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A Predictor-Corrector-Method for the Robot-assisted Assembly of Optical Systems

Motivation 

Nowadays, an automated and individual assembly of optical system is not possible yet. This is due to the increasing demand on miniaturization and high rejection rates during the assembly. To overcome this problem, optical systems integrate active and/or passive adjustment mechanisms for each critical optical component to maintain the high demands on tolerances (such as in interferometric devices) in order to preserve the system’s functionality. In turn, this also leads to increased production and labor costs.

Objectives

This research project deals with the function-oriented assembly of optical system which in turn leads to lower tolerances on optical components and positioning systems. Furthermore, lower rejection rates of optical components can be realized during the assembly .

Approach

In order to realize the aforementioned goals, a predictor-corrector-method is employed for the assembly process. At all times In the sequential assembly process, a simulation model runs in parallel that constantly adapts to changes in reality with the help of identification methods. This enables prediction for the future position of optical components (prediction step). By analyzing demanded tolerances (such as wavefront deviations), a correction of the nominal position can be calculated on the basis of the simulation model (corrector step) in order to ensure the functionality of the system at the end of the manufacturing process.

Experimental Setup

The experimental setup consists of a positioning and optical system which needs to be assembled. A macro-micro-manipulator including gripper is employed as positioning system which is characterized by a large workspace and high positioning precision. A Michelson interferometer is prototypically utilized as optical system which has a wavefront sensor at the end of its optical train. The wavefront sensor can be utilized to infer the position of optical components via identification methods and therefore adapt the simulation model to adhere to reality.

 

Contact: Dipl.-Tech. Math. Christopher Schindlbeck

Image-based control of an optomechanical derotator for the measurement of rotating components

Image derotator at the Institute of Measurement and Automatic Control
Result of a measurement on a roller bearing carried out with an infrared camera: thermographic measurements are an excellent example for measurements in the presence of motion blur due to the long exposure time prescribed by the temperature range
Result of a measurement on a blisk model carried out with a laser Doppler vibrometer: the measurement beam can be focused on one point on the measurement object with the help of the derotator enabling more precise vibration measurement results

Rotating machine components are widely spread in technical applications. To guarantee efficient and safe operation it is necessary to perform regular investigations of the components. Proper functioning of machine components is best demonstrated by metrological investigations especially carried out contactless during operation. Thus, the results are precise since they are not falsified by the measurement system.

Conventional measurement methods quickly reach their limits for machine components in motion, in particular if the components are rotating. In case measurement data is acquired by standard high speed cameras capturing visible light or thermographic cameras the reason lies in the occurrence of motion blur. If objects are rotating with a high velocity or long exposure times are necessary (due to bad illumination condition or the operation principle of the camera) streaking of the moving object is apparent. Furthermore, focusing measurement methods emitting measurement beams (like laser Doppler vibrometry) into one point on an object is impracticable due to the rotational movement making measurement results imprecise. A solution to this challenge is provided by on optomechanical derotator. The derotator optically generates a stationary image of a rotating object by a rotating reflector assembly.

For stationary images it is essential that the position/angular velocity of the derotator are synchronized with half the position/angular velocity of the measurement object. At the Institute of Measurement and Automatic Control (IMR) this is achieved by an image-based highly dynamic controller using full state feedback. Features on the measurement object (either specific structures on the object or additional makers) are first extracted and then tracked to calculate the angular position/velocity. The so calculated values are then used as the feedback source for the controller of the derotator.

Besides, it is necessary to align the optical axis of the derotator with the rotational axis of the measurement object. For this purpose the derotator is mounted on a hexapod, which is able to precisely change its position in six degrees of freedom. At the IMR optical methods are utilized to eliminate translational and rotational deviations of the derotator in a two-stage process.

If those conditions are met the derotator can be used for a variety of measurement tasks. At the IMR those metrological investigations are set up, carried out, analyzed and evaluated. Especially measurements in the presence of

  • motion blur due to long exposure times and high rotational velocities
  • emitting measurement beams (e.g. laser vibrometry)
  • high frame rates

are improved or even made possible with the help of the derotator.