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Development of a low cost profile measurement system using a CD pickup head and RTAI Linux

Gregor Gerstorfer, Institute for Measurement Technology, Johannes Kepler University, Linz, Austria
Bernhard Zagar, Institute for Measurement Technology, Johannes Kepler University, Linz, Austria

The development of a low cost profile measurement system suggests the use of an open source control system. In this work, the authors combine a commercially available compact disc (CD) pickup head, the National Instruments NI-PCI 6221 data acquisition card, and a PC running RTAI Linux. The setup will be used in a lab by students to get to know rapid control prototyping systems and open source alternatives to expensive industrial systems.

The setup consists of a CD pickup head (Mitsumi PXR 550 X), whose laser beam is focused onto a CD. The pickup head itself consists of a laser diode, a polarizing beam splitter, a collimator lens (which is movable by the voice coil motors), an anamorphic lens and the detector array. In this work, a controller drives the voice coil motor with the mounted collimator lens, so that the laser beam is reflected in the focal plane. Therefore the signals from the detector array have to be acquired, which is done by the NI PCI 6221, a 16 bit analogue input/output device.

The detector array is a photo detector with four areas arranged clockwise on the detector. The four detector areas deliver photo currents, proportional to the impinging light. The currents are converted into the voltages A, B, C, and D which form the so called focus error signal FE=(A C)-(B D). Due to the anamorphic lens the FE signal has a characteristic s-shape, when a reflecting object is moved through the focal plane towards the lens. This s-shape shows a linear range of about 3µm with the focal plane in the centre of the s-curve.

The controller is activated after the lens was moved to where the beam reflection takes place in the focal plane. The controller’s input is then the focus error signal, which should be zero. The controller was designed in Matlab/Simulink, using the RTAI expansions for Simulink. After code generation, the real time code is transferred to the controlling PC (Master). The Master is running Ubuntu where the generated realtime code is compiled and executed.

For modifying the controller parameters, for the visualization of the acquired signals and for data recording, a virtual machine running Ubuntu with loaded RTAI modules is used. Via TCP/IP, XRtaiLab (the software for visualization and parameter adjustment) in the virtual machine connects to the controlling PC (Master). The Master runs Ubuntu too, with loaded RTAI and COMEDI modules. COMEDI provides the drivers for the data acquisition card NI-PCI 6221.

The current setup is used for measuring the distance between tracks on a CD. Therefore, a CD is moved in radial direction along the pickup head. The evaluation of the acquired data results in a distance between two tracks of approximately 1.6µm, which corresponds to the specification of the CD standard.

In our proposed paper we will demonstrate the application of using RTAI Linux in a low cost profile measurement system to determine the profile of several specimens.