ÅKERBLOM, Bengt (Vårby Allé 23, Vårby, S-143 40, SE)
CLAIMS
1. Device for measuring thickness of a layer or a coating layer on an elongated strip (10) of material passing by, comprising means for measuring the thickness in different positions across the width of the elongated strip (10), said means for measuring the thickness is placed in radial direction from a longitudinal axis (21) through a roller (20) that is supporting the elongated strip (10), said means for measuring the thickness comprises a sensor head (12) arranged to rest on a gas cushion generated between the sensor head (12) and the surface of the elongated strip (10), characterised in that at the outer periphery (25) of the roller (20) is curved in axial direction with a larger diameter towards the ends of the roller (20).
2. Device for measuring thickness according to claim 1, wherein at least an outer section (23) in radial direction of the roller (20) is elastic.
3. Device for measuring thickness according to claim 2, wherein the entire roller (20), except the axis (21) through the roller (20) is elastic.
4. Device for measuring the thickness according to claim 2 or 3, wherein the elastic section (23) are made of an elastic material.
5. Device for measuring thickness according to claim 4, wherein the elastic material is rubber.
6. Device for measuring thickness according to claim 4 or 5, wherein the section (23) of elastic material is provided with passages or holes (24) extending within the region (23) to increase the elasticity of the region (23).
7. Device for measuring thickness according to any of the previous claims, wherein the curved outer periphery (25) is achieved by dividing the roller into segments (26) in axial direction, and that each segment (26) has a diameter that generates the desired curvature.
8. Device for measuring thickness according to claim 7, wherein the segments (26) have a circular peripheral surface and are placed side by side on a roller centre portion (22).
9. Device for measuring thickness according to claim 8, wherein the inside surface of the segments (26) and the outside surface of the roller centre portion (22) are shaped in a not circular shape to prevent that the segments (26) rotate on the roller centre portion (22).
10. Method for manufacturing the device according to any of claim 7-9, comprising the steps:
- manufacture segments (26) with different outer diameters;
- packing the different segments (26) side by side in an order that generates the curved peripheral shape. |
Device for measuring a layer thickness and a method for manufacturing said device
TECHNICAL FIELD
The claimed invention relates to a device for measuring a layer thickness and a method for manufacturing said device.
BACKGROUND OF THE INVENTION
Several different materials like paper and sheet-metal are produced in elongated stripes.
During production are the elongated stripes running over and around a number or rollers along the production line. Several different production steps are performed on the stripe along the production line in order to obtain a product with the desired features. One of them is for example coating the stripe with paint for esthetical reasons and/or to avoid corrosion on sheet metal. In order to ensure that the coated layer has the desired thickness it is necessary to measure the final coating thickness on the elongated stripe. A reliably device for measuring layer thickness is therefore requested.
One way of doing this is to use the device disclosed in WO 2005/116576 Al. The disclosed device includes a transmitter, movably mounted in a transmitter housing. The transmitter housing is attached to a device that is movably transverse to the longitudinal axis of the elongated strip in order to make it possible to measure the layer thickness in different positions across the strip. The transmitter comprises a sensor head with a sensor for measuring the layer thickness. Gas is blown out from an opening in the centre of the sensor towards the surface of the elongated strip to form a gas cushion that the sensor head is resting on. The gas cushion makes it possible to position the sensor head at a constant and small distance above the surface of the elongated strip. The gas cushion makes it possible to keep the sensor head separated from the elongated strip surface thereby avoiding damages to the strip surface and/or the measuring device. It is also important for the measuring accuracy that the transmitter is easily movable within the transmitter housing, which preferably is achieved by gas-mounting of the transmitter in the transmitter housing, whereby the position of the transmitter and consequently also the sensor head can be adjusted to the particular thickness of the measured object.
Using the device described above for measuring of the layer thickness does however suffer from a considerably sever drawback. The drawback is that since the elongated strip is edge long and the roller camber formed, the strip is not fully supported on the roller. The pressure exerted from the gas cushion on the elongated strip generates a deformation of the elongated strip underneath the sensor head. This deformation (illustrated in figure 1) is devastating for the accuracy of the measuring, since the layer thickness of the coating is in the size of μm and even a small deformation will have an impact on the measured thickness. A deformed elongated strip in relation to a not deformed strip is disclosed schematically in Figure 1.
During the production of the elongated stripe, the longitudinal edges of the stripe tend to become slightly longer that the central portion of the strip. This effect is unavoidable and unfortunately devastating for the layer thickness measurement since it causes non- alignment of the strip to the roller surface. The distance between the elongated strip and the surface of the roller is substantially zero in the centre and increases towards the longitudinal edges of the elongated strip. This non-alignment has a considerably negative impact on the measuring accuracy since the pressure applied on the elongated strip from the gas cushion and the sensor head result in a deformation of the elongated strip. The deformation of the elongated stripe is consequently, because of the increasing distance between the stripe and the roller surface, substantially zero in the centre of the strip and increases towards the longitudinal edges of the elongated strip.
A measuring device with improved accuracy in the conducted measuring is therefore requested.
OBJECT OF THE INVENTION
The object of the present invention is to provide a measuring device that improves the accuracy of the measured layer thickness.
SUMMARY OF THE INVENTION
The accuracy of the measured layer thickness is improved by a measuring device defined in the independent claims.
The present invention is a device for measuring thickness of a coating layer on an elongated strip of material passing by the measuring device. The device comprises means for measuring the thickness in different positions across the width of the elongated strip. The means for measuring the thickness is placed in radial direction from the longitudinal axis through a roller that is supporting the elongated strip, and comprises a sensor head (12) arranged to rest on a gas cushion generated between the sensor head (12) and the surface of the elongated strip (10), The outer periphery of the roller is curved in axial direction with a larger diameter towards the ends in order to compensate for the slightly longer edges of the elongated strip. The curved outer periphery of the roller ensures that the entire width of the strip aligns the surface of the roller thereby avoiding deformation of the strip,
A further improved measuring device comprises a roller that in combination with the curved outer periphery has at least an outer section in radial direction that is elastic. The elastic outer section provides a roller that ensures that the entire strip aligns the roller surface, is flexible to adopt length differences in the elongated strip without causing extra elongation of the strip.
The elastic section is preferably made of an elastic material like for example rubber or a soft plastic material. If a more flexible roller is required, the section of elastic material could be provided with air pockets, passages or holes within the section to increase the elasticity further.
One favourable way of generating the curved outer periphery of the roller is to divide the roller into segments in the axial direction of the roller and provide each segment with an outer diameter that generates the desired curvature. This means that the segments are shaped like rings with a circular peripheral surface placed side by side on a roller centre portion. The inside surface of the segments and the outside surface of the roller centre portion are preferably shaped in a not circular shape to prevent that the rings rotate on the roller centre portion.
BRIEF DESCRIPTION OF THE DRAWINGS
One schematic embodiment of the present invention is disclosed in the accompanying drawings, in which:
Figure 1 Discloses a schematic illustration of a deformed elongated strip supported by a prior art roller;
Figure 2 Discloses an axial view one side of a roller according to the present invention; and,
Figure 3 Discloses a side view and a cross section through a roller according to the present invention.
DETAILED DESCRIPTION OF THE INVENTION
Figure 1 discloses a small section of an elongated strip 10 of material. The strip 10 is supported by a conventional roller 11 that is rotating around a not illustrated axis placed in the centre of the roller 11. At a small distance above the surface of the elongated strip 10 is a sensor head 12 positioned. The sensor head 12 is provided with a measuring sensor for detecting then coating layer thickness when the layer is passing the sensor. The sensor head 12 is furthermore provided with a gas outlet, not disclosed, connected to a device for supplying gas, not disclosed. Gas is flowing out via the gas outlet, indicated with arrows in figure 1, in order to generate a gas cushion between the sensor head surface facing the elongated strip and the surface of the coated layer on the elongated strip 10. The pressure from the gas cushion and the sensor head 12 is generating a deformation in the elongated strip due to non alignment between the elongated strip 12 and outer surface of the roller 11. This deformation will have a considerable impact on the accuracy of measured coating layer thickness.
In figure 2 is a side view of a roller 20 according to the present invention illustrated. The roller 20 has an axis 21 placed in the centre of the roller 20. The axis 21 is surrounded by a roller centre portion 22 and outside the roller centre portion 22 is an outer portion 23. The outer portion 23 is made of an elastic material and provided with openings 24 arranged in the outer portion 23 in order to increase the elasticity further. The material in the outer portion is for example rubber but also other materials with elastic features could be used. The openings 24 are in this embodiment substantially rectangular and extending in axial direction inside the outer portion 23 but several different shapes would also be usable.
In figure 3 is a side view of the roller 20 illustrated. The roller has an outer periphery 25 that is curved in axial direction, and the roller 20 diameters are larger towards the ends of the roller 20. The curvature is substantially the same along the roller length and the difference in the roller diameter between the smallest diameter in the centre of the roller 20 and the largest diameter in the ends of the roller 20 are within the range of 100mm -
1000mm. The curvature could however be alternated depending on the type of material in the elongated strip 10 and the width of the strip 10. The difference in roller diameter is achieved by dividing the outer portion 23 of the roller 20 in axial direction into a number of segments 26. In figure 3 is a schematic roller 20 comprising ten different segments 26 disclosed. The appropriate number of segments 26 depends on the desired roller shape and the material in the outer region 23. Each segment 26 has a circular periphery but the outer diameter are selected so that the desired curvature is generated when the segments 26 are placed side by side in a predetermined order on the roller centre portion 22.