Laser TRACER calibration

A technique commonly used to accurately locate aircraft in flight could improve the method of measuring complex precision components for military and civilian aerospace applications.

Laser TRACER, developed by the National Physical Laboratory (NPL) determines the accuracy of co-ordinate measuring machines (CMMs) that are needed to measure the dimensions of precision components on the production line.

The system uses the principle of multilateration, which determines the co-ordinates of a target on an X-Y-Z grid by measuring the distance to the target from multiple fixed locations.

Multilateration is commonly used in civil and military surveillance to accurately locate an aircraft emitter by measuring the time difference of arrival of a signal from the emitter at three or more receiver sites.

But in this application, instead of locating an aircraft, Laser TRACER locates a small reflective target attached to a CMM spindle. A laser beam hits the target from multiple positions as the spindle moves through its calibration path. A beam then bounces back after each hit.

The system senses the displacement of the target with the use of an internal interferometer — a device for separating and re-combining beams of light. Interferometers consist of a complex arrangement of mirrors and beam splitters that randomly reflect some photons in one direction and transmit others in a different direction. The change in intensity of the re-combined beam provides the basis for the measurement.

‘We have developed software that will take this digital data and put it into a meaningful trace of the machine’s envelope size,’ said Laser TRACER product manage David Lowther. He said that the software calculates all the errors at high speed using the Monte Carlo system, a computational algorithm that relies on repeated random sampling to compute its results.

The CMM’s internal measurement data is then compared to those recorded by the Laser TRACER system, which will calibrate the results and print out a test report or United Kingdom Accreditation Service calibration certificate.

‘The machine operator will then have the fundamentals of the machine’s inaccuracy,’ said Lowther.

These fundamentals can then be used to perform error compensation. ‘We can feed that information into the computer that drives the machine,’ he said.

All of this, according to the company, can be completed in less than three hours. With other calibration methods, it could take up to two days to determine the accuracy of a CMM.

‘With our multilateration method you have loads of information coming in at the same time because the laser is chasing the target as it’s moved through its calibration path,’ said Lowther. ‘With other methods you have to set up the target and aim a laser at it, and you can only achieve one value at a time.’

He said the system has been proven to achieve sub-micron accuracy, which is partly due to the device’s NPL-patented internal design that is mechanically and thermally de-coupled from the tracking mechanism.

Another reason for Laser TRACER’s accuracy is its advanced self-calibrating software. ‘It takes into account any temperature or barometric changes that would affect the laser system,’ said Lowther.

The software was the result of an NPL research programme that aimed to develop a range of software tools for multi-sensor metrology network use. The organisation said that the techniques developed are applicable to any measurement network involving multiple sensors that are distance, displacement or angle-based.

This flexibility makes the tools particularly useful for assessing the performance of new classes of large-volume dimensional measurement systems such as laser radar and indoor GPS.

For now, though, the concentration is on production, which comes with many of its own challenges. The NPL researchers designed Laser TRACER to verify not only the accuracy of CMMs but also machine tools.

Lowther said accuracy is extremely important with 5-axis machining centres used to manufacture complex castings such as military equipment.

‘One of the worries is that if you get it nearly right, you can’t put it back on the machine because it is too close to being right and it’s still not correct,’ said Lowther. ‘You might have to scrap it, or produce new components to fit that error.’

He added that complex castings are not produced in high volumes like automotive parts, so getting it right is even more vital.

NPL and its partner ETALON will unveil Laser TRACER at MACH 2008, from 21-25 April at the NEC. It will later be available for purchase or lease.