Calibrating and operating rotary devices, in particular for rotating probe heads and/or probes of coordinate measuring machines
A measuring structure is calibrated for determining rotational positions of a rotary device that has a first part and a second part which could be rotated relative to the initial part about a rotational axis. Rotational positions of the initial part relative to the second part or rotational positions of the second part relative to the initial part are detected using a plurality of detectors distributed about the rotational axis. A respective measurement signal corresponding to every detected rotational place is generated such that redundant data on the rotational positions of the first part and the second part relative to each other is supplied. The redundant data on the rotational position(s) are analyzed these that effects of a translational movement of the first and the second part relative to each other are adjusted, the translational movement running laterally into the extension of the rotational axis.
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Field of the Invention
The invention relates to a way of calibrating a measuring arrangement for determining rotational positions of a rotary device that has a first part and a second part, which can be rotated relative to the initial part about a rotational axis.The invention additionally relates to a way of managing a rotary device that has a measuring arrangement for determining rotational positions. The rotary device is, specifically, a component part of a coordinate measuring machine that is designed formeasuring coordinates of a workpiece, hereinafter CMM for short. Furthermore, the invention relates to a calibrating structure for calibrating said measuring structure as well as the rotary device.
In US 2001/0025427 A1, as an example, a description is given of rotary apparatus for coordinate measuring machines that have two rotational degrees of freedom. The rotational axes of the rotary device are approximately perpendicular to a anotherand ensure it is possible like this to bring a stylus that’s connected to the rotary device into different positions over a wide range with differing positioning of the stylus. However, the invention isn’t restricted to rotary apparatus with two rotationaldegrees of freedom. Rather, the scope of the invention also even includes a rotary device with only one rotational degree of freedom, i.e. that the 2 parts of the rotary device are rotatable relative to one another about a rotational axis. The inventionalso comprises rotary devices with more than two rotational degrees of freedom of movement.
The usage of styluses and probe heads on CMMs is not only famous from US 2001/0025427 A1. The workpiece to be measured is probed by styluses or other probes, i.e. a probing element of the probe makes contact with the surface of the workpiece. Forvarious measuring jobs, it is positive here in case the alignment of the probe with a rotary apparatus can be diverse. But it is important in this regard to know the alignment, and consequently the rotational position, just and/or to have the ability to set itreproducibly. At least in the case of conventional CMMs, the orientation of the probe or the corresponding rotational position of these sections of the rotary device has to be taken into consideration in the determination of the coordinates of their probed pointon a surface. Corresponding calculations are revealed for example by the mentioned US 2001/0025427 A1.
However, the invention isn’t restricted to rotary apparatus that serve the aim of rotating probes for probing workpieces automatically. Optical probes or other measuring apparatus of a CMM which are employed in the measuring of coordinates of aworkpiece can be coupled to a rotary device, so that their alignment can be altered with a corresponding rotational motion of the rotary device. Specifically, the coordinate measuring device may be an arrangement with one or more sensors that are usedfor determining the location, alignment and/or deflection of a probe by a place of rest. This kind of arrangement is also referred to as a probe head, to which in turn a probe may be coupled. However, as an alternative, the sensor of the probehead may also be integrated in the rotary device.
Rotary devices normally have an integrated detector that is capable of measuring the rotational place. A known measuring principle is described for example in EP 1 923 670 A1. Accordingly, it’s a general principle of the scanning of aperiodic scale branch that a scanning head using a detector unit scans one or more periods of the scale branch of a measuring body on the other portion of the rotary device. The rotary apparatus in accordance with the present invention can also have onesensor that finds a rotational position of a first portion of the rotary device relative to the second portion of the rotary apparatus, the detector detecting in particular markers onto a measuring body that move through the detectable assortment of the sensor through arotational motion of the rotary apparatus. In this case, the markings may be for example markings in the form of lines that extend in a radial direction with respect to the rotational axis or that extend parallel to the rotational axis. Correspondingmeasuring bodies are also known as graduated discs. Such markings are often detected by optical sensors. Ideally, you will find a multiplicity of markings distributed around the rotational axis at exactly the exact angular spans from one another.
Alternately, other markings on the body could possibly be used for detecting the rotational motion. Potential, by way of example, are magnetic markings, provided for instance by an arrangement with magnetic elements distributed around therotational axis. The corresponding detector for detecting the magnetic markings may be for example a magnetoresistive sensor. However, it is also possible to use Hall sensors or other sensors that are capable of detecting the strength or direction of amagnetic area.
However, the invention isn’t limited to inkjet devices which have an angle measuring device that detects markings on a measuring body. Rather, the measuring body might rather have such as at least one magnet, and so that acorresponding magnetic field is rotated during the rotational motion of the rotary device about the rotational axis or, conversely, a rotatable portion of the rotor apparatus is rotated relative to the stationary magnetic field. At least one sensor detectsthe magnetic field and the rotational position is determined therefrom. 1 conceivable configuration has a plurality of magnetic field sensors that are dispersed around the rotational axis of the rotor apparatus. Every one of these magnetic field sensorsis capable of determining the place of a magnetic field present at any time at the magnetic field sensor. Additional a measuring body with at least one magnet is provided. If both parts of the rotary device that are rotatable relative toone another are rotated, the direction of this magnetic field varies at all the magnetic field detectors. It is theoretically possible to determine the rotational place by only a single such magnetic field sensor. For reasons of precision, however,multiple magnetic field detectors are used, and consequently redundant information concerning the rotational position is accessed and evaluated. Consequently, calibration of the rotary device permits a more accurate, and in particular more accuratelyreproducible, worth to be ascertained for the rotational place.
In the case of this detector system with magnetic field detectors, components of the rotary device and also components of other devices could alter the magnetic field of the measuring body. This happens depending on where rotational place therotary device is situated. In regards to the parts of the rotary device itself, the consequence of these parts can be largely eliminated by calibration. By comparison, the impact of components of other devices, which are only arranged in the area ofthe removable apparatus when the removable device is being used for its intended purpose, cannot be eliminated by calibration.
Errors also occur in the case of the aforementioned detector system with markings spread around the rotational axis and corresponding sensors for detecting the mark. Components of external devices admittedly don’t affect the positionof the mark relative to another (at least not if just small mechanical forces act from external on the rotary device). However, the spacing of the mark is not completely constant or known. Subsequently, the effects can however be largelyeliminated by using additional sensors that produce redundant details. This is explained in the aforementioned EP 1 923 670 A1. In this case, the additional sensors may either just be utilised in the calibration or be used at least partiallyduring the operation of the rotary device for its intended purpose.
A corresponding calibrating technique is described in the publication by Ralf B. Geckeler et al.”Calibration of angle encoders using transfer functions”, published in Meas. Sci. Technol. 17 (2006) 2811-2818. It’s also described therein thatan further measuring system might be installed on the removable device to the calibration. Geckeler describes a method for calibrating a rotary device in which a plurality of studying heads randomly varying angular intervals from one another are distributedaround the rotational axis. Differences in the angles of these rotational positions are formed from the measuring signs provided by the hearing heads. Furthermore, a Fourier transform is done and also a transfer function that describes how the Fouriercoefficients of the angular intervals can be calculated in the Fourier coefficients of the gaps in angles is implemented. Estimated values of the Fourier coefficients for each gap in angle are then combined, with appropriate weighting. Theerror of this measuring method to be calibrated is obtained from a number of the Fourier coefficients within the gaps in angles. Geckeler thus observed a decrease in the error by a factor of up to four.
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