Laser interferometer of new generation

Laser interferometer for length measurements ranging from a nanometre up to a few metres

The compact SP-NG interferometer for a diagonal measurement in a machining centre
The compact SP-NG interferometer for a diagonal measurement in a machining centre

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

The long-range interferometer for measuring ranges up to 80 m
The long-range interferometer for measuring ranges up to 80 m

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

SP-NG laser interferometer systems as built-in version
SP-NG laser interferometer systems as built-in version

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.

Compact calibration interferometer up to 6 m

Calibration interferometers for measuring of straightness, angle and  position

Two calibration interferometers were developed for measuring lengths up to 6 m especially for narrow spaces. The high-precision straightness measurement makes them also suitable for the alignment of machine components.

Calibration interferometers are used, wherever highly accurate detection of rotational and translational guidance deviations of linear positioning axes, machine tools and coordinate measuring machines is required. In addition to the high measurement resolution and linearity achievable, the traceability to the metre specification is a decisive advantage of the interferometric measurement method. This is ensured by the frequency connection of the laser source used as a benchmark.

Calibration interferometer SP 5000 C5 2D, compared to the proven SP 15000 C5 system

The high-precision requirements of a system can only be achieved if the existing deviations are known. These can be documented, for example, in the form of acceptance reports. Nevertheless, what is much more important is the fact that the accuracy can be increased by a compensation of the existing deviations – be it linear axis, coordinate measuring machine or machine tool.

As with all interferometers of SIOS Messtechnik GmbH, the basic concept of the calibration interferometers presented here is the use of a fibre-coupled sensor that avoids any heat generation at the measurement site. The measuring reflectors are completely passive, which means they are electrically and magnetically neutral. Another important advantage is the simultaneous acquisition of several measured variables, a principle that SIOS has been following for more than 10 years, since multicomponent measurements allow a mutual assignment (also random proportions) of guidance deviations and also drastically reduce the required measuring times.

The SP 15000 C5 laser Interferometer has already proven itself on the market as a system that records the linear position up to a distance of 50 m, the pitch and yaw angle as well as a straightness component by laser interferometry in a single measurement process. Special advantages of the system are the long range in the high angular resolution as well as the large measuring ranges for the angle and straightness measurement. This and the integrated alignment aids make system alignment child’s play.

However, in a number of applications, the space available in the machine is limited, or the permissible mass of the measuring reflector is limited due to the system design or measurement task (dynamic analysis). In addition, the attachment of the components of the measuring system can be difficult especially in machine tools.

Since the required measuring range is less than 6 m in many cases, two new calibration interferometers with a compact sensor head and a compact measuring reflector for measuring lengths up to 6 m have been developed on the basis of the tried-and-tested SP 15000 C5.

The position resolution and the resolution of the interferometric straightness measurement remain the same as with the SP 15000 C5, the angular resolution is very high at 0.001 arc seconds. Since the systems are to be used even in narrow spaces, focus was on easy handling of the compact system components. Thus, appropriate alignment aids are also included here.

Calibration interferometer with interferometric straightness measurement

Compact sensor head and measuring reflector of the calibration interferometer SP 5000 C5

With the new SP 5000 C5 calibration interferometers, all measured variables are recorded by interferometric method. The set-up and functioning are analogous to the large SP 15000 C5. The significantly more compact sensor head detects the position, the pitch and yaw angles as well as the horizontal and vertical straightness.

With dimensions of 50 mm x 50 mm x 50 mm and a weight of 240 g, the measuring reflector used is small and light.

The straightness components are measured in two passes without realignment of the system. The measurement resolution of the interferometric length measurement is 5 pm over a measuring range of up to 6 m. The system is characterised by very high angular and straightness measuring ranges and accuracies.

Calibration interferometer with 2D straightness measurement

The SP 5000 C5 2D calibration interferometers (Figure 2) are designed to further simplify handling and reduce measuring times. The straightness mirror is eliminated, so that the calibration is made easier and functionality is significantly improved. The compact, fully passive measuring reflector allows simultaneous measurement of position, pitch and yaw angle as well as straightness components in a single pass. Due to the absolute, micrometre-accurate straightness measurement, the SP 5000 C5 2D is also ideal for the alignment of machine components.

High-performance software solutions for standardised machine acceptance, volumetric correction of machine tools for all common machine control systems, dynamic component analysis and measuring system-supported alignment of machine components exist for all measuring systems.

Use of C5 and C5 2D series multicomponent interferometers significantly reduces the time required for acceptance inspections, calibrations, system analyses and correction range adjustments. This allows cost reduction, while increasing accuracy.