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Camera-based determination of the work offset

Quicker series production

To ascertain the zero point of a workpiece with a high degree of accuracy, touch-trigger probes combined with Sinumerik measuring cycles are the first choice. For smooth, efficient series production, cameras offer a quicker, albeit slightly less precise, alternative.

The digital processing of camera data has been successfully applied to many areas of automation technology. In machine tools, too, cameras can have a multitude of uses. They can monitor and analyze manufacturing processes in areas otherwise difficult to see, or digitally document the production process for training or quality assurance
purposes.
However, new high-resolution cameras are now being used for measuring tasks in time-optimized series production.

Object and pattern recognition

A typical example of the new generation of cameras is the Simatic MV440 1-D/2-D code-reading system. Specially developed for industry, it can be used in even difficult machine spaces thanks to its compact design, its flexible lighting options, and its many communication and connectivity alternatives. Combined with a range of software options, the code-reading system can also be used for text and object recognition. To determine a zero point, the camera records as an image the contours of the blank or workpiece before analyzing the information. By comparing the state of the blank with the predefined size of the reference object, the offset and rotation can be ascertained in two dimensions. The automatic correction of the zero point is carried out using ATRANS and AROT in the NC program.

Precalibrating the camera

To calibrate the camera for automated measuring using a reference part, the operator must first connect the camera via Profinet to the Sinumerik 828D CNC, mounting it parallel to the axis over the machine table. Then a predefined z distance from the camera to the reference part is approached and the x and y position as well as the zero offsets are saved as user data (GUD). The calibration process can be made easier for the operator with the help of menu images rendered in the Run MyScreens tool. The operator then captures images of the reference object with the camera and defines distinctive features in the images to aid measurement using the camera’s software. The coordinates of the distinctive features are recorded as pixel values in x and y and then transferred to the Sinumerik CNC. Using a predefined scaling factor, the pixel values can be converted into a traversing path

Measurements in automatic mode

In automatic mode the camera moves into the precalibrated reference position and creates an image of the blank. The analysis software compares this image with the reference image and determines the offset and rotation of the blank compared to the reference object using the distinctive features. The zero offset is then automatically corrected in the program via ATRANS and AROT. This saves time compared to measuring the position with a touch-trigger probe. Deviations must be taken into account in determining repeat accuracy. This is dependent on lighting, contrast and surface quality. The accuracy of position measurement is +/-0.02 mm, and that of rotation measurement +/-0.2°. For many applications in series production, however, the time gained is sufficient compensation for this loss of accuracy.

Further information and download

The programms and the documentation are available at the Siemens Support-Portal: