Glue measurement & control

Computer Thomographic scanners contain an array of image sensors on one side and a röntgen source on the other side. The patient is positioned in the centre of the scanner.
detector The array of image sensors and the röntgen source are connected to a rotating device so a lot of cross-section images of the patient can be created.
A scintillator is glued on front of the image sensor at a very accurate distance.
This scintillator converts the röntgen radiation into UV light. The image sensor is sensitive for this UV light so an röntgen image can be created. Until now the scintillator crystal is manually glued to the images sensor.
To do so, the scintillator and the sensor are positioned in a product holder.

The distance between the scintillator and the image sensor is about 100 micron.
The product holder is positioned under a certain angle with respect to the horizon.
Using a dispenser the glue is drop wise applied. When a drop is too late applied an air bubble will be visible in the röntgen image. During production the glue quality can not be measured.

glue unit with cameras Glue dispenser Only when the CT-scanner is tested, air bubbles in the glue gets visible. Too much glue will cause problems when the sensor is built-in in the CT-scanner.
To improve the glue quality a vision controlled production equipment is designed. The production unit contains 8 sensor-scintillator pairs. The camera-glue dispenser unit contains two cameras and two illuminations.
One camera is positioned on the dispenser side to control the dispenser so a glue drop is applied at the right time.
The second camera is positioned on the backside to check if sufficient glue is in the gap between the scintillator and the sensor.

Because the glue is very thin and transparent special illumination is used to visualize the contour of the glue.

Glue process

Proces animation

After positioning the first sensor-scintillator pair under the cameras-dispenser unit, three drops are applied at the gap. First drop left, second in the middle third drop on the right side op the sensor-scintillator pair.
Next step is an automatic glue check on the dispenser side and the backside using the vision system.
When enough glue is applied, the second sensor-scintillator pair is positioned under the camera-dispenser unit.
After the glue on the second sensor-scintillator pair is applied, the first sensor-scintillator pair will be positioned under the cameras and the amount of glue is measured.
If necessary extra drops of glue are applied and on the backside the second camera checks if the gap is filled with glue.

These steps are repeated until the production unit with 8 sensor-scintillator pairs are glued.

Detection algorithm

The camera of the vision system is positioned above the jig (sensor-scintillator holder) and perpendicular to the surface of the scintillator crystal and the sensor (photo sensitive diode).
Calculate large glue area Calculate small glue area Between the camera and the jig a coaxial led illumination is positioned which illuminates the scintillator crystal and the sensor.
The optics on the cameras is focused on the surface op the scintillator crystal. The diaphragm of the optics is used to create the required brightness of the image.
Both cameras are positioned with their topside to each other so the images acquired on the front and backside of the sensor are comparable.

When the cameras are not in the middle but shifted to the right or left side, the edge of the scintillator crystal and sensor are visible. When the edge of the crystal and the diode is visible on the left side of the front camera the edge of the crystal and diode is on the right side of the back camera and visa versa.

Algorithm

  • Detection of the scintillator crystal edge: in the fixed positioned search area several rulers (=edge detectors) detects the edge of the crystal. The edge of the crystal is the best fit of the detected edges.
  • Verifying the crystal edge: based on the found edge line of the crystal verification is done by detecting the vertical bars of the crystal. There fore several horizontal rules in the search area above and under the crystal edge detecting the edges of the vertical lines.
    The distances of these edges are compared with the distance of the vertical lines of the crystal. After this detection also the grey level (brightness) of the crystal is measured and compared with the required nominal level. This is an extra check.
  • Based on the scintillator crystal edge the search area for glue detection is defined. This search area is divided in several small sub areas.
    Per sub area the average grey level (brightness) is measured and compared with a threshold. When the measured grey level is lower then the threshold level, glue in that sub area is detected. In case of a higher brightness then the threshold level in that sub area no glue available.
    The total amount of glue in the detection area is calculated and the (maximum) height and width of the glue is calculated.
  • When no glue is detected a second detection algorithm is used for detecting a very small glue line on the edge of the crystal. Visible as a thin dark line very close to the crystal edge.
  • Small glue area detection: this detection detects the edge of the crystal very accurate e.g. outliers of the fitted edge are rejected and a new edge line of the crystal is calculated. Glue production unit In the search area rules (edge detection) detecting the edge of the glue. For every detected glue edge the distance to the accurate calculated crystal edge is calculated and scaled to millimeters.
    The area, height and width are also calculated for this small glue line.

  • The description of this project is published in Bits & Chips of December 2008 and in Mechatronica Magazine .

    (only in Dutch, sorry)