U.S. patent application number 11/182506 was filed with the patent office on 2006-02-16 for laser sheet generator.
Invention is credited to Adiano Cunha, John Keightley, Eric Rechner.
Application Number | 20060033935 11/182506 |
Document ID | / |
Family ID | 4168289 |
Filed Date | 2006-02-16 |
United States Patent
Application |
20060033935 |
Kind Code |
A1 |
Keightley; John ; et
al. |
February 16, 2006 |
Laser sheet generator
Abstract
A laser line generator having a laser diode, or other laser
source, emits a laser which is passed through an aspherical lens to
create a two-dimensional fan-shaped light sheet. This light sheet
is reflected off a flat mirror to expand the width of the light
sheet without increasing the size required by the optical path. The
light sheet is then reflected by parabolic mirror to create a
straight, planar light sheet.
Inventors: |
Keightley; John; (Langley,
CA) ; Rechner; Eric; (Woodlawn, CA) ; Cunha;
Adiano; (Surrey, CA) |
Correspondence
Address: |
ROBERT E. KREBS;THELEN REID & PRIEST
P.O. BOX 640640
SAN JOSE
CA
95164-0640
US
|
Family ID: |
4168289 |
Appl. No.: |
11/182506 |
Filed: |
July 14, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10061570 |
Feb 1, 2002 |
6947152 |
|
|
11182506 |
Jul 14, 2005 |
|
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Current U.S.
Class: |
356/638 |
Current CPC
Class: |
G01B 11/04 20130101 |
Class at
Publication: |
356/638 |
International
Class: |
G01B 11/08 20060101
G01B011/08 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 2, 2001 |
CA |
2,334,375 |
Claims
1. An apparatus for emitting a linear, planar sheet of light,
comprising: a laser which emits a beam of light; an aspherical lens
which converts said beam of light into a fan-shaped sheet of light;
and a parabolic mirror which reflects said fan-shaped sheet of
light into a linear, planar sheet of light.
2. The apparatus according to claim 1, further comprising a flat
mirror located between said aspherical lens and said parabolic
mirror which reflects said fan-shaped sheet of light from said
aspherical lens into said parabolic mirror.
Description
RELATED APPLICATIONS
[0001] This application is a Continuation in Part of U.S. patent
application Ser. No. 10/061,570, filed on Feb. 1, 2002, entitled
"High Speed Laser Micrometer" which claims priority to Canadian
Application No. 2,334,375, filed on Feb. 2, 2001, entitled "Laser
Micrometer", both of which are hereby incorporated by
reference.
FIELD
[0002] The invention relates to laser measurement systems.
BACKGROUND
[0003] A laser micrometer provides dimensional information about
objects placed in the path of a sheet of laser light which is
detected by, or scanned across, a detector. The width of the
"shadow" created on the detector provides a dimension of the
object,
[0004] Existing "laser micrometers" are typically designed for
small objects, with a maximum dimension of 6 inches. However, laser
micrometers offer accuracy better than 1/1000.sup.th of an inch, at
a scan rate of up to a thousand samples per second. A
[0005] In a laser micrometer, a laser light sheet is usually formed
using either static refractive lens elements to form a laser sheet
or a rotating mirror to scan the beam. The limitation on the
maximum size of the micrometer is the limitation on the size of
these light sheet forming elements.
[0006] Alternative systems, known as "light curtains" are usually
used in applications such as safety monitoring for preventing
access to hazardous or secured areas. However, a laser curtain can
be adapted to provide measurements of approximately 1/8.sup.th of
an inch accuracy and resolution. The scan rate from a light curtain
is typically 100-200 samples per second.
[0007] In a light curtain system, the light sheet is typically a
series of independent light beams emitted from a linear array of
light emitting diodes (LEDs) spaced at the desired measurement
resolution. An array of matching photodiode detectors completes the
system. The light curtain design is limited in resolution by the
physical spacing between the LEDs. The maximum scan rate is limited
by the need to strobe the LEDs in segments to avoid crosstalk
arising from adjacent photodiodes "seeing" the wrong LED. The
maximum scan rate is also reduced as the size of the light curtain
increases due to the large amount of data produced and the
limitations of the typical interface and data encoding scheme.
[0008] Therefore, there is a need for a large format, high speed
laser micrometer that is capable of scanning large objects with a
high scan rate and high degree of accuracy.
[0009] It is an object of this invention to provide a large format,
high speed laser micrometer to scan large objects with a high scan
rate and a high degree of accuracy. It is an additional object of
this invention to provide a parabolic mirror assembly for forming a
collimated laser light sheet.
SUMMARY OF THE INVENTION
[0010] A laser line generator having a laser diode, or other laser
source, emits a laser which is passed through an aspherical lens to
create a two-dimensional fan-shaped light sheet. This light sheet
is reflected off a flat mirror to expand the width of the light
sheet without increasing the size required by the optical path. The
light sheet is then reflected by parabolic mirror to create a
straight, planar light sheet.
[0011] One application of the laser line generator is in a large
profile, high speed laser micrometer. The micrometer is formed from
a light source unit comprised of a plurality of emitter modules
that combine to emit a laser sheet and a detector array comprised
of a plurality of detector modules. Each of the emitter modules is
aligned with a corresponding detector module such that an object
passing between the light source unit and the detector array can be
measured. Preferably, each of the emitter modules is comprised of
two or more laser line generators arranged in an overlapping
stair-step fashion to prevent gaps in the laser sheet emitted by
the emitter module. Each of the detector modules is comprised of
two or more linear CIS detectors, equal to the number of laser line
generators, arranged in an overlapping stair-step fashion
corresponding to said laser line generators.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] The invention itself both as to organization and method of
operation, as well as additional objects and advantages thereof,
will become readily apparent from the following detailed
description when read in connection with the accompanying
drawings:
[0013] FIG. 1 is a plan view of the laser sheet mirror
generator;
[0014] FIG. 2 is a plan view of the laser sheet generator;
[0015] FIG. 3 is a side view of the laser sheet generator;
[0016] FIG. 4 is schematic of a test setup for the laser sheet
generator;
[0017] FIG. 5 is a front view of a large format laser micrometer;
and
[0018] FIG. 6 is a side view of the laser line generator on an
optical table.
DETAILED DESCRIPTION
[0019] The laser optics of the laser line generator 42 are shown in
detail in FIGS. 1 and 2. Laser light 70 is passed through an
aspherical lens 72 (e.g. Kodak Part # LG-11) to collect the light
from the laser diode 68 and create a two-dimensional fan-shaped
light sheet 80. This light sheet is reflected off a flat mirror 74
to expand the width of the light sheet without increasing the size
required by the optical path. The light sheet is then reflected by
parabolic mirror 76 to create a straight, planar light sheet. The
term `parabolic mirror` refers to a mirror curved in one plane to a
parabola form--not to be confused with a paraboloidal mirror which
is curved in two planes (although the term parabolic is often used
to describe a paraboloidal mirror).
[0020] The laser diode 68 and lens 72 are supported on a laser
plate 84. The angle of the laser plate 84 can be precisely adjusted
so as to target the laser and to control the angle of incidence at
mirrors 74 and 76. The laser plate 84, diode 68, lens 72, flat
mirror 74 and parabolic mirror 76 are all supported on channel 86.
Channel 86 can be fixed or mounted in a number of different ways
depending on the application for which the laser line generator 42
is to be used.
[0021] Referring to FIG. 4, in one embodiment the laser line
generator 42 has a thin first surface mirror 76 which is formed
against a 20'' radius cylinder or parabola. The mirror is secured
to a 1/16'' FR-4 backing plate using Durabond E-20NS epoxy. Two
methods of making this mirror are presently feasible: vacuum
forming against the appropriate form, and clamping to the form by
securing end points of the mirror only. The laser diode 68 is set
at a distance from the mirror 76 for best collimation, near the
focus of 10''. The collimated laser light illuminates the target
100, and the resulting shadow of the target is viewed against the
screen 110. The target 100 consists of a packaging strip of surface
mount resistors which gives a line of 1 mm holes spaced at 4 mm
apart. The screen 110 consists of a vertical foamcore sheet with an
attached plot of vertical lines spaced 4 mm apart against which the
shadow of the target can be evaluated. The deviation from true
collimation is recorded, and then the target 100 is shifted
laterally by 2 mm to increase the resolution of the measured
collimation performance to an effective sampling interval of 2
mm.
[0022] Referring to FIG. 6, the alignment of one embodiment of the
laser line generator 42 is described, using a procedure to obtain a
collimated 8.5'' wide sheet of laser light. The laser line
generator 42 is assumed to be complete, with the exception of the
parabolic mirror 76 and final alignment, i.e. all spacers are yet
to be inserted under the laser plate, the laser has yet to be
secured to the laser plate, etc. The alignment consists of aligning
the laser plate angle and position, and adjusting the parabolic
mirror assembly to obtain a collimated sheet of light. The laser
plate 84 must be aligned close to the correct, final angle. The
final position and angle of the laser plate 84 will vary slightly
due to differences in the parabolic mirror 76 (e.g. backing
plate/mirror angle) and in the case of slight angular offsets in
the flat mirror 74 on the emitter channel 86. For efficiency, a
batch of several laser line generators 42 should be aligned at a
time: [0023] 1) Mount the channel 86 to the steel angle fixture 120
on the optical table 130; [0024] 2) Adjust the laser plate 84
position for a nominal centred position, with no skew angle to the
laser diode 68 and lens 72. [0025] 3) Power on the laser diode 68;
[0026] 4) Place the alignment target 140 adjacent to the edge of
the channel 86 and verify the laser sheet 82 is exiting close to
the first horizontal line on the target 140. [0027] 5) Move the
alignment target to 400 mm range from the channel 86. [0028] 6)
Adjust the angle of the laser plate 84 until the laser sheet 82
rises 19.3 mm .+-.0.5 mm over 400 m from the edge of the channel
86.
[0029] The laser line generator 42 is to give a well collimated
(but slightly diverging) laser sheet 82. There should also be no
skew in the plane of the laser sheet 82.
[0030] The forward/backward position of the laser plate 84 adjusts
the collimation of the laser sheet 82, while the lateral
(side/side) or angular position (skew) of the parabolic mirror 76
adjusts the centre of the laser sheet 82 and the skew angle of the
laser sheet 82.
[0031] In one embodiment of the invention the parabolic mirror 76
consists of a thin glass mirror epoxied to a steel backing plate.
The mirror 76 is formed to a parabolic profile by a combination of
the steel backing plate which is machined to the parabolic form and
an aluminum parabolic form against which the mirror 76 is pressed
during the epoxy process.
[0032] The large format laser micrometer 10 shown in FIG. 5 is
defined by a light source unit 20 and a detector array 30. The
light source unit 20 is connected to a power supply (not shown) for
activating and deactivating the light source unit 20. The detector
array 30 is connected to a central processing unit (CPU) 34 (not
shown) for receiving and interpreting data received from the
detector array 30. The CPU 34 may be a dedicated hardware unit
provided with a summary display of the micrometer output, a
personal computer (PC) or a proprietary system. More generally, any
system that interprets and presents the results, preferably while
providing control options to the user, will suffice.
[0033] The light source unit 20 is comprised of a number of emitter
modules 40. Each emitter module 40 has at least one laser line
generator 42 (see FIGS. 1-3). In one embodiment the laser line
generators 42 are arranged in a stair-step configuration with a
slight overlap to eliminate any gaps between the laser line
generators. Each laser line generator 42 forms a sheet of light
equal in width to the laser line generator 42. The overall result
is a laser sheet with an effective width equal to that of the
emitter module 40.
[0034] In normal operation, each emitter module 40 will be either
on (emitting light) or off. However, a pulsed mode of operation
should also be provided to allow for alignment and adjustment of
the detector array 30 at a lower (less than saturated) signal
level.
[0035] The detector array 30 is comprised of a number of detector
modules 50. Each detector module 50 is comprised of a number of
linear CIS detectors arranged in a stair-step configuration to
match the laser line generators 42 in the corresponding emitter
module 40. An optical filter covers the CIS detectors to prevent
signal interference from ambient or stray light sources.
[0036] The detector array 30 also includes one or more data
processing units 32. As shown in FIG. 5, one data processing unit
32 is connected to three detector modules 50. The data processing
units 32 are used to receive, interpret, and transmit the signals
from the detector modules 50 to the CPU.
[0037] An object 16 passing between the light source unit 20 and
the detector array 30 causes an interruption in the path of the
laser light incident on the detectors. The resulting transition in
the detector is recorded by the data processing unit 32 and passed
to the CPU (not shown). The CPU then interprets the transition data
and reports it to the user, either in a raw form, or as a
calculated measurement of size, whichever is required.
[0038] While each detector module 50 may include its own data
processing unit 32, it is preferable to have more than one detector
module 50 coupled to a data processing unit 32, to reduce cost and
system bandwidth requirements. In one configuration, there are two
"slave" detector modules coupled to a "master" detector module, one
to either side. The "master" detector module houses the data
processing unit 32, which receives detector signals from the
"master" unit and the two adjoining "slaves".
[0039] While more "slaves" can be connected to one "master", there
will be a threshold based on the available data bandwidth for
transmitting signals. If the number of "slaves" is too large, there
will be signal loss at the data processing unit and gaps or errors
will result. In a similar vein, while a separate data processing
unit 32 could be used for all detector modules 50, the data
bandwidth requirements for the CPU 34 make this configuration
unsuitable for a detector array 30 with a large number of detector
modules 50. The described array using one "master" with two
"slaves" represents a balanced approach that should work with the
majority of detector array configurations.
[0040] The data processing unit 32 is the interface between the
detector array 20 and the CPU. The data processing unit 32 receives
timing signals and commands from the CPU and transmits transition
data and gray-scale "video" (if required) back.
[0041] With a standard detector at 200 dots per inch (dpi)
resolution, the detector resolution is 0.005'' per pixel. The
practical limit on the resolution is determined by the collimation
quality of the laser light sheet and ambient or spurious lighting
effects on edge definition. It may be preferable for the data
processing unit to ignore every other pixel to reduce the number of
spurious transitions and the data bandwidth requirements. The
result is an effective resolution of 0.01''.
[0042] The data processing unit 32 requires some logic to account
for conditions that produce a large number of transitions in a
single scan line. For example, if a sharp edge of the object being
measure is coincident with the longitudinal axis of one of the
detectors it would result in a gray edge, a series of pixels
rapidly exchanging between ON and OFF states, producing numerous
transitions reported from the data processor. This could result in
an overload of the data buffers and a consequential loss of data
from the scan line and subsequent scan lines.
[0043] A second potential problem is excessive or false triggering
resulting from interference from dust and other small particulates.
Again, the repeated random transitions could overload the data
buffers and result in a loss of subsequent data.
[0044] The solution is to provide for a user-defined value for
transitions below which the transition should be ignored. For
example, setting the value to one means that single pixel
transitions are ignored i.e. a neighboring pixel must also undergo
a transition at the same time for the transition to be recorded and
the transition data transmitted.
[0045] The data buffer problem must also be considered in the
context of available bandwidth both to and from the data processing
unit 32. If each detector module 50 has a data processing unit 32,
bandwidth to the data processing unit 32 is not a problem, however,
bandwidth from the data processing unit 32 (to the CPU) becomes a
larger factor. The combination of "master" and "slave" detector
modules alleviates the situation, however, too many "slaves" and
not enough "masters" creates the opposite scenario, in that
bandwidth to the data processing unit 32 is now at a premium, and
bandwidth from is not a concern.
[0046] The limitations on the system, therefore, lie in the
bandwidth capabilities of the data processing unit 32 and CPU. One
"master" and two "slaves" is presented herein as a example that
provides efficient data handling capabilities. Obviously, systems
with a higher bandwidth can use a larger "slave" to "master" ratio,
which will permit a larger array, fewer "masters" to reduce cost,
or both.
[0047] The laser micrometer 10 is intended for profiling,
inspection, and process control applications where the outside
profile and/or location and size of holes is required. Typical
applications are:
[0048] Large punched sheet metal part inspection
[0049] Web guiding and detection of transmissive defects
[0050] Lumber gaging--board length and width, log diameter
[0051] The laser micrometer 10 provides significant advantages when
compared to existing discrete photodiode based systems:
[0052] No crosstalk between sensors that limits measuring
resolution and scan rate
[0053] True 0.005'' pixel spacing
[0054] 2 KHz scan rate independent of system width
[0055] 36 edges detected simultaneously per 72'' of width in
standard configuration--more in custom versions.
[0056] In the preferred embodiment of the laser micrometer 10 the
emitter modules 40 produce a visible light laser sheet made up of
three overlapping 8.5'' wide collimated (<3mr) beams, which
provide complete 24'' wide coverage per module.
[0057] In the preferred embodiment the detector modules 30 are made
up of 200 dpi CMOS arrays, a master module with an optional slave
unit on either side, and fibre optic control and data connection
between the master module and the hub.
[0058] In the preferred embodiment of the micrometer 10 the hub is
made up of a Motorola MPC8260 PowerPC based motherboard, a VxWorks
Operating System, a fibre optic interface board which supports up
to 4 Master detector modules, a fibre optic quadrature encoder and
choice of two fibre optic object `detect` inputs, a fast Ethernet
connection (RJ-45 or optional fibre optic) to an NT host.
[0059] The laser line generators 42 may be useful, individually or
in combination, in a number of applications beyond the
above-described micrometer.
[0060] Accordingly, while this invention has been described with
reference to illustrative embodiments, this description is not
intended to be construed in a limiting sense. Various modifications
of the illustrative embodiments, as well as other embodiments of
the invention, will be apparent to persons skilled in the art upon
reference to this description. It is therefore contemplated that
the appended claims will cover any such modifications or
embodiments as fall within the scope of the invention.
* * * * *