGS. 1A-B illustrate side 
and front views, respectively, of an undamaged mirror 10 having a laser 
beam 12 transmitted therethrough. As shown in FIGS. 1A-B, as laser beam 12 
is transmitted through mirror 10, a small magnitude of radial leakage will 
occur, as diagrammatically indicated by small arrows pointing radially 
outward from beam 12. 
FIGS. 2A-B illustrate side and front views, respectively, of a damaged 
mirror 14 having a laser beam 16 transmitted therethrough. A damaged 
portion 18 of mirror 14 illustrates that a coating, such as an 
anti-reflection coating, of mirror 14 has been damaged. Such coatings are 
well known in the art and typically used in high power beam transmission. 
As beam 16 is transmitted through damaged mirror 14, increased radial 
leakage occurs as diagrammatically indicated by large arrows extending 
radially outward from beam 16. 
FIGS. 3A-B illustrate scanning a damaged mirror 20 with a laser beam 22. As 
shown in FIG. 3A, an optical sensor 24, such as a photodiode or 
phototransistor, is mounted to mirror 20. Such mounting may be achieved by 
using an optically transmissive adhesive or a mounting bracket which 
maintains the photodiode in contact with the optical component. Mirror 20 
also includes a damaged portion 26. Beam 22 was scanned across mirror 20 
along an axis parallel to arrow 28 and through a center of mirror 20. 
Therefore, beam 22 passed through damaged portion 26. 
FIG. 3B illustrates an electrical signal generated by optical sensor 24 
during the scan operation. In the FIG. 3B graph, the x-axis is assigned 
units of laser beam position on mirror 20 and the y-axis is assigned 
arbitrary units of radial leakage intensity. Positions X.sub.1 and X.sub.2 
along the axis indicate locations at which damage portion 26 begins and 
ends, respectively. As is clear from FIG. 3B, once beam 22 encounters 
damaged portion 26, radial leakage increases. This increase in radial 
leakage is detected by optical sensor 24 which generates, as a result, an 
increased magnitude signal. The increased radial leakage continues until 
beam 22 has fully passed through damaged portion 26. Once the beam has 
fully traversed through the damaged portion, radial leakage intensity 
decreases to its normal magnitude thereby indicating a normal optical 
quality surface. 
The increase in radial leakage as illustrated in FIGS. 3A-B may result from 
damage to a mirror coating or other abnormal component operation, such as 
melting of an optical component substrate. It should be understood, 
therefore, that the present invention is not limited to practice with 
mirrors having coatings. Rather, the present invention detects an increase 
in radial leakage regardless of whether the component has such a coating. 
FIG. 4 illustrates a first embodiment of a monitoring system in accordance 
with the present invention. The monitoring system in FIG. 4 is shown in 
combination with a laser source which includes a back cavity spherical 
mirror 100, a laser crystal 102, a laser output coupler mirror 104, and a 
beam expanding unit including optical lenses 106 and 108. The back cavity 
spherical mirror, the crystal, and the coupler mirror compose a laser 
resonator. It should be understood that the present invention is not 
dependent upon specific components and can be utilized in combination with 
many other laser sources including many other optical components. Further, 
the present system could be implemented in an optical fiber output 
coupler, beam multiplexer, and many other laser system components. 
As shown in FIG. 4, the present monitoring system includes optical sensors 
such as photodiodes 110, 112, 114, and 116 mounted to each optical 
component. The photodiodes may, for example, be photodiode model # 
YAG-100A manufactured by EG&G of Salem, Mass. The photodiodes, however, 
need not be mounted to each optical component and could be mounted to a 
limited number of selected optical components. The photodiodes may be 
adhesively attached to the optical components by utilizing a light 
transmissive adhesive or the photodiodes may be mechanically maintained in 
contact with the components, for example, by adapting a lens holder to 
include an opening for the photodiode. 
Each photodiode includes a filter which transmits a single wavelength. For 
example, in an Nd:YAG based system, which system generates a beam having a 
wavelength of 1.06 .mu.m, each photodiode filter would be configured to 
transmit only 1.06 .mu.m wavelength signals. Therefore, only signals 
directly related to the laser beam would be transmitted through the filter 
and all other signals would be blocked. Each photodetector is coupled to a 
photodetector bias unit 118 which is coupled to a threshold comparing unit 
120. The threshold comparing unit is coupled to a power source interrupt 
unit 122 which, as shown in FIG. 4, is coupled to a laser power supply 
interlock, such as a relay for controlling the opening and closing of a 
switch which interrupts energizing the laser flashlamps. 
Before operation, a threshold value for threshold comparing unit 120 must 
be determined. The threshold value may be determined, for example, by 
initially operating the laser source at a low power level. The threshold 
level may be adjusted during the low power operation so that the threshold 
level is just above the detected normal operating radial leakage. As the 
power level of the source is increased, the operator simultaneously 
increases the threshold level so that laser flashlamp power is not 
interrupted. When the high power operating level is reached, the system 
user sets the threshold at a level which allows operation up to a maximum 
amount of allowable radial leakage, typically at a level just above radial 
leakage detected at the high power operating level. 
In operation, laser flashlamps (not shown) optically pump crystal 102 which 
emits a first beam 124 which is reflected by spherical mirror 100 back 
into crystal 102. Crystal 102 also emits a second beam 126 which is 
transmitted, partially, by mirror 104. Beams 124 and 126 actually compose 
a single laser beam, but are described conceptually as two beams to 
facilitate an understanding of operation. Although the laser source is 
shown operating as a laser oscillator, the present monitoring system could 
be utilized when the source operates as a laser amplifier. The beam 
expanding unit including lenses 106 and 108 expands the beam transmitted 
by the mirror and forms a beam 128 which may be utilized for materials 
processing. Further optical components such as a fiber injection unit 
could be provided for focusing beam 128 into an optical fiber, as is known 
in the art, for transmission of the laser beam to an output coupler. 
If each optical component of the laser source is undamaged, then each 
photodiode coupled to an optical component will generate a small magnitude 
signal. These signals are transmitted to the photodetector biasing unit 
which amplifies the signals received by the photodetectors. The amplified 
signals are then transmitted to the threshold comparing unit which 
compares the optical sensor signals with the predetermined threshold 
level. If an optical sensor signal exceeds the predetermined threshold, an 
output signal is generated by the threshold circuit and transmitted to 
power source interrupt unit 122. The interrupt unit then triggers the 
laser power supply interlock which, as hereinafter described, interrupts 
delivery of power to the laser flashlamps. Optical pumping of crystal 102 
will stop thereby stopping lasing. 
As described above, the present system detects the onset of abnormal laser 
processing component operation by detecting an increase in optical signal 
radial leakage through optical components. The present invention 
facilitates taking timely action, such as turning off power supplied to 
the laser flashlamps, to prevent damage to optical components of the laser 
system. Further, the present invention monitors, in substantially real 
time, laser materials processing components in a manner that does not slow 
laser materials processing. 
As pointed out above, the system can be utilized for monitoring components 
within a laser source as well as components throughout an entire laser 
system optical train. Photodiodes simply would be mounted to other optical 
components throughout the laser system and coupled to the photodetector 
biasing unit. 
FIG. 5 illustrates a second embodiment of a monitoring system in accordance 
with the present invention. The monitoring system in FIG. 5 is shown in 
combination with a laser source, such as the laser source illustrated in 
FIG. 4. The last two digits of reference numbers for components 
illustrated in FIG. 5 have the same last two digits as reference numbers 
for corresponding components, if any, shown in FIG. 4. 
In the second embodiment illustrated in FIG. 5, and in addition to the 
previously described components, laser diodes 230, 232, 234, and 236 are 
mounted to respective optical components at substantially 180.degree. from 
respective photodetectors. Each laser diode is coupled to a laser diode 
biasing unit 238 which supplies power to the laser diodes as known in the 
art. Each laser diode, as shown in FIG. 5, is mounted so that it emits a 
beam which traverses, in close proximity, a surface of a respective 
optical component. For each optical component, the surface which the laser 
diode beam traverses typically is the surface of the component at which a 
beam emitted from the crystal first encounters. The photodetectors are 
disposed so as to sense the laser diode emitted beam. As with the first 
embodiment, before operation, a threshold level must be predetermined. The 
threshold level may, for example, be set at a level just below the 
magnitude of the laser diode beam signal received by the photodetectors in 
normal operation. 
In operation, each respective laser diode transmits a beam in very close 
proximity to the optical surface of each respective optical component. The 
photodetectors detect these beams, and as a result, generate a high 
magnitude signal. Each photodetector includes a filter so that only 
signals having a wavelength of a laser diode beam emitted by a laser diode 
affect the signal transmitted to photodetector bias unit 218. 
The threshold comparing unit is set to trigger the interrupt unit when a 
signal from a photodetector falls below the predetermined level. 
Specifically, as damage to an optical component occurs, a beam transmitted 
by an associated laser diode will not be fully transmitted to the 
respective photodetector. When a lens or mirror becomes damaged, a plume 
typically results on the surface of the optical component where the laser 
beam is intercepted. The damaged portion of the lens, therefore, will 
interrupt, or at least partially interrupt, the laser diode generated 
beam. Therefore, the photodetector will detect a decrease in the signal 
transmitted by the respective laser diode. The result of these conditions 
will be a decrease in magnitude of the photodetector signal. As previously 
stated, if the photodetector signal falls below the predetermined 
threshold level, the power source interrupt unit will be triggered to cut 
off power to the laser flashlamps. 
FIG. 6 illustrates a third embodiment of a monitoring system in accordance 
with the present invention. The last two digits of reference numbers for 
components illustrated in FIG. 6 have the same last two digits as 
reference numbers for corresponding components, if any, shown in FIG. 4. 
In FIG. 6, optical fibers 340, 342, 344 and 346, each being coupled 
between a respective optical component and a respective photodetector, are 
utilized for transmitting optical signals from the optical components to 
the optical sensors. The optical fibers transmit, to the respective 
photodetectors, signals representative of the magnitude of radial leakage. 
Each optical fiber input end may be connected to an optical component by, 
for example, a light transmitting adhesive or utilizing mechanical holder. 
Utilizing optical fibers rather than directly coupling photodetectors to 
the optical components may be preferred in some situations where it is 
desirable to have the remaining portion of the safety monitoring system 
disposed remotely from the laser source. Operation of the embodiment 
illustrated in FIG. 6 is substantially similar to operation of the first 
embodiment illustrated in FIG. 4. 
FIG. 7 illustrates one embodiment of a biasing, threshold comparing, and 
power supply interruption circuit in accordance with the present 
monitoring system. The circuit schematic illustrated in FIG. 7 could be 
utilized in the first and third embodiments illustrated in FIGS. 4 and 6, 
respectively. For the second embodiment illustrated in FIG. 5, the 
threshold comparing circuit must be altered so that the switch is normally 
open. In this manner, and as described hereinafter in more detail, when 
the circuit is being driven, the switch is closed and power is supplied to 
the laser flashlamps. 
An optical sensor is shown in the circuit schematic in FIG. 7 as a 
photodiode 402. It should be understood, of course, that the optical 
sensor could be a phototransistor or any optical sensor which generates an 
electrical signal which varies in magnitude according to the intensity of 
sensed light. As previously described, the photodiode is mounted to an 
optical component (not shown) such as a lens or mirror. A resistor 404 is 
coupled between the photodiode and ground, and capacitor 406 is coupled 
across photodiode 402 and resistor 404. A first biasing voltage labeled 
"+Vbias1" is applied at a node 408. 
The non-inverting input for an operational amplifier 410, such as a 741 
operational amplifier, is obtained from between photodiode 402 and 
resistor 404. The numerical markings shown in association with operational 
amplifier 410 in FIG. 7 correspond to pins of the 741 operational 
amplifier which is well known in the art. A threshold-adjust resistor 412, 
which preferably is an adjustable resistor, is coupled to a terminal 414 
which, in turn, is coupled to a second biasing voltage "+Vbias2". The 
particular setting for resistor 412 is selected in accordance with a 
desired threshold level and may be determined, as previously described, 
through experimentation. The output from operational amplifier 410 is 
coupled as an inverting input to an operational amplifier 416, which also 
may be a 741 operational amplifier. As with operational amplifier 410, the 
numerical indications associated with operational amplifier 416 designate 
the pin locations of the 741 operational amplifier. A feedback resistor 
418 and a resistor 420, coupled to ground, are connected to the inverting 
input of operational amplifier 410. 
A diode 422 and a resistor 424 are coupled between operational amplifier 
416 and a transistor 426. Resistor 428 is coupled between transistor 426 
and ground. A diode 430 and a relay 432 coupled to transistor 426, as 
hereinafter explained, provide a means for interrupting the supply of 
power to laser flashlamps (not shown). A switch 434 is shown as being 
coupled to relay 432. A third biasing voltage, "+Vbias3", serves to bias 
the relay circuit. 
An initial threshold level setting for resistor 412 is performed as 
hereinbefore described and resistor 412 is adjusted to prevent the switch 
434 from being opened under normal conditions thereby interrupting the 
supply of power to the laser flashlamps. In operation, switch 434 is 
normally closed thereby allowing power to be supplied to the laser 
flashlamps from the power source. If the optical component becomes 
damaged, however, photodiode 402 will generate a higher magnitude signal 
due to increased radial leakage from the damaged optical component. This 
increased radial leakage increases the intensity of light impinging upon 
photodiode 402. As photodiode 402 generates an increased magnitude 
electrical signal, the voltage across resistor 404 increases. If the 
voltage across resistor 404 exceeds the predetermined threshold, 
operational amplifier 410 will generate an output signal. The output 
signal from operational amplifier 410 drives operational amplifier 416 
which, in turn, energizes transistor 426. When transistor 426 turns on, 
relay 432 is energized. When the relay is energized, switch 434 is forced 
from its normally closed position to an open position, thereby 
interrupting the supply of power to the laser flashlamps. 
For the second embodiment illustrated in FIG. 5, the switch in FIG. 7 must 
be configured to be normally open. In normal operation, the circuit will 
drive the relay so that the switch is closed. If the photodetector signal 
decreases, however, the voltage across resistor 404 also will decrease. 
When the voltage across the resistor falls below the predetermined 
threshold level, operational amplifier 410 will turn off thereby causing 
amplifier 416 to discontinue driving transistor 426. The switch therefore 
will open and power to the laser flashlamps will be interrupted. 
Many other configurations for the circuit shown in FIG. 7 are possible. 
While the present invention has been described with respect to specific 
embodiments, many modifications, variations, substitutions, and 
equivalents will be apparent to those skilled in the art. For example, the 
threshold could be set to a range of values rather than a discrete value. 
This could be achieved using well-known comparing circuits or even 
implemented in a computer. Accordingly, the invention is to be considered 
is limited only by the spirit and scope of the appended claims.