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@ -2055,25 +2055,24 @@ This approach allows combining extended travel with high precision and good dyna
**** Introduction :ignore: **** Introduction :ignore:
The advent of fourth-generation light sources, coupled with advancements in focusing optics and detector technology, imposes stringent new requirements on sample positioning systems. The advent of fourth-generation light sources, coupled with advancements in focusing optics and detector technology, imposes stringent new requirements on sample positioning systems.
For the ID31 beamline, the smallest anticipated beam size is approximately 200 nm (horizontal, Dy) by 100 nm (vertical, Dz).
To effectively utilize such beams, the positioning system must maintain the sample's point of interest within the beam profile throughout the experiment.
This translates to required peak-to-peak positioning stability better than 200 nm in Dy and 100 nm in Dz, corresponding to RMS values of approximately 30 nm and 15 nm, respectively.
Furthermore, tilt errors (Ry) must be controlled to below approximately 1.7 µrad peak-to-peak (250 nrad RMS).
Crucially, these specifications must be met even when considering high-frequency vibrations, owing to the use of high-frame-rate detectors. With ID31's anticipated minimum beam dimensions of approximately $200\,\text{nm}\times 100\,\text{nm}$, the primary experimental objective is maintaining the sample's point of interest within this beam.
These demanding stability requirements must be achieved within the specific context of the ID31 beamline, which necessitates building upon the existing micro-station infrastructure, accommodating a wide range of experimental configurations requiring high mobility, and handling substantial payloads up to 50 kg. This necessitates peak-to-peak positioning errors below $200\,\text{nm}$ in $D_y$ and $200\,\text{nm}$ in $D_z$, corresponding to RMS errors of $30\,\text{nm}$ and $15\,\text{nm}$, respectively.
Additionally, the $R_y$ tilt angle error must remain below $0.1\,\text{mdeg}$ ($250\,\text{nrad RMS}$).
Given the high frame rates of modern detectors, these specified positioning errors must be maintained even when considering high-frequency vibrations.
These demanding stability requirements must be achieved within the specific context of the ID31 beamline, which necessitates the integration with the existing micro-station, accommodating a wide range of experimental configurations requiring high mobility, and handling substantial payloads up to 50 kg.
The existing micro-station, despite being composed of high-performance stages, exhibits positioning accuracy limited to approximately $10\,\mu m$ and $10\,\mu\text{rad}$ due to inherent factors such as backlash, mechanical play, thermal expansion, imperfect guiding, and vibrations.
The existing micro-station, despite being composed of high-quality stages, exhibits positioning accuracy limited to approximately $10\,\mu m$ and $10\,\mu\text{rad}$ due to inherent factors such as backlash, mechanical play, thermal expansion, imperfect guiding, and vibrations.
While individual stage encoders can correct motion along their primary axis, they do not compensate for parasitic motions in other degrees of freedom.
The primary objective of this project is therefore defined as enhancing the positioning accuracy and stability of the ID31 micro-station by roughly two orders of magnitude, to fully leverage the capabilities offered by the ESRF-EBS source and modern detectors, without compromising its existing mobility and payload capacity. The primary objective of this project is therefore defined as enhancing the positioning accuracy and stability of the ID31 micro-station by roughly two orders of magnitude, to fully leverage the capabilities offered by the ESRF-EBS source and modern detectors, without compromising its existing mobility and payload capacity.
**** The Nano Active Stabilization System Concept **** The Nano Active Stabilization System Concept
To address these challenges, the concept of a Nano Active Stabilization System (NASS) is proposed. To address these challenges, the concept of a Nano Active Stabilization System (NASS) is proposed.
As schematically illustrated in Figure ref:fig:introduction_nass_concept_schematic, the NASS comprises four principal components integrated with the existing micro-station (yellow): a 5-DoF online metrology system (red), a 5- or 6-DoF active stabilization platform (blue), and the associated control system and instrumentation (purple). As schematically illustrated in Figure ref:fig:introduction_nass_concept_schematic, the NASS comprises four principal components integrated with the existing micro-station (yellow): a 5-DoF online metrology system (red), an active stabilization platform (blue), and the associated control system and instrumentation (purple).
This system essentially functions as a high-performance, multi-axis vibration isolation and error correction platform situated between the micro-station and the sample. This system essentially functions as a high-performance, multi-axis vibration isolation and error correction platform situated between the micro-station and the sample.
It actively compensates for positioning errors measured by the external metrology system. It actively compensates for positioning errors measured by the external metrology system.
The overarching goal is to improve the effective positioning accuracy from the micro-station's native $\approx 10\,\mu m$ level down to below $100\,\text{nm}$, while preserving the full mobility and 50 kg payload capability of the underlying stages.
#+name: fig:introduction_nass_concept_schematic #+name: fig:introduction_nass_concept_schematic
#+caption: The Nano Active Stabilization System concept #+caption: The Nano Active Stabilization System concept
@ -2082,33 +2081,32 @@ The overarching goal is to improve the effective positioning accuracy from the m
**** Online Metrology system **** Online Metrology system
The performance of the NASS is fundamentally reliant on the accuracy and bandwidth of its online metrology system, as the active control is based directly on these measurements. The performance of the NASS is fundamentally reliant on the accuracy and bandwidth of its online metrology system, as the active control is based directly on these measurements.
This metrology system must fulfill several criteria: measure the sample position in 5 DoF (excluding rotation about the vertical Z-axis); ideally, measure position relative to the X-ray focusing optics; possess a measurement range compatible with the micro-station's extensive mobility and continuous spindle rotation; achieve accuracy commensurate with the sub-100 nm positioning target; and offer high bandwidth for real-time control. This metrology system must fulfill several criteria: measure the sample position in 5 DoF (excluding rotation about the vertical Z-axis); possess a measurement range compatible with the micro-station's extensive mobility and continuous spindle rotation; achieve an accuracy compatible with the sub-100 nm positioning target; and offer high bandwidth for real-time control.
#+name: fig:introduction_nass_metrology #+name: fig:introduction_nass_metrology
#+caption: 2D representation of the NASS metrology system. #+caption: 2D representation of the NASS metrology system.
[[file:figs/introduction_nass_metrology.png]] [[file:figs/introduction_nass_metrology.png]]
An initial concept, depicted in Figure ref:fig:introduction_nass_metrology, involves a spherical reflector with a flat bottom surface fixed beneath the sample. A proposed concept (illustrated in Figure ref:fig:introduction_nass_metrology) features a spherical reflector with a flat bottom attached below the sample, with its center aligned to the X-ray focus.
The sphere's center is intended to coincide with the X-ray focus. Fiber interferometers target both surfaces.
Fiber interferometers are directed at both the spherical and flat surfaces. A tracking system maintains perpendicularity between the interferometer beams and the mirror, such that Abbe errors are eliminated.
A tracking system is needed to maintain interferometer alignment, eliminating Abbe errors by measuring directly relative to the point of interest. Interferometers pointing at the spherical surface provides translation measurement, while the ones pointing at the flat bottom surface yield tilt angles.
Translational positions (XYZ) are derived from measurements on the spherical surface, while tilt angles (Rx/Ry) are determined from measurements on the flat bottom surface.
The development of this complex metrology system constitutes a significant mechatronic project in itself and is currently ongoing; it is not further detailed within this thesis. The development of this complex metrology system constitutes a significant mechatronic project in itself and is currently ongoing; it is not further detailed within this thesis.
For the work presented herein, the metrology system is assumed to provide accurate, high-bandwidth 5-DoF position measurements. For the work presented herein, the metrology system is assumed to provide accurate, high-bandwidth 5-DoF position measurements.
**** Active Stabilization Platform **** Active Stabilization Platform Design
The active stabilization platform, positioned between the micro-station top plate and the sample, must satisfy several demanding requirements. The active stabilization platform, positioned between the micro-station top plate and the sample, must satisfy several demanding requirements.
It needs to provide active motion compensation in 5 DoF (Dx, Dy, Dz, Rx, Ry). It needs to provide active motion compensation in 5 degrees of freedom ($D_x$, $D_y$, $D_z$, $R_x$ and $R_y$).
It must possess excellent dynamic properties to enable high-bandwidth control capable of suppressing vibrations and tracking desired trajectories with nanometer-level precision. It must possess excellent dynamic properties to enable high-bandwidth control capable of suppressing vibrations and tracking desired trajectories with nanometer-level precision.
Consequently, it must be free from backlash and play, and its active components (e.g., actuators) should introduce minimal vibrations. Consequently, it must be free from backlash and play, and its active components (e.g., actuators) should introduce minimal vibrations.
Critically, it must reliably support and actuate payloads up to 50 kg. Critically, it must accommodate payloads up to 50 kg.
A suitable candidate architecture for this platform is the Stewart platform (or hexapod), a parallel kinematic mechanism capable of 6-DoF motion. A suitable candidate architecture for this platform is the Stewart platform (also known as "hexapod"), a parallel kinematic mechanism capable of 6-DoF motion.
Stewart platforms are widely employed in positioning and vibration isolation applications due to their inherent stiffness and potential for high precision. Stewart platforms are widely employed in positioning and vibration isolation applications due to their inherent stiffness and potential for high precision.
Various designs exist, differing in geometry, actuation technology, sensing methods, and control strategies, as exemplified in Figure ref:fig:introduction_stewart_platform_piezo. Various designs exist, differing in geometry, actuation technology, sensing methods, and control strategies, as exemplified in Figure ref:fig:introduction_stewart_platform_piezo.
A central challenge addressed in this thesis is the optimal mechatronic design of such an active platform tailored to the specific requirements of the NASS. A central challenge addressed in this thesis is the optimal mechatronic design of such an active platform tailored to the specific requirements of the NASS.
A more detailed review of Stewart platform and its main components will be given in Section [...].
# TODO - Review of Stewart platform ref:sec:detail_kinematics_stewart_review # TODO - Review of Stewart platform ref:sec:detail_kinematics_stewart_review
#+name: fig:introduction_stewart_platform_piezo #+name: fig:introduction_stewart_platform_piezo
@ -2129,30 +2127,32 @@ A central challenge addressed in this thesis is the optimal mechatronic design o
#+end_subfigure #+end_subfigure
#+end_figure #+end_figure
**** MIMO robust control strategies **** Robust Control
The NASS inherently involves multi-input, multi-output (MIMO) feedback control. The control system must compute the position measurements from the online metrology system and computes the reference positions derived from each micro-station desired movement.
The control system must process position measurements from the online metrology system and reference positions derived from the desired micro-station movements, commanding the active platform in real time to stabilize the sample and compensate for all error sources, including stage imperfections, thermal drifts, and vibrations. It then commands the active platform in real time to stabilize the sample and compensate for all error sources, including stage imperfections, thermal drifts, and vibrations.
Ensuring the stability and robustness of these feedback loops is paramount, especially within the demanding operational context of a synchrotron beamline, which requires reliable 24/7 operation with minimal intervention. Ensuring the stability and robustness of these feedback loops is crucial, especially within the demanding operational context of a synchrotron beamline, which requires reliable 24/7 operation with minimal intervention.
This contrasts with many traditional synchrotron instruments built using proven, passively stable components like stepper motors and conventional bearings.
Several factors complicate the design of robust feedback control for the NASS. Several factors complicate the design of robust feedback control for the NASS.
The system must perform reliably across diverse experimental conditions, including different scan types (tomography, linear scans) and velocities (slow drifts to fast fly-scans). First, the system must operate under across diverse experimental conditions, including different scan types (tomography, linear scans) and payloads' inertia.
The continuous rotation of the spindle introduces gyroscopic coupling effects and means actuators rotate relative to stationary sensors, altering the system dynamics. The continuous rotation of the spindle introduces gyroscopic effects that can affect the system dynamics.
Perhaps the most significant challenge is the wide variation in payload mass, from potentially 1 kg up to 50 kg. As actuators of the active platforms rotate relative to stationary sensors, the control kinematics to map the errors in the frame of the active platform is complex.
High-performance positioning controllers often assume a fixed, well-characterized payload, as seen in applications like lithography or atomic force microscopy (AFM). But perhaps the most significant challenge is the wide variation in payload mass (1 kg up to 50 kg) that the system must accommodate.
Designing for robustness against large payload variations typically necessitates larger stability margins, which can compromise achievable performance. Designing for robustness against large payload variations typically necessitates larger stability margins, which can compromise achievable performance.
Furthermore, unlike many systems where the active stage and sample are significantly lighter than the underlying coarse stages, the NASS payload mass can be substantially greater than the mass of the micro-station's top stages. Consequently, high-performance positioning stages often work with well-characterized payload, as seen in systems like wafer-scanners or atomic force microscopes.
This leads to strong dynamic coupling between the active platform and the micro-station structure, resulting in a more complex MIMO system with significant cross-talk between axes.
Furthermore, unlike many systems where the active stage and sample are significantly lighter than the underlying coarse stages, the NASS payload mass can be substantially greater than the mass of the micro-station's top stage.
This leads to strong dynamic coupling between the active platform and the micro-station structure, resulting in a more complex multi-inputs multi-outputs (MIMO) system with significant cross-talk between axes.
These variations in operating conditions and payload translate into significant uncertainty or changes in the plant dynamics that the controller must handle. These variations in operating conditions and payload translate into significant uncertainty or changes in the plant dynamics that the controller must handle.
Therefore, the feedback controller must be designed to be robust against this plant uncertainty while still delivering the required nanometer-level performance. Therefore, the feedback controller must be designed to be robust against this plant uncertainty while still delivering the required nanometer-level performance.
**** Predictive Design / Mechatronics approach **** Predictive Design / Mechatronics approach
The overall performance of the NASS will be determined by the interplay of numerous factors, including sensor accuracy and noise, actuator force and bandwidth, mechanical design stiffness and resonances, and the achievable control bandwidth. The overall performance achieved by the NASS is determined by numerous factors, such as external disturbances, the noise characteristics of the instrumentation, the dynamics resulting from the chosen mechanical architecture, and the achievable bandwidth dictated by the control architecture.
To navigate this complexity and ensure the final system meets its stringent specifications, a predictive design methodology, specifically a mechatronics approach, is essential. Ensuring the final system met its stringent specifications requires the implementation of a predictive design methodology, also known as a mechatronics design approach.
The goal is to rigorously evaluate different concepts, predict performance limitations, and guide the design process towards an optimal solution that functions correctly upon first assembly, given the significant cost and complexity involved. The goal is to rigorously evaluate different concepts, predict performance limitations, and guide the design process.
Key challenges within this approach include developing appropriate design methodologies, creating accurate simulation models capable of comparing different concepts quantitatively, and converging on a final design that demonstrably achieves the target performance levels. Key challenges within this approach include developing appropriate design methodologies, creating accurate models capable of comparing different concepts quantitatively, and converging on a final design that achieves the target performance levels.
* Original Contributions * Original Contributions
**** Introduction :ignore: **** Introduction :ignore: