Remote Optical View

Thanks for the prompt response.

The path would be curved, and an image size ratio of 1:1 would be desirable. In the interest of economics, I was thinking something that would transmit a light/dark pattern, that would accurately represent this gap, not necessarily a high resolution, with an external light source (flashlight).

My intent here is to allow the inspection of a required gap in an internal cable entry gland without disassembly of a machine,with the device permanently attached.

Here are two approaches using coherent fiber bundles, depending on the gap size and how much the gap distance varies. In the second approach, two less expensive fiber bundles are used, since only gap edges need be inspected. For gaps exceeding 1/8" this approach is probably more economical than using a larger bundle which spans the whole gap.

Note that if only one edge of the gap varies, then only one bundle need be used. The bundle diameter must be at least as large as the expected variance.

Note also that a flat coherent bundle may be used, since the gap distance varies in only one dimension. Depending on the precision with which the gap distance must be measured, a flat bundle may be assembled from individual fibers whose diameter provides a resolution of at least twice the required precision.

Europium,

The information you have provided is very helpful. Based on this information, I see that a go/no go type of indication would be most practical in this application.

To offer further detail, the subject machines are used in explosive mine atmospheres and as such, must be inspected for this required 1/8" gap. I am trying to offer a means to positively confirm this gap while avoiding the disassembly of the machine, without compromising the explosion-proof integrity.

Thank you for your time.

An alternative method would be to place a light source of constant brightness on one side of the gap, and a photodiode on the other. The amount of light reaching the detector would then be a function of the width of the gap. After a simple calibration you could determine the gap width by measuring the raw output of the detector.

You're most welcome!

Given the conditions in which your machine operates, a passive, non-powered solution is what you want. Fiber bundles meet this need quite nicely.

A few more questions, though:

• How much is the gap distance expected to vary?
• How much precision do you require for the gap measurement?
• Is the machine subject to severe vibration? If so you may opt to use plastic fibers or bundles made from them. Edmund Optics' bundles are made from fused glass fibers and could break under conditions of severe vibration. Plastic fibers generally have larger diameters than do glass or quartz fibers and thereby reduce the resolution of the transmitted image. However, they are flexible and generally much less expensive than glass.

One source for plastic & glass fiber optics.

Take care,

-e

In answer:

• The gap distance is set during assembly and is not expected to vary.
• To illustrate the degree of precision, the mine inspectors carry a length of 1/8 drill-rod to use as a "feeler gauge". They are checking for a MINIMUM gap distance of 1/8" and are not specific about the maximum distance, thus my reference to a go/no-go type measurement. This required gap stems from a time when all machines had threaded packing gland type seals,whereby the 1/8" gap would tell the inspector that the gland was properly packed. Although the subject machines use a flange type set-compression elastomer seal, the regulations have not been changed to accommodate this.
• Equipment in an underground mine is subject to shock and vibration loads.

The latest link you provided mentions "light pipe" which may be sufficient to transmit a light/dark pattern representing the gap.

Thank you.

Unredundant, here's your go/no-go light-pipe, and it's cheap, too.

Check out these 1 mm dia. stepped-index plastic optical fiber cables from Avago Technologies. This link directs you to Newark Electronics' page. Newark is a US distributor for Avago Technologies, who bought this product line from Hewlett-Packard. These cables are designed for data transmission between two optical transceivers, but this imposes no constraints on your particular application.

The cable costs $13.56 USD in quantities of one. It is a duplex (two-fiber), 2 m zip cable which you can cut to length with a pocket knife. From the pic I gather that it comes with connector hardware, but you need not assemble this to the cable in your application.

I've used this cable myself and it's easy to work with. And you can't beat the price.

Now that we have a better idea of your requirements, I think we've got you a cheap and effective solution. I hope this works out. Let's us know, 'K?

Newark part number: 74K5128

Avago part number: HFBR-RMD002Z

Europium,

Should I say the link to Newark was enlightening?

Plus, there was a sidebar add on the page for Banner retro reflectors!

I will keep you in the "loop".

Thank you.

You're welcome. Glad to be of help!

For your retro-reflector you may also wish to consider simple retro-reflective adhesive tape such as found in many hardware stores. 3M makes a wide range of reflective tapes in different colors.

Banner's reflector is pricey at 18+ bucks and may be more than you really need for a go/no-go indicator. Their reflectors are basically arrays of small molded-plastic corner-reflector cells. This may not be the best kind of reflector for an optical fiber whose diameter is on the order of the cell size because the reflected light does not exit the cell at exactly the same point at which it enters. For a 1 mm fiber the exit point may actually be outside the fiber's "field-of-view," so to speak, and so you won't get much light back unless the fiber and cell share a common centerline. With reflective tape you won't encounter such alignment problems.

Hi,

you need an illumination to make a high contrast of gap/components.

You need then an imaging optics from gap to detector.

This imaging optics may be a lens and a plane mirror (curved path needs the mirror) or an imaging mirror.

Or this may be consisting of the glass-fibers discussed by Europium.

Any dirt there?

RHABE

Hello RAHBE,

Yes, the light source would have to be external, perhaps a retro-reflective arraignment.

As far as contamination (dirt) it would only be on the external surface, which I don't see as a problem.

See my reply to Europium for further detail, and thank you for your input.