 |
Description  |
|
|
PRIOR ART AND BACKGROUND
Optical measuring systems utilizing light from a laser beam are known. One
such system is the co-pending application, Ser. No. 566,413, for "Reticle
Calibrated Diameter Gauge," by David E. Harris, et al., now U.S. Pat. No.
4,043,673 and assigned to the same assignee. In that co-pending system the
occultation of the measuring beam by the object being measured generates a
signal operative to control counting of the calibration pulses as an
indication of the dimension being measured.
There are many other variations of the split beam reference/measuring
signal comparison variations known to the prior art.
Each of the prior art optical measuring systems, although perhaps operable
for their intended purpose, do have attendant disadvantages. These prior
art systems generally utilize, for instance, a scanned photo diode matrix
or complex vidicon. Particularly, the need for calibration and constant
recalibration, incapacity for a low measurement rate, high resolution
capabilities, in stability, accuracy, and dependence from surface color
and composition errors have been encountered with these prior art optical
measuring systems. These disadvantages in turn are reflected as
tremendously costly.
SUMMARY OF THE INVENTION
The present invention is an optical gauging system utilizing the position
of a scanned laser beam to determine the position of an unknown surface
which can be used to gauge material thickness by optical triangulation.
The laser light beam passes through a focusing lens to a scanning mirror.
The scanner, in turn, deflects the beam onto a beam splitter where the
beam is divided into a measuring beam and a reference beam. The measuring
beam from the splitter passes through a collimating lens to assure that
the beam striking the workpiece is parallel and not diverging.
The reference beam is directed to a reticle--a photo transparency of a
series of lines and spaces. In this way the reference beam is alternately
transmitted and blocked as it passes through the reticle to a lens for
focusing on a photo detector. The photo detector converts the movement
across the reticle into a series of electrical calibration pulses.
The light striking the surface of the workpiece is back scattered and the
back scattered light is focused onto a split photo detector. As the
measuring beam scans across the surface of the workpiece, the focused back
scattered light is imaged into a spot of light travelling back and forth
across the split in the photo detector.
The output of the photo detector is fed to a balanced differential
amplifier. When the light spot is on one half of the photo detector, there
is a positive output; and when the light spot is on the other side of the
photo detector, there is a negative output.
The number of pulses, counted by the calibration circuitry, that occur
while the focused spot of light is on one half of the split photo detector
is an accurate digital measurement of the position of the surface of the
workpiece.
OBJECTS
It is accordingly a principal object of the present invention to provide a
new and improved laser beam optical measuring system for the thickness
measurement having particular application to the food, rubber, and
plastics industry.
It is a further object to provide such an optical distance measuring system
utilizing triangulation means that are independent of surface color or
composition, capable of high resolution, stability, permanently
calibrated, and reflect simplicity in construction and cost.
Other objects and features of the present invention will become apparent
from the following detailed description when taken in conjunction with the
drawings, in which:
BRIEF DESCRIPTION OF DRAWINGS
FIG. 1 illustrates schematically the components that comprise the optical
portion of the gauging system of the present invention.
FIG. 2 is a measurement beam video circuit comparator in block schematic,
and
FIG. 3 is the reticle video process and phase locked loop in block
schematic.
FIG. 4 is the overall system block diagram.
DETAILED DESCRIPTION OF DRAWINGS
With particular reference now to FIG. 1, there is illustrated in block
schematic the optical components of the gauging system of the present
invention.
The laser 2, excited by a suitable power source or other light source that
may comprise those known to the art such as galium arsinide diodes,
halogen arc light, or other finely focused light beams. In that preferred
embodiment there was used a helium neon laser light source capable of
producing a light beam of the order of 50 ml. diameter. By suitable
reduction by lens 3 the beam is reduced to a beam of the order of 15 ml.
diameter at the reticle plane 10.
In operation of the components shown schematically in FIG. 1, the laser
light beam strikes beam scanner 4. This scanner comprises a mirror 4 with
a suitable drive comprising an oscillator. The scan mirror deflector 4
deflects the laser beam through an arc onto a beam splitter 6. In a
conventional manner the beam splitter divides the scanned beam into two
light components.
The first light component is the measuring beam x that strikes the
workpiece 14. A collimating lens 7 is located in the path of the measuring
beam between the beam splitter 6 and the workpiece 14 and located at its
focal length away from the center of rotation of mirror 4. In this way the
light beam x passing through focusing lens 7 is parallel, and not
diverging.
The other component of light passing through the beam splitter 6 is the
reference beam. The light passing through beam splitter 6 strikes mirror 8
and is reflected on to the reticle 10. The reticle 10 is a photo
transparency of a series of lines and spaces of approximately 10 mils.
width each. This reticle 10 does not form a part of the instant invention
and may be that illustrated and described in U.S. Pat. No. 4,043,673,
issued Aug. 23, 1977.
The light beam y, passing through reticle 10, is focused by lens 11 onto
the photo detector 12. This photo detector 12 converts the movement of the
calibration beam y into a series of electrical calibration pulses. Each
pulse is calibrated to represent 20 mils. of movement of beam y across the
reticle 10 and a much smaller--but proportional, movement of the
measurement beam x across the surface of the workpiece 14.
The light back scattered light from surface 14 is focused by lens 15 onto
split detector 16. The focused light z images into a tiny spot of light
traveling back and forth across the split in the photo detector 16. The
signal from the split detector 16 is fed into a ballanced differential pre
amp 17.
As the measurement beam scans across the surface of the workpiece 14 and
the light spot is on one half of detector 16, the output is positive from
the pre amp 17. As the spot transitions to the other side of the split of
the detector 16, the output of the pre amp 17 swings to negative. The
number of pulses from detector 12 and the pre amp 13 that occur while the
focused spot of light is on one half of detector 16 are an accurate
digital measurement of the position of surface 14, relative to lens 7. The
count of calibration pulses is dependent on the suitable selection of lens
15, together with the angle from which it views the surface, and by proper
selection of the reticle spacing and optical path lengths. The resolution
of the system may be further increased by using a phase locked loop
(reference FIG. 3), that is, locking the voltage controlled oscillator 48
(FIG. 3) in phase with a harmonic multiple of the reticle pulse frequency.
With reference now to FIG. 2, there follows discussion of the measurement
beam video circuit and the auto-trigger point circuit. The back scattered
light is focused on the split photo detector (16 of FIG. 1). Several prior
art photo detectors have proven satisfactory, such as silicon photo
diodes, cadmium sulphate cells, and silicon solar cells. The preferred
embodiment utilizes a silicon solar cell which has proven to be a fairly
sensitive high speed detector with a reasonable signal to noise ratio. Two
such solar cells are cemented next to each other to make up the split
detector 16. These cells are connected with short leads to a high gain
differential amplifier 20. Since the amplifier 20 has complementary
inputs, one positive and one negative, any ambient light that falls on
both detectors will be subtracted out and accordingly, only the difference
in the lighting of the two halves of the detector will be amplified.
The pre amp builds the video signal from several mili volts up to several
volts where it can be sent back to the system mainframe for signal
processing. In the mainframe, the signal may be further amplified by
amplifier 22. This amplifier 22 is A.C. coupled to prevent amplifying any
drifts or offsets in the pre amp or power supply to make a repeatable
decision about when the video indicates that the spot of light has shifted
from one side to the other on the detector 16, the signal is buffered in
buffer 24 and then sent into a pair of detectors 32 and 34. One detector
stores the maximum positive excursion of the signal and the other stores
the maximum negative excursion. These stored peak voltages are sent
through a voltage divider, resistors 36 and 38, to establish the voltage
that represents the halfway point between the two extremes. The trigger
output voltage from divider is sent to a voltage comparator 26 having the
video signal at its other input. In determining the transition, the result
is independent of the amplitude of the video signal, and therefore
independent of the color of these surfaces we are measuring.
With reference now to FIG. 3, a discussion may now be had of the reticle
video circuit and its signal processing including the phase locked loop.
The reticle video signal found on photo detector 12, of FIG. 1, is only a
few millivolts in amplitude. It is amplified by a pre amp 36 to a level of
around a volt for transmission back to the system mainframe. Due to the
internal capacity of the silicon solar cell used for detector 12, the
higher the frequency of the reticle pulses, the lower their amplitude. The
scanning mirror 4 of FIG. 1 used to deflect the laser beam through its arc
is a taut band device, that tends to scan in a sine wave motion at its own
mechanical resonate frequency. The amplitude of the scan is not important
as long as we scan the entire reticle and off its ends 9. The ends 9 of
the reticle 10 are masked and since the mirror must go to zero velocity at
its maximum angles, this "edge" pulse is of much longer duration, and
therefore greater amplitude than the other reticle system pulses.
In order to determine the beginning and end of a given scan, these edge
pulses must be detected. A simple voltage comparator FIG. 3 38 is set up
to trigger whenever the voltage drops below a certain value, and the video
signal from the reticle pre amp 36 is fed into this comparator 38. The
resulting pulses, during the turning around of the scanning mirror, are
called the window pulses. These pulses are used throughout the system to
time and average.
Since the scan is sinusoidal, the reticle pulses are not all of equal
amplitude, nor is the photo reticle always perfect. In order to make the
reticle pulses more uniform with the variables, the video signal is
differentiated by differentiator 40. The resulting pulses are
symmetrically distributed about the zero volt line and can be easily used
to trigger a zero crossing detector 42. The output of the zero crossing
detector 42 is a series of pulses of uniform amplitude with full digital
logic levels being maintained.
With continued reference to FIG. 3, the scanned laser dimension comparator
reticle video processing may be described. Since each reticle pulse
represents 20 mils. of calibration beam travel, and that corresponds to 5
mils of measurement beam movement, the reticle pulses alone could be
counted to produce a digital representation of the measurement. In today's
control technology however, greater resolution is expected of most
measurement devices. In order to increase the resolution of the scanned
laser dimension comparator, a phase locked loop 50 comprised of 44, 46, 48
and 53 is used to multiply the effective number of reticle pulses from
zero detector 42. With reference to FIG. 4 the scanned laser beam passing
through the reticle and reticle focusing lens falls on to the reticle
detector 51 where it generates a series of pulses representing beam
velocity and position. These low level electrical pulses are amplified by
the reticle pre-amplifier 52 to a level of several volts. The amplified
reticle video, as this signal is called, is sent into two different
circuits. One circuit, the scan edge or "window" circuit 53 determines
when the beam has scanned off the edge of the reticle. One window pulse is
generated at each edge of the reticle. A digital divider 54 takes in 200
such "window" pulses, and puts out one window pulse in every 200, as a
timing pulse. This timing pulse represents 100 complete "round trip" scans
of the laser beam across the reticle. The timing pulse is sent to the
latch pulse generator 55 and also to the reset pulse generator 56. These
two generators are one shot multivibrators and are triggered into
generating one very short duration pulse for each timing pulse input they
receive.
The latch pulse generator puts out one short pulse on the leading edge of
the timing pulse and the reset pulse generator one short pulse on the
trailing edge. These pulses are used to reset a counter chain 64 and
update a set of digital storage latches 65 that are connected to the
counter outputs.
The reticle signal from the reticle preamp is also sent to the pulse
multiplier 57 which converts the several volts of video into a full
"logic" level pulse burst, and then, through the use of a phase locked
loop, multiplies the frequency of those pulses to increase system
resolution.
The image of the scanned measurement beam, as back scattered from the
surface being measured, and focussed by a viewing lens onto the "split"
detector 59, causes that detector and its pre-amp to generate a signal
that is positive and then negative, depending on which half of the
detector is illuminated by the image of the scanning spot of light. The
level of this signal leaving the measurement video preamp 60 will be
approximately 2 volts, but may vary considerably depending on several
factors including laser power and the color and texture of the surface
being measured. In order to insure adequate signal levels, AGC, Automatic
Gain Control can be added to the preamp circuit (these techniques are well
known to anyone familiar with radio and TV receiver circuits).
To insure a repeatable length video pulse with a possible drift in
amplifier DC levels or changes in the signal level from any cause, the
automatic trigger level circuit 61 shown in detail in FIG. 2 is employed.
This circuit stores the peak positive and negative excursions of the video
signal and then establishes video trigger voltage half way between the two
extremes. The trigger level circuit 61 of FIG. 4 and its output trigger
voltage is fed into one side of the measurement video voltage comparator
FIG. 4, 62 while the original measurement video is fed into the other
input. The output of voltage comparator 62, is a logic level pulse of a
duration the same as the measurement video signal but with fast rise time
and greater voltage level. The output of 62 is referred to as Video.
The video signal is fed to a NAND gate 63 with the multiplied reticle
pulses from the phase locked loop on the other input. The resulting output
from NAND Gate is a pulse burst containing all the multiplied reticle
pulses that occured while the Video signal from comparator 62 was at Logic
1 and no pulses from where it was at logic zero.
The gated pulses from gate 63 are divided by divider 68 and counted by
counter 61 for one complete scan cycle, the number of pulses counted
provide an accurate digital count from counter 61 that is representative
by display 66 of the relative position of surface 14 FIG. 1 with respect
to the optical gauging system. This count is then stored in display latch
65 and displayed via decoder display 66.
Although a certain and specific embodiment of the present invention has
been shown and described, it must be appreciated that modifications may be
had thereto without departing from the true spirit and scope of the
invention.
* * * * *
|
|
 |
|
 |
Description |
|
