Patent Publication Number: US-2010118305-A1

Title: Process and device for measuring the rotation angle of a rotating object

Description:
The invention relates to a process for measuring the rotation angle of a rotating object according to the preamble of patent claim  1  and to a device for measuring the rotation angle of a rotating object according to the preamble of patent claim  6 . 
     In many technical applications it is necessary to measure the rotation angle of a rotating object. Generally the rotation angle of the rotating object is measured in relation to a stationary object, e.g., the rotation angle of a rotating wheel in relation to a stationary motor part or machine part. However, the invention also includes cases in which a rotating object revolves in relation to another rotating object and the relative rotation angle between the two objects is to be measured. According to the invention, the measurement of the rotation angle also includes the measurement of magnitudes derived from the rotation angle, e.g., the angular velocity and the angular acceleration. Measurement of the rotation angle, as specified by the invention, also includes cases in which a linear motion is converted into a rotational motion, e.g., by a gear, and the position, velocity, acceleration, etc. of the linear motion is to be measured using the rotation angle of the rotating object. 
     To measure the rotation angle, it is known to scan a material measure assigned to the rotating object, where this material measure is coded in absolute fashion or is incrementally scanned. In particular, it is known to utilize the polarization of light to measure the rotation angle. Here light that has been linearly polarized using a polarizer is emitted by a transmitter and reaches a receiver after passing through an analyzer that revolves in correspondence with the rotating object. If the analyzer and its polarization plane lie parallel to the polarizer, the light will enter the receiver, while no light will enter the receiver if the polarization plane of the analyzer lies perpendicular to the polarization plane of the emitted light. The receiver thus delivers a signal that is dependent on the rotation angle of the analyzer and thus is dependent on the rotation angle of the rotating object. 
     In an arrangement known from DE 10 2005 031 966 A1, unpolarized light is emitted by a transmitter and strikes a polarizing filter which revolves with the rotating object. The light passing through the polarizing filter is polarized, and its polarization plane revolves with the polarizing filter. The light strikes a receiver, on whose light-intake side four polarizing filters are positioned in an array; their polarization planes are each rotated 45° relative to each other. With each 45° turn of the rotating polarizing filter, its polarization plane coincides with the polarization plane of a polarizing filter belonging to the receiver, so that there is a maximum in received optical intensity and thus a maximal receiver signal. 
     In an arrangement known from DE 201 02 192 U1, a transmitter emits linearly polarized light, which strikes a polarizing filter that has a reflector positioned behind it and that rotates with the rotating object. Two receivers with polarizing filters rotated 90° relative to each other receive the reflected light, and the signals from the two receivers create a signal that is dependent on object&#39;s angle of rotation. 
     The invention is based on the problem of providing a process and a device for measuring the rotation angle of a rotating object, in order to permit an improvement in signal evaluation by using optical polarization as a tool. 
     The invention solves this problem with a process having the features of patent claim  1  and a device having the features of patent claim  6 . 
     Advantageous embodiments of the invention are indicated in the secondary claims. 
     In accordance with the invention, the transmitter gives out at least two linearly polarized light rays, whose polarization planes are rotated relative to one another. If the analyzer and the transmitter revolve relative to each other in correspondence with the rotation of the object that is being measured, the polarization planes of the polarized light rays from the transmitter will successively coincide with the polarization planes of the analyzer, so that the light striking the receiver has an intensity that is dependent on the angle of rotation. In addition, however, the intensities of the light rays emitted by the transmitter are modulated, and specifically modulated in a phase-shifted fashion relative to each other. The intensity of the light striking the receiver, and therefore the signal strength, are thus influenced not only by the rotation angle, but also by the modulation of the light rays in their intensity. The receiver signal, which corresponds to the optical intensity of the light striking the receiver, thus depends on the modulation frequency of the lights rays emitted by the transmitter and on the rotation angle being measured. In particular, a receiver signal is produced which contains the modulation frequency of the light rays emitted by the transmitter and which is phase-shifted in accordance with the rotation angle being measured. 
     In evaluating the signals of the receiver it is advantageous if simple trigonometric functions are selected to modulate the intensity of the emitted light rays. It is also advantageous for the polarization planes of the radiated light rays to have simple trigonometric relationships among themselves. 
     For this reason, it is useful for the intensity of the light rays emitted by the transmitter to be modulated in sinusoidal fashion. A phase shift of 360°/n, where n is the number of light rays, can be advantageously chosen for the modulation of the individual light rays. 
     Furthermore, it is advantageous if the polarization planes of the light rays emitted by the transmitter are each rotated 180°/n relative to each other, where n is again the number of emitted light rays. 
     In a simplest embodiment, two linearly polarized light rays are emitted by the transmitter. When there is a larger number of light rays, the evaluation will benefit if the number n of light rays is a whole-number multiple of two. 
     With a view to expense and the advantageous evaluation of the signals, an embodiment is preferred in which four linearly polarized light rays, whose intensity is in each case modulated in sinusoidal fashion, are emitted by the transmitter, such that the phase shift in the modulation of the individual light rays in each case is 90°. The polarization planes of these light rays are each rotated 45° relative to each other. 
     In principle, the solution according to the invention can be so realized that the transmitter revolves in dependence on the rotating object while the analyzer remains stationary; or the transmitter may remain stationary while the analyzer revolves in dependence on the rotating object. 
     If the transmitter revolves in dependence on the rotating object, a suitable energy supply must be provided for the rotating transmitter. When the analyzer is positioned in stationary position, the receiver can be simply positioned in stationary fashion behind the polarizing filter of the analyzer. In the simplest embodiment, the polarizing filter can be positioned directly on the surface of the receiver. 
     If the transmitter is positioned in stationary fashion, its energy supply becomes easier to provide. The analyzer will then revolve in correspondence with the revolution of the rotating object. When the receiver revolves with the analyzer, difficulties arise in transmitting the analog receiver signals. Consequently, an embodiment is preferred in which only the analyzer revolves and the light passing through the analyzer is deflected to a receiver positioned in stationary fashion. 
    
    
     
       The invention is next described in greater detail on the basis of exemplary embodiments depicted in the drawing. Shown are: 
         FIG. 1  an initial embodiment of the invention, schematically depicted in a perspective view 
         FIG. 2  a diagram showing the optical intensity received by the receiver as dependent on the modulation frequency and the rotation angle 
         FIG. 3  a diagram showing the relationship between the rotation angle and the phase displacement of the receiver signal 
         FIG. 4  the device of  FIG. 1 , in a schematic axial section 
         FIG. 5  a second embodiment, also in a schematic axial section 
         FIG. 6  a third embodiment, also in a schematic axial section 
         FIG. 7  a fourth embodiment, also in a schematic axial section. 
     
    
    
     The basic principle of the invention is explained on the basis of an embodiment shown in  FIG. 1 , in which a transmitter emits four linearly polarized light rays, whose polarization planes are each rotated 45° relative to another. 
     In the embodiment shown in  FIG. 1 , which is also schematically depicted in  FIG. 4 , the rotation angle (or a value dependent on the rotation angle) of a rotating object  10 , e.g., a revolving shaft, is measured in relation to a reference system, e.g., the housing of a motor. 
     A stationary transmitter  12  emits at least two—and in the present exemplary embodiment four—linearly polarized light rays a, b, c, and d. The light rays a, b, c, and d basically run in axially parallel fashion relative to a central axis and are positioned around this central axis with an angular spacing of 90°, one relative to another. Serving to produce the light rays a, b, c, d are light sources  12 . 1 ,  12 . 2 ,  12 . 3 ,  12 . 4 , which are advantageously provided with collimating lenses. The light rays a, b, c, d pass through a polarizer  14 , which is fixed in position in front of the transmitter  12  and is composed of four polarizing filters  14 . 1 ,  14 . 2 ,  14 . 3 ,  14 . 4 . The polarizing filters are positioned around a central axis in an array of four quadrants, whose angular position corresponds to the light sources  12 . 1 ,  12 . 2 ,  12 . 3 , and  12 . 4 . The polarization planes of the polarizing filters  14 . 1 ,  14 . 2 ,  14 . 3 ,  14 . 4  are each rotated 45° one from the other. Consequently the light rays a, b, c, d are polarized in linear fashion after passing through the polarizer  14 , in such a way that their polarization planes are successively rotated by 45°. 
     The central axis of the transmitter  12  is basically oriented so as to be axially flush with the rotation axis of the rotating object  10 . An analyzer which revolves with the object is positioned in the incident area of the light rays a, b, c, d, concentric with the rotation axis of the rotating object  10 , e.g., an axle shaft. In the exemplary embodiments shown in  FIGS. 1 and 4  the analyzer consists of a polarizing filter  16 , e.g., a polarizing film, which is fixed in position on the rotating object. The light passing through the polarizing filter  16  reaches the receiver  18 , which measures the intensity of said light and coverts it into a corresponding electrical signal. 
     Proceeding in the axial direction, the receiver  18  may be positioned behind the polarizing filter  16 . If the rotating object  10  is tubular in shape, the receiver can be positioned in a fixed, coaxial position within the rotating object. If the receiver  18  revolves with the object  10  and the polarizing filter  16 , then the receiver signals must be uncoupled in an inductive or capacitive manner. 
     In the exemplary embodiment shown in  FIGS. 1 and 4 , however, a diffusion disk is positioned between the polarizing filter  16  and the rotating object, and this diffusion disk may consist, e.g., of glass. As can be seen in  FIGS. 1 and 4 , the diffusion disk  20  is advantageously applied to the axial face of the rotating object  10 , e.g., adhesively, while the polarizing filter  16  is in turn applied to that side of the diffusion disk  20  that faces the transmitter  12 , e.g., glued on as a film. The light passing through the polarizing filter  16  is diffusely scattered by the diffusion disk, so that the scattered light leaves the diffusion disk in radial fashion and reaches the receiver  18 , which is laterally positioned outside of the diffusion disk  20 . Since the polarizing filter  16  and the diffusion disk  20  are positioned in rotationally symmetrical fashion relative to the rotational axis of the object  10 , the intensity of the scattered light radially leaving the diffusion disk is independent of the rotation angle, and the receiver  18  can be positioned in stationary fashion. 
     The advantage conferred by the arrangement of the embodiment in  FIGS. 1 and 4  rests specifically in the fact that the light sources of the transmitter  12  and the receiver  18  are positioned in stationary fashion and can therefore be electrically connected in a stationary manner. It is not necessary to provide an electrical connection for the rotating parts. Furthermore, the transmitter  12  with the polarizer  14 , and the polarizing filter  16  with the diffusion disk  20  have a concentric design and small radial dimensions, with the result that the device can be miniaturized. The transmitter  12  and the polarizing filter  16  do not require a perfectly flush alignment in the axial direction; consequently these elements can be mounted independent of each another and without a great degree of adjustment. 
     To measure the rotation angle of the rotating object  10 , the light sources  12 . 1 ,  12 . 2 ,  12 . 3 , and  12 . 4  of the transmitter are modulated in sinusoidal fashion with respect to their transmitting power, so that the intensity of the light rays a, b, c, d is sinusoidally modulated. This modulation is such that the intensity of the light rays a, b, c, d is phase-shifted by 90° for each ray, relative to its predecessor. If the amplitude of the light ray&#39;s intensity is designated as U 0  and the modulation frequency as ω 0 , the following intensity-functions result for the light rays: 
         a=U   0 (1+sin ω 0   t ) 
         b=U   0 (1+cos ω 0   t ) 
         c=U   0 (1−sin ω 0   t ) 
         d=U   0 (1−cos ωhd  0   t )  (1) 
     The light rays a, b, d, d, each rotated in its linear polarization relative to the next, pass unweakened through the polarized filter  16  of the analyzer if the polarization plane of the rotating polarized filter  16  coincides with the polarization plane of the individual light ray. If the polarization plane of the rotating polarized filter  16  runs perpendicular to the polarization planes of the light rays, these rays will not be allowed to pass through. As dependent on the rotation angle φ of the polarized filter  16 , the intensities of the individual light rays a, b, c, d, behind the polarized filter  16  prove to be: 
         a (φ)= U   0 (1+sin ω 0   t )(1+sin φ) 
         b (φ)= U   0 (1+cos ω 0   t )(1+cos φ) 
         c (φ)= U   0 (1−sin ω 0   t )(1−sin φ) 
         d (φ)= U   0 (1−cos ω 0   t )(1−cos φ)  (2) 
     Since the polarization planes of the polarizing filter and of the light rays in each case coincide after the polarizing filter has been rotated 180°, the rotation angle Θ of the rotating object  10  is consequently Θ=2φ. 
     The total quantity X of the diffused light admitted by the polarizing filter  16  and scattered in the diffusion disk is given by the sum of the intensities of all the rays a, b, c, d which pass through for this rotation angle φ, and thus equals: 
     
       
         
           
             
               
                 
                   
                     
                       
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     Applying the conversion theorems of trigonometry: 
       sin α·sin β=½(cos(α−β)−cos(α+β)) 
       cos α·cos β=½(cos(α−β)+cos(α+β))  (4) 
     provides the following formula for the optical intensity of the measured light: 
         X=U   0 (4+(cos(ω 0   t −φ)−cos(ω 0   t +φ)+cos(ω 0   t −φ)+cos(ω 0   t +φ)) 
         X=U   0 (4+2 cos(ω 0   t −φ))  (5) 
     In accordance with the received optical intensity, a signal of the receiver  18  is thus obtained which is sinusoidal, contains the modulation frequency ω 0  of the light rays a, b, c, d, and is phase-shifted in accordance with the rotation angle φ of the polarizing filter and thus of the rotating object. In the process, the phase per revolution of the object  10 , or of the polarizing filter  16 , is shifted by one period. The effective phase shift of the receiver signal at any given moment thus represents a measure for the effective rotation angle at that moment. The number of periods by which the receiver signal is phase-shifted corresponds to the number of the object&#39;s revolutions. 
     If, for example, a modulation frequency of ω 0 =1 MHz is selected for the light rays a, b, c, d and if the object, e.g., a motor shaft, turns at a rate of 6000 revolutions/minute, or 100 revolutions/second, there is a phase shift of 100 periods per second and thus a received output signal with a frequency of 999.900 Hz. 
       FIG. 2  provides a corresponding schematic depiction of the dependency of the frequency of the measured receiver signal X on the rotating speed of the object  10 . 
       FIG. 3  shows the relationship between the rotating angle φ of the polarizing filter  16  and the phase shift of the receiver signal over one revolution of the rotating object  10 . 
     The embodiment shown in  FIG. 1 , with four light rays which are modulated in sinusoidal fashion and phase-shifted by 90° and whose linear polarization in each case is rotated by 45°, permits a signal evaluation which is particularly simple, as shown by the trigonometric derivation given above. It is clear to the specialist, however, that the invention can also be realized with a different number of light rays, with a different modulation, and with a different phase-shift in the modulation, as well as with a linear polarization of the light rays that is displaced by a different angle. However, this is generally associated with more complicated evaluating algorithms. Simple solutions are also provided when there are two light rays, however. The same simple evaluation that is described above also arises when the number of light rays is a multiple of four. The result then is a corresponding number of periods for the phase shift per revolution of the rotating object. 
       FIG. 4  again shows the device of  FIG. 1  in a schematic depiction. Here the transmitter  12 , the polarizer  14 , and the light rays are only schematically depicted, without showing the division involving four light sources, four polarizer fields, and four light rays.  FIG. 4  shows the compact design of the device, which permits it to have small dimensions. In particular, the polarizing filter  16  and the diffusion disk  20  can be positioned on the axial face of the shaft  22  of a motor  24 .  FIG. 4  also shows that a number of receivers  18 , e.g., photodiodes, can also be positioned around the diffusion disk  20 , to provide a better light output and thus a better reception signal. 
       FIG. 5  depicts a second embodiment. In this embodiment, the transmitter  12  and the polarizer  14  are firmly secured to the rotating object, e.g., the shaft  22  of a motor  24 , and rotate with the shaft  22 . The analyzer is accordingly placed in fixed position. The analyzer, which is formed, e.g., by a polarizing filter, can be positioned directly on the stationary receiver  18 . Since in this case the light sources and their modulation circuit rotate along with the shaft  22 , a capacitive or inductive energy supply to the rotating circuit is necessary. 
       FIG. 6  shows a third embodiment, which represents a modification of the embodiment of  FIGS. 1 and 4 . To divert the light that passes through the polarizing filter  16  of the analyzer radially and outward into the receiver(s)  18 , a conical mirror  26  is coaxially positioned behind the polarizing filter  16 . The mirror  26  can be molded into a transparent cylinder  28  positioned coaxially with the rotating object, such that the polarizing filter  16  is applied to the face of this cylinder  28 . This results in a better light output than is the case with the diffuse scattering provided by the diffusion disk  20 . However, the conical mirror  26  increases the overall axial length as compared to the diffusion disk  20 . 
       FIG. 7  shows a fourth embodiment, which is a modification of the embodiment of  FIG. 6 . Positioned behind the polarizing filter  16  is a mirror  30 , which lies parallel to the polarizing filter  16 . The light passing through the polarizing filter  16  is reflected back by this mirror  30  and reaches the receiver  18 , which in the present case is positioned next to the transmitter  12 . 
     It is clear to the specialist without further explanation that the employed analyzer does not need to be a polarization filter, but may also be a polarization-sensitive device that admits or reflects the polarized light in dependence on the rotating angle. 
     LIST OF REFERENCE NUMERALS 
     
         
           10  rotating object 
           12  transmitter 
           14  polarizer 
           16  polarizing filter 
           18  receiver 
           20  diffusion disk 
           22  shaft 
           24  motor 
           26  mirror 
           28  cylinder 
         a light ray 
         b light ray 
         c light ray 
         d light ray