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This paper investigates the motion control system of an optical telescope system used for precision satellite tracking and ranging applications. The system uses direct-drive permanent magnet synchronous motors (PMSMs) for high precision positioning. To overcome the performance limitations due to system dynamics and position dependent plant variations, a disturbance observer based control system is utilized. This paper contributes the detailed analysis, design and implementation of such an advanced control concept for the performance improvement of precision satellite tracking systems. Satellite tracking experiments are conducted to verify the performance of the proposed system. Utilizing the proposed control concept, the RMS servo error is reduced by a factor of 3.8 to well below the arcsecond range, achieving seeing limited tracking.

This paper investigates the tracking accuracy of an optical telescope system used for satellite tracking and laser ranging applications. The investigated system uses a high precision motion controller and a pointing model based on spherical harmonics to achieve high accuracy. To overcome the limitations due to local pointing model inaccuracies and dynamic effects during tracking, an iterative trajectory learning algorithm is proposed. The implementation, as well as the stability analysis of the proposed concept is presented. Satellite tracking experiments are conducted to verify the accuracy of the proposed system. Utilizing the proposed iterative trajectory learning concept, the tracking error is reduced by a factor of 11 and is ultimately limited by the uncertainty of the orbit prediction.

In hybrid reluctance actuators for high precision motion control, the magnetic hysteresis in the ferromagnetic stator and mover usually affects the precision. Therefore, this paper proposes magnetic flux estimation and control to compensate for the hysteresis. To measure the magnetic flux at various frequencies, the output signal of a search coil is combined with that of the current monitor of the employed current amplifier in a magnetic flux estimator. Finally, a magnetic flux controller is designed and implemented. For evaluation of the hysteresis compensation, the performance of the hybrid reluctance actuator is compared with both flux and current control. Experimental results show that flux control successfully compensates for some of the nonlinearities of the hybrid reluctance actuator, as the hysteresis is reduced by up to a factor 8.2 and the phase lag of the mover position is reduced by up to 6 deg in comparison with the case of current control.

Mechanical vibrations resulting from wind, ground vibrations and actuator imperfections lower the performance of optical telescope systems used for astronomical applications, as well as free-space optical communication. This paper proposes a high-bandwidth tip-tilt compensation system for optical telescopes, in order to reduce the influence of these vibrations. A typical, high precision, motorized telescope system is used as a base for this evaluation. By utilization of a tip-tilt sensor, the resulting tip-tilt errors are measured and analyzed in the frequency domain. A fast steering mirror integrated into the optical path allows to compensate for the measured tip-tilt errors. The performance of the fast steering mirror based system is compared to the original, uncompensated system, as well as to a compensation loop, which uses the telescope mount to counteract measured errors. Experiments demonstrate a significant reduction of the tip-tilt errors due to vibrations by a factor of 5 (altitude axis) and by a factor of 4.6 (azimuth axis) utilizing a compensation bandwidth of up to 810 Hz.

Midsized telescopes currently receive a lot of interest due to their application in free space optical communication and space debris observation. This paper analyzes an active mirror support system of a very thin meniscus with a diameter of 1 m and 25 mm thickness for the application in a lightweight, low-cost telescope. An analytic model for self-weight surface deflection is used to determine the number of concentric continuous support rings and find their radii to sufficiently support the mirror. The obtained radii are used as initial value for a finite element analysis to include shear effects and the presence of a central hole for optical access, enabling further improvement. Following that, a transition from continuous support rings to point supports is made. With a number of 24 support points, a surface RMS error of 10.9 nm can be achieved. Additionally, the requirements for an active support to correct low order aberrations are determined.

We investigate the design of an active support system for the thin primary mirror of a mid-sized telescope system used for optical satellite communication and space debris observation. To handle the complexity of this task, a general design methodology is proposed. The design for the axial and lateral support is separated into several subtasks to reduce the number of design variables in every design step. Due to the independence of mirror geometry and material, this methodology is also applicable to larger mirrors. Utilizing the proposed procedure, an active support for a 1-m meniscus mirror with 25 mm thickness and the requirement to achieve diffraction-limited optical performance is developed. The final system consists of 32 axial and 8 lateral actuators supporting the mirror with a maximal simulated RMS error of 8.8 nm and PV error of 48.0 nm when pointing to zenith. Simulations show that the obtained design ensures the required performance even under commonly occurring mirror deformations.

Atmospheric turbulences limit the achievable performance of free-space optical (FSO) satellite communication systems. Particularly in retro-reflective FSO satellite communication, tip-tilt disturbances are a dominant source of performance degradation and thus prevent the exploitation of the full potential of this communication system. This publication investigates the total tip-tilt error of a terrestrial optical communication platform for reflective optical communication using a 14-inch telescope with tip-tilt compensation. The compensation system consists of a fast steering mirror (FSM), a quad photo diode (QPD) and a controller. Dynamic error budgeting is used to systematically analyze the system components’ interplay and their contribution to the total error. Based on the results of the system analysis, a feedback controller for the compensation system is designed and tuned for disturbance rejection. The system’s performance is evaluated with a reflective FSO communication link over a distance of 600 m in urban environment. The atmospheric aberration statistic is put into relation with comparable measurements using satellite to earth communication links. Measurement results successfully demonstrate the system’s performance, effectively reducing the tip-tilt error up to a factor of 10.

A six degree of freedom, magnetically levitated metrology platform is proposed and implemented to enable nano-scale measurements directly in a production environment by providing vibration isolation. The metrology platform maintains a constant distance between sample and nano-metrology tool, forming a nano-scale laboratory environment directly in the production line. This paper presents the design of the proposed metrology platform. Tracking of the sample is achieved by using six position sensors, a six degree of freedom actuator and feedback control. Experimental results demonstrate positioning of the platform in six degrees of freedom at a bandwidth of 35 Hz in the translational directions and at a bandwidth of more than 15 Hz in the rotational directions, respectively. This results in a tracking error that is smaller than 50 nm rms. This paper denotes the first successful attempt for six degree of freedom vibration isolation to enable in-line nano-metrology.

Zusammenfassung In einer Produktionsumgebung beeinflussen mechanische Vibrationen den Relativabstand zwischen Inspektionsgerät und Werkstück, was für Messungen im Nanometerbereich hinderlich ist. Dieser Beitrag stellt eine Metrologieplattform zur Montage eines Inspektionsgerätes vor, die mittels eines H∞-Reglers den Relativabstand zum Werkstück konstant hält. Ein Laboraufbau mit einem Freiheitsgrad zeigt, dass in der Produktionsumgebung vorkommende Vibrationen um eine Größenordnung reduziert werden können, was eine Oberflächencharakterisierung im Nanometerbereich direkt in der Produktion ermöglicht.