Should We Use Laser-Based Sensors?

Readers help a reader solve this problem. Next: Can we go wireless in hazardous areas?

 

A Reader Writes:

We have a possible application to use laser-based sensors in a high-precision (10 um) triangulation solution with a range of about 2 ft. It seems that this is a borderline range, and perhaps a less-expensive LED-based solution makes more sense. I'd like some advice on the pros and cons of both, or an alternative to these solutions, if there is one.

--from February 2003 Control Design

Answers:

Exceeds Any Reasonable Limits

This application is impossible with the standoff you propose. It really boils down to the technical limitation and physics.

...Normally to measure off of a shiny surface at that resolution, you would need to be no more than an inch or so from the surface. It is important to direct a precise coherent laser beam off the surface and form an optimal reflection into the imaging optics, which refocus the light onto the PSD or CCD element.

...Maybe a interferometer would work but I do not have a lot of experience with that style measurement device. There is an outside chance that our SLS-5000 would work but I would only recommend up to a 100 mm standoff.

Michael Snow, Regional Sales Manager

LMI Technologies (USA) Inc., Cumming, Ga.

Can't Do What You Ask

Your reader may be asking for something that is impossible to deliver. Despite the fact that our company makes more than 12 sizes of laser triangulation sensors, we know of nobody who can deliver 10 µm accuracy over 2 ft. Your reader is about an order of magnitude off. Also, we know of no LED sensor that can get anywhere near the accuracy of laser. A laser interferometer may be able to measure the relative distance changes within 10 µm over 2 ft.

David Kepniss, Marketing Manager

Acuity Research, Menlo Park, Calif.

Where Else to Look for Help

The above answers are typical of what we received. We were unable to clarify the problem statement further, so the best thing we can do in this case is to direct readers to some basic technical papers about the design, performance intentions, and applications of laser-based triangulation technology.

The LMI Technologies web site (

www.lmitechnologies.com) has a white paper by Walt Pastorius, "Triangulation Sensors, an Overview" that describes the benefits of this non-contact sensing technology, namely better data collection rates, no probe wear or bounce considerations, and the ability for larger standoff distances. The paper covers the basic principles of triangulation, defines terms, and discusses surface effects

The Acuity Research home page (

www.acuityresearch.com) directs visitors to "Laser Displacement Sensors Explained," where they'll find definitions; a discussion of accuracy, repeatability, and resolution; an explanation of laser spot size and divergence; a section about visible and infrared lasers; methods for measuring the distance; and the performance of optical Sensors on specular and diffuse targets.

In addition, there's a white paper, "Online Geometry Measurement of Extruded Tire Components," on the web site of Bytewise Measurement (

www.bytewise.com) that details the use of triangulation-based sensing solutions for a more macro-scale application measuring tire profiles.

August's Problem:

Can We Go Wireless in Hazardous Areas?

We've been providing wireless communications solutions as alternatives where additional device-level nodes are needed in difficult-to-wire locations. We've not been involved with hazardous locations until now. Can we use wireless in Class I, Div. 1 and Class I, Div. 2 areas?

Send us your comments, suggestions, or solutions for these problems. We'll include them in the August 2003 issue. Send visuals, too,a sketch is fine. Have a problem you'd like to pose to the readers? E-mail us at CDTheAnswer@putman.net or mail to The Answer to Your Problems, CONTROL DESIGN, 555 W. Pierce Rd., Suite 301, Itasca, IL 60143. You can also fax to 630/467-1124. Please include your company, location, and title in the response.

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