Abstract:
The invention relates to a process and device for determining the alignment, with respect to a reference direction, of a cylindrical body ( 10 ) mounted to rotate around its lengthwise axis ( 22 ). The device including a position measurement probe ( 20 ), which is calibrated to the reference direction, being attached on the end face ( 12 ) of the body or on a surface essentially parallel to the end face, which probe gathers measurement data in at least three measurement positions around the lengthwise axis each position of which differs from the other by an angle of rotation of the body, such that one position measurement at a time is being taken. Then the alignment of the body with respect to the reference direction is computed from the determined measurement data.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a process and device for determining the alignment of a body with respect to a reference direction in which the body, particularly a roller of a printing press, is mounted for rotation around a lengthwise axis. 
     2. Description of Related Art 
     It has been suggested that a printing press be modified in order check the alignment of its rollers such that the axes or axis pieces of the rollers are provided with highly-planar end faces or provided with a high-precision adapter as an extension of the axial direction of the roller. Utilizing the end faces of the roller or the adapter attached to the roller the components of the angular alignment of the roller are determined with respect to a reference direction. This is done by means of a two-dimensionally acting angular position transducer, such as an optical gyro, which operates with high precision to determine the angular alignment of the roller with respect to a reference direction. 
     One problem in this approach is that it is extremely difficult to produce the end faces or the corresponding adapter with the necessary precision. That is, to achieve the required measurement accuracy, the end surfaces or the adapter must be produced with a surface quality which has a roughness measure must be much less than one micron. 
     SUMMARY OF THE INVENTION 
     A primary object of this invention is to devise a process and a device for determining the alignment of a body, which is mounted to rotate around its lengthwise axis, such that precise alignment measurements are possible even for roughly fabricated surfaces on the end faces of the body or on a surface, such as a high precision adapter, which is essentially parallel thereto. 
     This object is achieved by a process and a device described herein. In the process of the invention, precise determination of the alignment of a body depends neither on precisely fabricated end faces of the cylindrical body nor on a precisely fabricated adapter, since possible mis-orientation of the position measurement probe with respect to the lengthwise axis of the body can be reduced or eliminated by the evaluation of several measurement positions. 
     In a preferred embodiment, the position measurement probe is attached to the end face of the body or to a surface parallel to the end face of the body by means of magnetic forces. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a side view of the alignment measurement device of the invention attached to the roller to be measured; 
     FIGS. 2-4 each show a front view of the arrangement from FIG. 1 with the measurement device in a respective one of different measurement positions; and 
     FIGS. 5A &amp; 5B are graphical representations of measurements obtained using the measurement device illustrated in FIGS.  1 - 4 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     As shown in FIG. 1, a roller  10  has an axis piece  14  with a ground end face  12  to which the position measurement probe  20  is attached. The direction of the lengthwise axis of the roller  10  is labeled  100  in FIG.  1 . The inevitable geometric deviations of the end face  12  and the attachment surface  16  of the probe  20  from an ideal surface alignment is illustrated, in a highly exaggerated representation, in FIG. 1 by the amount of error f 1 , angular deviation of the orientation of the axis  24  or  200  of the position measurement probe  20  with respect to the axial direction  22  and  100  of the rollers arises. Such a deviation occurs generally in the two directions in space which are perpendicular to one another and to the roller axis  100 . 
     In order to provide a firm, but easily detachable, attachment of the probe  20  to the end face  12 , the probe  20  is provided with a relatively powerful permanent magnet  18  which is located within the probe housing. In an economical embodiment, the permanent magnet  18  is preferably composed of a neodymium-iron-boron material. This provides the probe  20  with a magnetic foot or a magnetic adapter. 
     The probe  20  preferably comprises three optical gyros, for example, fiber optic or laser gyros, each of which are capable of forming an optical ring, each optical gyro acquiring a rotation around the axis perpendicular to the plane of the ring. Initially, the probe  20  measures the revolutions around the three axes which are stationary in the initial coordinate system of the probe  20 , i.e., the normal line to each plane of the ring. As the actual measurements are achieved however, the probe does not deliver the respective angle of revolution of the roller with respect to these three axes, but rather the respective angle of revolution has been computed by coordinate transformation around three axes which are perpendicular to one another and which are stationary in the initial laboratory coordinate system, i.e., the initial coordinate system of the factory supporting the roller  10  to be measured. With respect to the three axes, the probe  20  is calibrated before the start of the measurement. To achieve this one reference direction is selected which for example can be the orientation of a second roller. The probe  20  then measures the torsion of the roller  10  with respect to this reference direction. The angle of revolution or torsion with respect to the reference direction is hereinafter called the “roll angle” while the angles of revolution with respect to the two other axes of the laboratory coordinate system are called the “pitch angle” and the “yaw angle”. 
     It is hereinafter assumed that the axis  22  corresponds to the reference direction, i.e., the roller  10  is aligned exactly in the reference direction. The roll angle then indicates the torsion of the probe  20  with respect to the axis  22 . Further, even if the roller  20  has a mis-orientation, this mis-orientation corresponds, i.e., within the framework of the amount of error f 1  and the mis-orientation of the roller  10 , approximately to the twisting of the probe with respect to the axis  24 . Thus, the coordinate system of the probe  20  and the initial laboratory coordinate system have an axis approximately in common, specifically the axis of the roll angle. 
     In the following detailed explanation, the terms “pitch”, “yaw” and “roll” angles label the instantaneous or current rotation of the measurement probe  20  around three axes which are perpendicular to one another and which are stationary in the initial laboratory coordinate system. Before starting the measurement, the corresponding calibration has been done with respect to the reference direction. 
     The roller  10  is precisely mounted to rotate around its lengthwise axis  22 . The roller  10  is preferably a print roller of a printing press or the roller in a machine for producing films, foils or thin sheets. 
     The purpose of the measurement is to determine the orientation of the roller  10 , i.e., the orientation of its lengthwise axis  22  and  100  with respect to the reference direction in the initial laboratory coordinate system, for example, by the reference direction provided by the orientation of another roller, and such that the horizontal and vertical angle deviation with respect to the given reference direction will be obtained as the measurements are achieved. Initially, assuming ideal contact surfaces between the axis  14  and probe  20 , the desired result could be obtained from a single measurement by means of the position measurement probe  20  by using the horizontal or vertical plane in the initial reference coordinate system as the calibration for the “pitch” angle or the “yaw” angle. However, based on the mis-orientation of the probe  20  which is shown exaggerated in FIG. 1, when only a single measurement is taken an unacceptably large systematic measurement error arises, that is, the probe axis  200  deviates in the vertical direction from the roller axis  100 , even though the latter is aligned exactly horizontally. 
     This systematic measurement error is reduced or eliminated by taking at least three measurements at different angles of revolution of the roller  10  and evaluating the results of the measurements taken in the different angular positions in order to determine the final result of the position measurement. To do this, the roller  10  with the measurement probe  20  attached thereto is rotated around its axis  22 , then in different rotary positions of the roller  10  measurements of the roll angle, the pitch angle and the yaw angle are taken. The measured roll angle generally corresponds with relative precision to the angle of revolution of the roller  10  and thus of the probe  20  around the axis  22 . 
     The measurement results can be evaluated easily when the measurements are taken in four measurement positions each turned 90° from the previous position. The values of the pitch angle and of the yaw angle measured by the probe  20  are then averaged, the result of which reproduces the true orientation of the roller  10  in a relatively precise manner. This true orientation can be represented as the systematic error resulting from the mis-orientation of the measurement probe  20  with respect to the end face  12  which is shown in vector form and, depending on the angle of revolution, is distributed conically around the true orientation of the roller to be measured. The properties of symmetry of the pertinent cone are used to determine the true angular orientation of the roller  10  in space. In determining the true angular orientation, it can be assumed that the axis of symmetry of the indicated cone of the error distribution reproduces the true orientation of the roller  10 . This is apparent in FIG. 1 where reference number  200 ″ identifies the relative orientation of the lengthwise axis  24  of the probe  20 , in FIG. 1, when the roller  10 , and thus the probe  20 , are turned by 180° with respect to the position shown in FIG.  1 . The true orientation of the axis  22  of the roller  10 , labeled  100 , is exactly in the middle between the two orientation directions  200  and  200 ″. 
     When measurements are taken in four different measurement positions, between which the roller  10  continues to turn 90°, then averaging the results of the measurement positions, which are offset by 180° the desired angle offset of the roller axis can be determined in the horizontal or vertical direction with respect to the reference direction. 
     In FIGS. 2 to  4 , a measurement process is shown by way of example, in which measurements are taken in three different measurement positions, each figure showing another of the measurement positions. The position measurement probe  20  before the start of the measurements is placed on the end face  12  and is kept securely in place during the measurements by means of the magnet  18 . 
     It can be seen in FIGS. 2 to  4  that the position measurement probe is connected by means of a flexible connecting cable  30  via an interface (plug)  32  which in turn is connected to the evaluation and output unit  33  shown in the FIG.  3 . The double arrows in FIGS. 2 to  4  label the reference system of the position measurement probe  20  (i.e., the initial laboratory coordinate system) which remains stationary during rotation of the probe. 
     FIGS. 5A and 5B show a sample evaluation of the measurement results, the pitch angle (FIG. 5B) and the yaw angle (FIG. 5A) measured by the probe  20  being plotted over the roll angle measured by the probe. The results obtained by the probe  20  measurement positions shown in FIGS. 2 to  4  are labeled in FIG. 5B with the reference numbers  60 ,  61  and  62  for the pitch angle and in FIG. 5A with the reference numbers  70 ,  71  and  72  for the yaw angle. In the evaluation of the measurement results, the basic approximation principle relied upon is that the true value of the mis-orientation, when the roller  10  is rotated with the position measurement probe  20  having an erroneous amount the pitch angle and the yaw angle acquired by the position measurement probe  20  as a function of the acquired roll angle, oscillates sinusoidally or cosinusoidally around the true value, i.e. the horizontal and vertical mis-orientation of the roller axis  22  and  100 . This approximation principle applies to small mis-orientations of the measurement probe axis with respect to the roller axis  22 . Thus, the average of the pitch angle and the yaw angle and therefore the vertical and horizontal mis-orientation of the roller axis  22  with respect to the reference direction can be easily ascertained from knowledge of the sine and cosine function. In the example shown in FIGS. 5A and 5B, the average is zero both for the pitch angle and yaw angle, i.e. the roller axis  22  shows no mis-orientation with respect to the reference direction. The representation shown in FIGS. 5A and 5B corresponds to the lengthwise axis  24  of the probe  20  rotating around axis  22  on the envelope of the cone. 
     Generally, the sine curve or the cosine curve is unequivocally determined by three points. In this respect, measurement of the pitch angle and the yaw angle at three different roll angles, i.e. in three different rotary positions of the roller  10 , is sufficient to determine the true pitch angle and the yaw angle by the determination of the sine or cosine function fixed by the measurement points. A higher accuracy can be achieved when more measurement points are available. This is suggested in FIGS. 5A and 5B by the measurement points  63  to  66  and  73  to  76 . When there are more than three measurement values, the pertinent sine and cosine function is then determined by curve matching or a compensation calculation, for example, the square of the deviations can be minimized. 
     To achieve a higher measurement accuracy, it is necessary for the probe measurements to be taken in the individual measurement positions within a relatively short time interval so that the calibration of the position measurement probe  20  is not lost. When an even higher precision measurement accuracy is desired, it is necessary to consider the rotation of the earth in the conventional manner.