Method and apparatus for controlling shot peening

A shot peening process may be controlled by

FIELD

The aspects of the disclosed embodiments relate to controlling shot peening.

BACKGROUND

Shot peening may be used for processing the surfaces of critical metallic components, e.g. gas turbine blades, toothed gears, or axles. A shot peening process may be verified by using so called Almen strips. The use of Almen strips may involve considerable amount of manual work.

SUMMARY

Some versions may relate to verification of a shot peening apparatus. Some versions may relate to a device for verifying a shot peening process. Some versions may relate to monitoring operation of a shot peening apparatus. Some versions may relate to a device for monitoring a shot peening process. Some versions may relate to controlling operation of a shot peening apparatus. Some versions may relate to a device for controlling a shot peening process. Some versions may relate to a shot peening apparatus, which comprises an optical device for verifying operation of the shot peening apparatus.

According to an aspect, there is provided a method, comprising:using a first shot peening unit to provide a particle jet,exposing one or more test strips to the particle jet such that the first shot peening unit provides a particle jet according to a first set of operating parameters,measuring one or more first deformation values of the test strips,illuminating at least a portion of the particle jet with illuminating light,capturing images of said portion during a measurement time period, wherein the first shot peening unit provides a particle jet according to said first set of operating parameters during said measurement time period,determining at least one velocity value of particles of the particle jet by analyzing the captured images, anddetermining a model based on the one or more first deformation values and based on the at least one velocity value.

According to an aspect, there is provided a method, comprising:providing a model which establishes a relationship between a velocity value of a particle jet and a deformation value,using a first shot peening unit to provide a particle jet,illuminating at least a portion of the particle jet with illuminating light,capturing images of said portion,determining one or more velocity values of particles of the particle jet by analyzing the captured images,determining an estimate of an arc height value from the one or more velocity values by using the model, andclassifying a shot peening operation as valid or invalid by checking whether the estimate of the arc height value is in a predetermined range.

According to an aspect, there is provided a method, comprising:providing a model which establishes a relationship between a velocity value of a particle jet and a deformation value,using a first shot peening unit to provide a particle jet,illuminating at least a portion of the particle jet with illuminating light,capturing images of said portion,determining at least one velocity value of particles of the particle jet by analyzing the captured images, andadjusting one or more operating parameters of the shot peening unit based on the velocity value.

According to an aspect, there is provided an apparatus, comprising:an illuminating unit to provide an illuminating light beam,an image sensor to capture images of a particle jet illuminated by the illuminating light beam, andan interface to receive one or more deformation values,
wherein the apparatus is configured to determine one or more velocity values of particles of the particle jet by analyzing the captured images, and to determine a model based on the one or more first deformation values and based on the one or more velocity values.

The method may comprise:using a shot peening unit (700) to provide a particle jet (JET1),exposing one or more test strips (S1) to the particle jet (JET1),measuring one or more deformation values (hAS) of the test strips (S1),illuminating at least a portion (RG0) of the particle jet (JET0) with illuminating light (LB0),capturing images (IMG2) of said portion (RG0),determining at least one velocity value (vAVE, vRMS) of the particle jet (JET1) by analyzing the captured images (IMG2), anddetermining a model (MODEL1) based on the one or more deformation values (hAS) and based on the at least one velocity value (vAVE, vRMS).

The shot peening unit may be arranged to provide a particle jet, which comprises particles moving at a high velocity. The particle jet may be directed to an object so as to process the surface of said object by shot peening.

The monitoring device may comprise an illuminating unit and an imaging unit. The illuminating unit may illuminate a predetermined region of the particle jet. The imaging unit may capture digital images of particles located within the illuminated region. The method may comprise estimating an arc height value (hAS) and/or a time equivalent value (TINT) by analyzing the captured digital images. The device may comprise a data processing unit, which may be configured to analyze the captured images.

The capability of the particle jet to cause irreversible plastic deformation of a surface may be quantitatively measured by using the test strip AS1, which may also be called e.g. as the Almen strip. Arc height values and/or time equivalent values may be measured by exposing the test strip to the particle jet such that the particle jet causes bending of the test strip. The shape of the test strip AS1after a shot peening test may be defined e.g. by an arc height value hAS.

The method may comprise measuring at least one velocity value by analyzing the captured images. The method may comprise determining a model, which describes a relationship between measured velocity values and the corresponding arc height values. The model may be determined based on measured arc height values and based on measured velocity values.

The method may comprise measuring at least one velocity value by analyzing the captured images. A corresponding deformation of an Almen strip may be subsequently estimated from the measured velocity value by using the model, without deforming the Almen strip.

The monitoring device may provide one or more measured velocity values based on a high number of detected particles. The measurement result may be based on a statistically meaningful portion of all particles which impinge on a target during a shot peening process. Analysis of the captured images may provide a statistically meaningful result within a reasonable time.

The present method may allow substantially continuous monitoring during a shot peening process. The present method may allow detecting transient disturbances of the jet. The present method may allow reducing the amount of manual work needed for handling the Almen strips. The present method may also allow determining the coverage of the particle jet.

Thanks to using the present imaging method, the use of the test strips may be reduced or avoided. Estimating the arc height values hASby using the measurement device and by using the model may reduce the number of test strips AS1needed for verifying a shot peening process. The operation of the shot peening unit may be monitored and/or verified before as shot peening process, during the shot peening process and/or after the shot peening process. The operation of the shot peening unit may be monitored and/or verified in real time. The need for manual work may be reduced or avoided. The operation of the shot peening unit may be monitored several times during a shot peening process or even continuously without increasing the amount of manual work needed for the monitoring. The verification may be performed more often and/or with a higher accuracy. Consequently, the quality of a shot peened product may be improved.

Estimating the arc height values hASby using the measurement device and by using the model may allow on-line control of a shot peening process.

DETAILED DESCRIPTION

Referring toFIG. 1, the surface SRF2of an object OBJ1may be processed by shot peening.

Shot peening may be used. e.g. for increasing the operating life of a component OBJ1, which is intended for use in demanding conditions. Shot peening may produce compressive residual stress in the surface layer SRF2of the component OBJ1. The compressive stress may reduce the risk of propagation of microscopic cracks in the surface layer SRF2. Shot peening may increase operating life of the parts, e.g. by reducing the risk of fatigue failure.

Shot peening may be performed by accelerating macroscopic particles P0to a high velocity, and directing the moving particles P0to the surface SRF2of an object OBJ1. The particles P0may hit the surface SRF2and may cause plastic deformation of the surface layer of the object. The moving particles P0may be called e.g. as the “shots”. The particles may be e.g. steel balls or ceramic balls. Shot peening may comprise providing a particle jet JET0, which comprises a plurality of particles P0k, P0k+1, P0k+2, . . . moving at high velocities vk, vk+1, vk+2, . . . . A shot peening unit700may be arranged to provide the particle jet JET0. The particle jet JET0may be provided e.g. by accelerating the particles with a high velocity gas stream. The particle jet JET0may also be provided e.g. by accelerating the particles with a rotating mechanical element.

The object OBJ1may also be called e.g. as a target. The surface SRF2may be exposed to particles of a particle jet JET0. A shot peening unit700may be arranged to provide the particle jet JET0. The jet JET0may comprise a plurality of particles P0k, P0k+1, P0k+2, . . . . The particles may be e.g. metal balls, pieces of metal wire, or ceramic beads. In particular, the particles may be steel balls.

The jet may be directed to the surface SRF2of the target. The target OBJ1may be e.g. a part of a machine, engine and/or a vehicle. The target OBJ1may be e.g. a mechanical component of a device.

The particle jet JET0may have central axis AX0. The particles of the jet may propagate substantially in the direction of the axis AX0. The jet may also be slightly diverging such that the particles have a significant velocity component in the direction of the axis AX0.

SX, SY, and SZ denote orthogonal directions. The axis AX0of the jet may be parallel e.g. with the direction SZ. The reference position POS0denotes a point where the axis AX0intersects the surface of the object. POS(x,y,z) may denote an arbitrary position. The position POS(x,y,z) may be specified e.g. by coordinates x,y,z with respect to the reference position POS0.

L2denotes a distance between the shot peening unit700and the surface SRF2. In particular, L0may denote a distance between the accelerating nozzle of the shot peening unit700and the surface SRF2. The position of the shot peening unit700and/or the orientation of the target OBJ1may be selected such that the surface SRF2is substantially perpendicular to the axis AX0.

Referring toFIG. 2a, the capability of a particle jet JET0to cause peening may be measured experimentally by using a test strip AS1. The test strip may be called e.g. as the Almen strip. The capability of the particle jet to cause the plastic deformation may be quantitatively measured by using the test strip AS1. The efficiency of the particle jet JET0may be measured by using the test strip AS1. The impacts of the particles may cause bending of a test strip so that the test strip AS1forms an arc. The height (hAS) of the arc may depend on the operating parameters of the shot peening unit700, on the distance (L1) between the shot peening unit700, and on the duration of the peening.

The dimensions of the test strips AS1and/or the details of the experimental set-up may be defined e.g. in a standard SAE J442, J443, J2277, J2597, AMS2430, and/or AMS2432. SAE means Society of Automotive Engineers, an organization based in the United States of America.

The particles may hit an exposed area AREA1of the surface SRF1of the test strip AS1. L1may denote the distance between the shot peening unit700and the test strip AS1. In particular, L0may denote the distance between the particle accelerating nozzle of the shot peening unit700and the reference area AREA0. The axis of the particle jet JET0may coincide with the center of the area AREA1.

Referring toFIG. 2b, the shot peening unit700may provide a particle flux through a reference area AREA0. L0may denote the distance between the shot peening unit700and the reference area AREA0. In particular, L0may denote the distance between the particle accelerating nozzle and the reference area AREA0. The reference area AREA0may have e.g. a width Δx and a height Δy. The reference area AREA0may be perpendicular to the axis AX0of the particle jet. The measuring device500may be arranged to measure the total kinetic energy of particles which pass through the reference area AREA0during a measurement time period TMEAS. The measuring device500may be arranged to capture images of particles which pass through the reference area AREA0. The measuring device500may be arranged to monitor the particle jet in the vicinity of the reference area AREA0.

The reference area AREA0may be positioned e.g. such that the distance L0is substantially equal to the distance L1. The size of the reference area AREA0may be equal to the exposed area AREA1of the test strip AS1. The width Δx of the reference area AREA0may be equal to the width of the exposed area AREA1of the test strip AS1, and the height Δy of the reference area AREA0may be equal to the height of the exposed area AREA1of the test strip AS1. The jet may have a diameter (wJET0) at the position of the reference area AREA0.

Referring toFIG. 3a, the measuring device500may comprise an illuminating unit100, an imaging unit200, and a data processing unit400. The illuminating unit100may be arranged illuminate a predetermined region RG0of the particle jet JET0. The imaging unit200may be arranged to capture digital images IMG2of particles located within the illuminated region RG0. The imaging unit200may be arranged to capture a plurality of images at a high frame rate. The imaging unit200may be a video camera.

The measuring device500may be arranged to measure one or more spatial distributions by analyzing the captured images (FIG. 5e). For example, the device500may be arranged to measure a spatial particle density distribution. The measuring device500may be arranged to measure a vertical density distribution by analyzing the captured images. The density distribution may provide e.g. particle density as a function of the vertical position with respect to the axis AX0of the jet JET0. The vertical position may be specified e.g. by y-coordinate in the direction SY.

The measuring device500may be arranged to measure a spatial velocity distribution by analyzing the captured images. A particle P0may have a velocity component vzin the axial direction SZ. The particle P0may also have a transverse velocity component vxin the direction SX and/or a velocity component vy in the direction SY. The measuring device500may be arranged to measure e.g. the velocity components vzand vyfor each particle appearing in a captured image. The measuring device500may be arranged to measure a spatial velocity distribution for the axial velocity components vzas a function of the vertical position y. The measuring device500may be arranged to measure a spatial velocity distribution for the transverse velocity components vyas a function of the vertical position y.

The measuring device500may be arranged to measure a spatial velocity probability distribution by analyzing the captured images.

The measuring device500may be arranged to measure a spatial distribution of mass flow by analyzing the captured images.

The measuring device500may be arranged to measure a spatial distribution of flux of kinetic energy by analyzing the captured images. The spatial distribution may provide information e.g. about an effective width of the particle jet.

The illuminating unit100may provide an illuminating light beam LB0. The particles P0may reflect, refract and/or scatter light LB1towards the illuminating unit100. The particles P0may provide reflected light LB1by reflecting, refracting and/or scattering the illuminating light LB0.

The imaging unit200may comprise focusing optics210and an image sensor SEN1. The focusing optics210may be arranged to form an optical image IMG1on an image sensor SEN1, by focusing the light LB1received from the particles. The image sensor SEN1may convert one or more optical images IMG1into a digital image IMG2. The data processing unit400may be configured to analyze one or more digital images IMG2obtained from the image sensor SEN1. The data processing unit400may be configured to perform one or more data processing operations e.g. for determining a model, for verifying a shot peening operation, for controlling operation of the shot peening unit, and/or for providing an indication if one or more measured velocity values are outside a specified range. The image sensor SEN1may be e.g. a CMOS sensor or a CCD sensor. CMOS means Complementary Metal Oxide Semiconductor. CCD means Charge Coupled Device. The image sensor SEN1may comprise a plurality of light detector pixels arranged in a two-dimensional array. The digital image IMG2may have a width ξIMGand a height υIMGin the image space defined by directions Sξ and Sυ. The image of the axis AX0may be e.g. substantially parallel with the direction Sξ. The direction Sυ may be perpendicular to the direction Sξ.

The field of view of the imaging unit may allow a considerable variation of the position of the particle jet. Thus, the position of the monitoring device of the does not need to be set with a high accuracy with respect to the axis of the particle jet.

The imaging unit200may have an optical axis AX2. The measurement region RG0may have a thickness d0in the direction of the optical axis AX0. The direction of the illuminating beam LB0may be specified e.g. by an axis AX1.

The axis AX2may be e.g. substantially perpendicular to the axis AX0and substantially perpendicular to the axis AX1. The illuminating light beam LB0may have e.g. a thickness d0in the direction of the optical axis AX2. The illuminating unit100may be arranged to provide e.g. a substantially planar light beam. The illuminating light beam LB0may be a light sheet. The illuminating unit100may comprise e.g. one or more lasers and/or light emitting diodes to provide the illuminating light beam LB0. Illuminating the jet by the light sheet may allow defining the thickness d0and/or position of the measurement region RG0accurately.

The method may comprise illuminating the particle jet JET0with the illuminating light LB0such that the thickness d0of the measurement region RG0is smaller than the diameter (wJET0) of the particle jet JET0. Thus, each captured image may represent a single slice (RG0) of the particle jet. The method may comprise determining a two-dimensional and/or a three dimensional spatial velocity distribution of the particle jet by analyzing the captured images. The method may comprise determining a two-dimensional and/or a three dimensional spatial particle density distribution of the particle jet by analyzing the captured images. Using the thin (d0<wJET0) measurement region (RG0) may facilitate determining the spatial distributions.

The illuminating unit100may be arranged to modulate the illuminating light beam LB0. The illuminating unit100may be arranged to modulate the optical intensity of the illuminating light beam LB0according to control signal S100. The measuring device500may be arranged to provide a control signal S100for modulating the illuminating light beam LB0. The control signal S100may comprise e.g. timing pulses for controlling timing of operation of the illuminating unit100. The illuminating unit100may be arranged to provide one or more illuminating light pulses LB0.

The position of the illuminating unit100may be defined e.g. by a mechanical frame with respect to the imaging unit200. The units100,200may be attached to a common frame. The device500may optionally comprise a robot for setting the position of the illuminating unit100and/or for the position of the imaging unit200. The device500may optionally comprise a robot for setting the position of the measurement region RG0with respect to the shot peening unit700.

The device500may comprise a memory MEM1for storing computer program code PROG1. For example, the code PROG1may, when executed by one or more data processors, cause a system or the device500to determine a total energy value by analyzing the images IMG2captured by the imaging device200. For example, the code PROG1may, when executed by one or more data processors, cause a system or the device500to estimate an arc height value hASby analyzing the images IMG2.

The device500may comprise a memory MEM2for storing one or more parameters of a model MODEL1.

The device500may optionally comprise a memory MEM3for storing one or more output values OUT1determined by using the model MODEL1. The output values OUT1may comprise e.g. one or more arc height values hAS,1, hAS,2, hAS,3and/or peening intensity rating values TINT.

The device500may comprise a user interface UIF1for receiving user input from a user and/or for providing information to a user. The user interface UIF1may comprise e.g. a keypad or a touch screen for receiving user input. The user interface UIF1may comprise e.g. a display for displaying visual information. The user interface UIF1may comprise e.g. a display for displaying one or more parameter values determined by analyzing the images. The user interface UIF1may comprise e.g. a display for displaying an indication when one or more parameters measured by the device are outside an acceptable range. The user interface UIF1may comprise e.g. an audio output device for providing an indication if one or more velocity values measured by the device are outside an acceptable range. The user interface UIF1may be configured to provide a visual alarm and/or an alarm sound if one or more velocity values measured by the device are outside an acceptable range.

The device500may comprise a communication unit RXTX1for receiving and/or transmitting data. COM1denotes a communication signal. The device500may be arranged to communicate e.g. with the shot peening unit700via the communication unit RXTX1. The device500may be arranged to communicate e.g. with a control unit of the shot peening unit700via the communication unit RXTX1. The device500may receive process data via the communication unit RXTX1. The process data may indicate e.g. when the shot peening unit is operating. The process data may indicate e.g. one or more process parameter values of the shot peening unit700. The device500may send process control data via the communication unit RXTX1. The process control data may comprise e.g. data for adjusting one or more process parameters of the shot peening unit700.

The device500may be arranged to receive measured data from a second measuring instrument via the communication unit RXTX1. The second measuring instrument may be e.g. an Almen gage.

The imaging unit200may form an image P1of each particle P0, which is located in the measurement region RG0during an exposure time Texof the image sensor SEN1. The optical image IMG1formed on the active area of the image sensor SEN1may comprise a plurality of sub-images P1. Each sub-image P1may be an image of a particle P0. The image sensor SEN1may convert an optical image IMG1into a digital (captured) image IMG2.

The image IMG2captured by the imaging unit200may represent a region RG0of the particle jet JET0. An average number of particles appearing in a single captured image may be e.g. in the range of 2 to 1000. An average number of particles appearing in a single captured image may be e.g. in the range of 10 to 100. The sub-images P1of the particles P0may be detected by an image analysis algorithm. The particles P0may be moving at a high velocity during capturing of an image IMG2. The velocity of each particle appearing in a captured image may be determined from the displacement value Δu and from the timing of the exposure and/or illumination. The optical image P1of each particle P0may move during capturing of the image IMG2. The movement of the optical image may define a displacement value Δu, which may be determined from the captured image IMG2by image analysis. Each substantially sharp image P1of a particle P0may be associated with a displacement value Δu. The velocity vkof a particle P0kmay be determined from the displacement value Δukand from the duration (TF) of illumination and/or from the exposure time period Tex.

When using illuminating pulse sequences, the velocity vkof a particle P0kmay be determined from the displacement value Δukand from the timing (e.g. t5−t1) of illuminating light pulses LB0. In particular, the axial velocity of a particle may be substantially proportional to Δuk/TF.

Referring toFIG. 3b, the angle between the axis AX1and the axis AX2may also substantially deviate from 90°, e.g. in order to provide high optical scattering coefficient when the particles P0provide the light LB1towards the optics210of the imaging unit200from the illuminating light LB0.

FIG. 4ashows, by way of example, temporal evolution of optical intensity of illuminating light LB0in the measurement region RG0. The particles P0may be illuminated by a single light pulse LB0during an exposure time period Tex. A first exposure time for capturing a first image IMG2t0may start at a time t0. A first illuminating light pulse LB0may start at a time t1. TFmay denote the duration of the illuminating light pulses LB0. A second exposure time for capturing a second image IMG2t0′may start at a time t0′. A second illuminating light pulse LB0may start at a time t1′.

Referring toFIG. 4b, the digital image IMG2may comprise e.g. sub-images P1k, P1k+1, P1k+2. The sub-image P1kmay be an image of a particle P0k. The sub-image P1k+1may be an image of a particle P0k+1. The sub-image P1k+2may be an image of a particle P0k+2. The length Δu of each sub-image P1may be substantially proportional to the velocity of the corresponding particle P0. The sub-image P1kmay have a dimension Δukin the direction Sξ. The sub-image P1k+1may have a dimension Δuk+1. The sub-image P1k+2may have a dimension Δuk+2. The velocity of each individual particle P1may be calculated from the dimension Δu of the corresponding sub-image P1, and from the timing or duration TFof the illuminating light pulses LB0. For example, the velocity vkof the particle P0kmay be substantially proportional to the value Δuk/TF.

The sub-images P1k, P1k+1, P1k+2may be detected e.g. by an image analysis algorithm. The device500may be configured to detect the sub-images P1k, P1k+1, P1k+2by using an image analysis algorithm. The device500may be configured to determine the dimensions Δuk, Δuk+1, Δuk+2from one or more captured images IMG2by using an image analysis algorithm.

The digital image IMG2may have a width ξIMG and a height υIMGin the image space defined by directions Sξ and Sυ. The image of the axis AX0may be parallel with the direction Sξ. The direction Sυ may be perpendicular to the direction Sξ.

The width ξIMGmay be e.g. equal to 1024 pixels, and the height υIMGmay be e.g. equal to 512 pixels.

The velocity of the particles may also be measured by using continuous illuminating light, i.e. light, which is not pulsed. In that case the velocity vkof the particle P0kmay be substantially proportional to the value Δuk/Tex.

The use of pulsed illumination may allow high instantaneous intensity and/or may allow precise timing for forming the sub-images.

Referring toFIGS. 4cand 4d, the illuminating unit100may be arranged to provide pulse sequences SEQ1, SEQ2, e.g. in order to facilitate detection of the sub-images P1by an image analysis algorithm. A pulse sequence SEQ1may comprise e.g. two or more pulses. A first pulse sequence may comprise e.g. pulses starting at times t1, t2, t3, t4, t5. A second pulse SEQ2sequence may comprise e.g. pulses starting at times t1′, t2′, t3′, t4′, t5′.

The exposure time Texmay temporally overlap several light pulses so that each particle P0may be represented by a group GRP, which is formed of two or more sub-images P1appearing in the digital image IMG2. For example, the particle P0kmay be represented by a first group GRPkformed of sub-images P1k,t1, P1k,t2, P1k,t3, P1k,t4, P1k,t5. The distance between adjacent sub-images P1k,t1, P1k,t2may depend on the velocity vkof the particle P0kand on the timing of the light pulses. Consequently, the velocity of each particle appearing in the image IMG2may be determined by analyzing the image IMG2. The sub-images P1k,t1, P1k,t2, P1k,t3, P1k,t4, P1k,t5may together form a combined shape, which may facilitate reliable detection of the sub-images P1k,t1, P1k,t2, P1k,t3, P1k,t4, P1k,t5, when analyzing the captured image IMG2. A second particle P0k+1may be represented by a second group GRPk+1formed of sub-images P1k+1,t1, P1k+1,t2, P1k+1,t3, P1k+1,t4, P1k+1,t5.

Referring toFIG. 4e, the particle jet JET0may be illuminated by an illuminating pulse sequence SEQ1. The pulse sequence may comprise e.g. three or more illuminating light pulses, which may be emitted at times t1, t2, t3, . . . .

FIGS. 4fand 4gshow, by way of example, a (digital) image IMG2, which was captured by using the illuminating pulse sequence.FIGS. 4gand 4fshow the same captured image IMG2. When using three or more illuminating pulses, the captured image IMG2may comprise easily discernible substantially linear groups GRP of sub-images (e.g. P1k,t1, P1k,t2, P1k,t3), wherein each group GRP may represent a single moving particle (e.g. P0k) which was illuminated by the pulse sequence during the exposure time period Texof the captured image IMG2. The position of the first sub-image P1k,t1of the first group GRPkmay be specified e.g. by image coordinates (ξk,υk). The position (ξk,υk) may indicate the position of the particle P0kwhen the image IMG2was captured.

The velocity of the particles may be determined by analyzing the captured images. For example, the velocity of a first particle P0kmay be determined from the dimension Δukof a first group GRPkformed of the sub-images P1k,t1, P1k,t2, P1k,t3. For example, the velocity of a second particle P0k+1may be determined from the dimension Δuk+1of a second group GRPk+1formed of the sub-images P1k+1,t1, P1k+1,t2, P1k+1,t3.

The method may comprise counting the number of particles appearing in a single captured image. The method may comprise counting the number of particles appearing in the captured images. The particle density may be determined from the counted number of particles. Thus, the particle density may be determined by analyzing the captured images.

The imaging unit200may have a certain depth of field (DoF) such that particles which are within the depth of field may have sharp sub-images on the image sensor SEN1, and particles which are outside the depth of field may have blurred sub-images on the image sensor SEN1. The captured image may comprise blurred sub-images e.g. if the thickness of the illuminating light beam LB0is greater than the depth of field (DoF). On the other hand, sharper images may be provided when the thickness of the illuminating light beam LB0is smaller than or equal to the depth of field (DoF).

The groups (e.g. GRPk) formed of the sub-images (e.g. P1k,t1, P1k,t2, P1k,t3) may be detected by using a pattern recognition algorithm. Each particle P0may be assumed to have a substantially constant velocity during the exposure time Tex.

A candidate group representing a particle may be accepted if the sub-images of said group are aligned in a substantially linear manner and if the distance between adjacent sub-images of said candidate group match with the timing (t1,t2,t3) of the illuminating light pulses LB0.

A candidate group may be e.g. discarded if the sub-images of said group are not aligned in a linear manner and/or if the distance between adjacent sub-images of said candidate group do not match with the timing (t1,t2,t3) of the illuminating light pulses LB0.

AX0′ may indicate the position of the axis AX0of the jet JET0. AREA0′ may indicate the position of the reference area AREA0. The position of the projection of the reference area AREA0may be indicated by a line AREA0′, which may be superposed on the captured image IMG2. The position of the projection of the axis AX0may be indicated by a line AX0′, which may be superposed on the captured image IMG2.

FIG. 4hshows, by way of example, a plurality of arrow symbols, which indicate velocity vectors of particles. The velocity vectors may be determined by analyzing the captured image ofFIG. 4g. The method may comprise determining the direction movement of a particle by analyzing one or more captured images. The length of each arrow symbol may be proportional to the speed of a particle, and the direction of the arrow symbol may indicate the direction of movement of the particle.

FIG. 5ashows, by way of example, velocity distributions measured by analyzing the captured images. The upper histogram ofFIG. 5ashows a first velocity distribution measured when the shot peening unit700operated according to a first operating parameter value (e.g. pacc=500 kPa). The lower histogram ofFIG. 5ashows a second velocity distribution measured when the shot peening unit700operated according to a second operating parameter value (e.g. pacc=350 kPa). A change of the pressure paccof the accelerating gas may have an effect on the average velocity of the particles. A change of the pressure paccof the accelerating gas may cause a change of the peak velocity vPEAKof the velocity distribution.

Np/bin may indicate the number Npof particles whose velocity is within a velocity range associated with a bin BIN1, BIN2, BIN3, . . . . For example the height of the vertical bar marked with the symbol BIN2may represent the number Npof particles P0whose velocity was within the range defined by the velocities vBIN1and vBIN2during a measurement time period TMEAS. For example the height of the vertical bar marked with the symbol BIN3may represent the number Npof particles P0whose velocity was within the range defined by the velocities vBIN2and vBIN3during a measurement time period TMEAS. The predetermined velocity ranges (e.g. from vBIN2to vBIN3) may be called e.g. as the velocity bins.

The number Npassociated with a bin may be indicative of a probability that a (randomly selected) particle of the jet has a velocity, which is within said bin. The velocity distributions ofFIG. 5amay also be called as velocity probability distributions.

Referring toFIG. 5b, the upper histogram may represent a measured velocity distribution, and the lower histogram may represent a measured energy distribution. The energy distribution may be determined from the measured velocity distribution.

The method may comprise:determining a velocity distribution by analyzing the captured images, anddetermining an energy distribution from the velocity distribution.

Np/bin may indicate the number Npof particles whose kinetic energy is within an energy range associated with a bin BIN21, BIN22, BIN23, . . . . For example the height of the vertical bar marked with the symbol BIN22may represent the number Npof particles P0whose kinetic energy was within the range defined by the energy values EBIN21and EBIN22during the measurement time period TMEAS. The height of the vertical bar marked with the symbol BIN23may represent the number Npof particles P0whose kinetic energy was within the range defined by the values EBIN22and EBIN23during said measurement time period TMEAS. For example, the energy bin BIN23may represent energy values, which are within the range from 24 mJ to 35 mJ, and the number Npof particles having the kinetic energy within said range may be approximately equal to 490 during the measurement time period TMEAS.

The method may comprise fitting a regression function to the measured data.FIG. 5cshows, by way of example, a probability density function pv(v) obtained by fitting a regression function to the histogram data ofFIG. 5a. The probability density function pv(v) may be optionally normalized such that the integral of the probability density function pv(v) over all possible velocities is equal to one. The probability density function pv(v) may represent a measured velocity distribution of the particles of the jet JET0. The probability density function pv(v) may have a peak value pMAXassociated with a velocity vPEAK. The velocity vPEAKmay denote the most probable velocity of the particles P0. The velocity distribution pv(v) may have a width ΔvFWHM, which may be defined by a first velocity vLand a second velocity vH. The velocities vL, vHmay be selected such that the velocity distribution pv(v) is equal to 50% of the maximum value pMAXat the velocities vLand vH.

The velocity distribution pv(v) may also sometimes have two or more peaks.

FIG. 5dshows, by way of example, a probability density function pE(E) obtained by fitting a regression function to the energy distribution shown inFIG. 5b. The probability density function pE(E) may be optionally normalized such that the integral of the probability density function pE(E) over all possible energy values is equal to one. The probability density function pE(E) may represent a measured energy distribution of the particles of the jet JET0.

Referring toFIG. 5e, the captured images may also be partitioned into two or more regions, which may be analyzed separately so as to provide spatial distributions. The uppermost curve ofFIG. 5emay represent a spatial distribution n(y) of particle density. y0may denote the (vertical) position of the axis AX0of the jet JET0. y1may denote the (vertical) position of an arbitrary point of the jet JET1. The second curve from the top may represent a spatial distribution vAVG(y) of average velocity vAVGof the particles. The third curve from the top may represent spatial distribution vRMS(y) of RMS velocity vRMSof the particles P0of the jet JET0. The lowermost curve may represent the spatial distribution Φ(y) of kinetic energy flux of the particles P0of the jet JET0. The energy flux Φ may mean the total kinetic energy of particles passing through unit area per unit time.

The spatial distributions ofFIG. 5emay be determined by analyzing the captured images. The spatial distributions ofFIG. 5emay be determined by determining the velocities of the particles from the captured images.

FIGS. 6aand 6bshow a test strip AS1before exposure and during exposure to the particle jet JET0. The strip AS1may initially be substantially flat and straight. The test strip AS1may have a width wASand a thickness tAS. REF1denotes the initial position of the surface SRF1of the test strip AS1.

The particles P0hitting the surface SRF1may slightly deform the surface SRF1. The particles P0may irreversibly deform the surface SRF1. For example, a particle P0kmay cause a first microscopic dent D0kin the surface SRF1of the test strip AS1. For example, a particle P0k+1may cause a second microscopic dent D0k+1in the surface SRF1of the test strip AS1. The particles may cause residual compressive stress in the surface layer of the test strip AS1such that the test strip is bent. The surface of the strip may have a plurality of dents after it has been exposed to the particle jet. The strip may be curved after it has been exposed to the particle jet. The shape of the test strip AS1may be defined e.g. by an arc height value hASand/or by a radius of curvature R1. The arc height value hASmay be measured according to a standardized method e.g. by using a measuring instrument called as the Almen gage.

The operating parameters of a (first) shot peening unit700may comprise e.g.:average size of the particles,average mass of the particles,mass flow rate of the particles,mass flow rate of accelerating gas,orientation of the axis AX0of the jet with respect to gravity.

A set of operating parameters of the shot peening unit700may refer e.g. to the following group of parameters:average size of the particles,average mass of the particles,mass flow rate of the particles,mass flow rate of accelerating gas,orientation of the axis AX0of the jet with respect to gravity.

A relationship between operating parameter values and corresponding arc height values hASmay be described by a model MODEL1. The method may comprise determining one or more parameter values of the model MODEL1.

A change of a parameter value may have an effect on the total kinetic energy of particles passing through the reference area per unit time. Thus, said change of a parameter value may have an effect on the capability of the particle jet to cause deformation of a surface. The model MODEL1may be determined experimentally. The effect of an operating parameter on the total energy may be determined experimentally by varying the operating parameter and by using the measuring instrument500for measuring corresponding total energy values EMEAS. The effect of said operating parameter on the arc height value may be determined experimentally by varying the operating parameter and by exposing a test strip AS1to the particle jet. A data point (e.g. D1inFIG. 6c) may comprise an energy value EMEASand an arc height value hASsuch that the energy value EMEASand the arc height value hASare obtained by using the same set of operating parameters.

The device500may be configured to receive one or more measured arc height values hASe.g. via the user interface UIF1and/or via the communication unit RXTX1. For example, a user may input one or more measured arc height values hASvia the user interface UIF1. For example, the communication unit RXTX1may receive one or more measured arc height values hASfrom an Almen gage and/or from another measuring instrument. The communication unit RXTX1may also be called as a communication interface.

The apparatus500may comprise:an illuminating unit100to provide an illuminating beam LB0,an image sensor SEN1to capture images IMG2of a particle jet JET1illuminated by the illuminating beam LB0, andan interface UIF1, RXTX1to receive one or more deformation values hAS,
wherein the apparatus500may be configured to determine one or more velocity values (e.g. vAVE, vRMS) of particles P0of the particle jet JET1by analyzing the captured images IMG2, and to determine a model MODEL1based on the one or more first deformation values hASand based on the one or more velocity values (vAVE, vRMS).

The method may comprise obtaining one or more data points D1, D2such that a first data point D1is obtained by using a first set of operating parameters. The model MODEL1may be determined by e.g. fitting a function based on the data point D1.

The method may comprise obtaining two or more data points D1, D2such that a first data point D1is obtained by using a first set of operating parameters, and a second data point D2is obtained by using a second different set of operating parameters. The model MODEL1may be determined by e.g. fitting a function to the obtained data points D1, D2.

A change of an operating parameter of the shot peening unit700may have an effect on the total energy value EMEAS, which in turn may have an effect on the corresponding arc height value hAS. Thus, the model MODEL1may also describe the relationship between total energy values EMEASand the corresponding arc height values hAS. The model MODEL1may be used for estimating an arc height value hAS=hAS(EMEAS), which is likely to correspond to a measured energy value EMEAS.

Determining the model MODEL1may comprise determining a first data point (D1), which comprises a first measured total energy value EMEAS,1, and a first measured arc height value hAS,1. The first height value hAS,1may be measured by exposing a test strip AS1to the particle jet during a first measurement time period TMEAS,1A. The first measured total energy value EMEAS,1may be determined from one or more velocity values obtained by analyzing images IMG2captured during a second measurement time period TMEAS,1B. The second measurement time period TMEAS,1Bmay also be called e.g. as a first auxiliary time period. The particle jet may be provided by a first shot peening unit700during the measurement time periods TMEAS,1A, TMEAS,1Bby using a first set of operating parameters. The distance L1between the first shot peening unit700and the test strip AS1may be substantially equal to the distance L0between the first shot peening unit700and the reference area AREA0during the measurement time periods TMEAS,1A, TMEAS,1B. In other words, the measuring device500may be arranged to operate such that the measured energy value EMEAS,1substantially corresponds to the integrated energy of the particle flux passing through a reference area AREA0, wherein the distance between the first shot peening unit700and the reference area AREA0is substantially equal to the distance L1.

A second data point D2may comprise a second measured total energy value EMEAS,2, and a second measured arc height value hAS,2. The second height value hAS,2may be measured by exposing a second test strip AS1to the particle jet during a second measurement time period TMEAS,2A. The second measured total energy value EMEAS,2may be determined by analyzing images IMG2captured during a second auxiliary time period TMEAS,2B. The particle jet may be provided by the first shot peening unit700during the measurement time periods TMEAS,2A, TMEAS,2Bby using a second set of operating parameters. The distance L1between the first shot peening unit700and the test strip AS1may be substantially equal to the distance L0between the first shot peening unit700and the reference area AREA0during the measurement time periods TMEAS,1A, TMEAS,1B, TMEAS,2A, TMEAS,2B.

A third data point D3may comprise a third measured total energy value EMEAS,3, and a third measured arc height value hAS,3. The third height value hAS,3may be measured by exposing a third test strip AS1to the particle jet during a third measurement time period TMEAS,3A. The third measured total energy value EMEAS,3may be determined by analyzing images IMG2captured during a third auxiliary time period TMEAS,3B. The particle jet may be provided by the first shot peening unit700during the measurement time periods TMEAS,3A, TMEAS,3Bby using a third set of operating parameters. The distance L1between the first shot peening unit700and the test strip AS1may be substantially equal to the distance L0between the first shot peening unit700and the reference area AREA0during the measurement time periods TMEAS,1A, TMEAS,1B, TMEAS,2A, TMEAS,2B, TMEAS,3A, TMEAS,3B.

An estimate (e.g. hE) for an arc height value may be subsequently determined from a measured energy value EMEASby using the model hAS(EMEAS). The measured energy value EMEASmay correspond e.g. to a point F1of the regression curve CRV1.

Table 1 shows, by way of example, measured values associated with the measurement time periods TMEAS,1A, TMEAS,1B, TMEAS,2A, TMEAS,2B, TMEAS,3A, TMEAS,3B. The measurement time periods listed in Table 1 have the same length.

paccdenotes a pressure of accelerating gas of the shot peening unit700. kPa means kiloPascal. The pressure paccmay have an effect of the initial velocity of the particles. The velocity of the accelerating gas may depend on the pressure pacc. The mass flow rate of the accelerating gas may depend on the pressure pacc.

hASdenotes the arc height value of the Almen strip AS1after the strip AS1has been exposed to the particle jet during the measurement time period TMEAS,1A, TMEAS,2A, or TMEAS,3A.

NMEASdenotes the number of particles which pass through the reference area AREA0during the measurement time period TMEAS,1B, TMEAS,2B, or TMEAS,3B. The number NMEASmay be determined by analyzing the images captured by the measuring device500during the measurement time period TMEAS,1B, TMEAS,2B, or TMEAS,3B.

vAVEdenotes the average velocity of particles which pass through the reference area AREA0during the measurement time period TMEAS,1B, TMEAS,2B, or TMEAS,3B. The average velocity vAVEmay be determined by analyzing the images captured by the measuring device500during the measurement time period TMEAS,1B, TMEAS,2B, or TMEAS,3B.

EMEASdenotes the total kinetic energy of particles which pass through the reference area AREA0during the measurement time period TMEAS,1B, TMEAS,2B, or TMEAS,3B. The energy values EMEASmay be determined by analyzing the images captured by the measuring device500during the measurement time period TMEAS,1B, TMEAS,2B, or TMEAS,3B.

The model MODEL1may be determined from the one or more experimentally measured data points D1, D2, D3. The model MODEL1may be determined by fitting a function to the one or more determined data points D1, D2, D3. The model MODEL1may be a regression function hAS(EMEAS), which may be fitted to the data points D1, D2, D3. The model MODEL1may be e.g. a polynomial function, which is fitted to the data points D1, D2, D3. The function hAS(EMEAS) may be represented e.g. by a curve CRV1shown inFIG. 6c. The model MODEL1may determine a relationship operating parameter values and the corresponding arc height values hAS.

The model MODEL1may be used for estimating an arc height value hAS,Efrom a measured energy value EMEAS. The method may comprise:determining a model MODEL1,capturing images of particles passing through the measurement region RG0during a measurement time period TMEAS,determining a measured energy value EMEASby analyzing the captured images, anddetermining a corresponding arc height value hASfrom the measured energy value EMEASby using the model MODEL1.

NMEASmay denote the number of particles hitting the test strip AS1during a measurement time period TMEAS. NMEASmay also denote the number of particles passing through the reference area AREA0during a measurement time period TMEAS. The length of the measurement time period TMEASmay be e.g. in the range of 1 s to 1000 s. Ekmay denote the kinetic energy of an individual particle P0k. mkmay denote the mass of the individual particle P0k. The kinetic energy Ekof an individual particle P0kmay be calculated from the velocity vkof said particle P0kby using the equation:

The total kinetic energy EMEASof particles P0k, P0k+1, P0k+2, . . . passing through the reference area AREA0during the measurement time period TMEASmay be calculated by using the following equation:

The particles P0k, P0k+1, P0k+2, . . . may have a narrow size distribution. For example, more than 90% of the total mass of the particles may be represented by particles, whose mass is in the range of 70% to 150% of the average mass of the particles. For example, more than 90% of the total mass of the particles may be represented by particles, whose diameter is in the range of 70% to 150% of the mass median diameter of the particles.

The square (vRMS)2of the RMS velocity vRMSmay be defined and calculated by using the following equation:

RMS means root mean square. The RMS velocity vRMSmay be determined by analyzing images IMG2captured during the measurement time period TMEAS.

The number of particles appearing in each captured image may be proportional to the instantaneous number density of particles of the jet. The number of sub-images P1k, P1k+2, Pk+2, . . . may be proportional to the instantaneous number density of particles of the jet. The number NMEASmay be determined by analyzing images IMG2captured during the measurement time period TMEAS.

The total number NMEASmay be estimated e.g. according to the following equation:

NIMG,AVEmay denote an average number of particles appearing in a single captured image. Cgmay denote a proportionality constant, i.e. a coefficient. The coefficient Cgmay depend e.g. on dimensions of the measuring region RG0in the directions SX and SZ. The size of the measuring region RG0may depend on the field of view of the imaging unit200and on the optical magnification of the imaging unit200.

vAVEmay denote the average velocity of the particles. To the first approximation, the number density of particles in the jet may be inversely proportional to the average velocity vAVEof the particles, in a situation where the mass flow rate of the particles is substantially constant.

d0may denote the thickness of the measurement region RG0in a direction, which is parallel to the optical axis AX2of the imaging unit200. To the first approximation, the relative fraction of particles passing through the reference area AREA0without passing through the measurement region RG0may be inversely proportional to the thickness d0of the measurement region RG0.

The coefficient Cgmay also be determined experimentally e.g. by positioning an aperture to the jet, collecting all particles which pass through the aperture during a test period, determining the total mass of the collected particles by weighing, and by dividing the total mass by the average mass of single particles. The coefficient Cgmay be determined experimentally and/or theoretically for each measurement set-up.

Equation (7) may be re-arranged e.g. into the following form:

The values NIMG,AVE, vRMS, and vAVEassociated with the measurement time period TMEASmay be determined by analyzing the images captured by the measuring device500. The total energy EMEASmay be calculated from the values NIMG,AVE, vRMS, and vAVEe.g. by using the equation (8). A corresponding arc height value hASmay be subsequently estimated from the total energy EMEASby using the model MODEL1.

The velocity value vRMSand the velocity value vAVEmay be determined separately e.g. in order to improve the accuracy of the estimated energy value.

However, the velocity value vRMSmay also be calculated from the velocity value vAVEby using information about the velocity probability distribution function. The velocity value vAVGmay be calculated from the velocity value vRMSby using information about the velocity probability distribution function. The velocity probability distribution function may be measured e.g. by analyzing the captured images. The velocity probability distribution may also be assumed to match with a predetermined function. The velocity probability distribution may be assumed to match e.g. with a Gaussian function.

The model MODEL1may also be determined based on the measured values vRMS, vAVEand NIMG,AVEand based on one or more measured arc height values hASsuch that it is not necessary to separately determine the value of the coefficient Cg. The contribution of the coefficient Cgmay be incorporated in the model by fitting the regression function to the experimentally measured data vRMS, vAVE, NIMG,AVE, hAS. The method may comprise determining a model hAS(NIMG,AVE,vAVE,vRMS) which may provide the arc height values as the function of the measured values NIMG,AVE,vAVE,vRMS. The arc height value hASmay be subsequently estimated from the measured values vRMS, vAVEand NIMG,AVE, by using the model MODEL1.

Some particles of the jet JET0may travel though the measurement region RG0such that they are not directly detected by the measuring device500. Some particles of the jet JET0may travel though the measurement region RG0such that the sub-images of those particles do not appear in any digital image captured by the imaging unit200. Some particles may travel outside the depth of field (DoF) of the imaging unit200. Some particles may travel through the measurement region RG0when the jet is not illuminated by the illuminating unit. Some particles may travel through the measurement region REG0when the image sensor SEN1is not in the active light-detecting state, i.e. between two consecutive exposure time periods. The un-detected particles may be taken into consideration by using the coefficient Cg.

The number NMEASmay also be determined e.g. by measuring the mass flow and/or volumetric flow of the particles supplied to the shot peening unit700. The number NMEASmay also be determined e.g. by collecting and weighing the particles after they have been accelerated by the shot peening unit700. However, even in that case determining the particle density from the captured images may improve the reliability of the method.

A shot peening process may need to be verified when producing critical parts. A shot peening process may need to be verified e.g. when producing critical parts of an airplane. Shot peening may e.g. relieve tensile stresses built up in a grinding or welding process and replace them with beneficial compressive stresses. Depending on the part geometry, part material, shot material, shot quality, shot intensity, and shot coverage, shot peening may increase fatigue life e.g. more than 100%, or even more than 1000%.

FIG. 6dshows, by way of example, the effect of processing time on the arc height value of a test strip AS1. The curve CRV2may depict the arc height value hASas the function of duration of the shot peening, when using a first set of operating parameters. The curve CRV2may be determined experimentally by using the test strips AS1and/or by using the model MODEL1. For example, the points B1to B5shown inFIG. 6dmay be measured experimentally by using the test strips AS1.

The curve CRV2has a first point C1and a second point C2. The points C1and C2may be determined by using the model MODEL1. The first point C1has an arc height value hC1, and the second point C2has an arc height value hC2. The first point C1is attained at the processing time TC1, and the second point C2is attained at the processing time TC2.

The points C1and C2may be selected such that the following two conditions are simultaneously fulfilled:

When the points have been selected such that the equations (9a) and (9b) are fulfilled, then the value TC1is equal to a time equivalent value TINTof the shot peening process, when using said first set of operating parameters. The time equivalent value TINTmay also be called e.g. as the “intensity” of the particle jet JET0. The time equivalent value TINTmay also be called e.g. as the “peening intensity rating”. The peening intensity rating TINTmay be valid for said first set of operating parameters, at the position of the reference area AREA0. Each peening intensity rating TINTmay be associated with a specified position and with a specified set of operating parameters.

FIG. 7ashows, by way of example, method steps for determining a model MODEL1, which may define a relationship between measured velocity values and corresponding deformation values.

The particle jet JET0may be provided according to a first set of operating parameters (step1010). For example, the pressure pACCof accelerating gas may be set to a first value.

A plurality of images of the particle jet may be captured (step1015) when the particle jet JET0is provided according to the first set of operating parameters.

One or more velocity values (e.g. vRMS, vAVG) may be determined by analyzing the captured images (step1020). The velocity distribution and the particle density may be determined by analyzing the captured images. The energy flux and/or total energy may be determined from the one or more measured velocity values (step1030).

One or more test strips AS1may be exposed to the particle jet JET0when the particle jet JET0is provided according to said first set of operating parameters (step1040).

A deformation value may be obtained by measuring the deformation of a test strip AS1after it has been exposed to the particle jet JET0. The deformation value may be e.g. an arc height value (hAS).

One or more deformation values may be obtained by measuring the deformation of one or more test strips AS1. For example, a first test strip may be exposed to the particle jet during a first time period, and a second test strip may be exposed to the particle jet during a second time period.

The model MODEL1may be determined by fitting one or more parameters of a regression function to the measured deformation value and to the one or more measured velocity values (step1060). An energy value may be determined from the one or more measured velocity values. The model MODEL1may be determined by fitting one or more parameters of a regression function to the measured deformation value and to the energy value.

The step1040may be performed after performing the step1015or before performing the step1015. The steps1015and1040may also be performed simultaneously. Performing the step1015may temporally overlap performing the step1040.

FIG. 7bshows, by way of example, controlling the shot peening process based on the one or more measured velocity values.

The particle jet JET0may be provided in step1110.

A plurality of images of the particle jet may be captured in step1120.

One or more velocity values (e.g. vRMS, vAVG) may be determined by analyzing the captured images (step1130). The energy flux and/or total energy may be determined from the one or more velocity values (e.g. vRMS, vAVG), in step1140. The velocity distribution and/or the particle density may be determined by analyzing the captured images.

A deformation value may be estimated from the measured velocity distribution and from the measured particle density by using the model MODEL1(step1150). The deformation value may be e.g. an arc height value (hAS).

The length of a processing time period may be selected according to the estimated deformation value (step1160).

A value of an operating parameter may also be selected based on the estimated deformation value in step1160. For example the pressure of accelerating gas may be selected and/or adjusted based on the estimated deformation value.

The surface SRF2of an object OBJ1may subsequently be processed according to the selected length of a processing time period (step1170).

FIG. 7cshows, by way of example, method steps for verifying the shot peening capability of a particle jet JET0.

The particle jet may be provided according to selected operating parameters (step1210).

A plurality of images IMG2may be captured by the imaging device500(step1220). The particle jet JET0may be illuminated with a sequence SEQ1of illuminating light pulses such that a captured image IMG2comprises two or more adjacent sub-images of the same particle. In particular, the particle jet JET0may be illuminated with a sequence SEQ1of illuminating light pulses such that a captured image IMG2comprises three or more adjacent sub-images of the same particle.

One or more velocity values (e.g. vRMS, vAVG) may be determined by analyzing the captured images (step1230).

The velocity distribution and the particle density may be determined by analyzing the captured images. The energy flux and/or total energy may be determined from the one or more measured velocity values. The images may be captured when the shot peening unit700is operated according to said selected operating parameters.

The measured values obtained by analyzing the images may be compared with one or more reference values in order to check whether the shot peening capability of the jet is in a predetermined range (step1240).

An energy value may be determined by analyzing the captured images, and a deformation value may be determined from the energy value by using the model MODEL1. The deformation value may be compared with a reference value in order to check whether the shot peening capability of the jet is in a predetermined range. The deformation value may be e.g. arc height value hASor a time equivalent value TINT. The energy value may represent e.g. the flux of kinetic energy of particles passing through the reference area AREA0or the total kinetic energy of particles passing through the reference area AREA0during a predetermined time period. The method may comprise determining the energy value from the measured velocity distribution and from the measured particle distribution.

A deformation value may be determined from the measured velocity distribution and the particle distribution by using the model MODEL1, and the deformation value may be compared with a reference value in order to check whether the shot peening capability of the jet is in a predetermined range. The deformation value may be e.g. arc height value hASor a time equivalent value TINT.

The method may comprise:providing a model (MODEL1) which establishes a relationship between a velocity value (vAVE, vRMS) of a particle jet (JET0) and a deformation value (hAS),using a first shot peening unit (700) to provide a particle jet (JET1),illuminating at least a portion (RG0) of the particle jet (JET0) with illuminating light (LB0),capturing images (IMG2) of said portion (RG0),determining a velocity value (vAVE, vRMS) of particles (P0) of the particle jet (JET1) by analyzing the captured images (IMG2),determining an estimate of an arc height value (hAS) from the velocity value (vAVE, vRMS) by using the model (MODEL1), andclassifying a shot peening operation as valid or invalid by comparing the estimate of the arc height value (hAS) with one or more reference values.

The method may comprise:providing a model (MODEL1) which establishes a relationship between a velocity value (vAVE, vRMS) of a particle jet (JET0) and a deformation value (hAS),using a first shot peening unit (700) to provide a particle jet (JET1),illuminating at least a portion (RG0) of the particle jet (JET0) with illuminating light (LB0),capturing images (IMG2) of said portion (RG0),determining a velocity value (vAVE, vRMS) of particles (P0) of the particle jet (JET1) by analyzing the captured images (IMG2),determining an estimate of an arc height value (hAS) from the velocity value (vAVE, vRMS) by using the model (MODEL1), andclassifying a shot peening operation as valid or invalid by checking whether the estimate of the arc height value (hAS) is in a predetermined range.

The estimate may be compared with one or more reference values in order to determine whether the estimate is in the predetermined range. The shot peening operation may refer to a method which comprises operating the first shot peening unit (700) according to a specified set of operating parameters during a specified time period.

An energy value may be determined by analyzing the captured images, and the energy value may be compared with a reference value in order to check whether the shot peening capability of the jet is in a predetermined range. The deformation value may be e.g. arc height value hASor a time equivalent value TINT.

The measured velocity distribution may be compared with one or more first reference values, and/or the measured particle distribution may be compared with one or more second reference values in order to check whether the shot peening capability of the jet is in a predetermined range.

FIG. 7dshows, by way of example, method steps for controlling operation of a shot peening unit700.

The steps1210,1220,1230and1240may be performed as discussed above with reference toFIG. 7c. The method may further comprise adjusting one or more operating parameters of the shot peening process based on the comparison (step1250).

The adjustable and/or selectable parameters of the shot peening process may comprise e.g. one or more of the following:pressure paccof accelerating gas,(mass) flow rate of accelerating gas,(mass) flow rate of particles P0passing via an accelerating nozzle of the shot peening unit700,length of a processing time period,relative transverse movement speed of the axis of the jet with respect to the object,distance L2between the nozzle of the shot peening unit700and the object OBJ1.

The velocity of a particle may have significant transverse component, i.e. the velocity is not always parallel with the axis AX0of the jet. The velocity vkof a particle may have an axial component vk,zand a transverse component vk,y. The axial component vk,zis parallel with the axis AX0, and the transverse component vk,yis perpendicular to the axis AX0. When evaluating the shot peening capability, the kinetic energy of each particle may be calculated from the axial component vk,z, by omitting the transverse component vk,y. The capability of a particle P0kto deform a surface SRF1may mainly depend on the axial velocity component vk,zof said particle. The velocity values (vRMS, vAVE) used e.g. in equations (1) to (8) may be determined from the axial velocity values vzof the individual particles P0. The axial velocity values vzof the individual particles P0may be determined from the captured images.

The velocity of an individual particle P0kmay also be determined by capturing a first image by using first single illumination pulse at a time t1, and capturing a second image by using a second single illumination pulse at a time t2. The first image may comprise a first sub-image P1k,t1of the particle P0k. The second image may comprise a second sub-image P1k,t2of the particle P0k. The spatial displacement Δukassociated with the particle P0kmay be determined by comparing the first image with the second image. The velocity of the particle P0kmay be determined from the displacement Δukand from the time difference t2−t1.

The method may comprise determining an angular divergence of the particle jet JET0by analyzing the captured images IMG1.

The method may comprise determining a width and/or a radial dimension of the particle jet JET0by analyzing the captured images IMG1.

Shot peening may be used e.g. for processing a gear part, camshaft, clutch spring, coil spring, leaf spring, suspension spring, connecting rod, crankshaft, gearwheel, part of an aircraft, part of a landing gear, components of an engine of an aircraft, engine housing, rock drill and/or turbine blade.

For the person skilled in the art, it will be clear that modifications and variations of the devices and the methods according to the present invention are perceivable. The figures are schematic. The particular embodiments described above with reference to the accompanying drawings are illustrative only and not meant to limit the scope of the invention, which is defined by the appended claims.