QMT Features: October 2010
Switch on - let’s go!
Absolute means ready to go! A look at Absolute Innovation Technology integrated into Hexagon Metrology’s new Absolute Arm and Absolute Tracker.

Speed of measurement is a critical enabling process that helps prevent defective product being produced and aids process control on the shopfloor. And where measurement adds value, faster measurement adds more value in any given period. So when  technology comes which promises instant measurement, such as Absolute Measurement,  then the benefits to manufacturing are obvious - not to mention those to engineering and R&D.

Global metrology company, Hexagon Metrology have introduced a range of products that employ absolute measurement technology. The ROMER Absolute Arm portable measuring system features absolute encoders. The distinguishing feature of the absolute encoder is that it reports the absolute position of the encoder to the electronics immediately upon power-up with no need for indexing. These encoders, therefore, allow the ROMER arm to be used straight away by the operator without the need for referencing prior to measurement. “Switch on and start measuring - whereas in the past you always had to zero the encoders which involved a routine with the Arm.” explains Steve Shickell, Portable CMM manager for Hexagon Metrology, UK.

 But its not just absolute measurement technology that Steve is keen to emphasise that makes the arm so special, “We have just launched a new integrated laser scanner for the new ROMER Absolute Arm. The certified laser scanner is an integral part of the arm’s wrist and is fitted to a seven axis version of the arm. It is an all purpose metrology tool for a multitude of applications. Changing from laser scanning to point probing is possible at the flick of a switch. 3D digitising, 3D modelling, point cloud inspection, reverse engineering, rapid prototyping or copy milling are the most frequent laser scanner applications for the ROMER  Arm.

In terms of accuracy, this is the first arm and scanner to be specified as a “complete system.” Steve explains that in the past, all arm manufacturers quoted the arm and  scanner specification by taking both individual specifications and simply adding the two together. “This may appear to be logical, but doesn't take into consideration the alignment uncertainty of the scanner to the arm, whereas the new ROMER Absolute Arm with integrated scanner is specified as a complete system, so our goal was to achieve high quality, and true accuracy which can be verified by VDE or VDI Standards.”

Absolute portability
Laser Trackers have been the benchmark tool for large scale high accuracy alignment and inspection tasks for the last two decades. 20 years ago Leica Geosystems introduced the world to the SMART 310. This first laser tracker was capable of measuring to +/- 10ìm per meter over a range of 25 meters. Although it was incredibly accurate, it wasn’t exactly portable. It required a tracker processor and a controller box (essentially two additional large computers) in addition to the running application PC. The system required mains power for operation and had a 30 minute warm-up period before the operator could start to take measurements.

On a similar time line the tracking total station was starting to emerge in the surveying world. This was a total station that could follow a moving reflector allowing an operator to measure by themselves. In the mid 1990’s Leica Geosystems released the TDA5000. This was their first high accuracy tracking total station. It introduced an all new Automatic Target Recognition (ATR) technology that allowed the total station to automatically target a reflector and track it while moving. These sensors were battery powered and sealed for use in the most demanding environments (including rain), but in even the best conditions were only accurate to around 0.25 mm. While laser trackers were getting smaller and lighter, tracking total stations were becoming faster and more functional.

Move forward 20 years and picture is quite different. Now Leica Geosystems have recently bridged the gap between traditional Laser Trackers and modern tracking Total Stations. By merging these two technologies Leica Geosystems has been able to create an ultra large scale truly portable CMM, the Leica Absolute Tracker AT401. It is  powered by its own internal battery and is able to work in the most demanding environment, yet maintains the highest level of precision and the largest ever work envelope. So by harnessing all of the benefits of high accuracy optical tracking total stations, and by employing the Absolute Distance Meter from the Leica Absolute Tracker AT901, the new AT401 is able to achieve high accuracy distance measurements with typical accuracy over an 80m range of +/- 5 µm.

Also developed on the AT401 to aid continuous, uninterrupted, measurement is a unique feature called PowerLock. This is an onboard vision technology which detects a reflector and automatically locks the laser beam onto it, even when the target is moving. The laser beam moves to the target, not the other way around. ”In the past operating a laser tracker was almost a ‘black art’.

It took time and experience for an operator to learn how to efficiently use the sensor without breaking the beam. However PowerLock changes this completely and makes the handling of a laser tracker much easier.” says Steve Shickell. “With a weight of less than 15 kilo (including its case), IP54 rating for harsh environments, battery operated and the ability to be operated completely wirelessly over a WiFi network, this new Laser Tracker truly is Absolutely Portable – just  switch on and its ready to go!”l

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Rob Tremain Photographer
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