Patent Publication Number: US-2007124061-A1

Title: Device for the absolute measurement of the linear or rotational position of an object

Description:
FIELD OF THE INVENTION  
      The invention relates to a device for the absolute measurement of the linear or rotational position of an object, in accordance with the preamble of patent claim  1 .  
     BACKGROUND OF THE INVENTION  
      To measure the linear or rotational position of an object it is known to couple the measured object with a material measure, which is scanned cyclically by a scanner. When the measured object is a rotating one it is thereby possible to determine the absolute position over several revolutions of the object. When the measured object moves in linear fashion a periodically returning material measure can be scanned over several periods. Here a device is used which records the object&#39;s position in coded fashion over a plurality of scanned cycles. To this end, the coding unit exhibits an input wheel, which is coupled to the measured object by an effective mechanical link. The effective mechanical link is generally realized by a toothed gearing. When the object being measured is a rotating one, the input wheel, which takes the form of a gearwheel, engages with the teeth of a gearwheel rotating with the measured object. When the object being measured is moved in linear fashion, the input wheel designed as a gearwheel engages with a toothed bar that is moved with the measured object.  
      In these known devices a problem is posed by the abrasion or wear that is experienced by the effective mechanical link, particularly by the gearings that engage with each other. In many cases this problem is a serious one, specifically because the abrasion is strongly dependent on the toothed pairing that creates the effective link. The greater the diameter of the driving gearwheel compared to the diameter of the driven input wheel, the greater is the abrasion. For structural reasons the diameter of the driving gear is often considerably greater than the diameter of the driven input wheel. For example, the diameter of the driving gearwheel must have a relatively large diameter if this driving gearwheel is positioned on, e.g., the hollow-shaft motor that represents the measured object. The diameter of the input wheel of the counting device, on the other hand, is usually small, in order to reduce the structural dimensions of the apparatus. In the extreme case of a measured object moved in linear fashion, the linear toothing corresponds to an infinitely large diameter on the driving gearwheel.  
     BRIEF SUMMARY OF THE INVENTION  
      The invention is based on the problem of creating a device which records the object&#39;s position in coded fashion over a plurality of scanned cycles, while minimizing the problem of attrition in the mechanical coupling of the coding unit.  
      The invention solves this problem with a device exhibiting the features of patent claim  1 .  
      Advantageous embodiments of the device are indicated in the secondary claims.  
      The basic idea of the invention consists in providing a relatively large degree of mechanical play to the effective mechanical link for driving the input wheel of the coding unit and in allowing the input wheel to be driven by its own electrical motor. The scanning of the material mass allows the drive of the input wheel by this motor to be synchronized with the motion of the measured object in such a way that the effective mechanical link occurs within this degree of play, with this result that there is no mechanical engagement which would cause attrition. During operation of the device the effective mechanical link between the measured object and the input wheel is non-functional and the input wheel is driven exclusively by its own motor. The effective mechanical link is replaced by the synchronization provided by this motor drive. If the measured object is moved without the device being switched on, the input wheel is not driven by the electrical motor. In this case, the effective link is mechanically engaged and drives the input wheel and thus the coding unit. The coding unit thereby records the number of rotations or cycles in conventional fashion, in order to determine the absolute position during the subsequent engagement of the device. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING  
      The FIGURE shows an embodiment of a device according to the present application for the absolute measurement of the linear or rotational position of an object. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      Next the invention will be described in greater detail on the basis of an exemplary embodiment, which is depicted schematically in the single FIGURE of the attached drawing.  
      In the schematic depiction the absolute angular position of a measured object  10  is be measured over a number of rotations. The measured object  10  can be, e.g., the shaft of a servomotor or a rotating object that requires positioning. The measured object  10  can also be the input shaft of a positional indicator, which is coupled to such a servomotor shaft or such an object being positioned.  
      Coupled to the measured object  10  in angularly secure fashion is a material measure, which the figure does not depict. The material measure can be, e.g., a coding disk, which is connected to the measured object  10  in torque-proof fashion and is scanned either optically, magnetically, or in some other fashion. The scanner  12  detects in absolute fashion the angular position of the material measure, and thus of the measured object  10 , over one rotation of the material measure. The signal given by the scanner  12  is fed to an electronic evaluating unit  16  over a signal line  14 .  
      In order to absolutely determine the angular position of the measured object  10  over a plurality of rotations, there is a coding unit  18 , which detects and encodes the rotations of the measured object  10 , or of the material measure, through use of a reduction gear. The coded information on the number of rotations is also fed to the evaluating unit  16 . In the evaluating unit  16 , the information on the position within a rotational cycle and the information on the number of rotations are consolidated into the absolute multi-turn positional value, which is then released by an interface  20 .  
      The device thus far accords with the prior art, and a detailed description is not necessary.  
      The coding unit  18  is coupled to the measured object  10  over an effective mechanical link. To this end, a gearwheel  22  is connected to the measured object  10  in torque-proof fashion. Engaging with the gearwheel  22  is an input wheel  24 , in the form of a gearwheel belonging to the coding unit  18 . The toothings of the gearwheel  22  and of the input wheel  24  are designed so as to mutually engage with a relatively large degree of play in the rotating direction. The rotation of the input wheel  24  is driven by an electric motor  26  belonging to the input wheel  24 . The motor  26  is piloted by the evaluating unit  16  over a control line  28 , in such a way that the input wheel  24  rotates synchronously with the gearwheel  22 . The scanning signals of the scanner  12  are used to achieve this synchronization.  
      When the device is running and the evaluating unit  16  and the motor  26  are operating, the synchronized rotation of the input wheel  24  and the gearwheel  22 , in conjunction with the relatively large degree of play between the teeth, ensure that the toothings of the driving gearwheel  22  and the driven input wheel  24  do not engage and transmit a drive torque. As a result, there is almost no wear on the toothings.  
      If the measured object  10  is moved when the device is not operating, the material measure and the gearwheel  22  are rotated with the measured object  10 . Because the toothings are engaged, the gearwheel  22  entrains the input wheel  24 , which in this case is not driven by the motor  26 , because the latter is shut off. The engagement of the gearwheel  22  with the input wheel  24  actuates the coding unit  18 , and the revolutions of the measured object  10  are recorded in coded form. When the device is later turned on, the absolute angular position measured over a plurality of revolutions is thus immediately available at the interface.  
      It is evident that the invention can be employed in the same manner when the measured object  10  is not a rotating object, but an object moving in linear fashion. Here the material measure can be positioned in a linear way, and the absolute positional values repeat periodically and succeed each other cyclically during scanning. The coding unit  18  serves to absolutely record the scanned cycles in coded fashion.  
      When the invention is applied in this way a linear gearing is used instead of a rotating gearwheel  22 , e.g., in the form of a toothed bar connected to the measured object. The toothed bar then engages with the input wheel, with an appropriate degree of play between the gearings.  
     LIST OF REFERENCE NUMERALS  
     
         
           10  measured object  
           12  scanner  
           14  signal line  
           16  electronic evaluation unit  
           18  coding unit  
           20  interface  
           22  gearwheel  
           24  input wheel  
           26  motor  
           28  control line