GB2087082A

GB2087082A - Electrically testing for straightness and evenness

Info

Publication number: GB2087082A
Application number: GB8132739A
Authority: GB
Original assignee: Carl Zeiss Jena GmbH
Current assignee: Jenoptik AG
Priority date: 1980-11-03
Filing date: 1981-10-30
Publication date: 1982-05-19
Also published as: DD159362B1; JPS57114804A; DD159362A1; GB2087082B; FR2493512A1; FR2493512B1; CH654915A5; DE3131084A1

Abstract

An arrangement comprises two intercommunicating liquid-filled vessels (6, 7) arranged respectively on a surface (2) to be tested and a reference surface (20). Each vessel contains a diaphragm (10, 11) and a sensor (12, 13) whose output represents the height or expansion of the associated diaphragm. The outputs are fed to a difference forming device (23), the result of the subtraction being fed via amplifier (24) and filter (25) to an indicator (26). To correct for unwanted variations in the volume of the vessels, the sensor outputs pass to device (23) via respective evaluation stages (34, 35) which are adjustable as to modify the sensor outputs in dependence upon the characteristics of the diaphragm (10, 11) of the respective sensor (12, 13). <IMAGE>

Description

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

A testing technique is known from East German Patent Specification 125440 in which two or more inter-communicating vessels contain a liquid which in each case deflects a diaphragm connected to a sensor according to the relative vertical positions of the vessels. Electrical signals are obtained from these deflections and are amplified, filtered and indisated or evaluated after subtraction. A disadvantage of this testing technique is that the precision which can be achieved is considerably impaired by virtue of the fact that the measurement signals are affected by the volume effects in the vessels. These volume effects are manifested by changes of volume which occur particularly as a result of deformation of the -eible tube during movements or rhe vessels.The larder are the areas to be tested, and the greater is the distance between the test points, that is to say, between the vessels, the greater is the effect of the aforementioned factors.Thus, accuracy of QP meas- urement and the applicability of the test system are greatly reduced. Moreover, volume changes in the vessels can lose caused by climatic conditions such as i-he effects of temperalure oF the vessel wells and of the liquid. The effect on the measurement signal is explained by the fact that, contrary to the case of a free liquid surface, the result signal is invariant by virtue of subtraction of the sensor signals as a result of equal values of the rise or fall of the liquid surface on both sides of the communicating system as a result of volume changes, and that the spring con stands of the diapl1r2gnìs acting integrally upon the surface of the liquid are not exactly equal and thus differing displacements of the diaphragms occur in the event of volume changes.Differing displacements of the diaphragms result in differing sensor signals, thereby impairing the test result The object of the present invention is to increase the accuracy and reliability of measurn-ment and to widen the range of use, particularlyfortl1etestirg of large areas.

In accordance with the present invention, there is provided an arrangement for testing for strnightness and evenness, comprising at least two intercommunicating liquid-filled vessels which are arranged on a surface to be tested and on a reference surface, respectively, and each of which contains a diaphragm and a sensor, the sensor outputs carrying electrical signals which are dependent upon the height or expansion of the associated diaphragms, the signals from the sensors being passed via respective first evaluation stages to a difference forming device whose output is passed to an indicator stage or a further evaluation stage, each said first evaluation stage being adjustable to modify the sensor signals in dependence upon the characteristics of the diaphragm associated with the respective sensor.

Volume effects caused by volume variations, which falsify the measurement result, are caused by differing characteristics of the diaphragms in the inter-communicating vessels. These differing characteristics result in differing electrical signals at the sensors in response to the change of volume. In accordance with the invention, the electrical signals of the sensors are influenced such that they can be adjustably controlled in each case in dependence upon the characteristics of the associated diaphragms. This control is effected in the evaluation stages which are electrically connected to the outputs of the respective sensors. In this manner, before subtraction, the sensor signals are modified by compensating quantities which correspond to the mechanical characteristics or differing mechanical characteristics of the diaphragms.Preferably, these compensating quantities are ascertained by using lsnown means for creating an artificial volume change (such as liquid displacement). The differing electrical signals caused by the differing characteristics of the diaphragms are evaluated from this volume change, and the compensating quantities are derived therefrom.

Advantageously, a differential coil system can be used for the evaluation stages, the excitation of the coils being controllable by the variable core of the differential coil system, such that the values of the sensor signals are varied relative to one another.

Advantageously, the subtracted can at the same time be realised in a bridge circuit of the sensors and of the differential coil system. The linearity of the measuring system, that is to say, a linear relationship between the mechanical change of height and the detection, measurement and indication thereof, is a basic requirement both for the accuracy of the measuring operation itself and alsoforthe precision of the compensation of the sensor signals in the evaluation stages. The sensor and its connection to the diaphragm constitute a critical location for ensuring this linearity. Therefore, it is advantageous to use Inown, very linear inductive or capacitive transducers as sensors, without variations of the air gap, dependent upon deflection, falsifying the transducer signals as a result of non-linearities.Mechanical followers should then be rigidly connected to the movable diaphragm which is stabilized by spring force for frictioniess movement in the direction of move ment.

The range of linearity of the basic test arrange- ment is not unlimited. By virtue of the invention, the known test technique is extended to the testing of very large surFaces with corresponding accuracy and reliability, since the measurement result is no longer falsified by volume expansion effects, particularly by deformation of the flexible pipe. It is possible, howewever, that the range of linearity of the test system may be taxed to the limits or beyond when testing surfaces whose geometry departs considerably from the flat.

In order, nevertheless, to ensure the linearity for the accuracy and reliability of the test result itself, and particularly for the compensation, in accordance with the invention, upon which this precision is also based, it is advantageous for the vertical co-ordinate of the reference surface to be variable.

Thus, different levels of the surface are detected with corresponding precision by point-by-point sensing by the principle for testing for evenness.

Moreover, an indication of the variations of the surface to be tested can at the same time be obtained by measuring the readjustment of the vertical coordinate.

The invention will be further explained hereinafter, by way of example only, with reference to the accompanying drawings, in which: Fig. 1 shows the basic arrangement for the measurement of straightness and evenness, having the evaluation stages in accordance with the invention; Fig. 2 shows a differential coil system as an example of an advantageous embodiment of the evaluation stages; and Fig. 3 shows the coupling of an inductive transducer, serving as a sensor, by rigid connection to the diaphragm in a vessel having a carrier frame.

Figure 1 shows an arrangement for testing for straightness and evenness which utilises the basic principle of the test arrangement described in East German Patent Specification 125,440.

Two vessels 6 and 7 filled with liquids 8 and 9 are interconnected by way of a tube 16 so as to communicate with one another. The top of each vessel 6,7 is sealed by a diaphragm 10 and 11, respectively.

Carrier frames 14, 15 are mounted on the vessel 6,7.

Sensors 12,13 are disposed in the carrierframes 14,15 respectively and having measuring elements 30 and 31 connected to the diaphragms 10,11 respectively. The vessel 6 has a follower 20 which is arranged to rest on a surface 2 to be tested, while the vessel 7 rests on a carrier frame 19 which is fixedly disposed on a foundation 20.

The tube 16 has a chamber 32 which is provided with a setscrew 33. The outputs ofthe sensors 12,13 are connected by way of leads 21 and 22 and evaluation stages 34,35 to a subtractor 23 whose output is connected to a test amplifier 24 by way of a lead 27.

The output signals from the test amplifier 24 are applied to a recording unit 26 by way of a filter 25.

The vessel 6 is vertically displaceable by the surface 2 by way of the follower 29. The diaphragms 10 and 11 are deflected by the intercommunicating vessel system containing the liquids 8 and 9 in dependence upon the relative vertical positions of the vessels 6 and 7. These deflections are detected by the sensors 12, 13 and are converted to electrical signals. Since the vessel 7 is fixedly mounted on the foundation 20, the output signals of the sensors 12 and 13 connected to the leads 21 and 22 contain information concerning the vertical co-ordinates of the surface 2 or its evenness, so that the surface geometry can be determined by the hose levelling principle. The difference between these signals is formed and subsequently amplified in the test amplifier 24.This measurement signal, a quasi direct voltage, is separated in the filter 25 from the spectrum of the interference signals which are caused particularly by vibrations at the place of installation. After the filtering operation, the measured variable is recorded or indicated by means ofthe recording unit 26. The indicated measured value is directly indicative of the geometry of the surface 2.

The diaphragms 10 and 11 are expansible structures whose spring constants acting integrally upon the liquids 8 and 9 are not exactly equal. The characteristics of the diaphragms are dependent upon a large number of factors, such as material, shape, technological parameters etc. However, differing characteristics of the diaphragms cause a difference in the extents to which the respective diaphragms 10 and 11 are deflected as a result of the differing extents to which the diaphragms expand. This results in falsifications of the measurement result when the volume of the liquids 8 and 9 changes.

Such changes of volume can be caused by climatic conditions and also by deformation of the tube 16. In order to compensate for these effects of errors, the leads 21 and 22 incorporate evaluation stages 34 and 35 which are connected to the outputs of the sensors 12 and 13 respectively and in which the measured signal is controlled in dependence upon the characteristics of the diaphragms 10 and 11 associated with the sensors 12 and 13 respectively. Advantageously, the electrical correction variables for the mechanically differing characteristics of the diaphragms are ascertained in the chamber 32 by artificially varying the volume by means of the set-screw 33. After the electrical signals from the sensors have been corrected, the differential signal on the lead 27 is independent of the effects of the changes of volume.

Thus, the test arrangement can be used universally for the testing of very large surfaces and even under differing climatic conditions, the tube 16 preferably being realized by a flexible tube. This movability and length of the flexible tube involves only slight risk of deformation.

Fig. 2 shows an advantageous development of the evaluation stages, each evaluation stage comprising two coils. Two coils 36 and 37 form the evaluation stage 34, while the evaluation stage 35 is realized by two coils 38 and 39. The coils 36,37,38,39 are arranged as a differential coil system in which a core 40 is movable. The inductance and thus the inductive resistance of the coils 36 and 37 and of the coils 38 and 39 are in each case varied in pairs by displacement of the coil 40, so that it is possible to influence the sensor signals relative to one another by adjusting the core 40. Figure 2 shows a bridge circuit of the sensors 12,13 with the differential coil system comprising the coils 36,37,38,39. Two coils 41 and 42 symbolize the variable inductances of an inductive transducer serving as a sensor 12. In the same manner, the sensor 13 is represented by two further variable coils 43,44. A supply voltage Uv is applied to this bridge circuit by way of two leads 45 and 46. The interconnected terminals of the coils 43 and 44 of the sensor 13 are connected to earth, while the interconnected terminals of the coils 41 and 42 are connected to the test amplifier 24 by way of the lead 27 (see Fig.

1). Advantageously, the subtractor 23 is at the same time realized by this bridge circuit.

By virtue of the elimination of the detrimental effect of the diaphragm characteristics on the measurement result when the diaphragms expand to different extents, the test arrangement can also be used with corresponding accuracy for large deflections of the diaphragms.

However, this requires a corresponding range of linearity in the detection, processing and indication of the vertical displacement: Figure 3 shows an advantageous coupling 10 between the sensor 12 and the diaphragm 10 for the purpose of ensuring this linearity. The sensor 12 is an inductive transducer in which a core 50 is moved in a coil system 47 having at least two terminals 48 and 49. The core 50 is rigidly connected to the diaphragm 10 by way of a carrier 51 having two fastening elements 52 and 53.

The carrier 51 is guided by means of a leaf spring 55 which is secured to the carrier frame 14 by means of a fastening element 54. In this manner, the movement of the core 50 is stabilized to a single direction of movement, since the diaphragm 10 constitutes a floating structure. The coil system 57 is arranged on the carrier frame 14 such that the core 50 can carry out frictionless movements in the coil system. This coupling between the diaphragm and the sensor is distinguished by a very linear characteristic, particularly since non-linearities do not occur as a result of a capacitive or inductive air gap. Alternatively, the sensor can be a capacitive transducer whose electrodes enter one another, for example vertically and parallel. It will be appreciated that the range of linearity during testing, and thus during compensation for the effects of changes in volume, is not unlimited. Differences in height, which tax the range of linearity to the limit and possibly therebeyond, occur when the geometry of the surface is not an exact plane, particularly when testing very large surfaces for which the test method can be used by virtue of the means in accordance with the invention. In order, nevertheless, to ensure, for the accuracy of measurement, complete linearity of the detection of the measured value up to indication, it is advantageouts to make the vertical position of the vessel 7 variable (re-adjustable) by means not shown in the drawings. Moreover, an indication of the variations in the height of the surface 2 to be tested can be obtained by highly accurate measurement in the locating of this vertical position of the vessel 7. Thus, the testing of uneven surfaces is based on point-bypoint or region-by-region comparative testing of planes.

Claims

1. An arrangement fortesting for straightness and evenness, comprising at least two intercommunicating liquid-filled vessels which are arranged on a surface to be tested and on a reference surface, respectively, and each of which contains a diaphragm and a sensor, the sensor outputs carrying electrical signals which are dependent upon the height or expansion of the associated diaphragms, the signals from the sensors being passed via respective first evaluation stages to a difference forming device whose output is passed to an indicator stage at a further evaluation stage, each said first evaluation stage being adjustable to modify the sensor signals in dependence upon the characteristics of the diaphragm associated with the respective sensor.

2. An arrangement as claimed in claim 1, wherein each evaluation stage comprises two coils, the coils of the evaluation stages being arranged as a differential coil system having an adjustable core for energizing the coils in pairs in each case.

3. An arrangement as claimed in claim 1 or 2, wherein means are provided for varying the volume in the communicating vessels for the purpose of obtaining control variables for the evaluation stages.

4. An arrangement as claimed in claim 1,2 or 3, wherein each sensor comprises an inductive transducer whose core is movable in a coil and is connected to the associated diaphragm and vertically guided by means of a spring.

5. An arrangement as claimed in claim 4, wherein each said spring is a leaf spring.

6. An arrangement as claimed in claim 1, wherein each sensor is a capacitive transducer whose movable electrode moves vertically and parallel to a fixed electrode, is rigidly connected to the diaphragm, and is vertically guided by means of a spring, preferably a leaf spring.

7. An arrangement as claimed in claim 1, wherein the vertical co-ordinate of the reference surface is measurably variable for surfaces to be tested, particularly surfaces having large vertical variations.

8. An arrangement for testing for straightness and evenness substantially as hereinbefore described with reference to the accompanying drawings.