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Calibration, Identification and Control of an Optomechanical Image Derotator

The main focus of this research project is the identification and control of an image derotator, which was developed at the institute. Compared to conventional derotators this derotator is not equipped with a glass prism but with a rotating reflector device. Such kind of reflector device prevents aberrations and enables thermographic investigations.

The investigation of rotating objects during operation is only possible, if the optical axis of the reflector device corresponds with the rotation axis of the measurement objekt. To automate this complex process of adjustment, the derotator is mounted on a hexapod (6-axis parallel-kinematics). The deviation from the target position is determined with a high-speed camera. An important milestone of the project is to coordinate the interaction of the camera and the hexapod with a real-time system (National Instruments LabVIEW Real-Time). Algorithms will be developed, which enables the drawing of conclusions from the image data about the deviation between target and actual values (feature-based system identification).

In a further step, the cascaded control structure of the derotator will be deactivated to enable the identification of the system. Based on this identification advanced control strategies can be implemented to improve the quality of the control during transient operating conditions of the measurement object. The advanced control concept will be realised on the realtime platform.

In order to simulate measuring tasks under laboratory conditions, the test bench will be equipped with a highly dynamic synchronous motor, which simulates the behaviour of the measurement object through preset velocity profiles. The control of this motor will be implemented on the realtime system. So the hole test bench is monitored and controled centrally.

Contact person: Dr.-Ing. Christian Pape

Control Concepts for Image-Guided Object Movements

The testing stand for 3D-inverted pendulum’s stabilization was constructed at the Institute of Measurement and Automatic Control. The reference point’s and top’s position must be detected for the identification of the pendulum. The position of the top – the white ball – will be measured with help of the high-speed camera. This CMOS-camera makes 148 frames per second. The camera’s calibration will be primarily done for the definition of the relationship between the 2D-image coordinates and the 3D-world coordinates. The position of the reference point or the cart will be measured with the stepper motor’s counter.
In the control of 3D inverted pendulum two problems occure, the regulation problem and the tracking problem. The aim of the regulation problem is to stabilize the pendulum and maintain the cart at the middle of the table and the other is to stabilize the pendulum while the cart is tracking a circular path. The control schemes for the problems are PID, state feedback control, model reference adaptive controler (MRAC) using full state feedback and non-linear controler. The controlling was technically realized using an xPC-Target Toolbox from MATLAB. The control algorithms are execute on the Target-PC. The image processing is done on the Host-PC  and its results are send to the Target-PC through the Ethernet.
These control concepts can be used for the stabilization of the patient's table in the radiation theraphy.

Contact person: Dr.-Ing. Christian Pape