WO2015075305A1

WO2015075305A1 - Chipper machine knife monitoring

Info

Publication number: WO2015075305A1
Application number: PCT/FI2014/050853
Authority: WO
Inventors: Seppo Silenius; Mikko Pesonen
Original assignee: Andritz Oy
Priority date: 2013-11-19
Filing date: 2014-11-12
Publication date: 2015-05-28
Also published as: UY35849A; AR098485A1; FI20136150A; FI125771B

Abstract

Knife quality monitoring in chipper ma- chines is an important task as the knife quality has a significant effect on the quality of the chips. The knives are arranged into a rotational knife disk and the impacts caused by each of the knives can be measured by using sensors. When the impact caused by the first knife is known, the impacts of each knife can be determined. The measured impacts can be compared during one revolution or between multiple revolutions and the knife quality can be analyzed from the measured impacts.

Description

CHIPPER MACHINE KNIFE MONITORING

FIELD OF THE INVENTION

The invention relates to chipper machines and in particularly to knife quality monitoring of such machines.

BACKGROUND OF THE INVENTION

Chipper machines are used to cut wood into smaller chips. Machines come in different sizes according to the en^¬ vironment where they are used. Chipper mills are large plants capable of receiving large amounts of raw material and they are typically used as a first phase of a pulp mill production process. The raw material is typically logs or other wood material that is continuously fed into a chipper machine. The feed material flow comprises a number of pieces of logs of different size and shape. The quality of chipped wood and the operating speed are essential for providing good raw material to the pulp mill. Pulp mills are typically huge and in continuous operation. Thus, unplanned production breaks need to be avoided. In order to provide time for maintenance of the chipper, chips can be stored temporarily so that there is supply material for the pulp mill even if the chipper is not operating.

Large chippers use large knife disks which typical^¬ ly have a diameter of 2 to 4 meters. The chipping knives are mounted substantially radially on the face of the disk. The logs are fed typically by gravity or horizontally against the face of the disk. Defects to knives can be caused, for example, by regular wearing or instantly by hitting foreign bodies, such as stones, metal objects or other similar hard material that may cause damage to the knives.

In order to provide good quality raw material at desired speed, the knives of the chipper mill have to be in good condition. Knives are typically rotationally arranged so that each of the knives cuts a slice of the ends of the fed logs during a revolution of the knife disk if there is wood to cut. The slices will at the same time break into chips of desired size. When the chip is cut with a damaged knife, the cut quality is not good since there will be plen- ty of undesired-sized particles. The pulping process condi^¬ tions are adapted to chips big enough and of quite equal size. This affects the share and quality of accepted fibers that will be produced from the raw material. Chips not hav- ing an optimum size distribution reduce the quality of the produced raw material for the pulping process. Thus, it is essential that the knives are in good condition.

It is natural that occasionally the knives do not have material to cut, for example, when there is a space be- tween logs fed into the chipper. Within one rotation of the disk there may be only a few knives which cut chips.

The huge variation in the diameter of the logs from some centimeters to over half a meter creates also different kinds of cutting situations. A cutting impact of a big log is quite different from that with an equal combined length or diameters of cutting of several small logs at a time. If there is a defect in a knife, it may not do much of cutting if it hits an area close to a place between two logs. That will not happen so easily with the biggest logs. The biggest logs can be so big that there will be two knives at the same time cutting the log. This means that there is a problem of addressing the impacts to the right knifes.

Conventionally knives are inspected and replaced manually at predetermined intervals or after a certain amount of chipped logs. When the size of the chips is moni^¬ tored, size distribution may be found to be undesired. That also can indicate a need to check the knives. In more ad^¬ vanced embodiments, sensors are used to measure the overall level of the impulses or sounds generated by the cutting knives. The drawback of such embodiments is that the materi^¬ al to be chipped varies even within one revolution. For ex^¬ ample, when cutting a log there might be softer and harder sections and they cause different measurement results. The variation is essentially larger when more than one log is fed simultaneously. The logs to be chipped also vary exten^¬ sively in amount and diameter and this has a huge impact to the measurement results. Thus, it is hard to detect whether the deviation is caused by a damaged knife, worn knives, or ordinary variation in the supply material. Furthermore, in conventional embodiments, the signals measured are not aligned to specific knives. Thus, the maintenance person knows that one or more knives may be worn or damaged but he doesn't know which one.

SUMMARY OF THE INVENTION

The invention discloses a method for knife quality monitoring in chipper machines which is an important task, since the knife quality has a significant effect on the quality of the chips. Also, without disc monitoring, a dam^¬ aged knife may cause other damages and expensive repair costs and a long shutdown period.

The knives are arranged into a rotational knife disk and the impacts caused by each of the knives can be measured individually by using sensors. The measured impacts can be compared within one revolution and/or between multiple revolutions and condition of the knives can be analyzed from measured impacts. The impact means most simply a maxi^¬ mum amplitude that is measured and addressed to a working sector. Instead of maximum amplitude, other relevant signal filtering algorithms may be used. In practice the maximum amplitude is an applicable choice in the implementation of the invention. If the maximum amplitude of impact of a work^¬ ing sector is lower than a determined noise level, no impact value is addressed to a working sector. If such a situation happens to one or more working sectors, the data related to that revolution may be omitted.

In an embodiment of the invention a method for de^¬ termining knife quality information in a chipper machine is disclosed. The chipper machine comprises a rotational knife disk comprising a plurality of knives. In the method a plu^¬ rality of working sectors is determined. Then measurement results from at least one monitoring sensor are received. In addition to the measurement results, at least one position indication of said rotational knife disk for each revolution of said rotational knife disk is received. Lastly said meas^¬ urement results of said each revolution are addressed to said working sectors based on at least one of said received indication . In a further embodiment of the present invention a computer program is disclosed. The computer program is configured to perform the method disclosed above when executed in a computing device.

In a further embodiment of the invention a chipper machine is disclosed. The chipper machine comprises a rota^¬ tional knife disk comprising a plurality of knives, an axle coupled to said rotational knife disk configured to cause the rotational movement, at least one monitoring sensor con- figured to measure an impact caused by a cutting knife, at least one angle locator configured to indicate a plurality of working sectors for each revolution of said rotational knife disk and a controller configured to receive measure^¬ ment results from said sensors and address said measurement results of said each revolution into working sectors.

The expression "working sector" means in this patent application a sector of the knife disk determined by embodiment basis. A beginning of a monitored revolution typ^¬ ically corresponds with the beginning of the first working sector of that revolution. In most common implementations the number of sectors is the same as the number of knives in the disk and each sector represents a typical or full angu^¬ lar cutting sector of a knife. For example, if the disk comprises 18 knives, typically 18 working sectors are defined. Sectors may be defined so that they are continuous and non- overlapping. Thus, each of the working sectors in the embod^¬ iment of 18 knives would be 20 degrees, forming a full revo^¬ lution. However, it is possible to determine small gaps be^¬ tween the working sectors or the sectors may be overlapping each other. For example, in the example of 18 knives the working sector may be, for example, 19 degrees leaving one degree between each of the working sectors or it may be 21 degrees so that each of the working sectors overlaps with the previous and the next working sector by one degree. When the sectors are overlapping, the sectors can represent a full cutting sector of a blade which sector has a larger angle than a revolution divided by the number of knifes of the knife disk. The overlapping may mean that the last sector of a revolution may end after the start of the next revolution. The overlapping of the first and last sectors may also be omitted to simplify addressing of the data of successive revolutions. If the working sectors are not overlapping, they can be calibrated to represent typical cutting sectors of the knives.

A person skilled in the art understands that the number of working sectors and the properties of these work^¬ ing sectors may be chosen on embodiment basis. A reason for choosing working sectors comprising more than one knife is that the larger working sectors are not so susceptible for addressing errors caused by the size variation of the raw material. For example, there are situations where one knife hits logs more than once per revolution or hits of particu^¬ lar knives do not always happen within their own sectors but in an adjacent knife's sector due to very large or very small diameter of logs being chipped.

Comparative analysis of monitoring of the knives can be facilitated by giving points to one or several work^¬ ing sectors that signal the most intensive impact of the revolution. The value of a point may be an integer or a floating point number. The value of the given points may re^¬ flect the comparative or absolute intensity of the impacts. The value of the points may also reflect intensity of im^¬ pacts that are over an average value of the same revolution or an average of several revolutions. Points can also be given if the impact is over a predetermined value which may depend from an average value. The points addressed to each working sectors are preferably accumulated (to scores) and they are compared to other accumulated points (or scores) of other working sectors to detect damaged or worn out knives.

The present invention provides a possibility to monitor knives of a chipper machine by dividing and direct^¬ ing the measured sensor data individually to represent the condition of each of the knives or a set of two or more suc- cessive knives within a same working sector. Thus, the im^¬ pacts caused by the knives within a working sector can be compared to impacts of other working sectors during the same revolution and/or, for example, by comparing an impact of a working sector to the impact of the same working sector dur- ing other revolutions. Also, it is possible to compare sums or averages of impacts of each working sector to the sums of averages of other sectors or to an average impact of all sectors .

These comparisons provide a possibility to detect defects in knives faster and the chipper maintenance can keep all of the knives sharp. Detecting by this comparison with statistical relevance that in one of the sectors the chipping causes more intense impacts than in other sectors, the target of detecting the need to inspect and replace the damaged knife ( knives ) is fulfilled. Sharp knives improve the quality of the processed material. As the quality of chips has an effect on the quality of the end product it is de^¬ sired to keep the quality as high as needed.

A further benefit of the invention is that in addi^¬ tion to the improved quality also the quantity is increased. When the knives of a chipper machine are sharp, the machine can produce more chips of acceptable quality from the re^¬ ceived raw material. When knives are dull, more fibers are lost, for example, in form of saw dust. A further important benefit of the present invention is preventing bigger mechanical damages to tooling around the knives by detecting broken knives early enough. BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to provide a further understanding of the invention and constitute a part of this specification, illustrate embodiments of the invention and together with the description help to ex- plain the principles of the invention. In the drawings:

Fig. 1 is a block diagram of an example embodiment of the present invention, and

Fig. 2 is a flow chart of a method according to an example embodiment of the present invention,

Fig. 3 is an illustration of possible measurement data,

Fig. 4 is an illustration of a view to the mouth of a chipper machine, and Fig. 5 is an illustration of a possible method for determining a damaged knife.

DETAILED DESCRIPTION OF THE INVENTION

Reference will now be made in detail to the embodi^¬ ments of the present invention, examples of which are illus^¬ trated in the accompanying drawings .

In figure 1 a block diagram of a chipper machine according to the present invention is illustrated. A person skilled in the art understands that figure 1 is a simplified illustration emphasizing parts that are necessary for the present invention and the chipper machine can be implemented in various manners. In the following examples it is assumed that the number of working sectors corresponds with the num- ber of knives in the knife disk.

In the chipper machine of figure 1 there is a rota^¬ tional knife disk 10 that is connected to a driven axle 11 that causes the rotational movement. The knife disk 10 com^¬ prises a plurality of knives that are configured to cut and slice wood to small pieces from the end face of the fed wood material. There is normally an angle between the axle of the knife disk 10 and the axis of fed logs. Therefore the knives will cause also an axial feeding force when they are slicing the logs. In a typical configuration sliced chips go through holes in the knife disk 10 and will be then transported away; however, the present invention works also with other configurations. As the cutting force generates also feeding force, one indication of worn knives has been that the feed^¬ ing speed of the fed material is slowed down. Then the chips are formed smaller than normally. Wood is brought downward to the knife disk 10 in accordance with arrow 16 by using a conveyor 15. Typically the conveyor and knife disk form an angle of less than 90 degrees. This can be done for example as shown in figure 1, or by tilting the knife disk 10.

The axle is coupled to condition monitoring sensors

12 and 13 which are used to collect data to be used in ana^¬ lyzing the condition of the knives. The sensors are prefera^¬ bly attached to bearing housings. The sensors typically com^¬ prise a plurality of individual sensors. For example, there may be acceleration and/or acoustic emission sensors that can measure the impacts of the cutting knives of the knife disk 10. Acoustic emission sensors typically are tuned to resonate in high ultrasound frequencies. Therefore they physically filter out hearable noises. Acceleration sensors often detect only one direction of accelerations. To get a full three dimensional, or four dimensional data as time is also logged with the data, all planes (x, y and z) should be sensed. In knife condition monitoring the use of 3d data is not necessary, however, it may be used when detecting defects. Frequencies that are not related to condition moni^¬ toring of the knives but other functions of the chipper may be filtered out from the acceleration data.

Data from each sensor can be analyzed separately to detect defected knives. The amplitudes of two or more sen^¬ sors can also be accumulated together and then the data is used for the condition monitoring analysis. These different ways can help in filtering relevant data for the one or more parallel analysis processes for each specific application.

One or more angle locator 17 is/are used for ena^¬ bling the addressing of the collected condition monitoring data to each particular working sector. This sensor may be, for example, a rotary encoder, an optical sensor, a magnetic sensor, a capacitive detector or similar, which indicates one or more angular positions of the knife disk. Based on these positions it is possible to directly indicate or cal^¬ culate the positions of working sectors. By knowing the po^¬ sitioning of the sectors or at least the beginning of the first working sector the positioning indications are used to split and address the condition monitoring data to each sec^¬ tor of a revolution of the knife disk.

A rotary encoder may, for example, give one thou^¬ sand signals per revolution and it can indicate the rota^¬ tional angle of the knife disk accurately. It may be rota- tionally connected to the axle 11 or to another position within a driving arrangement. The rotary encoder will quite accurately continuously indicate in which angular position the knife disk is at a certain moment. The angular position data can be used after calibration to continuously indicate when a particular knife can be cutting wood. This angular positioning data can therefore be used to address data from the condition monitoring sensors to a particular working sector of the knife disk. The condition monitoring data do not need to be stored first to represent a revolution but it can be directly addressed to a certain sector as the begin^¬ ning and end points are already known without intermediate calculations and successive splitting of a data of a full revolution .

A single angle locator will indicate a start of any revolution. It still may not indicate a beginning of the cutting sector of the first knife or the defined first work^¬ ing sector but it is advantageously positioned to define that position. If that is not the case, a calibrated nega- tive or positive offset time value can be determined to be used to define the beginning of the first working sector. The offset time value represents calibrated angular differ^¬ ence between the indication point and the beginning of the first working sector. The offset time value is therefore relative to the rotation speed of the knife disk.

After defining the beginning of the first working sector, the data given by the condition monitoring sensors can be split to time intervals and addressed to the particu^¬ lar working sectors. As the rotational speed of the knife disk is substantially constant, the data from condition mon^¬ itoring sensors between two successive indicated or defined starts of the first working sector can be split to the same number of equal time intervals.

If the number of the angle locators is the same as the number of the working sectors defined on the knife disk the angle locators can be calibrated to indicate the start of every particular working sector, the data from condition monitoring sensors can be directly addressed to the particu^¬ lar working sector. A rotary encoder may be used as an angle locator. When a rotary encoder is used it may be calibrated correspondingly to indicate the start of every particular working sector.

Condition monitoring sensors 12 and 13 are coupled to a computing device 14 that is configured to store all measurements. As a start of the first working sector is de^¬ termined by using an angle locator, position sensing sensor or sensors, the measurements can be stored so that each rev^¬ olution of a knife disk 10 can be analyzed by the computing device 14. The measurement data of a revolution may also be directly split and addressed to particular working sectors of a particular revolution for analysis.

In figure 2 a method according to an embodiment of the present invention is disclosed. In the embodiment a chipper machine is processing incoming wood, such as logs, and chipping them into chips by using a rotational knife disk comprising a plurality of knives. Every time when a knife hits or cuts logs, or anything else that is coming in^¬ side, the hit causes a measurable impact. This impact and the hit causing the impact are explained in more detail lat^¬ er with reference to figure 3, which discloses an example of a measurement result. Typically this impact is measured by using acceleration sensors or acoustic sensors, step 20. The measurable impact varies depending on the knife sharpness and chipped material. Sometimes there is sand, rocks, metal or other undesired material and the hit thereof causes a considerably different impact than wood. Correspondingly, for example hitting a metal object can damage one or several knives very fast. Only one hit may cause a damage that should be repaired. Sometimes metal or other foreign bodies can cause damage to a plurality of knives. Furthermore, a blunt knife causes a different impact than a sharp knife. Furthermore, as the measurement period is defined by the working sector, there may be more than one impacts in each of the measurement results.

After measuring these impacts they are stored to a computer device so that data concerning each revolution of the rotational knife disk can be separated. This is done by determining the beginning of the first working sector of each revolution, steps 21 and 22. This is done by the angle locator (s) , for example, by using a rotary encoder, an optical mark that is detected, or any similar mechanism as discussed above with reference to figure 1. When the condition monitoring data of a complete revolution is stored into a computing device it must be ad^¬ dressed into working sectors so that the measurement results for each working sector can be detected, step 23. This can be done, for example, so that the time needed for one revo^¬ lution is known or it is measured from the impulses of the angle locator (s). Furthermore, in addition to the indication of a new revolution, i.e. the beginning of the first working sector, the rotary encoder may be used for determining be- ginnings and ends of other working sectors. As the working sectors are known, the measurement data can be directly ad^¬ dressed to any particular working sector comprising one or more knives. Another option is to split the measured meas^¬ urement data of a revolution into pieces in accordance with the number of working sectors. If the operating speed is constant, the division is easy to make. When there is a sta^¬ tistically relevant amount of measurement data, the condi^¬ tion of the knives can be recognized from the measurement deviations between the measurements of working sectors, step 23. When a defected knife is suspected, an automatic alarm or a system shutdown can be performed, step 24.

A person skilled in the art understands that the method discussed above is applied with an arrangement where^¬ in a plurality of working sectors is defined for a chipper machine. The method of figure 2 is a continuous process. The predetermining of the alignment of the sectors does not need to be continuously updated but it can be changed during the monitoring process to represent the situation when there are excessively thick or thin logs which therefore are out of a typical cutting sector of the knives. By using a digital camera technology or other known optical or mechanical meth^¬ od to detect the height of the pile of logs, the determining of the alignment of the working sectors may be adapted to better represent the actual cutting sectors.

In Figures 3 and 5 examples with five knives are illustrated. A person skilled in the art knows that the num^¬ ber of knives is typically higher and may be chosen case- specifically. Thus, the number here is chosen only for the sake of the clarity. In Figure 3 an example of a measurement data is disclosed. In the x-axis is the time from the beginning of a revolution and y-axis shows the measured amplitudes. In the figure only first five working sectors 31 - 35 are illus- trated. A beginning of a new revolution marker 30 is also illustrated. From the marker 30 a new revolution begins.

If we take a look at the first revolution we can see a base noise and five local peak values that differ from the base noise level. These peaks with higher amplitude have been caused by impacts caused by the cutting knives. One can see that the peaks do not have the same amplitude because the amplitude varies depending on the wood quality. Impacts 31 - 34 are normal impacts, which have been caused when a knife of the knife disk slices chips from the raw material. Impacts 31 - 34 of the first revolution are caused by slic^¬ ing with a sharp knife. The last impact 35 is caused by a dull knife. It can be seen that the amplitude caused by a dull knife is remarkably higher. Based on the above a person skilled in the art understands that the expression "impact" in this application means a measurable event caused by a knife when slicing chips.

Revolutions 2 - 4 are further illustrations, where revolution two is similar to revolution one, and the impact of knife 35 is significantly higher and differs substantial- ly from other impacts of the same revolution. Revolution 3 is similar, with the exception of knife 34, which has not had any hit during revolution 3. In revolution 4 it can be seen that knife 32 has higher amplitude than knife 35. An example of the utilization of this information is disclosed with reference to figure 5.

It is important to understand that impacts do not appear at the exactly same moment of time in every revolu^¬ tion. The small differences are typical because of the raw material. Wood arrives at the chipper in different quality and quantity. For example, in the extreme situation there may be just one thin piece of wood and later a number of thick logs. When the monitoring system and especially the alignments of the working sectors are calibrated, also an oscilloscope can be used to check that the main impulses caused by a certain knife will happen well within the determined working sector.

It must be acknowledged that figure 3 and the de^¬ scription above are just examples. So-called silent periods between impacts may be shorter or longer. It is possible that in some rare cases a knife hits at least twice a high^¬ est level of one log in the same revolution as there can be logs of different size in the feed chute and at different distances from the center of the knife disk. The amplitude or impact of next hits are typically smaller as typically the next hit cuts a smaller piece of wood; however, it is not necessarily always so if a dull point of the knife hits a log as the latter hit happens. The multiple hit occur^¬ rence, however, does not require any specific processing as the embodiment disclosed in the above provides correct re^¬ sults even in case of multiple hits.

Figure 4 is an illustration of a view to the mouth of a chipper machine. In the figure a rotational knife disk 40 is shown. The knives 44 - 46 of the knife disk are shown only through the mouth 43 of the chipper machine. The mouth 43 is an opening in front of the knife disk 40 for feeding logs 41, 42 to the chipper machine for cutting. In the figure 4 knife 45 is still cutting logs 41 and 42. Thus, the knife disk 40 is still in the working sector associated with the knife 45. As it was explained, the position of the knife disk may be determined by using a angle locator, the working sector may be determined from the information received from angle locator sensor. The working sector may be determined, for example, as a start point of the working sector so that the end point is the start point of the next working sector. Typically actual locations are determined by calibrating the chipper machine so that the working areas are such that in most cases only one knife is cutting, however, as explained a person skilled in the art understands that several hits may occur within one working sector intentionally or unintentionally.

In figure 5 a simple method for determining knife quality is disclosed. In the method one point is given to a working sector that is having the biggest amplitude in a revolution where each of the working sectors have a measure^¬ ment result caused by a knife hitting the wood. As we can see from figures 3 and 5, the fifth working sector 35 has a dull knife, which is the fifth knife in the example, and it causes the highest amplitude during the first two revolu^¬ tions. Then at the third revolution, the fourth working sector did not have a measurable impact. Thus that revolution was not full and is not counted in. The method is arranged to maintain the data, for example in the memory of a con- troller or server connected to the chipper, as explained earlier. The data is typically statistically analyzed for a predetermined number of last measured revolutions. The stored data is analyzed and in accordance with the proper^¬ ties of the chipper machine by a maintenance person. For ex- ample, if the maintenance person has decided to analyze 10000 last revolutions and the knife disk has 18 knives, a warning limit could be set to 800 points and automatic stop to 1300 points?. When all knives are sharp, highest ampli^¬ tudes distribute randomly to all knives and the limits of for further actions are not met.

The principles mentioned above may be used for providing a plurality of different analyzing periods. For example, it is possible to monitor the last 240 and 480 sim^¬ ultaneously in order provide different analyzing results. Furthermore, the above mentioned high number of revolutions is used only as an typical example. The analysis may also be based on one revolution only, wherein the alerting or stopping is performed, for example, in the case that measurement result deviation exceeds a threshold specifically determined for one revolution. As discussed above with regard to 240 and 480 revolution analysis periods, it is possible to use these longer periods together with one revolution analysis. Furthermore, instead determining the period as a number of revolutions it is possible to determine the analyzing period as a time interval, for example in seconds or minutes. The above disclosed method is the simplest but may provide ade^¬ quate results. A person skilled in the art understands that more sophisticated methods with similar basic principles can be designed. For example, one or more points may be given to more than one working sector of a revolution. For example, points can be given to two highest amplitudes so that the highest gets two points and the second one point. Points may also be given to every working sector in which the highest amplitude is over a certain remarkably high limit value. The points may be given also so that when the amplitudes of two or more sector are substantially the same but differ signif^¬ icantly from the average amplitude, the same points will be given to all of such sectors having a high enough amplitude.

In a further embodiment the points have a value that indicates a difference from the average. That kind of points or reference values may be given to all sectors or only those above a certain limit or exceeding substantially, for example 30 % the average of maximum values of sectors of the revolution or multiple prior revolutions. For example, a point may be given to each working sector that is closer to the maximum amplitude than the average amplitude. If there is more than one working sector closer to the maximum than the average, points will be assigned in accordance with the distance to the maximum. A person skilled in the art under^¬ stands that by using different methods, the limits for warn^¬ ing and automatic stop must be selected in the actual pro^¬ cess conditions. That can be done by following up the actual process with the monitoring system selecting limit values that are able to point out that at least one knife is dam^¬ aged or worn out.

In a further embodiment the measured highest ampli^¬ tude values or otherwise filtered impacts or points of a particular working sector are accumulated together. The pe- riod of accumulation may be a constant floating interval of last data. The accumulation may also be concerning all of the data and/or results that are collected after a replace^¬ ment of at least one knife or all of the knives. This is ad^¬ vantageous for informing about the overall wear of the knives. The accumulated value is then compared with a prede^¬ termined threshold value that is defined case-specifically. For example, a person skilled in the art may determine such a threshold limit based on his professional experience. If these sums are divided by the number of accepted revolu- tions, an average of the highest value of the particular sector is calculated. It can be compared to an average of all working sectors and if the average of one or more sec^¬ tors deviates substantially, for example 20 %, from the av- erage of all sectors the alarming, or with 30 % deviation, the shutdown is activated. It must be understood that this threshold value or the difference in averages may change if the raw material quality is changed, for example, from a wood species to another.

Furthermore, a person skilled in the art under^¬ stands that even if in the above example the number of work^¬ ing sectors is the same as the number of knives, the inven^¬ tion may be implemented so that the number of the working sectors is different from the number of the knives. Advanta- geously the number of knives is a multiplicity of the number of working sectors.

The above mentioned methods may be implemented as computer software which is executed in a computing device able to communicate with a chipper machine or being a part of such a machine. The software is embodied on a computer- readable medium so that it can be provided to the computing device. The computing device mentioned may be, for example, a controller, an ordinary computer, a server or similar. The computing device may be incorporated into the chipper ma- chine or it may be located remotely and coupled with an ap^¬ propriate network connection to the chipper machine for receiving the information. The network connection can be wireless or wired.

As stated above, the components of the exemplary embodiments can include computer readable medium or memories for holding saving/storing instructions programmed according to the teachings of the present invention and for holding data structures, tables, records, and/or other data de^¬ scribed herein. Computer readable medium can include any suitable medium that participates in providing instructions to a processor for execution. Common forms of computer- readable media can include, for example, a floppy disk, a flexible disk, hard disk, magnetic tape, any other suitable magnetic medium, a CD-ROM, CD±R, CD±RW, DVD, DVD-RAM, DVD1RW, DVD±R, HD DVD, HD DVD-R, HD DVD-RW, HD DVD-RAM, Blu- ray Disc, any other suitable optical medium, a RAM, a PROM, an EPROM, a FLASH-EPROM, any other suitable memory chip or cartridge, a carrier wave or any other suitable medium from which a computer can read.

It is obvious to a person skilled in the art that with the advancement of technology, the basic idea of the invention may be implemented in various ways. The invention and its embodiments are thus not limited to the examples de- scribed above; instead they may vary within the scope of the claims .

Claims

1. A method for determining knife quality information in a chipper machine, wherein said chipper machine comprises a rotational knife disk comprising a plurality of knives, characterized in that the method comprises:

determining a plurality of working sectors;

receiving measurement (21, 22) results from at least one monitoring sensor;

receiving (21, 22) at least one position indication of said rotational knife disk for each revolution of said ro^¬ tational knife disk; and

addressing (23) said measurement results of said each revolution to said working sectors based on said received at least one indication.

2. The method according to claim 1, wherein the method further comprises determining quality of knives that perform cutting in a working sector (24) from measurement deviations between measurement results of each of the work^¬ ing sectors (24 ) .

3. The method according to claim 2, wherein assigning for at least one working sector in accordance with the predetermined measurement deviation.

4. The method according to claim 1, wherein the method further comprises accumulating measurement results for each working sector respectively over a plurality of revolutions and determining knife quality from measurement deviations between accumulated measurement results of each of the working sectors.

5. The method according to any of the preceding claims 1 - 4, wherein determining knife quality by comparing measurement deviations over a plurality of revolutions.

6. The method according to any of the preceding claims 1 - 5, wherein ignoring measurement results of a rev^¬ olution are ignored if at least one of the working sectors did not indicate a cutting impact during said revolution.

7. A computer program comprising code adapted to cause the method according to any of claims 1 - 6 when exe^¬ cuted on a data-processing system.

8. A chipper machine, which machine further comprises :

a rotational knife disk (10) comprising a plurality of knives ;

an axle (11) coupled to said rotational knife disk (10) and configured to cause the rotational movement;

at least one monitoring sensor (12, 13) configured to measure an impact caused by a knife hit;

at least one angle locator (17) configured to indicate a plurality of working sectors for each revolution of said ro^¬ tational knife disk (10);

characterized in that

a controller (14) configured to receive measurement re^¬ sults from said sensors (12, 13, 14) and split said measure- ment results of said each revolution into working sectors.

9. The chipper machine according to claim 8, where^¬ in said at least one monitoring sensor (12, 13) configured to measure an impact caused by a cutting knife hit is pro^¬ vided with an acceleration sensor or an acoustic emission sensor.

10. The chipper machine according to claim 8 or 9, wherein at least one angle locator configured to indicate the measurement result of the first knife is a rotary encod^¬ er, optical sensor, magnetic sensor or capacitive sensor.

11. The chipper machine according to any of the preceding claims 8 - 10, wherein said controller (14) is configured to determine knife quality from measurement devi^¬ ations between measurement results of each of the working sectors belonging to the same revolution.

12. The chipper machine according to claim 11, wherein said controller (14) is configured to assign a point for at least one working sector in accordance with the meas^¬ urement deviation.

13. The chipper machine according to any of the preceding claims 8 - 12, wherein said controller (14) is configured to accumulate measurement results for each work^¬ ing sector respectively over a plurality of revolutions and to .

14. The chipper machine according to any of the preceding claims 7 - 13, wherein said controller (14) is configured to determine knife quality by comparing measure^¬ ment deviations over a plurality of revolutions.

15. The chipper machine according to any of the preceding claims 7 - 14, wherein said controller (14) is configured to ignore measurement results of a revolution if at least one of the working sectors did not indicate a cut^¬ ting impact during said revolution.