Abstract:
The invention relates to a guiding device ( 3 ) for guiding a displaceable machine element ( 1 ) of a machine. Said guiding device ( 3 ) in connected in a mechanically fixed manner to a machine bed ( 4 ). A material measure ( 6 ) is arranged along at least one part of the guiding device ( 3 ) in order to determine a position of the machine element ( 1 ). The guiding device ( 3 ) and the material measure ( 6 ) can be displaced in relation to each other and at least one part of the material measure ( 6 ) is in contact with the guiding device ( 3 ) either directly or via a bearing ( 9 ). The inventive guiding device ( 3 ) enables the displaceable machine elements to be positioned in a precise manner even when the machine bed ( 4 ) is vibrated. The invention also relates to a machine comprising said type of guiding device ( 3 ).

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to a guiding device for guiding a moveable machine element of a machine and also to a machine having such a guiding device. 
     In machines such as machine tools, production machines and/or robots, a moveable machine element such as a guiding carriage frequently has to be displaced in a position-regulated manner along a guiding device which may be in the form of a guiding track, for example.  FIG. 1  schematically shows a commercial machine tool, where, for reasons of clarity, only those elements which are necessary to understand the invention are shown. A moveable machine element  1 , which is designed in the exemplary embodiment in the commercial machine in the form of a guiding carriage, is guided by a guiding device  3 , which is provided in the exemplary embodiment in the form of a guiding track. The machine element  1  is equipped with a drive, for example a linear drive, which is not shown for reasons of clarity and is used by an open-loop and/or closed-loop control system to displace the machine element  1  in a position-regulated manner along the guiding device  3 . The guiding device  3  is connected in a mechanically fixed manner to a machine bed  4  which stands on a floor  5 . 
     A measuring scale  6 , which may be, for example, in the form of a strip having a regular graduation, is connected in a mechanically fixed manner to and along the guiding device  3  in order to determine the position of the machine element  1 . The measuring scale  6  has, for example, a “graduation” in the form of lines, which graduation can be read by a reading head  2  connected to the machine element  1 . The measuring scale  6 , in conjunction with the reading head  2 , therefore permits an exact determination of the position of the machine element  1 . 
     In a commercial machine, further fixtures, for example in the form of a spindle with tools, which can be displaced along the guiding device  3  using the machine element  1 , are provided on the latter, which is in the form of a “guiding carriage” in the exemplary embodiment. In a machine of this type, the workpiece to be machined is often anchored directly to the floor. The machine element  1  and the fixtures attached thereto form a “tool carriage”. If desired, a tool carriage can have multiple guiding carriages rather than just one guiding carriage. In machines such as machine tools, production machines and/or robots, the machine carriages must be positioned with a high degree of precision and at high speed, while maintaining prescribed displacement profiles. The prescribed displacement profiles include speed changes which necessitate high acceleration forces. The quick change in the acceleration forces causes the mechanical system of the machine, in particular the machine bed  4 , to oscillate. The machine bed  4  is often stressed very strongly by the reaction forces of the movement guidance. The influence of the acceleration forces of the tool carriage causes the machine bed to give way in the opposite direction. In the process, it is frequently deformed and/or begins to oscillate. Even if the machine element  1  can be positioned exactly in relation to the machine bed  4 , the machine bed  4  itself still oscillates with respect to the floor  5 . If the machine is a portal milling machine, for example, in which the workpiece to be machined is fixedly anchored on the floor, the oscillations of the machine bed  4  can be seen from the workpiece, even if the machine element  1  maintains exactly the prescribed position in relation to the machine bed  4 . 
     The oscillations of the machine bed  4  frequently have such an unfavorable effect that the closed-loop control characteristics of the movement guidance of the machine element  1  are strongly compromised and not even a movement guidance of the machine element  1  in relation to the machine bed  4  that is sufficiently precise can be achieved. In that case the oscillations of the machine bed  4  occurring during the movement guidance have a doubly disadvantageous effect. 
     The prior art strives to suppress undesired oscillations of the machine bed by means of a correspondingly rigid and heavy design of the machine bed. The machine bed, however, becomes heavy, complex and expensive on account of this. 
     If a direct linear drive is used as the driving system for the machine element, it is possible to separate the reaction surface of the motor from the machine bed. This fact is disclosed by the laid-open specification DE 198 10 996 A1. The document proposes decoupling of the motor reaction surface from the machine bed, with the reference point of the axis control remaining on the machine bed. The document proposes to anchor the motor reaction surface with respect to the machine bed such that it can move. This causes the reaction force which arises as a direct consequence of the acceleration operations to be absorbed in the motor reaction surface of moveable design and in this manner to be kept away from the machine bed. The machine bed therefore remains at rest even given severe changes in speed of the tool carriage, which significantly improves the accuracy of the movement guidance of the tool carriage. The reference point for the determination of position remains here unchanged on the machine bed for the closed-loop control, for example. Feed drives designed according to this technical teaching are used in machine tools for example in which the tool carriage is subjected to high accelerations. It has been shown, however, that the necessary moveable anchoring of the motor reaction surface of the linear direct drive results in significant costs and complexity. They are therefore not used especially as a device for avoiding oscillations in machine beds of cost-effective and light-weight construction. 
     The laid-open specification DE 198 10 996 A1 proposes to decouple the motor reaction surface from the machine bed in order to improve the movement guidance in a machine having a machine bed which is able to oscillate. 
     It is known from the laid-open specification WO 91/16594 to integrate a position measurement system into a rolling element guiding means. The measuring scale of the position measurement system is here fixedly anchored to the guide track of the rolling element guiding means. 
     The journal “Werkstatt und Betrieb” [Workshop and Operations], page 160 to 164, year 133 (2000)6, Carl Hansa Verlag, Munich, discloses a guiding device located on a first portal for guiding a moveable machine element, wherein a measuring scale is attached to a second portal which is physically separate from the first portal. The measuring scale and the guiding device are physically separate from one another on account of the spacing between the two portals. The physically wide and relatively irregular spacing between the two portals entails that the spacing between measuring scale and the reading head of the machine element is subject to fluctuations. The measurement system, however, reacts with great sensitivity to change in the spacing between measuring scale and guiding device because, for example, the graduation marked on the measuring scale (a graduation may be provided for example in the form of lines marked on the measuring scale) can no longer be read off exactly by the reading head. This has a very strongly negative effect on the positioning accuracy of the machine element. 
     SUMMARY OF THE INVENTION 
     The invention is based on the object of providing a guiding device for guiding a moveable machine element of a machine, which enables exact positioning of the moveable machine element even if the machine bed oscillates. 
     This object is achieved by virtue of a guiding device for guiding a moveable machine element of a machine, wherein the guiding device is connected in a mechanically fixed manner to a machine bed, and wherein a measuring scale for determining a position of the machine element is arranged along at least part of the guiding device, wherein the guiding device and the measuring scale can move in relation to one another, and with at least part of the measuring scale is in contact with the guiding device directly or via a bearing. 
     A first advantageous refinement of the invention is characterized in that the bearing is in the form of a sliding contact bearing. Sliding contact bearings can be provided especially cost-effectively as compared to other types of bearing. 
     It also proves advantageous if the bearing is in the form of a rolling element bearing, since this ensures a particularly exact guidance of the measuring scale along the guiding device. 
     It furthermore proves advantageous if the measuring scale is connected to an anchor arranged outside the machine bed. This ensures that the measuring scale is anchored with respect to the oscillating machine bed such that it is at rest. 
     It furthermore proves advantageous if the anchor is a workpiece to be machined, since in that case the position of the machine element with respect to the workpiece is measured directly. 
     It furthermore proves advantageous if a hold-down exerts pressure on the measuring scale in the direction of the guiding device, since in that case the measuring scale is continuously in uniform contact with the bearing. 
     It proves advantageous in this context to equip a machine with the guiding device according to the invention. 
     Use of the guiding device according to the invention is especially advantageous in machine tools, production machines and/or robots, since in these technical fields high positioning accuracy of the machine element is often necessary. However, the guiding device according to the invention can of course also be used in machines in other technical fields. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
       An exemplary embodiment of the invention is shown in the drawing and is explained in more detail below. In the figures: 
         FIG. 1  shows a schematically illustrated machine according to the prior art, 
         FIG. 2  shows a schematically illustrated machine with the guiding device according to the invention, 
         FIG. 3  shows a cross-sectional drawing of the guiding device according to the invention, and 
         FIG. 4  shows a plan view of the guiding device according to the invention. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       FIG. 2  shows a schematic illustration of a machine with the guiding device according to the invention. With respect to the basic design, the embodiment illustrated in  FIG. 2  corresponds substantially to the embodiment described previously in  FIG. 1 . Identical elements are therefore provided with the same reference symbols in  FIG. 2  as in  FIG. 1 . The essential difference with respect to the embodiment according to  FIG. 1  involves the measuring scale  6  not being connected in a mechanically fixed manner to the guiding device  3 , but the guiding device  3  and the measuring scale  6  being able to move in relation to one another, with at least part of the measuring scale  6  being in contact with the guiding device  3  directly or via a bearing. The bearing can be used firstly to move the guiding device  3  and the measuring scale  6  in relation to one another and, secondly, it is used to determine the spacing between reading head  2  and the measuring scale  6  exactly along the entire measuring scale  3 , and the spacing is not subject to any fluctuations. Furthermore, in the embodiment according to  FIG. 2  as compared to the embodiment according to  FIG. 1 , the measuring scale  6  is connected to an anchor which is arranged outside the machine bed  4  and, in the exemplary embodiment, is in the form of the two supports  7   a  and  7   b . Since the anchor is physically separate from the machine bed  4 , said anchor does not oscillate with the machine bed and thus ensures that the measuring scale  6  is anchored at rest. The measuring scale  6  does not always have to be arranged along the entire guiding device  3  as in the exemplary embodiment, however, but can also be arranged along only part of the guiding device  3 . The guiding device  3  is connected in a mechanically fixed manner to the machine bed  4 . 
       FIG. 3  illustrates the guiding device  3  according to the invention in the form of a cross-sectional drawing. The guiding device  3  is, in the exemplary embodiment according to  FIG. 3 , in the form of a guiding track. The moveable machine element  1 , in the exemplary embodiment in the form of a guiding carriage, is moveably connected to the guiding device  3  via four rows of balls  11   a ,  11   b ,  11   c  and  11   d . The movement of the machine element  1  along the guiding device  3  is guided by these. The drive system of the machine element  1  for displacing the machine element  1  along the guiding device  3  is not shown for reasons of clarity, since it is not essential for understanding the invention. The fixtures which are usually present on the machine element  1 , such as a spindle and/or tools, are not illustrated for reasons of clarity and because they are not necessary for understanding the present invention. The reading head  2  is screwed onto the machine element  1  by means of two screws  12   a  and  12   b . The reading head  2  scans the measuring scale  6 , which, in the exemplary embodiment, is in the form of a strip, with the aid of a light beam  8 . The measuring scale  6  is supported on the guiding device  3  via a bearing  9  and moveably connected to the guiding track  3  in this manner. 
     In the exemplary embodiment according to  FIG. 3 , the bearing  9  is in the form of a rolling element bearing. Thus, a multiplicity of rolling bodies are provided via which the measuring scale  6  is in contact with the guiding device  3 . The spacing between reading head  2  and measuring scale  6  is identical along the entire measuring scale  6  and is not subject to any fluctuations on account of the fact that the measuring scale  6  is supported against the guiding device  3  by means of the bearing  9 . The measuring scale  6  can thus be read off exactly by the reading head  2  at any desired position. Two hold-downs  10   a  and  10   b , which are connected, in the exemplary embodiment, to the reading head  2  and exert slight pressure on the measuring scale  6  in the direction of the guiding device, ensure that the measuring scale  6  continuously maintains uniform contact with the bearing  9 . Rather than the two hold-downs  10   a  and  10   b , it is of course also possible for only one hold-down to be provided, or additional hold-downs can be used. 
     In the embodiment according to  FIG. 3 , the bearing  9  is in the form of a rolling body bearing, wherein balls or rollers, for example, can be used as rolling elements. As an alternative, however, the bearing  9  can also be in the form of a sliding contact bearing, wherein an oil-containing substance, for example, can be used as lubricant between guiding device  3  and measuring scale  6 . As an alternative, the guiding device  3  and/or measuring scale  6  can also be coated with an anti-friction coating, such as Teflon, and a sliding contact bearing can be provided even without lubricant in this manner. 
     As an alternative, for example the measuring scale  6  and/or the guiding device  3  itself may also have a smooth surface even without coating, for example by means of corresponding polishing treatment, so that the necessary sliding characteristics between guiding device  3  and measuring scale  6  are provided even without the use of a bearing. The measuring scale  6  is, in this case, in direct contact with the guiding device  3 . 
       FIG. 4  illustrates the guiding device  3  according to the invention in the manner of a plan view. The machine element  1  can be moved along the guiding device  3 . The reading head  2  is screwed onto the machine element  1  with the aid of two screws  12   a  and  12   b . The measuring scale  6  is screwed, with the aid of two screws  13   a  and  13   b , onto the respectively associated supports  7   a  and  7   b , which, according to  FIG. 2 , are located outside the machine bed, so that the measuring scale  6  is anchored to the supports  7   a  and  7   b , which form an anchor, such that it is at rest. 
     It is of course also conceivable to provide the anchor for the measuring scale in the form of a workpiece to be machined. In that case, the position of the machine element  1  is measured with direct reference to the workpiece. 
     With the aid of the present invention it is no longer necessary to design the machine bed itself such that it is correspondingly resistant to oscillations in order to ensure exact positioning of the machine element in relation to a reference system at rest. The ability of the machine bed to oscillate can be tolerated to the benefit of a simple and cost-effective design of the machine bed, without the quality of a workpiece to be machined being affected by it. Since, on account of the guiding device according to the invention, the machine bed oscillations no longer have a negative effect on the measurement of the position of the moveable machine element of the machine, it is also possible, for example, for the controller gain of the position controller responsible for the positioning of the machine element to be increased such that a more dynamic movement guidance of the machine element can be achieved, which enables faster processing times. 
     As an alternative to the described guiding device according to the invention it is also conceivable, in order to achieve the object according to the invention, to use the guiding device according to  FIG. 1 , in which the measuring scale  6  is connected in a mechanically fixed manner to the guiding device  3 . The reading head  2  according to  FIG. 1  here produces a first position signal. A measuring transducer is attached to a support arranged outside the machine bed, as is drawn, for example, in  FIG. 2 , which measuring transducer measures the position between the support at rest and the oscillating machine bed and in this manner produces a second position signal. Furthermore, the two position signals are then added in an electronic circuit and in this manner the position of the machine element with respect to a reference system at rest is determined, which means with respect to the support, in this case. The signal processing time of the electronic circuit should be short so as not to disadvantageously affect the closed-loop control processes connected downstream.