Method for determination of roll density

1. In a method for determination of roll density, in which density of a wound roll formed of a web is determined at a multiplicity of occasions while reeling the web by measuring a parameter corresponding to at least one value selected from the group consisting of: measured web length in the roll (l), measured roll diameter (D), and basis weight of the web (b); and using a known relation between variables: measured web length in the roll (l), measured roll diameter (D), initial roll diameter(D o ), basis weight of web (b) and density (ρ) of the wound roll;

l

=

πρ

⁡

(

D

2

-

D

0

2

)

4

⁢

b

the improvement comprising:

eliminating distortions in said measured parameter caused by noise and other disturbances, by using a recursive time variant least squares method in the processing of said measured parameter.

2. The method of claim 1 wherein a method of mathematical statistics is applied to the least squares method, whereby a relation is obtained between the at least one measured value exactness and the method's built-in elimination of noise and disturbances.

3. The method of claim 1 wherein statistical confidence limits are determined for the at least one measured value.

4. The method of claim 1 , wherein before beginning a measurement occasion, initializing the at least one measured value by two points determined by a nominal density or by a preceding measurement; and further comprising carrying out the steps of:

updating a correlation matrix and a regression vector;

reversing the correlation matrix to calculate a paper thickness;

calculating density of the wound roll based on the paper thickness, followed by;

updating an error variance, followed by;

calculating a confidence limit for a selected probability;

if the confidence limit is below a selected lower limit or above a selected upper limit then changing a buffer length;

determining a speed of oblivion based on the buffer length; and

beginning another measurement occasion after a selected web length has been wound or unwound from the wound roll.

5. The method of claim 1 wherein the recursive time variant least squares method uses an extended time variant Kalman filter for further processing of the measurement results and for measuring the internal stresses of the roll.

6. The method of claim 1 wherein the roll diameter is determined by contact-free measurement.

7. The method of claim 6 wherein the diameter is measured by pulse measurement, ultrasound or a laser sensor.

8. The method of claim 1 wherein the density is measured separately for each of a plurality of stations based on the roll diameter and web length given by a linear sensor.

9. The method of claim 1 wherein the method is applied in connection with winding-in of the web.

10. The method of claim 1 wherein the method is applied in connection with unwinding of the web.

11. The method of claim 1 wherein the method is applied in connection with winding of a paper or board web.