Such refiners are used for refining fibrous material. The refiner normally comprises refining members in the form of discs which rotate in relation to each other and between which refining material passes from the inner periphery of the refining members where the refining material is supplied, to the outer periphery of the refining members, through a refining gap formed between the refining mem- bers. Often one of the refining discs is stationary while the other one rotates. The refining discs are generally constructed from segments provided with bars. The inner segments have a coarse pattern and the outer segments have a finer pattern in order to achieve fine refining of the refining material.

To ensure good quality refining material when refining fibrous material, the disturbances in operating conditions that continually occur for various reasons are corrected by continuous control of the various refining parameters to optimal val- ues. This can be achieved, for instance, by altering the supply of water to give greater or less cooling effect, changing the flow of refining material or adjusting the distance between the refining members, or a combination of these measures. To enable the necessary adjustments and corrections careful determination of the energy transmitted to the refining material is necessary, and also of the distribution of the energy transmitted across the surface of the refining members.

In order to determine the energy/power transmitted to the refining material it is already known to attempt to measure the shearing forces occurring in the refin- ing zone. What is known as a shearing force occurs when two surfaces move in relation to each other with a viscous liquid between them. Such a shearing force is also created in a refiner when refining wood chips mixed with water. It can be imagined that the wood chips are both sheared and rolled between the refining discs and also that collisions occurs between wood chips and bars. The shearing force is dependent, inter alia, on the force of the discs as they are brought together and also the friction coefficient. Furthermore the normal force exerted on the sur- face varies with the radius.

A method and a measuring device are already known through WO 00/78458 for measuring stress forces in such refiners, comprising a force sensor that meas- ures the stress forces over a measuring surface constituting a part of a refining disc and where said measuring surface comprises at least parts of more than one bar and is resiliently arranged in the surface of the refining disc. However, this

measuring device has proved to be very sensitive to temperature variations, which are usual in the relevant applications, and it therefore often gives incorrect values for the stress, which cannot be used to control the refining process for instance.

The object of the present invention is primarily to solve the problems men- tioned above and thus provide a method and a measuring device that provides a more reliable result than devices already known.

The object is achieved by means of a method as defined in patent claim 1 and having the features defined therein, and by means of a measuring device as defined in patent claim 3.

In accordance with the method of the present invention, the measurement takes place through the measuring surface being resiliently mounted in a direction parallel to the surface of the refining disc and, in the event of a stress force, being movable in said direction in relation to two rigidly mounted force sensors with which the measuring surface is connected and which are arranged to produce oppositely directed deflection when the measuring surface is influenced by said stress forces, and said stress forces are calculated on the basis of the difference between the deflections measured for respective force sensors on each occasion. Using two force sensors offers the important advantage that a value can be obtained for the stress forces that is not affected by the temperature variations occurring. This is done by utilizing the difference between the deflections measured for respective force sensors on each occasion as a value of the stress forces. This value can then be used to calculate the magnitude and distribution of the power transmitted to the refining material and these calculations can then be utilized to control the refining process.

The use of two sensors, in the manner described in accordance with the present invention, gives the advantage that any error in measurement is halved.

A preferred embodiment of the measuring device in accordance with the present invention is characterized in that the device comprises members that measure the stress forces in the form of two force sensors arranged to produce oppositely directed deflection when the measuring surface is influenced by said stress forces, so that said stress forces can be calculated on the basis of the dif- ference between the deflections measured for respective force sensors on each occasion, and it also comprises a body connecting said sensors to the measuring surface. The advantages of using two force sensors have been described above and are of great significance in this context.

In accordance with an advantageous embodiment the force sensors are ar- ranged symmetrically, i. e. symmetrically in relation to a central axis of the measur- ing surface that is perpendicular to the measuring surface.

The sensors, which are preferably piezoelectric force sensors (also known as transducers), constructed out of quartz crystal (so-called quartz sensors), also contribute to an extremely rigid measuring device being possible. The preferred sensors can handle up to 200°C and are also linear up to this temperature.

Other advantages and features are revealed in the subordinate claims.

The present invention will now be described with reference to the embodi- ment illustrated in the accompanying schematic drawings in which Figure 1 shows a perspective view of a refining segment in a refining disc provided with measuring devices in accordance with the present invention, Figure 2 shows a basic diagram of a measuring device in accordance with the present invention, Figures 3a and 3b illustrate the force ratios applicable to the invention, and Figure 4 shows a section through a measuring device in accordance with the present invention.

Figure 1 thus shows a part of a refining disc in the form of a refining seg- ment 1 provided with a pattern comprising a number of bars 3 extending primarily in radial direction. In this figure measuring devices 5 in accordance with the inven- tion have been schematically indicated. These measuring devices preferably have a circular measuring surface with a diameter in the order of 30 mm, for instance, but the measuring surface may also have some other geometric shape. The measuring devices are preferably arranged at different radial distances from the centre of the refining disc, and segments at different distances from the centre preferably also have measuring devices. The measuring devices may also advan- tageously be peripherally displaced in relation to each other, these measures being aimed at being able better to determine the power distribution in the refiner and thus better to control the refining process. When a measuring device is influenced by a force parallel to the surface of the refining disc/segment, each force sensor of the measuring device will generate a signal that is proportional to the load.

The measuring device in accordance with the invention functions in accor- dance with the principle illustrated in Figure 2. This shows a disc segment 1 from the side, provided with bars 3. A measuring device 5 is also shown which, for the sake of simplicity, is shown as comprising only one force sensor 10, and a measur- ing surface 7 in the form of a part of the surface of the disc segment, which is pro- vided with a number of bars 6, or at least parts thereof. When the refining disc is

subjected to a shearing load F the measuring device 5 (the sensor) will take up a load Fm which is represented by the following expression: <BR> <BR> <BR> <BR> <BR> <BR> <BR> <BR> Fm=F.<BR> <BR> <BR> <BR> <BR> <BR> <BR> <P>F. =F.- (1) 12 where 12 is the distance between the location where the sensor 10 is attached in the measuring device and a joint 8 in the device, and where 11 is the distance be- tween the measuring surface 7 of the measuring device and the joint 8. This for- mula is valid provided the joint 8 does not take up any torque and that the pressure distribution over the measuring surface 7 subjected to the shearing force is not too uneven. In principle the joint 8 consists of a plate that is so thin that it contributes negligibly to the total rigidity of the measuring device while at the same time being able to withstand the loads it is subjected to. The thickness of the plate may be relatively great since the rigidity of the sensor is relatively great, thus resulting in slight deflection of the plate. The dimensions of the joint 8 shall thus be suitable for withstanding the vertical load arising while at the same time absorbing only a neg- ligible part of the lateral load the screw and the sensor shall absorb. See also the detailed description with reference to Figure 4.

The models in Figures 3a and 3b depict how high or low rigidity affects the function of the measuring device through the rigidity of the sensor, attachment screw (attachment member by which each sensor is secured in relation to the measuring surface and the body, see Figure 4) and joint. The force and the torque absorbed by the sensor/attachment screw and joint, respectively, are controlled by the equation F sensor = k2 o and M = k3-A (p, where M is the torque in the joint. k2 is here the rigidity of the spring 15, i. e. the sensor 10 together with the attachment screw 20, and the rigidity k3 is the rigidity of the support point/joint 8. The equation shows clearly that if F is constant and k2 increases then 6 will decrease, as will also M since the torque is directly proportional to the deflection 6 for small angles.

In the present case k2 is large, which means that equation (1) is valid.

It should be emphasized that by relatively high rigidity of the sensor/attach- ment screw is meant in the present case high rigidity in relation to the load the sensor/screw shall absorb. The load may vary considerably over the refining zone- from some 20N to some 150N, for instance. With an estimated average value of about 40N displacements of the measuring surface obtained in the present case can be measured in the order of hundredths of a millimetre. As mentioned earlier, these small displacements facilitate sealing of the device from the surrounding en-

vironment, for instance. As regards the body 17, this can be deemed completely rigid in a direction perpendicular to the measuring surface.

Figure 4 shows a preferred embodiment of a measuring device in accor- dance with the present invention. The measuring device 5 comprises a measuring surface 7 provided with bars 6, or parts of bars, which measuring surface consti- tutes a part of a disc segment, as illustrated in Figure 1. As can also be seen in Figure 1, the measuring device preferably has a circular measuring surface.

The measuring surface 7 is in direct contact with a body 17, preferably of steel, which extends through the interior of the device. The measuring surface is preferably firmly screwed in the body 17. A short distance below the measuring surface the body 17 is provided with a transverse recess in which two force sen- sors 10, 11 are arranged, preferably quartz sensors. The sensors 10, 11 are fixed in relation to the body 17 by means of attachment screws 20 arranged to clamp each sensor against the body 17 on diametrically opposite sides thereof, as will be further described below. The attachment screws and any intermediate elements are preferably shaped so that a uniformly distributed load is obtained on each sen- sor, and preferably with a certain pre-stress. In accordance with this embodiment the sensors are arranged symmetrically in relation to a centre line extending through the measuring surface 7 and the body 17. The sensors will thus produce oppositely directed deflection when influenced by a force. When the pressure on the measuring surface increases, therefore, the load will increase on one of the sensors and will simultaneously decrease on the other. Naturally it would be possi- ble to arrange the sensors in some other way in relation to each other and still have their deflection oppositely directed. Other attachment devices for the sensors 10,11 are naturally also possible.

The body 17 preferably has circular cross section. Further down, below the sensors the body 17 assumes a narrowing, flattened shape within an surface cor- responding to the joint 8, mentioned previously and described with reference to Figures 2,3a and 3b.

The load Fm which the measuring device will take up through the sensors 10, 11 when it is subjected to a shearing force F is calculated in this case as: S2-S1 Fm = k (2) 2

where Si is the shearing force indicated by the first sensor 10, S2 is the shearing force indicated by the second sensor 11 and k is a scale factor based on previous calibrations.

This means that the shearing load F influencing the refining disc can be cal- culated to: <BR> <BR> I2 S2-S1<BR> F = # #k (3) 2 This is the equation used to calculate the magnitude and distribution of the power transmitted to the refining material, these calculations then being utilized to control the refining process.

The sensors 10, 11 and the body 17 are arranged in a protective housing 22. This housing has an opening at the top abutting the surrounding refining seg- ment, which is closed by the measuring surface 7, a seal 12 surrounding the measuring surface, and by a sleeve 13 in which the seal is arranged. The seal 12 consists of a particularly suitable, somewhat yielding material such as rubber, so that it can permit the small movements caused by the shearing forces in the measuring surface while still achieving a good seal that prevents steam and pulp from penetrating into the device. The seal preferably also has a damping effect on the vibrations that arise during operation. The purpose of the sleeve 13 is primarily to facilitate closing of the measuring device since the measuring surface and the seal are first mounted in the sleeve which can then easily be inserted partially into the housing 22. It is naturally possible to omit the sleeve.

The housing 22 also has a function when it comes to fixing the sensors 10, 11 in relation to the measuring surface 7. The sensors are thus attached in the housing by means of attachment screws 20. Finally, the body 17 is attached in the housing at the end opposite to the measuring surface.

The invention shall not be considered limited to the embodiments illustrated by way of example but can be modified and altered in many ways within the scope of the appended claims, by one skilled in the art.