Brown & Sharpe's K Series optical CMMs use advanced
optical triangulation technology to gather accurate
dimensional data quickly and efficiently in any production
These compact, portable measuring systems combine
three CCD cameras in an environmentally stable carbon-fiber
frame with an ergonomically designed, LED-driven probing
device to measure large parts, such as sheet-metal
assemblies, dies, molds, car bodies, automotive interiors
and fixtures, where they're located on the shop floor.
Optional LEDs can be attached to the part, and software
can be added to the system to transform the CMM into
a tool for dynamic measurement. With this option,
the motion of parts within their assemblies, such
as the movement of a door in an automobile body, can
be easily measured, calculated and graphically displayed.
This new technology will compete in the market now
served by articulated-arm CMMs and laser trackers.
Compared to arms and trackers, K Series optical CMMs
provide easier setup, fixturing and operation, along
with increased accuracy, measurement speed and overall
efficiency. These CMMs also eliminate the recurring
problem of accidentally interrupting a laser beam
during measurement--which requires the operator to
begin the measurement again.
Operators measure workpiece features using a hand-held
Space Probe equipped with nine infrared LEDs. The
operator records dimensional data by touching the
workpiece feature with the probe, triggering its LEDs
and sending a signal back to the CCD cameras. The
data point is recorded through the triangulation process.
Using the dynamic referencing feature, a workpiece's
initial alignment is saved by placing LEDs directly
on the part. Even if the workpiece and the system
are moved, the part can be realigned automatically,
saving time and improving inspection throughput.
K Series optical CMMs are available in two models.
The K400 has a measuring volume of 160 ft3 (4.5 m3);
the larger K600's measuring volume is 600 ft3 (17
m3). Additional temperature compensation utilities
are included in both systems to deal with changing
The K400 and K600 optical CMMs exclusively use PC-DMIS
measurement and inspection software. This comprehensive
software includes a suite of analysis and reporting
tools and an adaptable shop-floor user interface.
PC-DMIS is fully compatible, through its Direct CAD
Interface option, with most major CAD systems. This
allows complex part programs to be constructed using
the part's original CAD design data.
The K Series includes both portable and mobile configurations.
The portable configuration includes a camera, controller,
Space Probe, portable industrial PC and industrial
camera tripod. The mobile configuration includes a
camera, industrial mobile camera trolley and a mobile
workstation complete with preinstalled controller,
PC, printer, Space Probe holder and universal power-supply
With the automotive industry's
continuing push to reduce inspection cycle times, traditional
tactile probe and data-gathering technology is often too
slow to support most full-body and body-assembly inspection
requirements for timely process-control applications. Consequently,
automakers and coordinate measuring machine manufacturers
are continually evaluating improved methods for gathering
and analyzing dimensional data.
One new approach focuses on high-speed scanning using
CMMs that automatically measure the shape and form of workpieces.
The CMMs then incorporate the information with CAD systems
to provide insight into the manufacturing operation. High-speed
scanning is simply a way of automatically collecting a large
number of data points to define a part's shape quickly and
accurately. Such noncontact gaging--when used for subassembly
and body-in-white inspection applications--offers accurate
real-time monitoring of quality output from the production
line as well as the flexibility to handle line changes.
For several years, CMMs have been used as in-line measurement
and inspection systems for body-in-white applications. In
many of these cases, however, CMMs are equipped with tactile
analog probes to gather dimensional data. These probes are
durable, heavy-duty electronic sensors that provide a high
degree of data accuracy, albeit at a relatively slow speed.
Scanning technology was initially developed during the
mid- to late 1970s. Beginning in the late 1980s, microprocessor-based
controllers and firmware were introduced. At about this
time, the RS-232 connection between the CMM and host computer
changed to Ethernet, and clock rates increased to 200 hertz.
Today, more PC-like controllers and 500+ hertz clock speeds
have made high-speed data manipulation possible. An Ethernet
connection between the CMM and the host computer can transfer
information at 100 MB/sec or faster. Because of these advances,
as well as the widespread use of finite element and modal
analysis in coordinate measuring, CMMs can now gather thousands
of data points per second using advanced sensor technology.
Corresponding with improvements in CMMs and controller
technology are improvements in the field of sensor technology.
In particular, two areas of noncontact sensor technology
for measuring car bodies and subassemblies have received
much attention: laser stripe scanners and optical sensors
that use a CCD camera combined with a suitable lighting
Both of these sensors integrate a light source and photoelectric
detector and work on the principle of triangulation. The
light source emits a precisely focused laser or infrared
light beam that strikes the workpiece, creating diffused
or scattered light that is then focused on a photoelectric
array. Any variation on the surface distance from the sensor
results in a change in the spot image's position on the
The technology has been successfully adapted to measure
all typical body-in-white features, such as surface point,
center of gravity, character points, edge point, large and
small holes, curvature radius, slot, square slot, and flush
Laser sensors work by projecting a strip of light on a
part's surface to make a virtual copy of an existing part
or shape. This virtual 3-D copy consists of a cloud of measured
points, each with its own XYZ coordinate in space; the point
cloud is used to perform inspections. Although laser sensors
are highly accurate data-gathering devices, they cover less
area during the data-gathering process than CCD-type optical
sensors. A typical laser sensor offers a field of view of
20 X 20 mm, a maximum resolution of 40 X 60 µm and
accuracy in the range of 15 µm. Laser sensors generally
capture 500 data points per line and scan at a speed of
25 lines per second.
The CCD optical sensor works by relating the pixels of
the image or picture taken by the sensor to the corresponding
3-D points of the framed part area. It allows a virtually
simultaneous reconstruction of all the points inside its
field of view. The sensor's scanning speed is directly proportional
to its field of view. For example, a typical optical sensor
of this type has a field of view of 59 X 42 mm, a standoff
distance of 117 mm, a field depth of 8 mm and a resolution
of 8 µm/pixel. The CCD optical sensor's broad data-acquisition
area allows inspection routines to be performed very quickly.
Compared to a tactile probe measuring the same feature,
the cycle time is significantly shorter: for example, 1.5
seconds for a noncontact sensor compared with 16 seconds
for the tactile probe.
Some noncontact sensors integrate a structured light source
that emits a plane of light. When this plane intersects
the part, a line of light forms along the part's contour.
The image sensor detects this line and transforms it into
a measurable digital image. Distance measurements can then
be obtained based on the shape and position of the line
on the part surface. Structured light sensors are able to
simultaneously triangulate and calculate the XYZ coordinates
of hundreds of points along the line.
Using optical CMMs is another approach to noncontact inspection
of body assemblies (see story on page 24). These CMMs offer
not only extremely fast data gathering but also portability.
Although noncontact sensors are used in a variety of measurement
and inspection applications, applying their technology to
the 3-D dimensional control of car bodies on the production
line has been challenging. The first relatively successful
application of the technology was with a stationary multisensor
inspection cell that uses a number of CCD camera/laser light
sensors to gather dimensional data. The system meets production
cycle-time requirements by simultaneously measuring a number
of body elements, determined by the number of sensors installed
in the cell.
However, the stationary multisensor approach has some
limitations. Due to the large number of sensors required
in each cell, it's expensive to manufacture, and maintenance
is often complex and costly. The system generates relative,
rather than absolute, measurement data and requires gold
masters that must be certified and carefully stored to support
the periodic certification of cell performance. In addition,
the system can't be easily reconfigured to meet varying
The solution lies in combining CMM flexibility with noncontact
sensor data-gathering technology. CMMs used for body-in-white
inspections are called "measuring robots." These
are generally designed as horizontal arms, the only structure
capable of reliable dynamics, high accuracy and the long
measuring strokes needed for full car-body inspection. The
design is also well-suited for adding a second arm to create
a multi-axis inspection cell. With such a configuration,
cycle time is dramatically reduced, throughput is maximized
and measuring errors are minimized.
Measuring robots are fast, accurate and rugged machines
designed to operate in the most severe production environments.
They can be easily integrated, in-line or side-by-side,
with production equipment. They also resist ambient shop-floor
conditions such as temperature gradients, airborne contaminants
and vibrations. Internal positive air-pressure systems keep
critical components clean. Machines are constructed from
materials that ensure an optimal rigidity-to-weight ratio
and react well to thermal variations.
The addition of an optical sensor to a measuring robot
is a solution that combines high-speed data gathering and
all the typical capabilities of a CMM, such as easy programming,
quick reconfiguration, high accuracy and full consistency
of results, with off-line CMMs equipped with traditional
probes. It meets production cycle-time requirements by using
the noncontact sensor to measure elements based on single-image
acquisition through single positioning, and by utilizing
the CMM's high-speed characteristics for car-body inspections
in production environments.
In a typical measuring routine--for example, the position,
diameter and orientation of a through hole in a car body--the
system executes the following steps:
Using the CMM's flexibility, the sensor is moved to a position
over the hole.
The sensor acquires an image of the hole, projecting it
over the part surface.
The 3-D position of the hole is calculated from the deformation
induced on the sensor light by the presence of the hole.
The system reconstructs both the hole plane and the hole
shape without repositioning the sensor or moving the CMM.
Probe orientation is critical for accurate, efficient
subassembly and body-in-white measurements. However, part
accessibility limitations often require frequent orientation
of the head during the scanning process, and unless the
sensor is mounted in an articulated holder, results in less
effective overall system throughput.
Continuous two- and three-axis servo wrists solve this
problem. These are designed to quickly orient the probe
to any attitude, following precise 3-D trajectories. The
system controller continuously monitors the servo wrists'
speed and motion to reach maximum machine efficiency. Servo
wrists guarantee full-part accessibility--by extension bars,
if necessary, for inside car-body measurements--with no
loss of accuracy. The sensor is functionally connected to
the CMM through the three-axis wrist and is recognized by
the CMM like any other standard probe; therefore, all sensor
functions are integrated and managed by the software. Probe
changers and magazines are fully supported. The sensor 3-D
measurements are referred to the CMM coordinate system.
Advanced software compensation routines ensure high probe-positioning
accuracy without the need for calibration.
Advanced measurement and inspection software has evolved
along with high-speed scanning CMMs, optical sensors and
probe holders. Software, such as Brown & Sharpe's PC-DMIS
measurement and inspection software, supports both laser
and CCD optical sensors. The CMM is controlled by a distributed
processing architecture that includes the controller and
PC-DMIS software manages all machine functions, the operator
interface, part programming and 3-D data analysis. It also
provides an interface between the sensor and computer for
efficient data exchange. PC-DMIS includes special data-analysis
tools for automobile body features, as well as special programming
that relates data from both arms of a dual-arm measuring
robot to a common axis. It calculates the most suitable
wrist angle for measurement, based on the theoretical location
and vector of a part feature. These features are accessible
through a graphical user interface that requires the operator
simply to point and click on the program and operation of
Brown & Sharpe is currently working with industry
groups to develop an optical sensor interface standard so
that any type of optical sensor will work effectively with
any type of CMM.
The ultimate value of metrology is its ability to forge
a link between design intent and manufacturing capability.
CMMs, particularly those capable of scanning, are an extremely
flexible and cost-effective way for manufacturers to take
advantage of the benefits of metrology for improved quality
and process control.
David H. Genest is director of marketing and communications
for Brown & Sharpe in North Kingstown, Rhode Island.
Genest has bachelor's and master's degrees in mechanical
engineering and has been involved in product engineering
and development during his career. His background in metrology
system design and development includes the introduction
of Brown & Sharpe inspection systems for shop floor
applications. He is a member of the Metrology Automation
Association board of directors. Letters to the editor about
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