The invention relates to an apparatus for reducing the friction between two bodies and, in particular, for reducing the friction between the sensor and the object to be measured of a thickness measuring apparatus according to the preamble of the independent claim 1.
In previous designs for reducing the friction, thickness sensors for plastic foils, for example, are coated with materials which glide well on these foils (e.g. PTFE, i.e., polytetrafluorethylene, and/or Teflon™), are executed as rolling thickness sensors (EP 01182423) or an air pocket is built up between the sensor and object to be measured (UK 1 436 698).
For measuring the thickness of foils on which bodies such as sensors glide very poorly, so-called matte foils or foils which are even sticky, for example, with EVA (ethylene vinyl acetate) in the outer layer contacting the sensor, a surface coating with almost ideal gliding material (e.g. PTFE) is also not sufficient. Rolling thickness sensors and extreme contact-sensitive thickness sensors with air cushions are complicated in construction and are distance-sensitive.
The thickness measurement of plastic foils for blown foil lines is carried out for procedural reasons, for flat foil lines for cost reasons ideally with a reflective measuring operation. Known measuring methods are those which function, for example, capacitively, optically or with ionising radiation.
In all of these measuring methods the distance to the object to be measured has a certain influence on the measuring precision. The contacting measurement operation (distance 0) would be metrologically optimal. In certain limits the distance from sensor to foil can be measured and the measurement signals can be arithmetically compensated by computation (mathematically) (EP0801290, EP1681531).
The object of the invention is to reduce the friction between two bodies which are in contact with each other or only a slight distance of one millimeter at most exists between them e.g. between the sensor surface and the object to be measured such that very matte foils contacting (distance 0) or almost contacting (distance almost 0) can be measured.
This object is solved according to the invention with an apparatus having the features in the characterizing portion of the independent claim. Further, the invention relates to a device for manufacturing plastic foils with such an apparatus. The depending claims relate to advantageous embodiments of the invention.
The vibration movements can be carried out in all three dimensions, transversely, rotating, wobbling or also combined. The lift, the frequency, angle of rotation and the movement pattern, e.g. rectangle, saw tooth or sinus of the vibration, have a decisive influence on the reduction of the friction.
The vibration can, for example, be generated by means of piezo actors, drive with an electromagnetic alternating field or with an imbalance at a rotating part.
The invention will be explained in more detail on the basis of both drawings which schematically show an example of a measuring apparatus at a plastic blown foil.
In the drawings:
If the vibration in the direction of z is executed transverse or rotating around an axis parallel to the direction of the y axis through point M in the distance r1, r2, then it is possible that the sensor only occasionally contacts the foil. If the vibration is executed in the y direction, then the contact pressure or the distance to the foil remains constant. Vibrations in the x direction vary the relative speed between foil and sensor.
If the vibration is executed as a rotation around an axis on the measuring head surface, then the contact pressure and/or the distance to the foil changes on all points of the sensor next to the axis.
The apparatus is suited to reduce the friction between two bodies 1, 2 moving relative to each other. The two bodies 1, 2 which move relative to each other, can contact each other constantly or occasionally or have a small distance from one another of, for example, up to one millimeter. The distance can vary. With the relative movement of both bodies 1, 2, for example, the one body can be a sensor 1 which is firmly installed, and the second body can be a foil 2, which moves with considerably constant speed. The device has a vibration device 3, 4, 5, 6 for vibrating at least one of the bodies 1, 2. The vibration movements of the vibrated bodies can have any pattern.
Number | Date | Country | Kind |
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06405461.2 | Nov 2006 | EP | regional |