Diffracting-based encoder resolves to increase accuracy

Can imperfections bring you closer to perfection? They can by measuring those imperfections accurately.For instance, using the interference between 1st diffraction orders, a patented encoder technology from MicroE (Natick, Mass.) can produce nearly perfect sinusoidal signals from a photodetector array inserted in the fringe pattern.

10/01/1998


Can imperfections bring you closer to perfection? They can by measuring those imperfections accurately.

For instance, using the interference between 1stdiffraction orders, a patented encoder technology from MicroE (Natick, Mass.) can produce nearly perfect sinusoidal signals from a photodetector array inserted in the fringe pattern. The signals are then electronically interpolated, allowing detection of displacement that's only a fraction of the optical fringe period. This means more precise position sensors, which means more accurate, less expensive, and smaller motion control systems.

A laser diode serves as the encoder's light source, and its diverging beam is first collimated by a lens and sized by an aperture. The beam passes through a phase grating, which diffracts the light into discrete orders and also suppresses the "0th" order of diffraction and all even orders (see diagram). 0th order suppression creates a region beyond the diverging 3rd order where only the

A photodetector placed in this region will produce four channels of sinusoidal output that are also nearly pure when relative motion occurs between the grating and the detector. Standard electronics can then be used to amplify, normalize, and interpolate the detector's output to a desired resolution level.

Simplicity allows precision

MicroE reports its diffraction-based encoders can deliver 0.6-nm resolution at a range of about 0.25 m. The company adds that the simplicity of its design produces several advantages over both Michelson interferometers and traditional chrome-on-glass encoders. First, the MicroE encoder's ability to secure measurements with only a laser diode and collimating optics—the diffractive phase grating and detector array—results in a compact encoder that can be inserted into many applications typically too small for conventional encoders.

Second, MicroE states that the direct relationship between the grating and the fringe movement desensitizes its encoder from environmentally caused errors that affect Michelson encoders. Third, using interference in the

In addition, because the region of interference is relatively large and nearly sinusoidal interference is obtained everywhere in it, MicroE says alignment tolerances are an order of magnitude more relaxed than those required by either of the competing technologies.

For more information visit www.controleng.com/info .





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