Motion sub-system forms basis of ultra-precision scanner

The Damian ultra-precision scanning system at the Royal Observatory of Belgium (ROB) fulfils a significant part of a global programme for the digitisation of astrometric plates and aerial photographic images that span over a century and has shaped our understanding of the world, the solar system and the universe. With more than three million plate images estimated to be available in the astronomical community alone, the programme was borne out of a need to provide a centralised archive for the digital age, to enhance the accuracy of predictive ephemerides and to preserve the heritage of this important work. As part of an international network of institutions involved in similar work, the ROB has become a global centre of excellence, particularly where extreme levels of accuracy are required - such as astrometry plates.

One particular exacting example involves digitising thousands of photographic plates for the US Naval Observatory. Taken over a period of 30 years several decades ago, using a 26-inch refractor telescope, the photographs record the moons of Mars, Jupiter and Saturn. By combining the process with highly sophisticated measurement and prediction software, Damian can actually fill in many gaps that have hitherto been difficult to calculate. By improving this understanding of the interactive motions and internal structure models of these solar system bodies, the accuracy of calculation for their future positions over time is improved significantly. Other work carried out has similar significant implications for galactic kinematics, space surveillance and other areas of research in high-precision aerial mapping.

The ROB began development of the Damian (Digital Access to Metric Images Archives Network) digitiser with a study that produced design specifications for a 350mm x 350mm X-Y scanning motion system. To ensure true reproducibility of the original analogue photographic images, the positional accuracy and repeatability targets needed to be an order above the inherent accuracy for the plates. This value was interpreted in the region of 0.5 microns over the whole scanning area with respect to a fixed telecentric objective of a sophisticated digital camera system. To ensure the fastest possible throughput, the motion system required a full move displacement of 10mm in less than half a second, including acceleration, deceleration and settling time and once in position, the stability (jitter) requirement was just 20 nanometres. These factors would ensure that the plates could be fully scanned and archived in a matter of minutes, unlike older methods that were less accurate and required hours of processing. The machine specification also called for automatic loading of the glass photographic plates and film rolls, with a working duty cycle close to 24/7 continuous and unattended operation - so high reliability, low maintenance and a long working life were also critical factors.

Motion sub-system

To realise such extreme precision and dynamic performance, the ROB chose Aerotech for a complete motion sub-system based on its ABL3600 series open-frame air bearing table in a custom-engineered system that included a lapped granite base plate, a granite bridge with a vertical focussing axis for the camera and optical assembly, plus a photographic plate holder, plate storage tower and fully automatic transport system.

Now delivered and fully commissioned, the Damian digitiser is housed in a clean room that is maintained to within 0.1degC and 1 per cent RH. Aerotech's ABL3600 series X-Y stage includes a preloaded, high-stiffness air bearing system featuring powerful dual brushless and slotless linear motors on each axis with glass scale linear encoder servo feedback. Two granite rails form a reference for the lower axis and both axes are referenced to a lapped granite surface plate with passive air isolation to maximise vibration stability. In combination with this positioning system, and fundamental to the machine's overall dynamic performance, Aerotech's linear technology servo amplifiers guarantee exceptionally smooth motion, high bandwidth and outstanding in-position stability with zero backlash or hysteresis. The friction-free mechanics also provide the added benefit of very low maintenance and an essentially limitless working life. The large through aperture of the open-frame design allows back illumination for the plates and images. The design of the ABL3600 has a pedigree of successful applications in semiconductor fabrication and test, and is highly suitable for demanding scanning microscopy, imaging and step-and-repeat positioning applications.

For the ROB, Aerotech extended the ABL3600's normal working travel range from 250mm to 350mm in both axes. For such high-precision applications, Aerotech's HALAR calibration firstly ensures optimal levels of accuracy, bi-directional repeatability, straightness and flatness for each axis, then performs error mapping with laser-based measurement systems at Aerotech's state-of-the-art metrology lab - with the calibration file pre-configured on the motion controller. During tests at Aerotech and on-site at the ROB, object locations on a calibrated test plate were repeated to within 70 nanometres over a usable X-Y travel range of 335mm - this far exceeded the original working specification. Other impressive results included geometric tests for accuracy and repeatability to better than +/-0.1 micron and the displacement speed and in-position stability performance requirements were fully met.

The Damian ultra-precision scanning system includes a full cable management system and Aerotech's A3200 Automation Platform motion control system, which controls the main X-Y axes as well as several other motion axes for film wind, plate stacking and tray height adjustment. The ROB took care of interfacing the A3200 to its own imaging software using a step-and-repeat procedure with alignment accuracy ensured using selected objects on the photographic media. Images are recorded at standstill, hence the need for such high in-position stability.

Aerotech's PC-based, software-only motion and machine controller provides position, velocity and time information to Firewire-interfaced linear technology servo drives. A range of optional modules include HMI, soft PLC and vision control. The fully deterministic controller can be programmed using Aerotech's own AEROBasic, RS274 G-code and Labview. Alternatively, programmers can call upon the powerful Microsoft .NET development environment to help reduce project development timescales with C, C++, VisualBasic or Delphi-based programming.

This application is fairly typical of Aerotech's engineered systems approach in which customers provide their own very specialised expertise in combination with a fully tested and certified motion sub-system.