What Is a Raster Scan?

Moire fringes, the basis of metrological grating technology, was first proposed by British physicist L Rayleigh. It was not until the 1950s that grating moire fringes were used for precision measurements.

Grating technology

Right!

The basis of metrology grating technology

From the 1950s to the 1970s, the grid measurement system was from

In 1950, the German Heidenhain pioneered the DIADUR replication process, that is, the photolithographic replication process of evaporation of chrome on glass substrates, which can produce high-precision, low-cost grating scales, so grating measuring instruments were accepted by customers and entered the product. market. In 1953, Ferranti of the United Kingdom proposed a 4-phase signal system, which can achieve 4 times frequency subdivision in a Moire fringe cycle and can identify the direction of movement. This is the 4 frequency phase detection technology, which is the basis of the grating measurement system. And has been applied to this day.

In the early 1960s, the German company Heidenhain began to develop

Lasers can be classified into geometric gratings (amplitude gratings) and diffraction gratings (phase gratings) according to the principle of forming moire fringes, and they can also be classified into transmission gratings and reflection gratings according to different optical paths. Micrometer and submicron gratings are measured using geometric gratings. The grating pitch is 100 m to 20 m, which is much larger than the wavelength of the light source. Diffraction can be ignored. When the two gratings move relative to each other, low-frequency beats occur to form moire fringes. The measuring principle is called the image principle. Nano-scale grating measurement uses a diffraction grating with a grating pitch of 8 m or 4 m. The width of the grating line is very close to the wavelength of light. Then diffraction and interference phenomena occur to form moire fringes. The measurement principle is called the interference principle. The three measurement principles adopted by the German Heidenhain company's products are now introduced.

(1) Principle of image measurement with four-field scanning (transmission method)

The vertical incidence optical system is a 4-phase signal system, which divides the four windows of the indicating grating (scanning mask) into 4 phases, and the grid lines of each phase are sequentially shifted by 1/4 grid pitch on the received 4 photoelectric elements. An ideal 4-phase signal is obtained, which is called the principle of image measurement with four-field scanning. Heidenhain's LS series products use this principle. The grid pitch is 20 m, the measurement step is 0.5 m, the accuracy is ± 10, ± 5, ± 3 m, the maximum measurement length is 3 m, and the carrier is glass.

(2) Principle of image measurement with quasi-single-field scanning (reflection method)

The reflection scale grating is a steel strip with a grating pitch of 40 m. The indication grating (scanning mask) is composed of two phase gratings that are interlaced and have different diffraction performance. To this end, a scanning field can generate a phase shift of 1/4 grating. The distance of the four images is called the principle of image measurement of quasi-single-field scanning. Because only one scanning field is used, the local pollution of the scale grating makes the change of the light field intensity uniform and has the same effect on the four photoelectric receiving elements, so it will not affect the quality of the grating signal. At the same time, the gap and the gap variance of the index grating and the scale grating can be larger. Heidenhain LB and LIDA series metal reflection gratings use this principle. The LIDA series open grating has a grating pitch of 40m and 20m, a measurement step of 0.1m, an accuracy of ± 5m, ± 3m, a measurement length of up to 30m, and a maximum speed of 480m / min. The LB series closed grating has a pitch of 40 m and a maximum speed of 120 m / min.

(3) Interferometric principle of single-field scanning

For gratings with very small pitches, the indicator grating is a transparent phase grating, the scale grating is a self-reflecting phase grating, and the light beam is diffracted by the double grating. The beams at each level interfere with each other to form a Moire fringe. Among them, the +1 and -1 level interference fringes are fundamental fringes, and the period of the fundamental fringes changes synchronously with the grating pitch. Optical modulation generates three measurement signals with a phase difference of 120 °, which are received by three photoelectric elements, and then converted into a universal sinusoidal signal with a phase difference of 90 °. Heidenhain LF, LIP, LIF series encoders work according to the principle of interference. The carriers of the encoders are steel plates, steel strips, glass and glass ceramics. These series of products are sub-micron and sodium-meter, with a minimum resolution of 1 Nanometer.

In the late 1980s, the transmission grating LID351 (resolution: 0.05 m) with a grating pitch of 10 m had stricter clearance requirements (0.1 ± 0.015) mm. Due to the use of a new interferometric measurement principle, the installation tolerances for nanometer-level diffraction gratings are widened, for example, the gap and parallelism between the indicator grating and the scale grating are wide (see Table 1).

Table 1 Gap and parallelism between indicator grating and scale grating

Grating type-signal period (m)-resolution (nm)-gap (mm)-parallelism (mm)

LIP3720.21810.3 ± 0.02

LIP471250.6 ± 0.02

LIP5714500.5 ± 0.06

Only the grating grating LIP372 has a grating pitch of 0.512 m. After optical frequency doubling, the signal period is 0.128 m, and other grating pitches are 8 m and 4 m. The signal period obtained after optical double frequency is 4 m and 2 m. Its resolution It is 5nm and 50nm, the system accuracy is ± 0.5m and ± 1m, and the speed is 30m / min. LIF series grid pitch is 8m, resolution is 0.1m, accuracy is ± 1m, and speed is 72m / min. The carrier is a glass ceramic with a temperature coefficient close to zero or a glass with a temperature coefficient of 8 ppm / K. The diffraction grating LF series is a closed grating ruler with a grating pitch of 8 m, a signal period of 4 m, a measurement resolution of 0.1 m, a system accuracy of ± 3 m and ± 2 m, a maximum speed of 60 m / min, and a measurement length of 3 m. The carrier is made of steel. Ruler and glass with the same coefficient of steel expansion (10ppm / K).

Several key issues of grating measurement systems

(1) Measurement accuracy (accuracy)

The measurement accuracy of the grating line displacement sensor depends first on the quality of the scale division of the scale grating and the quality of the indication raster scan (clearness of the edges of the raster line is important), and then the quality of the signal processing circuit and the indication grating along the scale. Guided error. What affects the measurement accuracy of the grating ruler are the position deviation over the entire measuring length of the grating and the position deviation within a signal period of the grating.

The accuracy (precision) of the scale is represented by the accuracy level. Heidenhain is defined as: the maximum value Fmax of the position deviation based on the average value in any 1m measurement length section falls within ± a (m), Then ± a is the accuracy level. Heidenhain accuracy levels are divided into: ± 0.1, ± 0.2, ± 0.5, ± 1, ± 2, ± 3, ± 5, ± 10, and ± 15 m. It can be seen that the accuracy level of the Heidenhain scale has nothing to do with the measurement length. This is a very high requirement. At present, no manufacturer can achieve this level.

At present, Heidenhain glass transmission grating and metal reflection grating have a pitch of only 20 m and 40 m, and a diffraction grating pitch of 4 m and 8 m. The signal period after optical double frequency is 2 m and 4 m. Heidenhain requires that the position deviation of one signal period of the open grating is only ± 1%, and that of the closed grating is only ± 2%. See Table 2 for the signal period and position deviation of the grating.

Table 2 Grating signal period and position deviation

Grating category-signal period (m)-position deviation within a signal period (m)

Geometrical gratings20 and 40open scales ± 1%, that is, ± 0.2 to ± 0.4; closed scales, ± 2%, that is, ± 0.4 to ± 0.8

Diffraction gratings-2 and 4-Open scales ± 1%, ie ± 0.02 to ± 0.04; Closed scales ± 2%, ie ± 0.02 to ± 0.08

(2) Signal processing and grid pitch subdivision

Grating measurement is the combination of absolute measurement within one cycle and incremental measurement outside the cycle. That is to say, the grid pitch is subdivided to perform absolute measurement within one cycle. Continuous incremental measurement. In order to ensure the accuracy of the measurement, in addition to the quality of the grating's scribing and motion, there must also be certain requirements for the quality of the moire signal of the grating, because this affects the accuracy of the electronic subdivision, that is, it affects the measurement of the grating Signal subdivision (octave number) and measurement resolution (measurement step). The subdivision number and accuracy of the grid pitch also affect the accuracy and measurement step of the grating measurement system. The requirements for the moire fringe signal quality are mainly that the sineness and orthogonality of the signal are good; the DC level drift of the signal is small. The photoelectric conversion circuit and the subsequent digital interpolation circuit in the reading head require good frequency characteristics to ensure high measurement speed.

Heidenhain company specially designed a grating frequency multiplier for the connection between the grating sensor and crc. The sinusoidal signal (one period is a grating pitch) output by the grating sensor is interpolated and digitized to give a square wave with a phase difference of 90 °. The number of subdivisions (multipliers) are 5, 10, 25, 50, 100, 200, and 400. Taking into account the 4th octave of the CNC system, the subdivisions of the grid pitch are 20, 40, 100, 200, 400, 800 and 1600, can realize the measurement step from 1nm to 5m, the choice of frequency multiplier depends on the quality of a grating period of the grating signal. As the frequency multiplier increases, the output frequency of the grating sensor must decrease. The relationship between the frequency multiplier subdivision number and the input frequency is shown in Table 3.

Table 3 Frequency multiplier subdivision and input frequency

Frequency multiplier subdivision number: 0-2-10-25-25-50-100-200-400

Input frequency (KHz): 600-500-200-100-100-50-25-25.25-6.25

Choose different octave numbers to get different measurement steps. In Heidenhain's digital display table, you can set as many as 15 multipliers, and the highest frequency can reach 1024, which is 1, 2, 4, 5, 10, 20, 40, 50, 64, 80, 100, 128, 200, 400, 1024. The maximum multiplier of digital graphics cards used on a microcomputer can reach 4096.

(3) Parameter marking and absolute coordinates of the grating

Establishment of the absolute position of the grating

The encoder is an incremental measurement, and the absolute position of the encoder is determined using a reference mark (zero position). The width of the reference mark signal is consistent with the signal period of a grating pitch. After processing by the subsequent circuit, the pulse width of the reference signal is consistent with one measurement step of the system. In order to shorten the distance of returning to the zero position, Heidenhain designed a reference mark that is coded by distance within the entire measurement length. After two reference marks are passed, the absolute position of the scale can be determined, such as gratings with a grating pitch of 4 m and 20 m. The absolute position can be determined after the ruler scanning unit is moved 20 mm relative to the ruler. The grating ruler with a grid pitch of 40 m must be moved 80 mm to determine the absolute position.

Absolute coordinate sensor

In order to measure the absolute position at any time, Heidenhain designed and manufactured the LC series absolute encoders. It uses seven incremental code channels to obtain the absolute position. Each code channel is different. The grid pitch of the finest code channel has There are two types, one is 16m and the other is 20m. The resolution can be 0.1m, the accuracy is ± 3m, the measurement length can reach 3m, and the maximum speed is 120m / min. The photoelectric scanning principle used by it is the same as the commonly used transmission grating, which is an image measurement principle with four-field scanning.

(4) the carrier of the grating

The scale is manufactured in an environment of 20 ° ± 0.1 ° C. The thermal performance of the scale directly affects the measurement accuracy. The thermal performance of the scale is best consistent with the thermal performance of the device under test. Taking into account different operating environments, the carriers of Heidenhain scales have different coefficients of thermal expansion. Available materials are glass, steel, and zero-expansion glass-ceramic. The expansion coefficient of ordinary glass is 8ppm / K. Now Heidenhain has adopted glass with expansion coefficient like steel. These materials are insensitive to vibration and shock, have certain thermal characteristics, and are not affected by changes in air pressure and humidity. Glass, glass-ceramic and steel are used as the carrier materials of the grating ruler whose measurement length is less than 3m, and steel belt is used for more than 3m. By selecting the material and corresponding structure of the scale carrier, the thermal performance of the grating scale and the component under test can be optimally matched.