Fig. 1. Optical system set-up. The 3 telecentric lenses, each quipped with a band-pass filter, are imaging the object being back-illuminated by light of the same colour as that of the filter.

On-line system
The scope of the machine was the non-contact measurement of all the relevant dimensions of precisely turned cylindrical pins made of polished metal, whose shapes may vary and be also very complicated.
In order to obtain the maximum measurement accuracy, in the range of +/- 2 microns, it was necessary to take images of object areas as small as possible. To ensure this, each piece was imaged through 3 different cameras looking at three different object regions: the first camera was looking at the top of the pin, the second at the middle, the third at the bottom of the part. 1/2" detector industrial cameras, with Megapixel resolution, were used: a 1/2" detector size was chosen instead of 2/3" in order to keep the magnification not too high and preserve optimal depth of focus, necessary to look at object of different diameters and to take in account nominal position deviation due to part placement tolerances.

Due to the reduced part dimensions it was not possible to place the three cameras one over the other, aligned on the same axis, but a 30° displacement of the 3 viewing points was necessary.
Each camera was equipped with an Opto Engineering telecentric lens: the choice of telecentric lenses was mandatory for this application where low distortion and good field depth are necessary, but the most important advantage was the capability to make precise measurement even when the part was not exactly placed in its nominal position. The adoption of telecentric lenses has nearly tripled the system accuracy, compared to what achieved in the prototypal stage of this application, initially employing common entocentric lenses.

On the back side of the piece a white LED backlight illuminator, with side light injection technology, was placed. Accurately measuring a polished cylindrical metal part can be very difficult because this kind of objects reflect on their edges light coming from the sides and, therefore, the diameter seems to be much lower than actually is, because the cylinder sides seem to be as bright as the background.

Fig. 2. Light coming from the surroundings can be reflected on the edges of a cylindrical reflective object and make it to appear smaller than actually is.

To overcome this problem, the typical solution is using a collimated source or reducing the area of the back light illuminator; in our application, collimated source could not be used because of mechanical constraints, while a reduction of the backlight area was not possible because the object was observed from 3 different viewpoints.

To eliminate the border effects caused by a large backlighter, each lens mounted a different bandpass optical filter (red, green and blue) and the back lighting area was also divided into three regions, each covered by the same kind of interferential filter mounted on the lens pointing to that illuminator zone.
In this situation, each camera is reached only by the light coming from the corresponding area on the backlight, and no light reflected on the cylinder edges is coming from the surrounding areas.

Fig. 3. Schematic layout of the optical system.

Each image captured by the cameras was analysed and every dimensional information was checked at sub-pixel resolution.

Fig. 4. Application user interface showing object images, measurements and pieces out of tolerances.

The tracing of measurement lines on the image took in account the inclination of the part with reference to x and y axes. To tune the dimensional scale, in each image a calibration reference with known dimensions was placed on the side of the object in order to be able to define the exact pixel-to-millimetre conversion.
These calibration references made also possible the correlation among the measures of each image, allowing the determination of the overall piece dimensions.

Each production line is equipped with 2 complete systems, each with 3 cameras, allowing the simultaneous control of 2 pieces. All the 6 cameras are connected to the same PC with a GigaEthernet connection and are grabbing an image simultaneously following the same trigger signal; this makes possible a faster acquisition with no synchronisation problems. Each frame coming from each single camera is analysed in order to ensure the measurements are within the defined mechanical tolerances for part acceptance; the PC can also send a signal to a PLC to trigger the management of the defective pieces, which must be scrapped from the production line.
The control rate reaches 120 pieces/minute, taking in account also the movement mechanism.

For the human supervisor, the system shows the 6 images and corresponding measurement results, displaying green and red flags for parts within or out of the specified tolerances.

Off-Line System
Besides calibrating overall measurements of on-line system, an off-line system is necessary in order to control the online system accuracy, define the shape model of parts and to make a cross-check control on 1% of production.

Fig. 5. Off-line system.

This system is also equipped with a digital Megapixel camera; the part is positioned by the user above an anti-reflection coated glass plate with a red LED, side-injection technology, back-light behind. To reduce border effects, the area of the back light can be windowed by 2 couples of sliding black sheets, partially covering the emitting surface of the illuminator. A TC 23 64 Opto Engineering telecentric lens is placed above the part, taking care about the alignment of the optical axis which must be perfectly orthogonal to the glass plate in order to achieve the best performances. The measurement is compared with the theoretical shape model parameters stored in a database.
It is also possible to change the telecentric lens in order to resize the imaged object area and tune the resolution; after calibrating the system with a geometric pattern and a calibration software tool, the system can be used to control with the desired geometrical accuracy object with sizes ranging from 2 mm up to 144 mm.