SE536088C2 - Method and apparatus for determining the state of a gear by combining torque and angular representations - Google Patents

Abstract

lO 27 Abstract An improved method and apparatus for creating a combinedrepresentation of transferred torque ru over rotation angle dfor a torque transferring system is disclosed, where thetorque transferring system includes at least two meshinggears. According to the invention, at least two individualrepresentations of said transferred torque rm and saidrotation angle d are determined, wherein each individualrepresentation includes at least one transferred torque ru andat least one corresponding rotation angle d of an individualsystem operation. Each individual representation iscompensated for an increasing transferred torque rn during theindividual system operation. Then, the combined representationis created by combining the at least two compensated individual representations. Fig. l

Description

lO METHOD AND APPARATUS FOR COMBINING TORQUE AND ANGLEREPRESENTATIONS Field of the invention The present invention relates to a method for creating acombined representation of transferred torque rm over rotation angle d as defined in the preamble of claim I.

The present invention also relates to an apparatus arrangedfor creating a combined representation of transferred torqueru over rotation angle d as defined in the preamble of claim l3.

The present invention also relates to a computer program and acomputer program product implementing the method of the invention.

Related art and background of the invention In systems transferring torque, such as tools, for examplenut-runners, transmission systems, such as gear boxes, andother system transferring a torque, it is important to be ableto determine a condition of the torque transferring systems.It is, for example important to be able to determine acondition of one or meshing gears being included in the torquetransferring systems. Such meshing gears include spiral bevelgears, consisting of a pinion gear and a crown gear, fixedring gears, sun gears, planet gears, and essentially any othergears being used for transferring the torque through the system.

In this document, the invention will be described mainly forits implementation in a tool, such as a nut-runner. However, the invention is generally applicable to essentially any other lO system transferring and/or monitoring torque by means of meshing gears.

A nut-runner provides a torque to a nut in order to tighten orloosen the nut, thereby tightening or loosening a jointincluding the nut. The nut-runner includes meshing gears. Thepurpose of these gears is to increase the amount of torquewhich can be provided to the nut by the nut-runner. Themeshing gears are thus used for decreasing the angularrotational speed at the nut, whereby the torque being providedat the nut by the nut-runner is increased. Thus, the angularrotational speed being provided by a motor in the nut-runneris, by the meshing gear, geared down in steps being related tothese meshing gears, whereby the torque being applied to the nut is increased in corresponding steps.

Thus, the meshing gears are a vital part of such torquetransferring systems, and it is very important to be aware of their condition.

Torque transferring systems, such as tools, are widely used ine.g. manufacturing and repair processes. Such manufacturing orrepair processes depend on reliable tools for securingcontinuous manufacturing or repairing. In order to avoidmanufacturing or repairing pauses, that are both annoying andexpensive for the manufacturer or repairer, it is important toknow the condition of the tools being used in the manufacture or repair process.

In known solutions to this problem, the tools being used aretaken out of use, e.g. out of the manufacturing or repairingprocess, and are analyzed under controlled conditions in anexternal analysis apparatus for tool condition determination.Thus, an external apparatus is here needed for performing this determination, which of course adds to the manufacturing or lO repairing costs. In these known solutions, information isusually collected for the whole torque transferring system,i.e. for a complete rotation of all of the meshing gearsincluded in the system. To collect information of the completerotation takes some time. Also, for torque transferringsystems being implemented in large apparatus, such as a gearbox in a truck, it can involve a substantial amount of work todisassemble the large apparatus in order to be able to extractthe torque transferring system and then to install the torque transferring system into the external analysis apparatus.

Also, since the tools have to be taken out of use when beinganalyzed, either e.g. the manufacturing of repairing processhas to be stalled during the analysis, or there has to be anabundance of tools present at the manufacturing or repairingsite, such that a replacement tool could be put to work when the analysis is performed.

Both stalling of the manufacturing or repairing and purchasingof replacement tools add to the production or maintenancecosts for the items being produced or repaired. Since cost isa crucial competition factor in manufacturing and repairingprocesses, it is important to minimize these additional costs resulting from tool condition control.

Aim and most important features of the invention It is an object of the present invention to provide a methodand an apparatus that solves the above stated problem byproviding a possibility to determine a condition for thetorque transferring system during normal operation of the torque transferring system.

The object is achieved by the above mentioned method for creating a combined representation of a transferred torque IH lO and a rotation angle d, according to the characterizing portion of claim l.

The object is also achieved by the above mentioned apparatusarranged for creating a combined representation of atransferred torque rm and a rotation angle d, according to the characterizing portion of claim 13.

The object is also achieved by the above mentioned computerprogram and a computer program product implementing the method of the invention.

The method and apparatus according to the present inventionare characterized in that they create a combinedrepresentation of the transferred torque ru and the rotationangle d for the torque transferring system by combininginformation from at least two individual system operations,where each individual representation includes at least onetransferred torque ru and at least one corresponding rotationangle d of an individual system operation. According to theinvention, each one of these individual representations hasbeen compensated for an increasing transferred torque IHduring the individual system operation to which it relates.The compensation makes it possible to join subsequentindividual representations together, since each compensatedindividual representation simulates an essentially constanttransferred torque rn during that individual system operation,thereby resulting in a combined representation having high quality.

Thus, from a number of individual representations, each ofwhich being too short for being used as a basis fordetermining a gear condition, a combined representation iscreated, which has a length being long enough for being utilized when determining a gear condition. Thereby, the lO apparatus can gather information under normal operation of thetorque transferring system, and when enough information hasbeen gathered, i.e. when a combined representation having acertain length has been created, a gear condition can bedetermined without taking the torque transferring system outof production. Thus, the present invention makes it possibleto perform real time gear condition determinations for a torque transferring system, such as a nut-runner.

According to an embodiment of the present invention, each oneof the individual representations is detrended, such that ithas a zero mean amplitude. The elimination of the trend of theindividual representations facilitates the combination of theindividual representations. Zero mean data is also preferredin many signal analysis measures that can be utilized for further analysis of the combined representation.

According to an embodiment of the invention, an absolute angledflß for the torque transferring system is utilized whencombining two or more angularly subsequent individualrepresentations. Since the absolute angle dæß is known with ahigh enough sample rate, the joints between the individualrepresentations can be made very exact, thereby resulting in acombined representation of high quality, which in turn resultsin that a high quality determination of the gear condition is possible.

According to an embodiment of the invention, the combinedrepresentation includes information of a transferred torque IHover an angle interval corresponding to a complete rotationfor all of the meshing gears of the torque transferringsystem. Hereby, a complete presentation of the possible deterioration of the gears can be provided. lO The invention further presents a determination of a gearcondition for the torque transferring system. Here, a combinedrepresentation, having been created in accordance with theinvention, is analyzed, whereby a deterioration of any one ormore of the meshing gears is identified if deterioration hasoccurred. This condition determination can, by the use of thepresent invention, be performed by utilizing only data beingavailable from sensors located within the torque transferring system.

The gear condition determination according to the inventionmakes it possible to essentially continuously monitor thedeterioration. This gives an indication of the state of healthfor the torque transferring system, and can also be used forpredicting a failure of the torque transferring system.Thereby, the time to failure can be estimated during normaloperation, which minimizes the risk for breakdown e.g. on the assembly or repair line.

Also, the gear condition determination according to theinvention can be utilized for, during normal operation,determining a suitable time for maintenance of the torquetransferring system, such that failure of the torque transferring system can be avoided.

The analysis of the combined representation can include atransformation of a time domain signal of the combinedrepresentation into a frequency domain, and then analyzing the amplitude levels of the frequency domain signal.

Since the created combined representation of the transferredtorque rm and the rotation angle d for the torque transferringsystem has been created by adding a number of individualrepresentations together, thereby achieving a combined representation of appropriate length, the frequency domain lO signal of the combined representation can very clearly anddistinctly indicate if one or more meshing gears aredeteriorated. The length of the combined representationreduces the noise of the frequency domain signal and alsoenhances the resolution of the signal, making the analysis of the signal very efficient and reliable.

Detailed exemplary embodiments and advantages of the methodand the apparatus according to the invention will now bedescribed with reference to the appended drawings illustrating some preferred embodiments.

Brief description of the drawings Fig. l shows a flow chart for the method of the invention.

Fig. 2 shows increasing amplitude alterations with increasing absolute rotating angle.

Fig. 3 shows a window function.

Fig. 4 shows a linear portion of an individual system operation.

Fig. 5 shows a compensated and detrended individual representation.

Fig. 6 shows a combined representation.

Fig. 7 shows a frequency domain signal of the combined representation for a healthy gear.

Fig. 8 shows a frequency domain signal of the combined representation for a deteriorated gear.

Detailed description of preferred embodiments The present invention provides a method for, and an apparatusarranged for, making it possible to provide real-time gearcondition information, i.e. to provide gear conditioninformation during normal operation, of a torque transferringsystem. To be able to determine such gear conditioninformation, a representation of a transferred torque rm and arotation angle a for the torque transferring system is needed.Such a representation, which also can be denoted a trace, thusincludes one or more values for a transferred torque rm at oneor more rotation angles a, respectively. From thisrepresentation, the gear condition can be determined for the meshing gears of the torque transferring system.

One problem being related to providing a gear condition duringnormal operation of the torque transferring system is that ittakes some time for the system to make a complete rotation ofall of the meshing gears being included in the system. Forexample, a nut-runner tightening one nut as an individualsystem operation may not perform a complete rotation,especially for a stiff nut joint being tightened. Generally,information from a complete rotation is desired for thecondition determination, which makes it difficult to perform areal time gear condition determination due to the frequentlyoccurring short individual system operations. One single nuttightening very seldom provides information relating to such a complete rotation.

According to the present invention, a combined representationof the transferred torque rm and the rotation angle a for thetorque transferring system is created by combining informationfrom at least two individual system operations. More in detail, for each individual system operation, one individual representation of the transferred torque ru and the rotationangle d is determined. Each such individual representationincludes at least one transferred torque ru and at least onecorresponding rotation angle d of the individual systemoperation. Each one of these individual representations iscompensated for an increasing transferred torque rn during the individual system operation to which it relates.

Thereafter, the combined representation of the transferredtorque rm and the rotation angle d for the torque transferringsystem is created by combining the at least two compensatedindividual representations, whereby the combination results inthe combined representation, which has a length being longenough for being utilized for determining a gear condition.Thus, the individual representations are joined togetherappropriately, possibly over complete rotations, to create the combined representation.

Figure 1 shows a flowchart for the method according to theinvention. In a first step 101 of the method, at least twoindividual representations of the transferred torque rm andsaid rotation angle d are determined, wherein each individualrepresentation is related to an individual system operation.In a second step 102 of the method, each individualrepresentation is compensated for an increasing transferredtorque IH during the individual system operation. In a thirdstep 103 of the method, the combined representation is createdby combining the at least two compensated individual representations.

Thus, according to the present invention, the combinedrepresentation of the transferred torque ru and the rotationangle d for the torque transferring system is created by combining a number of individual representations relating to a number of individual system operations. Thereby, it ispossible to utilize a number of individual representations,each individual representation itself including too littleinformation to be useful for determination of a gearcondition, to create a combined representation includingenough information to determine the gear condition. To be ableto combine these individual representations, and to keep thesignature of the torque transferring system constant, eachindividual representation is, according to the presentinvention, compensated for the increase in torque amplitudebeing caused by increased load during that individual systemoperation. The compensated individual representations can thenbe combined, since the influence of individual load levels has been removed.

The problem related to the increased load is illustrated infigure 2 for a torque transferring system being a nut-runner.As can be seen in figure 2, the amplitude of the torquealterations increases with increasing absolute rotating angle.Each individual system operation has a corresponding increasein torque alteration amplitude, which would cause errors infollowing analysis steps if the individual representations ofthe transferred torque ru and the rotation angle d were to becombined without any compensation. This problem is solved bythe present invention, by compensating for the increasedtransferred torque rn during each individual system operation,thereby resulting in individual representations of thetransferred torque rn and the rotation angle d for eachindividual system operation having peak amplitudes beingunaffected by the increasing torque during the individualsystem operation, i.e. simulating an essentially constant transferred torque rm during the individual system operation. lO ll The at least two individual representations of the transferredtorque rn and the rotation angle d, can then, since theincrease in torque during the individual system operations iscounteracted, be easily combined into the combinedrepresentation of the transferred torque ru and the rotation angle d.

Thereby, a combined representation of the transferred torqueru and the rotation angle d can be created from a number ofindividual system operations, whereby there is no need fortaking the torque transferring system out of its normal usefor determining this combined representation. Thus, thepresent invention provides a combined representation of thetransferred torque ru and the rotation angle d, which can be utilized for a real time condition determination.

According to an embodiment of the invention, the compensationof the individual representation of the transferred torque rmand the rotation angle d is achieved by applying a windowfunction W'to the individual representation. One example ofsuch a window function W'is shown in figure 3. As can be seenin figure 3, the window function W has a shape including anamplitude decreasing with increasing sample numbers, where thedecrease essentially correspond to the increase in torquecausing the increased alteration amplitude peaks of theindividual representations. According to an embodiment, thewindow function W decreases linearly, from a start amplitudevalue being dependent of a difference in amplitude levels ofstart and end points of the individual representation, to an end amplitude value of one (l).

Thus, when applying this window on the individualrepresentations, they are compensated for the increased torque such that the compensated individual representation simulates lO l2 an essentially constant transferred torque rn during theindividual system operation. The application of the windowingfunction can also be seen as an amplitude normalization of theindividual representations, resulting in a normalizedindividual representation simulating essentially constanttransferred torque rm over the individual system operation.The windowing function is adaptable, i.e. not fixed. Thewindowing function is individual for each individualrepresentation, and will change depending on the load duringeach individual system operation and on the torque being preferred for normalizing to.

According to an embodiment of the present invention, therotation angle d and the corresponding transferred torque IHfor each individual representation are chosen so as toindicate an influence of teeth of the meshing gears as arelationship between the rotation angle d and correspondingtransferred torque rm of the individual system operation.Thus, the cropped data hereby includes the valuableinformation of the meshing gears teeth influence for each individual system operation.

Normally, only one segment of an individual system operationincludes linearly increasing values for transferred torque IHover rotation angle d. These linearly increasing valuesusually include the valuable information of the meshing gearsteeth influence. For e.g. an electrical nut-runner, thissegment is linearly increasing because the joint here behaves linearly in relation to the rotation angle d.

However, also if the valuable information of the meshing gearsteeth influence is not included in the linearly increasingsegment, the information could still be usable through compensation and detrending of the information. lO l3 Figure 4 shows one example of torque and absolute angle for anindividual system operation for a nut-runner, wherein theindividual system operation includes one nut tightening. Theinteresting portion, which includes the valuable informationof the individual system operation, lies between the showncircular markers in figure 4. The left marker indicates astarting angle dl of the interesting portion of the individualsystem operation, and the right marker indicates an end angle dg of the interesting portion.

The starting angle dl is, according to an embodiment, chosensuch that the selected interesting portion corresponds to atransferred torque rm above a signal to noise level (SNR) ofthe apparatus. The end angle dg is chosen such that iscorresponds to a transferred torque rm being close to a targetfor the transferred torque rn. Such a target is normally usedby the torque transferring system and is thus available foridentification of the interesting region including thevaluable information of the meshing gears teeth influence.According to this embodiment, the interesting portion isextracted from the individual system operation and is used fordetermining the individual representation of the transferredtorque ru and the rotation angle d. This can also be seen ascropping the individual system operation data to itsinteresting part. Thus, the individual representation shown infigure 2 corresponds to the interesting region of the individual system operation shown in figure 4.

According to an embodiment of the invention, the determinationof the individual representations includes removing a trendfrom each one of the individual representations. Thedetrending causes each detrended individual representation tohave an amplitude which has a mean torque of approximately zero (O) Nm, i.e. the detrended individual representation of 14 the transferred torque ru and the rotation angle d includes azero-mean individual representation. Because this zero-meancharacteristic of the detrended individual representations, they can easily be joined into a combined representation.

According to an embodiment of the invention, this detrendingis performed by subtracting a polynomial from the individualrepresentation of the transferred torque ru and the rotationangle d, thereby obtaining a detrended individualrepresentation being centered around a torque rm ofapproximately zero Nm, i.e. having a mean value being approximately zero.

Figure 5 shows an example of a compensated and detrendedindividual representation corresponding to the aboveindividual system operation of figure 4. As can be seen infigure 5, this detrended individual representation has a meanvalue being zero and simulates an error signal from anessentially constant transferred torque rn during theindividual system operation, i.e. the detrended individualrepresentation is centered around a transferred torque ru ofzero Nm to only simulate the influence of the meshing gears.In comparison, the signature meshing of gear teeth would bedifficult, if not impossible, to obtain without a load being present on the torque transferring system.

A detrended individual representation, as the one shown infigure 5, can easily be joined together with at least oneother such detrended individual representation, therebyproviding a combined representation, which can be utilized fordetermining the condition of the gears in the torque transferring system.

According to an embodiment of the invention, an absolute angle dflß is utilized for determining an angle relationship between two angularly subsequent individual representations beingcombined according to the invention. Here, the rotation angled used in each individual representation of the transferredtorque rm and the rotation angle d is an absolute, i.e. non-relative, angle dæß. This makes it possible for the twoangularly subsequent individual representations to be joinedtogether at a correct rotation angle d, thereby achieving acombined representation having a very high quality in thejoint, which also makes it possible to make a very exact gearcondition determination. The two angularly subsequentindividual representations need here not be related to two intime sequentially performed individual system operations.Thus, the use of the absolute angle dæß also makes it possibleto keep track of where to insert a specific individualrepresentation into a combined representation in order tocreate a complete combined representation, possibly overcomplete periods of the system rotation, as will be explained below.

According to an embodiment of the present invention, thedesired combined representation of the transferred torque IHover the rotation angle d, which includes information from atleast two individual system operations, includes informationof a transferred torque ru over an angle intervalcorresponding to a complete rotation for all of the meshinggears of the torque transferring system. Thus, a number ofindividual system operations have been made, each resulting inan individual representation covering an interval of angles d.If enough individual system operations are performed in orderto produce a number of intervals of angles d that togethercover a complete rotation of the meshing gears, the combinedrepresentation of the transferred torque ru and the rotation angle d is a complete rotation representation including 16 information of the complete rotation. Thus, the completerotation representation of the transferred torque rm and therotation angle d is then usable to determine a condition forall of the possible cogging events from all of the meshing gears of the torque transferring system.

Here, it is important to be able to keep track of the correctangle of the individual representations in order to correctlyjoin them. Each individual representation has to be fittedinto the combined representation in the exact right positionin order to result in a combined representation being true tothe gear teeth. The absolute angle dæß must be utilized forkeeping track of the exact position for where an individualrepresentation is to be inserted, such that all angles of acomplete rotation of the meshing gears are represented in thecomplete combined representation after a sufficient number of individual representations have been added to the combined representation.

Figure 6 shows an example of such a combined representation ofmeshing gear teeth over a number of angular degrees, in whichtransferred torque rm and over absolute rotation angle dæß fora number of individual representation have been combined. Thecircles indicate joining points. If further individual systemoperations are performed, and the corresponding individualrepresentations are appended with angle values beingaccurately placed where they belong in relation to a period ofrotation for the nut-runner, the resulting combinedrepresentation can provide information from all angles of acomplete rotation of the meshing gears of the nut-runner.Thus, the combined representation can then be utilized fordetermining the condition of all of the meshing gears of the nut-runner. This will be described more in detail below. 17 According to an embodiment of the present invention, thecombined representation of the transferred torque ru androtation angle d, which has been created according to theinvention as described above, is utilized for determining agear condition. The combined representation is here analyzed,and if any of the meshing gears of the torque transferringsystem is deteriorated, this one or more meshing gear can beidentified. Thus, by utilizing this embodiment of theinvention, the combined representation is utilized fordetermining a gear condition, which can be performed duringnormal operation of the torque transferring system, i.e. in real time.

According to an embodiment of the invention, the analysis ofthe combined representation includes a transformation of atime domain signal of the combined representation into afrequency domain signal of the combined representation. Thetransformation can for example be performed by the use of aFourier transform, such as a Fast Fourier Transform (FFT).From this frequency domain signal, the gear condition caneasily and reliably be determined, as will be described morein detail below. Thanks to the length of the combinedrepresentation, i.e. thanks to the combination of a number ofindividual representations into a combined representation ofsufficient length, the transformation can be made with highaccuracy, resulting in a frequency domain signal of the combined representation having a low noise level.

According to an embodiment of the invention, an equidistantre-sampling is performed on the time domain signal of thecombined representation before it is transformed into thefrequency domain. The equidistant re-sampling can also beperformed on the time domain individual representations before combining them into a combined representation. The equidistant 18 re-sampling achieves a simulated speed of operation, andconstant angle distance between the samples of the combinedrepresentation, which removes the effects of increasing speedduring the individual system operations, thereby making itpossible to represent the information of the combined representation in the frequency domain.

After the transformation, the condition of the meshing gearscan be determined by amplitude analysis of the combinedrepresentation in the frequency domain. Figure 7 shows anexample of a combined representation signal in the frequencydomain for a healthy bevel gear, and figure 8 shows an exampleof a combined representation signal in the frequency domainfor a deteriorated bevel gear. From analysis of figures 7 and8, it is clear that a deteriorated bevel gear generates gearmeshing frequencies of higher amplitude than that of a healthybevel gear in the frequency domain signal. This is by theinvention utilized for identifying the condition of themeshing gears of the torque transferring system. However,determination of a state of health from the combinedrepresentation can be obtained in a number of different ways.Amplitude comparison is only one such analysis method. Theaccuracy and level of detail for the combined representationsignal in the frequency domain is dependent on the length ofthe combined representation. Since the combined representationaccording to the present invention includes a number ofindividual representations, such that the combinedrepresentation has a considerable length, the characteristicsof the combined representation signal in the frequency domainare very distinct. Therefore, deteriorations of the meshing gears can be easily detected.

As is shown in figures 7 and 8, a deteriorated gear results in a far higher amplitude than a health gear, which can be 19 utilized in the determination of the deterioration. Thus, anincrease in amplitude can here be utilized for identifying a deteriorated gear.

According to another embodiment of the invention, the timedomain signal of the combined representation is utilized fordetermining the gear condition. As for the frequency domaincondition determination, amplitude analysis of the time domainsignal is here used for identifying deteriorated gears. Forthis analysis, amplitude values, and more specifically changesin amplitude values, can be monitored to follow the gear condition.

The different steps of the method of the invention describedabove can be combined or performed in any suitable order. Acondition for this, of course, is that the requirements of astep, to be used in conjunction with another step of themethod of the invention, in terms e.g. of availableinformation, such individual representations or combined representations, must be fulfilled.

The above described method and embodiments of the inventioncan be implemented in by a computer program, having codemeans, which when run in a computer causes the computer toexecute the steps of the method. The computer program isincluded in a computer readable medium of a computer programproduct. The computer readable medium may consist ofessentially any memory, such as a ROM (Read-Only Memory), aPROM (Programmable Read-Only Memory), an EPROM (ErasablePROM), a Flash memory, an EEPROM (Electrically Erasable PROM), or a hard disk drive.

According to an aspect of the invention, an apparatus arrangedfor creating a combined representation of a transferred torque rm and a rotation angle d for a torque transferring system is lO presented, where the torque transferring system includes at least two meshing gears.

The apparatus includes determination means, which is arrangedfor determining at least two individual representations of thetransferred torque ru and the rotation angle d, as describedabove. Each one of these individual representations includesat least one transferred torque rm and at least onecorresponding rotation angle d of an individual systemoperation, i.e. each individual representation is a sub-tracefor an individual system operation. The apparatus alsoincludes compensation means, which is arranged forcompensating each individual representation for an increasingtransferred torque rm during the individual system operation.The apparatus also includes creation means, which is arrangedfor combining the at least two compensated individual representations, thereby creating the combined representation.

According to an embodiment of the invention, the apparatus isalso arranged for determining a gear condition for the torquetransferring system. The apparatus here includes, in additionto the means arranged for creating the combined representationmentioned above, analysis means. The analysis means isarranged for analyzing the created combined representation,and is arranged to, based on this analysis, identify apossible deterioration of at least one of the meshing gears in the torque transferring system.

The torque transferring system is, according to an embodiment,a nut-runner system. The individual system operation is thenan individual nut tightening being performed by the nut-runner. As described above, such a nut tightening is oftenperformed rapidly, whereby only a portion of a complete rotation of the meshing gears of the torque transferring lO 2l system is made. Typically, for stiff joints, only a smallportion of the complete rotation is performed during an individual tightening.

According to an embodiment of the invention, the creatingmeans is arranged for including information of a transferredtorque ru over an angle interval corresponding to a completerotation for all of the at least two meshing gears in thecombined representation. Thus, the creation means combines ahigh enough number of individual representations in order togenerate information corresponding to a complete rotation ofthe meshing gears of the system. In other words, individualrepresentations are joined together by appending them suchthat the combined representation has a length which covers acomplete rotation. For example, if a complete rotation of ameshing gears of a nut-runner needs the nut to have beenturned ll turns, then the combined representation shouldinclude enough individual representations adding up to anangel of ll*360°=3960°. An absolute angle dæß can here beutilized for keeping track of where in the combinedrepresentation for the complete rotation an individualrepresentation should be inserted, in order to cover all angles d of the complete rotation.

The above described apparatus can be implemented in a controlunit being situated either at the torque transferring system,e.g. within a nut-runner, or separated from the torquetransferring system, e.g. in a stationary or portableprocessing unit, such as a computer or the like, which is incontact with the torque transferring system, e.g. via radio of cables. 22 The method and apparatus according to the invention may bemodified by those skilled in the art, as compared to the exemplary embodiments described above.

As is obvious for a skilled person, a number of otherimplementations, modifications, variations and/or additionscan be made to the above described exemplary embodiments. Itis to be understood that the invention includes all such otherimplementations, modifications, variations and/or additions which fall within the scope of the claims.

Claims (16)

lO 23 Claims

1. l. Method for creating a combined representation oftransferred torque ru over rotation angle d for a torquetransferring system including at least two meshing gears, saidmethod being characterized by the steps of: - determining at least two individual representations of saidtransferred torque ru over said rotation angle d, eachindividual representation including at least one transferredtorque ru and at least one corresponding rotation angle d ofan individual system operation; - compensating each individual representation for anincreasing transferred torque rn during said individual systemoperation; and - creating said combined representation by combining the at least two compensated individual representations.

2. Method as claimed in claim l, wherein said at least onerotation angle d and said corresponding at least onetransferred torque rm for each individual representation arechosen to indicate an influence of teeth of said meshing gearsas a relationship between said rotation angle and corresponding transferred torque rtr.

3. Method as claimed in anyone of claims l-2, wherein saiddetermination of said at least two individual representationsincludes removing a trend from each one of said at least twoindividual representations, such that each detrendedindividual representation has an amplitude being centered around a torque of approximately zero Nm.

4. Method as claimed in anyone of claims l-3, wherein saidcompensating step results in an amplitude compensated individual representation, said amplitude compensated lO 24 individual representation simulating an essentially constant transferred torque rn during said individual system operation.

5. Method as claimed in anyone of claims l-4, wherein anabsolute angle dæß is utilized for determining an anglerelationship between two angularly subsequent individualrepresentations being combined, thereby allowing said twoangularly subsequent individual representations to be joined together at a correct rotation angle d.

6. Method as claimed in anyone of claims l-5, wherein saidcombined representation includes information of a transferredtorque ru over an angle interval corresponding to a complete rotation for all of said at least two meshing gears.

7. Method for determining a gear condition for a torquetransferring system including at least two meshing gears, saidmethod including the steps of: - creating a combined representation of transferred torque IHover rotation angle d according to anyone of claims l-6; and - analyzing said combined representation, wherein adeterioration of at least one of said at least two meshing gears is identifiable.

8. Method as claimed in claim 7, wherein said analyzing stepincludes a transformation of a time domain signal of saidcombined representation into a frequency domain signal of said combined representation.

9. Method as claimed in claim 8, wherein an equidistant re-sampling is performed on said time domain signal of said combined representation or on at least two time domain signalsof said at least two individual representations, respectively, before said transformation.

10. Method as claimed in anyone of claims 7-9, wherein saiddeterioration is identified by comparing an amplitude of saidfrequency domain signal of said combined representation with at least one amplitude threshold.

11. Computer program, characterized in code means, which whenrun in a computer causes the computer to execute the method according to any of the claims 1-10.

12. Computer program product including a computer readablemedium and a computer program according to claim 11, whereinsaid computer program is included in the computer readable medium.

13. Apparatus arranged for creating a combined representationof transferred torque ru over rotation angle d for a torquetransferring system including at least two meshing gears,characterized by: - determination means, arranged for determining at least twoindividual representations of said transferred torque ru oversaid rotation angle d, each individual representationincluding at least one transferred torque rm and at least onecorresponding rotation angle d of an individual systemoperation; - compensation means, arranged for compensating eachindividual representation for an increasing transferred torquern during said individual system operation; and - creation means, arranged for creating said combinedrepresentation by combining the at least two compensated individual representations.

14. Apparatus as claimed in claim 13, wherein said creationmeans is arranged for including information of transferred torque ru over an angle interval corresponding to a complete 26 rotation for all of said at least two meshing gears in said combined representation.

15. Apparatus arranged for determining a gear condition forsaid torque transferring system, said apparatus including: - means for creating a combined representation of transferredtorque ru over rotation angle d according to anyone of claims13-14; and - analysis means, arranged for analyzing said combinedrepresentation, wherein a deterioration of at least one of said at least two meshing gears is identifiable.

16. Apparatus as claimed in anyone of claims 13-15, whereinsaid torque transferring system is a nut-runner system, andsaid individual system operation is an individual nut tightening.