Measuring Arrangement for the Clear Scanning of at Least One Reference Mark Allocated to a Graduation

The present invention relates to improvements to a measuring device for the measurement of the relative position of two objects, of the type having a scale which defines a graduation and at least one reference mark, absolutely allocated to the graduation, as well as a scanning unit which operates to scan the scale and to generate reproducible scanning pulses which act upon a counter. Such measuring devices typically include at least one scanning plate in the scanning unit for scanning the graduation and the reference marks of the scale, and the separation between the scanning plate and the scale in the direction travelled by the light beam is held at a predetermined spacing.

In such measuring arrangements the reference marks are typically used to generate electric control pulses which can be used in various ways. For example, such control pulses can be used to set the counter to a zero position, to load a preselected position value into the counter at the commencement of the measuring operation, and to monitor the measuring value against interfering impulses. Furthermore, such electric control impulses can be used to act upon and control a control arrangement engaged on the outlet side.

In code measuring systems, of the type which do not include reference marks, on occasion several graduation tracks with differing grid constants are used. In this case, it is a known practice to provide differently sized spacings between the graduation tracks and the associated scanning fields. In this system, it is the grid constants of the graduation tracks and the wavelength of the reading light of the illuminating arrangement that determine the appropriate spacings. See, for example DE-OS 15 48 874.

German patent DE-PS 29 52 106 describes an incremental angle measuring system in which a scale is provided which includes a measuring graduation and a number of reference marks, each of which defines a unique line group distribution. The individual reference marks are scanned by respective scanning fields included in a scanning unit, and each scanning field of the scanning unit is clearly and unambiguously allocated to a respective one of the reference marks. This is because the allocated scanning field defines the same line group distribution as that of the associated reference mark. Because of the irregular line group distributions, the distance between the scale and the scanning plate is preferably at most about 4d.sup.2 /.lambda., where d is the width of the narrowest line of the line graduation of the reference mark and .lambda. is the center of gravity wavelength of the light. Typically, this spacing is desirable for clear and error free scanning of the reference marks.

In addition, it is known that for the scanning of a regular, periodic incremental graduation of a scale, the scanning distance between the scale and the scanning plate does not need to be maintained at a single definite spacing. Rather, a number of differing spacings are possible. When the graduation of a scale is traversed by light in a parallel beam path, diffraction patterns of the graduation of the scale are generated which can be scanned with a scanning graduation of like grid constant. Only at certain planes behind the graduation plane of the scale does the interference associated with the diffraction patterns provide an optimal pattern for scanning. In general, for a graduation with a grid constant P.sub.M and a center of gravity wavelength .lambda. of the light, the optimal scanning planes are separated from the graduation plane of the scale by a distance equal to n.multidot.P.sub.M.sup.2 /.lambda. (n=0, 1, 2, . . .). Thus, optimal scanning signals can be generated at a separation equal to n.multidot.P.sub.M.sup.2 /.lambda. between the graduation plane of the scanning plate and the graduation plane of the scale (see Machine Shop Magazine, April, 1962, p. 208). Larger spacings between the scale and the scanning plate bring advantages in that the measuring device is less sensitive to mechanical influences, such as for example, processing chips or shavings. In the event of small spacings between the scanning plate and the scale, such processing chips or shavings can more readily become jammed between the scale and the scanning plate and can lead to damage to the graduations of the scale and the scanning plate. Furthermore, when larger separations are used the scale and the scanning plate can more readily be cleaned in the event of fouling. Yet another advantage of larger separations between the scale and the scanning plate arises from the fact that spacing tolerances are also greater with these greater spacings. For this reason, lower demands can be placed on the precision of guidance of the scanning plate with respect to the scale. Furthermore, in the event larger separations are used between the scale and the scanning plate, the periodic scanning signals generated in connection with the incremental graduation of the scale have a more sinusoidal signal form, so that the signal period of the scanning signal can better be subdivided by interpolation.

In the past, the utilization of very large spacings between the scale and the scanning plate in the scanning of the incremental graduation has been possible without interposed imaging optics only in measuring devices which include no reference marks on the scale. This is because for the clear scanning of the reference marks, as set out above, a predetermined small distance between the scale and the scanning unit must not be exceeded.

The present invention is directed to an improved measuring device of the type which includes a scale that defines reference marks, which makes possible a clear scanning of the reference marks as well as an optimal scanning of the graduation of the scale.

According to this invention, a measuring system of the type described initially above includes at least one scanning plate which defines at least one graduation scanning field and at least one reference mark scanning field. Means are provided for establishing a phase difference between (1) the optical path length between the graduation and the graduation scanning field, and (2) the optical path length between the reference mark and the reference mark scanning field. An alternate form of this invention utilizes graduation and reference mark scanning fields arranged such that the separation between the graduation and the graduation scanning field is equal to the separation between the reference mark and the reference mark scanning field. In this alternate embodiment, an imaging optic is positioned between the scanning plate and the scale and aligned with the reference mark scanning field such that reliable scanning of the reference mark is provided, at the prevailing separation between the reference mark and the reference mark scanning field.

The present invention provides important advantages in that a measuring device of the type described above is provided with remarkably simple means that allow the most favorable spacings required for trouble free scanning of both the incremental graduation and the reference marks of the scale. These separations can be selected substantially independently so that the measuring accuracy and the degree of subdivision in an interpolation of the measuring values are improved. Furthermore, by providing greater spacings between the graduation of the scale and the associated scanning fields of the scanning plate, lower demands can be placed on the guidance accuracy of the scanning plate with respect to the scale. This is true since such larger spacings are typically associated with larger spacing tolerances. With such a measuring device, in which a predetermined small distance between the reference marks and the associated scanning fields must not be exceeded, no high precision guides have to be provided. For this reason, a simply constructed and economical measuring arrangement is provided. Furthermore, the preferred embodiments described below do not result in any increase of the outside dimensions of the measuring system, so that the flexibility of use of these measuring systems is not impaired. Further advantageous features of the invention are set forth in the dependant claims.

The invention itself, together with further objects and attendant advantages, will best be understood by reference to the following description, taken in conjunction with the accompanying drawings.

FIG. 1 is a schematic representation of an incremental measuring device.

FIG. 2 is a partial perspective view of the scale of the measuring device of FIG. 1, which defines a measuring graduation and a number of reference marks.

FIG. 3 is a plan view of a scanning plate which defines scanning fields and is included in the scanning unit of the measuring device of FIG. 1.

FIG. 4 is a graphic representation of the amplitude of light modulation measured at various spacings between the scale and scanning plate of FIGS. 2 and 3.

FIGS. 5a-5f are schematic representations of five preferred embodiments of the improvement of this invention suitable for use in the measuring device of FIG. 1.

Turning now to the drawings, FIG. 1 schematically shows a photoelectric incremental length measuring device that includes a measuring scale M and a scanning unit A. The scale M and the scanning unit A are connected in each case (in a manner not shown) with objects to be measured, such as two machine parts of a processing machine. The scale M defines an incremental graduation T in the form of a line grid (FIG. 2) which is scanned in direct light, without contact, photoelectrically by the scanning unit A. Alongside the graduation T there are arranged two reference marks R.sub.1, R.sub.2 which are positioned at predetermined absolute positions with respect to the graduation T. Each of the reference marks R.sub.1, R.sub.2 is made up of a respective group of lines having an irregular line distribution. The line group distributions of the two reference marks R.sub.1, R.sub.2 are preferably as dissimilar as possible from one another. The scanning unit A generates periodic scanning signals S.sub.1, S.sub.2 as the scanning unit A moves in the measuring direction X. These scanning signals S.sub.1, S.sub.2 are amplified in the scanning unit A and converted into square wave signals S.sub.1 ', S.sub.2 ', which are applied to control an electronic counter Z which displays the measurement value in digital form. The square wave signals S.sub.1 ', S.sub.2 ' are displaced with respect to one another by a quarter of the grid constant P.sub.M (graduation period) of the graduation T in order to allow the direction of scanning to be determined. Reference signals RS.sub.1, RS.sub.2 are generated in response to the reference marks R.sub.1, R.sub.2, and these reference signals RS.sub.1, RS.sub.2, are converted into square wave signals RS.sub.1 ', RS.sub.2 ' which are similarly applied to the counter Z.

The reference signals RS.sub.1, RS.sub.2 can be used to trigger various functions in the counter Z. For example, by evaluating the reference signals RS.sub.1, RS.sub.2, an incremental measuring system can be made into a quasi-absolute measuring device if to each reference mark RS.sub.1, RS.sub.2 a number is allocated which represents its absolute position with respect to an invariable zero point. Furthermore, a predetermined one of the reference marks RS.sub.1, RS.sub.2 can be used to set the counter Z to the value zero when the associated one of the reference signals RS.sub.1, RS.sub.2 obtained from that reference mark R.sub.1, R.sub.2 is detected.

A scanning plate AP such as that shown in FIG. 3 is provided in the scanning unit A for the scanning of the scale M. This scanning plate AP defines two scanning fields AT.sub.1, AT.sub.2 for the scanning of the graduation T. These two scanning fields AT.sub.1, AT.sub.2 are displaced with respect to one another by a quarter of the grid constant P.sub.M of the graduation T. The two scanning fields AT.sub.1, AT.sub.2 are otherwise identical to the graduation T. Photosensors (not shown) are included in the scanning unit A and aligned with the scanning fields AT.sub.1, AT.sub.2 in order to generate the scanning signals S.sub.1, S.sub.2. The scanning plate AP also includes scanning fields AR.sub.1, AR.sub.2 for the scanning of the reference marks R.sub.1, R.sub.2. The line group distributions of the individual scanning fields AR.sub.1, AR.sub.2 are identical with the line group distributions of the corresponding reference marks R.sub.1, R.sub.2. For this reason, the reference signals RS.sub.1, RS.sub.2 are generated when the reference marks R.sub.1, R.sub.2 are aligned with the respective ones of the scanning fields AR.sub.1, AR.sub.2. Because the line distributions of the reference mark R.sub.1 and the scanning field AR.sub.1 correspond and the line distributions of the reference mark R.sub.2 and the scanning field AR.sub.2 correspond, it is assured that the scanning signal RS.sub.1 is only generated when the scanning field AR.sub.1 is aligned with the reference mark R.sub.1, and that the reference signal RS.sub.2 is only generated when the scanning field AR.sub.2 is aligned with the reference mark R.sub.2. If, for example, the scanning field AR.sub.1 is moved past the nonassociated reference mark R.sub.2, no reference signal is generated.

When collimated light passes through the periodic incremental graduation T of the scale M, refraction patterns of the graduation T of the scale M are generated in certain planes behind the plane of the graduation T of the scale M by virtue of interference effects of the light rays refracted by the graduation T. With a grid constant P.sub.M of the graduation T of the scale M and a center of gravity wavelength .lambda., the spacings of these planes is given by the formula n.multidot.P.sub.M.sup.2 /.lambda. (n=0, 1, 2, . . .) with respect to the plane of the graduation T of the scale M. FIG. 4 represents the amplitudes J of light modulation arising during relative movement between two graduations of equal grid constant as a function of the separation a between the two graduations. As shown in FIG. 4, maximum modulation and therefore optimal scanning signals S.sub.1, S.sub.2 are generated only at the discrete distances n.multidot.P.sub.M /.lambda. of the plane of the scanning fields AT.sub.1, AT.sub.2 of the scanning plate AP from the plane of the graduation T of the scale M.

Because of the irregular line group distributions used for the reference marks R.sub.1, R.sub.2 on the scale M, optimum scanning of the reference marks R.sub.1, R.sub.2 is obtained when the distance between the reference marks R.sub.1, R.sub.2 and the associated scanning fields AR.sub.1, AR.sub.2 on the scanning plate AP, measured in the direction of travel of light, is at most about 4d.sup.2 /.lambda., where d signifies the width of the narrowest line of the grid graduations of the reference marks R.sub.1, R.sub.2. However, as explained above, optimal scanning of the graduation T of the scale M requires that the distance between the graduation T and the associated scanning fields AT.sub.1, AT.sub.2 on the scanning plate AP in the direction of the light beam be about equal to n.multidot.p.sup.2 /.lambda. (n=1, 2, 3, . . .).

According to this invention, these conflicting requirements are met by adjusting appropriately on the one hand the optical path length of the light rays passing through the graduation T of the scale M and the associated scanning fields AT.sub.1, AT.sub.2 of the scanning plate AP, and on the other hand the optical path length of the light rays passing through the reference marks R.sub.1, R.sub.2 of the scale M and the associated scanning fields AR.sub.1, AR.sub.2 of the scanning plate AP. This is accomplished in certain embodiments of this invention by providing a phase difference h between these two optical path lengths, in which h is equal to the difference between the selected graduation scanning distance n.multidot.P.sub.M.sup.2 /.lambda. and the reference mark scanning distance 4d.sup.2 /.lambda.. This phase difference h is equal to c.multidot.l, where l is the difference in path lengths and c is the index of refraction obtaining in this path length difference.

In order to achieve the desired phase difference h=c'.multidot.l', according to FIG. 5a the scanning fields AT.sub.1 ', AT.sub.2 ' and the scanning fields AR.sub.1 ', AR.sub.2 ' of the scanning plate AP' are displaced parallel to one another in the direction of the light rays by the interval l'. The graduation T' and the reference marks R.sub.1 ', R.sub.2 ' of the scale M' lie in a single plane. c' signifies the index of refraction of the medium traversed by the light rays.

FIG. 5b shows a second preferred embodiment in which the same phase difference h=c'.multidot.l' is obtained by displacing the graduation T' by the interval l' with respect to the reference marks R.sub.1 ', R.sub.2 ' of the scale M' in the direction of the light beam path. The graduation T' and the reference marks R.sub.1 ', R.sub.2 ' remain parallel to one another in this embodiment but offset by the distance l'. The scanning fields AT.sub.1 ', AT.sub.2 ' and the scanning fields AR.sub.1 ', AR.sub.2 ' of the scanning plate AP' lie in a single plane.

FIG. 5c shows a third preferred embodiment in which a desired phase difference h=c".multidot.l" is achieved even though the scanning fields AT.sub.1 ", AT.sub.2 " and the scanning fields AR.sub.1 ", AR.sub.2 " of the scanning plate AP" lie in a single plane. Furthermore, the graduation T" and the reference marks R.sub.1 ", R.sub.2 " of the scale M" also lie in a single plane. The desired phase difference h is achieved by a transparent layer U" having a layer thickness l" in the light beam direction and a refraction index c". This transparent layer U" is arranged in front of the scanning fields AT.sub.1 ", AT.sub.2 " so as to obtain the desired phase difference between the graduation optical path and the reference mark optical path.

FIG. 5d shows a fourth preferred embodiment for the achievement of the desired phase difference h=c'".multidot.l'". In this embodiment the transparent scanning plate AP'" defines a layer thickness l'" in the light beam direction and a refraction index c'". The scanning fields AR.sub.1 '", AR.sub.2 '" are arranged on the surface of the scanning plate AP'" facing the scale M'", and the scanning fields AT.sub.1 '", AT.sub.2 '" are arranged on the surface of the scanning plate AP'" facing away from the scale M'". As shown in FIG. 5d the graduation T'" and the reference marks R.sub.1 '", R.sub.2 '" are coplanar. In this embodiment, it is the scanning plate AP'" itself which provides the desired difference in phase between the scanning paths of the graduation T'" and the reference marks R.sub.1 '", R.sub.2 '".

FIG. 5e shows a fifth preferred embodiment for the achievement of the desired phase difference h=c"".multidot.l"". In this embodiment, the scanning fields AT.sub.1 "", AT.sub.2 "" and the scanning fields AR.sub.1 "", AR.sub.2 "" of the scanning plate AP"" lie in a single plane which is inclined toward the plane of the graduation T"" and of the reference marks R.sub.1 "", R.sub.2 "" of the scale M"". The inclination of the scanning plate AP"" is about an axis of rotation running in the measuring direction X in such a way that the center point M.sub.1 "" of the scanning fields AT.sub.1 "", AT.sub.2 "" in the light beam direction is displaced by the interval l"" with respect to the center point M.sub.2 "" of the scanning fields AR.sub.1 "", AR.sub.2 "". As before, the symbol c"" is used to indicate the index of refraction of the medium traversed by the light rays.

FIG. 5f shows a sixth preferred embodiment of this invention in which the graduation T.sup.5 and the reference marks R.sub.1.sup.5, R.sub.2.sup.5 of the scale M.sup.5 have the same distance (for example n.multidot.P.sub.M.sup.2 /.lambda.) from the associated scanning fields AT.sub.1.sup.5, AT.sub.2.sup.5, and AR.sub.1.sup.5, AR.sub.2.sup.5 of the scanning plate AP.sup.5. In order to obtain optimal scanning of the reference marks R.sub.1.sup.5, R.sub.2.sup.5, there is provided an imaging optic O such as a converging lens which has optical properties to enable appropriate scanning of the reference marks R.sub.1.sup.5, R.sub.2.sup.5 in the face of greater than optimal separation between the reference marks R.sub.1.sup.5, R.sub.2.sup.5 and the associated scanning fields AR.sub.1.sup.5, AR.sub.2.sup.5.

The present invention can also be used with particular advantages in other measuring systems, in which the graduations of the scale and of the scanning fields of the scanning plate are not identical. For example, in a three grid measuring system, in which the graduation of the scanning field is twice as great as the graduation of the scale, particular advantages are obtained. In such a three grid measuring system (as described for example in German DE-OS 25 11 350), using a non-parallel beam path there can be achieved very large spacings with relatively great spacing tolerances. Of course, it should be understood that a wide range of changes and modifications can be made to the preferred embodiments described above. It is therefore intended that the foregoing detailed description be regarded as illustrative rather than limiting, and that it be understood that it is the following claims, concluding all equivalents, which are intended to define the scope of this invention.