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

Prototypical setup of a Michelson interferometer
A misaligned optical component leads to a distorted wavefront
Macro-micro-manipulator with gripper for placing optical components

Motivation 

Nowadays, an automated and individual assembly of optical system is not possible yet. This is due to increasing demands 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 in order to preserve the system’s functionality (such as in interferometric devices). Unfortunately, in turn, the active adjustment may also lead to increased production and labor costs.

Objectives

This research project deals with the function-oriented assembly of optical system which shall lead to lower tolerances on optical components and positioning systems. Furthermore, lower rejection rates of optical components are to 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 (correction 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 system and an 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 employed 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 to the simulation model to adhere to reality as close as possible.

 Contact: Dr.-Ing Christian Pape

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.

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