Abstract:
An apparatus for the photoelectric measurement of an essentially planar measurement original includes a photoelectric scanner and a positioning arrangement for the two dimensional movement of the scanner over the measurement original positioned on a supporting surface and the positioning at defined measurement locations on the measurement original. The positioning arrangement is constructed as a type of SCARA robot and includes a stationary base, a first and a second movable robot arm and a positioning control. The first robot arm is mounted on the stationary base for rotation by a motor about a first axis of rotation perpendicular to the supporting surface. The second robot arm is mounted at the free end of the first robot arm for rotation by a motor about a second axis of rotation perpendicular to the supporting surface and carries the scanner, and the positioning control is constructed for rotation of the two robot arms according to externally supplied positioning commands. The apparatus requires relatively little mounting space, is suitable for both remission and transmission measurements, and can be realized at relatively low construction cost.

Description:
FIELD OF THE INVENTION 
   The invention relates to an apparatus for photoelectric measuring of an essentially planar measurement. 
   BACKGROUND ART 
   In the graphics industry, so called test charts must often be densitometrically or calorimetrically measured, for which hand scanners are used. Such test charts include a large number of special measurement fields on a single sheet onto which a scanning head of the hand scanner is positioned for the measurement. Especially for test charts with a large number of measurement fields (e.g., (several hundred), an automatic positioning of the scanner would significantly reduce the amount of time and effort required for positioning the scanning head. 
   Different solutions for this automatic positioning are already on the market. One of the best known solutions includes an X-Y Table which can receive a hand scanner and drive the hand scanner across the measurement original according to the principle of an X-Y slide table in two orthogonal coordinate directions under the control of a computer. The mechanics required to drive the hand scanner, such as motors and guides, require a relatively large amount of space and are therefore normally positioned under the table for the original. However, with this arrangement, a transmission measurement is then not possible. When transmission measurements are required, the mechanics for driving the hand scanner need to be positioned beside the support for the original, whereby the overall measurement arrangement becomes relatively large even for small sized originals including for example A4 size (210 mm×297 mm). 
   SUMMARY OF THE INVENTION 
   It is an object of the invention to improve such a prior art apparatus in such a way that it requires a minimal overall mounting space while being useful for both remission as well as transmission measurements. Furthermore, manufacture of the apparatus is mechanically realized mechanically in a simple and correspondingly cost efficient manner. 
   These objects of the invention are realized with the measurement apparatus in accordance with the invention characterized by the features of the independent claim. Especially advantageous embodiments and further developments of the measurement apparatus in accordance with the invention are the subject of the dependent claims. 
   This application claims priority to Application No. 04 009 331.2, filed Apr. 20, 2004, in the European Patent Office, the contents of which are incorporated by reference in their entirety. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention is further described in the following by way of the drawings. 
       FIG. 1  shows a perspective view of an embodiment of a measurement apparatus in accordance with the invention; 
       FIG. 2  shows a longitudinal section through the apparatus according to  FIG. 1 ; 
       FIG. 3  is a sketch for the explanation of the kinematics of the robot arms; and 
       FIG. 4  is a block schematic of the position control of the apparatus in accordance with the invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The measurement apparatus illustrated in  FIG. 1  is constructed on a base plate  1  which simultaneously forms the supporting surface for an original  2  to be measured. 
   The apparatus includes a base  10 , which carries or houses all of the remaining components of the apparatus. The base  10  is removably fastened to the base plate  1  by way of a clamping arrangement (not illustrated), including for example, clamping bolts, but not is limited thereto. If the apparatus is to be used for transmission measurements on an illumination table, the base  10  can be fastened to the illumination table for example by a double-sided adhesive tape. In that case, the transparent cover plate of the illumination table would serve as the supporting surface for the original. 
   A first robot arm  20  is supported at one end on the base  10 , which is stationary on the base plate or supporting surface  1 , for rotation by a motor about a first axis of rotation A 1  perpendicular to the base plate or supporting surface  1 . A second robot arm  30  is supported on the free end of the first robot arm  20  for rotation by a motor about a second axis of rotation A 2  perpendicular to the supporting surface  1 . The second robot arm  30  carries a photoelectric hand scanner  40 , including for example, an Eye-One hand spectrophotometer commercially available from Gretag-Macbeth AG. A positioning means (not illustrated) is adapted to the outer shape of the scanner  40  to ensure, in a generally known manner, a defined positioning and a secured fastening of the scanner to the robot arm  30 . The scanner  40  is oriented so that a scanning head  41  of the scanner  40  is located in the vicinity of the free end of the second robot arm  30 . A suitable opening is thereby provided in the robot arm  30  through which the scanning head  41  of the scanner  40  passes. The optical axis of the scanning head  41  is identified by  41   a  ( FIG. 2 ). 
   As is immediately clear from  FIG. 1 , the scanning head  41  of the hand scanner  40  can be moved to any position of the measurement original  2  by suitable rotation of the two robot arms  20  and  30 , whereby the working range which can be covered is of course limited by the dimensions of the two robot arms  20  and  30  and their possible angular positions. The base  10  together with the two robot arms  20  and  30  forms a dual-axis robot as principally generally known, for example, under the term SCARA-Robot (“Selective Compliant Assembly Robot Arm”). 
   For adjustment of the height of the scanner  40  above the base plate  1 , spacing disks (not illustrated) can be provided which are inserted between the base  10  and the base plate  1 . The apparatus can thereby be adapted for the measurement of measurement originals of any thickness (“height”). 
   According to a preferred embodiment, the second robot arm  30  is equipped with a hinged support  31  which is hinged (e.g., rotatably supported) at the rear part  32  of the robot arm  30  about a hinge axis A 3  essentially parallel to the base plate or supporting surface  1  and enables movement of the scanner  40  or the scanning head  41  perpendicular to the supporting surface  1  or original  2 . In an exemplary embodiment, hinged support  31  enables limited movement of the scanner  40  or the scanning head  41  perpendicular to the supporting surface  1  or original  2 . The spacing between the scanning head  41  and the supporting surface  1  or the original  2  thereon can thereby be maintained constant even when the supporting surface  1  or the measurement original  2  are not completely planar or when the spacing must be adapted for thicker originals. The pivotal movement of the hinged support  31  is downwardly limited to a maximum of about 1 mm. A soft driving of the scanner  40  up onto the edges of even very thick originals is thereby guaranteed. 
   According to a further aspect of the invention, the second robot arm  30  or its hinged support  31  carrying the scanner  40  is provided with further openings (not illustrated) through which slide legs or slide rollers or balls  42  ( FIG. 2 ) are receivable therein so that the scanner  40  can be supported thereby on the original  2 . This not only ensures the correct spacing of the scanning head  41  from the measurement original, but results in a load relief of the two robot arms  20  and  30 , which again enables a less costly mechanical construction. However, the device or the robot arms are principally self-supporting, and therefore, the supporting of the scanner  40  via balls  42 , for example, on the original  2  serves primarily only for the maintenance of a constant measurement distance. 
     FIG. 2  shows various construction details of the measurement apparatus by way of a cross-section through the base  10  and the first robot arm  20 . 
   A stationary hollow shaft  11  is co-axially mounted in the base  10 , as well as a first direct current motor  12 . Furthermore, a circuit board  13  is housed in the interior of the base  10 , which carries the electronic components for a positioning control  50  ( FIG. 4 ) to be described later in more detail. 
   The first robot arm  20  is rotatably mounted on the hollow shaft  11  by way of a ball bearing  14 . A spur gear  21  is non-rotatably connected with the first robot arm  20  and is coupled by way of a toothed-belt  22  with a pinion  23  on the direct current motor  12  and is driven thereby. A bowl  15  for the receipt of electrical connecting conduits is disposed on top of the hollow shaft  11 . 
   Above the ball bearing  14 , a first coding disk  16  is rigidly mounted on the hollow shaft  11 . An edge of the first coding disk  16  is surrounded by a first photoelectric incremental encoder  17 . The incremental encoder  17  is connected with the first robot arm  20  and moves upon rotation of the latter along the circumference of the coding disk  16  and thereby scans the coding pattern found thereon. The first coding disk  16  and the first incremental encoder  17  together form a first angle encoder, which allows in a generally known manner the capture of the angular positions of the first robot arm  20  relative to the base  10  and the supply of the angular positions to the positioning control  50 . 
   A second direct current motor  25  with a pinion  26  is rigidly positioned in the first robot arm  20  in the vicinity of its supported end. At the free end of the first robot arm  20 , a second ball bearing  27  is incorporated which rotatably supports a further hollow shaft  28 . The rear portion  32  of the second robot arm  30 , and thereby the whole robot arm, is hung from this hollow shaft  28 . A spur gear  29  is rigidly connected with the hollow shaft  28  and coupled by a toothed-belt  33  with the pinion  26  of the second direct current motor  25  and driven thereby. 
   A second coding disk  34  is rigidly connected with the hollow shaft  28  in the upper region thereof. A circumference of the second coding disk  34  is surrounded by a second incremental encoder  35  and scanned. The second incremental encoder  35  is rigidly positioned in the second robot arm  20 . The second coding disk  34  and the second incremental encoder  35  together form a second angle encoder, which allows in a known manner, capture of the angular positions of the second robot arm  30  relative to the first robot arm  20  and supply of the angular positions of the second robot arm  30  relative to the first robot arm  20  to the positioning control  50 . A further bowl  36  is disposed on the hollow shaft  28  to receive electrical connecting conduits. 
   An electrical plug-in connection is provided in the rear portion  32  of the second robot arm  30  which engages a plug-in connection on the hand scanner  40  for the remote control of the scanner by an external computer. A cable leads from this plug-in connection in the second robot arm  30  through the hollow shaft  28 , spirally through the bowls  36  and  15  and finally through the hollow shaft  11  into the base  10  and from there to a plug-in connection provided therein for an external computer. 
   The electrical connecting conduits from the first incremental encoder  35 , from the second direct current motor  25  and from the first incremental encoder  17  to the positioning control found on the circuit board  13  are also guided through the hollow shaft  11  into the base  10 . 
   The two encoding disks  16  and  24  are preferably constructed as a film and provided with such a coding pattern that a resolution of about 8000 increments result, corresponding to an angular resolution of about 0.05°. The incremental encoder may include a HEDS-9000 series unit commercially available from Agilent Technologies, for example. The integration of the two angle encoders  16 - 17  and  34 - 35  into the support of the relatively mutually rotatable parts is constructively simple and realized in a cost efficient manner, in accordance with the invention. 
   The kinematic connections of the measurement apparatus are illustrated in  FIG. 3 . Axes x and y are thereby the axes of a Cartesian coordinate system, the origin of which is located on the first axis of rotation A 1  of the first robot arm  20 . The angle q 1  represents the angular position of the first robot arm  20  relative to the x-axis. The angle q 2  represents the angular position of the second robot arm  30  relative to the first robot arm  20 . When L 1  is the relevant length of the first robot arm  20  defined by the distance from the first axis of rotation A 1  to the second axis of rotation A 2  and L 2  is the relevant length of the second robot arm  30  defined by the distance from the second axis of rotation A 2  to the optical axis  41   a  of the scanning head  41 , the following conditions obviously result between the Cartesian coordinates x and y and the optical axis  41   a  and the angles q 1  and q 2 :
 
 x=L 1*cos( q 1)+ L 2*cos( q 1 +q 2) y=L 1*sin( q 1)+ L 2*sin( q 1+ q 2)  [1]
 
   The already mentioned positioning control  50  is illustrated in a block diagram in  FIG. 4 . The positioning control  50  includes essentially a track position generator  51 , a kinematics stage  52  and two control circuits  53  and  54 . The two control circuits  53 ,  54  each respectively include an angle comparator  531 ,  541 , a proportional-integral-differential-controller (PID)  532 ,  542  and a motor driver  533 ,  543  and further include respectively one of the direct current motors  12 ,  25 , as well as respectively one of the angle encoders  16 - 17 ,  34 - 35 . 
   First, the size and orientation of the measurement original  2  located on the base plate  1  is determined. For this, a visible reference point (e.g., a sight) at the free end of the second robot arm  30  is moved manually, or under electronic control to three corner points of the measurement original  2 . The reference point thereby has a defined spacing from the optical axis  41   a  of the scanning head  41  of the scanner  40 . The corresponding x-y coordinates in relation to the scanning head  41  or its optical axis  41   a  are then calculated (in an external computer) from the angular positions q 1  and q 1  at these three corner points in connection with the defined spacing reference point-optical axis  41   a , and stored. The size and orientation of the measurement original  2  is directly calculable from the x-y coordinates of the three corner points. The electronically controlled movement of the reference point to the three corner points of the measurement original can be realized, for example, in such a way that commands for the movement of the scanning head  41  in x and y directions are given to the positioning control  50  by an external computer (not illustrated) by pressing suitable keys. The alternate manual movement can also be easily achieved, since the supports of the robot arms  20 ,  30  have only a small friction moment when the drive motors are not electrified. When the reference point is at the desired position above a corner point, the associated angular positions can be transferred to an external computer upon the operation of a suitable key at the computer and calculated into absolute x-y coordinates. These x-y coordinates, as already mentioned, are in relation to the optical axis  41   a  of the scanning head  41 . 
   As in the known X/Y—table solutions for many common commercial measurement originals (test charts), the geometric data (dimensions) and the (relative) coordinates of the measurement fields present, as well as information on their construction, are stored in the external computer. When, as described above, the absolute x-y coordinates of the corner points of the concrete measurement original are known, relative coordinates stored for the concrete measurement original can be converted into absolute coordinates with the help of these corner point coordinates. The absolute coordinates are then used as positioning commands for the control of the measurement apparatus. 
   After these preparatory steps, the apparatus is ready for the automated measurement of the measurement original. 
   Positioning commands are supplied by an external computer to the track position generator  51  through a connection (not illustrated). These positioning commands represent the (absolute) Cartesian coordinate values x s , y s  of a starting point (on the measurement original), the Cartesian coordinate values x e , y e  of an end point and a speed value v. The track position generator  51  now produces from these positioning commands a temporal sequence of Cartesian coordinate values x(t) and y(t), which represent a linear track of the scanner  40  (more precisely of the optical axis  41   a  of the scanning head  41 ) between the starting point and the end point (on the measurement original). 
   The kinematics stage  52  calculates from these coordinate values x(t) and y(t) a corresponding temporal sequence of nominal angular positions q 1 ( t ) soll  and g 2 ( t ) soll  for the two robot arms  20  and  30 . This calculation is carried out simply by resolving the above two equations [1] for q 1  and q 2 . 
   The two control circuits  53 ,  54  finally control the two direct current motors  12  in such a way that the actual angular positions q 1 ( t ) ist  and q 2 ( t ) ist  captured by the angle encoders  16 - 17  and  34 - 35  correspond to the preset nominal angular positions q 1 ( t ) soll  and q 2 ( t ) soll , and the scanning head  41  of the scanner is thereby moved over the measurement original  2  at the commanded speed from the commanded starting point to the commanded end point. During the movement of the scanning head  41 ; the measurement original  2  is scanned in a generally known manner and the measurement data are transmitted to the connected external computer. 
   For use of the measurement apparatus for different measurement applications, the base plate  1  can be made white on one side and black on the other. The apparatus is then mounted on one or the other side of the base plate  1 , as desired. 
   A (physical) white reference is normally necessary for the calibration of the scanner  40 . This can be provided, for example, on the base  10 , whereby it naturally must be reachable by the scanning head  41  of the measurement apparatus. However, the white reference can also be provided, for example, on the hinged support  31  to be mechanically moveable into the measurement beam of the scanner  40 , for example, by way of a pin. 
   Although the present disclosure has been provided with reference to exemplary embodiments thereof, the present disclosure is not to be limited thereto. Rather, modifications, enhancements and/or variations to the disclosed devices, systems and features are contemplated, and such modifications, enhancements and/or variations will not depart from the spirit or scope of the present disclosure.