DE102004003329B3 - Method of geometrically measuring a surface covered with a transparent material film especially a fluid film such as for use in surgery where at least two measurements are made for each measurement point - Google Patents

Abstract

The method involves directing two separate light bundles (4) onto the surface (5) from two starting points spaced a given distance from each other in a reference system (8). For a first measurement, the two light bundles are applied to the surface at a measurement point (6) for coverage. The position of the two light bundles in the reference system is determined to derive the distance of the measurement point from a reference point in the reference system using triangulation. For each measurement point, at least a second measurement is carried out with a different positioning of the two starting points. The distance between the starting points and the reference point is determined taking into account the refraction in the film (7), from the detected positions and the change in starting points.

Description

Technical application

The The present invention relates to a geometric measurement method one with a transparent material film, in particular a liquid film, covered surface, at the two separate light bundles of two starting points, which in a reference system a predetermined Distance apart, be directed to the surface, the two light beam for one first measurement on the surface in a measuring point to cover be brought and covered recorded in the measuring point, the position of the two light bundles in the reference system is going to over Triangulation the distance of the measuring point to a reference point of the reference system.

The geometric measurement of surfaces with optical measuring methods is used in many technical areas. A special Application area represents the field of surgery on which in Increasingly, navigation systems are being used. this concerns especially the minimally invasive surgery but also conventional Interventions, for example in robot-assisted applications. navigation systems enable an orientation of the surgeon in the guidance of instruments within of the human body, without the surgeon having an immediate perception of the intervention site must have. Many such navigation systems use preoperatively created, three-dimensional image data from computed tomography or Magnetic resonance imaging. Furthermore, intraoperative measuring methods are also available known in which the environmental information during the procedure directly detected at the point of intervention and displayed for navigation. Examples of this are measurement methods with optical 3D scanners at the endoscope tip, which is continuously a current, three-dimensional reconstructed Provide image of the surrounding tissue from the endoscope tip.

Next This special medical application can be optically Method for geometric measurement of a surface as well in other technical areas, such as for positioning of elements, for quality assurance in production or in the field of technical endoscopy.

From the DE 100 37 771 A1 A method for geometrically measuring a surface is known, in which two separate light bundles are directed from two starting points, which have a predetermined distance from one another in a reference system, onto the surface to be measured. The two light beams are then brought to overlap on the object in a measuring point. When the two light spots produced by the light bundles are covered on the surface, the position of the two light bundles in the reference frame is detected. From the detected position of the light beams and the distance of the two starting points at the time of the overlap, the distance of the measuring point on the object to a reference point of the reference system is determined. This can be done by calculation with triangulation or by comparison with already pre-calculated for the device used by triangulation table values. The document also specifies a device for carrying out the method, which can be integrated in particular into an endoscope. By measuring a large number of measuring points on the surface, the surface profile can be reconstructed.

Just when using such a device in the field of surgery However, there is the problem that the surfaces to be measured during the Operation often by natural body fluids or rinsing fluids are covered. This thin one liquid films on the surface can falsify the measurement by refraction effects. The same is true for others Areas of application, for example in technical endoscopy or industrial Applications with flooded surfaces where the ones to be measured surfaces with a thin one transparent material film are covered.

The DE 44 34 822 C1 describes a device for non-contact measurement of distances to reflective interfaces, in particular for non-contact measurement of the thickness of measurement objects made of transparent material. In this device, two counter-rotating beams from two radiation sources are guided by two beam splitters and a deflection obliquely on the surface of the measurement object. The deflector consists of a body of transparent material with reflective and refractive interfaces. The partial beams reflected at the front and rear surfaces of the transparent measurement object are directed by the deflection device and the beam splitters onto two spatially resolving detection devices. From the points of impact of the reflected rays then the distance to the measurement object can be or determine the thickness of the transparent measurement object.

Also the EP 0 452 665 A2 describes an apparatus and method for measuring the thickness of transparent coatings. Here, two lasers are used whose beams are superimposed on the surface of the layer to be measured. With a CCD sensor, the radiation of one laser reflected at the layer and the radiation of the other laser scattered at the layer are detected. From the detected signal, the thickness of the layer is then derived.

outgoing from this prior art, the object of the present The invention is to provide an optical measuring method that the geometric measurement of a transparent material film covered surface with high accuracy.

Presentation of the invention

The Task is solved by the method according to claim 1. advantageous Embodiments of the method are the subject of the dependent claims or can be understood from the following description and the embodiments remove.

at the present method for geometrical measurement of a a transparent material film, in particular a liquid film, covered surface become two separate light bundles of two starting points, which in a reference system a predetermined Distance apart, directed to the surface. The two light bundles will be for one first measurement on the surface in a measuring point to cover brought and the location of the two light bundles in the reference system at coverage recorded at the measuring point. The method is characterized in that for the Measuring point at least one second measurement at a defined other Location of the two starting points is performed relative to the surface and from the detected positions of the light bundles, the defined change in position the two starting points and the distance of the two starting points the distance of the measuring point to a reference point of the reference system considering the refraction of light in the material film is determined. The default Of course, the distance between the two starting points remains the change of position receive.

For the survey the entire surface Measurements are made at several over distributed the surface Measured points, wherein each measurement point two measurements with different Location of the starting points carried out become. The measuring points must not explicitly specified, for example, as markings. The respective change in position the two starting points between the two measurements is defined made, so they for the evaluation is known. Such a change in position is preferably carried out through a change the distance of the starting points to the surface by a defined distance. By double measuring each measuring point with this different Location of the starting points leaves the influence of the refraction of the light bundles at the boundary layer to determine transparent material film and thus the exact distance the reference point to the surface determine. In the same way, from this measurement, the thickness of the transparent material film.

Consequently allows the present method, the geometric measurement of a a transparent material film covered surface with high accuracy. Measurement errors, which in the description introduction mentioned methods due to refraction effects in flooded surfaces especially in the direct distance between the device and the surface as well additionally at an angle Radiation to the surface can be generated by a lateral offset, in the present method be compensated. The process thus expands the range of applications such optical measuring systems on applications with special Environmental conditions.

at The concept of the present proceedings was recognized to be that caused by a transparent film of material on the surface Refraction by double measurement of each measuring point with different Location of the starting points considered during the reconstruction of the surface can be. The number of unknowns to be resolved is included twice the measurement of each measurement point less than the number the equations resulting from the geometric situation. The refractive index of the material film is known as provided. This is in the applications mentioned in usually the case.

When determining the distance of the measuring point from the reference point from the two measurements represents the normal vector to the surface of the transparent material film is an important size. Preferably, this is. Normal vector determined numerically from the respective equations for the two measurements.

In cases in which the numerical determination due to measurement inaccuracies leads to problems a rough starting value is preferably given for this normal vector, from which the numerical determination is performed. In an advantageous Embodiment of the method, this starting value from a conventional Surveying the surface won. Here are first all measuring points recorded with one or both measurements, below also called scan, and without taking into account the transparent Material layer over simple triangulation from one of the measurements a surface course determined. For this surface course is for every measuring point a preliminary one Normal vector determined. Subsequently, based on each measurement point both measurements according to the present Method performed the exact range determination, wherein in the determination of the normal vector, the previously determined preliminary normal vector is specified as a rough start vector.

at Use of a device for carrying out the method, in which the areal Scanning the surface over one rotatory drive is performed, the defined change in position also caused by the measurement of each measuring point be that the connecting axis between the starting points eccentric is arranged to the axis of rotation. A change of position is then by a slight rotation of the axis of rotation achieved.

Preferably, in the present method, a device is used as shown in the already explained DE 100 37 771 A1 is known. This device only has to be expanded so that a displacement of the carrier element for the two scanning mirrors is made possible by a defined distance in the direction of the axis of rotation (z-axis). This shift can be done automatically by a corresponding drive when performing the measurement. Furthermore, the associated evaluation device must be designed in accordance with the present method for the exact determination of the distance to the reference point from the two measurements.

Short description of drawings

The The present method will now be described with reference to an exemplary embodiment explained in more detail in conjunction with the drawings. Hereby show:

1 a schematic representation of the basic structure for geometrical measurement of a surface with an optical measuring method;

2 the geometric conditions in the measurement of a surface with a transparent material layer; and

3 strongly schematized a section of the optical and mechanical structure of an apparatus for performing the present method.

Ways to execute the invention

1 shows very schematic the basic structure for geometric measurement of a surface 5 with an optical measuring method, as in the DE 100 37 771 A1 is described. In this example, the measuring device, hereinafter also referred to as scanner, consists of two mirrors which can each be tilted about parallel axes of rotation 1 . 2 Together, on a common support element 3 are stored. The two mirrors come with a light beam 4 in particular a laser beam, illuminated by the mirror on the surface to be measured 5 is steered. By suitable control of the mirror 1 . 2 The two of them are above the mirrors on the surface 5 generated light spots at the desired measuring point 6 brought to the overlay. This process is controlled by a camera. The spatial position of the light bundles striking the surface 4 is about the adjustment angle 11 the mirror 1 . 2 known. From the known distance of the axes of rotation of the mirror 1 . 2 to each other, the predetermined distance of the starting points for the on the surface 5 directed light beam 4 corresponds, and the respective spatial position of the light beam 4 The distance between a reference point of the scanner and the measuring point 6 on the surface 5 be calculated. This corresponds to the known triangulation technique.

Is however the surface 6 additionally with a transparent material film 7 , especially one Liquid film, coated, so the known simple triangulation technique does not provide the correct distance.

The conditions in such a case are based on 2 shown. Due to the refraction on the surface of the liquid film 7 are the mirror positions to a case without this liquid film 7 when overlapping the two by the light bundles 4 changed on the surface generated light spots. The beam path over the mirror 1 respectively. 2 deflected light beam 4 can in the reference system 8th of the scanner in vector notation are described as plane equation: e S1,2 : o → S1,2 + λ S1,2 · R1 → S1,2 + μ S1,2 · R2 → S1,2 ,

wobei β_S1 und β_S2 die Spiegelwinkel darstellen.The direction _vector r2 → _S1,2 depends on the following parameters r2 → S1,2 = f (r1 → S1,2 , ν F , n F ) where ν _{F is} the refractive index of the liquid film 7 and n → _{F is} the normal vector to the surface. It is assumed that the normal vector is in the measuring point 6 on the surface to be measured 5 is congruent with the two normal vectors of the liquid surface at which the over the mirror 1 and 2 deflected laser beams 4 through the surface, ie the planes are parallel in this area. This assumption can be made with short measuring distances to the scanner, small liquid layer thicknesses and small mirror distance. Based on this assumption, the coefficients λ _S1,2 and μ _{S1,2 can} each be determined by the distance of the scanner reference system 8th to the liquid surface H → _F or to the surface to be measured H → _G are expressed with the normal vector:

where β _S1 and β _{S2 represent} the mirror angles.

From the last two equations, assuming the flat surface, λ _{S1 can be expressed} by λ _S2 and μ _S1 by μ _S1 (or vice versa). Thus, with a known normal vector with the above plane equation, 3 equations with 2 unknowns are available.

However, this ratio is insufficient for an unknown normal vector (3 additional unknowns). To solve this problem, therefore, in the present method, another scan is performed, in which the scanner mirrors are displaced either by a defined offset amount along the z-axis or in a construction in which the connecting axis of the mirror, ie the support member 3 , eccentric (eg offset in y-direction) to the axis of rotation 9 the scanner is mounted, a rotation is made by a defined angle. The camera optics remains fixed so that the position of the measuring point 6 does not change in the camera image. This puts two measurements from one measurement point 6 and thus three more equations for solving the problem.

Of the Normal vector is approximated numerically. All can do this potential Normal vectors are tried until the equation systems a common solution to have. The description of this solution of course done in different ways, as well as the way in which the equations are resolved become. In one possible The equation systems thus become a single equation reformulated that with a proper choice of the normal vector the result of this equation becomes 0.

By Measurement error can be distorted in practice the result, however, so that the equation never exactly gives the value 0. Through this Deviation can also with other normal vectors the results of the equation system have local minima. A distinction of the wrong from the correct results are made more difficult. To this problem should avoid one as possible exact selection of a starting value for the normal vector, so that the search is in the right place, d. H. near the actual Minimums is started.

This Start vector, d. H. the rough direction of the normal vector becomes preferably from the surface normals the uncompensated, d. H. as would no liquid present, calculated and therefore shifted surface points approximated. With this starting value and the equations from the additional measurement let the actual distances without further details Knowledge about the actual Liquid thickness calculate each measuring point. The prerequisite for such an approximation of the Normal vector is a surface scan, so that the measuring method in this case requires a surface multipoint measurement.

From the corresponding equations, not only the distance to the measuring point can be 6 on the surface 5 but also the thickness of the liquid film 7 determine. This opens up numerous hitherto unspecified applications in which the thickness of such a surface film is to be measured.

3 Finally, also schematically shows a part of a scanner for carrying out the present method, which consists of two on a support element 3 attached mirrors 1 . 2 which are illuminated horizontally exactly on its axis of rotation from the scanner center. The entire bar-shaped carrier element 3 is around the z axis 9 rotatably mounted, so that in combination with the swivel angles 11 the mirror 1 . 2 in front of the scanner a three-dimensional volume can be scanned. A y-offset can be compensated by rotation around the z-axis (fine adjustment) for each individual measuring point, with the camera remaining fixed to measure the overlap.

In the present example, the respective second measurement for each measuring point by displacement of the carrier element 3 generated in the z-direction, as indicated by the arrow 10 is indicated. There are thus two independent angle measurements for the same overlap point. From these two measurements, the normal vector is first roughly calculated from the measured angles without refractive compensation and then determined exactly with an approximation algorithm. As a result, both the thickness of the liquid layer 7 as well as the distance of the scanner reference point to the overlapping or measuring point 6 receive.

1

first mirror

2

second mirror

3

support element

4

light beam

5

to measuring surface

6

measuring point

7

transparent Footage

8th

reference system

9

axis of rotation (Z-axis)

10

shift around the z-axis

11

setting angle

Claims (5)

Method for geometric measurement of a transparent material film ( 7 ) covered surface ( 5 ), in which - two separate light bundles ( 4 ) of two starting points in a reference system ( 8th ) a preference distance from one another to the surface ( 5 ), - for a first measurement the two light bundles ( 4 ) on the surface ( 5 ) in a measuring point ( 6 ) are brought to overlap and - the position of the two light bundles ( 4 ) in the frame of reference ( 8th ) at coverage in the measuring point ( 6 ) is detected in order to use triangulation to determine the distance of the measuring point ( 6 ) to a reference point of the reference system ( 8th ), whereby for each measuring point ( 6 ) at least one second measurement is carried out at a defined changed position of the two starting points and from the detected positions of the light bundles ( 4 ), the defined change in position of the two starting points and the predetermined distance of the two starting points, the distance of the measuring point ( 6 ) to the reference point of the reference system ( 8th ) taking into account the refraction of light in the film of material ( 7 ) is determined. A method according to claim 1, characterized in that the respective change in position of the two starting points between the two measurements by a change in the distance of the starting points to the surface ( 5 ) is made by a defined distance. A method according to claim 1, characterized in that the respective change in position of the starting points between the two measurements when using a scanner device for performing the method, in which a planar scanning of the surface ( 5 ) takes place via a rotary drive and a connecting axis of the two starting points is arranged eccentrically to the axis of rotation, is achieved by a rotation of the axis of rotation. Method according to one of claims 1 to 3, characterized in that in determining the distance of the measuring point ( 6 ) from the reference point from the two measurements a normal vector to the surface of the transparent material film ( 7 ) in the frame of reference ( 8th ) is determined numerically. Method according to Claim 4, characterized in that a start vector is used for the numerical determination of the normal vector, which indicates a rough direction of the normal vector, the start vector being calculated by calculating a surface course from one of the measurements for a multiplicity of measuring points ( 6 ) without consideration of the material film ( 7 ) and derivation of the surface normals at the respective measuring point ( 6 ) is obtained from the calculated surface profile.