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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: M.Sc. Bettina Altmnn

Macro-Micro-Kinematics to the microassembly

General Information

The main focus of this research project is to studying methods for the development of manipulation systems, which should constitute the basis for a clinical high-precision assistance system. A resolution of 1 µm in a 10 mm³ work space is intended.

The technical implementation is made by linking a piezo actuator and a 6-axis-precision-robot (µKRoS316). The robot moves the tool in the entire work space (macro-positioning). The micro-positioning-unit has to compensate deviations of the robot end-effector from the reference position and to provide the high-precision-movement of the tool.

The whole work schedule can be divided into five working areas:

  • Controller design for the coupled system
  • Development of an external measurement system for realtime-6D-measurement
  • Improvement of the 6D-accuracy of robots
  • Matching of the coordinate systems of positioning units and measurement system and path-planning
  • Development of a set of tools for the micro-milling

Coupled Control

In this project, the coupling of the robot with a piezo actuator is studied. The control of the 6-axis robot μKRoS and xyz-piezoelectric stage has to be improved. Furthermore, the interaction between the piezoelectric stage and the robot and their influence on the overall accuracy are investigated. The project includes a modeling of the robot, including the nine torque-motors and the piezo-table. Finally, the verification of the whole system will be carried out in machining tests.

Setup of the Measurement System

The control of coupled system is not possible without a proper measurement system. This system has to measure the position and the orientation of the robot’s end-effector in real time, so that xyz-piezoelectric stage can compensate the positioning error of the tool. Two cameras with telecentric lenses, whose optical axis are oriented rectangular to each other can be used for this task. This measurement system makes it possible to determine the 6D-position with a sub pixel accuracy of 1.5 µm. But these cameras have a limited frequency.

Calibration of Positioning Units

In industrial applications the absolute accuracy is enhanced by calibrating the kinematic parameters and compensating for manufacturing errors. The accuracies are less than 0.7 mm and do not comply with the latest medical demands. To comply with the requirements, that the robot should be applied as a manipulation system. It is planned to obtain an absolute accuracy less than 0.1 mm respectively 0.1° in a 10 mm³ work space.

Two main kinds of errors have to be compensated:

  • Compensation from geometrical influence factors like deflection or home position errors
  • Compensation of the 6D-temperature drift (non-geometrical errors)

Due to the fact that the work space is limited to 10 mm³, it may be accepted that the robot is linear and can be described using polynomial functions. Initial researches have shown that absolute accuracies less than 0.087 mm respectively 0.09° can be achieved.

The axes of the piezoelectric stage have to be calibrated too. This means that these axes are not aligned perpendicular to each other and their orientation to the camera measurement system and to the robot end-effector are unknown. Through an appropriate calibration method the parameters will be estimated to eliminate the problems mentioned above.

Matching of Coordinate Systems and Path-planning

The machining of a work piece must be carried out in an absolute coordinate system. For this purpose coordinate systems of every single component – coordinate systems of the robot, measurement system, piezoelectric stage as well as coordinate systems of the work piece and of the tool – must be transformed to a global coordinate system. In this coordinate system the trajectory of the tool must be generated. To compensate the positioning error of the robot with piezoelectric stage an algorithm will be developed, that holds the tool in the nearest to the reference path position. This method must also completely exclude communication delays between the robot and the piezoelectric stage.

Tool-set

To implement the calibration methods and matching of coordinate systems mentioned above a tool-system must be developed, that takes into account the special features of the robot construction on the one hand and allows the detection of the work piece’s and tool’s parameters on the other hand.

Contact personDr.-Ing. Christian Pape