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Photo-media digitiser is the last word in accuracy

11 April 2011

Nanometre accuracy and extreme stability were among the exacting specifications that Aerotech had to meet when its engineered motion sub-system was selected for an ultra-precision plate scanner at the Royal Observatory of Belgium

The ‘Digital Access to Metric Images Archives Network’ (DAMIAN) ultra-precision scanning system at the Royal Observatory of Belgium (ROB) plays a significant role in the global programme to digitise astrometric plates and aerial photographic images spanning more than a century. With in excess of three million plate images estimated in the astronomical community alone, the programme was borne out of a need to provide a much needed centralised digital archive in circumstances where media deterioration has become an increasing problem.

One project example at the ROB involves digitising thousands of photographic plates for the US Naval Observatory. Taken over a period of 30 years, using a 26in 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.

The ROB began development of the DAMIAN digitiser with a study that produced design specifications for a 350mm x 350mm X-Y scanning motion system. To ensure the perfect 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 rather than older methods which 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 long working life were also critical factors.

The precision and dynamic performance is being met by a motion sub-system based on Aerotech’s ABL3600 series open-frame air bearing table. The custom engineered system includes 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 digitiser is housed in a temperature and humidity controlled clean room, maintained to within 0.1ºC and 1%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.

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 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 – far exceeding the original working specification.

Other impressive results included a geometric test for accuracy and repeatability to better than +/- 0.1 micron and the displacement speed and in-position stability performance were fully met.

The ROB took responsibility for 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 stand-still, thus the need for such high in-position stability.

The  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 Microsoft .NET development environment to help reduce project development time with C, C++, VisualBasic or Delphi based programming.

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