Is an Imaging Sensor Sensible?

How Will Precision and Accuracy Compare to Using a Presence Sensor?

By Control Design Staff

Offering customers the latest technologies is one way we try to make a name for ourselves, and improve their operations. We've been pretty traditional with our presence sensors, but suppliers are pushing imaging solutions more aggressively since cost is becoming less of an issue. We like the added versatility an image sensor might offer. However, the machines operate in an environment that varies significantly in temperature and humidity, so we're unsure how precision and accuracy will compare. Anyone have observations or experiences to share?

—From May '13 Control Design

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Answers

Careful What You Wish For
The question, "Do you offer a vision system?" comes up frequently. I believe this is because most people look for one solution to solve as many problems as possible. And in everyday life situations, it makes perfect sense to approach a problem in this manner.

Apply this thinking to an industrial automation process and one might conclude that a vision system is the most logical choice for a wide array of applications.

This couldn't be further from the truth. For the vast majority of industrial applications, a vision system tends to be overly expensive, unnecessarily complex and simply overkill for a large number of applications.

Industrial vision systems definitely have a place in certain applications, especially when the requirements are too complex for traditional presence sensors.

While one vision system could be versatile enough to solve a multitude of applications, it is not without many limitations of its own, making traditional presence sensors, more than ever, advantageous for many reasons.

Presence sensor advantages include:

  • Low initial investment. The cost of a presence sensor combined with the installation time is very low. Even though the cost of a vision system has decreased over the years, there is still a considerable amount of time required for testing, installation and personal training.
  • Special illumination is not a requirement for presence sensors. Without the appropriate illumination, the vision system will not perform as intended.
  • Presence sensors serve a simple purpose: object presence or object absence. An intimate understanding of the overall production process, in-depth knowledge about the product to be detected and high-level programming skills are not typical requirements. In most cases, you power the sensor and wire the output to the appropriate location.
  • High operating range. Continual innovation in photoelectric and ultrasonic sensing technology allow for the operation of presence sensors at extremely high operating distances, well beyond 1,000 ft.
  • High environmental resistance. Presence sensors are well equipped to deal with the numerous and oftentimes unfavorable environmental conditions found in even the most demanding industrial settings, operating within a wide temperature range, and powering through airborne contaminants such as dust or fog.

Rather than focusing on the application solution in its entirety, consider breaking it up into smaller, more manageable pieces. By breaking up the process into smaller tasks, extremely complex solutions start to become a little simpler and oftentimes, are easily within the capabilities of a traditional presence sensor and corresponding PLC.

I strongly believe that a vision system be recommended only after all other options have first been exhausted.

Michael Turner,
product manager,
Pepperl+Fuchs

[Editor's note: The following are responses to posting this problem on LinkedIn's Industrial Automation and Controls Network.]

Operating Variables Usually Not the Issue
As a distributor of sensing solutions (including image sensors), I can relate to this reader's position. However, this is what jumped out at me: "...the machines operate in an environment that varies significantly in temperature and humidity, so we're unsure how precision and accuracy will compare..."

Unless the humidity and temperature range is fogging a lens and/or heating up the part to a point where it starts throwing off photons, I don't usually see temperature and humidity as the issue. Maybe this application is precision gauging? I'd have expected to see something related to repeatable fixturing and ambient lighting before hearing concerns with temperature and humidity. The machine vision camp is saying, "Sure, easy." The traditional sensor folks say, "Don't use more technology than you need." Both can be true.

Jim Hausch,
account representative,
Power/mation

Sensor Setup Simpler
Don't overlook the cost of time required to setup and configure the sensing device. A photo sensor or proximity sensor installs and aligns quickly. A vision system usually requires some configuration and setup beyond simple alignment.

Also, do your techs have the tools and training to configure the vision systems?

In the right applications, vision systems are excellent, but as a presence sensor, a simpler solution has advantages in cost and time. I once ran into an application where the controls engineer had used a photo sensor to trigger a vision system to detect the presence of a product on a production line. Huh?

Layne Carruth,
president,

Control Systems Engineering

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