Geometrical instruments – Gauge – Coordinate movable probe or machine
Reexamination Certificate
2000-03-08
2003-11-25
Gutierrez, Diego (Department: 2859)
Geometrical instruments
Gauge
Coordinate movable probe or machine
C033S556000, C033S559000
Reexamination Certificate
active
06651351
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS:
Not Applicable
BACKGROUND OF THE INVENTION
1. Technical Field of the Invention
The invention relates to a method for measurement of an object by means of a feeler element used with a coordinate measuring instrument and extending from a feeler extension, where the feeler element is brought into contact with the object and its position is then determined. The invention further relates to a coordinate measuring instrument for measurement of structures of an object by means of a feeler used with a coordinate measuring instrument and comprising a feeler element and a feeler extension, a sensor for optical determination of the feeler element and/or at least one target directly assigned thereto, and an evaluation unit using, wherein the structures can be calculated from the position of the optical system relative to the coordinate system of the coordinate measuring instrument and from the position of the feeler element and/or of the target measured directly using the optical system.
2. Description of Related Art
For measurement of the structures of an object, coordinate measuring instruments with electromechanically operating feelers are used with which the structure position is determined indirectly, i.e. the position of the sensing element (ball) is transmitted via a feeler pin. The attendant deformations of the feeler pin in conjunction with the active friction forces lead to a falsification of the measurement results. Because of the strong force transmission, measurement forces also result that are typically in excess of 10 N. The geometric design of such feeler systems limits these to ball diameters greater than 0.3 mm.
The three-dimensional measurement of small structures in the range of a few tenths of a millimeter and the sensing of easily deformed test specimens is therefore problematic, if not impossible. As a result of the not completely known error influences due to deformation by the feeler pin and feeler element, and the unknown sensing forces due to stick-slip effects for example, measurement uncertainties occur that are typically in excess of 1 &mgr;m.
It is known from WO 93/07443 to indirectly determine the structure of an object by means of optical sensors, where a rigid feeler has at least three targets, which are measured for determination of a coordination measurement point using an angle sensor.
Another possibility for optical measurement of the structures of a body is described in WO 88/07656 by an interferomter system. This system comprises a feeler with a rod-like feeler extension at the end of which a ball is arranged that is brought into contact with the body whose position is to be determined. The feeler extension extends from a plate-like holder that is adjustable in three dimensions relative to the object. Retroreflectors extend from the holders and are subjected to beams emitted by interferometers. The reflected beams are then measured by the interferometers in order to permit measurement of the optical axis between the interferometers and the retroreflectors for the determination of the position of the object.
The publication US-Z.:
Quality
, April 1998, p. 20 ff contains the proposal of measuring structures of an object by means of a feeler element by determining its position with an optical sensor. Here it is important that the feeler is sharply imaged.
It is known from the publication US-Z.:
American Machinist
, April 1994, p. 29-32, to use various measuring systems for the determination of the geometry of a workpiece. In this case it relates to the possibility, one the one hand, of measuring a surface with a video camera, and on the other hand, of performing a tactile measurement, these being treated as alternatives.
In US-Z.:
Tooling & Production, October
1990. p 76-78, a feeler is used optionally for purely tactile, i.e. mechanical measurement and for optical measurement to determine structures. In this case, the feeler contacting the body must also be clearly optically imaged at all times.
A corresponding mechanical-feeler coordinate measuring instrument is shown for example in German Patent 43 27 250 A1. Here a visual check of the mechanically sensing process can be made with the aid of a monitor by observation of the feeler head using a video camera. This feeler head can if necessary be designed in the form of a so-called oscillating crystal feeler that is cushioned upon contact with the workpiece surface. The video camera therefore permits bracing and control on the monitor of the position of the feeler ball relative to the workpiece or to the hole therein which is being measured. The measurement proper is conducted electromechanically, so that the above drawbacks remain valid.
An optical observation of a feeler head in a coordinate measuring instrument is also shown in German Patent 35 02 388 A1.
To determine the precise position of the machine axes of a coordinate measuring instrument, at least six sensors are attached on a sleeve and/or to a measuring head in accordance with German Patent 43 12 579 A1, for enabling the distance from a reference surface to be determined. The sensing of the object geometries is not dealt with in detail here, instead a proximity-type process as a substitute for the classic incremental path measurement systems is described.
U.S. Pat. No. 4,972,597 describes a coordinate measuring instrument with one feeler, of which the feeler extension is pretensioned in its position by a spring. A feeler extension section passing inside the housing has two light-emitting elements located at a distance from one another for determining by means of a sensor element the position of the feeler extension, and hence indirectly that of a feeler element arranged on the outer end of the feeler extension. The optical system here also replaces the classic path measurement systems of electromechanical feeler systems. The sensing process proper is again achieved by force transmission from the feeler element to the feeler pin via spring elements to the sensor. The aforementioned problems with bending and sensing force remain here too. This method is indirect.
To measure large objects such as aircraft components, feeler pins with light sources or reflecting targets are known, the positions of which are optically measured (German Patents 36 29 689 A1, 26 05 772 A1, and 40 02 043 C2). The feelers themselves are moved manually or by using robotics along the surface of the body to be measured.
With this method, the position of the feeler element is stereoscopically determined in its position by triangulation or similar means. The resolution of the overall measurement system is hence directly limited by the sensor resolution. The use of such systems is therefore possible only in the case of relatively low requirements as regards the relationship of measurement area and accuracy. In practice their use is limited to the measurement of large parts.
Aiming at the position of the feeler element using a microscope is also known. In this case, the transmitted-light method is used, so that only structures such as all-through holes or similar can be measured in respect of their diameters. In view of the visual evaluation in the microscope and the separate arrangement of feeler element and optical observation system, neither measurement of more complex structures (distances in complex geometries, angles etc.) nor automatic measurement is possible. Systems of this type are as a result highly prone to faults and are therefore not offered on the market.
SUMMARY OF THE INVENTION
The problem underlying the present invention is to develop a method and a coordinate measuring instrument of the type mentioned at the outset such that any structures can be determined with a high degree of measurement accuracy, with the aim of precisely determining the position of the feeler element to be brought into contact with the object. In particular, it should be possible to measure out bores, holes, undercuts or similar, and to determine structures in the range between 50 and 100 &mgr;m with a measuring accuracy of at least &plusm
Christoph Ralf
Schwenke Heinrich
Trapet Eugen
Dennison, Schultz & Dougherty
Gonzalez Madeline
Werth Messtechnik GmbH
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