Abstract:
An apparatus for measuring film bubbles ( 10 ), includes a guide track ( 22 ) that extends in a circumferential direction of the film bubble with a constant spacing relative to the latter, the guide track guiding a carriage ( 20 ) which carries a measuring head ( 12 ) facing the peripheral surface of the film bubble ( 10 ), the measuring head ( 12 ) is connected to a free end of a cantilever ( 18 ) via an articulated joint ( 16 ), the cantilever having another end pivotally connected to the carriage ( 20 ), and a compensating drive mechanism ( 30 ) for correcting a change in the orientation of the measuring head ( 12 ) resulting from a pivotal movement of the cantilever ( 18 ) is associated with the articulated joint ( 16 ).

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to an apparatus for measuring film bubbles, comprising a guide track that extends in circumferential direction of the film bubble with a constant spacing relative to the latter, said guide track guiding a carriage which carries a measuring head facing the peripheral surface of the film bubble. 
     In the art of film blowing, it is desirable to perform measurements on the film bubble during the production process. In particular, it is desired to continuously monitor the thickness distribution of the film over the periphery of the film bubble, so that the cooling and/or extrusion temperature may be feedback-controlled for obtaining a constant thickness profile. In this case, the measuring head may for example be a capacitive thickness gauge, that hovers on an air cushion over the outer peripheral surface of the film bubble as has been described in WO 2009/027037. 
     EP 1 674 821 A1 describes a measuring device of the type indicated above wherein two carriages are guided on the guide track, and these carriages are connected to one another by a beam. The measuring head is mounted in the center of the beam. The beam consists of two parts that are pivotally or telescopically connected to one another, so that the distance between the two carriages on the guide track may be varied and, consequently, the radial position of the measuring head may be adapted to the actual diameter of the film bubble, while the measuring head stays facing the peripheral surface of the film bubble. 
     SUMMARY OF INVENTION 
     It is an object of the invention to provide a measuring device of this type which has a simpler construction. 
     According to the invention, this object is achieved by the features that the measuring head is connected, via an articulated joint, to a free end of a cantilever the other end of which is pivotally connected to the carriage, and that a compensating drive mechanism correcting a change of the orientation of the measuring head resulting from the pivotal movement of the cantilever is associated with the articulated joint. 
     According to the invention, only a single carriage per measuring head needs to be mounted on the guide track. For adapting the position of the measuring head to different diameters of the film bubble, the cantilever is pivoted relative to the carriage. However, this pivotal movement is also accompanied by a change in the orientation of the measuring head relative to the peripheral surface of the film bubble. This change is reversed by the compensating drive mechanism, so that the measuring head will always be oriented in parallel with the peripheral surface of the film bubble at the position where it measures the film. 
     Advantageous details of the invention are indicated in the dependent claims. 
     Depending upon the embodiment, the cantilever may be pivotable in a horizontal plane, i.e. in parallel with the plane of the guide track, or in a vertical plane, i.e. normal to the plane of the guide track. In the former case, a pivotal movement of the cantilever will in general also result in a change of the azimuth of the measuring position where the measuring head measures the film. In order to be able to measure and record the thickness profile over the periphery of the film bubble, it is necessary to know the azimuth of the measuring position for each instant when a measurement is made. When the cantilever is pivoted in order to adapt the apparatus to a different diameter of the film bubble, calculations are performed for correcting the resulting change in the azimuth. 
     The compensating drive mechanism may be an active drive mechanism, e.g. in the form of a suitably controlled servo motor. However, it is also possible to provide a passive drive mechanism which is formed by coupling the pivotal movement of the measuring head about the pivotal axis of the articulated joint to the pivotal movement of the cantilever relative to the carriage. The coupling may for example be achieved by means of an articulated control linkage assembly or by a belt and pulley arrangement. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       An embodiment example will now be described in conjunction with the drawings, wherein: 
         FIG. 1  shows a schematic plan view of a measuring apparatus according to the invention; and 
         FIG. 2  shows an enlarged plan view of essential parts of the measuring apparatus in a different operating state. 
     
    
    
     DETAILED DESCRIPTION 
     The apparatus shown in  FIG. 1  serves for performing measurements on a tube-like film bubble  10  which has been shown in horizontal cross-section in  FIG. 1 . As is known in the art of film blowing, a film tube is extruded from an annular nozzle and is inflated with internal air so as to form the film bubble  10  which will then be drawn-off upwardly, flattened, and wound on a coil. In the zone where the diameter of the film bubble increases as a result of internal air being blown in, the film bubble is usually cooled by blowing air against the film bubble from outside, until the plastic material loses its plasticity at the so-called freeze limit, whereafter it can no longer be stretched. The peripheral areas of the film bubble  10  which are cooled less intensely remain at higher temperature and, hence, are easier to be stretched, so that the film is thinned here to a larger extent, whereas a thickened region is formed in the film in peripheral areas where the cooling is more intense. Thus, non-uniform cooling conditions can lead to an uneven thickness profile of the film. The measuring apparatus that has been shown here has the main purpose to continuously monitor the thickness profile of the film above the freeze limit on the entire periphery of the film bubble  10 , so that the cooling conditions can be controlled in a closed feedback loop. 
     To that end, the periphery of the film bubble  10  is engaged by a measuring head  12 , e.g. a capacitive measuring head which slides over the film surface on an air cushion. The measuring head  12  is mounted at an end of a holder  14  which itself is pivotally mounted to the free end of a cantilever  18  via a joint  16 . 
     The other end of the cantilever  18  is attached to a carriage  20  that is guided on a guide track  22 . In the example shown the guide track  22  is annular and surrounds the entire periphery of the film bubble  10 . The carriage  20  can be moved along the guide track  22  by means of locomotive drive  24 , so that the measuring head  12  may revolve around the periphery of the film bubble  10 . 
     In order for the position of the measuring head  12  to be adaptable to varying diameters of the film bubble  10 , the cantilever  18  is pivotable about a vertical axis  26  (in parallel with the axis of the film bubble  10 ) relative to the carriage  20 . To that end, in the example shown, a pivotal drive  28  has been provided which may be configured as a spindle drive or, optionally, a hydraulic or pneumatic piston/cylinder unit. 
     When, however, the cantilever  18  is rotated by a certain angle about the axis  26  in order to adjust the measuring head  12  to a different diameter of the film bubble, the orientation of the measuring head  12  will change accordingly. If no countermeasures were taken, the measuring surface of the measuring head  12  facing the film surface would no longer be oriented parallel to the film surface, and a correct measurement would not be possible. For this reason, the joint  16  is associated with a compensating drive mechanism  30  which automatically corrects the orientation of the measuring head  12 . 
     When, as in the condition shown in  FIG. 1 , the cantilever  18  does not extend exactly tangential to the guide track  22 , and then the cantilever is pivoted about the axis  26 , the movement of the joint  16  at the free end of the cantilever has also a component in circumferential direction of the guide track  22 , and this has the consequence that the azimuth AZ of the measuring position is changed although the position of the carriage  20  remains unchanged. This change of the measuring position must be taken into account when the measurement results provided by the measuring head  12  are recorded, so that the film thickness may correctly be associated with the respective segments of the periphery of the film bubble. 
     Moreover, the change of the azimuth AZ has also the consequence that the peripheral surface of the film bubble  10  at the position of the measuring head  12  has a different orientation. This is why, in general, the compensating drive mechanism  30  must not only turn back the holder  14  by the angle about which the cantilever  18  has been rotated relative to the carriage  20 , but must also compensate for the change in the orientation of the surface of the film bubble that has been caused by the change in azimuth. 
     In the example shown, the compensating drive mechanism  30  is a passive drive mechanism which does not have a motor but mechanically couples the pivotal movement of the cantilever  18  relative to the carriage  20  to a rotation of the holder  16  about the joint  16 . To that end, a pulley  32  is mounted on the holder  14 , and a belt  34  runs over the periphery of the pulley. One end of the belt  34  is fixed at the pulley  32 . The other end is fixed at a cam disk  36  and passes over a control contour that is formed by this cam disk  36 . The cam disk  36  is non-rotatably connected to the carriage  20 . 
     A tension spring  38  is excentrically attached to the pulley  32  for holding the belt  34  under tension. When, now, the cantilever  18  is rotated about the axis  26  relative to the carriage  20 , the belt  34  is either wound-up or rolled-off from the control contour of the cam disk  36  so that its length is either increased or reduced, with the result that the pulley  32  and the holder  14  and the measuring head  12  are rotated accordingly about the joint  16 . The contour of the cam disk  36  has been designed such that this rotation compensates not only the rotation of the cantilever about the axis  26  but also the change in azimuth, so that the measuring surface of the measuring head  12  will again be oriented exactly in parallel with the peripheral surface of the film bubble  10 . 
       FIG. 2  illustrates a condition where the film bubble  10  has a larger diameter and, consequently, has a smaller spacing from the guide track  22 . Consequently, the cantilever  18  has been rotated outwardly. As a result, the belt  34  has been rolled-off from the control contour of the cam disk  36 , and the effective length of the belt  34  has increased. This increasing length has permitted a contraction of the tension spring  38 , so that the pulley  32  has been rotated about an angle that is determined by the geometry of the cam disk  36 . The amount of this angle is such that, in the new position, the measuring head  12  engages the surface of the film bubble  10  again in exact parallelism with this surface. 
     Mounted on the carriage  20  is an electronic control unit  40  which controls the locomotive drive  24 , the pivotal drive  28  for the cantilever  18 , as well as the operation of the measuring head  12 , and receives and records the measurement results. The control unit  40  also processes a displacement signal that is delivered by the locomotive drive  24  and indicates the position of the carriage  20  on the guide track  22 , as well as an angular increment signal provided by an angular position sensor (not shown) at the cantilever  18  and indicating the angular position of the cantilever  18  relative to the carriage  20 . In a preferred embodiment, a displacement sensor is integrated in the pivotal drive  28  (spindle drive) for measuring the (linear) displacement of the spindle drive, and the angular increment signal is calculated from the signal of this displacement sensor. Based on these data, the control unit  40  calculates the azimuth position of the measuring head  12 , so that the results of the thickness measurement can correctly be associated with the pertinent circumferential position on the film bubble  10 . 
     When the measuring apparatus is converted and adapted to a different film bubble diameter, the cantilever  18  may optionally be pivoted manually. Thus, the pivotal drive  28  is not mandatory. 
     On the other hand, when a controllable pivotal drive  28  is present, the control unit  14  can control the pivotal movement of the cantilever  18  as a function of the actual shape and size of the film bubble  10 . To that end, the measuring head  12  may have an integrated force sensor which measures the force with which the film bubble presses against the measuring head. An example has been described in EP 1 191 305 B1. When the control unit  40  feedback-controls this force to a given target value, the measuring head will automatically adapt to the actual position of the film bubble. As an alternative, the position of the measuring head relative to the film bubble may also be detected and controlled by means of a distance sensor, e.g. an ultrasonic distance sensor.