GB2144538A

GB2144538A - Linear measurement apparatus

Info

Publication number: GB2144538A
Application number: GB08419733A
Authority: GB
Inventors: Gerd Jager; Hans Buchner; Eberhard Manske; Walter Schott; Kurt Schumacher
Original assignee: SUHL FEINMESSZEUGFAB VEB
Current assignee: SUHL FEINMESSZEUGFAB VEB
Priority date: 1983-08-03
Filing date: 1984-08-02
Publication date: 1985-03-06
Also published as: DE3426981C2; DE3426981A1; DD220698A1; GB2144538B; GB8419733D0

Abstract

In the apparatus, which enables digital measurement with maximum accuracy up to a range of measurement of approximately 20 mm, an optical divider, comprising two component prisms (1,2), is illuminated with parallel light and monochromatic light. The two beams are deflected by means of a 90 DEG reflecting prism (3), which is vibrated, and each beam enters a respective triple prism (4,5), is again deflected, reflected from a silvered face of the optical divider, covers the distance to the triple prism again and interferes. The two triple prisms are secured to a prism- retaining yoke (13) which is connected to a measuring plunger (8). The measuring plunger is guided. The apparatus can also digitally measure all the values which can be depicted over a travel. <IMAGE>

Description

SPECIFICATION Linear measurement apparatus The present invention resides in a linear measurement apparatus which serves for accurate linear measurement.

Various devices and apparata for linear measurement are known. Inductive measuring transducers, such as are produced by VEB Feinmesszeugfabrik Suhl, are frequently used for electrical linear measurement. The selfinductance or the mutual inductance varies in dependence upon the travel to be measured.

These measuring transducers supply an analog output signal and have accuracy limits of approximately + 0.5%. For this reason, they can only be used over ranges of measurement for precision measurement with errors of +0.1 ,um.

Furthermore, apparata are known which have photoelectrically scanned incremental scales (pulse scales) as measuring means.

Measuring accuracies of + 1 ,um are obtaied with such measuring systems (see supplier's survey of the firm Heidenhain, 1982).

Greater accuracy in the case of measuring ranges of approximately 20 mm is difficult to obtain owing to the errors of graduation of the pulse scales, guidance errors and thermal expansion.

Interferometers of greatly varying construction are used for the precision measurement of great lengths (approximately 1 m) (Lit.

Laser Interferometer Metrials M100E, Brochure of Soro, France; Hewlett-Packard Brochure on 5526 A Laser measurement Sys tem).

The above involve very expensive apparata having large geometrical dimensions. Hence, they are unsuitable for the measurement of short lengths.

An object of the present invention is to provide an apparatus for linear measurement which enables maximum accuracy in the case of ranges of measurement of approximately 20 mm.

The invention aims to avoid the disadvantages of the known solutions that either maximum accuracy cannot be obtained in the case of ranges of measurement of approximately 20 mm, or that the economic expense of the measuring apparatus is unjustifiably high.

Another object of the invention is to provide a linear measurement transducer (precision indicator) which enables measurement of small distances of up to approximately 20 mm with a high resolution of X 32 Hence, linear measurements can be made with maximum accuracy which is not possible either with inductive measuring transducers or with incremental measuring stylii, only small expenditure on apparata being required relative to the accuracy obtained.

In accordance with the invention, there is provided an apparatus for linear measurement, comprising a laser, an expansion system, an optical divider, a 90 reflecting prism, triple prisms, optical fibre light guides and photodetectors, the optical divider comprising two component prisms each having angles of 30 , 60 and 90 and one being half silvered on its face opposite to said 30 angle, which face is cemented to the corresponding face of the second component prism such that the faces opposite the 30 angles are located in one plane, the 60 and 90 angles of the two component prisms being varied such that the exact angle between the hypotenuse face and the corresponding face of the second component prism differs from 60 by an amount corresponding to the desired interference fringe spacing, the coplanar faces of the component prisms each having on a half thereof a reflecting layer such that the separating edge between the silvered and non-silvered surface portions is at right angles to the cemented faces, the optical divider being rigidly connected to a holding plate by means of a spacer ring and the holding plate being rigidly connected to a base plate and having a flexible tongue, the 90 reflecting prism being rigidly disposed on the flexible tongue by means of a spacer ring and a piezoelement having an oscillating weight being disposed on the flexible tongue, a stressing stem in which a measuring plunger having a contact tip is guided being rigidly mounted on the base place and a triple-prism-retaining yoke being rigidly connected to the measuring plunger, the triple prisms being rigidly disposed on the triple prism retaining yoke by means of spacer rings with the central beams of the triple prisms in alignment with the axis of symmetry of the measuring plunger, the material of the spacer rings having a coefficient of thermal expansion which lies between those of the materials of the base plate on the one hand and the optical divider and the 90 reflecting prism on the other hand, and between the coefficients of thermal expansion of the materials of the triple-prism-retaining yoke on the one hand and the triple prisms on the other hand, the angles between the one edge and the reflecting surface of the triple prisms being not equal to 90 and the angle between the two surfaces of the triple prisms intersecting at said one edge being 90', half the sum of the distances between the entry faces of the triple prisms and the apex of the contact tip being equal to the radius of the contact tip, the optical fibre light guides being disposed in the interference pattern, and the photodetectors being connected to the optical fibre light guides.

The coherent monochromatic light beam is divided into two equal component beams in the optical divider. The component beams are in each case reflected through 90 by means of the 90 reflecting prism and each enters a respective triple prism. After reflection in the triple prisms, the component beams are reflected from the silvered short faces after reflection by the 90 reflecting prism, pass through the triple prisms again and are reu nited in the optical divider.

For the purpose of further processing electronically, the interference phenomenon is scanned by the fibre optic light guides beyond which are connected the photodetectors.

The triple prism retaining yoke is made preferably from silica glass and is rigidly connected to the measuring plunger which is preferably made from invar. The measuring plunger carries the contact tip for sensing the object to be measured. During a measurement travel, the two triple prisms are moved in opposite directions relative to the optical divider, so that double sensitivity is effected.

The radius R of the contact tip is chosen such that half the total of the distances between the entry faces of the triple prisms and the apex S of the contact tip at the point of contact with the object to be measured, is equal to the radius R of the contact tip. An arrangement of this kind minimizes the measurement errors occurring as a result of guidance errors when sensing planar surfaces to be measured. Troublesome interference phenomena always occur in interferometer arrangements as a result of reflection from the boundaries between two media having differing reflecting indices.In order to suppress the influence of such troublesome interference phenomena, either one of the angles between the reflective surfaces of the triple prisms or the angles which pertain to possible combinations, are made unequal to 90 , the angles of the two triple prisms having to be equal in pairs. The deviation from the 90 angle is chosen such that the troublesome interference fringe spacings are so small that they are no longer resolved in a troublesome manner by the photodetector.

A further reduction of the measurement errors resulting from guidance errors is achieved by an arrangement in which the central beams of the two triple prisms are in alignment with the axis of symmetry of the measuring plunger.

When the flexible tongue of the holding plate is vibrated by means of the piezoelectric oscillator, the output signal can be additionally modulated, hence simplifying the electronic evaluation.

The optical components, optical divider tri ple prisms and 90 reflecting prism are connected to their base by means of spacer rings by, for example, cementing or glueing. Advantageously, the spacer rings are made of a material whose coefficient of thermal expansion lies between the values of the coefficients of thermal expansion of the optical compo nents and the base plate in order to reduce mechanical stresses. Securing by utilizing the spacer rings enables defined cemented joints, which in turn results in minimum temperature dependencies.

The invention will now be described further hereinafter, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a diagrammatic view of one embodiment of an apparatus in accordance with the present invention; Figure 2 is a partial plan view of the apparatus of Fig. 1; Figure 3 is a section through line A-A of Fig. 2; and Figure 4 is a section through line B-B of Fig. 2.

Referring to Fig. 1, an optical divider comprising component prisms 1, 2 is illuminated with coherent monochromatic light by means of a laser 7 and an expansion lens system 6.

The component prism 1 has angles of 90', 60 and 30 which are produced with the maximum technical accuracy. The side face of the component prism 1 opposite the 60 angle is half silvered. The corresponding side face of the component prism 2 is cemented or glued to the aforesaid side face. The component prism 2 has the same geometry as the component prism 1. The two component prisms are cemented to one another such that the side faces opposite the 30 angles are located in one plane.

The 60 angles of the two component prisms 1 and 2 are varied in a suitable manner, such that the angle a between the hypotenuse face of the component prism 2 and the two side surfaces of the component prisms 1 and 2 opposite the 30 angles differ from 60 , such that a desired interference fringe spacing is produced. Each of the side faces of the two component prisms opposite the 30 angles carries half of a fully reflecting layer 1 8. The coherent, monochromatic beam is divided into two equal component beams in the optical divider. The component beams are reflected through 90 by a 90 reflecting prism 3. A component beam is reflected in a triple prism 5, while the other component beam is deflected in a triple prism 4. After further reflection from the 90 reflecting prism 3, the two component beams are reflected from the fully reflecting layer 18, pass through the triple prisms 4 and 5 again! and are unified in the optical divider. For the purpose of, for example, direction-dependent counting, the interference phenomenon occurring is scanned by two fibre optic light guides 14 which are disposed so as to be out of phase by 90 . Photodetectors 1 5 are disposed downstream of the fibre optic light guides 14.

As will be seen in Fig. 2, the triple prisms 4 and 5 are rigidly cemented to a triple-prismholding yoke 13 by means of spacer rings 21 and 22. The triple-prism-holding yoke 1 3 is made from silica glass and is rigidly connected to a measuring plunger 8 which is made from invar. The measuring plunger 8 is held in a clamping cylinder 9 which is held in a base plate 11. The measuring plunger 8 carries a contact tip 10 for ensing an object 24 to be measured. Referring to Fig. 2, the radius R of the contact tip 8 is chosen such that a, + a2 R = 2 a, and a2 are the distances between the entry surfaces of the triple prisms and the apex S serving as the point of contact with the object to be measured.In order to suppress troublesome interference, the angle ss in the two triple prisms 4 and 5 is chosen to be ss = 90 + 3' in Fig. 4, and the angles ss4 and ss5 are chosen to be ss4 = ss5 = 90 in Fig. 1.

The arrangement of Fig. 1 is such that the central beams of the triple prisms 4 and 5 are in alignment with the axis of symmetry of the measuring plunger 8.

As is shown in Fig. 3, the optical divider 1, 2 is rigidly connected to a holding plate 1 2 by means of a spacer ring 20 by cementing, the holding plate 1 2 in turn being rigidly disposed on the base plate 11. As is shown in Fig. 1, one location of the holding plate 1 2 is necked down to form a flexible tongue 19.

The reflecting prism 3 is secured to the free end of the flexible tongue 1 9 by means of a spacer ring 23. Furthermore, a piezo flexible element 1 6 with an oscillating weight 1 7 is disposed at the free end of the flexible tongue 1 9. The piezo flexible element 16 is oscillated electrically.

The optical components, that is to say, the optical divider comprising the component prisms 1 and 2, the triple prisms 4 and 5, as well as the 90 reflecting prism 3, are made from optical glass of the same type, such as BK7. The spacer rings 20, 21, 22 and 23 are made from a material whose coefficient of thermal expansion lies in the value between the coefficients of thermal expansion of the base plate 11 and the triple-prism-holding yoke 1 3 on the one hand, and the optical divider, comprising the component prisms 1 and 2, the 90 reflecting prism 3 and triple prisms 4 and 5 on the other hand.

The apparatus of the invention can be used to advantage when resoluting of up to A 32 when large measuring ranges of 10 to 20 mm and over are required. Also, all the values which can be delineated along the travel of the plunger 8 can be measured digitally.

Claims

1. An apparatus for linear measurement, comprising a laser, an expansion system, an optical divider, a 90 reflecting prism, triple prisms, optical fibre light guides and photodetectors, the optical divider comprising two component prisms each having angles of 30 , 60 and 90 and one being half silvered on its face opposite to said 30 angle, which face is cemented to the corresponding face of the second component prism such that the faces opposite the 30 angles are located in one place, the 60 and 90 angles of the two component prisms being varied such that the exact angle between the hypotenuse face and the corresponding face of the second component prism differs from 60 by an amount corresponding to the desired interference fringe spacing, the coplanar faces of the component prisms each having on a half thereof a reflecting layer such that the separating edge between the silvered and non-silvered surface portions is at right angles to the cemented faces, the optical divider being rigidly connected to a holding plate by means of a spacer ring and the holding plate being rigidly connected to a base plate and having a flexible tongue, the 90 reflecting prism being rigidly disposed on the flexible tongue by means of a spacer ring and a piezoelement having an oscillating weight being disposed on the flexible tongue, a stressing stem in which a measuring plunger having a contact tip is guided being rigidly mounted on the base plate and a triple-prism-retaining yoke being rigidly connected to the measuring plunger, the triple prisms being rigidly disposed on the triple prism retaining yoke by means of spacer rings with the central beams of the triple prisms in alignment with the axis of symmetry of the measuring plunger, the material of the spacer rings having a coefficient of thermal expansion which lies between those of the materials of the base plate on the one hand and the optical divider and the 90 reflecting prism on the other hand, and between the coefficients of thermal expansion of the materials of the triple-prism-retaining yoke on the one hand and the triple prisms on the other hand, the angles between the one edge and the reflecting surface of the triple prisms being not equal to 90 and the angle between the two surfaces of the triple prisms intersecting at said one edge being 90 , half the sum of the distances between the entry faces of the triple prisms and the apex of the contact tip being equal to the radius of the contact tip, the optical fibre light guides being disposed in the interference pattern, and the photodetectors being connected to the optical fibre light guides.

2. An apparatus as claimed in claim 1, wherein the measuring plunger is made from invar.

3. An apparatus as claimed in claim 1 or 2. wherein the triple-prism-retaining yoke is made from silica glass.

4. An apparatus for linear measurement constructed and adapted to operate substantially as herein described, with reference to and as illustrated in the accompanying drawings.