GB2156989A

GB2156989A - Device and method for measuring and adjusting lengths

Info

Publication number: GB2156989A
Application number: GB08508212A
Authority: GB
Inventors: Gerd Schuchardt; Hans-Joachim Freitag; Friedrich Tzschach; Rolf Thiemer; Lothar Voigt
Original assignee: Carl Zeiss Jena GmbH
Current assignee: Jenoptik AG
Priority date: 1984-04-02
Filing date: 1985-03-29
Publication date: 1985-10-16
Also published as: DD225497A1; GB2156989B; CH669254A5; JPS6144320A; IT8567290A0; IT1184929B; GB8508212D0; DE3506537A1; IT8567290A1

Abstract

A method and device for measuring and setting of lengths, comprising a fixed scalp (3), a movable scanning device (4) which includes two detector arrangements 7, 71 and 8, 81, and an evaluation unit. The second of the detector arrangements is separated from the first detector arrangement by a very precise distance A. It is on the basis of the distance A that measurements are established by the device. In operation, eg of a movable support 1 along a lathe bed, primary pulses produced by one detector arrangement are compared with secondary pulses produced by the other detector by means of the electronic evaluating unit to modify the position measuring output to take account of inaccuracies in the operating of the scale 3, eg caused by temperature variations. <IMAGE>

Description

SPECIFICATION Device and method for measuring and adjusting lengths The present invention relates to a device and method for measuring and adjusting lengths for use in translatory incremental measuring systems, and is particularly, but not exclusively,suitable for measuring long lengths.

It is known to use incremental measuring systems for measuring and adjusting lengths and angles.

The known measuring systems comprise at least one scale, which serves as a length or angle measure, provided on a scale mount and a scanning device, the one being displaceable relative to the other. In the course of relative displacement, a sequence of signals is derived which corresponds to the relative positions of the scale and the scanning device. The electrical output signals of the measuring system are fed into an electronic evaluation circuit.

The precision of the measuring system depends substantially on the precision of the scale devision of the scale and on the material properties of the mount. A plurality of solutions in this regard are known from US-PS 3 100 345, DE-AS 1 051 519, DE-AS 1 266 986, CH-PS 474 049, DE-AS 1155 611 and other which concern the technology of applying very precise grating devisions, and the measuring technology for ensuring the quality and the detection of correction values.

The costs involved in producing very precise gratings increase with increasing lengths to be measured if the precision requirements are to be maintained.

The precision obtained in the manufacture of scale divisions, including the detected correction values, is dependent upon the state of the scale mount at the time of manufacture. However, the initial precision is not obtained indefinitely in the course of operation by the user as a result of aging and deformation of the grating mount.

Based on the aforegoing knowledge, particular requirements for the scale or grating mount have had to be realised up to now: namely a long-term constancy and sophisticated installation conditions. Known measuring systems employ materials for graduation and scale mounts having comparatively high thermal expansion coefficients (glass, steel, or=lO Fm/K m). Thus the indication of the position depends on the respective ambient temperature, unless special correction measures are provided for their compensation.

It is an object of the present invention to obviate the above disadvantages of the known measuring systems.

In accordance with a first aspect of the present invention, there is provided a device for measuring and adjusting lengths comprising a scale disposed on a mount, a scanning means which includes at least a first and a second detector arrangement, and an evaluation unit, said second detector arrangement having, considered in the direction of the scale division, a very precise distance A relative to the first detection arrangement, the distance A representing the basis on which measurement is made by the device.

It is advantageous when the distance A is embodied by a material path, the thermal expansion coefficient of which approximates to zero.

In accordance with a second aspect of the present invention, the second detector arrangement, which is provided at a distance A relative to the first detector arrangement in the scanning direction, derives correction signals in interaction with a primary sequence of signals which correspond to the position of the scale division relative to the scanning means, the correction signals being at least at one time evaluated by the evaluation unit for correcting the primary sequence of signals in the course of a progressive relative movement between the scale and the scanning means on a path B equal to the distance A, the evaluated correction signals being a measure of the difference of the path B to the value of the primary sequence of signals, which is equivalent to the path and which results when the first detector arrangement moves to the position of the second detector arrangement and wherein the primary sequence of signals is evaluated by the evaluation unit in such a manner that the sequence of secondary signals delivered from the device for all relative positions related to any initial position are formed from the sequence of primary signals by involving correction values derived from at least one second detector arrangement and evaluated in the evaluation unit.

In the present arrangement, the scale is not used for making the actual measurement itself but serves rather to provide bases to which, in the course of a measuring operation, the actual measuring information is associated.

The actual basis for the measurement is the very constant distance A between the first and the second detector arrangements, the first detector arrangement of which delivers primary signals related to a definite starting position according to the principle of known scale systems, and the position equivalent association of which primary signals depends on the defective scale divisions.

These primary signals, however, are not the starting signals for the device according to the invention.

By means of the second detector arrangment, information is produced by evaluation of the signals generated thereby indicative of the distance between the sections of the scale scanned by the first and the second detector arrangement related to the distance A of the two detector arrangements. When the first and the second detector arrangements, in the course of a progressing movement, sweep the grating scale, the signal from the second detector arrangement is sampled exactly when the first detector ar rangement delivers its counting pulses, for example, when the grating divisions are opto-electronically scanned in a known manner by triggering the analog voltage zero passage, then it is feasible to detect the length which the first detector arrangement and, hence, the entire scanning device has covered to arrive from the photoelectrical centre (K) of its scanned section, that is, from the position where the counting pulses have just been triggered, at the photoelectrical enter (K+1) of the section which has just been sampled by the second detector arrangement.

This length is the result of the distance A plus the signed product of signal slope and signal size, that is, of the correction signal from the second detector arrangement of the moment of scanning.

When this operation is repeated at the moment when the first detector arrangement is positioned at the photoelectrical centre K±l and delivers its counting pulses from there, the distance to the position of the photoelectrical center of the section where the second detector arrangement is positioned can be detected.

In a continuous repetition of this operation at all positions K-l, K, K+1, K+2 etc. and starting from a definite origin position, the required information can be obtained in the evaluation device by use of known hardware and a suitable algorithm (soft-ware) where these designated positions are factually independent of the number of delivered primary signals from the first detector unit.

These data are provided in the sense of a linked measure where a term of the sum consists of a constant value A and of the departure therefrom measured by the second detector arrangement.

By use of the evaluation device it is now feasible by suitable electronic means to arrange for the number of known secondary pulses which are delivered as output values by the device to correspond to the respective actual position. This is achieved in that correction pulses are added to or deducted from the sequence of primary pulses only when this is necessary within the frame of the precision of the system to establish accordance of an indication in a respective indication unit of the system with an actual position.

In order to obtain advantageous effects with detailed problems of information transfer and information processing it is feasible and reasonable also to perform a signal evaluation of the second detector arrangement between the disignated positions K, K+1, K+2 etc. and to exploit the advantages of this redundancy of determination.

It is feasible in the course of realising a forward and a backward movement according to the disclosed principle to provide a detector arrangement having an analogors function and in addition to the second detector arrangement at the other side considered in the direction of division at a distance A adjacent the first detector arrangement.

From this, the information for formation of the secondary pulses can be derived for both operation directions.

It is of particular advantage in cases where the scales comprise a plurality of quasi-identical groups of graduation marks that: a) the width of the countergratings of at least the first detector arrangement or, in the event of a matrix, the effective width of its scanning groups has to be equal to the width of the group of graduation marks; b) the distance A has to be substantially equal to the width of the group of graduation marks; and c) the designated positions (K-l), K, (K+1), (K+2), etc. mentioned hereinbefore have to be so arranged by a suitable association of the defined initial positions relative to the position of the groups of graduation marks that they coincide with the centres of the groups of graduation marks.

Under these conditions a particularly favourable adaptation of the secondary pulses according to the actual position is feasible, since a linear course of the position error of the primary signals is obtained between the positions (K), (K+1), (K+2), etc.

With reservations, this is also valid when under the conditions a) to c) to width of the group of graduation maks is not used as a criterion but an integral multiple thereof.

The use of a matrix arrangement of a plurality of countergratings with the first detector arrangement is particularly to be preferred, on the one hand, to exploit its advantages in known manner, as concerns sweeping stability and error correction, and on the other hand to satisfy the conditions a) and b) when the first detector arrangement simultaneously includes a plurality of countergratings and scanning groups, respectively, of different phase positions in order to form the forward-backward information in known manner.

Said correction signal will result, as already mentioned, as a signed product of signal slope and signal size from the second detector arrangement (and a third, etc. respectively). Therefore, it is necessary to have said signal slope as a constant computation value at one's disposal in the evaluation unit.

Its numerical association is exactly correct when, by use of same operation paths and scales having different division errors (which can be achieved, for example, by differently tensioning an elastic scale mount), equal indication values will result in the device.

Hence, the numerical association is modified until this is achieved.

The reference value for the abovementioned linked measure is the distance A. It is not necessary to exactly adjust this size between the first and the second, respectively, any further detector arrangement or to detect it by a direct measurement. It is sufficient to approximate this distance within the frame of, for example, the criterion mentioned under b); however, a long time constancy has to be ensured. The exact association to a measure can be performed in that an approximated value for A is fed into the evaluation unit and used for formation of the secondary pulses and the indication of the device is compared to a norm by a definite procedure. A computation size results from the comparison which size is subsequently stored in the evaluation unit and which permits one to associate the device with the actual position and to the unit of length, respectively.

It can be appropriate in the course of a measuring and positioning operation, respectively, in which repeated forward and backward movements take place, to store the detected correction signals from the second detector arrangement in the evaluation unit and, when the respective positions of the grating are swept again, to omit a new evlauation of the signals from the second detector arrangement but to utilize the stored correction signals.

The constancy of the basis A could be deteriorated by the zero drift of the detector channels and of its illumination device which participate in the formation of the correction signals. Therefore, it is advantageous to connect and to calibrate these dectector channels with their illumination device before or after a measurement with respect to their zero point position and/or signal slope by means of areas which have a definite transparency of a definite reflectance.

In order to render superfluous a very highly precise positioning setting, it is advantageous to make these areas homogeneous over a sufficient width, that is, free of gradients with respect to their transparency and reflectance, respectively. These areas can be secured to the scale or, in order to start with a measurement at any desired position of the operation range of the device and to define the zero point connection there, it can be associated with the scanning device.

Since in a device according to the invention the scale mount with its scale division is only an auxiliary scale and does not serve as a measuring norm, it is feasible to employ elastic materials and to install these scale mounts so as to be pretensioned in use so that, in contrast to known devices, the division precision can be lost.

Thus, it is achieved that all division marks remain at their geometrical position even during a measurement where temperature variations of the scale mount occur provided that the pretensioning is sufficient.

In this manner, temperature influences are substantially eliminated.

Thus, an advantageous feature of the present invention is to provide a method and device for measuring lengths which, though maintaining the precision of the measuring system itself, permits a reduction of the requirements in the scale division precision, in particular, to the scale mount and to the technology of applying the scale division and the associated measuring technology.

It is another advantageous feature of the present invention to provide a method and device for measuring lengths particularly large lengths greater than 1 metre, which renders the precision of the system independent of aging systems, deformations of the scale mount and of temperature influences on the latter.

In order that the invention may be more readily understood, reference is now made to the accompanying drawings which illustrate diagrammatically and by way of example only one embodiment thereof and wherein: Figure 1 is a schematic view of a length measuring device installed in a lathe; Figure 2 is a schematic, enlarged view of the detector arrangements of the length measuring device of Fig. lad Figure 3 is a block-schematic drawing of the signal processing members of the length measuring device.

In Fig. 1 a device for measuring and adjusting lengths is provided on a lathe for measuring displacement of a support 1 relative to a bed 2. A scale 3 is attached to the bed 2 in a longitudinal horizontal direction. A scanning device 4 is secured to the support 1 so as to be narrowly parallelly spaced from the scale 3. The support 1 and the scanning device 4 are commonly displaceable in directions indicated by a double arrow L. The scanning device 4 is provided with a window 5 for indication of the actual position of the support 1 relative to the bed 2.

The scanning device 4 comprises, about an optical axis (not shown) which is at right angles to the drawing plane and perpendicular to the scale 3, a light source (not shown), a telecentrical optical imaging system (not shown), a detector arrangement (Fig. 2),and an electronic evaluation unit (Fig. 3).

The scale 3 is a lithographically produced grating which consists of a regular sequence of substantially equally spaced alternating opaque and transparent division lines which are at right angles to the longitudinal extension direction of the scale body 3 in the plane of the surface of the scale body 3.

Furthermore, the scale 3 can comprise individual units of quasi-identical graduation which are fitted together to yield the scale 3 shown in Fig. 1.

It is to be understood that this scale need not be a perfect one as will be explained hereinafter in connection with the operation of the invention.

It is also feasible to construct the scale of an elastic material which will be tensioned to the machine bed 2.

Fig. 2 shows the scale 3 and a part of the detector arrangement in more detail and at right angles to the abovementioned optical axis defined by the telecentrical illumination system. The optical axis passes the detector arrangement at 0. To a transparent mount 9, which is disposed at a narrow and parallel space to the surface of the scale 3, four gratings 7, 7', 8, 8' are attached, namely, a countergrating 7 having regular and parallel division lines alternatingly transparent and opaque, a countergrating 7' arranged beside the grating 7, the opaque and transparent division lines being displaced to those of the countergrating 7 to effect a 180 phase shift. The countergratings 8 and 8' are adjacently and subsequently arranged to the countergratings 7 and 7', respectively, considered in the direction of displacement indicated by the double arrow L (Fig. 1).Again the division lines on grating 8 are staggered to those of grating 8' to effect a phase shift of 180 . The countergratings 7, 7', 8, 8' are commonly displaceable by means (not shown) relative to and along the scale 3. Each of the countergratings 7, 7', 8, 8' is provided with (not shown) focussing means and photodetectors. Thus, the countergrating 7 is followed by a first focussing means and a (first) photodetector, which, in turn, is connected via an interpolation device 10 (Fig. 3) to an electronic evaluation device 11. A similar arrangement is provided for the countergrating 7', which is also folowed by a (second) focussing means and a (second) photodetector which is also connected to the electronic evaluation device 11 via the interpolation device 10.

In contrast to the latter arrangement, the countergratings 8 and 8' are connected in-line via a third and a fourth focussing means, repectively, and a third and a fourth photodetector, respectively, to the evaluation device 11.

The scale 3 (Fig. 2) comprises a plurality of units of quasi-identical graduation, only the units 30, 31, 32, 33 of which are visible in the drawing.

The reference numerals 30, 31, 32, 33 correspond to the designations K-2, K-1, K, and K+1, respectively. Each of the units 30, 31, 32, 33 includes an arrangement of division lines of alternating opaque and transparent stripes op and tr.

An important feature of the invention lies in the constant distance A between the detector arrangements 7, 7', and 8, 8', corresponding to the distance between the photoelectric centre C of the detector arrangement 7, 7' and the centre C' of the detector arrangement 8, 8'.

In a practical embodiment of the measuring device, the units 30, 31, 32, 33 of the grating 3 each have a length of 12mm and carry 1500 division line (4 pm line, 4 am gap). The two detector arrangements 7, 7' and 8, 8' each have a respective length of 12mm.

The distance A = 12 mm is considered from the photoelectric centre C of the detector arrangement 7, 7', to the centre C' of the detector arrangement 8, 8'.

In operation, the scanning device 4 is displaced with the support 1 relative to the scale 3 on the bed 2.

Since the scanning device includes the detector arrangements 7, 7' and 8, 8', the first detector arrangement 7, 7' produces a sequence of primary pulses when sweeping the scale 3 which are fed into the interpolation unit 10. The sequence of primary pulses comprises one pulse per 0.5 ,um (T = 0.5 slum, where T is the resolution of the interpolation unit 10).

When, for example, the detector arrangement 7, 7' moves from the position XK 1 to the position XK, the path covered is XK - XK1 = M + S. UK where M = the base of measurement which approximately equals the distance A, S = signal slope (voltage per variation of path) of the second detector arrangement 8, 8', UK = signal voltage of the second detector arrangement 8, 8' when the first detector arrangement 7, 7' delivers its scanning pulses at the position XK-11 digitized by means of an A/D conversion.

The length of the measuring base M which is, as mentioned hereinbefore, substantially equal to A, is also 24.000 x 0.5 Am = 12 mm (N = 24.000 is the number of primary increments when sweeping an entire unit of quasi-identical graduations). By introducing a correction constant K, one obtains: M=N.T+K Hence, a number of correction pulses KIK is obtained which permits the formation of a secondary se quence of pulses from the primary sequence of pulses in the interval XK " XK as an amount of the next lower multiple integer of the term K + S. UK + R,1 r T where RK is the remainder from the preceding interval as a result of the quantisation by 0.5 ,sum.

Hence, the subsequent connection RK becomes RK = (K + S. UK + RK-1) - KIK. T < 0.5 ijm.

The grating division of the scale 3 is produced in such a manner that the correction signals are always negative (too short a division) so that the correction only consists in a suppression of pulses rather than an addition. This correction is performed by the evaluation unit 11 which includes a computer with suitable ancillary hardware.

The secondary pulse sequence which corresponds to the actual progress, that is movement of the scanning means 4 relative to the scale 3, results from the primary pulse sequence by suppression of KIK pulses in the respective interval, for example XK, from which the value KIK results by use of known electronic means so that exactly N-KIK secondary pulses are delivered, for example, between XK-1 and XK.

Starting from an actually realized slope of the second detector arrangement 8, 8', the slope S is a computation value in the evaluation algorithm which, when the system starts operation, is matched until the same indication is visible each time in the window 5 produced by the secondary pulse sequence for a definite path of operation for example, XK 1 independent of the division errors which the scale 3 may have.

The correction value K is obtained in that, over a path of operation of for example 1200 mm, the difference between the output of the system displayed in the window 5 and this path of operation is formed and is associated according to the measuring basis: indication -1200 mm K= 100 By inputting this value K to the respective storage location of the evaluation device 11 and, hence, into the evaluation algorithm, the "metrological connection" of the system is obtained, independent of the division errors of the scale 3 divisions with which the scanning device 4 is operated.

The term "metrological connection" used herein is to be understood as the unique association of the measuring system to the meter norm.

This is achieved, for example, by employing a laser path measuring system. Under control of the latter, the support 1 is exactly displaced by 1.2 meter.

indication -1200 mm The correction value K = 100 is now the new correction value for the evaluation unit 11, which ensures the exact metrological connection.

To this end, the previous K is cancelled and the new precise one is fed into the respective storage location in the evaluation unit 11.

Furthermore, the very precise distance A might suffer from zero drift of the first, second, third and fourth photodetectors and the respective channels connected thereto. To counter this, the scale 3 has an area 12 (framed in Fig. 1 by dashed lines) at one end portion of the scale 3. This area 12 is at least of the size of the two detector arrangements 7, 7', and 8, 8' and has either a transparent or a reflective surface.

The reflectance or transparency of the area 12 has a definite proportion. However, this proportion is very homogeneous so that no gradients exist. This definite proportion is suitable to set and adjust the zero passage of the electrooptical channels and the slope of the switching pulses in said channels. In the latter case the area 12 comprises at least two sub-areas of definite but different reflectance or transparency.

Claims

1. A device for measuring and setting of lengths, comprising a grating disposed on a grating mount, a scanning device which includes at least two detector arrangements, and an evaluation unit, a second of said detector arrangements having, considered in the direction of the grating divisions, a very precise distance A relative to the first detector arrangement, the distance A representing the basis for the metrological connection of the device.

2. A device as claimed in claim 1, wherein a further detector arrangement is provided in the opposite direction to the direction of division and wherein the distance between the further detector arrangement and the first detector arrangement corresponds to the distance A.

3. A device as claimed in claims 1 and 2 in which the grating comprises at least two quasi-identical grating units and in which the distance A equals an integral multiple, the simple length included, of the grating units.

4. A device as claimed in claims 1 and 2, wherein the distance A is embodied by a material path, the thermal expansion coefficient of which is substantially zero.

5. A device as claimed in claims 1 and 2, in which the grating is scanned by the scanning device by use of optoelectronic means, and in which the device includes at least one homogeneous reference area of a definite reflectance or transparency which serves to adjust the zero point position and signal slope of at least those optoelectronic channels which are involved in the detection of the correction signals, said areas being connectable to said measuring device before each measurement.

6. A device as claimed in claims 1 and 2, wherein the reference faces are associated with the scanning device and, hence, the optoelectronic channels are connnectable at any desired position within the operation range of the device.

7. A method for measuring and setting lengths with a device which comprises a grating disposed on a grating mount, a scanning device which includes at least two detector arrangements at a mutual distance A, and an evaluation unit, wherein from at least a first detector arrangement of said two detector arrangements a primary signal sequence is derived corresponding to the relative positions of the grating and the scanning device, and wherein at least a second detector arrangement of said two detector arrangements, which is disposed at said distance A relative to the first detector arrangement in the scanning direction, derives correction signals in interaction with a primary sequence of signals which correspond to the position of the grating relative to the scanning device, the correction signals being at least at one time evaluated by the evaluation unit for correcting the primary sequence of signals in the course of progression of the scanning device relative to the grating on a path B equal to the distance A, the evaluated correction signals being a measure of the difference of the path B to the path equivalent value of the primary sequences of signals, which results when the first detector arrangement moves to the position of the second detector arrangement, and wherein the primary sequence of signals is evaluated by the evaluation unit in such a manner that the sequence of secondary signals delivered from the device for all relative positions related to any initial position are formed from the sequence of primary signals involving correction values derived from said at least one second detector arrangement and evaluated in the evaluation unit.

8. A method as claimed in claim 7, wherein the distance A which is the basis for the metrological connection of the device is determined with respect to its very precise connection to a unit of length in that, by a comparison performed between the indication of the device to a calibration norm over a definite path of operation, a computation size is obtained and stored in the evaluation unit, and ensures the connection of the device to the unit of length in the frame of an evaluation algorithm.

9. A method for measuring and setting lengths, substantially as herein before described with reference to the accompanying drawings.

10. A device for measuring and setting of lengths, substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.