Software adds flexibility to laser interferometry

The ML10 laser interferometer system has been used for many years for the calibration of machine tool positional accuracy and repeatability, in line with both national and international standards.

Technological advances in traditional metal cutting and developing industries such as semiconductor processing, flat panel production and biotechnology have led to an ever-increasing number of specialised processes and machines with which to achieve them.

The increasing miniaturisation and competition in these industries, allied to huge increases in demand have placed severe demands on process accuracy and throughput.

The overall picture is one where processes have to deliver the highest possible reliability and repeatability over long periods.

Many incorporate high accuracy and often high speed x-y stages or multi-axis motion systems and the analysis of the motion characteristics of these stages, and the systems into which they are built, is critical to their operational performance.

Many system developers have turned to laser interferometer based measurement systems to give them the information they need to improve the design of production process machinery and shorten product development cycles.

Increasingly, Renishaw's ML10 laser interferometer measurement system has been used by some of the largest OEMs in the industries mentioned above.

The unique properties of the ML10 laser interferometer mean that the system gives a position reading resolution of just 1.24nm combined with a measuring range of many metres.

This combination of resolution and measuring range makes the laser interferometer an ideal tool for characterising the performance of motion systems.

All ML10 system measurements are interferometric and based on the wavelength of a known and regulated laser light source.

Distance measurements based on the wavelength of light are recognised internationally as the primary measurement standard, and give the user assured traceability of measurement back to those at national standards laboratories.

It is therefore not surprising that the ML10 laser interferometer system has been used for many years for the calibration of machine tool positional accuracy and repeatability, in line with both national and international standards, with thousands of users worldwide.

Until today, users of Renishaw's laser measurement system have used software that is largely optimised for machine tool calibration or have developed their own custom analysis software.

Now, a unique new software package from Renishaw (QuickView) makes the ML10 laser interferometer measurement system an even more flexible and powerful analysis tool, suitable for machine developers and builders, but which will also appeal to engineering and research establishments as well as academic institutions.

QuickView is a simple-to-use and intuitive software package to capture, review and save dynamic data from the ML10 laser interferometer system.

Essentially, Renishaw has combined the high accuracy and resolution of its interferometer system with the ease of use of a conventional oscilloscope.

For years, electronic engineers have relied on oscilloscopes to study high-speed variations in voltage or current.

Now, Renishaw's new QuickView software provides mechanical engineers with a similar capability, allowing them to study minute variations in linear or angular displacements, velocities or accelerations.

Its key feature is the ability to display continuous "streaming" data from the ML10 laser interferometer, on a live oscilloscope type display allowing real-time analysis of motion and positional characteristics.

A simple, graphical interface provides for flexible operation, avoiding the need for predefined measurement targets and sequences - just point and measure.

QuickView software provides users with the following functionality: live data display in an oscilloscope style format; three modes of data capture: free running, single shot trigger and multi-shot trigger; data capture rate of 5kHz; easily exportable CSV data format allowing detailed off-line analysis; cursors for measurement of amplitude, time and frequency; linear, angular and straightness measurement options; distance, velocity and acceleration display modes; pan and zoom function allowing "close up" analysis of selected data; and selectable "capture" timebase up to 20s.

QuickView software, running on Window XP, reads the laser data 5000 times a second and displays the result as a position/time trace on the screen in real time.

Conventional time-base and gain controls are available to adjust the time-base (x-axis) from 10ms to 20s and the position axis from 100nm to 5m.

Additional on-screen buttons allow selection of AC or DC coupling and a range of lowpass filters with response times of 0, 1, 2, 5 and 10ms.

AC coupling is especially useful when measuring vibration since it allows any slow drift in position (due to thermal expansion for example) to be rejected.

DC coupling is useful when checking positional accuracy.

Unlike a conventional oscilloscope, the software has the ability to differentiate the incoming data to obtain velocity or acceleration/time traces.

Velocities are obtained by calculating the differences between adjacent laser position readings and accelerations by calculating the differences between adjacent laser velocity readings.

The lowpass filters are especially useful in removing noise from velocity and acceleration traces.

The display options and the inherent low noise of the ML10 laser interferometer measurement system allows users to see features on screen down to the 1nm (linear) or 0.01arc-sec (angular) resolution of the laser system.

Captured data can easily be exported in CSV format into supporting applications such as MathCAD, Mathmatica and Excel for further analysis or, alternatively, in Renishaw's own format for use with their Laser10 software (allowing FFT analysis).

Accelerometers and laser vibrometers have found many applications in engineering.

Compared with an accelerometer, Renishaw's interferometer based system provides far more accurate linear and angular accuracies, making it extremely useful for assessing dynamic positioning performance and repeatability.

For example, linear positional data is accurate to +/- 0.7ppm (eg +/- 0.7um over a 1m move).

The laser also provides the advantages of no trailing cables and true differential measurement.

Renishaw believes that the flexibility of the ML10 laser interferometer and QuickView software offers a very cost effective alternative to accelerometers and vibrometers, particularly for existing ML10 users, while still retaining the advantages of measurement range (up to 40m).

The software is designed for use with Renishaw's new DX10 interface which connects the laser and environmental compensator to the PC via an industry standard USB connection.

The DX10 avoids the need for PCMCIA (laptop) or internal PCI (desktop) interfaces, increasingly important as these are phased out of mainstream PCs in favour of USB connectivity.

With the addition of QuickView software, Renishaw has further enhanced its ML10 laser interferometer measurement system, providing a comprehensive suite of hardware and software to meet the measurement and calibration needs of the precision engineering world.

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