The wavelength of the 57Fe Mössbauer radiation is measured with a relative uncertainty of 0.19 ppm by using almost exact Bragg backscattering from a reference silicon crystal. Its value is determined as lambda(M) = 0.860 254 74(16)x10(-10) m. The corresponding Mössbauer photon energy is E(M) = 14 412.497(3) eV. The wavelength of the 57Fe Mössbauer radiation is easily reproducible with an accuracy of at least 10(-11)lambda(M) and could be used as a length standard of atomic dimensions.
Self-optimizing control is a strategy for selecting controlled variables. It is distinguished by the fact that an economic objective function is adopted as a selection criterion. The aim is to systematically select the controlled variables such that by controlling them at constant setpoints, the impact of uncertain and varying disturbances on the economic optimality is minimized. If such a selection leads to an acceptable economic loss compared to perfectly optimal operation then the chosen control structure is referred to as "self-optimizing". In this comprehensive survey on methods for finding self-optimizing controlled variables, we summarize the progress made during the last one and a half decades. In particular, we present brute-force methods, local methods based on linearization, data and regression based methods, and methods for finding nonlinear controlled variables for polynomial systems. We also discuss important related topics such as handling active constraints. Finally, we point out open problems and directions for future research.
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