Novel Sample Stage Alignment Hexapod for X-ray Microscopy and Tomography

Hexapod with Additional Linear Encoders to Achieve the Highest Precision

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1. Application: X‑Ray Microscopy / Tomography

For synchrotron-based sub-micrometer hard X‑ray Tomography, the sample rotation axis must be aligned very precisely in at least five degrees of freedom with respect to the field of view, the lens, the detector or the beam itself. High stiffness and stability are required, too. A hexapod-type parallel kinematic machine (PKM) is well suited for this purpose. It provides six degrees of freedom, the pivot point can be chosen arbitrarily (for example, along the rotation axis), and it is very stiff and compact compared to a serial kinematic stack of stages.

Basic principle of Hexapod PKM and serial kinematics 6 DOF system. Hexapod structures are stiffer and lighter, with no accumulation of guiding errors of individual axes. However, hexapods requires more sophisticated controllers. (Image: PI)
Basic principle of Hexapod PKM and serial kinematics 6 DOF system. Hexapod structures are stiffer and lighter, with no accumulation of guiding errors of individual axes. However, hexapods requires more sophisticated controllers. (Image: PI)
Ovali Hexapod with customized top plate. In addition to the 6 active struts for positioning, 6 more linear-encoder equipped sensor struts are implemented to achieve new levels of accuracy. (Image: PI miCos)
Ovali Hexapod with customized top plate. In addition to the 6 active struts for positioning, 6 more linear-encoder equipped sensor struts are implemented to achieve new levels of accuracy. (Image: PI miCos)

For the example in focus, we implemented a customized hexapod. It features an oval baseplate to fit into an actual X-ray microscope and a top platform which provides mounting interfaces to other components on top of the device. But, the main challenge was to meet the very demanding requirements regarding its precision.

2. Challenge: Load / Position-Independent Position Measurement

The position of the center of mass of the payload will necessarily change when the PKM is moving. In the given application, the mass of the device mounted on top of the PKM is around 60 kg and in addition, it moves along the longitudinal axis independently from the hexapod movements. Therefore, the legs and their respective joints are facing varying mechanical loads. Rotary encoders implemented in the driving legs can only infer the linear extension by measuring the rotary motion of the screw drive inside the leg. This measurement does not take into account that deformations of the joints are varying and that the thermal impact, caused by motor, gearbox, and the stiff roller screw drive, causes expansions that affect the overall accuracy at the point of interest at the platform. This situation is independent of the kinematic model; serial kinematic stacks of individual single-axis positioners are even more affected.

To overcome the limitations, the task was to find a way to enable load- and travel-history-independent platform position measurement for all six degrees of freedom at large travel ranges.

3. Solution: Hexapod with 6 Additional Sensor Legs

As basis a tried and proven off-the-shelf design was chosen, the HP-550 high-load hexapod. It was modified with six additional non-motorized measurement struts. These linear sensors are directly attached between the bottom and top platforms and not affected by varying mechanical loads and thermal influences in driven legs. The measurement struts were designed carefully with play free joints and linear scales. A lightweight space-frame structure was chosen to support the linear measuring systems, reducing forces and torque caused by gravity to a minimum.

The high precision Ovali hexapod with six motion legs, 6 linear encoder struts and additional integrated motion axes. (Image: PI miCos)
The high precision Ovali hexapod with six motion legs, 6 linear encoder struts and additional integrated motion axes. (Image: PI miCos)

4. Results / Figures

Equipped with this solution, the hexapod achieves better overall positioning precision and repeatability  independent of the travel-history. A positive side-effect was generated due to the oval design of the PKM. The achieved minimum incremental step size (MIM) for movements around the transverse axis (RotY) is less than 2 µrad, which is a great result for a hexapod in this payload-class.

The prototype of the Ovali hexapod achieved the results stated in Table 1. It has to be emphasized that travel ranges were customized to best fit the specific application. For some DoF, the required motion ranges were much smaller than for others – different requirements can be met with a modified design.

Table 1: Achieved metrology results

  Travel Range Minimum Incremental Motion Repeatability Speed
X ±1 mm 100 nm 144 nm 3.4 mm/s
Y ±50 mm 100 nm 60 nm 3.4 mm/s
Z -40 mm +60 mm 100 nm 11 nm 3.4 mm/s
rotX ±3° 3.5 µrad 7 µrad 1.2°/s
rotY ±1° 1.7 µrad 5 µrad 1.2°/s
rotZ ±15° 3.5 µrad 5 µrad 1.2°/s
Load (base plate horizontal / any orientation) 60 kg/30 kg
The Ovali hexapod is part of a complex precision positioning system with 56 motion axes. (Image: PI miCos)
The Ovali hexapod is part of a complex precision positioning system with 56 motion axes. (Image: PI miCos)
 

The Ovali Hexapod is used inside the MiQA System – a 56-Axis, 20-Ton Nanopositioning System for X-ray Microscopy & Quality Assurance of Optical Elements.
The Ovali Hexapod is used inside the MiQA System – a 56-Axis, 20-Ton Nanopositioning System for X-ray Microscopy & Quality Assurance of Optical Elements.

Video: More applications for hexapods in Beamline Instrumentation.
Video: More applications for hexapods in Beamline Instrumentation

Author: Dr. Markus Simon and Tim Naumann, PI miCos GmbH, Eschbach

> More Hexapod application stories

> More Beamline Instrumentation stories

> Standard Hexapod product overview

Learn more about PI Precision Motion Systems, Stages, Components: www.pi-usa.us/products/index.php

 

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About PI

PI (Physik Instrumente) is a leading manufacturer of precision motion control equipment, piezo motors, air bearing stages and hexapod parallel-kinematics for semiconductor applications, photonics, bio-nano-technology and medical engineering. PI has been developing and manufacturing standard & custom precision products with piezoceramic and electromagnetic drives for 4 decades. The company has been ISO 9001 certified since 1994 and provides innovative, high-quality solutions for OEM and research. PI is present worldwide with fifteen subsidiaries, R&D / engineering on 3 continents and total staff of more than 1,000.

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