Laser interferometers for length measurements
From nanometer to 80 meters
Increasing demands on the accuracy 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 measuring 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 size of the sensor head is determined only by optical elements, and it is very compact.
The basis fot the wide range possibilities for length measurement with an interferometer lies in the relationship between the measuring resolution in the sub-nanometre range and large measuring lengths of a few metres. This relationship is possible because the measuring systems have several metrologically relevant options within the device concept.
Length measurements over longer distances are primarily characterized by their requirements for exact measurement of the environmental parameters in the air, the wavelength stability and the temperature of the material. Therefore, the SP-NG interferometers are equipped with a highly accurate environmental compensation, which is crucial to compensate for the measurement deviation.
For short measuring distances, frequently neglected and non-measuring-equipment-related alignment errors can significantly influence measurement uncertainty. If the measurement direction and movement direction of an object in a short guide do not match, these errors can strongly affect 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 movement direction is visualized. The geometric alignment of the system can be done without errors.
The following table summarises different requirements for length measuring systems and options of an SP-NG interferometer required for accurate length measurement. The specified measurement tasks are only examples, but they are derived from empirical values.
Measurement task
Measuring system requirements:
High accuracy of the measuring standard
Solutions with the SP-NG interferometer:
Long-term, frequency-stabilized 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 minimized.
Sturdy housing
Measuring system requirements:
High accuracy of the measuring standard
System alignment
Solutions with the SP-NG interferometer:
Long-term stable, frequency-stabilized lasers
Highly accurate detection of air tmperature, material temperature and air pressure
Integrated alignment optics
Minimization of Abbe errors through a beam-defined measurement site
Measuring system requirements:
High system stability
High resolution and linearity
Aligned measurement and movement axes
Abbe-error-free measurement
Solutions with the SP-NG interferometer:
Integrated alignment optics
Low-noise signal evaluation with sub-nanometre resolution
Stable build and mounting of the sensor head
Optional material selection for the sensor head
Based on the Michelson interferometer principle with an interferometer dead-path defined by this
With the SP-NG interferometer the measurement and calibration process can also be synchronized with the positioning controller 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 the user to control the recording of measured values. 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 measurement of long distances
A lense option and a special design of the SP-NG sensor head allow it to make measurements up to 80 m.
Various different optical reflectors can be used with this long-range interferometer. A special reflector, in which no glass medium is enclosed 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 errors in the measured values due to the measuring beam transitioning between the air and the reflecting medium. The reflectors angle of inclination around its centre can be up to ±22.5°. The use of such reflectors with highly tilt-invariant reflectors 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 important for longer distances. Therefore, the systems can optionally be equipped with several wired or 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 devices and machine tools as well as on 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 optional long-term 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.
During installation the measuring systems in this version are aligned in an array and the calibration remains temperature and stress-resistant for long periods of time. In the OEM segment a version made of materials adapted to a specific measurement set-up is possible. This is 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 stand out with their high precision and resolution as well as high immunity to interference. The fibre-optic coupler that transmits the laser light avoids heat sources being present 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 devices and machine tools. These measuring systems are extremely versatile and can be easily adapted to a wide variety of measuring tasks.
Our webinar tip:
High precision length measurement
In this webinar, our experts talk about the theory and practice of interferometric length measurement.
Suitable products and solutions
Long-Range laser interferometer
for highly accurate length measurements over long distances (long range measurements)