I hate to sound like a complete fool but can someone tell me the difference between the glass and mag scale DROs?
While much of the information on Wikipedia is ok and commonly provides a quick introduction to technologies, many times it is inaccurate or outdated. For example, in the above link it states that magnetic scales are much coarser (low resolution) than they currently are. My PM Mag DRO on my PM1440GT have 1 micron and 5 micron resolution which is similar to the best of the optical scales. Any higher resolution would essentially be useless.
There are three main sensor technologies for these digital linear scales: Optical, magnetic, and capacitive. There are other types, for shorter scales or less accurate scales. These three, in some broad sense, all use lines on a substrate which are effectively counted as the scale is moved. Hence, you can think of them as a ruler where one counts the rule lines. But they are usually more complex having multiple rule lines of different densities. Anyway, they are all relative distance scales, meaning that they measure from one point to another. They are not absolute scales as they do not retain an absolute starting point, origin. The technique for detecting the lines as the sensor is moved over the lines can differ and can be simple or complex for more accuracy. 1) Optical measurements essentially uses visible or near visible light which is sent through, or reflected from, the glass substrate for detecting the lines. For a course optical scale this could be this simple. For a higher accuracy scale this commonly monitors multiple lines at one time and produces interference patterns on a detector of multiple elements. The resolution is limited by the physics of the optical wave length but is sometimes extended by non-digital extrapolation techniques. One cannot easily measure optical patterns that are much smaller than the wavelength of the light. Visible light has a wave length of 0.5 microns or larger (deep blue light is about 0.450 microns while dark red is about 0.800 micron in wave length). Optical disk, like DVDs, use blue or near blue wave lengths and marks are molded into the disk. The spacing between the marks contains the data information. 2) Magnetic scales, also uses lines, but these lines are really magnetic patterns of the magnetized direction of the magnetic particles on a substrate. The sensors are either Hall effect or magneto resistive, and here again, can measure individual lines transitions or multiple. (Hall sensors, which have current flowing in them, yield a voltage based upon the magnetic field strength applied. Magneto-resistive sensors have a change in their resistance based upon the orientation of its magnetization. The orientation is sensitive to the magnetic field coming from the patterns on the magnetic DRO rule). This technology is similar to magnetic recording and the resolution is limited by how close the sensor can get to the magnetic particles. Very , very, roughly stated if the sensor is within 1 micron it can measure 1 micron spaced transitions in magnetization patterns. If it is closer it can measure smaller distances. In today's hard drives the sensor is place at extremely close proximity to the disk surface and transitions spacing as small as 0.01 microns are common. Far smaller than optical spots. Most dirt particles are huge compared to this so effectively no dirt is allow in the hard disk drive. At this scale roughness of the substrate is of great concern. However, the dirt and the physical geometry of a lathe DRO does not allow such close spacing. Likewise, the magnetic coating in a DRO is composed of magnetic particles in a plastic binder/tape so the thickness of this binder tends to limit the resolution if not the spacing between the surface and the sensor surface. This spacing is very much an engineering issue. A virtue of the magnetic DRO is that the length of the rule can easily be shortened by just cutting it off. 3) Capacitive sensors utilize metal electro patterns which on the surface of the DRO rule. These electrode patterns on the DRO rule play the role of the lines on the ruler. These are not usually used for lathe DRO's as the debris can short out the electrodes. The sensor is also a set of a counter electrode patterns that effectively must align with the DRO rule electrodes. AC Voltages are usually applied to the electrodes and the capacitance is constantly measure to see if is changing as the DRO sensor head is moved along the rule. However, this technology is commonly used in vernier calipers where the environment is both cleaner and shorter. (By the way, a similar capacitive effect is used in the devices used to sense and find the studs in a wall, i.e. "Stud Finder". Here, though, the change in the capacitance is use to the presence of the wood stud changing the dielectric constant of the material between two electrodes on the hand held device rather than a counter electrode.)
If you have a lot of money to spend (~$many thousands) you can get something called a heterodyned laser interferometer, for example invented by folks at Hewlett Packard/Agilent/Keysight Technology. These things have a resolution of 1 Angstrom. (10,000 Angstroms in micron). I have an old one of these, and while the laser and all of the optics still works the electronics have failed.
Each of these linear measurement systems represent old technology and so most of the fundamental patents are expired. Hence, anyone can built these devices that wants to do so. However, while it is far more difficult to search for, and read patents, the details of how these things function are usually better explained in the patent literature than on casual web sites. Patent are interesting beasts. Lawyers and inventors must write them in such a manner to teach how the technology works in order to be awarded a valid patent, but at the same time try not to divulge any more details than absolutely necessary to achieve their protection goal while keeping competitors at bay!
Dave L.
