Abstract:
An apparatus for angle measurement that includes a graduation carrier with a code track disposed concentrically to a center point of the graduation carrier. The apparatus includes a first scanning unit having a first interface and a second scanning unit having a second interface, wherein the first and second scanning units ascertain angle values of the graduation carrier by scanning the code track. The apparatus further includes a control unit having: 1) a device interface that is in communication with a follower electronics unit, 2) a control unit interface that is in communication with the first and second interfaces and 3) a processing unit. By which the processing unit, angle values of the first and second scanning units can be requested and processed into a corrected angle value, and the corrected angle value can be transmitted to the follower electronics unit via the device interface.

Description:
RELATED APPLICATIONS 
       [0001]    Applicants claim, under 35 U.S.C. §119, the benefit of priority of the filing date of Jul. 28, 2011 of a German patent application, copy attached, Serial Number 10 2011 079 961.3, filed on the aforementioned date, the entire contents of which is incorporated herein by reference. 
       BACKGROUND 
       [0002]    1. Technical Field 
         [0003]    The present invention relates to an apparatus for angle measurement, and to a method for angle measurement. An apparatus and a method according to the present invention for angle measurement can decisively improve the accuracy in the measurement of the angular position of shafts with large diameters, in both rotary tables and telescopic applications. 
         [0004]    2. Background Information 
         [0005]    High-precision angle measurement devices for measuring the angular position of a shaft, such as a shaft of a rotary table, are known. Variants that are self-supported and have a shaft on the end toward the measuring system are described in the book entitled “Digitale Längen-und Winkelmesstechnik” [Digital Length and Angle Measurement Technology] by A. Ernst, published by the Verlag Moderne Industrie, 3 rd  edition, 1998, pp. 61-64. To achieve high accuracy, first, highly accurate and hence very expensive precision bearings must be used in the measuring system. Second, connecting the measuring system shaft, which has a graduation plate with a radial measurement graduation, by a suitable coupling to the shaft that is to be measured requires major effort and expense. 
         [0006]    Also, from pages 64-70 of the above mentioned book, angle measurement devices without self-support are known, in which a rotationally symmetrical measurement graduation or a corresponding graduation plate is disposed directly on a shaft that is to be measured. Corresponding scanning units for scanning the measurement graduation are disposed in stationary fashion relative to the rotating graduation plate. 
         [0007]    In these angle measurement devices, incremental graduations with up to 36,000 radial lines are used as a measurement graduation, which further increases the angular resolution by interpolation. 
         [0008]    In the ideal case, the accuracy of the angle measurement in such angle measurement devices depends on both the precision with which the measurement graduation was applied to the graduation plate, and on the measurement error of the scanning unit. In reality, due to manufacturing tolerances, the rotary motion of the graduation plate and, thus, of the measurement graduation, always has both an error in eccentricity and an error in wobble. This is due to the fact that: 1) the center point of the graduation plate can never lie exactly on the axis of rotation of the shaft to be measured, and 2) the axes of rotation of the graduation plate and of the shaft to be measured can never be disposed in exact alignment. As a consequence, the spacing and position of the measurement graduation relative to the scanning unit vary within one revolution of the shaft to be measured, resulting in a measurement error in the angle measurement. 
         [0009]    To reduce the high expense for 1) precision bearings and precision couplings, especially in large angle measurement devices for shafts of large diameters, and 2) the mechanical calibration, German patent disclosure DE 199 07 326 A1 of the present Applicant proposes scanning the measurement graduation at a plurality of scanning points distributed over the circumference of the graduation plate, and evaluating the resultant sinusoidal signals and correcting errors in eccentricity and wobble. 
         [0010]    Modern angle measurement devices, though, preferably use an absolutely coded code track instead of the incremental track. This code track is, for example, a multi-track code, such as a gray code, or a single-track incremental code, known as a “pseudo-random code” (PRC). This has the advantage that at every moment the absolute angle position can be determined directly by scanning the code track. However, the signal processing proposed in DE 199 07 326 A1 cannot be used for absolutely coded code tracks. 
       OBJECTS AND SUMMARY OF THE INVENTION 
       [0011]    It is therefore an object of the present invention to disclose an absolute angle measurement device with improved accuracy. 
         [0012]    This object is attained by an apparatus for angle measurement, wherein the apparatus includes a graduation carrier that has a code track disposed concentrically to a center point of the graduation carrier. The apparatus includes a first scanning unit that has a first interface and a second scanning unit that has a second interface, wherein the first and second scanning units ascertain angle values of the graduation carrier by scanning the code track. The apparatus further includes a control unit having: 1) a device interface that is in communication with a follower electronics unit, 2) a control unit interface that is in communication with the first and second interfaces and 3) a processing unit. By which the processing unit, via the control unit interface and a request for angle values, angle values of the first and second scanning units can be requested and processed into a corrected angle value, and the corrected angle value can be transmitted to the follower electronics unit via the device interface. 
         [0013]    An apparatus for angle measurement in the form of an angle measurement device is now proposed, including
       a graduation carrier, which has a code track disposed concentrically to its center point;   at least two scanning units for ascertaining angle values of the graduation carrier by scanning the code track;   a control unit, having a device interface for communication with a follower electronics unit and having at least one interface for communication with interfaces of the scanning units;
 
in which the control unit includes a processing unit, by which via the at least one interface angle value of the scanning units can be requested and processed into a corrected angle value, and the corrected angle value can be transmitted to the follower electronics unit via the device interface.
       
 
         [0017]    It is a further object of the present invention to disclose a method with which the accuracy of this kind of absolute angle measurement device can be improved. 
         [0018]    This object is attained by a method for angle measurement having an apparatus for angle measurement that includes a graduation carrier that has a code track disposed concentrically to a center point of the graduation carrier. The apparatus includes a first scanning unit that has a first interface and a second scanning unit that has a second interface, wherein the first and second scanning units ascertain angle values of the graduation carrier by scanning the code track. The apparatus further includes a control unit having: 1) a device interface that is in communication with a follower electronics unit, 2) a control unit interface that is in communication with the first and second interfaces and 3) a processing unit. By which the processing unit, via the control unit interface and a request for angle values, angle values of the first and second scanning units can be requested and processed into a corrected angle value, and the corrected angle value can be transmitted to the follower electronics unit via the device interface. The method includes requesting, via a request of angle values, angle values from the first and second scanning units and processing, via the processing unit, the angle values into the corrected angle value. The method further includes transmitting the corrected angle value to the follower electronics unit. 
         [0019]    A method for angle measurement having an apparatus for angle measurement is now proposed, having the following processes:
       requesting angle values from at least two scanning units of the apparatus;   processing the angle values into a corrected angle value via a processing unit of the apparatus; and   transmitting the corrected angle value to a follower electronics unit of the apparatus.       
 
         [0023]    Further advantages and details of the present invention will become apparent from the ensuing description in conjunction with the drawings. 
         [0024]    In the drawings: 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0025]      FIG. 1  is a block diagram of a first exemplary embodiment of an angle measurement device in accordance with the present invention; 
           [0026]      FIG. 2  is a block diagram of a first embodiment of a control unit to be used with the angle measurement device of  FIG. 1  in accordance with the present invention; 
           [0027]      FIG. 3  is a block diagram of a second embodiment of a control unit to be used with the angle measurement device of  FIG. 1  in accordance with the present invention; 
           [0028]      FIG. 4  is a block diagram of a third embodiment of a control unit to be used with the angle measurement device of  FIG. 1  in accordance with the present invention; and 
           [0029]      FIG. 5  is a block diagram of a further exemplary embodiment of an angle measurement device in accordance with the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0030]      FIG. 1  shows a block diagram of a first exemplary embodiment of an angle measurement device  10  of the present invention. The angle measurement device  10  includes as a graduation carrier a circular or annular code disk  20 , and the code disk has a code track  25 , disposed radially around the center point of the code disk  20 . In addition, the angle measurement device  10  includes four scanning units  30 ,  31 ,  32 ,  33  and a control unit  40 . 
         [0031]    For the operation of the angle measurement device  10 , the coded disk  20  is connected in a manner fixed against relative rotation to a shaft that is to be measured, so that it rotates about its center point M together with the axis of rotation of the shaft to be measured. The code disk  20  has a code track  25  that is capable of being scanned photoelectrically, magnetically, capacitively, or inductively. According to the present invention, the code track  25  includes an absolute coded graduation, for instance in the form of a multi-track code (such as a gray code), or a single-track incremental code (pseudo-random code or PRC). To increase the resolution of the angle measurement device  10 , a further track with an incremental graduation (not shown) can additionally be disposed parallel to the absolute encoded graduation. 
         [0032]    As described at the outset above, because of unavoidable tolerances involved in mechanically connecting the shaft to be measured to the code disk  20 , errors of eccentricity and wobble occur. This is true both for angle measurement devices  10  without self-support, in which the code disk  20  is disposed directly on the shaft that is to be measured. It is also true for self-supported angle measurement devices  10  that themselves include a shaft on which the code disk  20  is secured in a manner fixed against relative rotation and wherein such shaft is in turn connected via a shaft coupling to the shaft that is to be measured. 
         [0033]    The scanning units  30 ,  31 ,  32 ,  33  are suitably embodied for scanning the code track  25  and for ascertaining absolute angle values of the code disk  20 , and, thus, of the shaft to be measured, from the scanning signals. The physical scanning principle is not of significance for the present invention. In the present example, the scanning units  30 ,  31 ,  32 ,  33  are disposed in stationary fashion, distributed at defined angular spacings over the circumference of the code disk  20 . For instance, the angular spacing between each of the scanning units  30 ,  31 ,  32 ,  33  is 90°. In ascertaining the corrected angle value, the offset that results in the ascertaining of the absolute angle values of the scanning units  30 ,  31 ,  32 ,  33  can be taken into account either in the scanning units  30 ,  31 ,  32 ,  33  or the control unit  40 . 
         [0034]    Although in the exemplary embodiment in  FIG. 1  four scanning units  30 - 33  are used, the present invention is not fixed at that number. What is important for ascertaining the corrected angle value is that there are at least two scanning units. 
         [0035]    For outputting the absolute angle values, the scanning units  30 ,  32 ,  32 ,  33  have digital interfaces  35 ,  36 ,  37 ,  38 . Both parallel interfaces, with preferred data widths of 4 bits (one nibble), 8 bits (one byte), or 16 bits (one word), and serial interfaces can be used. Especially suitable serial interfaces are known standard interfaces for position measuring devices, such as EnDat or SSI. However, field bus systems (such as CAN bus, Interbus-S, or SERCOS) or interfaces that can be summarized by the term “real-time ethernet” can also be used as interfaces  35 - 38 . Accordingly, the interface connections, as shown in  FIG. 1 , can be either point-to-point connections or bus connections with a linear or annular structure. 
         [0036]    For the communication between the control unit  40  and the scanning units  30 - 33 , interfaces  45 - 48  are provided in the control unit  40 , which are connected to the interfaces  35 - 38  of the scanning units  30 - 33  via suitable signal lines. The signal lines can be conventional electrical lines, but when there are great spatial distances between the control unit  40  and the scanning units  30 - 33 , it can be advantageous to use optical waveguides as the signal lines and optical interfaces for both the interfaces  45 - 48  of the control unit  40  and the interfaces  35 - 38  of the scanning units  30 - 33 . The absolute angle values, which are ascertained in the scanning units  30 - 33 , can now be transmitted to the control unit  40  via the interface connections inside the devices. 
         [0037]    For communication with a follower electronics unit  100 , the control unit  40  further includes a device interface  42 . This interface is advantageously also a serial data transmission interface; prominent examples for this are again EnDat, SSI, or real-time ethernet interfaces. 
         [0038]    A processing unit  41  is also provided in the control unit  40 . It serves to request angle values from the scanning units  30 - 33  via the corresponding interface connections, for forming a corrected angle value using the requested angle values, and to transmit the corrected angle value to the follower electronics unit  100  via the device interface  42 . 
         [0039]    The processing unit  41  can moreover check the angle values arriving from the scanning units  30 - 33  for plausibility. If deviations that exceed a fixed tolerance threshold are found, the processing unit  41  can send a warning or error report to the follower electronics unit  100  via the device interface  42 . 
         [0040]    For security-relevant applications, it is advantageous to design the processing unit  41  such that at least two corrected angle values, which are based on the angle values from various scanning units  30 - 33 , can be generated and transmitted to the follower electronics unit  100 . In this way, by comparison in the follower electronics unit  100 , errors in the generation or transmission of the corrected angle values can reliably be discovered. 
         [0041]    Particularly in very large angle measurement devices  10 , in which the code disk  20 , the scanning units  30 - 33 , and the control unit  40  cannot be disposed together in one housing, but instead are installed separately as individual components, it is advantageous to combine the control unit  40  and one of the scanning units  30 - 33  (in  FIG. 1 , the scanning unit  30 ) into a master scanning unit  50 . By this, the assembly of the angle measurement device  10  can be made easier on the one hand, and, on the other hand, the interface connection of the corresponding interfaces  35 ,  45  can be embodied more simply. 
         [0042]    A particular advantage of an angle measurement device  10  of the present invention is that from the standpoint of the follower electronics unit  100  with regard to communication with the device interface  42 , no difference from communication with an angle measurement device of the kind known from the prior art is apparent, yet substantially improved measurement accuracy is nevertheless attained. Thus, even existing systems can be retrofitted with an angle measurement device  10  of the present invention and can achieve improved accuracy without having to make changes in the follower electronics unit  100 . 
         [0043]      FIG. 2  shows a block diagram of a control unit  40   a  that can be used in the angle measurement device  10  of  FIG. 1  in accordance with the present invention. Function blocks that have already described in conjunction with  FIG. 1  are identified by the same reference numerals. 
         [0044]    For ascertaining the corrected angle value, the processing unit  41  a includes an extrapolation unit  410 , a clock generator  420 , a time measuring unit  430 , and a correction unit  440 . 
         [0045]    At time intervals which are determined by the clock signal of the clock generator  420 , the extrapolation unit  410  requests actual angle values from the scanning units  30 - 33  via the interfaces  45 - 48 . If a positioning request command arrives from the follower electronics unit  100  via the device interface  42 , then the extrapolation unit  410  ascertains extrapolated angle values from the at least two most up-to-date angle values of each scanning unit  30 - 33 . The time required for the extrapolation between the request for the latest angle value per scanning unit  30 - 33  and the time when the positioning request command arrives is measured by the time measuring unit  430 . The extrapolated angle values are delivered to the correction unit  440 , which processes them into the corrected angle value, for instance by finding the average of the extrapolated angle values. This corrected angle value is output to the follower electronics unit  100  via the device interface  42 . 
         [0046]      FIG. 3  shows a block diagram of a control unit  40   b  that can be used with the angle measurement device  10  of  FIG. 1  in accordance with the present invention. Once again, function blocks that have already described in conjunction with  FIG. 1  are identified by the same reference numerals. 
         [0047]    In this exemplary embodiment, the processing unit  41  b includes a correction unit  510  and a correction value ascertaining unit  520 . Optionally, the processing unit  41   b  can also contain a clock generator  530 . 
         [0048]    If a positioning request command arrives at the control unit  40   b  from the follower electronics unit  100 , the correction value ascertaining unit  520 , via the interfaces  45 - 48 , requests actual angle values from the scanning units  30 - 33  and ascertains a correction value which is suitable for correcting the actual angle value arriving from a leading scanning unit  33 . For that purpose, the correction value ascertaining unit  520  is advantageously designed such that the request for actual angle values is made with the least possible time lag and simultaneously for all the scanning units  30 - 33 . The ascertained correction value is delivered to the correction unit  510 , which corrects the actual angle value arriving from the leading scanning unit  33  and outputs it to the follower electronics unit  100  via the device interface  42 . 
         [0049]    With regard to the reaction time of the angle measurement device  10 , or, in other words, the time between the arrival of the positioning request command and the outputting of the actual corrected angle value, it is advantageous if, as the correction value is forwarded to the correction unit  510  for ascertaining the corrected angle value, the correction value ascertained upon arrival of the previous positioning request command is used, rather than the correction value ascertained from the angle values arriving at the time. The reason this can be done is that the errors of eccentricity and wobble are long-period errors; that is, at small changes in angle, the correction value varies only insignificantly. Moreover, angle measurement device  10  is typically requested cyclically at short time intervals, or, in other words, with a high request frequency when the follower electronics  100  is in operation. In this way, especially when the shaft to be measured is rotating at a slow rpm, or at a high request frequency of the follower electronics unit  110 , an improved reaction time can be attained without a significant reduction in the measurement accuracy, since at the moment the positioning request command arrives the correction value is already available. 
         [0050]    In a further improvement for this purpose, the request of actual angle values from the scanning units  30 - 33  and the ascertainment of correction values in the correction value ascertaining unit  520  can be controlled by the optional clock generator  530 . As a result, it is ensured that new correction values are ascertained continuously, and, therefore, upon arrival of a positioning request command, very up-to-date correction values are always already available. 
         [0051]    To avoid chronologically overlapping arrivals of requests of actual angle values in the leading scanning unit  33  that are controlled internally by the clock generator  530  and externally via the device interface  42 , the leading scanning unit  33  can be excluded from the ascertainment of new correction values. In that case, new correction values are ascertained using only angle values from the remaining scanning units  30 - 32 . 
         [0052]      FIG. 4  shows a block diagram of a control unit  40   c  to be used with the angle measurement device  10  of  FIG. 1  in accordance with the present invention. Once again, function blocks that have already described in conjunction with  FIG. 1  are identified by the same reference numerals. 
         [0053]    This embodiment is based on the recognition that in cyclical operation of the angle measurement device  10 , the interval of time between the arrival of positioning request commands from the follower electronics unit  100  via the device interface  42  is known. In addition to the correction unit  610 , a trigger unit  620  is now provided, which controls both the requesting of new angle values from the scanning units  30 - 33  and the ascertainment of corrected angle values. To that end, the trigger unit  620  sends trigger signals to the correction unit  610 . The timing of the trigger signals is structured such that the trigger signals are generated a predetermined time after the latest arrival of a positioning request command from the follower electronics unit  100 , but still before the arrival of an actual positioning request command. As a consequence, in cyclical operation an actual corrected angle value can be sent to the follower electronics unit  100  immediately in response to a positioning request command. The information about when the trigger unit  620  has to send a trigger signal to the correction unit  610  can be imparted to the trigger unit  620 , for instance, via the device interface  42 . 
         [0054]      FIG. 5  shows a block diagram of a further exemplary embodiment of an angle measurement device  10   a  of the present invention. In a departure from the exemplary embodiment of  FIG. 1 , the angle measurement device  10   a  includes as a graduation carrier a ring  200 , on the cylindrical outer surface of which a code track  25   a  is disposed. In this exemplary embodiment, the code track  25   a  includes an absolute coded graduation  125  as well as an incremental graduation  126 . The ring  200  is made from steel, for example, and in professional circles it is also known as a “drum.” Advantageously, the lengths of the lines of the graduation tracks  125 ,  126  as measured transversely to the measurement direction are embodied such that they are greater than the scanning length required by the scanning units  30 - 33 , so that axial displacements of the ring  200  relative to the scanning units  30 - 33  do not cause any incorrect measurement values. 
         [0055]    Especially when the outer dimensions of the ring  200  are large, the absolutely coded graduation  125  and the incremental graduation  126  are not applied directly to the ring  200 . Instead, the graduations  125  and  126  are in the form of a steel band that carries the graduation tracks  125 ,  126  and is placed in a groove located in the outer diameter of the ring and is fastened with a turnbuckle. In principle, as shown in detail A of  FIG. 5 , this creates a transition point S. As a scanning unit  30 - 33  passes over the transition point S, an abrupt change in the angle value that is read out can occur. To minimize its effect on the formation of the corrected angle value, it is advantageous if the processing unit  41  blanks out angle values from scanning units  30 - 33  which are located in the vicinity of the transition point S and does not use them for ascertaining the corrected angle value. 
         [0056]    It should also be pointed out that in the implementation of the control unit  40 , particularly in the exemplary embodiments described in conjunction with  FIGS. 2-4 , it is not absolutely necessary, for ascertaining the correction values, to transmit the angle values from all the scanning units  30 - 33  in their entirety to the control unit  40 . Deviations in the angle values that are caused by wobble errors and errors of eccentricity move within a relatively narrow range of values. It is therefore usually sufficient to transmit only the actual angle value from a leading scanning unit  30 - 33  in its entirety. For the correction thereof to transmit only as many of the less significant bits of the remaining scanning units  30 - 33  as needed so that a maximum error can be reliably corrected. 
         [0057]    In general, it is advantageous if the control unit  40  is designed such that the scanning units  30 - 33  can be addressed by the follower electronics  100  via the interface  42  individually as well. To that end, interface commands can, for instance, be provided that make it possible to select, describe, or read out individual scanning units  30 - 33 . Fundamentally, however, still other selection and switchover mechanisms are also possible. 
         [0058]    It is also advantageous to embody the control unit  40  such that individual scanning units  30 - 33  are deactivatable, or that their angle values are not taken into account in ascertaining the corrected angle value. In that case, the angle measurement device  10  can continue to be operated, with reduced accuracy, even if one scanning unit  30 - 33  fails or furnishes erroneous values. Particularly in connection with the exemplary embodiment of a control unit  40  described in conjunction with  FIG. 3 , it is advantageous to embody the leading scanning unit  33  in selectable fashion. 
         [0059]    The foregoing description is provided to illustrate the present invention, and is not to be construed as a limitation. Numerous additions, substitutions and other changes can be made to the present invention without departing from its scope as set forth in the appended claims.