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Input / Output Devices
Linear Encoder Information & Linear Encoders
A typical linear encoder consists of a scanning unit and a scale. The linear encoder's scale is generally glass and is cemented to a support, usually an aluminum extrusion. Linear encoders also have a scanning unit, which contains a light source, photocells, and a second graduated piece of glass called the scanning reticle. This scanning reticle sits a short distance from the scale.
In operation, a parallel beam of light produced by the light source and lens passes through four windows on the scanning reticle, through the glass scale, and onto a set of photosensors. When the scanning unit moves, the scale modulates the light beam, creating sinusoidal outputs from the photosensor.
The four windows in the linear encoder scanning reticle are each phase shifted 90° apart. The linear encoder system combines the phase-shifted signals to produce two symmetrical sinusoidal outputs phase shifted by 90°. A fifth pattern on the scanning reticle has a random graduation that, when aligned with an identical pattern on the scale, creates a reference signal.
To obtain high resolution, linear encoders often use a fine-scale pitch. Because of the diffraction effects of the scale grating, spacing between the fixed scale and scanning reticle must be extremely narrow and constant. Consequently, the entire linear encoder scanning unit mounts on a carriage that runs on ball bearings along the glass scale. Linear encoders have a scanning unit , which connects to the machine slide via a coupling that compensates for alignment errors between the scale and the machine guideways.
In linear encoders for NC applications, external electronics interpolate sinusoidal signals from the linear encoder head. The effect is to subdivide the basic line spacing on the scale to gauge smaller increments of motion. Standard linear encoders using this principle come in a variety of form factors. Some linear encoders have small exterior dimensions while others fasten along the length of the scale with screws, permitting higher accuracy (up to ±2 µm/m). These linear encoders may incorporate a mechanical corrective device, which contains correction points at 100-mm (4-in.) intervals. This feature allows linear encoders to compensate for errors caused by imperfections in the guideways.
Linear encoders described above are limited to measuring lengths of about 3 m or so. Linear encoders that measure longer distances contain scales made from steel tape. A typical linear encoder has tape that is about 0.3 mm thick and contains a grating of highly reflective gold lines with a pitch of about 100 µm.
Linear Encoders: Linear Encoder Information
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