DE9321368U1 - Photoelectric length or angle measuring device - Google Patents

Description

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DR. JOHANNES HEIDENHAIN GmbH 21 June 1993

Photoelectric length or angle measuring device

The invention relates to a photoelectric length or angle measuring device according to the preamble of claim 1.

Such measuring devices are used in particular in processing machines to measure the relative position of a tool in relation to a workpiece to be processed and in coordinate measuring machines to determine the position and dimensions of test objects.

From DE 19 05 392 B and DE 40 06 789 A1 ₇ , from which our invention is based, an incremental photoelectric measuring system is known in which the scanning grid is applied directly to the light-sensitive surface of a semiconductor substrate. The scanning grid is printed or glued directly onto the flat light receiving surface.

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Z. ·

It is also known to manufacture the grid lines of the scanning grid itself as receiving elements in the form of light-sensitive strips. Such a scanning unit is described in DE 19 62 099 B. The lines of the scanning grid are alternately transparent and non-transparent, where the non-transparent lines are light-sensitive on one side. For this purpose, positively and negatively doped semiconductor layers are applied one above the other in strip form on a substrate.

A similar device is described in DE 32 09 043 A1. The scanning grating is a semiconductor element on which active light-sensitive lines as receiving elements and light-shielding passive lines are arranged alternately in the measuring direction.

In many cases it is necessary to generate several scanning signals that are phase-shifted relative to one another in order to form a value that determines the current position. For example, in order to detect the direction of movement it is necessary that at least two scanning signals that are phase-shifted by 90° are generated. It is also advantageous that a scanning signal that is phase-shifted by 180° is generated for one scanning signal in order to obtain a zero-symmetrical signal from both scanning signals. According to DE 19 05 392 B, four grating divisions that are phase-shifted relative to one another by 1/4 of the division period are provided for this purpose. These grating divisions are arranged one above the other perpendicular to the measuring direction. This has the disadvantage that each opening in the grating division only

scans a fraction of the length of a scale line. The intensity of the scanning signals is therefore very low and partial contamination of the scale distorts the scanning signals considerably. 5

In DE 40 06 789 A1, the scanning gratings are arranged offset from one another by a fraction of a graduation period to generate phase-shifted scanning signals. The light-sensitive surfaces assigned to the scanning gratings are also spaced apart from one another by this fraction of a graduation period. With small grating constants, this arrangement has the disadvantage that crosstalk causes the scanning signals to influence one another, which leads to measurement errors. Furthermore, only a small area of the graduation of the scale is used to generate a scanning signal.

In incremental measuring systems, the phase-shifted scanning signals are particularly uniform and independent of partial contamination of the scale if all phase-shifted scanning signals are generated by the smallest possible scale area. This type of scanning is also referred to as single-field scanning. A measuring system of this kind is known from GB-PS 1311275. The scanning unit consists of several groups of light-sensitive strips arranged one behind the other as receiving elements. To generate scanning signals that are phase-shifted relative to one another, the strips within a group are arranged out of phase. The strips of the groups, each of which is in phase, are connected in parallel to one another. This arrangement has the major disadvantage that with small

Lattice constants the scanning signals are distorted by crosstalk.

In DE 33 08 841, several adjacent receiving elements with the same phase position form a group. Several groups with different phase positions are arranged according to certain symmetry conditions. This has the disadvantage that inhomogeneities in the light source can distort the scanning signals. In contrast, the invention has the advantage that by arranging many groups one after the other, inhomogeneities in the light source and the scale have no negative influence on the scanning signals and thus on the measurement accuracy.

In order to avoid crosstalk, it was further proposed in DE 33 08 841 A1 to arrange the individual receiving elements at a distance that is a multiple of the graduation period of the scale. However, this has the disadvantage that the individual receiving elements are difficult to manufacture for small grating constants.

The invention is based on the object of creating a position measuring device in which the scanning signals have a high quality and thus a high measurement accuracy is guaranteed.

This object is achieved according to the invention by the features of claim 1.

The advantages achieved with the invention are in particular that the phase-shifted signals are derived from a small scale range through a quasi single-field scanning and thus partial contamination does not distort the measurement. Furthermore, crosstalk from

from one receiving element to the other, even with very small division periods. The direct formation of the scanning grating on the semiconductor substrate ensures good light conversion with a high degree of efficiency.

Advantageous embodiments of the invention are specified in the dependent claims.

Embodiments of the invention are explained in more detail with reference to the drawing.

It shows

Figure 1 shows schematically a photoelectric length measuring device according to the invention;

Figure 2 is a plan view of the scanning plate of Figure 1;

Figure 3 shows a cross-section of the scanning plate according to Figure 2;

Figure 4 is a plan view of a second embodiment of the scanning plate;

Figure 5 shows a cross-section of the scanning plate according to Figure 4 and

Figure 6 shows a further schematic representation of a length measuring device designed according to the invention.

Figure 1 shows the functional principle of a photoelectric length measuring device. This length measuring device consists of a light source 1, the light of which is bundled by a lens 2 and directed onto a scale 3. The scale 3 is a glass body, on the surface of which an incremental graduation 4 in the form of transparent and opaque lines made of chrome is applied. The width of a transparent and opaque line is called the graduation period and is designated Tl. The light from the light source 1 falls through the transparent lines of the graduation 4 onto a scanning plate 5. The scale 3 is movable relative to the scanning plate 5 in the measuring direction X, whereby a modulation of the light that falls on the scanning plate 5 occurs.

The scanning plate 5 is a semiconductor substrate on which several scanning graduations are applied that are phase-shifted with respect to one another. The scanning plate 5 is shown in detail in Figures 2 and 3. The scanning plate 5 consists of a base body 6 made of glass, semiconductor material or metal. An N-type semiconductor layer 7 is applied to this base body 6, in which P-type regions 8 are formed that are spaced apart from one another. Due to the PN junction thus formed, these regions 8 are light-sensitive surface areas. The width B of one of these surface areas 8 in the measuring direction X is selected so that it can be easily manufactured using known methods such as diffusion. Typical values for the width B are in the range 20 to 1000 Lzm. Typical values for the distance A between the individual light-sensitive surface areas 8 are 20 to 50 μm or greater, so that a

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Crosstalk from one surface area 8 to the adjacent surface area 8 is excluded.

In the example shown, two scanning signals are to be generated that are 180° out of phase with one another. In order to achieve a quasi single-field scanning, the first light-sensitive surface area 8.1 is used to form the 0° scanning signal and the following light-sensitive surface area 8.2 is used to form the 180° scanning signal. A scanning grid 9 is applied to each of the surface areas 8, which consists of alternating transparent and non-transparent areas with the division period T2. The non-transparent grid lines are aluminum strips that serve as electrodes for discharging the electrical charge. The width B of the light-sensitive surface areas 8 is a multiple of the division period T2 of the scanning grid 9. Typical values of the division period T2 are in the range from 4 to 20μηη. The structuring of the scanning grating 9 is possible using known, simple photolithographic processes. The distance A between the light-sensitive surface areas 8 is also a multiple of the division period T2 of the scanning grating 9. The term multiple does not mean a restriction to whole-number multiples.

In order to be able to form scanning signals of high intensity, many light-sensitive surface areas 8.1, 8.3 or 8.2, 8.4 are provided for each scanning signal of a certain phase position, which are electrically connected to one another. This also has the advantage that an averaging of the

scanning signal takes place over a large scale range and thus partial contamination or division errors of the scale 3 do not influence the scanning signal too much. In the example shown in Figure 2 , the length of the scanning range in which a group of light-sensitive surface areas 8.1 and 8.2 are arranged to generate scanning signals that are phase-shifted with respect to one another is designated by C. On the scanning plate 5, two groups are arranged one behind the other in the measuring direction X.

The surface areas 8.1 and 8.2 form the first group and the surface areas 8.3 and 8.4 form the second group. The surface areas 8.1 and 8.3 with the same phase position are electrically connected to each other. The same applies to the surface areas 8.2 and 8.4.

The scanning grating 9 is applied to the light-sensitive surface areas 8 within a group in a phase-shifted manner by a fraction of a graduation period T2.

A group of consecutive surface areas can also consist of more than two surface areas. For example, of four surface areas for generating scanning signals that are phase-shifted by 90° relative to one another, or of three surface areas for generating scanning signals that are phase-shifted by 120° relative to one another. Likewise, more than two groups are advantageously formed consecutively in the measuring direction X in a common semiconductor substrate.

A particularly advantageous embodiment of the invention is shown in Figures 4 and 5. The

The scanning plate 5 shown also consists of areas 8 of type P which are formed in a semiconductor layer 7 of type N. Instead of the electrodes 9 shown in Figures 2 and 3, in this case electrical contacts 10 are provided at the edge of the P areas 8 to discharge the electrical charge. The scanning grid 9 is applied as opaque strips made of chrome to a subsequent insulating layer 11. The dimensions T2, A, B and C are identical to the design according to Figure 2. This design has the advantage that a standard semiconductor substrate can be used, onto which a scanning grid 9 with any fine graduation period T2 is applied in the last step.

The light-sensitive surface areas 8 are designed as photodiodes in the examples shown. By forming a further PN junction in the light-sensitive surface areas 8, phototransistors can also be realized. Further explanations are unnecessary, since details on the production of light-sensitive surface areas or elements are described in the cited prior art.

Figure 6 shows a further example of the invention in principle. In this case, the scale 3 is illuminated by a grid-shaped semiconductor light source 12. This light source 12 can be a flat light-emitting semiconductor element on which the grid 13 is applied. Such a light source 12 is described in US Pat. No. 5,155,355. The light source 12 can also consist of individual spaced-apart

consist of line-shaped light-emitting individual elements that are formed in a semiconductor substrate.

In order to obtain harmonic-free scanning signals during the relative movement between the light source 12, scanning plate 5 and scale 3, the "bridge-gap" ratio of the light source 12 differs from that of the scanning plate 5. The ratios are selected, for example, as described in DE 12 82 988 B. The width L (gap) of the light-emitting lines is

L = m2 * - ,

the distance S (bar) of the light-emitting lines is
20

S = ml * - ,
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where P is the division period of the light source 12, η is the ordinal number of a harmonic of the scanning signals to be eliminated and ml and m2 are integers that satisfy the condition ml + m2 = η. To eliminate another harmonic, the scanning grid 9 of the scanning plate 5 has a different "bridge-gap" ratio. Here, the distance K is defined as the bridge and the width V as the gap of the opaque strips of the scanning grid 9.

The scanning grid 5 is, for example,

K1
Vl

divided so that all even-numbered harmonics are suppressed. The light source 12 is, for example,

S2

L1

so that the 3rd, 6th, 9th, etc. harmonics are additionally suppressed.

The invention can be used in length and angle measuring systems.

In a manner not shown, the scanning grid of each light-sensitive area can be designed in such a way that a harmonic-free scanning signal is generated from each area, i.e. that the scanning grid has a filter function. This can be achieved by limiting the outer contour of the light-sensitive surface areas in a sinusoidal manner, similar to the frequency filter aperture according to DE 19 41 731 C. Another possibility is to arrange the opaque strips of the scanning grid of an area according to a sine function, as is known in principle from EP 0 157 177 B1 and EP 0 541 827 A1.

Claims (9)

DR. JOHANNES HEIDENHAIN GmbH 21 June 1993 Claims 1. Photoelectric length or angle measuring device for determining the position of gratings that can be moved relative to one another, in which a scanning plate is illuminated by a light source to generate at least one position-dependent scanning signal, the scanning plate (5) being a semiconductor substrate in which several light-sensitive surface areas (8) spaced apart in the measuring direction (X) are formed, on the surface of each of which a scanning grating (9) with several graduation periods (T2) made up of alternating transparent and non-transparent areas is applied, characterized in that the distance (A) between the individual light-sensitive -|5 len surface areas (8) is a multiple of the graduation period (T2) of the scanning grating (9). 2. Photoelectric length or angle measuring device according to claim 1, characterized in that several light-sensitive surface areas (8.1, 8.2; 8.3, 8.4) following one another in the measuring direction (X) form a group which generate scanning signals that are phase-shifted with respect to one another, and that several of these groups are arranged one behind the other in the measuring direction (X). 3. Photoelectric length or angle measuring device according to claim 2, characterized in that the light-sensitive surface areas (8.1, 8.3; 8.2, 8.4) of the groups with the same phase position of the scanning gratings (9) are electrically connected to one another to form a common scanning signal. 4. Photoelectric length or angle measuring device according to one of the preceding claims 1 to 3, characterized in that a flat light-emitting semiconductor light source (12) is provided for illuminating the scanning plate (5), on the light-emitting surface of which a grid (13) with opaque strips spaced apart from one another in the measuring direction (X) is applied. 5. Photoelectric length or angle measuring device according to one of claims 1 to 3, characterized in that a light-emitting semiconductor light source is provided for illuminating the scanning plate (5), which consists of light-emitting strip-shaped semiconductor elements spaced apart from one another in the measuring direction (X). 6. Photoelectric length or angle measuring device according to claim 4 or claim 5, characterized in that the ratio of the width (S) of the non-light-emitting strips to the width (L) of the light-emitting strips of the light source (12) depends on the ratio of the width (V)
the opaque strips to the distance (K) this strip of the scanning grid (9) of the scanning plate (5) deviates. 7. Photoelectric length or angle measuring device according to claim 5, characterized in that the ratio S 1 1 L 2 1 and the relationship V K is. 8. Photoelectric length or angle measuring device according to one of claims 4 to 7, characterized in that a scale (3) is arranged in the beam path between the light source (12) and the scanning plate (5) so as to be displaceable in the measuring direction X relative to the light source (12) and the scanning plate (5). 9. Photoelectric length or angle measuring device according to one of the preceding claims, characterized in that a light-sensitive surface area and/or the scanning grid of a light-sensitive surface area is designed in such a way that the light-sensitive surface area and/or the scanning grid has a filter function for filtering out harmonics.