FIELD

Some versions may 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 according to claim 1 .

Further embodiments are defined in the other claims. 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 ( iAs) of the test strips (S1 ),

- illuminating at least a portion (RGO) of the particle jet (JETO) with illuminating light (LBO),

- capturing images (IMG2) of said portion (RGO),

- determining at least one velocity value (VAVE, VRMS) of the particle jet (JET1 ) by analyzing the captured images (IMG2), and

- determining a model (MODEL1 ) based on the one or more deformation values (hiAs) 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 (hiAs) 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 AS1 after a shot peening test may be defined e.g. by an arc height value iAs.

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 iAs by using the measurement device and by using the model may reduce the number of test strips AS1 needed 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 iAs by using the measurement device and by using the model may allow on-line control of a shot peening process. BRIEF DESCRIPTION OF THE DRAWINGS

In the following examples, several variations will be described in more detail with reference to the appended drawings, in which

Fig. 1 shows, by way of example, in a three dimensional view, processing a surface of an object by shot peening, Fig. 2a shows, by way of example, in a three dimensional view, exposing a test strip to the particle jet,

Fig. 2b shows, by way of example, in a three dimensional view, particles passing through a reference area, shows, by way of example, in a three dimensional view, an optical measuring device arranged to capture images of particles of the particle jet, Fig. 3b shows, by way of example, in a three dimensional view, an optical measuring device arranged to capture images of particles of the particle jet,

Fig. 4a shows, by way of example, timing of illuminating light pulses,

Fig. 4b shows, by way of example, an image captured by an image sensor of the measuring device,

Fig. 4c shows, by way of example, timing of illuminating light pulses,

Fig. 4d shows, by way of example, an image captured by the image sensor of the measuring device,

Fig. 4e shows, by way of example, timing of illuminating light pulses, Fig. 4f shows, by way of example, an image captured by the image sensor of the measuring device,

Fig. 4g shows, by way of example, sub-images obtained by using an illuminating pulse sequence,

Fig. 4h shows, by way of example, particle velocity vectors determined by analyzing the captured image of Fig. 4f, Fig. 5a shows, by way of example, a first velocity distribution associated with a first value of an operating parameter, and a second velocity distribution corresponding to a second value of said operating parameter, Fig. 5b shows, by way of example, a measured velocity distribution and an energy distribution corresponding to the measured velocity distribution,

Fig. 5c shows, by way of example, a velocity distribution function and an energy distribution function,

Fig. 5e shows, by way of example, spatial distribution of particle density, spatial distribution of average velocity, spatial distribution of RMS velocity, and spatial distribution of energy flux,

Fig. 6a shows, by way of example, in a side view, a test strip which is in the initial straight state,

Fig. 6b shows, by way of example, in a side view, geometric deformation of the test strip when the test strip is exposed to the particles of the particle jet,

Fig. 6c shows, by way of example, a regression curve fitted to experimental data, shows, by way of example, determining a time equivalent value (TINT) based on arc height values, shows, by way of example, method steps for determining a model, shows, by way of example, selecting a process parameter based on measurement results obtained by analyzing the captured images, shows, by way of example, method steps for verifying a shot peening process, shows, by way of example, method steps for controlling shot peening based on measurement results obtained by analyzing the captured images.

DETAILED DESCRIPTION

Referring to Fig. 1 , the surface SRF2 of an object OBJ1 may 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 SRF2 of 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 PO to a high velocity, and directing the moving particles PO to the surface SRF2 of an object OBJ1 . The particles PO may hit the surface SRF2 and may cause plastic deformation of the surface layer of the object. The moving particles PO may 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 JETO, which comprises a plurality of particles POk, P0k+i, POk+2, ... moving at high velocities Vk, Vk+i , Vk+2, .... A shot peening unit 700 may be arranged to provide the particle jet JETO. The particle jet JETO may be provided e.g. by accelerating the particles with a high velocity gas stream. The particle jet JETO may also be provided e.g. by accelerating the particles with a rotating mechanical element. The object OBJ1 may also be called e.g. as a target. The surface SRF2 may be exposed to particles of a particle jet JETO. A shot peening unit 700 may be arranged to provide the particle jet JETO. The jet JETO may comprise a plurality of particles POk, P0k+i, 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 SRF2 of the target. The target OBJ1 may be e.g. a part of a machine, engine and/or a vehicle. The target OBJ1 may be e.g. a mechanical component of a device.

The particle jet JETO may 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 AX0 of the jet may be parallel e.g. with the direction SZ. The reference position POS0 denotes a point where the axis AX0 intersects 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.

L2 denotes a distance between the shot peening unit 700 and the surface SRF2. In particular, L0 may denote a distance between the accelerating nozzle of the shot peening unit 700 and the surface SRF2. The position of the shot peening unit 700 and/or the orientation of the target OBJ1 may be selected such that the surface SRF2 is substantially perpendicular to the axis AXO.

Referring to Fig. 2a, the capability of a particle jet JETO to 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 JETO may 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 AS1 forms an arc. The height ( iAs) of the arc may depend on the operating parameters of the shot peening unit 700, on the distance (L1 ) between the shot peening unit 700, and on the duration of the peening. The dimensions of the test strips AS1 and/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 AREA1 of the surface SRF1 of the test strip AS1 . L1 may denote the distance between the shot peening unit 700 and the test strip AS1 . In particular, L0 may denote the distance between the particle accelerating nozzle of the shot peening unit 700 and the reference area AREAO. The axis of the particle jet JETO may coincide with the center of the area AREA1 .

Referring to Fig. 2b, the shot peening unit 700 may provide a particle flux through a reference area AREAO. L0 may denote the distance between the shot peening unit 700 and the reference area AREAO. In particular, L0 may denote the distance between the particle accelerating nozzle and the reference area AREAO. The reference area AREAO may have e.g. a width Δχ and a height Ay. The reference area AREAO may be perpendicular to the axis AXO of the particle jet. The measuring device 500 may be arranged to measure the total kinetic energy of particles which pass through the reference area AREAO during a measurement time period TMEAS. The measuring device 500 may be arranged to capture images of particles which pass through the reference area AREAO.The measuring device 500 may be arranged to monitor the particle jet in the vicinity of the reference area AREAO.

The reference area AREAO may be positioned e.g. such that the distance L0 is substantially equal to the distance L1 . The size of the reference area AREAO may be equal to the exposed area AREA1 of the test strip AS1 . The width Δχ of the reference area AREAO may be equal to the width of the exposed area AREA1 of the test strip AS1 , and the height Ay of the reference area AREAO may be equal to the height of the exposed area AREA1 of the test strip AS1 . The jet may have a diameter (WJETO) at the position of the reference area AREAO. Referring to Fig. 3a, the measuring device 500 may comprise an illuminating unit 100, an imaging unit 200, and a data processing unit 400. The illuminating unit 100 may be arranged illuminate a predetermined region RG0 of the particle jet JET0. The imaging unit 200 may be arranged to capture digital images IMG2 of particles located within the illuminated region RG0. The imaging unit 200 may be arranged to capture a plurality of images at a high frame rate. The imaging unit 200 may be a video camera.

The measuring device 500 may be arranged to measure one or more spatial distributions by analyzing the captured images (Fig. 5e). For example, the device 500 may be arranged to measure a spatial particle density distribution. The measuring device 500 may 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 AX0 of the jet JET0. The vertical position may be specified e.g. by y-coordinate in the direction SY.

The measuring device 500 may be arranged to measure a spatial velocity distribution by analyzing the captured images. A particle P0 may have a velocity component v _z in the axial direction SZ. The particle P0 may also have a transverse velocity component v _x in the direction SX and/or a velocity component vy in the direction SY. The measuring device 500 may be arranged to measure e.g. the velocity components v _z and v _y for each particle appearing in a captured image. The measuring device 500 may be arranged to measure a spatial velocity distribution for the axial velocity components v _z as a function of the vertical position y. The measuring device 500 may be arranged to measure a spatial velocity distribution for the transverse velocity components v _y as a function of the vertical position y.

The measuring device 500 may be arranged to measure a spatial velocity probability distribution by analyzing the captured images.

The measuring device 500 may be arranged to measure a spatial distribution of mass flow by analyzing the captured images. The measuring device 500 may 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 unit 100 may provide an illuminating light beam LB0. The particles P0 may reflect, refract and/or scatter light LB1 towards the illuminating unit 100. The particles P0 may provide reflected light LB1 by reflecting, refracting and/or scattering the illuminating light LB0. The imaging unit 200 may comprise focusing optics 210 and an image sensor SEN1 . The focusing optics 210 may be arranged to form an optical image IMG1 on an image sensor SEN1 , by focusing the light LB1 received from the particles. The image sensor SEN1 may convert one or more optical images IMG1 into a digital image IMG2. The data processing unit 400 may be configured to analyze one or more digital images IMG2 obtained from the image sensor SEN1 . The data processing unit 400 may 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 SEN1 may be e.g. a CMOS sensor or a CCD sensor. CMOS means Complementary Metal Oxide Semiconductor. CCD means Charge Coupled Device. The image sensor SEN1 may comprise a plurality of light detector pixels arranged in a two-dimensional array. The digital image IMG2 may have a width ξΐΜο and a height UIMG in the image space defined by directions 5ξ and Su. The image of the axis AXO may be e.g. substantially parallel with the direction 5ξ. The direction Su may be perpendicular to the direction 5ξ.

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 unit 200 may have an optical axis AX2. The measurement region RG0 may have a thickness dO in the direction of the optical axis AXO. The direction of the illuminating beam LB0 may be specified e.g. by an axis AX1 .

The axis AX2 may be e.g. substantially perpendicular to the axis AXO and substantially perpendicular to the axis AX1 . The illuminating light beam LB0 may have e.g. a thickness dO in the direction of the optical axis AX2. The illuminating unit 100 may be arranged to provide e.g. a substantially planar light beam. The illuminating light beam LB0 may be a light sheet. The illuminating unit 100 may 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 dO and/or position of the measurement region RG0 accurately.

The method may comprise illuminating the particle jet JET0 with the illuminating light LB0 such that the thickness dO of the measurement region RG0 is smaller than the diameter (WJETO) 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 (dO<wjETo) measurement region (RGO) may facilitate determining the spatial distributions. The illuminating unit 100 may be arranged to modulate the illuminating light beam LBO. The illuminating unit 100 may be arranged to modulate the optical intensity of the illuminating light beam LBO according to control signal S100. The measuring device 500 may be arranged to provide a control signal S100 for modulating the illuminating light beam LBO. The control signal S100 may comprise e.g. timing pulses for controlling timing of operation of the illuminating unit 100. The illuminating unit 100 may be arranged to provide one or more illuminating light pulses LBO.

The position of the illuminating unit 100 may be defined e.g. by a mechanical frame with respect to the imaging unit 200. The units 100, 200 may be attached to a common frame. The device 500 may optionally comprise a robot for setting the position of the illuminating unit 100 and/or for the position of the imaging unit 200. The device 500 may optionally comprise a robot for setting the position of the measurement region RGO with respect to the shot peening unit 700.

The device 500 may comprise a memory MEM1 for storing computer program code PROG1 . For example, the code PROG1 may, when executed by one or more data processors, cause a system or the device 500 to determine a total energy value by analyzing the images IMG2 captured by the imaging device 200. For example, the code PROG1 may, when executed by one or more data processors, cause a system or the device 500 to estimate an arc height value iAs by analyzing the images IMG2. The device 500 may comprise a memory MEM2 for storing one or more parameters of a model MODEL1 .

The device 500 may optionally comprise a memory MEM3 for storing one or more output values OUT1 determined by using the model MODEL1 . The output values OUT1 may comprise e.g. one or more arc height values hAs,i ,hAs,2, hAs,3 and/or peening intensity rating values TINT.

The device 500 may comprise a user interface UIF1 for receiving user input from a user and/or for providing information to a user. The user interface UIF1 may comprise e.g. a keypad or a touch screen for receiving user input. The user interface UIF1 may comprise e.g. a display for displaying visual information. The user interface UIF1 may comprise e.g. a display for displaying one or more parameter values determined by analyzing the images. The user interface UIF1 may 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 UIF1 may 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 UIF1 may 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 device 500 may comprise a communication unit RXTX1 for receiving and/or transmitting data. COM1 denotes a communication signal. The device 500 may be arranged to communicate e.g. with the shot peening unit 700 via the communication unit RXTX1 . The device 500 may be arranged to communicate e.g. with a control unit of the shot peening unit 700 via the communication unit RXTX1 . The device 500 may 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 unit 700. The device 500 may 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 unit 700.

The device 500 may 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 unit 200 may form an image P1 of each particle P0, which is located in the measurement region RGO during an exposure time T _ex of the image sensor SEN1 . The optical image IMG1 formed on the active area of the image sensor SEN1 may comprise a plurality of sub-images P1 . Each sub-image P1 may be an image of a particle P0. The image sensor SEN1 may convert an optical image IMG1 into a digital (captured) image IMG2.

The image IMG2 captured by the imaging unit 200 may represent a region RGO of 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 P1 of the particles P0 may be detected by an image analysis algorithm. The particles P0 may 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 Au and from the timing of the exposure and/or illumination. The optical image P1 of each particle P0 may move during capturing of the image IMG2. The movement of the optical image may define a displacement value Au, which may be determined from the captured image IMG2 by image analysis. Each substantially sharp image P1 of a particle P0 may be associated with a displacement value Au. The velocity Vk of a particle P0k may be determined from the displacement value Auk and from the duration (TF) of illumination and/or from the exposure time period T _ex . When using illuminating pulse sequences, the velocity Vk of a particle P0k may be determined from the displacement value Auk and 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 Auk TF. Referring to Fig. 3b, the angle between the axis AX1 and the axis AX2 may also substantially deviate from 90°, e.g. in order to provide high optical scattering coefficient when the particles P0 provide the light LB1 towards the optics 210 of the imaging unit 200 from the illuminating light LB0. Fig. 4a shows, by way of example, temporal evolution of optical intensity of illuminating light LBO in the measurement region RGO. The particles P0 may be illuminated by a single light pulse LBO during an exposure time period T _ex . A first exposure time for capturing a first image IMG2to may start at a time to. A first illuminating light pulse LBO may start at a time t1 . TF may denote the duration of the illuminating light pulses LBO. A second exposure time for capturing a second image IMG2to' may start at a time t0'. A second illuminating light pulse LBO may start at a time t1 '. Referring to Fig. 4b, the digital image IMG2 may comprise e.g. sub-images P1 k, P1 k+i , P1 k+2. The sub-image P1 k may be an image of a particle POk. The sub-image P1 k+i may be an image of a particle P0k+i . The sub-image P1 k+2 may be an image of a particle P0k+2. The length Au of each sub-image P1 may be substantially proportional to the velocity of the corresponding particle P0. The sub-image P1 k may have a dimension Auk in the direction βξ. The sub-image P1 k+i may have a dimension Auk+i . The sub-image P1 k+2 may have a dimension Auk+2. The velocity of each individual particle P1 may be calculated from the dimension Au of the corresponding sub-image P1 , and from the timing or duration TF of the illuminating light pulses LBO. For example, the velocity Vk of the particle POk may be substantially proportional to the value Auk TF.

The sub-images P1 k, P1 k+i , P k+2 may be detected e.g. by an image analysis algorithm. The device 500 may be configured to detect the sub-images P1 k, P1 k+i , P1 k+2 by using an image analysis algorithm. The device 500 may be configured to determine the dimensions Auk, Auk+i , Auk+2 from one or more captured images IMG2 by using an image analysis algorithm.

The digital image IMG2 may have a width ξιι ιο and a height UIMG in the image space defined by directions βξ and Su. The image of the axis AX0 may be parallel with the direction βξ. The direction Su may be perpendicular to the direction βξ.

The width ξΐΜο may be e.g. equal to 1024 pixels, and the height UIMG may 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 Vk of the particle POk may be substantially proportional to the value Auk/T _ex .

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

Referring to Figs. 4c and 4d, the illuminating unit 100 may be arranged to provide pulse sequences SEQ1 , SEQ2, e.g. in order to facilitate detection of the sub-images P1 by an image analysis algorithm. A pulse sequence SEQ1 may 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 SEQ2 sequence may comprise e.g. pulses starting at times t1 ', t2', t3', t4', t5'.

The exposure time T _ex may temporally overlap several light pulses so that each particle P0 may be represented by a group GRP, which is formed of two or more sub-images P1 appearing in the digital image IMG2. For example, the particle POk may be represented by a first group GRPk formed of sub- images P1 k,ti , P1 k,t2, P1 k,t3, P1 k,t4, P1 k,ts. The distance between adjacent sub- images P1 k,ti , P1 k,t2 may depend on the velocity Vk of the particle POk and on the timing of the light pulses. Consequently, the velocity of each particle appearing in the image IMG2 may be determined by analyzing the image IMG2. The sub-images P1 k,ti , P1 k,t2, P1 k,t3, P1 k,t4, P1 k,ts may together form a combined shape, which may facilitate reliable detection of the sub-images P1 k,ti , P1 k,t2, P1 k,t3, P1 k,t4, P1 k,t5, when analyzing the captured image IMG2. A second particle P0k+i may be represented by a second group GRPk+i formed of sub-images P1 k+i ,ti , P1 k+i ,t2, P1 k+i ,t3, P1 k+i ,t4, P1 k+i ,ts. Referring to Fig. 4e, the particle jet JET0 may 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. 4f and 4g show, by way of example, a (digital) image IMG2, which was captured by using the illuminating pulse sequence. Figs. 4g and 4f show the same captured image IMG2. When using three or more illuminating pulses, the captured image IMG2 may comprise easily discernible substantially linear groups GRP of sub-images (e.g. P1 k,ti, P1 k,t2, P1 k,t3), wherein each group GRP may represent a single moving particle (e.g. POk) which was illuminated by the pulse sequence during the exposure time period T _ex of the captured image IMG2. The position of the first sub-image P1 k,ti of the first group GRPk may be specified e.g. by image coordinates (ξ^υ^. The position (ξ^υ^ may indicate the position of the particle POk when the image IMG2 was captured.

The velocity of the particles may be determined by analyzing the captured images. For example, the velocity of a first particle POk may be determined from the dimension Auk of a first group GRPk formed of the sub-images P1 k,ti , P1 k,t2, P1 k,t3. For example, the velocity of a second particle P0k+i may be determined from the dimension Auk+i of a second group GRPk+i formed of the sub-images P1 k+i,ti, P1 k+i,t2, P1 k+i,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 unit 200 may 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 LB0 is greater than the depth of field (DoF). On the other hand, sharper images may be provided when the thickness of the illuminating light beam LB0 is smaller than or equal to the depth of field (DoF). The groups (e.g. GRPk) formed of the sub-images (e.g. P1 k,ti , P1 k,t2, P1 k,t3) may be detected by using a pattern recognition algorithm. Each particle P0 may be assumed to have a substantially constant velocity during the exposure time T _ex .

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 LBO.

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 LBO.

AXO' may indicate the position of the axis AXO of the jet JETO. AREAO' may indicate the position of the reference area AREAO. The position of the projection of the reference area AREAO may be indicated by a line AREAO', which may be superposed on the captured image IMG2. The position of the projection of the axis AXO may be indicated by a line AXO', which may be superposed on the captured image IMG2.

Fig. 4h shows, 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 of Fig. 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. 5a shows, by way of example, velocity distributions measured by analyzing the captured images. The upper histogram of Fig. 5a shows a first velocity distribution measured when the shot peening unit 700 operated according to a first operating parameter value (e.g. ρ ₃ ∞=500 kPa). The lower histogram of Fig. 5a shows a second velocity distribution measured when the shot peening unit 700 operated according to a second operating parameter value (e.g. ρ ₃ ∞= 350 kPa). A change of the pressure ρ ₃ ∞ of the accelerating gas may have an effect on the average velocity of the particles. A change of the pressure ρ ₃ ∞ of the accelerating gas may cause a change of the peak velocity VPEAK of the velocity distribution.

Np/bin may indicate the number N _p of 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 BIN2 may represent the number N _p of particles P0 whose velocity was within the range defined by the velocities VBINI and VBIN2 during a measurement time period TMEAS. For example the height of the vertical bar marked with the symbol BIN3 may represent the number N _p of particles P0 whose velocity was within the range defined by the velocities VBIN2 and VBIN3 during a measurement time period TMEAS. The predetermined velocity ranges (e.g. from VBIN2 to VBIN3) may be called e.g. as the velocity bins.

The number N _p associated 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 of Fig. 5a may also be called as velocity probability distributions.

Referring to Fig. 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, and - determining an energy distribution from the velocity distribution.

N _p /bin may indicate the number N _p of 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 BIN22 may represent the number N _p of particles P0 whose kinetic energy was within the range defined by the energy values EBIN2I and EBIN22 during the measurement time period TMEAS. The height of the vertical bar marked with the symbol BIN23 may represent the number N _p of particles P0 whose kinetic energy was within the range defined by the values EBIN22 and EBIN23 during said measurement time period TMEAS. For example, the energy bin BIN23 may represent energy values, which are within the range from 24 mJ to 35 mJ, and the number N _p of 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. 5c shows, by way of example, a probability density function p _v (v) obtained by fitting a regression function to the histogram data of Fig. 5a. The probability density function p _v (v) may be optionally normalized such that the integral of the probability density function p _v (v) over all possible velocities is equal to one. The probability density function p _v (v) may represent a measured velocity distribution of the particles of the jet JET0. The probability density function p _v (v) may have a peak value PMAX associated with a velocity VPEAK. The velocity VPEAK may denote the most probable velocity of the particles P0. The velocity distribution p _v (v) may have a width AVFWHM, which may be defined by a first velocity VL and a second velocity VH. The velocities VL, VH may be selected such that the velocity distribution p _v (v) is equal to 50% of the maximum value PMAX at the velocities VL and VH.

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

Fig. 5d shows, by way of example, a probability density function PE(E) obtained by fitting a regression function to the energy distribution shown in Fig. 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 to Fig. 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 of Fig. 5e may represent a spatial distribution n(y) of particle density, yo may denote the (vertical) position of the axis AXO of the jet JETO. yi may 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 VAVG of the particles. The third curve from the top may represent spatial distribution VRMs(y) of RMS velocity VRMS of the particles PO of the jet JETO. The lowermost curve may represent the spatial distribution Φ(ν) of kinetic energy flux of the particles PO of the jet JETO. The energy flux Φ may mean the total kinetic energy of particles passing through unit area per unit time.

The spatial distributions of Fig. 5e may be determined by analyzing the captured images. The spatial distributions of Fig. 5e may be determined by determining the velocities of the particles from the captured images. Figs. 6a and 6b show a test strip AS1 before exposure and during exposure to the particle jet JETO. The strip AS1 may initially be substantially flat and straight. The test strip AS1 may have a width WAS and a thickness†AS. REF1 denotes the initial position of the surface SRF1 of the test strip AS1 . The particles PO hitting the surface SRF1 may slightly deform the surface SRF1 . The particles PO may irreversibly deform the surface SRF1 . For example, a particle POk may cause a first microscopic dent DOk in the surface SRF1 of the test strip AS1 . For example, a particle P0k+i may cause a second microscopic dent D0k+i in the surface SRF1 of the test strip AS1 . The particles may cause residual compressive stress in the surface layer of the test strip AS1 such 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 AS1 may be defined e.g. by an arc height value iAs and/or by a radius of curvature Ri . The arc height value iAs may 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 unit 700 may 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 AXO of the jet with respect to gravity.

A set of operating parameters of the shot peening unit 700 may 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 AXO of the jet with respect to gravity. A relationship between operating parameter values and corresponding arc height values iAs may 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 MODEL1 may 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 instrument 500 for 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 AS1 to the particle jet. A data point (e.g. D1 in Fig. 6c) may comprise an energy value EMEAS and an arc height value iAs such that the energy value EMEAS and the arc height value iAs are obtained by using the same set of operating parameters.

The device 500 may be configured to receive one or more measured arc height values iAs e.g. via the user interface UIF1 and/or via the communication unit RXTX1 . For example, a user may input one or more measured arc height values iAs via the user interface UIF1 . For example, the communication unit RXTX1 may receive one or more measured arc height values iAs from an Almen gage and/or from another measuring instrument. The communication unit RXTX1 may also be called as a communication interface.

The apparatus 500 may comprise:

- an illuminating unit 100 to provide an illuminating beam LB0,

- an image sensor SEN1 to capture images IMG2 of a particle jet JET1 illuminated by the illuminating beam LB0, and

- an interface UIF1 , RXTX1 to receive one or more deformation values iAs, wherein the apparatus 500 may be configured to determine one or more velocity values (e.g. VAVE, VRMS) of particles P0 of the particle jet JET1 by analyzing the captured images IMG2, and to determine a model MODEL1 based on the one or more first deformation values iAs and based on the one or more velocity values (VAVE, VRMS).

The method may comprise obtaining one or more data points D1 , D2 such that a first data point D1 is obtained by using a first set of operating parameters. The model MODEL1 may 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 , D2 such that a first data point D1 is obtained by using a first set of operating parameters, and a second data point D2 is obtained by using a second different set of operating parameters. The model MODEL1 may 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 unit 700 may have an effect on the total energy value EMEAS, which in turn may have an effect on the corresponding arc height value iAs. Thus, the model MODEL1 may also describe the relationship between total energy values EMEAS and the corresponding arc height values iAs. The model MODEL1 may be used for estimating an arc height value iAs= hAs(EMEAs), which is likely to correspond to a measured energy value EMEAS.

Determining the model MODEL1 may comprise determining a first data point (D1 ), which comprises a first measured total energy value EMEAS.I, and a first measured arc height value iAs.i . The first height value iAs,i may be measured by exposing a test strip AS1 to the particle jet during a first measurement time period TMEAS.IA. The first measured total energy value EMEAS.I may be determined from one or more velocity values obtained by analyzing images IMG2 captured during a second measurement time period TMEAS.IB. The second measurement time period TMEAS.I B may also be called e.g. as a first auxiliary time period. The particle jet may be provided by a first shot peening unit 700 during the measurement time periods TMEAS.IA, TMEAS.I B by using a first set of operating parameters. The distance L1 between the first shot peening unit 700 and the test strip AS1 may be substantially equal to the distance L0 between the first shot peening unit 700 and the reference area AREA0 during the measurement time periods TMEAS.IA, TMEAS.I B. In other words, the measuring device 500 may be arranged to operate such that the measured energy value EMEAS.I substantially corresponds to the integrated energy of the particle flux passing through a reference area AREA0, wherein the distance between the first shot peening unit 700 and the reference area AREA0 is substantially equal to the distance L1 .

A second data point D2 may comprise a second measured total energy value EMEAS,2, and a second measured arc height value iAs,2. The second height value iAs,2 may be measured by exposing a second test strip AS1 to the particle jet during a second measurement time period TMEAS,2A. The second measured total energy value EMEAS,2 may be determined by analyzing images IMG2 captured during a second auxiliary time period TMEAS,2B. The particle jet may be provided by the first shot peening unit 700 during the measurement time periods TMEAS,2A, TMEAS,2B by using a second set of operating parameters. The distance L1 between the first shot peening unit 700 and the test strip AS1 may be substantially equal to the distance L0 between the first shot peening unit 700 and the reference area AREA0 during the measurement time periods TMEAS.IA, TMEAS.I B, TMEAS,2A, TMEAS,2B. A third data point D3 may comprise a third measured total energy value EMEAS,3, and a third measured arc height value iAs,3. The third height value hAs,3 may be measured by exposing a third test strip AS1 to the particle jet during a third measurement time period TMEAS,3A. The third measured total energy value EMEAS,3 may be determined by analyzing images IMG2 captured during a third auxiliary time period TMEAS,3B. The particle jet may be provided by the first shot peening unit 700 during the measurement time periods TMEAS,3A, TMEAS,3B by using a third set of operating parameters. The distance L1 between the first shot peening unit 700 and the test strip AS1 may be substantially equal to the distance L0 between the first shot peening unit 700 and the reference area AREAO during the measurement time periods

TMEAS. IA, TMEAS. I B, TMEAS,2A, TMEAS,2B, TMEAS,3A, TMEAS,3B. An estimate (e.g. ΙΊ Ε) for an arc height value may be subsequently determined from a measured energy value EMEAS by using the model IIAS(EMEAS). The measured energy value EMEAS may correspond e.g. to a point F1 of the regression curve CRV1 . Table 1 shows, by way of example, measured values associated with the measurement time periods TMEAS.IA, TMEAS.I B, TMEAS,2A, TMEAS,2B, TMEAS,3A, TMEAS,3B. The measurement time periods listed in Table 1 have the same length.

Table 1 : Examples of measured values associated with measurement time periods TMEAS. IA, TMEAS.I B, TMEAS,2A, TMEAS,2B, TMEAS,3A, TMEAS,3B.

Period Pace hAS Period Pace NMEAS VAVE EMEAS

(kPa) (mm) (kPa) (m/s) (J)

TMEAS. IA 200 1 .3 TMEAS.IB 200 3789 16.6 22.0

TMEAS,2A 350 4.0 TMEAS,2B 350 2207 30.2 70.9

TMEAS,3A 500 6.2 TMEAS,3B 500 1618 42.4 144 Pace denotes a pressure of accelerating gas of the shot peening unit 700. kPa means kiloPascal. The pressure ρ ₃ ∞ may have an effect of the initial velocity of the particles. The velocity of the accelerating gas may depend on the pressure p _aC c The mass flow rate of the accelerating gas may depend on the pressure Mace- iAs denotes the arc height value of the Almen strip AS1 after the strip AS1 has been exposed to the particle jet during the measurement time period

TMEAS. IA, TMEAS,2A, or TMEAS,3A.

NMEAS denotes the number of particles which pass through the reference area AREA0 during the measurement time period TMEAS. I B, TMEAS,2B, or TMEAS,3B. The number NMEAS may be determined by analyzing the images captured by the measuring device 500 during the measurement time period TMEAS.I B, TMEAS,2B, or TMEAS,3B.

VAVE denotes the average velocity of particles which pass through the reference area AREA0 during the measurement time period TMEAS. I B, TMEAS,2B, or TMEAS,3B. The average velocity VAVE may be determined by analyzing the images captured by the measuring device 500 during the measurement time period TMEAS. I B, TMEAS,2B, or TMEAS,3B.

EMEAS denotes the total kinetic energy of particles which pass through the reference area AREA0 during the measurement time period TMEAS. I B, TMEAS,2B, or TMEAS,3B. The energy values EMEAS may be determined by analyzing the images captured by the measuring device 500 during the measurement time period TMEAS. I B, TMEAS,2B, or TMEAS,3B.

The model MODEL1 may be determined from the one or more experimentally measured data points D1 , D2, D3. The model MODEL1 may be determined by fitting a function to the one or more determined data points D1 , D2, D3. The model MODEL1 may be a regression function iAs(EMEAs), which may be fitted to the data points D1 , D2, D3. The model MODEL1 may be e.g. a polynomial function, which is fitted to the data points D1 , D2, D3. The function IIAS(EMEAS) may be represented e.g. by a curve CRV1 shown in Fig. 6c. The model MODEL1 may determine a relationship operating parameter values and the corresponding arc height values iAs.

The model MODEL1 may be used for estimating an arc height value IIAS.E from a measured energy value EMEAS. The method may comprise:

- determining a model MODEL1 ,

- capturing images of particles passing through the measurement region RGO during a measurement time period TMEAS,

- determining a measured energy value EMEAS by analyzing the captured images, and

- determining a corresponding arc height value iAs from the measured energy value EMEAS by using the model MODEL1 .

NMEAS may denote the number of particles hitting the test strip AS1 during a measurement time period TMEAS. NMEAS may also denote the number of particles passing through the reference area AREAO during a measurement time period TMEAS. The length of the measurement time period TMEAS may be e.g. in the range of 1 s to 1000 s. Ek may denote the kinetic energy of an individual particle POk. nrik may denote the mass of the individual particle POk. The kinetic energy Ek of an individual particle POk may be calculated from the velocity Vk of said particle POk by using the equation: The total kinetic energy EMEAS of particles POk, P0k+i, P0k+2, ... passing through the reference area AREAO during the measurement time period TMEAS may be calculated by using the following equation:

^N MEAS \ ₂

^E MEAS ∑ ^m kVk (2)

k=l ^Z

The particles POk, P0k+i, 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.

Consequently, the mass nrik, nrvi, nrik+2, ... of each individual particle P0k, P0k+i, P0k+2, ... may be approximated by the average mass ITIAVG: ^m k ^{¾ m} AVG (3)

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

₂ l ^N MEAS 2

VRMS =— ∑ ^v k

^N MEAS k=l

RMS means root mean square. The RMS velocity VRMS may be determined by analyzing images IMG2 captured during the measurement time period TMEAS. Combining the equations (2), (3), (4) may provide:

1 2

^E MEAS = ^N MEAS ^{' ~ M} AVE ^{' V} RMS ( ⁵ )

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 P1 k, P1 k+2, Pk+2, ... may be proportional to the instantaneous number density of particles of the jet. The number NMEAS may be determined by analyzing images IMG2 captured during the measurement time period TMEAS.

The total number NMEAS may be estimated e.g. according to the following equation: ^N MEAS IMG, AVE ^{- T} MEAS (6)

N iMG, AVE may denote an average number of particles appearing in a single captured image. C _g may denote a proportionality constant, i.e. a coefficient. The coefficient C _g may depend e.g. on dimensions of the measuring region RGO in the directions SX and SZ. The size of the measuring region RGO may depend on the field of view of the imaging unit 200 and on the optical magnification of the imaging unit 200.

VAVE may 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 VAVE of the particles, in a situation where the mass flow rate of the particles is substantially constant. dO may denote the thickness of the measurement region RGO in a direction, which is parallel to the optical axis AX2 of the imaging unit 200. To the first approximation, the relative fraction of particles passing through the reference area AREA0 without passing through the measurement region RGO may be inversely proportional to the thickness dO of the measurement region RGO.

The coefficient C _g may 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 C _g may be determined experimentally and/or theoretically for each measurement set-up.

Combining (5) with (6) gives:

^E MEAS IMG, AVE ^{- T} MEAS ^{" m} AVE ^{" V} RMS

v _AVE - d0 Equation (7) may be re-arranged e.g. into the following form:

^E MEAS " N IMG, AVE · T _MEAS (8)

The values NIMG.AVE, VRMS, and VAVE associated with the measurement time period TMEAS may be determined by analyzing the images captured by the measuring device 500. The total energy EMEAS may be calculated from the values NIMG.AVE, VRMS, and VAVE e.g. by using the equation (8). A corresponding arc height value iAs may be subsequently estimated from the total energy EMEAS by using the model MODEL1 . The velocity value VRMS and the velocity value VAVE may be determined separately e.g. in order to improve the accuracy of the estimated energy value.

However, the velocity value VRMS may also be calculated from the velocity value VAVE by using information about the velocity probability distribution function. The velocity value VAVG may be calculated from the velocity value VRMS by 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 MODEL1 may also be determined based on the measured values VRMS, VAVE and NIMG.AVE and based on one or more measured arc height values iAs such that it is not necessary to separately determine the value of the coefficient C _g . The contribution of the coefficient C _g may be incorporated in the model by fitting the regression function to the experimentally measured data VRMS, VAVE, NIMG.AVE, r s. 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 iAs may be subsequently estimated from the measured values VRMS, VAVE and NIMG.AVE, by using the model MODEL1 .

Some particles of the jet JETO may travel though the measurement region RGO such that they are not directly detected by the measuring device 500. Some particles of the jet JETO may travel though the measurement region RGO such that the sub-images of those particles do not appear in any digital image captured by the imaging unit 200. Some particles may travel outside the depth of field (DoF) of the imaging unit 200. Some particles may travel through the measurement region RGO when the jet is not illuminated by the illuminating unit. Some particles may travel through the measurement region REG0 when the image sensor SEN1 is 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 C _g .

The number NMEAS may also be determined e.g. by measuring the mass flow and/or volumetric flow of the particles supplied to the shot peening unit 700. The number NMEAS may also be determined e.g. by collecting and weighing the particles after they have been accelerated by the shot peening unit 700. 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. 6d shows, by way of example, the effect of processing time on the arc height value of a test strip AS1 . The curve CRV2 may depict the arc height value iAs as the function of duration of the shot peening, when using a first set of operating parameters. The curve CRV2 may be determined experimentally by using the test strips AS1 and/or by using the model MODEL1 . For example, the points B1 to B5 shown in Fig. 6d may be measured experimentally by using the test strips AS1 .

The curve CRV2 has a first point C1 and a second point C2. The points C1 and C2 may be determined by using the model MODEL1 . The first point C1 has an arc height value hci, and the second point C2 has an arc height value hc2. The first point C1 is attained at the processing time Tci, and the second point C2 is attained at the processing time Tc2.

The points C1 and C2 may be selected such that the following two conditions are simultaneously fulfilled: ^h C2 ^{~ h} Cl (9a)

hCl

^T C2 - ^T C1 CI (9b)

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

Fig. 7a shows, 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 JETO may be provided according to a first set of operating parameters (step 1010). For example, the pressure PACC of accelerating gas may be set to a first value. A plurality of images of the particle jet may be captured (step 1015) when the particle jet JETO is 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 (step 1020). 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 (step 1030). One or more test strips AS1 may be exposed to the particle jet JETO when the particle jet JETO is provided according to said first set of operating parameters (step 1040).

A deformation value may be obtained by measuring the deformation of a test strip AS1 after it has been exposed to the particle jet JETO. The deformation value may be e.g. an arc height value ( iAs).

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 MODEL1 may 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 (step 1060). An energy value may be determined from the one or more measured velocity values. The model MODEL1 may be determined by fitting one or more parameters of a regression function to the measured deformation value and to the energy value. The step 1040 may be performed after performing the step 1015 or before performing the step 1015. The steps 1015 and 1040 may also be performed simultaneously. Performing the step 1015 may temporally overlap performing the step 1040.

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

The particle jet JET0 may be provided in step 1 1 10.

A plurality of images of the particle jet may be captured in step 1 120.

One or more velocity values (e.g. VRMS, VAVG) may be determined by analyzing the captured images (step 1 130). The energy flux and/or total energy may be determined from the one or more velocity values (e.g. VRMS, VAVG), in step 1 140. 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 (step 1 150). The deformation value may be e.g. an arc height value (h _AS ).

The length of a processing time period may be selected according to the estimated deformation value (step 1 160).

A value of an operating parameter may also be selected based on the estimated deformation value in step 1 160. For example the pressure of accelerating gas may be selected and/or adjusted based on the estimated deformation value.

The surface SRF2 of an object OBJ1 may subsequently be processed according to the selected length of a processing time period (step 1 170). Fig. 7c shows, by way of example, method steps for verifying the shot peening capability of a particle jet JETO.

The particle jet may be provided according to selected operating parameters (step 1210).

A plurality of images IMG2 may be captured by the imaging device 500 (step 1220). The particle jet JETO may be illuminated with a sequence SEQ1 of illuminating light pulses such that a captured image IMG2 comprises two or more adjacent sub-images of the same particle. In particular, the particle jet JETO may be illuminated with a sequence SEQ1 of illuminating light pulses such that a captured image IMG2 comprises 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 (step 1230).

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 unit 700 is 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 (step 1240).

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 iAs or a time equivalent value TINT. The energy value may represent e.g. the flux of kinetic energy of particles passing through the reference area AREA0 or the total kinetic energy of particles passing through the reference area AREAO during 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 iAs or 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 (JETO) and a deformation value (hiAs),

- using a first shot peening unit (700) to provide a particle jet (JET1 ),

- illuminating at least a portion (RG0) of the particle jet (JETO) 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 (hiAs) from the velocity value (VAVE, VRMS) by using the model (MODEL1 ), and

- classifying a shot peening operation as valid or invalid by comparing the estimate of the arc height value (hiAs) 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 (JETO) and a deformation value (hiAs),

- using a first shot peening unit (700) to provide a particle jet (JET1 ),

- illuminating at least a portion (RG0) of the particle jet (JETO) 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 ( iAs) from the velocity value (VAVE, VRMS) by using the model (MODEL1 ), and

- classifying a shot peening operation as valid or invalid by checking whether the estimate of the arc height value (hiAs) 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 iAs or 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. 7d shows, by way of example, method steps for controlling operation of a shot peening unit 700.

The steps 1210, 1220, 1230 and 1240 may be performed as discussed above with reference to Fig. 7c. The method may further comprise adjusting one or more operating parameters of the shot peening process based on the comparison (step 1250).

The adjustable and/or selectable parameters of the shot peening process may comprise e.g. one or more of the following:

- pressure ρ ₃ ∞ of accelerating gas, - (mass) flow rate of accelerating gas,

- (mass) flow rate of particles PO passing via an accelerating nozzle of the shot peening unit 700,

- length of a processing time period,

- relative transverse movement speed of the axis of the jet with respect to the object,

- distance L2 between the nozzle of the shot peening unit 700 and the object OBJ1 . The velocity of a particle may have significant transverse component, i.e. the velocity is not always parallel with the axis AX0 of the jet. The velocity Vk of a particle may have an axial component Vk, _z and a transverse component Vk, _y . The axial component Vk, _z is parallel with the axis AX0, and the transverse component Vk, _y is 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 P0k to deform a surface SRF1 may mainly depend on the axial velocity component Vk, _z of said particle. The velocity values (VRMS, VAVE) used e.g. in equations (1 ) to (8) may be determined from the axial velocity values v _z of the individual particles P0. The axial velocity values v _z of the individual particles P0 may be determined from the captured images.

The velocity of an individual particle P0k may 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 P1 k,ti of the particle P0k. The second image may comprise a second sub-image P1 k,t2 of the particle P0k. The spatial displacement Auk associated with the particle P0k may be determined by comparing the first image with the second image. The velocity of the particle P0k may be determined from the displacement Auk and from the time difference t2-t1 .

The method may comprise determining an angular divergence of the particle jet JET0 by analyzing the captured images IMG1 . The method may comprise determining a width and/or a radial dimension of the particle jet JETO by 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.