This paper describes the principle of precise length measurements using multi-wavelength interferometry. Three different examples are given in order to demonstrate PTB's ability in this field: 1) Measurements of absolute distances up to 10 m performed by combination of the variable wavelength interferometry with fixed wavelength interferometry using two wavelengths simultaneously, 2) a three-wavelength diode laser interferometer for precision measurements of surface profiles (< 100 µm) and 3) PTB's Precision Interferometer for length measurements of prismatic bodies, e.g. gauge blocks, using three wavelengths. The setup and principle of measurements and some results are presented and discussed in each case.

The generation of broadband microwave frequency comb from a femtosecond pulse train by direct photodetection opens the possibility for high-accuracy length measurements of long distances. We demonstrate a relatively simple realization of this measurement principle: an electronic distance measurement system based on a time-of-flight approach, driven by a femtosecond fibre laser source as a modulator. By the evaluation of the phase shifts of two distinct comb frequencies, a coarse and a fine measurement of the absolute distance can be performed. The range of the measurement system is demonstrated up to a length of 100 m. The experimental comparison of the femtosecond laser system with a conventional reference counting interferometer shows a precision better than ±10 μm at 100 m, corresponding to a relative measurement uncertainty of 1 × 10 −7 L. The limiting factors for the measurement uncertainty of the system are theoretically investigated and shown to be of the same order of magnitude.

A well-known method for compensation of the refractive index of air for length measurements based on the speed of light is a measurement with two different wavelengths and using the dispersion relation. No measurement of temperature, pressure and CO2 content of the air is necessary. However, this method is valid only for dry air and is practically not used. For moist air the partial pressure of water vapour has to be known as the only air parameter for the compensation of the refractive index. In practice it is not possible to measure the partial pressure of water vapour in air directly. Standard hygrometers indicate the relative humidity. To get the partial pressure the temperature has to be known. Experiments were performed with this modified compensation method where the air pressure and the humidity are measured conventionally. A homodyne interferometer was set up with a frequency doubled Nd:YVO4 laser with the wavelengths 1064 nm and 532 nm. The effective temperature in the measurement path is derived from the length results for both wavelengths. Our experiments indicate a length-dependent measurement uncertainty of below 1.2 × 10−7L for distances up to 30 m.

This paper analyzes control strategies for induction motors in railway applications. The paper will focus on drives operating with a low switching to fundamental frequency ratio and in the overmodulation region or six-step operation, as these are the most challenging cases. Modulation methods, efficient modes of operation of the drive and the implications for its dynamic performance, and machine design will also be discussed. Extensive simulation results, as well as experimental results, obtained from a railway traction drive, are provided.

The calibration and verification of high-precision electronic distance meters (EDMs) requires well-characterized and calibrated geodetic baselines. As the length measurements are performed typically over several hundred metres in air, a thorough understanding of the environmental conditions is necessary. In the course of a major refurbishment, the 600 m baseline of the Physikalisch-Technische Bundesanstalt at Braunschweig, Germany, was equipped with a dense environmental sensor network. This paper presents the characterization of this novel reference baseline, including the calibration of the inter-pillar distances, and identifies the major sources of uncertainty for such a length standard. A preliminary expanded standard uncertainty (k = 2) of is deduced for single-slope distance comparisons on the baseline. In the course of a full calibration, the additive constant cEDM of an EDM can currently be determined with an expanded uncertainty of U(cEDM)k = 2 = 6.1 × 10−5 m, and its scale correction sEDM with an expanded uncertainty of U(sEDM)k = 2 = 8.2 × 10−7. As an example, a femtosecond laser-based distance measurement over 600 m on this baseline is presented.