DK160592B

DK160592B - Method and apparatus for measuring the angular velocity difference between two rotating measurement objects

Info

Publication number: DK160592B
Application number: DK306488A
Authority: DK
Inventors: Finn Mengel
Original assignee: Dantec Elektronik Med
Priority date: 1988-06-06
Filing date: 1988-06-06
Publication date: 1991-03-25
Also published as: DK160592C; DK306488D0; DK306488A

Description

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Opfindelsen angår en fremgangsmåde til måling af vinkelhastighedsforskel mellem to måleobjekter, som udfører en roterende bevægelse ved hjælp af kohærent elektromagnetisk stråling.The invention relates to a method for measuring angular velocity difference between two measuring objects which perform a rotary motion by coherent electromagnetic radiation.

5 Fra EP-patentskrift nr. 0103422 og en artikel af T. Watanabe m.fl. "Optical Heterodyne Method For Measuring Rotational Velocity" publiceret i Optics Communications, Vol. 43, nr. 3, 1. oktober 1982 er det kendt at måle vinkelhastigheden for et roterende legeme f.5 From EP Patent No. 0103422 and an article by T. Watanabe et al. "Optical Heterodyne Method For Measuring Rotational Velocity" published in Optics Communications, Vol. 43, No. 3, October 1, 1982, it is known to measure the angular velocity of a rotating body f.

10 eks. til brug ved bestemmelse af torsionssvingninger i roterende maskindele, ved en optisk heterodynmåling, hvor to parallelle bundter af kohærent stråling rettes mod overfladen af det roterende legeme og de tilbagesendte spredningsstrålinger fra indfaldspunkterne ud-15 sættes for heterodyn frekvensblanding, hvorved modulationsfrekvensen bestemmes som udtryk for rotationshastigheden.For example, for use in determining torsional oscillations in rotating machine parts, by an optical heterodyne measurement where two parallel bundles of coherent radiation are directed to the surface of the rotating body and the returned scattering radiation from the points of incidence is subjected to heterodyne frequency mixing, whereby modulation frequency is determined. expression of the rotational speed.

Den foreliggende opfindelse tager sigte på at anvise en videreudvikling af dette kendte måleprincip 20 til brug ved bestemmelse af forskellen mellem to roterende måleobjekter vinkelhastigheder, hvadenten de to måleobjekter er separate fysiske genstande som f. eks. en remskive og en transmissionsrem, eller udgøres af aksialt forskudte sektioner eller dele af et og samme 25 roterende legeme, f. eks. en aksel.The present invention aims to provide a further development of this known measurement principle 20 for use in determining the difference between two rotational measuring objects angular velocities, whether the two measuring objects are separate physical objects, such as a pulley and a transmission belt, or constitute axially staggered sections or parts of one and the same rotating body, e.g., a shaft.

Som væsentlige men ikke-begrænsende eksempler på opfindelsens industrielle anvendelse kan således navnlig nævnes en nøjagtig måling af slip i en remtransmission, hvorved opfindelsen imødekommer et stort praktisk 30 behov, samt bestemmelse af torsionssvingninger i roterende aksler.Thus, as significant but non-limiting examples of the industrial application of the invention there may be mentioned, in particular, an accurate measurement of slip in a belt transmission, whereby the invention meets a great practical need, and determination of torsional vibrations in rotating shafts.

Med udgangspunkt i det ovenfor beskrevne kendte måleprincip er fremgangsmåden ifølge opfindelsen ejendommelig ved, at vinkelhastighederne for de to måleob-35 jekter bestemmes ved laser-doppler måling i to separate balancerede målekanaler, i hver af hvilke to rumligt 2Based on the known measurement principle described above, the method according to the invention is characterized in that the angular velocities of the two measurement objects are determined by laser-doppler measurement in two separate balanced measuring channels, in each of which two spatially 2

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adskilte laserstrålebundter hidrørende fra samme strålingskilde udsendes i i det væsentlige parallelle senderetninger mod to i det væsentlige i samme radialplan liggende indfaldspunkter på det respektive måleobjekt, 5 og at der efter detektering af de fra indfaldspunkterne i det væsentlige parallelle modtageretninger tilbagesendte spredningsstrålinger i hver kanal frembringes et Doppler-målesignal som udtryk for forskellen mellem Doppler-frekvenserne for de to spredningsstrålinger, 10 idet der i den ene kanal foretages en forskydning af Doppler-frekvensen for den ene spredningsstråling, hvorefter signalerne fra de to kanaler blandes og stødfrekvensen i det resulterende signal separeres som udtryk for vinkelhastighedsforskellen.separate laser beam bundles originating from the same radiation source are emitted in substantially parallel transmitting directions against two substantially incident points on the respective measurement object, 5 and, after detecting the scattering radiations returned from the incident points in substantially parallel receiver directions, in each channel measuring signal as an expression of the difference between the Doppler frequencies of the two scattering radiations, 10 in which one channel is displaced by the Doppler frequency of the one scattering radiation, after which the signals from the two channels are mixed and the shock frequency of the resulting signal is separated as expression. for the angular velocity difference.

15 I hver af de to målekanaler foretages laser- doppler-målingen efter i det væsentlige samme princip som ved ovennævnte kendte målemetode. Gennem forskydningen af Doppler-frekvensen for det ene strålebundt i kun den ene kanal indføres en bærefrekvens, som mulig-20 gør heterodynblanding af Doppler-målesignalerne fra de to kanaler, hvorved stødfrekvensen kan frasepareres som direkte udtryk for vinkelhastighedsforskellen.In each of the two measurement channels, the laser doppler measurement is performed according to essentially the same principle as in the above known measurement method. Through the displacement of the Doppler frequency for the one beam in only one channel, a carrier frequency is introduced which allows heterodyne mixing of the Doppler measurement signals from the two channels, whereby the shock frequency can be separated as a direct expression of the angular velocity difference.

Med henblik på udnyttelse af fremgangsmåden til bestemmelse af vinkelhastighedsforskel mellem to sepa-25 rate fysiske objekter med stærkt forskellig reflek-tionsevne har det vist sig hensigtsmæssigt at foretage en rumlig separering af modtageretningen og senderetningen for hvert strålebundt i hver kanal f. eks. ved at senderetningen og modtageretningen krydser hinanden 30 under en lille vinkel ved måleobjektets overflade.In order to utilize the method for determining angular velocity difference between two separate physical objects with greatly different reflectivity, it has been found appropriate to make a spatial separation of the receiver direction and the transmitting direction for each beam of beam in each channel, e.g. the transmitter direction and the receiver direction intersect at a small angle at the surface of the measurement object.

Til brug ved udøvelse af fremgangsmåden angår opfindelsen tillige et apparat til måling af vinkelhastighedsforskel mellem to måleobjekter som udfører en roterende bevægelse ved hjælp af kohærent elektromagne-35 tisk stråling, hvilket apparat er ejendommeligt ved, at det omfatter to separate balancerede laser-Doppler må- 3For use in practicing the method, the invention also relates to an apparatus for measuring angular velocity difference between two measuring objects which performs a rotary motion by coherent electromagnetic radiation, which is characterized in that it comprises two separate balanced laser-Doppler measuring devices. 3

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lekanaler for hver sit af de to måleobjekter, hvilke kanaler hver omfatter bundtdele- og fokuseringsorganer til udsendelse af to rumligt adskilte laserstrålebundter hidrørende fra samme strålingskilde i det væsentli-5 ge parallelle senderetninger mod to i det væsentlige samme radialplan liggende indfaldspunkter på det pågældende måleobjekt samt midler til detektering af de fra indfaldspunkterne i i det væsentlige parallelle modtageretninger tilbagesendte spredningsstrålinger midler 10 til bestemmelse af Doppler-frekvensforskydningen for hver af disse spredningsstrålinger og midler til frembringelse af et Doppler-målesignal som udtryk for forskellen mellem Doppler-frekvensforskydningerne for de to spredningsstrålinger, hvorhos der i den ene kanal 15 findes midler til forskydning af Doppler-frekvensen for den ene spredningsstråling og signaludgangene fra de to kanaler er forbundet til en subtraktiv frekvensblander med et efterfølgende separeringsorgan for den ved blandingen frembragte stødfrekvens som udtryk for vinkelha-20 stighedsforskellen mellem de to måleobjekter.play channels for each of the two measurement objects, each of which comprises bundle and focusing means for transmitting two spatially separated laser beam bundles originating from the same radiation source substantially parallel transmitting directions to two substantially radial plane incident points on said measurement object, and means for detecting the scattering radiations returned from the points of incidence in substantially parallel receiver directions means 10 for determining the Doppler frequency offset for each of these scattering radiations and means for generating a Doppler measurement signal as an expression of the difference between the Doppler frequency offsets for the two scattering radiations. means are provided in one channel 15 for displacing the Doppler frequency for the one scattering radiation and the signal outputs of the two channels are connected to a subtractive frequency mixer with a subsequent separating means for the mixing device. gene produced shock frequency as an expression of the angular velocity difference between the two measurement objects.

Opfindelsen forklares i det følgende nærmere under henvisning til den skematiske tegning, hvor fig. 1 og 2 illustrerer de geometriske forhold ved måling af slip i en remtransmission, 25 fig. 3 i hovedtræk viser et eksempel på den op tiske konfiguration af et apparat ifølge opfindelsen, fig. 4 et eksempel på behandlingen af Doppler-signalerne i de to målekanaler, fig. 5 en udførelsesform for en i den ene måle-30 kanal virksom frekvensforskydningsindretning, og fig. 6 og 7 udførelsesformer for den optiske konfiguration i en målekanal til frembringelse af to parallelle strålebundter og til rumlig separering af sende- og modtageretningerne.The invention is explained in more detail below with reference to the schematic drawing, in which fig. 1 and 2 illustrate the geometric conditions for measuring slip in a belt transmission; FIG. 3 is a general view of an example of the optical configuration of an apparatus according to the invention; 4 shows an example of the processing of the Doppler signals in the two measurement channels; FIG. 5 shows an embodiment of a frequency shift device operating in one of the measuring channels; and FIG. 6 and 7 are embodiments of the optical configuration in a measurement channel for generating two parallel beams and for spatially separating the transmitting and receiving directions.

35 Som eksempel på en praktisk industriel anvendel se af fremgangsmåden af apparatet ifølge opfindelsen er 435 An example of a practical industrial application of the method of the apparatus according to the invention is 4

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i fig. 1 og 2 vist en del af en remtransmission med en remskive 1 og en kilerem 2.in FIG. 1 and 2 show a portion of a belt transmission with a pulley 1 and a V-belt 2.

Undser belastning kan der i en sådan transmission optræde slip mellem remskiven 1 og remmen 2 så- 5 vel ved den effektafgivende remskive som den eller de effektforbrugende remskiver. I en given situation vil slippet i hovedsagen afhænge af faktorer som overfladebeskaffenhed, elasticitet og andre materialeegenskaber hos remmen 2.Under such a load, in such a transmission, slip between the pulley 1 and the belt 2 can occur both at the power-producing pulley and the power-consuming pulley (s). In a given situation, the release will depend mainly on factors such as surface texture, elasticity and other material properties of the belt 2.

10 Det skal imidlertid bemærkes, at den illustrere de slipmåling kun er et eksempel på et område, hvor der foreligger behov for bestemmelse af små forskelle i rotations- eller vinkelhastihed.10 However, it should be noted that illustrating the slip measurement is just one example of an area where there is a need for determining small differences in rotational or angular velocity.

Remskiven 1 antages at rotere med en tidsafhæn- 15 gig vinkelhastig Ω-j^t) og at være udsat for ligeledes tidsafhængige vibrationspåvirkninger V|-rans(t). Slippet kan beskrives som den relative hastighed mellem remskiven 1 og remmen 2, i deres kontaktflade, men da kontaktfladen normalt ikke vil være tilgængelig, kan slip-20 pet i praksis måles som forskellen i vinkelhastighed mellem remskiven 1 og remskiven 2, dvs. s = (Ω2-Ω1)/ΩχThe pulley 1 is assumed to rotate at a time-dependent angular velocity Ω-j (t) and to also be subject to time-dependent vibration effects V | -rans (t). The slip can be described as the relative velocity between the pulley 1 and the belt 2, in their contact surface, but since the contact surface will not normally be available, the slip can in practice be measured as the difference in angular velocity between the pulley 1 and the pulley 2, ie. s = (Ω2-Ω1) / Ωχ

Indføres vT1 og vT2 som udtryk for de tangentia-le hastigheder ved ydersiden af henholdsvis remskiven 25 l og remmen 2, R-^ som udtryk for remskivens radius og AR som udtryk for forskellen mellem remmen 2 og remskiven 1 fås Ω2 = Vij^/(Κχ H* AR) Ωχ = vT1/Rx 30 Ud fra disse definitioner kan hovedproblemerne ved slipmålingen opstilles som følger:Entering vT1 and vT2 as an expression of the tangential velocities at the outside of the pulley 25 l and the belt 2, respectively, R- as an expression of the radius of the pulley and AR as an expression of the difference between the belt 2 and the pulley 1 is obtained Ω2 = Vij ^ / ( Κχ H * AR) Ωχ = vT1 / Rx 30 From these definitions, the main problems in the slip measurement can be set as follows:

Ekcentricitet af remskiven 1 medfører, at radius Rx bliver tidsafhængig og vil ofte foreligge ved den for optisk måling tilgængelige kantdel af remskiven 35 l's yderside, for hvilken der ofte ikke stilles særlige tolerancekrav ved fremstillingen.Eccentricity of the pulley 1 causes the radius Rx to become time-dependent and will often be present at the edge portion available for optical measurement of the outer surface of the pulley 35 l, for which special tolerance requirements are often not made during manufacture.

55

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Den hastighed som faktisk kan måles i et givet punkt på overfladen er vektorsummen af den tangentiale hastighed v^ og den translatoriske hastighed v.j.rans, hvoraf sidstnævnte som følge af vibrationer ofte vil 5 være betydeligt større end hastighedsforskellen mellem remskiven 1 og remmen 2.The velocity that can actually be measured at a given point on the surface is the vector sum of the tangential velocity v ^ and the translational velocity v.j.rans, the latter of which, due to vibrations, will often be considerably greater than the velocity difference between the pulley 1 and the belt 2.

Variationer i remtykkelsen vil føre til belastningsafhængige variationer i radiusforskellen AR.Variations in belt thickness will lead to load-dependent variations in the radius difference AR.

Da det måleteknisk interessante område er slip-10 værdier omkring 1%, således at der skal måles en lille forskel mellem store nummeriske størrelser, må der stilles store krav til nøjagtigheden.Since the measurement area of interest is slip-10 values around 1%, so that a small difference between large numerical sizes must be measured, high demands must be placed on accuracy.

Da der til en fleksbel transmi s s ions rem sædvanligvis ikke vil kunne fastgøres nogen målesensor eller 15 detektor, vil optiske målemetoder i sig selv forekomme ideelle forsåvidt de kan tilpasses objekternes reflek-tionsevne. Da en transmiss ions rem ofte har sort overflade, vil der hertil kræves en betydelig lasereffekt.Since no measuring sensor or detector will usually be able to be attached to a flexible transmission strap, optical measurement methods per se will appear ideal in that they can be adapted to the reflectivity of the objects. Since a transmission belt often has a black surface, a significant laser effect will be required.

Navnlig vibrationsproblemet fører til, at en 20 konventionel lase-Doppler måling med hinanden krydsende strålebundeter vil vise sig mindre egnet i praksis, fordi fokuseringsdybden er for ringe.In particular, the vibration problem causes a conventional laser-Doppler measurement with intersecting beam bundles to prove less suitable in practice because the focusing depth is too low.

Den indledningsvis beskrevne kendte målemetode udmærker sig ved netop at undgå denne ulempe og elime-25 nerer herved i forbindelse med ren vinkelhastighedsmåling de problemer, der skyldes excentricitet og vibrationer.The known measurement method initially described is distinguished by precisely avoiding this disadvantage and thereby eliminates in connection with pure angular velocity measurement the problems caused by eccentricity and vibration.

I det stærkt skematiske diagram i fig. 3 er de to måleobjekter som f. eks. em remskive og en transmis-30 sionsrem vist som to separate roterende legemer 3 og 4. Fra en fælles strålingskilde i form af en laser 5 sendes et strålebundt mod en variabel bundtdeler 6, med hvilken lasereffekten fordeles på to målekaner 7 og 8, i hver af hvilke, der, som forklaret i det føl-35 gende, frembringes to rumligt adskilte, i det væsentlige parallelle strålebundter henholdsvis 9, 10 og 11, 6In the highly schematic diagram of FIG. 3, the two measuring objects such as pulley and transmission belt are shown as two separate rotating bodies 3 and 4. From a common radiation source in the form of a laser 5, a beam of radiation is sent to a variable beam divider 6 with which the laser power are divided into two measuring jugs 7 and 8, in each of which, as explained below, two spatially separated, substantially parallel beams of 9, 10 and 11, 6 are produced, respectively.

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12, som parvis rettes mod to i samme radialplan liggende målepunkter henholdsvis 13, 14 og 15, 16 på overfladen af hver sit af de to roterende legemer 3 og 4.12, which are directed in pairs to two measuring points located in the same radial plane 13, 14 and 15, 16 respectively on the surface of each of the two rotating bodies 3 and 4.

5 De tilbagesendte spredningsstrålinger fra lege merne 3 og 4 føres via bundtdelere 17 og 18 i kanalerne 7 og 8 til fotodetektorer 19 og 20.5 The returned scattering radiations from physicians 3 and 4 are fed through bundle dividers 17 and 18 in channels 7 and 8 to photodetectors 19 and 20.

I den ene af målekanalerne indgår i det viste eksempel en optisk frekvensforskydningsindretning 21, 10 som frekvensforskyder det ene af de udsendte strålebundter i den pågældende kanal og derved indfører en bærefrekvens i Doppler-signalet i denne kanal.In one of the measurement channels, in the example shown, an optical frequency offset device 21, 10, which frequency offsets one of the transmitted beams in the respective channel, includes a carrier frequency in the Doppler signal in that channel.

I principdiagrammet i fig. 4, hvor rotationshastighederne for legemerne 3 og 4 er angivet ved Q3 og 15 Q4 er den i kanalen 7 indførte frekvensforskydning betegnet med fs, hvorved der som Doppler-målesignaler fra fotodektorerne 19 og 20 fås henholdsvis f^+fø og fD4. Disse signaler føres til en subtraktiv frekvensblander 22 af den dobbeltbalancerede type, som herved 20 afgiver udgangssignalet fD3"^D4+f0/ hvorfra vinkelhastighedsforskellen ίΐ)3-%4 uden videre kan frasepareres i den efterfølgende Doppler-processor 23.In the principle diagram of FIG. 4, where the rotational speeds of the bodies 3 and 4 are indicated by Q3 and 15 Q4, the frequency offset introduced in channel 7 is denoted by fs, whereby as Doppler measurement signals from the photo detectors 19 and 20 are obtained f ^ + f0 and fD4, respectively. These signals are fed to a subbalanced frequency mixer 22 of the dual-balanced type, which thereby outputs the output signal fD3 + D4 + f0 / from which the angular velocity difference ΐ 3 - 4% can be separately separated into the subsequent Doppler processor 23.

Pig. 5 viser et eksempel på optisk frembringelse af den i målekanalen 7 virksomme frekvensforskydning.Pig. 5 shows an example of optical generation of the frequency offset operating in the measuring channel 7.

25 Den kohærente stråling fra strålingskilden 5, der til opnåelse af en tilstrækkelig effekt fortrinsvis er en Ar-laser føres i den variable bundtdeleindretning 6 gennem en λ/4-plade 24 til en bundtdeler 25, som fordeler lasereffekten mellem målekanalerne 7 og 8. I 30 det beskrevne praktiske tilfælde vedrørende slipmåling for en remtransmission foretages den af λ/4-pladen 24 og bundtdeleren 25 frembragte variable bundtdeling med fordel således, at størstedelen af effekten dirigeres til den målekanal 8, som er knyttet til transmis-35 sionsremmen 2, der har den ringeste reflektionsevne.The coherent radiation from the radiation source 5, which in order to obtain a sufficient power, is preferably an Ar laser is passed in the variable bottom part device 6 through an λ / 4 plate 24 to a bundle divider 25 which distributes the laser power between the measuring channels 7 and 8. In the described practical case concerning slip measurement for a belt transmission, the variable bottom distribution produced by the λ / 4 plate 24 and the bundle divider 25 is advantageously made so that most of the power is directed to the measuring channel 8 which is connected to the transmission belt 2, which has the slightest reflection ability.

I den anden kanal 7 føres delstrålebundtet fra bundtdeleren 25 gennem en λ/2-plade 26 til en polari- 7In the second channel 7, the partial beam bundle from the bundle divider 25 is passed through a λ / 2 plate 26 to a polar 7.

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serende bundtdeler 27, hvorfra to strålebundter med i forhold til hinanden orthogonale polarisationsretninger føres henholdsvis gennem en Bragg-celle 28 og en kvarts-kompensatorstang 29. Med Bragg-cellen 28 til-5 føres det pågældende strålebundt en frekvensforskyd-ningpå f. eks. 40 MHz. De orthogonalt polariserede strålebundter fra Bragg-cellen 28 og kvarts-kompensa-torstangen 29 føres igen sammen i en bundtdeler 30 og derfra til målekanalen 7.transmitting bundle portions 27, from which two beam bundles having mutually orthogonal polarization directions are passed through a Bragg cell 28 and a quartz compensator rod, respectively 29. With the Bragg cell 28, the beam beam in question is fed a frequency shift of, for example, 40 MHz. The orthogonally polarized beam bundles from Bragg cell 28 and quartz compensator rod 29 are again brought together in a bundle divider 30 and thence to the measuring channel 7.

10 I den ikke-frekvensforskudte målekanal 8 deles lasereffekten som vist i fig. 6 i en λ/2 plade 31 i to strålebundter med indbyrdes orthogonale polarisationsretninger, f. eks. 45° i forhold til tegningens plan. I et Wollaston-prisme 32 af bøj es de to stråle-15 bundter under en indbyrdes vinkel på f. eks. 2®. Wollaston prismet 32 befinder sig i brændplanet for en kombination af en bundtekspander 33 og en frontlinse 34, hvorfra de to parallelle, indbyrdes orthogonalt polariserede strålebundter 11 og 12 rettes mod måleob-20 jektet 4.10 In the non-frequency offset measuring channel 8, the laser power as shown in FIG. 6 in a λ / 2 plate 31 in two beams with mutually orthogonal polarization directions, eg 45 ° with respect to the plane of the drawing. In a Wollaston prism 32 of the two beam-15 bundles are bent at an angle of, for example, 2®. The Wollaston prism 32 is in the focal plane of a combination of a bottom expander 33 and a front lens 34, from which the two parallel, orthogonally polarized beam bundles 11 and 12 are directed to the measurement object 4.

Med en bundtekspansionsfaktor på 1,94 og en brændvidde på 600 mm for frontlinsen 34 opnås herved en afstand mellem bundterne 11 og 12 på 10,8 mm og en kalibreringsfaktor på 4,6 kHz/rpm. Til opnåelse af en 25 tilstrækkelig fin fokusering kan en yderligere bundtekspander 35 være anbragt foran Wollaston-prismet 33.With a bottom expansion factor of 1.94 and a focal length of 600 mm for the front lens 34, a distance between the bundles 11 and 12 of 10.8 mm and a calibration factor of 4.6 kHz / rpm is thereby obtained. To achieve a sufficiently fine focus, an additional bottom expander 35 may be disposed in front of the Wollaston prism 33.

Medens det ved de foran beskrevne kendte metoder til vinkelhastighedsmåling foreskrives, at sende- og modtageretningerne skal være sammenfaldende foretrækkes 30 ifølge opfindelsen en rumlig separering af sende- og modtageretningerne i hver kanal, således som det er vist i fig. 7. Herved undgås nemlig spredningslys fra de optiske overflader i bundtekspanderen 34 efter Wollaston-prismet 33.While it is prescribed by the known methods for angular velocity measurement described above that the transmitting and receiving directions should be coincident, a spatial separation of the transmitting and receiving directions in each channel, as shown in FIG. 7. This avoids scattering light from the optical surfaces of the bottom expander 34 according to the Wollaston prism 33.

35 Separering kan foretages således, at sende- og modtageretningerne danner en lille vinkel med hinandenSeparation can be made so that the transmitting and receiving directions form a small angle with each other

Claims

A method for measuring angular velocity difference between two measuring objects performing a rotary motion by coherent electromagnetic radiation characterized by determining the angular velocities of the two measuring objects (3, 4) by laser-Doppler measurement in two separate balanced measurement channels (7, 8) in each of which two spatially spaced laser beam bundles (9-12) originating from the same radiation-ski side (5) are emitted in substantially parallel transmitting directions to two substantially incident points (13-16) located at the same radial plane DK 160592B 9 the respective measuring object (3, 4) and, after detecting the scattering radiations returned from the incident points in substantially parallel receiver directions, in each channel, a Doppler measurement signal is generated as an expression of the difference between the Doppler frequencies of the two scattering radiations, in one channel (7), the Doppler frequency is shifted for one scattering beam ng, after which the signals from the two channels 10 are mixed and the shock frequency in the resulting signal is separated as an expression of the angular velocity difference.

Method according to claim 1, characterized in that said offset of the Doppler frequency is made as optical frequency offset of the one transmitted laser beam bundle.

Method according to claim 1 or 2, characterized in that for each beam of beam in each channel the receiver direction is spatially separated from the transmitter direction.

Method according to claim 3, characterized in that the transmitting direction and the receiving direction cross each other at a small angle.

Method according to one of claims 1-4, characterized in its use for measuring the slip of a pulley between a pulley and a transmission belt.

Method according to one of Claims 1 to 4, characterized by its use for torsion measurement on a rotating shaft.

Apparatus for measuring angular velocity difference 30 between two measuring objects (3, 4), which performs a rotary motion by coherent electromagnetic radiation in the practice of claim 1, 2 or 3, characterized in that it comprises two separate balanced laser beams. Doppler measurement channels (7, 8) 35 for each of the two measurement objects, each of which comprises bottom part and focusing means (31-34) for emitting two spatially separated laser beam bundles originating from the same radiation source (5) parallel transmitting directions against two substantially the same radial plane incident points on the respective measuring object and means (19-20) for detecting the scattering radiation returned from the incident points in the substantially parallel receiver directions, means for determining the Doppler frequency offset for each of these scattering radiation and means for producing a Doppler measurement design all as an expression of the difference between the Doppler frequency offsets for the two scattering radiations, wherein means (28) for displacing the Doppler frequency for the one scattering radiation and the signal outputs of the two channels (7, 8) are connected to a subtractive frequency mixer (22) with a subsequent separating means (23) for the shock frequency produced by the mix as an expression of the angular velocity difference between the two measurement objects.

Apparatus according to claim 7, characterized in that said means for displacing the Doppler frequency comprises a bundle divider (27) with a subsequent Bragg cell (28) disposed in the path of the laser radiation fed to the channel (7).

Apparatus according to claim 7 or 8, characterized by means for spatially separating transmitter direction and receiver direction for each beam of beam in each channel.

Apparatus according to one of claims 7-9, characterized in that the two measuring channels are designed as individual controllable units connected to the same laser radiation source over optical fiber transmission devices.

Apparatus according to one of Claims 4 to 7, characterized in that means are provided for individually adjusting the laser power in the two channels for compensating for differences in the reflectivity of the two measuring objects.