ÅKERBLOM, Bengt (Vårby Allé 23, Vårby, S-143 40, SE)
CLAIMS
1. Sensor head (12) for measuring the thickness of a layer coated on an elongated strip (10) of material passing by the sensor head (12), the sensor has a front surface (13) that is facing the surface of the elongated strip (10), said front surface (13) comprises an air outlet (14) placed in the centre of the front surface (13), said outlet (14) is connected to an gas supply device generating an gas flow out from the air outlet (14) directed substantially perpendicular from the front surface (13) in order to form a gas cushion that the sensor head (12) is resting on, characterised in that the front surface (13) further comprises two or four sensors (15) placed at the same distance from the centre of the front surface (13), with the two sensors (15) radially opposite to each other, and in case of four sensors (15) the additional two sensors (15) are placed radially opposite to each other between the first two sensors (15).
2. Sensor head (12) according to claim 1, wherein the sensor head front surface (13) comprises two sensors (15).
3. Sensor head (12) according to claim 1 or 2, wherein the sensor head (12) is substantially circular.
4. Sensor head (12) according to claim 1, 2, or 3, wherein each sensor (15) is placed between 5mm - 20mm from the centre of the front surface (13).
5. Measuring device for measuring the thickness of a layer coated on an elongated strip (10) of material passing by the device on a roller (11), said device comprises a housing in which a sensor head (12) is movably secured in axial direction and a gas supply device connected to an outlet (14) in the centre of the sensor head (12) in order to form a gas cushion that the sensor head (12) is resting on between the sensor head (12) and the surface of the elongated strip (10), characterised in that the sensor head (12) is a sensor head according to anyone of claims 1- 4.
6. Measuring device according to claim 5, wherein the front surface (13) on the sensor head (13) is placed substantially parallel to the surface of the elongated strip (10).
7. Measuring device according to claim 5 or 6, wherein the axial direction of the sensor head (12) is positioned in radial direction in relation to the longitudinal axis of the roller (11).
8. Method for calibrating the measuring device according to claim 5-7, comprising the steps:
C) determine a value of the coating layer thickness in a starting point;
D) turn the sensor head (12) a predetermined step around its longitudinal axis to a new position; E) repeat step C and D a number of times and then positioning the sensors
15 above the point in which the smallest coating layer thickness was determined. |
Sensor head, a measuring device comprising the sensor head and a method for calibrating the measuring device
TECHNICAL FIELD The claimed invention relates to a sensor head, a measuring device comprising the sensor head and a method for calibration the measuring device.
BACKGROUND OF THE INVENTION
Several different materials like paper and sheet-metal are produced in elongated stripes. During the production are the elongated stripes running over and around a number or rollers along the production line. Several different steps are performed on the stripe along the production line. One of them is for example coating the stripe with paint for esthetical reasons and/or to avoid corrosion on the 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.
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 Ln 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 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.
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 relates to a sensor head for measuring the thickness of a layer coated on an elongated strip of material that is passing by the sensor head. The sensor head has a front surface that is facing the surface of the elongated strip. The front surface comprises an air outlet placed in the centre of the front surface. The outlet is connected to an gas supply device generating an gas flow out from the air outlet and directed substantially perpendicular from front surface in order to form a gas cushion that the sensor head is resting on between the surface of the elongated strip and front surface. The front surface on the sensor head further comprises two or four sensors placed at the same distance from the centre of the front surface with two sensors radially opposite to each other, and in case of four sensors the additional two sensors are placed radially opposite to each other between the first two sensors.
The claimed sensor head comprising two or four sensors placed at a distance from each other makes it possible to measure the layer thickness in two or four different separated measuring points preferably placed in the area where the deformation from the gas cushion not affect, or at least has a smaller impact on the accuracy of the measured thickness. The positions of preferred measuring points are illustrated in figure 2.
A sensor head comprising two sensors is positioned so that a line through the two sensors is substantially parallel to the direction of movement of the elongated strip while a sensor head comprising four sensors is positioned substantially so that a line through two sensors placed side by side on the front surface is either substantially parallel or perpendicular to the direction of movement of the elongated strip.
The sensor head front surface is substantially circular with the gas outlet placed in the centre. The circular shape and position of outlet ensures that substantially the entire sensor head will rest on the gas cushion which is important to achieve a high accuracy in the measured layer thickness.
The two or four sensors placed on the sensor head front surface are placed at a distance of between 5mm - 20mm from the centre of the front surface in order to provide a range for adjustment of the measuring points.
The present invention is also related to a measuring device for measuring the thickness of a layer coated on an elongated strip of material passing by the device on a roller. The measuring device comprises a housing in which a the previously defined sensor head is movably secured in axial direction, and a gas supply device connected to an outlet in the centre of the sensor head in order to form a gas cushion that the sensor head is resting on between the front surface on the sensor head and the surface of the elongated strip.
The measuring device and the housing for the sensor head is arranged such that the front surface on the sensor head is placed substantially parallel to the surface of the elongated strip, and the axial direction of the sensor head is positioned in radial direction in relation to the centre axis of the roller. This configuration provides a correct position of the sensor head in relation to the elongated strip and roller which is desired to achieve an accurate measurement of the coating layer thickness.
The position of the measuring points disclosed in figure 2 could however vary depending on the material of the elongated strip, the pressure actuated by the gas cushion, the speed at which the elongated strip is moving etc. This makes it necessary to calibrate the measuring device in relation to the specific assumptions relevant for the measurement that will take place. Figure 2 is illustrating the impact from the gas cushion and the sensor head seen
from one direction, but the impact from the gas cushion and the sensor head is also affecting the elongated strip in the direction of the rotational axis of the roller. The use of four different sensors makes it possible to compensate for the impact from deformation in that direction as well.
This is done by the method for calibration defined in claim 7 comprising the steps C-E. The method defined in the claim solves the problem of finding the measuring points in which the impact from the gas cushion is as small as possible.
BRIEF DESCRIPTION OF THE DRAWINGS
One embodiment of the present invention, comprising two sensors, is disclosed in the accompanying drawings, in which:
Figure Ia Discloses a schematic side view of the elongated strip on a roller without a pressure applied from the gas cushion on the elongated strip;
Figure Ib Discloses a schematic side view of the deformed elongated strip on a roller with a pressure applied from the gas cushion on the elongated strip; Figure 2 Discloses a schematic cross-sectional view of the inventive sensor head; and, Figure 3 Discloses a cross-sectional view of one embodiment of the inventive sensor head;
DETAILED DESCRIPTION OF THE INVENTION
Figure Ia and Ib discloses schematically what is happening during measurement of a coating layer thickness during use of a sensor head resting on a gas cushion.
In figure Ia and Ib is a an elongated strip 10 of material placed on a roller 11. The elongated strip is rolling on the roller 11 at a fairly constant speed. A sensor head 12 is placed at a distance from the surface of the elongated strip 10 with the sensor head front surface 13 substantially parallel to the surface of the elongated strip 10. The front surface 13 of the sensor head 12 comprises a gas outlet 14 placed in the centre of the circular sensor head front surface 12 and two sensors 15. Each sensor 15 is placed at a distance from the centre of the front surface 13. Both sensors 15 are positioned at substantially the
same distance from the centre, and radially opposite to each other along a line through the centre of the front surface 13.
In figure Ia is the measuring device not in use and consequently is no gas flowing from the gas outlet 14 towards the surface of the elongated strip 10. hi figure Ib is the measuring device activated and gas is flowing from the gas outlet 14 and substantially perpendicular towards the surface of the elongated strip 10. The gas flow, illustrated with arrows in figure Ib 5 forms a gas cushion between the front surface 13 on the sensor head 12 which keeps the sensor head 12 at a constant distance above the surface of the elongated strip 10. The gas cushion and the sensor head 12 however generate a force on the elongated strip 10 which unfortunately deforms the elongated strip. This deformation is schematically illustrated in figure Ib. This deformation is devastating for the accuracy of the measuring result since the coating layer thickness is in the size of μm and every misleading factors will have a severe impact on the final result.
In figure 2 is an enlarged view the deformation of the elongated strip illustrated more in accordance with the real appearance. The roller 11 and the elongated strip 10 are moving in the direction of arrow M. The largest deformation of the elongated strip IO occurs directly underneath the gas outlet 14 on the sensor head 12. The deformation decreases in both directions along the periphery of the roller and in two points P is the effect of the deformation substantially zero. These points P are placed at approximately the same distance from the centre of the sensor head front surface 13 along the surface of the elongated strip 10. The sensors 15 on the front surface 13 are preferably positioned in radial direction from the axis of rotation for the roller 11 above the elongated strip 10. By placing the sensors 15 immediately above these points P, at a distance D from each other, is the impact from the deformation reduced considerably.
The positions of the two points P are however dependent on the specific material in the elongated strip 10, the speed of the elongated strip 10, the radius of the roller 11 etc. The sensors 15 position in relation to the elongated strip 10 must therefore be calibrated in relation to the specific conditions. This is done by turning the sensor head 12 within a sensor head housing, not illustrated, so that the sensors are positioned as close to the measuring points P as possible.
The calibration must be done when the elongated strip 10 is moving on the roller 11. The measuring points are found by making a first measurement of the coating layer thickness in any position of the sensor head. The sensor head 12 is then turned a predetermined step around its longitudinal axis into a new position and the thickness measured in the new position. These steps are repeated a number of times depending on the desired accuracy of the calibration. After the measuring is completed are the sensors 15 positioned above the measuring points in which the smallest measured value was achieved.
In figure 3 is a sensor head 12 illustrated in detail. The sensor head comprises the gas outlet 14, sensors 15 and the front surface 13. Opposite the front surface 13 is the sensor head provided with means for connecting the sensors to a computer device for determine and storing the measuring data. Furthermore is the sensor head 12 provided with means for connecting a gas delivery device (not illustrated in the figure) that is providing the gas flow through the gas outlet 14, and means for securing the sensor head 12 in an axially movable manner in a not illustrated sensor head housing. The sensor head 12 is movable in axial direction in order to be able to maintain its position at a substantially constant distance above the surface of the elongated strip 10. This could be achieved by a gas-mounting arrangement in the sensor head housing.