More about scales for a DRO. Magnetic and capacitive scales are analog while the glass scales are digital. This means that the position of the readhead is done differently depending on the scale. The magnetic and capacitive scales have a signal that changes gradually as the readhead passes by. Because of the way the signal changes the resolution can theoretically be very fine, much finer than what the display shows. But because the signal is also subject to electrical noise, which must be filtered, the practical resolution is much lower than theoretical resolution. Capacitive scales are so noisy they are limited to a practical resolution of .0005. Magnetic scales are way less noisy so the practical resolution can be much finer, even ten times finer. When I say the scale is noisy what I really mean is that the way the scale reading system works is noisy. Also, because of this electrical noise capacitive scales tend to not be as repeatable as other scale types. Magnetic and capacitive scales require some pretty complex electronics to be read, part of the reason for the noise in the electronics. On the other hand, glass scales are truly digital. They have discrete lines on the scales that are read by passing light through the scale. The light is either blocked or not. So the signal is either on or off. This means the scales are simple to read. It also means they have a certain fineness of resolution that cannot be made finer. They are typically read in quadrature because they have two LEDs and two photo receptors to do the reading. The LED and photo receptor pairs are staggered so that as the readhead is moved pair A senses the leading edge of a line on the scale, then pair B senses the leading edge, then pair A senses the trailing edge, then pair B senses the trailing edge. So, 4 simple on or off signals per line. This divides the spacing of the lines by 4, meaning that the scale can be read to a resolution 4 times finer than the marks on the scale. So even though the scale resolution does not change a finer resolution can be had by how the scale is read. The accuracy of the scale depends on the accuracy of the lines on the scale. These days it is possible to make very accurately placed lines on the scales for very little cost. Because the position of the lines on the scale cannot change, because no interpolation is done, glass scales repeat. You can always count on the readout showing the same position when the readhead is moved a certain amount. Even if the scale is inaccurate the inaccuracy will always be the same. Because the output from a glass scale is purely digital, just on or off, the signal is easily used for other things, like adding another display in another location so that you can have two readouts at once. Or the signal from the scale can be used to position a stepper motor.
Eric