Can I Measure Up on Color Variation?

I want to use machine vision to measure color variation and perform real-time control on our paint nozzles. I’m not an expert, so I need a machine vision system that will be relatively easy to design, install and maintain. Can I do this myself with assistance from the right vendor, or should I look for a system integrator that specializes in machine vision?

—from November ’07 Control Design

ANSWERS

Learn From the Integrator
[This comment was posted on Machine Builder Forum at ControlDesign.com/MBF]

This is a tough question, because it depends on your experience. If this is your first vision install, then use an integrator, ask for a guarantee of performance, and learn all you can.

In general, here are some things to watch for when “measuring” color.

1. Lighting is more important than any other component, so make sure the lighting is consistent in your operating environment.
2. Define how much color shift is allowed, how much is a fail condition and how you will test this vision system to see if it can detect your defined color differences. Can you produce “bad” samples at the edge of good and bad?
3. Get your quality people to sign off on a standard. This is tough, but it will save countless days of testing during startup.
4. Finally, you’ll want to do some homework on whether or not you need a relative color measurement or an absolute color measurement. There are ways to accomplish each, but the hardware and software are very different and not usually interchangeable.

GLENN ARCHER, director of business development,
Epic Vision Solutions

Not an Expert? Go With the SI

Cognex recommends that “non-experts” attempting to use machine vision for real-time process control, rather than simple pass/fail inspection, employ an integrator that has a successful track record of integrating similar systems.

JOHN LEWIS, public relations manager,
Cognex

Seeing is believing A color vision sensor connected to a PLC can measure everything from paint to marmalade. Photo by SICK

CVS Monitors RGB

A number of variables must be considered. It’s important to know whether problems occur when the colors change from one product to the next, or if this application requires continuous color data with multiple inspections of one signal unit. Since the options vary in complexity and overall ability to distinguish differences in color, it’s crucial to know the reason for the additional inspection. 

Since the customer has asked for a simple solution that could be installed and maintained by people in-house, a color vision sensor with an analog or serial output connected to a PLC would be ideal for this application. The CVS monitors the RGB values and triggers an output if the reading varies too far from the desired color.  Plus, the CVS has its own output based on a taught color. If the color fades or changes too much, the CVS can directly signal a paint controller to stop spraying.

JIM ANDERSON, machine vision product manager,
SICK

One Step at a Time

The application you describe can be done with assistance from the right vendor—one that offers machine vision testing lab facilities and can provide automation integration services. You should expect several things from a competent supplier.

First, to ensure the success of your machine vision application, an initial site survey is needed. This helps document all aspects of how your inspected object arrives at the inspection, how it’s presented to the camera, space limitations of the inspection area and speed of the line. It provides a reference to the critical parameters involved. Most vision suppliers ask 50 or 60 questions to simulate your operation and prepare an accurate assessment for proof of concept prior to recommending a system to test on your line.

Second, along with the site survey, you can help your supplier by providing a video and/or still photos of the process. Provide samples of good and bad pieces, identified with the problem in cases of bad pieces, to evaluate at proof-of-concept. During proof of concept, your supplier verifies the required elements necessary in the inspection process, identifies the illumination requirements and confirms the system’s capability to meet the specific application demands. After that, engineering identifies associated elements necessary for the inspection to generate a bill of materials and quotation for the solution.

Third, your application is highly dependent on proper and consistent illumination. Expect to be asked to identify QA/QC standards for color, how you implement them, and how you adhere to stipulated standards such as the color temperature of lighting, acceptable tolerance ranges and the frequency of standards testing to ensure lighting and paint composition are within spec. They also should ask about consistency of object presentation relative to the cameras. For example, the various shapes of car doors and body panels may require different areas of inspection and lighting to ensure accurate inspection of paint color.

Fourth, to integrate the inspection system in your control system, your supplier should ask about system dynamics and process lag, the plant’s environment, compliance with required protection or safety standards and adherence to preventive maintenance guidelines.