Laser interferometer for length measurements ranging from a nanometre up to a few metres
Increasing demands on the accuracies of coordinate measuring machines, machine tools and positioning devices with movement ranges over several metres pose a challenge to measurement systems when it comes to their acceptance and calibration. In this context, new generation laser interferometers offer unique properties that combine a large length measuring range with an extraordinarily high resolution. By using the wavelength of He-Ne lasers as a long-term stable measuring standard, the measuring systems can be traced back to national standards and are, therefore, ideally suited for calibrations and tasks in metrology. The basis for the interferometers is SIOS Messtechnik GmbH‘s single-beam concept, which combines excellent linearity over the entire measuring range with simple calibration.
The universal interferometer of the SP – NG series
The basic system of the SP-NG series is based on the proven concept of the compact single-beam laser interferometer from SIOS Messtechnik GmbH. These interferometers are distinguished by the fact that only one measuring beam, which is reflected by the measuring reflector back into itself, is used for the interferometric length measurement. This results in a defined sensing point on the measurement object. Therefore, it is possible to design the metrological arrangement so that the laser beam is exactly aligned with the measuring axis. This minimizes the Abbé error that is a typical source of error in all length measurements. With a small reflector that can tilt up to ±12.5°, measurement set-ups can be quickly and easily calibrated. The measuring principle of the interferometer also allows the use of a simple plane mirror as a reflector if there is considerable transverse displacement of the measurement object along the beam direction in the set-up. With the environmentally corrected light wavelength of a stabilized He-Ne laser as a highly stable natural measuring standard, these sensors have nanometre accuracy and excellent linearity. The light source is located outside the sensor in the evaluation electronics and the light is generally supplied via fibre-optic cables. As a result, the very compact size of the sensor head is determined only by optical elements.
The excellent options for length measurement with an interferometer are provided by the correlation between the measuring resolution in the sub-nanometre range and large measuring lengths of a few metres. This correlation is possible because the device concept of the measuring systems includes several metrologically relevant options.
Length measurements over longer distances are primarily characterised by the requirements for exact measurement of the environmental parameters in the air, the wavelength stability and the material temperature. Therefore, the SP-NG interferometers are equipped with a highly accurate environmental compensation, which is crucial for the measurement deviation.
For short measuring distances, frequently neglected and non-measuring-equipment-related alignment errors can significantly influence the measurement uncertainty. If the measuring direction does not match the direction of movement of an object in a short guide, these errors can dominate the measurement result. The standard version of the SP-NG interferometer has built-in alignment optics, which are indispensable in these applications. The reflected measuring beam is evaluated and a deviation between the measuring direction and the object movement is visualised. The geometric alignment of the system can be done without errors.
The following table summarises requirements for length measuring systems and options of an SP-NG interferometer, required for accurate length measurement. The specified measurement tasks are only shown as examples and are derived from empirical values.
|Measurement task||Measuring system requirements||Realization in the SP-NG interferometer|
|Length measurement and calibration over a few metres, > 0.5 m||– High accuracy of the measuring standard||– Long-term, frequency-stabilised lasers
– Highly accurate detection of air temperature, material temperature and air pressure
– Single-beam interferometer, measuring beam defines the measuring axis and Abbé errors can be minimised.
– Sturdy housing
|Length measurements and calibration in the range from 100 mm to 500 mm||– High accuracy of the measuring standard
– Alignment of the system
|– Long-term, frequency-stabilised lasers
– Highly accurate detection of air temperature, material temperature and air pressure
– Integrated alignment optics
– Abbé error minimisation through a beam-defined measurement site
|Length measurement and calibration up to 100 mm||– High system stability
– High resolution and linearity
– Measuring axis and movement axis are aligned
– Abbé-error-free measurement
|– Integrated alignment optics
– Low-noise signal evaluation with sub-nanometre resolution
– Stable mounting of the sensor head
– Material selection for the sensor is possible
– Michelson interferometer principle and a interferometer dead path defined by it
The measurement and calibration process with the SP-NG interferometer can also be synchronised with the positioning control of the system. Extensive options for electrically isolated triggering of the system such as:
- start/stop triggering
- triggering of individual measured values
- substitution of the sampling frequency
allow control of the measured value recording. As a result, measurements over longer distances can be carried out “on-the-fly” and thus, save time. The measured values are transmitted to a laptop or computer over a fast USB interface.
The concept for long distances
A lens option and a special design of the SP-NG sensor head allow measurements up to 80 m.
Various optical reflectors can be used for this long-range interferometer. A special reflector, in which no glass medium is trapped between the reflective surfaces is used for standard measurements over longer distances. This reflector consists of three highly accurate mirrors, which are arranged at an angle of 90° to each other. This prevents any additional falsification of the measured values due to a transition between the air and the reflecting medium for the measuring beam. The maximum angle of inclination of the reflector around the centre of the reflector can be up to ±22.5°. The use of such reflectors with high tilt invariance greatly simplifies the calibration and set-up of the interferometer. The permissible range of motion of the long-range reflector transversely to the measuring beam is up to ±1.5 mm. The measurement of environmental parameters is given high importance for longer distances. Therefore, the systems can optionally be equipped with several wired or also wireless temperature sensors to record the temperature distribution in large spaces.
Applications of long-range interferometers include laser interferometric measurements on guides, the calibration of high-precision axes on measuring machines and machine tools as well as coordinate measuring machines for double or multi-coordinate measurements.
The OEM concept
The concept of the SP-NG measuring system enables a sensor design with the option for long-term, stable calibration of the measuring beam. This solution is applied if the interferometers are used as an OEM component in a custom set-up. Figure 3 shows two aligned systems, whose beam adjustment is ensured by special optics.
The measuring systems in this version are aligned in an arrangement during installation and the calibration remains temperature and stress-resistant for long periods. In the OEM segment, a version made of the materials, adapted to a measurement set-up is possible to obtain a low-drift measurement set-up.
Maximum accuracy for large measuring ranges
The new generation laser interferometers are a valuable tool for all applications, where accurate length measurements are required. The systems are characterised by high precision and resolution as well as high immunity to interference. The fibre-optic coupler for transmitting the laser light prevents heat sources in the sensor head and allows its flexible arrangement in the room as well as quick and easy adjustment. A high tilt invariance and low sensitivity to a lateral offset of the measuring reflector make these interferometers a versatile tool for the commissioning, adjustment and calibration of precision guides, coordinate measuring machines and machine tools. These measuring systems are extremely versatile and can be easily adapted to a wide variety of measuring tasks.