With so many vision sensors
available today, it can be a daunting task to figure out
which one is right for your particular application. Will
the product you're evaluating handle the variable conditions
that exist in your plant? What are the latest networking
capabilities to look for? What types of accessories and
product support options are important?
Whether you're brand new to machine vision or an experienced
user, this guide will help you during your vision sensor
selection process. It provides answers to 10 of the most
important questions to consider, and offers valuable tips
on how to evaluate specific product features.
Part location tools, available with virtually all vision
sensors, are software programs used to find parts within
the vision camera's field of view. This is typically the
first step in any vision application--from the simplest
robot pick-and-place operation to the most complex assembly
verification task--and the one that usually determines whether
the application succeeds or fails.
It sounds simple enough, but locating parts in today's
production environments can be extremely challenging for
vision sensors. This is because many variable conditions
exist that can alter the way a part appears to a vision
sensor, which is trained to recognize parts based on a reference
or "model" image of the part. Variable conditions
include part rotation, changes in optical scale, inconsistent
lighting conditions and normal variations in part appearance.
Network communications provide a number of important benefits.
First, they enable vision sensors to communicate pass/fail
results data to PCs at the enterprise level. Second, they
enable vision sensors to communicate directly with PLCs,
robots and other factory automation devices.
When establishing a communication link between vision
sensors and PCs at the enterprise level, make sure the vision
sensor you're evaluating supports a broad range of standard
network protocols, including:
SMTP--Simple Mail Transfer Protocol capability enables electronic
notification of problems that occur on the production line.
For example, if 10 consecutive parts fail inspection, the
sensor can send an e-mail to a computer, pager or cell phone.
This provides both emergency notification that the line
may need to be stopped and a second level of inspection
FTP--File Transfer Protocol enables users to archive failed
inspection images without writing custom software.
DHCP--With Dynamic Host Configuration Protocol, each vision
sensor you link to the network is automatically assigned
an IP address, enabling true plug-and-play performance.
Sensors without this capability require an IP address to
be manually assigned, which often involves having to ask
an IT administrator for an available address.
DNS--This allows you to name each vision sensor. Without
DNS, it may be a daunting task to keep track of all the
vision sensors running on the line and may require labels
to be physically applied to each sensor with the IP address.
TCP/IP client/server--This enables vision sensors to initiate
the transfer of results to other devices directly over an
Ethernet without any code development.
Telnet--This Internet standard protocol enables remote login
and connection from host devices.
When connecting vision sensors to factory automation devices
such as PLCs and robots, look for support for the following
Ethernet/IP--This protocol enables vision sensors to link
to PLCs and other devices over a single Ethernet cable,
eliminating the need for complex wiring schemes and costly
ModBus/TCP--Another factory network protocol that permits
direct connectivity to other devices over an Ethernet.
Finally, as more and more vision sensors are used throughout
the manufacturing process, it becomes important to have
a centralized way of managing them. Ensure that the vision
sensor you're evaluating will allow you to manage and control
vision activity over the network from remote locations in
the plant and beyond.
Make sure the graphical user interface you create
can perform the following tasks:
Display images with graphics to allow for immediate
analysis of failed parts
Enable operators to easily turn the inspection on
and off and modify parameters
Display pass/fail results data
Repeatability can be tested by presenting a part
to the vision sensor and measuring the part at least
25 times without changing its position, lighting or
any other variables. From this, you should be able
to plot the repeatability of the measurements and
make sure that any variance in the results stays within
the measurement tolerance.
To evaluate industrial code-reading tools, first
test for the sensor's read rate--the percentage of
codes the vision sensor has read from all the codes
it has "looked at." To do this, present
a well-printed code to the vision sensor and have
it read the code hundreds of times under pristine
conditions. Make sure the read rate is 100 percent,
or you could face problems later. For example, a read
rate of 99.7 percent means that one code out of every
350 is not read. At a production speed of 2,000 parts
per hour, the sensor could discard 2,700 good parts
Once you've established the sensor's read rate,
you should run a reliability test to understand how
factors like line vibration, variable lighting conditions
and excessive line speeds might be affecting reading
performance. To do this, present a large sample of
good, bad and marginal codes to the vision sensor.
This will provide a good assessment of how the vision
sensor will withstand the range of real-world conditions
it will need to contend within production mode.
Vision applications don't usually require extensive HMIs,
but operators typically need to interact with the vision
sensor in order to make modifications during part changeovers,
change tolerance parameters and determine the cause of part
The vision sensor should allow you to create a custom
graphical user interface as you configure the system. Make
sure that this can be done without having to utilize Visual
Basic or some other higher-level language.
Image preprocessing tools allow the user to manipulate
the raw image in order to highlight desired features or
eliminate undesirable features. This ability can be a key
factor in the overall performance of a vision sensor and
should be a part of the standard offering.
Look for products with a suite of image preprocessing
tools that will enable you to provide a range of functions
Improving the contrast between the edges of a part and its
Filtering out insignificant features
Eliminating reflections that have been cast off the part
Smoothing rough textures
By optimizing image data in its raw form, the overall
accuracy and robustness of a vision sensor can be significantly
Whether you're reading stamped alphanumeric codes on automotive
parts or verifying date and lot code information on medicine
bottles, there are several capabilities to look for when
evaluating character reading and verification tools.
Statistical font training--This capability allows you to
create a single model or "reference image" from
a series of images. This enables the sensor to better handle
the range of normal variations in print quality, whether
it has to do with poor contrast, placement variations, degradations
or variations in stroke widths. Unless you can be positive
that every label will be printed with the exact quality
seen in the model, the ability to develop a statistical
model can be crucial to the success of your application.
Image preprocessing tools--As mentioned earlier, these allow
you to optimize a trained model by sharpening the edge contrast
of characters and filtering out any extraneous background
noise that exists in the image. Having optimized models
maximizes the reliability and repeatability of the vision
Instant image recall--This enables line operators and technicians
to quickly and easily view failed images on the monitor.
Whether the failure is caused by a camera knocked out of
position or a missing label, it's important to know immediately
why a package failed so corrective action can be taken if
If your application involves critical dimensional measurement,
you'll want assurance that the gaging tools are accurate
and perform with a very high degree of repeatability.
In addition to testing for repeatability, it's a good
idea to make sure that the vision sensor has a full suite
of gaging tools. This will eliminate the need to write scripted
programs to develop functions that are not part of the standard
Today's vision sensors should offer reliable, repeatable
performance on 2-D codes that have been poorly formed, degraded
or those that vary in position from part to part. They should
perform well no matter the type of marking method your parts
are subject to (dot peen marking, etching, hot stamping
and inkjet are among the most common methods) or the part
surface type, e.g., glass, metal, ceramic and plastic.
In terms of specific code-reading features, you may want
to ask about the following:
Code quality verification capability--Look for products
that can verify code quality to established standards. This
provides valuable information about how well the marking
process is working.
Read-per-second rate--Depending on your production line
speed and throughput requirements, you may want to verify
a vision sensor's read-per-second rate. The fastest vision
sensors can read up to approximately 50 codes per second.
To ensure that your system integration process is quick
and painless, look for a vision sensor with its own family
of compatible accessories. This places the burden on the
vendor to test each accessory and confirm that everything
works together without any problems.
Accessories to look for include:
Lighting accessories--Because there are many part surface
characteristics and ambient lighting conditions to contend
with, your vision sensor vendor should be able to offer
a variety of lighting options. A comprehensive family of
light modules includes ring light modules, which provide
soft, even illumination from all directions; back light
modules, which offer maximum contrast between a part and
its background; darkfield lights, which provide low-angle
illumination for imaging of part surface irregularities;
Communications accessories--Make sure that communications
peripherals such as I/O modules and network gateway modules
are offered that will enable easy, quick connectivity between
the vision sensor and PLCs, robots and other factory automation
devices and networks.
Monitors--Some vendors offer a number of sensor-compatible
displays. When selecting a monitor, look for an LCD display
with antiglare impact shielding and NEMA-rated mounting
bezel that provides a dust- and liquid-tight seal when mounted
in an enclosure.
Camera enclosures--Industrial camera enclosures offer protection
from dust, high temperatures and wash-down and should be
easily mounted and able to accommodate a variety of camera
lens sizes and types.
Your vendor should offer a stand-alone vision sensor that
does not require a PC--during configuration or in production
mode. The sensor should offer true plug-and-play performance
that enables you to quickly configure the application, from
start to finish, right out of the box. Just as important,
the vision sensor should not require you to roll a PC onto
the factory floor every time changes to the application
need to be made. Finally, a true stand-alone vision sensor
should enable you to hook up a monitor for live image display
without a PC.
When evaluating vision sensors, look for a vendor that
offers a wide range of product support and learning services.
These services start with the initial assessment of your
application. Important questions to consider include:
Is the representative a full-time machine vision specialist?
How will the application be evaluated, and by whom?
Is the vendor willing to expend the engineering resources
necessary to qualify my application, or will that responsibility
Once selected, what product support is available to ensure
my installation is a success?
Does the selected vendor offer cost-effective training alternatives
such as online "self help" support, online courses,
worldwide technical support and personalized on-site training?
Does the selected supplier have the track record and financial
stability to maintain their role as a long-term vision solutions
It's also important to look for a vendor with a global
network of offices offering both pre- and post-sales support.
This way, you can get the same consistent high level of
product support anywhere in the world. This can be especially
important if the system is commissioned in one location
and shipped to another.
The Ten Most Common Vision System Problems
Evan Lubofsky is a senior writer with Cognex Corp. He
has a bachelor's degree in journalism from the University
of Massachusetts. He has written about machine vision and
imaging technologies for more than seven years. Lubofsky's
articles have appeared in more than 40 trade publications.
Prior to joining Cognex, he worked for Data Translation.
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© Copyright 2002, Cognex Corp.