T 0625/01 10-04-2002

1. The appeal is admissible.

2. Amendments

The amendments in claims 1 and 6 defining that the comparison level is based on the output signals of all photodetectors are fairly supported by the embodiments in the Figures (see the circuits in Figures 8, 9, 23, 26A and 26B) and the corresponding passages in the description. Furthermore the board is satisfied that the amendments do not give rise to an objection under Art.123(2) or (3) EPC. Therefore the amendments are formally allowable.

3. Article 84 EPC

The objections by the respondent that claims 1 and 6 lack clarity are not shared by the board. Firstly, in the opinion of the board, in the present case the appeal procedure does not provide a formal basis for addressing the specific objections, because the objected features had already been present in the granted claims and are not a result of the amendments (see Decision T 0301/87, 1st Headnote and Points 3.7 & 3.8. of the Reasons). Secondly, in the board's view, also in substance the respondent's objections are not persuasive:

Claims 1 and 6 define that "a light-beam" is irradiated onto a first scale and -after modulation- is received and modulated by a second scale. The modulation process may result in the formation of plural secondary beams (e.g. by diffraction or refraction effects of the modulating structure), which beams are received by respective plural photodetectors. Therefore the claims define a predetermined set-up between a primary light beam from a light source, one or more secondary modulated beams, and respective photodetectors. Whether, as argued by the respondent, a further primary light beam from a further light source is not excluded by the wording of the independent claims is not pertinent to the question of clarity, because, according to the claims, also any further primary beams would have to be modulated, resulting in (further) respective secondary beams and being received by (further) plural photodetectors. It is noted that the respondent's references to the plural "beams" in the patent specification are all in the context of (plural) secondary beams, therefore the cited passages are not in conflict with the claimed subject-matter.

The respondent's objection that claims 1 and 6 erroneously define that the comparison level is formed from the sum of the photodetector signals is not shared by the board. The claims require that the comparison level is determined, this "determination being conducted based on the sum" of all the output signals, which condition implies that in the calculation of the comparison level the sum of the signals is taken into account. This view is also supported by the disclosure, see equations (5), (10) and (17).

4. Novelty

4.1. Document E2 which, in the respondent's view, anticipates the subject-matter of the independent claims, shows in Figure 5 an apparatus for measuring relative displacement information of a measured object, utilizing a first scale (slitted disk 1 ) and a second scale (slitted disk 2 ) of which one (disk 1 ) is mounted on a rotary axis. The apparatus in Figure 5 of E2 comprises three light sources (6, 7 and 24), which may be regarded as forming a "light-beam irradiating unit". As discussed in point 3 supra, claim 6 (and similarly claim 1) defines a structural relation between every single primary light beam, secondary modulated beams, and the photodetectors receiving these secondary beams. Differing from the requirement in claim 6, in the apparatus shown in Figure 5 of E2 every primary light beam from a respective light source (6, 7, 24) is -after modulation- received by one -and only one- respective photodetector (8, 9, 25). See also E2, page 15, lines 21 to 28, where the positional relation between the light emitting and receiving elements 6 & 8; 7 & 9; and 24 & 25 is explained. Therefore, although the measuring apparatus of E2 comprises "plural photodetectors" (namely: light receiving elements 8, 9 and 25), these three photodetectors are not arranged for receiving a modulated secondary light beam or beams generated by the same primary light beam. A further difference between the subject-matter of claim 6 and the apparatus disclosed in document E2 resides in the processing of the signals. In Figure 6 of E2, which shows the signal processing section of the apparatus in Figure 5, the comparison level signal is based on the combination of the signals "b" and "" from two of the three light receiving elements. Therefore the comparator level is not based on the sum of all the output signals from the plural photodetectors (8, 9 and 25).

4.2. Document E1 discloses in Figure 1 an apparatus for measuring relative displacement information of a measured object, utilizing a first scale (track 1); a second scale (mask 3), of which one ("movement sensor fixed to one of said members" or track 1; see claim 1 of E1) is mounted on the measured object; a light beam irradiating unit (light source 2) for irradiating a light beam onto the first scale (1), wherein the light beam is subjected to optical modulation by the first scale (1) and is introduced to the second scale (aperture plate 3); plural photodetectors (a, b) for receiving the light beam subjected to optical modulation in the second scale (3); a measuring unit for measuring the relative displacement information of the object from the output signals (201, 202; 211, 212) respectively obtained from several photodetectors (a and b); the measuring unit being adapted to measure the displacement information by the comparison of each of the output signals from the photodetectors with a comparator level in a comparator setting circuit (column 2, lines 57 to 62).

The comparison level in the apparatus shown in Figure 1 of E1 is based on the signal of the other photodetector (signal 201 compared with signal 202), or on a signal obtained by combining signals transmitted by a plurality of detectors (column 2, lines 60 to 62; claim 1, column 10, line 18). Therefore document E1 does not disclose that the comparator level is based on the sum of all the signals of the photodetectors.

4.3. The other known prior art documents are not relevant for the question of novelty, since in the apparatuses according to these documents the output signals from the diodes are directly used for the postion indication.

4.4. Therefore in the opinion of the board the subject-matter of claim 6 is novel. For the same reason the subject-matter of claim 1 is considered novel, because this claim defines a signal processing method with the corresponding method features as the apparatus of claim 6.

5. Inventive step

5.1. As set out in points 4.1 and 4.2 supra, documents E1 and E2 disclose apparatuses for measuring relative displacement between two scales of the generic type of the apparatus defined in claim 6 and the method defined in claim 1 of the patent in suit. These documents also address the same general problem of ensuring an accurate determination of the measured position if the received signal at the photodetector varies in amplitude. However, the concrete problem underlying document E2 differs from the one in document E1 and the patent in suit. Whereas the patent and document E1 address the fluctuations of the light source, document E2 discloses an optical encoder positioned in a transfer system of an automobile wherein variations of transmission may occur through the medium (oil) in which the encoder is placed. Furthermore, the apparatus in document E1 includes the same primary light source for all photodetectors in the same way as the device in claim 6 of the patent in suit, whereas in contrast the apparatus disclosed in document E2 includes separate light sources for every detector. Hence, document E1 appears closer to the claimed subject-matter and is considered to disclose the closest prior art.

5.2. The apparatus defined in claim 6 and the measuring apparatus known from E1 differ in the determination of the comparison levels (see point 4.2). This applies correspondingly to the method defined in claim 1. The objective problem deriving from the difference in determination of the comparator signals may be seen in the idea of using approximately the total amount of light transmitted by the optical scale, which should ensure a stable reference level which is insensitive to fluctuations in light output of the light source due to, for instance, temperature (patent specification, column 6, line 57 to column 8, line 22; in particular column 7, lines 40 to 43) or temperature-dependent variations in the sensitivity of the photodetectors (column 14, lines 56 to 57).

5.3. In the board's opinion, document E1 does not offer a hint or suggestion to the solution defined in claims 1 and 6, to determine the comparator level by generating a reference based on the sum of all the output signals of the photodetectors. In particular, in the embodiment of Figure 1 of E1 the square-wave signal 221 is obtained by comparing the signal provided by photodetector "a" with the one from the other photodetector "b" (column 2, line 63 to column 3, line 35), which provides "perfectly precise positions... whatever the fluctuations of the light source" are (column 3, lines 19 to 21). Since in this case, both photodetector signals are already used, there would be no incentive to modify the comparison by adding up the signals of these photodetectors.

The respondent has furthermore argued, that document E1 also teaches to base the comparison signal on a plurality of signals from different detectors (column 9, lines 10 to 12) and that, by extrapolation of this teaching, the step of using the signals from all photodetectors would then be obvious. In the board's understanding, the motivation for adding up a plurality of signals in the further embodiments of document E1 is not obtaining a more stable and time-independent signal, as in the patent, but increasing the positional resolution of the device, see the embodiment in Figure 5 and column 7, lines 8 to 34. For instance, in Figure 5, reference signal 524 is obtained by forming half of the sum of signals 412 and 414. It is readily understood that the higher positional resolution thus obtained is due to the higher frequent interpolated signal (see the double-sided modulated signal 524 compared to the original signals 412 and 414 in Figure 5). If, in this embodiment, the skilled person attempted to add up all photodetector signals for establishing a reference level, the high frequencies in the reference signal and thereby the positional information would be lost. It is therefore the board's opinion, that for the case of two photodetectors document E1 does not offer an incentive to modify that apparatus in the manner as defined in independent claims 1 and 6; and that for the case of more than two photodetectors, such a modification would go against the teaching of E1 (to increase the device's positional resolution). Hence, document E1 considered alone does not provide information to modify the measuring apparatuses disclosed in this document in the claimed way.

5.4. The respondent has also argued that the skilled person would consider to include the teaching of document E4 in the apparatus of E1.

In the board's view, a combination of the teachings of these documents would not seem very probable, because both documents offer complete and self-containing solutions for the problem of fluctuations in the light source. In document E4, the light source is stabilised via a feedback control loop. The feedback signal is obtained by summing up the signals of the photodetectors, which are compared with a fixed reference voltage for the feedback loop. The problem of light fluctuations thereby being solved, the photodetector output signals are used as the position indicating output of the transducer (E4, column 5, lines 42 to 44). A hypothetical combination of the teachings of E1 and E4 would include the idea of stabilising the light intensity of the light source by including a feedback loop as disclosed in E4; and to process the photodetector signals as disclosed in E1 (using the signals of a plurality of photodetectors, different from the measuring photodetector) in order to increase the positional resolution of the apparatus. Such a combination, however, would still not correspond to an apparatus with the combined technical features of claim 6, or teach the method defined in claim 1.

5.5. Therefore the method in claim 1 and the apparatus of claim 6 is not obtainable from the prior art in an obvious way.

5.6. Claims 2 to 5 and claims 7 to 15 are dependent on claims 1 and 6 and, therefore, their subject-matter also involves an inventive step.