**3. Length calibration systems**

Length calibration problems are inevitably connected with mastering and research of modern interferometric displacement measurement technologies, error compensation technique as well as digital microscopy achievements. These problems are often determined by the embedded metrology needs that can be met only by developing novel systems that absorb recent scientific and technical findings and optimally comply with specific calibration requirements as well as by improving existing calibration systems open to complying with fundamental principles of precision engineering. Satisfying more demanding requirements ultimately drives the demand to trace/validate the products on their manufacturing line as well as performing the graduation line detection and position measurements in a short time.

The ability to provide reference measurements at such levels of uncertainty requires developments beyond the current state of the art in each of three areas:

the physical artifact to be calibrated;


In addition, the three developments need be tied together in a measurement procedure that includes innovative measurement algorithms and methods.

For high precision line scale calibration and traceability to primary length standard optical comparators with movable optical line detection system or movable line scale are used. The interferometer is the instrument that transfers international standard of length into physical measurements. Digital measurement microscopes enable to perform precise positioning of length calibration systems, to estimate quality of line edges and precise location of lines. The system is placed in an air-conditioned environment.

The interferential comparator presented in Figs. 3, 4 was developed to calibrate both line graduation scales up to 3.5 m long and incremental linear encoders.

Fig. 3. Mechanical layout of the precision line scale comparator

The comparator consists of four main parts, namely the body of the machine, a laser interferometer, a translating system and a detecting apparatus. The body of the machine, which is made of granite surface plate, is used as the base of the machine and as a guide for the moving carriage. Measurement of the displacement of the carriage is realized by laser interferometer that consists of Zygo ZMI 2000 laser head and interferometer with the singlepass arrangement. The interferometer provides a resolution of 0.62 nm.

The comparator was designed to achieve expanded measurement uncertainties (k = 2) down to 7 × 10-7 m (L = 1 m) in dynamic regime. It enabled to trace the calibration of line scale of up to L ≤ 3.5 m long to the wavelength standard. The magnification and numerical aperture of the NIKON objective lens used was 20× and 50×, and numerical aperture – 0.4 and 0.55 respectively.

 the theoretical model of the systematic errors in measurement results arising from the interaction of the artifact and the measuring machine in the calibration process.

In addition, the three developments need be tied together in a measurement procedure that

For high precision line scale calibration and traceability to primary length standard optical comparators with movable optical line detection system or movable line scale are used. The interferometer is the instrument that transfers international standard of length into physical measurements. Digital measurement microscopes enable to perform precise positioning of length calibration systems, to estimate quality of line edges and precise location of lines. The

The interferential comparator presented in Figs. 3, 4 was developed to calibrate both line

The comparator consists of four main parts, namely the body of the machine, a laser interferometer, a translating system and a detecting apparatus. The body of the machine, which is made of granite surface plate, is used as the base of the machine and as a guide for the moving carriage. Measurement of the displacement of the carriage is realized by laser interferometer that consists of Zygo ZMI 2000 laser head and interferometer with the single-

The comparator was designed to achieve expanded measurement uncertainties (k = 2) down to 7 × 10-7 m (L = 1 m) in dynamic regime. It enabled to trace the calibration of line scale of up to L ≤ 3.5 m long to the wavelength standard. The magnification and numerical aperture of the NIKON objective lens used was 20× and 50×, and numerical aperture – 0.4 and 0.55

the measuring machine to do the calibration

includes innovative measurement algorithms and methods.

graduation scales up to 3.5 m long and incremental linear encoders.

Fig. 3. Mechanical layout of the precision line scale comparator

respectively.

pass arrangement. The interferometer provides a resolution of 0.62 nm.

system is placed in an air-conditioned environment.

Fig. 4. Precision interferometer-controlled line scale comparator with charge-coupled device (CCD) microscope

A moving CCD microscope serves as structure localisation sensor for the measurements of line scales. The microscope on the carriage guided on aerostatic bearings is moved with a controlled velocity of 1–10 mm/s.

The graduation line distances are measured during continuous motion. Average profiles of the graduation lines are formed by summing picture element intensities of each row of the CCD. Line centre is calculated as weighted mean from intensity profile of a line.

Air pressure, temperature, humidity are on line accessed to determine the refraction index of the air by the Edlen's formula. The angular control loop - together with the numerical procedure - was applied to compensate and reduce the Abbe uncertainty contribution.

The whole calibration process and all operations of the system are controlled by the PC, which runs according to specific operation program that includes also the error compensation.

The measured performances confirm that investigated measurement system can operate reliable at velocities up to 6 mm/s without appreciable loss in measurement accuracy.

In order to examine the calibration process in real time, the experiments of the line scale calibration with a moving CCD microscope have been carried out in specific operating modes, and the accuracy of dynamic calibration was analysed.
