GB1595949A

GB1595949A - Contact-free thickness measurement systems

Info

Publication number: GB1595949A
Application number: GB5002777A
Authority: GB
Original assignee: Siemens AG
Current assignee: Siemens AG
Priority date: 1976-12-01
Filing date: 1977-12-01
Publication date: 1981-08-19
Also published as: NL7713236A; IT1089106B; BE861356A; FR2373034A1; DK531677A; DE2654478A1; FR2373034B1

Description

(54) IMPROVEMENTS IN OR RELATING TO CONTACT-FREE THICKNESS MEASUREMENT SYSTEMS (71) We, SIEMENS AKTIENGESELLS CHAFF a German Company, of Berlin and Munich. Gennan Federal Republic, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The invention relates to a contact-free thickness measurement system using deflectable light beams.

Our United Kingdom Patent Application No. li'Si76 (Specification No. 1,535,352, describes and claims a system for measuring the thickness of a workpiece having two plane-parallel surfaces, wherein a single laser projects a beam via a single deflector device to a beam-splitting arrangement producing three or four beam components, two of which are conducted periodically across one of said surfaces and the other one or two of which are conducted periodically over the other of said surfaces. wherein said components are reflected in diffused fashion from said surfaces, wherein the reflected components are detected by detectors, which are arrranged at a distance from the workpiece surfaces, which correspond in number to the number of components into which the laser beam is split, and each of which is only able to receive light from a specific direction, and wherein the distances of the surface points from which the reflected light components are received by the detectors from a zero plane in which the components of the laser beam intersect at the beginning of each deflection period are determined by measuring the time intervals between the beginning r aid deflection period and the reception of a light signal in the corresponding detector, and wherein the thickness of the workpiece is determined by a measuring circuit from said distance values.

If a plurality of points are scanned on the X rk piece surface, the work piece may be tilted relative to a virtual reference plane during the measurement. In this case it is possible to determine the thickness of the work piece independently of the tilt.

In the above-mentioned system the laser beam is split into two subsidiary beam components, one serving to scan one face of the work piece, and the other to scan the other face thereof. In order to determine the position of a plurality of points on the two opposed surfaces of the work piece, these beam components are split into further subbeams by prisms.

One object of the present invention is eo provide a simplified system of contact-free thickness measurement.

The invention consists in a contact-free thickness measurement system in which a light source is provided to project a sharply focused light beam via a light deflector to a beam splitter, where two simultaneously and periodically deflectable light beam components can be produced, means being provided via which one of these light beam components is projected onto one surface of an object to be measured and another light beam component is projected onto the opposite surface of said object, a plurality of directionally selective photo-detectors being provided to receive light each from a specific respective direction, and each detector being used to determine the times at which the point of incidence of one of these components on an object surface comes into a position lying on its line of sight, the light beams being periodically moved back and forth over opposite surfaces of the object from a given zero position such that firstly a light beam component falls upon a point A on the surface of the object lying in the line of sight of a first detector, then the light beam component falls upon a point B Iying on the same surface of the object and on the line of sight of a second detector, whilst there is arranged at least one further detector with a line of sight which can be used to record the time at which a light beam component falls upon a point C which lies on the last mentioned line of sight, and on said opposite surface of the object, the detectors supplying signals to a measuring circuit to determine the thickness of the work-piece.

Thus. in a system constructed in accordance with the invention splitting of the subsidiary beam components into further beam components is superfluous.

The invention will now be described with reference to the drawing, which schematically illustrates one preferred exemplary embodiment.

The system utilises a light source which produces a sharply focused light beam, and continuous signal laser is particularly suitable for a light source of this type.

A light beam 10 produced by a light source I is directed into a beam deflector 2, in which it is periodically deflected as symbolised by an arrow 11 on the emergent light beam path 10.

The deflector device described in our United Kingdom Patent Specification No.

1,440,688 is particularly suitable as the light deflector 2. as this known device can produce large deflection angles, and is also advantageous in that the resonant frequency at which this device is operated is relatively stable.

On the beam path following the light deflector there is arranged a beam splitter 3 which divides the light beam 10 into two beam components of approximately equal intensity. Thus at successive instants beam components 100. 101 and 102 are projected in accordance with the instantaneous deflection of the light beam 10, together with beam components 1000. 1001 and 1002 are produced on paths diverging from those of the first components. The beam components 100 and 1000 correspond to a predetermined zero position of the lightdeflector.

A semi-permeable relfector can be used as the beam splitter 3 for example.

The beam components 100 to 102 are guided bv a stationary reflector 4 to one surface of an object 55. whose thickness is to be measured. The beam components 1000 to 1002 are guided towards this object from an opposite direction by a stationary reflector 40.

The object to be measured must be arranged within a measurement zone that is defined by a zero plane ZZ. and a further plane MM parallel thereto.

Outside of the measurement zone there are arranged pairs of photo-detectors 5 and 6.50 and 6(). which are provided with direction allv selective diaphragms. so that they are only able to receive light from a selected direction. as symbolised by corresponding lines of sight 5 51. 52. 6t and 62 respectively.

The lines of sight of the detectors 5 and 6 must lie in the same plane as the beam components 100, 101 and 102 reflected by the reflector 4. The lines of sight of the detectors 50 and 60 must lie in the same plane as the beam components 1000, 1001 and 1002 reflected by the reflector 40.

The line of sight 51 is incident on a surface of the object 55 at a point A, and the line of sight 61 is incident on this surface at a point B, the line of sight 52 being incident on an opposite surface at a point C, and the line of sight 62 at a point D on this opposite surface.

As soon as the points A, B, C and D come into the path of a respective beam component this is recorded by the relevant one of the detectors 5, 6 50 and 60. As every object reflects components of any incident beam in diffused fashion, it is unnecessary for the light beam and the corresponding lines of sight to possess the same angle relative to the axis of incidence on a surface of the object.

Thus the object can assume any arbitrary position within the measurement zone, and the detectors will in any case record the respective instants at which the points A, B, C and D are in the path of the relevant beam components, to supply signals to their associated measuring circuits.

The beam components 100 and 1000 are produced in a predetermined zero position of the light deflector 2. These beam components meet at a point 0 on the zero plane ZZ as long as there is no object positioned in the measurement zone.

The light deflector causes periodic scanning from the zero position. At a later point of time, in accordance with the deflection movement of the light detector, there are formed beam components 101 and 102 from one direction, and beam components 1001 and 1002 from the other direction. This process is repeated periodically in accordance with the periodic movement of the light deflector 2.

The times at which the light deflector assumes the zero position will be t;. These times can be recorded in a simple fashion by a photo-detector 7 into which a small component of the beam component 1000 is gated by means of a glass plate 8. This glass plate also gates out a weak light component from the later light beam components 1000 and 1002, although these later. weak light beams are not incident on the detector 7.

The detector 7 can simultaneously be used to count the deflection frequency of the beam components.

In order to be able to check the deflection speed, it is possible to provide a further detector (), on which a weak light beam component is incident at a later point of time when a limiting beam component 1004 is produced. This weak light beam component can itself be gated out of the beam component path 1004 by means of the glass plate 8.

The time difference between the time to which is signalled by the detector 7 and the time at which light is incident on the detector 9 thus constitutes a gauge for the deflection speed of the light deflector and can be utilised in the measuring circuits. If the deflection frequency is known, this time difference is also a gauge for the deflection range of this light deflector.

Thus the deflection movement of the light deflector 2 can be monitored and measured by means of the detectors 7 and 9.

This deflection movement can fundamen tally also be measured and monitored in a different manner. However, the represented method is particularly simple.

The thickness of the object 55 is measured in the following manner: at the time tf), as recorded by the detector 7, the upper surface of the object 55 is in the path of the light beam component 100 and the opposite surface thereof is in the path of the beam component 1000. At the points of incidence the beam components are reflected in diffused fashion, as indicated by arrows. As the points of incidence lie outside the lines of sight 51, 52. 61 and 62 ofthe detectors 5. 6, 50 and 60 these detectors do not produce a signal.

At a later time t,. the upper surface of the object 55 has a point A in the path of the light beam component IQI, and this light beam is reflected in diffused fashion by the object surface. As the point A lies on the line of sight 51 of the detector 5, the time t, is recorded by this detector, and the time difference If, - tA serves as a gauge for the distance of the point A from the zero plane ZZ.

At a further point of time In, the object surface has a point B in the path of the light beam component 102. This light beam com- ponent is reflected in diffused fashion from the object surface and as the point B lies on the line of sight 61 of the detector 6, the time t1 is recorded by this detector 6. The time difference t,-t0 serves as a gauge for the distance of the point @ from the zero plane ZZ.

The opposite surface of the object 55 is '-canned in corresponding manner.

Nt a time t,. the point C is in the path of the beam component 1001. and this light beam is reflected in diffused fashion by the object surtace. As the point C lies on the line of sight 32 of the detector i0. the instant t( is recorded b: detector SQ, and the time differcnce t -t serves as a gauge for the distance of the point C from the zero plane ZZ.

A a later time tD, the point D is in the path of the light beam component 1002. and as this light beam is reflected in diffused fashion from the surface and the point C) lies on the line of sight 62 clothe detector 60. the time tl, recorded by this detector 60, so that the time interval 1o - tu serves as a gauge for the distance of the point Et from the zero plane ZZ.

The thickness of the object can now be determined from the position of the points A, B, C, D relative to the zero plane ZZ. The object may be tilted by any arbitrary angle about an axis which is perpendicular to the drawing plane of the drawing.

In the drawing all the light beams and all the lines of sight are assumed to Iie in the same plane. If, in the thicknessmeasuwment of the object, the object is to be allowed to be arbitrarily tilted about arbitrary axes, the plane formed by the light beams 100, 101 and 102 reflected by the reflector 4 and: by the lines of sight 51 and 61 must be inclined relative to the plane formed by the light beams E0007 1001 and 1002 reflected by the reflector 40 and by the lines of sight 52: and 62.An expedient angle of inclination is 90", i.e. the planes are at right angles ta one another, This is easily possible by an appropriate arrangement of the reflector 40 and the detectors 50 and 60.

In this way it is ensured that the path described by the point of incidence of the light beams on the upper surface of the object (as shown) this path being determined by the straight line through the points A and B, is not parallel to the path described by the point of incidence of the light beams on the under surface of the object (as drawn), This path on the underneath of the object is determined by the straight line through the points C and D.

In the exemplary embodiment illustrated four detectors S, 6 50 and 60 are provided to detect a response from beam components scanning the object.. It is also possible to provide a larger number of detectors which, correspondingly, consecutively record the times at which surface points ofthe object lie on the relevant line of sight of the detectors are in the path of the light beam Thus it is possible to increase the redundancy of the device, and also possible to measure more complicated objects whose surfaces are not plane-parallel to one another In the case of objects havmg plane-parallel surfaces, it is sufficient to use three of the detectors 5, 6, 50 and 60 to determine the thickness of tyre object if this object is tilted at a predetermined angle relative to the zero plane, hn the event that the components scanning the upper surface of the object are detected by two detectors 3 and 6, whereas the beam scanning the under surface is detected by one of the detectors 50 or 60, this angle of tilt cannot lie in the plane deter mined by the lines of sight 51 and 61, The same applies to the angle of tilt and the plane determined by the lines of sight 52 azd 62 if the scanning af the under surface of the object is detected by Iwo detectors 50 and 6O, and that of the upper surface only by one of the detectors 5 or 6.

When an object having plane-parallel surfaces is subject to arbitrary tilts, two detectors are required for the upper surface and under surface of the object, i.e. a total of four.

WHAT WE CLAIM IS: 1. A contact-free thickness measurement system in which a light source is provided to project a sharply focused light beam via a light deflector to a beam splitter, where two simultaneously and periodically deflectable light beam components can be produced, means being provided via which one of these light beam components is projected onto one surface of an object to be measured and another light beam component is projected onto the opposite surface of said object, a plurality of directionally selective photodetectors being provided to receive light each from a specific respective direction, and each detector being used to determine the times at which the point of incidence of one of these components on an object surface comes into a postion lying on its line of sight the light beams being periodically moved back and forth over opposite surfaces of the object from a given zero position such that firstly a light beam component falls upon a point A on the one surface of the object lying on the line of sight of a first detector. then the light beam component falls upon a point B lying on the same surface of the object and on the line of sight of a second detector, whilst there is arranged at least one further detector with a line of sight which can be used to record the time at which a light beam component falls upon a point C which lies on the last mentioned line of sight, and on said opposite surface of the object, the detectors supplying signals to a measuring circuit to determine the thickness of the work-piece.

2. A system as claimed in Claim 1. in which said measuring circuits are provided to measure the time differences t,,-t,. t,-tB and t, - t"t,. where t , is the time at which the beam components commence a periodic scan from their zero position; tA is the time at which the relevant light beam component falls on the point A; tB is the time at which the relevant light beam component falls on the point B; and t,- is the time at which a light beam component falls on a point C on the opposite surface.

3. A system as claimed in Claim 1 or Claim 2. in which there are at least two detectors arranged with their lines of sight directed at one surface of the object and that there are at least two detectors arranged with their lines of sight directed at the other surface.

4. A system as claimed in any preceding (laim. in which the determination of the time to is established by a photo-detector into which a beam component is gated from the beam components when in their zero position.

5. A contact-free thickness measurement system substantially as described with reference to the drawing.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. and that of the upper surface only by one of the detectors 5 or 6. When an object having plane-parallel surfaces is subject to arbitrary tilts, two detectors are required for the upper surface and under surface of the object, i.e. a total of four. WHAT WE CLAIM IS:

1. A contact-free thickness measurement system in which a light source is provided to project a sharply focused light beam via a light deflector to a beam splitter, where two simultaneously and periodically deflectable light beam components can be produced, means being provided via which one of these light beam components is projected onto one surface of an object to be measured and another light beam component is projected onto the opposite surface of said object, a plurality of directionally selective photodetectors being provided to receive light each from a specific respective direction, and each detector being used to determine the times at which the point of incidence of one of these components on an object surface comes into a postion lying on its line of sight the light beams being periodically moved back and forth over opposite surfaces of the object from a given zero position such that firstly a light beam component falls upon a point A on the one surface of the object lying on the line of sight of a first detector. then the light beam component falls upon a point B lying on the same surface of the object and on the line of sight of a second detector, whilst there is arranged at least one further detector with a line of sight which can be used to record the time at which a light beam component falls upon a point C which lies on the last mentioned line of sight, and on said opposite surface of the object, the detectors supplying signals to a measuring circuit to determine the thickness of the work-piece.