GB2086588A

GB2086588A - Electrical transducer for testing for straightness and evenness

Info

Publication number: GB2086588A
Application number: GB8132738A
Authority: GB
Original assignee: Carl Zeiss Jena GmbH
Current assignee: Jenoptik AG
Priority date: 1980-11-03
Filing date: 1981-10-30
Publication date: 1982-05-12
Also published as: JPS5788303A; CH654658A5; DD154037A1; DE3128450A1; FR2493513A1; GB2086588B; FR2493513B1; DD154037C2

Abstract

An arrangement for testing a surface for straightness and evenness comprising a vertical displacement transducer which includes a vessel 6 containing liquid 9 and closed at its upper end by a diaphragm 10. A sensor 12 detects displacement of the diaphragm. The vessel 6 is connected by a tube 16 to a similar vessel of a second transducer on a reference surface. To improve the accuracy and reliability of measurement, the vertical displacement pick-up disposed on the surface to be tested is seated on an intermediate member 30, 35 on the underside of which is disposed a follower 36 whose displacement l relative to the axis 37 passing through the centre point of the diaphragm 10 is adjustable by means of a screw 33 and oversized hole 38. <IMAGE>

Description

SPECIFICATION Apparatus for testing for straightness and evenness The invention relates to an apparatus for testing for straightness and evenness, particularly for large areas to be tested.

An apparatus for testing for straightness and evenness is known from East German patent specification 125440 in which two vertical displacement transducers are arranged on a reference surface and on a surface to be measured, respectively. The apparatus includes a communicating vessel system which deflects diaphragms at the test location and at the reference location. Electric signals are produced in dependence upon these deflections of the diaphragms and are indicative of the vertical positions of the vertical displacement transducers relative to one another. The vertical displacement transducer at the location to be tested rests on the surface to be tested by way of a follower.

Theoretically, vertical displacement transducers are not affected by tilting if the diaphragms are sensed exactly centrally and the follower is mounted exactly centrally on the vessel. In practice, however, this cannot be achieved, since eccentricities greater than 0.1 mm ensue in operation as a result of inhomogeneities of the material of the diaphragms and as a result of manufacturing tolerances when mounting the sensor over the centre of the diaphragm to be monitored.In the event of tilting of the vertical displacement transducer associated with the test surface, for example, as a result of large unevenness on the surface to be tested, or as a result of inaccurate horizontal guidance of the vertical displacement transducer, the measurement result based on the expansion of the diaphragm is impaired by error components of the 1 st order, that is to say, errors proportional to the tilt. The recorded measurement signal of the vertical coordinate is therefore subject to error as a result of the effects of tilting, and the accuracy and reliability of measurement are limited.

An object of the present invention is to increase the accuracy and reliability of measurement such that the measurement result is substantially unimpaired by tilting phenomena at the vertical displacement transducer.

In accordance with the present invention, there is provided an apparatus for testing surfaces for straightness and evenness, particularly for large test areas, comprising at least two liquid-filled vessels which act as vertical displacement transducers and which, in use, are arranged on a surface to be tested and a reference surface, respectively, each vessel being sealed by a respective diaphragm which is connected to a respective sensor, the sensors providing electrical signals dependent upon deflections of the diaphragms, and the vertical displacement transducer engaging the surface to be tested by way of an intermediate member having on its underside a follower which is arranged to contact the surface to be tested, the follower being mounted on said intermediate member, so as to be displaceable relative to a position in which it is in vertical alignment with the centre of the diaphragm.

According to the manner in which the movable vertical displacement transducer on the test surface is guided, tilting of this transducer can occur as a result of large unevenness of the surface of the workpiece to be tested, or as a result of defects in the horizontal guidance arrangement. In the case of the diaphragm not being sensed centrally, which can be caused by diaphragm characteristics and also by manufacturing and assembly inaccuracies, error components would result in the test signal of the vertical displacement transducer in proportion to the tilt.These error components caused by changes of the height of the location at which the diaphragm is sensed as a result of tilting of the vertical displacement transducer are eliminated by intentionally causing, in additon to these unintentional changes of height, tilting movements by a device which is additionally disposed at the sensing location for the surface to be tested. The follower for the surface to be tested, which normally has to be disposed centrally on the vertical displacement transducer precisely in order to avoid the defects of tilting, is disposed so as to be offset relative to the vertical central axis of the vertical displacement transducer.

In this manner, further tilting movements are intentionally produced when sensing the surface to be tested. By adjusting the horizontal position of the follower (adjusting the eccentricity produced), the tilting movement is influenced such that, although it is equal in magnitude to that of the unavoidable detrimental effect of tilting, it is of opposite sign.

Thus, the measurement signal of the vertical displacement transducer is only affected by a tilting effect which results from the two tilting movements and which is zero or negligible in a range of tilt relevant to the test for evenness.

The invention will be further described hereinafter, by way of example only, with reference to the accompanying drawing which is a sectional view of one embodiment of a vertical displacement transducer in accordance with the present invention.

The illustrated vertical displacement transducer comprises a vessel 6 which is filled with a liquid 9 and which is sealed at the top by means of a diaphragm 10. A carrier frame 14 is mounted on the vessel 6 and accommodates a contactless sensor 12 whose sensing element is constituted by a ferromagnetic or electrically conductive layer 17 located on the diaphragm 10, and whose electrical output is conducted out of the carrier frame 14 by way of a lead 21. The vessel 6 has a tube 16 for communicating with a further similar vertical displacement transducer (not illustrated in the drawing) and is disposed on a measurement carriage 5 which incorporates a bore 28 for a follower 29 of the vessel 6.

The measurement carriage 5 and the follower 29 are seated on an intermediate member 30 having two lateral connection points 31,32 for horizontal guidance in the directions of the x and y coordinates. An intermediate plate 35 is secured to the underside of the intermediate member 30 by means of a screw 33 and a washer 34, a follower 36 in turn being disposed on the underside of the intermediate plate 35. The follower 36 is seated on a surface 2 to be tested. The two followers 29 and 36 are offset relative to one another by a distance I in the direction of vertical alignment.

The follower 36 senses the surface 2 to be tested.

Changes of the vertical coordinate of the surface 2 are transmitted to the vessel 6 by way of the follower 36, the intermediate member 30 and the intermediate plate 35, and the follower 29. According to the deflection of the diaphragm 10 by the fluid 9 in the inter-communicating vessel system, the sensor 12 supplies, by way of the lead 21, signals which are evaluated in a known manner in conjunction with signals from a vertical displacement pick-up disposed on a reference surface. The result of this evaluation is information concerning vertical positions of the points, lines or portions of the surface 2, contacted by the follower 36, relative to the vertical position of the reference surface (not illustrated in the drawing).

Like the sensor 12, the follower 29 is disposed coaxially of a central axis 37 of the vessel 6. By reason of technological conditions and characteristics of the material of the diaphragm 10, the location at which the sensor 12 senses the diaphragm 10 is, in practice, not exactly coaxial with the central axis 37.

Upon tilting of the vessel 6 and the carrier frame 14, this eccentricity would lead to variations of the vertical deflection of the diaphragm at the point at which the sensor 12 senses the diaphragm 10, and which the sensor 12 would record as errors when measuring the vertical coordinate of the surface 2 at any given time.

In order to avoid this, the two followers are offset relative to one another by the distance e with respect their vertical alignment so as to impart an intentional additional tilt to the intermediate member 30 to which the intermediate plate 35 is secured and which necessarily performs the same tilting movements as the vessel 6 and the carrier frame 14. The effect of this additional tilt can be influenced by setting a suitable distance e. For this purpose, the intermediate plate 35 is adjustably secured to the intermediate member 35 by means of the screw 33 in a bore 38 which is of correspondingly large dimension. The tilting effect additionally caused by this adjustment is arranged such that an effect resulting from the two tilt phenomena does not cause a variation in the vertical deflection of the diaphragm 10 recorded by the sensor 12 within a range of tilt relevant to the meausrement of evenness. Thus, the result of the vertical coordinate measurement of the surface 2 is independent of tilt phenomena of the vessel 6 and thecarrierframe 14.

Claims

1. An apparatus for testing surfacesforstraightness and evenness, particularly for large test areas, comprising at least two liquid-filled vessels which act as vertical displacement transducers and which, in use, are arranged on a surface to be tested and a reference surface respectively, each vessel being sealed by a respective diaphragm which is connected to a respective sensor, the sensors providing electrical signals dependent upon deflections of the diaphragms, and the vertical displacement transducers engaging the surface to be tested by way of an intermediate member heaving on its underside a follower which is arranged to contact the surface to be tested, the follower being mounted on said intermediate member, so as to be displaceable relative to a position in which it is in vertical alignment with the centre of the diaphragm.

2. An apparatus as claimed in claim 1 wherein the follower which contacts the test surface is connected to the intermediate member by way of an intermediate plate disposed beneath the intermediate member and adjustably connected thereto by means of a screw which extends through an oversized hole in the intermediate plate and is received in a correspondingly screw-threaded bore in the underside of the intermediate member.

3. An apparatus for testing for straightness and evenness substantially as hereinbefore described with reference to the accompanying drawing.