Incremental measuring instrument

An instrument for measuring the movement of a first object with respect to a second object is disclosed including a rotatable opaque disc mounted on the first object, which disc defines a plurality of translucent involute stripes proceeding from a base circle, a scale including a reflective grid pattern mounted on the second object, and an optical system for projecting an image of a portion of the grid pattern onto an image zone of the disc such that the stripes of the image of the grid are substantially perpendicular to a line drawn tangent to the base circle through the image zone.

BACKGROUND OF THE INVENTION 
The present invention relates to an instrument for measuring the 
displacement or the velocity of an object relative to a reference surface. 
In general terms, this instrument includes a rotating disc on which is 
formed an optical, spirally striped pattern, and which is mounted either 
on the reference surface or to follow the movement of the object. A scale, 
having an optical pattern of stripes preferably arranged perpendicular to 
the direction of movement of the object, is mounted either to follow the 
movement of the object or on the reference surface. By means of an optical 
arrangement an image of one of the striped patterns is projected onto the 
other striped pattern, and a photodetector is provided for measuring the 
light passed by the other striped pattern. Means are provided for 
generating a reference signal which is applied as an input to a circuit 
for evaluating the measuring signal provided by the photodetector and 
comparing it to the reference signal. 
In one device of this type known to the art, the optical, spirally striped 
pattern consists of a long, multiply winding translucent spiral in an 
otherwise opaque disc (German Pat. No. 1,177,353, FIG. 4). The pitch of 
the spiral is constant and corresponds to the spacing of straight, 
parallel bars of the striped pattern of a scale arranged near the disc. In 
order to properly orient the two striped patterns, at least in a small 
zone of intersection between the two patterns, the stripes of the pattern 
on the scale are arranged tangentially to the stripes of the pattern on 
the disc in the intersection zone and thereby substantially perpendicular 
to the radius of the disc which passes through the intersection zone. When 
the scale is stationary with respect to the center of rotation of the 
disc, the two striped patterns intersect to pass light to a photodetector 
once with every revolution of the disc. The frequency of the measuring 
signal given off by the photodetector is therefore equal to the rotation 
frequency of the disc. 
This often results in a relatively low frequency measuring signal, which 
can be an important disadvantage when circuits are used for evaluating the 
measuring signal in which the accuracy of the result is substantially 
proportional to the frequency of the measuring signal. This is often the 
case, for example, when digital circuits are used to compare the number of 
wave trains of the measuring signal and of a reference signal. With 
devices of this type, the disc must be driven at a very high velocity in 
order to achieve high accuracy, in practice resulting in a device which is 
often expensive to manufacture. 
In another known device of the prior art the stripes of the striped pattern 
of the disc are radially arranged (German Pat. No. 1,177,353, FIG. 2). A 
scale with a striped pattern having straight, parallel stripes is arranged 
near the disk and oriented such that the scale stripes run parallel to a 
radius of the disc passing through the midpoint of the image zone 
allocated to the photodetector. Here, the frequency of the measuring 
signal is a multiple of the frequency of the disc, corresponding to the 
number of radial stripes, and it is therefore sufficiently high for the 
previously mentioned circuits for the evaluation of the measuring signal. 
An important disadvantage here, however, is the fact that a complete match 
between the stripes of the two striped patterns is not possible, since the 
radial stripes which do not pass through the center of the image zone form 
an angle to the parallel stripes of the striped pattern of the scale which 
increases with distance from the center of the zone. Therefore, when an 
image zone large enough to produce a sufficiently large measuring signal 
is used, a measuring signal with a high interference signal constituent is 
often obtained. 
SUMMARY OF THE INVENTION 
The present invention is directed to an instrument of the type mentioned at 
the outset, in which the measuring signal corresponding to the brightness 
fluctuations of the image zone presents an amplitude and frequency 
sufficient for the further processing of the signal and in which the 
interference constituent of the measuring signal is low. 
In the instrument of this invention, the stripes of the striped pattern of 
the disc are arranged in the form of involutes, which proceed from a base 
circle centered at the pivot point of the disc. Furthermore, the 
projection of the one striped pattern on the other striped pattern is 
arranged such that the stripes of the striped pattern of the scale are 
oriented substantially perpendicular to a tangent passing from the image 
of these stripes at the disc onto the base circle of the involute stripes 
of the striped pattern of the disc. 
Because the involute stripes of the striped pattern of the disc have the 
same center of curvature in the image zone the involute stripes can be 
brought into virtually complete alignment with stripes arranged parallel 
to one another on the striped pattern of the scale. A striped pattern with 
a plurality of involute stripes can be arranged on the disk. The frequency 
of the measuring signal is higher than the frequency of the disk by a 
factor equal to the number of involute stripes on the disk. 
It is particularly advantageous to place the image zone in such a way that 
the length of the tangent between the base circle and the intersection of 
the striped patterns is large in the scale of the image in comparison to 
the length of the stripes of the striped patterns in the image. By reason 
of the small relative change of radius of curvature between the involute 
stripes in the image zone, the curvature of these stripes is virtually the 
same and these stripes can be brought into very good alignment with the 
stripes of the scale. The stripes of the scale can be formed either as 
adjacent, like-oriented, slightly curved stripes having a radius of 
curvature that corresponds to the mean radius of curvature of the involute 
stripes of the disk, or alternately, as straight, parallel stripes. The 
latter embodiment is especially simple to produce. 
The striped patterns may be formed of alternately translucent and opaque or 
alternately reflecting and absorbing stripes. By suitably combining 
translucent or reflecting striped patterns, a measuring device optimally 
adapted to the particular test geometry can be constructed. For example, a 
light-reflecting striped pattern is often to be preferred for a scale 
mounted on the object. The remaining parts of the device can be installed 
in a housing remote from the object. This is of particular importance in 
environments in which unfavorable conditions for measuring instruments 
(such as dirt, refrigerants or lubricants used with machine tools) are 
found. 
The light given off by the image zone comprises equally long periods of 
light and dark if the stripes of the two striped patterns are equally wide 
in the projected image. In this case, the measuring signal given off by 
the detector is suited especially well for evaluation by the circuit. 
When the object is set in motion, and thereby one of the two striped 
patterns, for example the scale pattern, moves with respect to the other, 
then the measuring signal changes in phase and frequency with respect to 
the measuring signal corresponding to the object at rest. The frequency 
change is proportional to the velocity of the object, and the phase 
displacement is proportional to the interval moved by the object. For the 
determination of the changes of the measuring signal arising from the 
movement of the object, the measuring signal is compared with a reference 
signal. It is possible to obtain a reference signal by scanning a 
particular optical reference pattern of the disk by means of a detector. 
This has the advantage that possible fluctuations in the rotational 
velocity of the disc work out in like manner on the measuring system as on 
the reference signal and, with suitable evaluation by the circuit, do not 
influence the measuring result. This approach offers the additional 
advantage that faults in trueness of rotation of the disc do not cause 
measuring errors, since the detector providing the measuring signal and 
the scanning arrangement providing the reference signal can be arranged in 
exact alignment in the measuring direction. 
A special reference pattern becomes unnecessary if the striped pattern of 
the disc is used as the reference pattern. If it is desired, however, to 
obtain a reference signal having as high a frequency as possible, then it 
is advantageous to form the reference pattern as a striped pattern with 
radially running stripes, since a specially produced reference pattern can 
be more densely occupied than the involute stripe pattern of the disc, and 
radially running stripes are especially simple to produce. 
In the case of a disc driven at constant velocity the scanning arrangement 
may be dispensed with if the reference signal is delivered by a function 
generator, as for example by a quartz-stablized multivibrator. 
The circuit for evaluating the reference signal and the measuring signal 
can be executed in such a way that the circuit includes a up-down counter 
for the formation of a digital value representing the displacement of the 
object, which counter adds and subtracts signals derived from the 
measuring signal and the reference signal. This circuit can be constructed 
simply to deliver a digital value representing the displacement of the 
body, which signal is very well suited for further processing in data 
processing installations such as machine tool controls. 
The accuracy of the measurement is increased if the up-down counter is used 
to count very high frequency signals derived from the measuring signal and 
the reference signal. This is achieved by the means that the circuit 
includes at least one subdivider that delivers an output signal with a 
frequency greater by a prescribed factor than an input signal. 
In a second embodiment of the invention, the rotating disc may be driven at 
a rotational velocity regulated in such a manner that the frequency of the 
measuring signal is held constant instead of the rotational velocity of 
the disc. 
In this case it is the reference signal indicative of disc velocity which 
is evaluated to determine the position or velocity of the object. In that 
the reference pattern may be formed with closely spaced stripes, this 
embodiment may produce a high frequency measurement signal suitable for 
processing in the evaluating circuit. 
The invention, together with further objects and attendant advantages, will 
be best understood by reference to the following detailed description 
taken in the conjunction with the appended drawings.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
Referring now to the drawings, FIG. 1 shows a scale 12 provided with a 
striped pattern 10 of alternately reflecting and light-absorbing stripes 
11, which is connected to an object 14, the movement of which is to be 
measured relative to a reference surface 16. For this, a casing 18 is set 
up at a distance from this scale 12 on the reference surface 16, which 
casing seals off the components necessary for the measurement in a dust 
tight manner. The casing also seals out external light except for that 
which enter through a measuring opening 20, and protects these components 
from mechanical damage. Inside the casing 18 a disc 26 is rotatably 
mounted, driven by a motor 22 by means of a coupling 24. The disc 26 is 
mounted free of play on a ball bearing or roller bearing 28. Inside the 
casing 18 there is arranged a first light source 30, preferably a 
semiconductor light source, which by means of a partially reflective 
mirror 32 and a system of lenses generates an image 40 of the striped 
pattern 10 of the scale 12 on the disc 26. For this the light source 30 is 
projected via a condenser lens 34 and the partially reflective mirror 32 
onto a lens 36. The lens 36 is located in the focus of a field lens 38 
mounted in the measuring opening 20, which projects the light source 30 
into infinity and therefore illuminates the striped pattern 10 uniformly. 
Lens 36 and field lens 38 are arranged so that the stripes 11 are sharply 
projected onto the disc 26. They form there the stripes 11' of the image 
40. Since the disc 26 has a striped pattern 42 (as described below) of 
alternately translucent and opaque stripes 44, light from the image 40 of 
the striped pattern 10 of the scale 12 can pass through the disc 26 and be 
measured by a photosensitive detector 46. Since the disc 26 rotates, there 
is obtained with object 14 at rest a measuring signal f.sub.1 with 
alternating amplitude corresponding to the alternating mutual covering 
over of the two striped patterns 40 and 42. In FIG. 2 there is seen the 
disc 26 from the side of the detector 46. The image 40 of the striped 
pattern 10 of the scale 12 projected onto the other side of the disc 26 is 
indicated with a broken contour line 48. The light coming from the zone 
enclosed by this contour line 48 is received by the detector 46. For the 
formation of a reference signal f.sub.2 independent of the movement of the 
object 14, there is arranged a second detector 50 behind the disc 26, 
which detector 50 scans a uniformly illuminated reference pattern 52 of 
the disc 26. For this the reference pattern 52 is illuminated by a second 
light source 54 via a condenser lens 56. Between light source 54 and 
detector 50 there is arranged a diaphragm 58, which has either a single 
diaphragm opening of the width of stripes 60 of the reference pattern 52, 
or has a striped pattern 56 corresponding to the reference pattern 52. The 
diaphragm 58 as well as the zone of the reference patterm 52 illuminated 
through the stripe pattern 62 of the diaphragm are shown in FIG. 2. This 
zone is indicated by the broken contour line 64. 
The striped pattern 10 of the scale 12 is composed of reflecting and 
light-absorbing stripes 11 of equal width of, for example, 0.5 mm, i.e. of 
a period of 1 mm. A rectangular field, for example 11.times.25 mm in size, 
of the striped pattern 10 is projected as image 40 onto the disc 26 in the 
zone bounded by the contour line 48. The lens system of lens 36 and field 
lens 20 has a projection ratio of 1:5.5, and the image 40 therefore has a 
length 1 of 4.5 mm and a width b of 2 mm. In FIG. 4 dotted lines are used 
to indicate some of the projected stripes 11' of the striped pattern 10 in 
the image 40. The period of these projected stripes is 0.18 mm. The 
stripes 11' are projected onto the involute stripes 44 of the striped 
pattern 42 of the disc 26. This is represented in FIG. 4, in which the 
curvature of the stripes 44 is exaggerated for the clarification of the 
drawing. The radius of curvature r of the stripes 44 is a function of the 
position of the image 40 on the disc 26. FIG. 3 shows a disc 27 in which 
the striped pattern 42 serves also as reference pattern, which is 
indicated by the contour line 64 inside the involute stripe pattern 42. In 
correspondence to FIGS. 1 and 2 the detector 50 scans the zone of disc 27 
bounded by this contour line 64. 
The stripes 42 follow involutes that describe the curves defined by the 
ends of different-length threads in winding off from a base circle 68. The 
radius of curvature of a point on an involute, for example, inside the 
contour line 48 of FIG. 3, is, therefore, the length of the line 72 
tangent to the base circle 68, between the point on the involute and the 
base circle 68. The base circle 68 is centered in the pivot point 70 of 
the disc 27. The stripes 44 are slightly arched with an arching height h, 
which should preferably be small with respect to the period t. This is 
achieved by the means that the radius of curvature r is chosen large with 
respect to the width b. The magnitude of the arching height h is given by 
the relation 
EQU h=(b.sup.2 /8r 
Thus, with a radius of curvature r, of, for example, 25 mm and a width b of 
2 mm, an arching height h of 0.02 mm is obtained. 
Since the tangent 72 is simultaneously radial for the stripes 44, the 
object 11 of the striped pattern 10 are projected into the image 40 in 
such a way that they stand perpendicular to the tangent 72. They are then 
oriented the same as the stripes 44 and give a sharp covering off of the 
two patterns 40 and 42. 
The signals f.sub.1 and f.sub.2 generated by the detectors 46 and 50, 
respectively, are fed to a circuit 74 (FIG. 1) which generates a reference 
signal g representating the displacement of the body 14. This signal can 
be fed to a display device or to a control, for example a machine tool 
control (not represented in FIG. 1). In FIGS. 5 and 6 there are shown two 
preferred embodiments 76 and 78 of the circuit 74. Both embodiments 76 and 
78 include an up-down counter 80, which adds rectangular pulses derived 
from the signal f.sub.1 and substracts from them rectangular pulses 
obtained from the signal f.sub.2. In order to improve the accuracy of this 
result, these rectangular (square) pulses have in each case a frequency 
greater by a prescribed factor than the signals f.sub.1 and f.sub.2. 
These high frequency rectangular pulses are obtained in the first 
embodiment 76 in the following manner. The signal f.sub.1 is fed to an 
alternating voltage amplifier 82 and thereupon to a Schmitt trigger stage 
84, which generate sinusoidal or rectangular pulses, respectively, of the 
same frequency as f.sub.1. 
Simultaneously a voltage-controlled multivibrator 86 generates a 
rectangular signal with a frequency 1000.times.f.sub.1 '. This signal is 
transformed by a divider 88 into a rectangular signal of the frequency 
f.sub.1 '. A multiplier 90 forms the product of f.sub.1 and f.sub.1 ' and 
feeds this to a low pass filter 92, which generates from this a direct 
current signal. This direct current signal is different from zero in the 
event that f.sub.1 and f.sub.1 ' are not identical with like phase 
position and regulates the voltage-controlled multivibrator 86 in such a 
manner that any phase deviation between the signals f.sub.1 and f.sub.1 ' 
is eliminated. The multivibrator 86 thereby provides an output signal with 
a frequency of 1000.times.f.sub.1. 
In like manner, a signal with the frequency of 1000.times.f.sub.2 is 
obtained from the signal f.sub.2. If a reference pattern is used with a 
spatial frequency that differs from that of the involute striped pattern 
42, a second frequency f.sub.10 must be obtained from the frequency 
f.sub.2 which is identical with the frequency f.sub.1 then the object 14 
is at rest. In this embodiment divider 89 is provided, which forms a 
signal having the frequency (1/1000).multidot.(f.sub.2 f.sub.10). The 
remaining components necessary for the formation of the signal 
1000.times.f.sub.10 from the signal f.sub.2 are the same as those already 
discussed for the evaluation of the signal f.sub.1 and are therefore 
designated with the same reference figures, but primed. 
Through the described embodiment 76 of the circuit 74 there is achieved a 
subdivision by the factor of a thousand, i.e., a subdivision of the 
striped pattern 10 with a period of 1 mm to a period of 0.001 mm. Other 
subdivision factors are possible with analogous applications of the 
principles described. 
If the rotation frequency of the disc 26 or 27 is sufficiently constant for 
the desired measurement precision, then the reference signal can be taken 
as signal f.sub.2 from a multivibrator 92 with the frequency 1000 f.sub.10 
instead of from detector 50, and can be fed to the updown counter 80, as 
shown in FIG. 5 with broken lines. 
The frequency of the signal f.sub.1, f.sub.2 which is not maintained 
constant should be as high as possible, so that the demands on the 
low-pass filter 92 and 92' of the particular subdivider circuit 86, 88, 
90, 92 or 86', 88', 90', 92' can be kept low. Since the reference pattern 
can be made with a very high number of stripes as a radial striped pattern 
52 lying on the outer edge of the disc, it is advantageous to regulate the 
frequency of the disc 26, 27 in such a way that the measuring signal 
f.sub.1 is constant and the frequency of signal f.sub.2 varies in 
correspondence to the movement of the object 14. The subdivider circuit 
86, 88, 90, 92 for the lower frequency f.sub.1, therefore, can be 
dispensed with and the constant signal f.sub.1 can be derived from a 
multivibrator. The lowpass filter 92' for the high frequency f.sub.2 can 
be simply constructed. 
The circuit arrangement for the realization of this measuring approach is 
shown in FIG. 6 as second preferred embodiment 78. The motor 22 is here 
regulated in such a way that the measuring signal f.sub.1 becomes equal to 
a prescribed constant signal f.sub.1 '. This constant signal f.sub.1 ' is 
derived from a multivibrator 94, which delivers a signal 1000 f.sub.1 '. 
From the signal 1000 f.sub.1 ' there is formed with the aid of two 
subdividers 96 and 98 the signal f.sub.1 ' and it is fed to a multiplier 
100. Simultaneously there is fed to the multiplier 100 the signal f.sub.1 
processed by an amplifier 102 and a Schmitt trigger stage 104 coupled to 
the output of the amplifier 102. The output signal of the multiplier 100 
conducted through a low-pass filter 106 regulates a voltage-controlled 
multivibrator 108, which, via an amplifier 110, drives the motor 22 of the 
disc 26, 27. The motor 22 is regulated in such a way that the deviations 
between signal f.sub.1 and f.sub.1 ' are eliminated. The signal 100 
f.sub.1 ' (or, if need be, 1000 f.sub.1 ' corresponding to FIG. 5) is fed 
to the up-down counter 80. 
The signal f.sub.2 can be subdivided by a large factor, as in the 
embodiment of FIG. 5. In the event that a lower accuracy is sufficient, 
the signal f.sub.2 can be subdivided by a smaller factor. In FIG. 6 there 
is shown twenty-fold subdividing divider 112 constructed as a simple 
network. To it there is fed the signal f.sub.2 as amplified by an 
amplifier 114. Here, a third signal f.sub.3 phase-shifted by 90.degree. 
with respect to signal f.sub.2 is used. Signal f.sub.3 is provided by a 
dertector 51 which scans the reference pattern 52 and is amplified by an 
amplifier 116 (broken line in FIG. 6). The output signal 20.times.f.sub.2 
of the divider 112 is fed to the up-down counter 80. 
For the measurement of large displacements of the object 14 the scale 12 is 
preferably composed of several sequents of, for example, 250 mm in length 
each. The production of individual segments is cheaper than the production 
of a whole scale 12. For economy of production it is also advantageous 
that the period of the striped pattern 10 be relatively great. If greater 
precisions are required, then a continous, precisely manufactured scale 12 
can be used. While the preceding embodiments utilize the signals f.sub.1 
and f.sub.2 provided by the detectors 46, 50 to measure the displacement 
of the object 14, it should be understood that a comparison of the 
frequencies of f.sub.1 and f.sub.2 can be made to determine the 
instantaneous velocity of the object 14. The device described herein is 
economical to manufacture and simple in construction, and it makes 
possible a reliable, accurate measurement of the movement of the object 
14. 
Of course, it should be understood that various modifications and changes 
to the preferred embodiments described herein will be apparent to those 
skilled in the art. Such modifications and changes can be made without 
departing from the spirit and scope of the present invention. It is, 
therefore, intended that such changes and modifications be covered by the 
following claims.