Linear Encoder Advancements
Make More Possible with Less

The photonics and semiconductor industries are very complex, and there is much interdependency. Machines related to both these industries (from fiber alignment stages to optical inspection systems) all have a few things in common despite their wide range of applications.

In both these industries, it is critical that their machines count very small increments and perform very accurate high-resolution movements. Whether the application is aligning two or more fibers together to launch a signal and then perform a joining process or applying a Bragg fiber grating through a window in a fiber, it is critical that highly accurate, very precise movements be made. To do this very fine measurement, the most important common component in both of these machine types is the linear encoder.

Today, major technological advancements in linear encoders make more possible with seemingly less. Encoders with more compact sizes along with additional functions are giving machine manufacturers capabilities that, previously, they could only imagine.

Introduction

Many applications in the electronics industry demand high traversing speeds and quality of velocity control, requiring signal periods of 4 µm with high signal quality for reliable interpolation. These high traversing speeds are attained through high acceleration and low mass in the components. This presents a demand for small dimensions and light construction of the encoders.

Recently, HEIDENHAIN Corporation was able to reduce the scanning head glass scale of a linear encoder to a size of 26 mm x 16 mm x 16.5 mm (LxWxH) (new model LIF 401). For lengths shorter than 1000 mm, the cross section is only 10 mm x 5 mm. Integration of an additional optical limit and homing functions in the smaller housing resulted in a reduction of weight, installation space, cabling, and installation time. This enables the machine designer to save a significant amount of money.

The tolerance of the encoders to contamination was also substantially improved. Thanks to the new SUPRADUR scales, the new exposed linear encoders of the LIF series are sturdy and, at the same time, highly precise measuring systems that provide increased security against such unplanned contamination (more on contamination and SUPRADUR later in this article).

(Figure 1) New, small LIF 481 with glass scale (center) and Zerodur scale (front) compared with the LIF 181 (back)

2 Limit and Homing
Incremental and reference-mark scanning is based on the well-proven interferential measuring principle, which has already found application in HEIDENHAIN’s LIP and LIF product families, and is characterized by large scanning areas, single-field scanning and the well-known hysteresis-free optical reference mark. A new feature of the LIF 401 is its limit-switch and homing signals, which are generated together with the reference mark optically from one track. The homing signal serves to determine the traversing direction when the reference mark is to be traversed. In conjunction with the limit switch function, the electronics must be able to detect the following four conditions reliably: the areas to the left and right of the reference mark, the left limit switch, and the right limit switch.

The optics were designed so that all four conditions can be detected in addition to the reference mark from the same track by applying a phase grating at the left of the reference mark but not at right. The change of grating characteristics results in a switching operation that is characteristic of the homing signal. The limit switch function is realized through adjustable aperture clips that the user can adjust and adhere to the desired position. When the limit switch is traversed, the limit switch signal reverses its algebraic sign. The homing signal then makes it possible to ascertain whether the left or right limit switch was triggered.

(Figure 2) Scale with limit and homing track and the signals from the modified reference mark track

Tolerance to contamination
The problem of contamination must be considered for light-absorbing or light-dispersing contamination. This category of contamination can occur from wear particles or PCB dust. For encoders that operate on the interferential measuring principle with single-field scanning and large scanning areas, this type of contamination results only in a reduction of signal amplitude. The functions of the encoder, in particular the interpolation, remain intact, as do the reference-mark, limit-switch and homing functions.

The second category is contamination that impairs the diffraction characteristics of the phase grating scales. This category can include fingerprints incurred during mounting or oil residue from guideways, which fill the gaps between steps in conventional step gratings. This disturbs the grating characteristics, reduces the quality of interpolation, and leads in the worst case to encoder failure. With its new development, the SUPRADUR graduation, HEIDENHAIN has now found a successful solution to this problem. Here the valleys between peaks are very precisely filled with a transparent dielectric to ensure the planarity of the scales. Liquid contamination such as oil films can only distribute themselves evenly on the grating. Therefore, the grating characteristics and all functions of the encoder remain largely unaffected.

(Figure 3) SUPRADUR graduation of the LIF 481 and its effects on toleration to contamination

In this way, the use of large scanning fields, single-field scanning, and SUPRADUR gratings increases the operational safety and consequently the reliability of the encoder during its service life while allowing reasonable mounting tolerances. Figure 4 shows the error of an LIF 481 from different smudges of oil, water, toner, dust from printed circuit boards, and fingerprints. The scanning method is so effective that the encoder continues to function in spite of this heavy contamination. The signal is not impaired enough to threaten the function of the evaluation electronics. Such performance is unrivaled among encoders with a signal period of 4 µm. Even the length error is only slightly changed by the contamination. The short-range error in this measurement increases from approx. ± 0.02 µm to ± 0.2 µm in the area of the contamination, but this is a very small effect for such massive contamination in the measured areas.

Figure 4: Measurement error of the LIF 481 with different types of contamination

Product family
The exposed linear encoders of HEIDENHAIN’s LIF family have been in use for years in a wide range of applications such as die and wire bonders, wafer steppers, probers, wafer saws and handling devices. The new LIF 401 is a supplement to this product family. Besides the improvements such as increased resistance to contamination and the integrated limit and homing functions, the availability of materials was enhanced. The scale with its SUPRADUR grating is now available on glass with an expansion coefficient of approximately 8 ppm/K as standard version, and in a special version on ZERODUR with an expansion coefficient of approximately 0 ppm/K. The accuracy grade of the scales is ± 3 µm. The available measuring lengths of the new, smaller scales range from 70 mm to 220 mm, in future up to 1000 mm. The LIF 481 permits a traversing speed of 1.2 m/s at a signal reduction of –3 dB, and 1.6 m/s at –6 dB. Vibration of 400 m/s² and shock of 700 m/s² are permissible for the scanning heads. The permissible operating temperature range is 0 °C to 50 °C.

Conclusion
The LIF 481 and LIF 471 represent a major advancement in the development of exposed linear encoders with 4 µm signal period. Limit and homing functions and a contamination-resistant scanning have been realized at the same time as a reduction of the overall volume. #

Authors:
Dr. Sebastian Tondorf, Michael Hermann, Fundamental Research