Sealed linear encoders offer higher positioning accuracy

Dean Palmer takes a look at how linear encoders with improved signal quality can help manufacturers improve positioning accuracy in machine tool, handling and automation tasks

Linear encoders are typically used as position measuring systems on machine tools, in handling and automation technology and for inspecting equipment. But demands on these units are increasing, driven by the customer. In harsh operating environments, for example, sealed versions are required to protect the encoder from dust, chips and splash fluids. More recent applications for sealed linear encoders include direct drives and assembly automation. The popularity of linear motors, for example, requires increased traversing speeds and precision machining centres require high positioning accuracy and an improved tolerance to contamination. Heidenhain, a manufacturer of encoders, touch probes and other measurement equipment, has developed a range of slim line, sealed linear encoders that, according to Stuart Jenkins, UK sales and marketing manager at Heidenhain, "use a scanning principle that is characterised by significantly reduced sensitivity to contamination and higher quality of the output signal". The encoders operate according to the principle of photo electrically scanning a regularly structured measuring standard. The type of scanning is crucial for the quality of the output signals and therefore both for the positioning accuracy and for traversing speed. For the first time, Heidenhain has applied the single field scanning principle to its LS sealed linear encoders. A structured scale moves relative to an opposed grating - the index grating - with an identical or similar structure. The incident light is modulated: if the gaps are aligned, light passes through. If the lines of one grating coincide with the gaps of the other, no light passes through. Photocells convert these variations in light intensity into electrical signals. With four-field scanning, the scanning reticle has scanning fields whose gratings are offset to one another by a quarter of a grating period each. The corresponding photocells generate sinusoidal signals, phase shifted to one another by 90°. These scanning signals are not at first symmetrical about the zero line. The photovoltaic cells are therefore connected in a push-pull circuit, producing two output signals in symmetry with respect to the zero line and electrically phase shifted by 90°. However, in single field scanning, the scanning reticle has one large area grating, whose grating period differs slightly from that of the scale. This generates an optical beat along the length of the scanning field. At some positions the lines coincide and permit light through. At other locations, the lines and gaps coincide, causing a shadow. In between, the gaps are only partially covered, which causes a type of optical filtering which allows homogenous signals of a shape very close to a sine wave. Rather than individual photocells, one large area, specially structured photosensor generates the four, 90° electrically phase-shifted scanning signals. So Heidenhain's sealed linear encoder acquires its accuracy from the high quality of the line grating on the glass scale. Its grating period is 20µm and permits measuring steps down to 0.1µm and finer, which can be produced by interpolation. Jenkins says the new scanning optics have considerable influence here. "The large scanning field and the special optical filtering generate scanning signals with constant signal quality over the entire path of traverse. This is the prerequisite for small position error within one signal period, high traversing speed and good control quality for direct drives." The large scanning area over the full width of the scale grating and the arrangement of several scanning fields in succession, make the encoders insensitive to contamination. In tests, Jenkins explained that, even when contamination over large areas was simulated, "the encoder continued to provide high quality signals and the position error remained far below the value specified for the accuracy grade of the encoder".