 |
Description  |
|
|
BACKGROUND
Referenced in this application are:
1. U.S. Pat. No. 4,373,804;
2. Copending application by the inventor entitled "Robotic Inspection
Improvements", Ser. No. 657,260 filed Oct. 3, 1984;
3. Copending application entitled "Triangulation Data Integrity", Ser. No.
657,261 filed Oct. 3, 1984; and
4. Copending application Ser. No. 207,081, now U.S. Pat. No. 4,453,082.
the disclosures of which are herein incorporated by reference.
Triangulation based optical sensors described in the above references are
very useful for the measurement of contoured dimensions on parts such as
sheet metal, body panels and the like. Such sensors usually have an
operable range, for example say 10 mm over which they can obtain accurate
distance data (say to 0.01 mm) from a standoff to a surface of say 50 mm.
When used in a manually positioned mode, these sensors can however have
certain disadvantages. First, many are constructed with infra-red light
sources whose spots, lines or other zones projected on the part cannot be
seen directly by the operator.
Second, even when visible sources are used, the range of the part to the
sensor is not often indicated except by looking at a display which can be
difficult for the operator to use. In addition, it is difficult for the
operator to known when he is within the range of the sensor to take valid
data since all such sensors have limited ranges over which they are
effective, eg. 10 mm in the above example, or even up to 25 mm where
lesser resolution is provided.
For this reason, the invention discloses several techniques including both
visual and aural for obviating these difficulties and providing workable
manual mode sensor operation. While generally of most use with optical
sensors, many of the techniques disclosed herein can be used with other
non-contact ranging sensors such as ultrasonic, inductive, etc.
Furthermore, use is not limited to coordinate measurement machines but can
be used for machines in general such as milling machines. In addition, the
disclosed ideas are of use on automatically servoed measuring machines to
keep the operator informed and are not limited only to manual operation.
The invention is illustrated in the following embodiment:
FIG. 1 is a schematic elevation view of a basic triangulation sensor based
embodiment of the present invention.
FIG. 2a is a schematic view of two reference detectors according to the
present invention.
FIG. 2b, 2d, and 2e are graphical representations of outputs of detectors
according to the present invention.
FIG. 2c is a schematic representation of zone image focus on a detector.
FIG. 3 is a schematic plan view of a semi-automatic embodiment of the
present invention.
FIG. 4a is a schematic plan view of a twin beam projection embodiment of
the present invention.
FIG. 4b and 4c are schematic representations of two range determination
methods of the present invention.
FIG. 1 for example illustrates a basic triangulation sensor using an
infra-red light source such as diode laser 1, focusing lens 2, imaging
lens 3, scanning photodetector array 4 and reference detector 5 in housing
6. A zone of light such as spot 10 is formed on part surface 11 and imaged
by lens 3 to form a spot image 15 on array 4 and, via beam splitter 20,
spot image 21 on reference detector 5.
Optional audible signals via audio generator 30 are returned when the part
11 is within (or alternatively not within) the desired measurement range.
An in-range condition is determined by microcomputer 40 either from
reference detector 5 or from a detector array 4 (e.g. a Reticon 256C) or
other equivalent image position detector output (e.g. an analog continuous
shuttling barrier type such as the UDT SC-10). Range is determined from
the array signal. A suitable photo detector array is a Reticon 1024C
linear photodiode array.
The primary intent of this device is to signal an operator 55 manipulating
the CMM arm 60 (to which the sensor is attached) when satisfactory data
can be read. As long as the detector is within range, an indication (or
optionally no indication) is produced. When it begins to get out of range,
a warning signal comes on. The data from array 4 is typically combined by
microcomputer 40 with sensor position as determined from encoder 61 on
member 60 to produce a combined result indicative of part surface
position.
Optionally a variable pitch or pulse rate audible signal can be provided by
generator 30 under control of microcomputer 40 to indicate that one is
getting either closer to the nominal range position or farther away from
it for example by decreasing the rate (or frequency) as one gets away from
nominal on either side until out of range (where no signal would normally
be provided).
Also optional is a light type indicator 50 to indicate that one is within
range. A pulsing light signal can indicate one is getting closer or
farther away by a change in the pulse rate. This light signal is visible
ideally at the end of the sensor (eg. 51) so that the operator can view
both the part and the end of the sensor simultaneously.
A change in color of the light source can also be used, e.g. by using red
and green LEDs 51 to indicate.
It is noted that the audio signal is generally best for surface contour
scanning, since the operator can then look wherever he pleases,
concentrating on providing the scanning motion required to keep the sensor
within range as it traverses the part surface.
The reference detector whose function and design is well described in
references 2 and 3 above, can be used to easily give a simple "in-range"
signal. If the reference detector voltage exceeds a threshold, the sensor
is in range, assuming the reference detector has a similar field of view
to the detector array--i.e. that an image spot hits the reference detector
when it also in on the array. (Note that reference detector 5 could be
incorporated with array 4, but no array chips now have such built in
reference detectors.)
For trend purposes (e.g. going out of range), this data could be derived
from the detector array output (distance to part) or by using a modified
reference detector as shown in FIG. 2a.
FIG. 2a illustrates two designs of reference detectors (5 and 20) in which
the spot image decreases the output of the detector as it approaches the
edges of the detector from nominal standoff illustrated schematically by
center line 25--i.e. the equivalent out of range condition as on array 4.
Detector 5 illustrates use of graded neutral density filters 101 and 102,
with highest attentuation near the detector ends, while the detector 20 is
shaped such that portions of the spot image 21 fall off at the ends. In
either case, the signal, such as voltage V out of the detector drops as
shown in FIG. 2b.
It is noted that the fall off function is provided naturally if the
reference detector is typically positioned to receive the same amount of
light as the main detector and is relatively narrow. Its signal can be
used to give a somewhat range determinate signal, i.e. when light level on
the detector is equal or greater than a value, the sensor is in range.
Light level decreases on the reference detector when the sensor is away
from its nominal standoff (generally the point 21' at which the incident
beam is best focused on the part and generally the point 21" where the
zone image is best focused on the detector). This is shown in FIG. 2c.
It is noted that the use of reference detectors with variant light level
output, however, is difficult as they can confuse the light power control
system (unless readings are taken with the system deactivated for the
purpose) attempting to maintain constant image spot intensity on the
photodetector array, unless steps are taken to control the light power
also in concert with range as well as part surface reflectivities
considerations.
To obviate these difficulties it is of interest to use a reference detector
of the continuous spot position monitoring type, such as a UDT SC-10.
These continuous detectors give a light level output (constant over the
range) from one port, while providing from a second port a voltage output
as shown in FIG. 2d indicative of position of the image zone on its face.
This can give even a two axis position indication if desired, although one
axis is generally enough.
Such detectors have high frequency response, and can, if desired, be
coupled to a machine control to automatically keep a sensor within
operating range, freeing the main detector (e.g. a detector array) to take
accurate position data.
It is noted that a reference detector itself can be a photodetector array,
preferably operating at a higher scan speed than the main detector such
that the approximate range and light intensity derived from its output can
be used advantagously to control light power, sensor position or main
array integration time for example. As an example, a Reticon 128C could be
used as the reference with a 17284 used as the main array.
Shown in FIG. 2e is an example of a possible audio output of generator 30
wherein frequency drops off as the sensor approaches the out of range
condition (where sound is extinguished). While the reference detector
conveniently provides this data, it could be derived from the
photodetector array based range data as well. Frequency is constant over a
give "good data" range about nominal.
FIG. 3 illustrates an embodiment of the invention for scanning a part;
Triangulation sensor 300 according to the invention is mounted to a
moveable member 320 of machine 301 having base 302 where part 305 is
located.
Horizontal arm 320 is movable horizontally and said arm position is
determined by encoder 308 connected together with sensor 300 to controller
310. Arm 320 generally is movable vertically along a vertical member 330
with vertical location encoded as well (as in the case of member 60 and
encoder 61 in FIG. 1).
In operation the operator often follows this procedure. He moves the arm
vertically downward until the sensor picks up the part surface at the
highest point, eg. point A, and he continues to move the sensor downward
until the sensor is nearest the part but still within its useful range. He
then locks the vertical axis and moves the sensor arm 320 horizontally to
sweep the surface until the sensor starts to go out of range, e.g. at
point B, creating a contour plot for example 311, using the sensor output
and the horizontal encoder postion.
As the range limit `B` is reached, the operator lowers the arm 320 on the
vertical axis again and continues the scan in the same manner. The
computer, if provided, puts the pieces of data from each sweep together if
desired.
It is noted that the axes can be motorized, e.g. by motor 321, to allow the
arm to automatically scan. A stop can be programmed when the sensor goes
of of range, or the vertical axis can be motorized as well.
FIG. 4a illustrates an embodiment of the invention used to provide a visual
indication on the part surface 195 itself.
Sensor 200 for example like 6 in FIG. 1, but with a visible laser source,
projects a visible laser beam 202 onto part 195.
In the first instance, a light beam projector 205 comprising incandescent
bulb 206 is focused by leans 207 to form spot 208 on part 195. The angle
.theta. is set up so that as long as the operator observes this spot to
overlap the main spot of laser beam 202 the sensor is within its operable
range.
Where the main beam 202 is invisible, a second visible projector 216, like
205, can be used to project second spot 218 onto the part. The angles 0
and 0 are adjested so that the spots 208 and 218 overlap when the sensor
is in range and are separated when out of range, as schematically shown in
FIG. 4b.
Possibilities other than overlap are possible, for example holding spots
218 and 208 within same zone. Indeed, as shown schematically in FIG. 4c, a
zone of limit lines 220 and 221 or spaced spots can be projected by one of
the projector units, such that the laser spot 202 or other line or spot
projector indication falls within lines 220 and 221 when the sensor is in
range, and to a near or far side from the projector when out of range.
For example, lines 220 and 221 can be projected by putting a suitable 2
slit reticle in front of bulb 206 and spot 202 can be maintained within
the lines by the operator to keep the sensor within range.
* * * * *
|
|
 |
|
 |
Description |
|
