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LINEAR MOTION
manufacturing. It’s another to find something that works, can keep up with the high demands, and resist wear and tear. The design stage is where the necessary components are identified, and specific ones are chosen. There may not be a better example of this than the high-precision, high-yield world of semiconductor production. For maximum productivity and efficiency, the control, motor, machine frame and the position linear encoder must work seamlessly together. In this high-stakes sector, the potential costs of a misfit or a component that can’t hold up can have dramatic effects that trickle down through the whole engineering process or production itself.
L i n e a r M o t i o n
HEIDENHAIN’s LIP 200 series is an optical scanning encoder using interferential scanning with diffracted light to generate its current signals allowing for a
greater amount of interpolation with a finer grating period (down to 2.048 µm) that is largely free of harmonics (interpolation error of ±0.4 nm). The scale is made up of OPTODUR grating on ZERODUR glass making it both resistant to contamination and providing a low thermal coefficient down to 0±0.1. This results in a measuring step of as low as 31.25 picometers.
Optical scanning linear encoders
From fabricating integrated circuits, wafer dicing and packaging testing, the robotics employed in these tasks need to move to designated positions rapidly, accurately and without overshoot or ringing. This makes direct-drive linear servo motors a common choice. However, their superior speed and control put steep demands on feedback signals, putting a premium on linear encoders for short- and long-term performance.
Clean, precise linear position feedback reduces vibration in the machine frame, eliminates velocitydependent motor resonances, and prevents additional heat generation, allowing the motor to realize its maximum mechanical power rating and efficiency of operation.
At semiconductor-level of precision, encoder signals need to be interpolated to reach a high enough resolution for useful feedback. Interpolation error is to be anticipated with any encoder. That is, periodic position error within one signal period of the encoder’s output signals. Even the highest quality encoders, those most often applied in these cases, include interpolation error, though just 1 to 2 percent of the signal period. That said, if the frequency of interpolation error increases too much during production, the resulting heating or noise can make it difficult for the drive to stay within its effective range.
So if this naturally occurring error has this much impact on motor efficiency and needs to be accounted for, that shows how important the right component choices are in the design process.
Optical scanning linear encoders
Optical scanning linear encoders are most often the best option in semiconductor production, but they are not created equal. These incorporate measuring standards or scales with periodic structures known as graduations. The substrate material is glass, steel, or — for large measuring lengths — steel strips. These fine graduations — periods from 40 µm to under 1 µm are typical — are manufactured in a photolithographic process. Characteristic properties are high-edge definition and excellent homogeneity which are prerequisites for accurate performance.
The optical scanning is often paired with exposed linear encoders, meaning the measuring standard is exposed. These can reach higher levels of accuracy and resolution and take up less space. Even in fabs or inspection facilities where federal clean standards are adhered to, at such precision demands, contamination can still affect feedback (in the form of fingerprints from mounting or oil accumulation from guideways, for example), and lead to
L i n e a r M o t i o n
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poor results. While their homogeneity and defi nition impact accuracy, the toughness of the gratings a ects contamination resistance. Contamination on the measuring standard infl uences the light intensity of the signal components, and therefore the scanning signal. Its level of e ect depends on the scanning method. Large, single-fi eld scanning is the best choice. With only one fi eld, the output signals will change in their amplitude, but not in their o set and phase position over the range of travel. They stay highly interposable, and the interpolation error remains small. The large scanning fi eld in relation to the graduation detail reduces sensitivity to contamination. (One of the largest scanning fi elds deployed in semiconductor work is 14.5 mm.2 ) Even with contamination up to 3 mm in diameter, linear encoders continue to provide high-quality signals with position error below the values specifi ed. This stable signal, even in the face of some contamination, maintains low interpolation error, high traversing speed, good control-loop performance and low heat in the drives. Just like feedback has to resist contamination, it also has to be able to handle temperature changes to remain within the machine’s working accuracy. The “operating temperature range” is the limit of ambient temperature within which the specifi cations of the encoder still comply, for one, thermal expansion of the encoder’s carrier. An encoder’s specifi ed expansion coe cient should expand or contract in a defi ned, reproducible manner, matching the thermal behavior of the machine frame. The thermal dynamics of the machine should be taken into account when deciding where to mount the encoder’s scale and scanning head. So for instance, it’s generally recommended to not put feedback near heat sources. Along those lines, here are a few more reasons why semiconductor machines must be designed with feedback in mind from the beginning:
• The mounting surface must meet fl atness requirements. • To facilitate adjustment of the scanning head to the scale, the scanning head should be fastened to a mounting bracket.
• To keep the resulting Abbe error as small as possible, the linear scale should be mounted parallel to the machine guideway. • To avoid vibration, the best mounting surfaces are solid and stable machine elements as opposed to hollow parts.
Finally, on mounting, with small signal periods come narrow mounting tolerances for the scanning gap, the space between the encoder’s scanning head and its scale. This is the result of di raction caused by the grating structures. Such di raction can lead to a signal attenuation of 50 percent upon a gap change of only ±0.1 mm. More workable mounting tolerances are possible with linear encoders that use an interferential scanning principle and innovative index gratings, a variation of optical encoders that use the imaging principle to measure displacement. DW
HEIDENHAIN | www.heidenhain.us/
HEIDENHAIN uses tough gratings manufactured in highly specialized, proprietary processes. In the company’s SUPRADUR process, a transparent layer is applied first over the reflective primary layer.
Then, an extremely thin, hard chrome layer is applied to produce a grating and is shown here on an LIF 400 linear encoder. These graduations have proven to be particularly insensitive to contamination because the low height of the structure leaves practically no surface for dust, dirt, or water particles to accumulate. This ensures the high signal quality that direct drives require.
