GB2137355A

GB2137355A - Tensioned-wire straight edge apparatus having compensation for the properties of the material of the tensioned wire system

Info

Publication number: GB2137355A
Application number: GB08402247A
Authority: GB
Inventors: Gerhard Meister
Original assignee: Carl Zeiss Jena GmbH
Current assignee: Jenoptik AG
Priority date: 1983-01-31
Filing date: 1984-01-27
Publication date: 1984-10-03
Also published as: DD213498A1; JPS59151002A; DE3340298A1; CH664013A5; GB8402247D0; GB2137355B

Abstract

A tensioned-wire straight edge apparatus, for use as a straightness and evenness standard in precision measuring technology and in manufacture, includes a stretched tensioned-wire system (11) which comprises a plurality of thin ferromagnetic wires scanned by means of inductive scanning systems (5,6). The wire system includes a locally defined permanent-magnetic mark (21) produced by magnetization. An adjusting member (4) for adjusting the scanning systems (5,6) to the location where the scanning output signal is invariant with respect to fluctuations of permeability, is provided on a carrier (3) receiving the scanning systems. <IMAGE>

Description

SPECIFICATION Tensioned-wire straight edge apparatus having compensation for the properties of the material of the tensioned wire system The invention relates to a tensioned-wire straight edge apparatus having compensation for the properties bf the material of the tensioned wire system.

East German Patent Specification No.139295 describes a tensioned-wire straight edge apparatus of high accuracy which includes a carrier in which the tensioned-wire system, comprising a plurality of thin, parallel wires, is stretched between holding and tensioning devices and includes scanning systems for scanning the wire system in its longitudinal direction. Means for exciting oscillation of the wire system to its natural frequency are also provided.

Taking into account the sag of the wire, evenness and straightness characreristics of the test object are ascertained in an evaluation device from the measured values supplied by the scanning systems which, for example, scan the wire system inductively or capacitively.

Only the sag of the wire is compensated for in this tensioned-wire striaght edge apparatus, and the wires are homogenized by demagnetization with a decaying alternating field. The remaining local fluc tuations of the permeability pR of the wire system which can falsify the measurements, are not detected.

In the arrangement specified in application WP Co 1 B/233748/4, the differential arrangement of two contactless, inductive displacement pickups have to be adjusted relative to the tensioned-wire system such that, so to speak, the measurement deflection is kept at zero, that is to say, the bridge circuit is only operated in a state adjusted to "zero" diagonal voltage. The tensioned-wire system would thereby be located in the geometrical centre, that is to say, the centre greatly affected by residual A Ar effects, between the end faces of the two displacement pickups (scanning systems) which are generally not identical.However, the electrical centre independent of A Ar along the tensioned wire system does not always conform to the geometrical centre between the displacement pickups, this also being caused by, inter alia, certain asymmetries in the construction of the displacement pickups.

An object of the present invention is to overcome the disadvantages of the prior art and to increase the accuracy of the tensioned-wire straight edge apparatus by compensating by single means for the effect of local differences in the magnetic and electrical properties of the wire on the measurements.

In accordance with the present invention, there is provided a tensioned-wire straight edge apparatus including a tensioned-wire system which comprises ferromagnetic wires, a frame in which the tensionedwire system is stretched in holders and which has a guide and supports in its longitudinal direction, there being displaceably mounted on the guide a carrier which is movable at right angles to the guide and which receives at least two scanning systems and on which is disposed a folower for sensing the surface of the test object, and an evaluation device electrically connected to the scanning systems, the tensioned-wire system including a locally defined permanent-magnetic mark, and there being provided on the carrier an adjusting member which enables the scanning systems to be adjusted to a location (electrical centre) which is constant with respect to fluctuations of permeability, that is to say, to the location at which the permanent magnetic mark is ineffective with respect to measurement operations, between end faces of the scanning systems and at right angles to the plane of the tensioned wire system.

Advantageously, the mark is a pulse-like limited variation of the magnetic permeability of the tensioned wire system whose permeability is varied by an amount < 10% relative to the mean value of the permeability of the wire system produced by demagnetization.

Hence, in order to adjust the scanning systems to the electrical centre, that is to say, to the A r invarient location, the carrier accommodating the scanning systems has an adjusting member by means of which the height associated with the geometrical centre between the scanning systems can be varied by a predetermined amount. This amount must be at least sufficiently large to enable compensation for the greatest possible difference between the geometrical and electrical centres and, with multiple traversing of the mark, is adjusted by a carrier frequency bridge circuit such that the mark causes neither a positive nor a negative deflection on a recording or indicating instrument.When the tensioned wire straight edge apparatus has been subjected to this basic adjustment, the testing of the test object is free to the greatest possible extent from A pJr-effects.

Hence, in addition to eliminating the effect of the sag of the wire and the inaccuracy of the guide carrying the carrier, this arrangement also eliminates the negative effect of the local fluctuations of the magnetic permeability of the tensioned wire system on the accuracy of measurement by the tensioned wire straight edge apparatus when measuring straightness, evenness and the surface parameters.

The invention will be further described hereinafter, by way of example only, with reference to the accompanying drawing, which ilustrates diagrammatically one embodiment in accordance with the present invention.

The illustrated tensioned-wire straight edge apparatus comprises a frame 1 having guides 2 on which a carrier 3 is displaceably mounted. an adjusting member 4, scanning systems 5,6 and a low-mass, preferably inductive displacement pickup 8 controlable by a follower 7, are disposed on the carrier 3.

Tensioning means 9,10 are located on the frame 1. A tensioned-wire system 11 comprising a large number of thin ferromagnetic wires is stretched between the tensioning means 9,10 and issuported relative to the test object 14 by suports 12,13 disposed on the frame 1.Advantageously, the supports 12,13 are vertically adjustable. The tensioned-wire system 11 has a locally defined permanent-magnetic mark 21 which is produced by magnetization and whose permeability is varied by an amount s. 10% relative to the mean value of the permeability of the wire system 11 produced by demagnetization. The entire tensioned wire straight edge can rest on the test object 14 by means of support elements 15, 16, or on a base on which the test object is located.As may be seen in the drawing, the coils 5' and 6' of the scanning systems 5 and 6 for scanning the wire system 11 are interconnected in a carrier frequency bridge circuit which incorporates a carrier frequency amplifier 17. The bridge circuit also incorporates balancing elements 18, 19 by which the measuring bridge can be balanced and by which an indicating instrument 20 connected to the carrier frequency amplifier 17 can be set to "zero". Preferably, the carrier frequency amplifier 17 is connected to the adjusting system 4' of the adjusting member 4, so that the electrical centre can be adjusted at the same time as the zero balancing of the bridge circuit.

In orderto eliminate the effect of the residual magnetism of the tensioned-wire system on the accuracy of measurement, the scanning systems 5,6 have to be adjusted to a location which is not affected by fluctuations of permeability, that is to say, to the electrical centre between the end faces of the scanning systems 5,6. For this purpose, the scanning systems 5,6 are guided cylically back and forth across the location of the mark 21, and the carrier frequency bridge circuit is adjusted by at least one of the balancing elements 18, 19 such that the mark 21 appears with neither a positive nor a negative sign in the measurement result or on the indicating instrument 20.The adjusting system 4' of the adjusting member 4 automatically ensures that the state of balance of the bridge circuit determined by the balancing elements 18, 19 is retained, and hence local y, fluctuations in the wire system 11 remain ineffective with respect to measuring operations during displacement of the scanning systems 5,6 along the wire system 11.

When set up in this manner, the measurements taken on the test object 14 are performed in a known manner and without being affected by fluctuations of permeability which still exist.

It may be mentioned at this juncture that, for example, when scanning thin, non-ferromagnetic but electrically conductive strips or tapes, analogous conditions exist for the local electrical conductivity of the eddy currents which are caused by the scanning systems and which, in this instance, replace the permeability.

Claims

1. A tensioned-wire straight edge apparatus including a tensioned-wire system which comprises ferromagnetic wires, a frame in which the tensionedwire system is stretched in holders and which has a guide and supports in its longitudinal direction, there being displaceably mounted on the guide a carrier which is movable at right angles to the guide and which receives at least two scanning systems and on which is disposed a follower for sensing the surface of the test object, and an evaluation device electrically connected to the scanning systems, the tensioned-wire system including a locally defined permanent-magnetic mark, and there being provided on the carrier an adjusting member which enables the scanning systems to be adjusted to a location (electrical centre) which is constant with respect to fluctuations of permeability, that is to say, to the location at which the permanent magnetic mark is ineffective with respect to measurement operations, between end faces of the scanning systems and at right angles to the plane of the tensioned wire system.

2. A tensioned-wire straight edge apparatus as claimed in claim 1, wherein the mark is a pulse-like defined variation of the magnetic permeability of the tensioned-wire system whose permeability is varied by an amount s 10% relative to the mean value of the permeability of the wire system produced by demagnetization.

3. A tensioned-wire straight edge aparatus as claimed in claim 1, wherein the adjusting member is connected to a carrier frequency bridge circuit for automatically ensuring the retention of a balanced bridge condition.

4. A tensioned-wire straight edge apparatus as claimed in claim 1,wherethe permanent-magnetic mark is formed by local magnetisation of said ferromagnetic wires.

5. A tensioned-wire straight edge apparatus substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.