a b c 5 Image analysis

5 Image analysis

The images captured by the camera A have been analysed with different methods looking for the right approach to the problem. A totally automatic analysis of the images seems to be the ideal way to obtain the desired measures, but the errors, due to the not ideal lighting and to the lack of alignment between the camera and the releasing plane, influence the results. Due to these reasons it has been chosen a semi-automatic procedure in which the image processing is able to find the edges and the operator gives to these edges the most suitable geometric shape.

5.1 Analysis with an image processing software [Matrox Edge Finder]

The images captured by camera A have been transformed and analysed using an image processing commercial software.

5.1.1 Elaboration of the images

Using the software Jasc Animation Shop 3, some frames have been selected and acquired as single images in the .tif format with dimensions 640x480. In order to obtain a more useful picture, with the software Matrox Inspector 4.1, each image has been changed into an 8 bit unsigned image, processed two times with the smooth 5x5 filter and finally has been applied the window leveling operation that allows taking a range of pixel values in the image and re-map the values to a different range, making the image binary. The file obtained using this sequence of operations can be processed with the utility Matrox Edge Finder that discerns the outlines of the objects and exports them as edge chains into a drawing interchange format file .dxf (shown in Figure 1c) that can be used with a cad software (for example Autodesk AutoCAD). Using the chain points it is possible to draw the ideal shapes of the objects: circles to identify the sphere and the meniscus’s lateral surface, a line for the plane edge.

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b

c

Figure 1 Phases of the elaboration of the images.

5.1.2 Tilting error

The shapes obtained from the previous procedures are not the real shapes because of various errors which occur during the process:

2 the objects appear in a different way depending on the lighting (direction and intensity); 2 the binarization of the image involves loss of information and consequently the width

and the shape of the objects result modified;

2 the camera’s tilting produces wrong shapes and placements of the items;

Various attempts have been done to decrease all these errors: various lighting conditions, different software filters and various plane and camera positions have been tried, so the condition used are the best conditions achieved. Naturally these errors still remain in the resulting image and they must be estimated.

The systematic camera’s tilting error has been evaluated analysing the image of the sphere laying on the plane when it has been released: because of the minimum energetic configuration the sphere has to be in contact with the plane. In the analysed images, instead, the plane’s contour found by the edge finder application is not tangent to the ideal sphere’s edge. The measure of the distance between the line representing the located plane’s edge and the sphere is the value of the error (Figure 2). This value is used to correct the other analysed frames (Figure 3).

Individuated plane

Real Plane

Meniscus

Sphere

X

Figure 2 Determination of the real plane using an image with a released sphere

Real plane

Sphere

Meniscus

Drop

Individuated

plane

X

X

a

b

5.1.3 Meniscus analysis

From the analysis of the images, some important parameters of the geometry of the meniscus, of the sphere and of the releasing drop (α, β, xa, ra, h and R referred to Figure 2

of chapter 3) have been obtained. That gives the possibility to evaluate the volumes involved in the process and than the capillary forces.

Sphere

Meniscus

Drop

Figure 4 Geometric parameters obtained from the image analysis

5.1.4 Centring effect

It is interesting to notice that the misalignment between the sphere barycentre and the barycentre of the water drop is nullified by the releasing mechanism (Figure 5). Actually, Figure 6, obtained with the edge finder application, shows that, when the sphere is released, its centre lays nearby the symmetry axis of the drop. The position ranges, marked in figure, don’t overlap, so this effect is not caused by errors in the image analysis. This centring effect, working until the positioning error between the sphere and the drop is about R/2 (R is the radius of the drop), entails that, for a correct assembly, the releasing drop must have a more accurate positioning than the necessary one for the grasping task.

Figure 5 Scheme of the centring effect (source [1])

Syringe axis

Drop axis

Position range of the sphere’s centre before releasing phase

Position range of the sphere’s centre after releasing

Figure 6 Example of misalignment recovering

5.1.5 Releasing height

Some videos have been analysed in order to obtain information about the detachment of the sphere from the tip. These videos have been chosen because they represents experiments in which the approaching speed between the sphere and the releasing drop is very low, so the conditions are close to the static ones.

To evaluate the process some frames have been caught between the instant before the sphere touches the drop on the plane and the instant after the syringes needle leaves the sphere and one frame only for the plane calibration. In order to estimate the height at which the releasing force and the weight overcome the picking force, the distances between the planes of subsequent frames and the distances along the y axis of the

centres of the ideal spheres have been measured (Figure 7). In the particular instant in which the sphere is grabbed by the drop on the releasing plane, the vertical velocity of the sphere is different from the velocity of the plane. After that, considering the measurement errors, the velocities of the two items are the same.

For example the analysis of the video named pick_33.avi shows that the sphere, when it touches the releasing drop, tends to detach from the syringe tip. The Figure 7 illustrates two subsequent frames: the centre of the ideal sphere moves towards the plane with a shift corresponding to 13 pixels, while the plane moves in the other direction of 2 pixels (about 0.01 mm). That means that the sphere has been grabbed by the releasing drop and that the contact height is the releasing height.

X Y

Figure 7 Shifts of the plane and of the sphere centre during releasing process.

The geometries of the meniscus of these two frames have been used in the simulation tool to validate the experiment.

5.1.6 Analysis of the depositions

Finally, some deposition tests have been done in order to evaluate contact angles and the volumes of the drops. Using a visual basic program, three series of ten deposition for each couple liquid-component that exists in the tests (water-aluminium, water-glass, oil-glass) have been executed. The elaboration of the images is the same of the previously described tests (par. 4.2). Tables 1, 2 and 3 report the values of some deposition tests.

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Figure 8 a, b, c Deposition tests: water on aluminium (a), water on glass (b) and oil on glass (c)

liquid Plane Trial Measured Angles (degrees) Drop Vol. [kpixel3] water aluminium 01 67 70 140,822 156,111 water aluminium 02 68 71 153,076 145,695 water aluminium 03 70 71 163,857 155,991 water aluminium 04 67 70 160,98 151,956 water aluminium 05 66 72 146,907 146,148 water aluminium 06 64 70 128,751 130,873 Angle Average 68,8 Volume Average 148,4 Table 1 Results of depositions of water on aluminium plane

Volume of water drop on aluminium plate

148,4 120 130 140 150 160 170 Trials Vol [kpixel 3 ] Trial 01 Trial 02 Trial 03 Trial 04 Trial 05 Trial 06

liquid plane trial measured angles (degrees) water glass 01 25 25 water glass 02 28 28 28 water glass 03 30 31 31 water glass 04 32 32 33 water glass 05 31 33 34 water glass 06 34 36 35 Average 30,94

Table 2 Some results of the depositions of water drops on a glass plane

liquid plane trial measured angles (degrees) oil glass 01 15 15 16 oil glass 02 15 15 16 oil glass 03 15 15 16 oil glass 04 14 15 15 oil glass 05 14 15 oil glass 06 14 16 Average 26,59

Table 3 Some results of the depositions of oil drops on glass plane

It is important to highlight that the angles measured with these tests are static contact angles, while the angles measured with the picking and releasing tests are dynamic. In [1] the author dwells upon static contact angle hysteresis as shown in Figure 10.

Figure 10 Contact angle hysteresis (source [1])

5.2 Other procedures for the analysis

In order to prevent some errors of the image analysis, two different kinds of analysis have been tried. The first type of analysis (manual analysis) prevents from the loss of information caused by the binarization process using not filtered images. The second type of analysis (model analysis) uses a software that try to recognise into the images the shapes of the models prepared before.

5.2.1 Manual analysis

The aim of the manual analysis is to reconstruct the geometry of the meniscus using the images captured by the camera directly in the cad program (actually the images have to be transformed in 8 bit unsigned format). In this way there is not image processing software but the process is manually done. In Figure 11 the geometries have been drawn interpreting the differences in the grey scale between neighbouring pixels. The comparison between the two methods (manual and edge finder analysis) is shown in Table 4.

Pick15_0021 Meniscus Volume [kpixel3] Meniscus Volume [mm3] Manual Analysis 2120 0,018

Edge Finder Analysis 1542 0,013 Diff. 28%

Table 4 Comparison between the manual analysis and the edge finder analysis for the video pick15_0021

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Figure 11 Manual analysis of a drop (a) and of the corresponding meniscus (b)

This analysis has not been developed because it has been considered too much subjective for a rigorous study.

5.2.2 Model analysis

With this procedure, before the analysis of the images (previously transformed in 8 bit unsigned), some models of the shapes that the software have to recognise have to be prepared. Models of sphere, of the drop and of the meniscus of different measures have to be prepared because the difference between the model and the identified shape has to be small. The vertical lines defining the syringe and the horizontal one representing the plane could be revealed by a tool of the model analysis software but it requires the right setting of some parameters.

An example of an image processed by the model analysis is reproduced in Figure 12. The measures of the geometries pointed out with this procedure refer to the model ones. This analysis has not been developed because the software reveals the meniscus shapes with difficulty because they are quite different from the models prepared. Furthermore, the results obtained are unsatisfying considering the great work required.

Figure 12 Example of Model analysis

5.3 Experimental results

The measurements of the sphere radius, of the contact angles and of the picking and releasing volumes have been introduced into the simulation tool in order to obtain plots of the corresponding forces. With this graphs it is possible to compare the experimental results with the theoretical ones, evaluating the difference between the corresponding release heights.

For example, analysing the images of the video named pick_33.avi , the values of the parameters of the meniscuses, reported in Table 5, have been measured:

Picking Releasing Sphere radius [mm]

α

β

α

β

Table 5 Measured values of the parameters

Using these values, taking 0.02 mN as weight of the sphere and as surface tensions the values 0.020 Nm-1 for the picking and 0.073 Nm-1 for the releasing liquid, the simulation tool returns the plots (shown in Figure 13 and in Figure 14).

Figure 13 Picking force

Releasing Force

h [mm]

Vol [mm

]

Fc [mN]

In order to release the sphere, the capillary force has to be greater than the difference between the picking force and the weight of the sphere. The picking force that have to be considered is the one corresponding to the height equal to zero, because, when the sphere is grabbed, the distance from the picking tip becomes zero. With these considerations the picking force results about 0.09 mN (Figure 13). In Figure 14, using the releasing volume and the required force (about 0.07 mN), the theoretical releasing height is obtained: about 0.035 mm .

The difference between the measured releasing height and the theoretical one is not negligible. As written before, the measures are disturbed by a lot of errors. First of all there are errors concerning the geometry of the system: we supposed the meniscus to be axial-symmetric but the spheres are quite irregular and there could be some impurities both on the plane and inside the liquids. Moreover the simulation supposes that the picking tool is a perfect plane and that the liquid-vapour interface is an arc. Furthermore the volumes of the meniscuses have been supposed to be constant but the evaporation could be considerable. In addition it is worth noting that these experiments are dynamic, so the contact angles change at any moment. A large number of errors afflicts also the image analysis: wrong lighting conditions, loss of information during image processing, tilting of the camera, etc.; these things change the shapes, the dimensions and the positions of the objects in the images.