FR2497568A1 - PHOTOELECTRIC METHOD AND RECEIVER DEVICE FOR DETECTING THE POSITION OF ANGULAR STRUCTURES TO BE MEASURED - Google Patents

Abstract

THE PHOTO-ELECTRIC RECEIVER DEVICE CONSISTS OF AT LEAST ONE CENTRAL RECEIVING SURFACE 1 AND AT LEAST ONE PERIPHERAL RECEIVING SURFACE 11, 12, 13, 14. THE RECEIVING SURFACES ARE ELECTRICALLY ISOLATED FROM ONE ANOTHER AND THE SIGNALS EMITTED BY THE DIFFERENT SURFACES. CAN BE LOGICALLY COMBINED TO CREATE THE MEASUREMENT SIGNAL WHICH IS RECORDED OR VIEWED AFTER AMPLIFICATION.

Description

The present invention relates to a method

  photo-electric device and a device for detecting the posi-

  angular structures to be measured which may have any orientation with respect to a fixed reference system (coordinate system). The method and the device according to the invention are used,

  for example, in optical measuring devices of pre-

  (coordinate measuring machines) for the implementation of a positioning process according to the

  coordinates x, y and q, while allowing a measurement

  tomatic structure of mobile structures, that is to say structural

  non-stationary tures. Another application of the invention

  tion concerns the recognition of structural positions

  simple geometrics on jigs or semi-chips

  conductors in the field of photolithography.

  Photoelectric systems are used in different forms for the production of light microscopes and path measurement systems and

  also for object measurement and image analysis.

  Known documents relating to this area, including

  Photolithography describes a large number of static and dynamic photoelectric processes and

  at the detection of the position of edges or edges and no-

  also flat and straight objects (for example by

  the technical journal "Optik 21 (1964) page 605 and the following

  and by the scientific bulletin of the University Friedrich Schiller, Jena, Math.-Nat., 25 of 1976 no 5,

page 683).

  These processes are very often complicated or they do not completely satisfy the requirements of the

  measure of coordinates (they require, for example, the pre-

  necessary reference marks or marks on

  the objects to be measured, and a preliminary orientation

port to the measuring system).

  There are also devices for detecting

  photo-electric the position of an object in which an analyzer tube is used as a converter (patent DD 128 637). A single or double scan or scan of the image in the direction of the x and y coordinates

  is then realized in the analyzer tube thanks to a com-

  appropriate direction of the exploration unit. An electronic evaluation device raises the erroneous position with respect to an ideal position in the x-direction. y and L I.e

  measuring time is less than 300 minutes ..

  Patent Application DE 2,405,102 discloses a method according to which a linear structure or a straight line is obtained by means of a probe emitting a light which is effected. e ts to circular and deviated mechanically. ú-e -iguaho du-; esur

  created in a photo-receiver.

  read, regarding their position of pirse by Yralpor

  to the rotational movement, to determine the positir! year-

  gular of the structure. The analysis of the frequency of the measurement signals gives the indication as regards

  position relative to the center of rotation.

  These last two solutions are characterized by a dynamic measurement principle. The use of mechanical scanning or scanning means limits the use of

  of these processes to quasi-stationary objects.

  However, the use of non-mechanical scanning means

  such as the use of analyzer tubes, creates

  problems due to limited local resolution.

  It is furthermore probable that the latter two mentioned methods lead to the appearance of highly erroneous measurements in the case of non-ideal structures, since the location of the structure is measured without

  take into account generally accepted definitions.

  DE 2 102 027 discloses a method for measuring and defining the location of edges or ridges at the location of the maximum blackening gradient of an edge-shaped intensity profile. In this

  effect the double deviation of the intensity profile is achieved

  optically. This method can however only be used for one or two measuring devices, as indicated (see patent application DE

2,017,400).

  It is an object of the present invention to provide rapid, objective and non-contacting of measurement quantities when measuring structures producing

  optical effects, which are not necessarily stationary

  compared to a fixed reference system and which in relation to the latter must not necessarily have a preferential orientation. The current

  invention should make it possible to obtain greater precision

  positioning and a higher speed of

  safe through the use of visual positioning methods and this especially for optical precision measuring devices such as coordinate measuring apparatus comprising a digital system for measuring

of journeys.

  The object of the invention is to create a photo

  an electrical device and receiver device for recognizing and measuring edge-shaped structures

  or straight lines that produce optical effects.

  When the location of the target structure coincides with the mean photometric definition, it shall be possible to derive from it an appropriate control signal for a peripheral electronic measuring device,

  for example, a device indicating a position of a system

displacement measurement system.

  The problems described above are solved

  according to the invention by a static photoelectric process.

  which, thanks to a two-dimensional photometric evaluation,

  of the radii emitted by the characteristic structural elements, makes it possible to determine the position of the structure independently of the orientation of the latter within a fixed reference system (coordinate system) with respect to a reference mark of represented aim (origin of coordinates). The method is characterized in that the minimum and maximum of the radiation flux

  are detected optically and in an integral and bi-

  dimensionally in the immediate vicinity of the transition region of the edge, in that the sum of all rays is formed either optically by means of a peripheral diaphragm surface (for example a circular ring) and by deriving a signal electric which is

  proportional to that sum, either by an

  different signal magnitudes which are proportional

  to each other as a result of

  passing through several partial areas of diaphragm

  me peripherals and in that all the rays of the zone

  transitions of the ridge are moreover captured by an over-

  center face of the diaphragm so that an electrical signal can be obtained again which is proportional to these

  rays. Then we proceed to a differentiation of

  resulting electrical and after pre-amplification

  the signals are evaluated according to the relationship between

  the peripheral diaphragm surface and the central surface

  slide of diaphragm. It is obvious that pre-amplification

  tion is superfluous when the surfaces are identical.

  The zero crossing occurring in the differential signal is preferably used for indicating a relative and defined position of the structure to be measured.

and the origin of coordinates.

  The rays emitted by the peripheral and central diaphragm surfaces are led to two receivers

  photoelectric devices by means of

  appropriate rays or they are gathered and visualized on a receiver via a switch of

rays (Chopper process).

  According to another embodiment of the invention, the diaphragm surfaces are formed by surfaces

  sensitive photoelectric receivers realized and

  posed appropriately. This chip-integrated or hybridized photoelectric receiver device has at least one central receiving surface and at least one peripheral receiving surface surrounding the central receiving surface. The receiving surfaces are arranged electrically isolated from one another and the signals of the different surfaces can be freely combined. The central receiving surface is preferably in a circular or square form and the peripheral receiving surface may have an annular shape or it may consist of four partial surfaces of identical shape. In this case each of the partial surfaces has an area which is identical to that of the central surface or the sum of the peripheral surfaces is equal to the area

of the central surface.

  The receiver device according to the invention allows in addition to the implementation of the method for the exploration independently of the direction, of the structures to be measured in the form of an edge thanks to the free combination of all the individual receiving surfaces, and also the implementation of the known two-cell differential method and

  intended for the exploration of rectilinear objects.

  Since the method used constitutes a static measuring method, the device is particularly suitable

  well to the measurement of mobile structures (non-stationary).

  Various other features of the present invention

  come out from the following detailed description.

  Embodiments of the subject of the invention are shown, by way of non-limiting examples, to

attached drawings.

  Fig. is the schematic diagram of a

  receiver consisting of five integrated receiver diodes

  having a square shape and identical dimensions.

  Fig. lb shows the appearance of the reception signal during detection by translation of an edge (t = o

or 45).

  Fig. lc shows the pace of the reception signal

  when detecting an edge by rotation.

  Fig. ld shows the pace of the reception signal

  when detecting a straight line by translation.

  Fig. 2a is a schematic diagram of a provision

  receiver with five integrated receiver diodes

  rectangular shape but of different geometry

  ferent. Fig. 2b shows the appearance of the reception signal

  when detecting a translation edge.

  Fig. 3a represents a receiver device com-

  poses two circular formine receiving surfaces, arranged coaxially in one another and having a

supersized ratio of 1: 1.

  Fig. 3b shows the reception signal during the

detection of an edge.

  Fig. 3c represents the disuoait: To iD recipe: inr according to fig. 2a associated with a disos: if devaluation

  signal for the detection of a rectii> line;

within a nominal range.

  Fig. 3d shows a receiving device comnpor-

  three receiving surfaces arranged coaxially

one inside the others.

  Fig. 3rd shows the reception signal during the

detection of a straight line.

  Fig. 4 represents a receiving device

  see fig. 2a but in which the two surfaces

  are subdivided into four sectors each

ticks.

  The receiver device shown in FIG. 1a is composed of five separate square receiving surfaces, namely, a central surface 1 "and four peripheral surfaces 11 ', 12'1-, 13: and 14' of lateral lengths A. This assembly can be realized by being integrated to a chip or as a hybrid construction The separation strips d between the receiving surfaces determine the x, y axes of the coordinate system.

  of this receiver in an optical measuring device for coordinating

  data it must be arranged in a training plan

  image or, for example, on the screen of

  jection so that its center is located in the line of sight. We generally choose the optical axis of the device

measurement as a target axis.

  The type of receiver shown allows 1. Obtaining a coincidence signal when setting up an optical effect edge without taking into account

  account of the position of the origin of the coordinates.

  2. Obtaining a parallelism signal or a recovery signal after having obtained the coincidence according to 1 and during a rotation of an edge with respect to

coordinate axes.

  3. Obtaining a coincidence signal in an uncontrolled positioning of the photometric center of optical effect lines and a width b. The width lines ba + 2 d and b 5a + 2d are placed relative to the origin of the coordinates and the db 2a + d lines with respect to a point of view on the coordinate axes and at a distance of 1/2 (a + d) of the origin of the coordinates

(see Fig. la).

  4. Obtaining a coincidence signal presented

  both during the angular displacement of a line of lines that extend parallel and at a distance of 1/2

(a + d) coordinate axes.

With regard to point 1.

  According to the invention, the positioning of

  edges by means of a receiver according to fig. the

  kills according to the equation: Z = where P1 represents the propagation of rays on surfaces 1i, 121, 13 'and 14' * 1 the propagation of rays on surface 1 ",

= O at the coincidence.

  The multiplication by four of the propagation of

  rays is obtained after the transformation by amplification

  appropriate cation of the signal s (r, 4) which is proportional

to the propagation of rays.

  4 Ji (q) - 4 s1 (rk 9): 5 (rklt) Z = 1 57- where s '4 represents the signals from surfaces 111, 12', 13 'and 14' sI sl] e signal from of the surface 1 "and

  rk - the coordinates of the location of the edge.

  During translation through the receptor

  of an edge-shaped structure and having any angular position with respect to the axes of the

  coordinates, a signal is obtained whose appearance is

  for the process. Differential signals s (rk,) for - = 0o or 2 = 45- are shown in FIG. lb. The receiver device shown in FIG. 2a works according to the same measurement principle. Legality

  between the sum of the signals emitted by the recep-

  these outside 21 ', 22', 23 ', 24' and the signal from

  of the inner receiving surface 2 "in an illuminated

  of all receiving surfaces, is then obtained by the same area of the sum of the

  surfaces 21 ', 22', 23 ', 24' and the surface 2 ".

  Fig. 2b shows the characteristic curves s (rk, A) of the signals. The differential signal of both

  present embodiments, regardless of the posi-

  angle of the ridge with respect to the axes of the

  data, a zero crossing during the coincidence between

  the edge and point of origin of the coordinates. At this

  both the edge obscures half the sum of the outer receiving surfaces as well as half of the inner receiving surface and the differential signals are

created.

  8 '-z (rk Z) (rk t) = according to FIG. lb or 4 - s2z (rk) - 4 s "2 (rk) = 0 Z = 1

according to fig. 2b.

  The detection or stop zone in the direction of the coordinates results from the equation Bxy = 2 (a'1 + d) + a "The detection or stop zone B indicates the zone

  on the receiver device for which s (rk, q) 40.

  When there are external dimensions

  ticks (circumscribed circular surface) the sensitivity is greater for the embodiment with equal surfaces according to FIG. the only for the embodiment

according to fig. 2a.

  In both receivers the sensitivity increases with the increase of 450 at the angle of the edge by

  refers to the axes of the coordinates (separation band).

With regard to point 2.

  The two receiving devices can always be used, in a simple way, for the indication of the coincidence during a rotation (edge parallel to the coordinate axis). For this purpose the surfaces 11 'and 13' or

  12 'and 14' of the device according to FIG. are, for example,

  connected to form a differential circuit. The differential signals s (x, 90 *) = s12 (x, 900) - s'14 (x 90) or s (y, o) = if (y, 0) - s13 (y, O) become zero when the angular positions indicated

above are reached.

  Obtaining the coincidence position by

  port to one of the senses of coordinates requires a

  position in accordance with paragraph 1. The appearance of the different

  ferential is shown in fig. the. The limit angle of the detection or arrest zone is generally obtained by the following relations: a max = arc tan 2 for h or max a d = arctan 2d for h = O (corncidence) max 2d a in which

  h represents the distance between the edge and. the point of origin

  gine coordinates in the direction of max coordinates the limit angle of the detection or stopping area of

  the edge with respect to the direction of the coordinates.

  With regard to the point The receiving device according to FIG. the whose

  surfaces have the same area advantageously

  indication of the nominal position of a line. AT

  this effect is used the static two-cell process.

  It is then possible to establish a differentiating circuit between the receptor receiving surfaces, 12 ', 13' and 14 'and the surface 1', for example with a view to using the passage

by zero of the differential signal.

  s (X) = s'12 (x) - s1 "(X) s (y) = if (x) - Si (x) for small line widths (s <b (2a + d) or s (X) = s12 (x) - s14 (X) s (y) = s11 (y) - s13 (y)

  for line widths s + 2d <b <3a + 2d.

11i

  Within a range of 0 01. t450 the step

  wise by zero of a pair of receiving surfaces deter-

  mined is dependent on the rotational position. When it comes to larger angles we must measure each time in the other direction of coordinates.

  The appearance of the signals as a function of the

  placement of the photometric center of the line is shown

in fig. id.

With regard to point 4.

  Provided that the positioning of the line has been carried out in accordance with point 3, it is possible to obtain a

  coincidence signal when the directions of the coordi-

  are achieved through the differential connection of

  partial receiving surfaces according to point 2.

  Another receiver device for exploration

  edges independent of the direction is represented in fig. 3a.

  This device is distinguished by a sensitivity that is independent of the orientation of the edge. The device is constituted by a receiving surface of circular shape

  and by another receiving surface of shape also circulating

  eyelike and which is arranged concentrically with respect to

  in the center of the circle of the first surface. The two sur-

  receiving faces must be connected in such a way that

  the electrical signals they emit are proportional

  incident light flux, these signals being then subtracted. The devices used for this purpose are known, for example by the patent application DE

  1,957,161, and require no explanation.

  According to the invention the length of the spokes must be chosen so that the annular surface 31 'is equal to the annular surface 3. This is the case when,

  while neglecting the dimensions of the width of separa-

  between the receiving surfaces, ri = \ / W ro.

  When an edge-shaped structure moves on the receiving surface as shown in FIG. 3a, the shape of the differential signal is that shown in FIG. 3b. It emerges from this figure that a zero signal appears which can be advantageously used for

  start of a length measurement process when

  that the money is in coincidence with the central point

  tral of the receiving device. At this point the arte structure obscures just half of the receiving surface, ie the other half is

  illuminated, so that the difference in the reception

  becomes zero, provided that the surfaces of that

  deny have the same sensitivity.

  The analytic relationship between the location of the

  rk and the differential signal s (x) for a separation width d - 0 results from the following equation: I rc.COS --- 'S (rk DirE.SD r (arc cos -arc cos + orrr rOrk (2 kV -) where SD represents the sensitivity of the receiving material and E the light intensity in the illuminated part of the transition of the argue.

  enlightened of the edge we admit that E = 0.

  For a receiving sensitivity of the device in the vicinity of the zero crossing (coincidence point), for example, values are obtained: E = 1500 lx, SD = 4.2 mA / lm and r1 = 100o m a value of S ( rkle) = 0.5 nA / Pm Ark Since the dark current of matter

  chosen receptor is approximately 0.07 nA at the time of

  Also, differences of less than 1 μm can be detected in the case of ideal structures (maximum contrast). The symmetry of revolution of the receiver device makes it possible to obtain a sensitivity of the device which is totally independent of the direction. The line of sight

  is determined by the center point of the circle. The provision

  may be used in a particularly advantageous way.

  to measure moving angular objects (non-stationary

  servants). According to an improvement of the invention, provision is made

  also a device which allows the detection, within the

  within a predetermined nominal range, of a structure

in the form of a line.

  When using the receiver type

* in fig. 3a it is necessary, for this purpose, to amplify

  electronically for a factor <3x (eg 2.5x) the reception signal from the annular surface

  before the recording of the different photoelectric current

  (Figure 3c). The same effect can however also be obtained, according to the invention, by connecting to the first annular surface a second peripheral annular surface (Fig. 3d). The connection of the receivers must be made in such a way that the resulting total signal

  s. combined with signals from different

  ge se rentes partial receiving surfaces according to the following relation: Sges (r, q) = s "3 (r,) s31 (rl 4) + s'32 (ri)] The switch S makes it possible to switch to the position

  "arete exploration" (switch position s).

  _ a | b S s (r) = S (k) s (r) = S (rs) When radius r2 is chosen (neglecting again the separation slot) so that it is slightly smaller than the double radius r of the receiver annular, the pace of the signal during the passage of a structure in

  form of line above this receiver, is that represented

  felt in fig. 3rd. The width b of the line is bl b2 <b3.

  It appears from this figure that there is a double zero crossing of the silnal voltage which is symmetrical at the point of coincidence (that is, the center of the line is in the line of sight). The distance of these zero crossings of the line of sight is independent of the line width and is determined solely by

  the chosen radius r2 (and the amplification factor of the

  signai folding). When the amplification factor approaches the value 3 (or the ratio of the radii of 3a

  value r2 / r. = 2), the zero crossings are approaching

  gressively from the line of sight and the negative co -iposarnts

  signal decreases which results in the recording of

  Zero crossings are no longer assured.

  The device described above therefore makes it possible to detect and measure a structure in the form of a line within a predetermined range. For example, a nominal range of 30 μ is obtained for example for a beam of 75 μm and a range of 140 μm.

nominal - 10 μm.

  This use also allows and without difficulty

  to measure moving objects thanks to the realization

of a static measurement process.

  Another advantageous embodiment of a disclosure

  positive receiver is shown in fig. 4. This receiver device is particularly suitable for positioning the photometric center in a line with respect to a point of view. For this purpose the sectors are connected in pairs to form a differential circuit by means of which the receiver signal responds to the following relations: s (r,) = s '41 + s' 42 = s "45 + s" 46- (s t43 + s' 44 + s "47 + s" 48) for 5 <(f) <o or s (r,) s4] + d 44+ S "45 + s" 48- (s'42 + 8'43 + s "46 +" s 47)

for 0- <(45.

  The separation strips determine the axes of the

coordinate system.

  The zero crossing of s (r, q) indicates the coincidence of the photometric center of the line with the center of the receiving device. The same branch is used to indicate the coincidence of an edge with the axes of the

coordinates during a rotation.

  The coincidence of a line (photometric center)

  with the axes of the coordinates during a rotation is in-

  when the following relations are satisfied: s'41 (r, c) -s'44 (r,) = 0 and

s42 (r,) -s' (r, -'43r, q) = 0.

  The last two uses mentioned need

  are placed in position on the point of origin of the coordinates. The device can also be used to directly determine the angular position of a line with respect to the direction of the coordinates. For this purpose and during the displacement of the object in the x direction, a differentiating circuit is established, for example, between the surfaces

  receivers 411, 44 'and receiving surfaces 42I, 43'.

  The zero crossings of the two differential signals are then recorded / and the angle is calculated from the

  corresponding path coordinates.

arc tan 2 (ro + d) + r1 - rO- d

= arc tan-

  _ x The positioning of an edge can be done using an embodiment that corresponds to that

  of the receiver device according to FIG. 3a.

  In this case angular positions t = 0 or Qc 90 must be avoided due to the presence of the separation groove (reduced measuring accuracy).

l6

Claims (19)

  1 - Photoelectric process for detecting the posi-   of edge-shaped structures to be measured by means of a two-dimensional photometric evaluation of radiation fluxes emitted by characteristic structural elements, characterized in that the minimum and maximum of the radiation flux are detected optically in an integral and two-dimensional way in the immediate vicinity of   transition zone of the ridge and regardless of the direction   talion inside a coordinate system, in that the sum of all these radiation fluxes is formed   optically using a peripheral diaphragm surface   and an electrical signal proportional to this sum   is derived or in the course of an electrical   than different signals which are proportional to the partial radiation fluxes passing through a plurality of partial diaphragm surfaces, in that all the radiation flux,   influenced by the change in intensity of the   sition of the edge, is captured entirely by a central diaphragm surface, in that an electrical signal is created which is again proportional to the sensed radiation,   in that we finally form the difference between   obtained, in that the electrical signals   are evaluated after pre-amplification according to   surface ratio between the central diaphragm surface   trale and the peripheral diaphragm surface and in that the disappearance of the difference indicates a relative position defined between the structure to be measured and the point original coordinates.   2 - Process according to claim 1, characterized in that the diaphragm surfaces are formed by   receiving surfaces of photoelectric receivers   appropriately arranged and arranged and that the   the signals delivered by the receiving surfaces   indicate a relative and determined position of the structure   to be measured or the coincidence of the latter with   the point of origin of the coordinates.   3 - Process according to claim 1, characterized   rised by the fact that radiation fluxes from   central and peripheral diaphragm faces are directed to two photoelectric receivers via appropriate ray guiding elements. 4 - Process according to claim 1, characterized in that the two radiation streams combined by guide elements are represented on a receiver   photo-electric through an inter-   breaking the rays and in that the magnitude of the electrical signal   modulated receiver delivered by the receiver indicates a specific relative position of the structure to be measured in relation to   at the point of origin of the coordinates.   - Photoelectric receiver device for the   implementation of the method according to one of the claims   1 to 4, characterized by at least one central receiving surface and at least one peripheral receiving surface surrounding the central surface, the receiving surfaces   being electrically isolated from one another and the   of the different surfaces which may be born freely.   6 - Device according to claim 5, characterized   risé in that it is made in one piece.   7 - Device according to claim 5, characterized   risé in that it is made in a hybrid form.   8 - Device according to one of claims 5 to   7, characterized in that the peripheral receiving surface is composed of at least four partial surfaces of the same configuration separated and in that each partial surface   has the same area as the central surface.   9 - Device according to one of claims 5 to   8, characterized in that it consists of a surface   central square (1 ") and four peripheral receiving surfaces   (11 ', 12', 13 ', 14i) identical to the central surface trale (1 ").   Device according to claim 9, characterized   in that the measuring signal is obtained, according to claim 1, by electrical formation of the sum of the signals (sei 2, s 3, se 4) produced by the peripheral receiving surfaces (11 ', 12 ', 13', 14 ') and by a subsequent differentiation of the sum signal and the signal (s "1) produced by the central and am- folded by a factor of 4.   11 - Device according to claim 9, characterized   in that the measurement signal is formed using   signals coming from two different surfaces each time   Peripheral Ceptresses] 2 ') and (14') respectively   and (13), and in accordance with the relation s (r, 1) = s14 <r () s14 (r) respectively sr, q) = s (r, sq) - s' (r)   12 - Device according to claim 9, characterized   rised by a differential connection of receiving surfaces   these devices facing each other or by the differential connection of a peripheral receiving surface and the central receiving surface (1 ").   13 - Device according to one of claims 5   7, characterized in that the peripheral receiving surface   is composed of at least four partial surfaces   separated, having an identical shape and whose super-   total value corresponds to that of the central surface.   14 - Device according to claim 13, characterized   risé in that it consists of a receiving surface   square (2 ') and four receiving surfaces   periphery (21 ', 22', 23 ', 24') also of square shape and whose total area corresponds to that   the central receiving surface (2 ').   Device according to Claim 14, characterized   in that the obtaining of the measurement signal following the   claim 1 results from the formation of sums of electricity   signals (s'21 's'22 s'23, s'24) from the peripheral receiving surfaces (21', 22 ', 23', 24 ') and a subsequent differentiation of the sum signal and the   signal is produced by the central receiving surface (2 ").   16 - Device according to claim 14, characterized   in that the measuring signal is obtained in accordance with   the process defined by claim 11.   17 - Device according to one of claims 5 to   7, characterized in that it is composed of a surface area   ceptrice (3 ") of circular shape and   receiving face (3 ') of annular shape and surrounding the surface (3 ").   18 - Device according to claim 17, characterized   characterized in that the measurement signal is created in accordance with claim 1 by a differential connection both receiving surfaces.   19 - Device according to claim 18, characterized   in that the signal produced by the receiving surface   ring is amplified by a factor (M).   Device according to claim 17, characterized   it is supplemented by the addition of one   x annular peripheral receiving surface (32 ') whose outer radius must not exceed twice the radius   the central receiving surface (3y) of circular shape.   21 - Device according to claim 20, characterized   in that the measurement signal is created by a different   ferentiation of the sum of the signals coming from the surfaces   receivers (31 ') and (32') and the signal produced by the central receiving face (3 ").   22 - Device according to claim 17, characterized   tered in that both the central receiving surface   that the annular peripheral receiving surface are sub-   each divided into four equal receiving surfaces in the form of sectors and electrically isolated from each other.   23 - Device according to claim 22, characterized   in that the measurement signal is created by a   logical combination of the signals according to claim 18.   24 - Device according to claim 22, characterized   in that the measurement signal is created by a different   ferentiation of sum signals from surfaces   receivers in the form of sectors and separated by bands.   Device according to claim 22, characterized   in that two measuring signals are created simul-   at the time of the differential connection each time of two sectors, for example (41 ') and (42') as well as   (43 ') and (44') of the annular receiving surface.