OPTIMALITY CHARACTERISTICS OF PI/PID CONTROLLERS: A COMBINED MIN-MAX/ISE INTERPRETATION

This communication addresses the tuning of PI and PID controllers on the basis of the IMC approach. The tuning is based upon a first order plus time delay (FOPTD) model and aims to achieve a step response specification. Through analysis it has been found that by using the IMC approach we get a PI or a PID depending on the rational approximation used for the time delay term. This article raises the question that the use of a PID instead of a PI controller should be based on another reason more related to the control objectives rather than the use of a better approximation for the time delay. An alternative tuning is presented here, from within the IMC formulation, based on a min-max optimization. From the tuning rule provided by this approach the optimum settings from an integral squared error criterion point of view are derived. The optimal controller results in being a PI controller. From this optimal controller as the starting point, the introduction of the derivative action can be seen as a detuning procedure that can increase the robustness of the controller. This approach provides further insight into the tuning of PI and PID controllers giving the (alternative) parameters a precise engineering meaning.     

Speciation of the gaseous phase of the HI section of the iodine sulphur thermochemical cycle by modeling and inversion of FTIR spectra

This paper presents a laboratory experiment for the study of the liquid–vapor equilibrium of HI–I₂–H₂O ternary mixtures that are involved in the iodine sulphur thermochemical cycle for the production of hydrogen. The apparatus is based on a heated tantalum pressurized vessel equipped with temperature and pressure transducers and carrying windows for optical soundings. It enables recordings of the infrared transmission of the vapor with a Fourier Transform Spectrometer (FTS). The spectra show the signatures of HI and H₂O through rovibrational absorption transitions in both the 4200–5800 cm⁻¹ (1.7–2.4 μm) and 6200–7800 cm⁻¹ (1.3–1.6 μm) regions. In order to analyze the FTS recordings, a spectra calculation software and a dedicated database of spectroscopic parameters have been built. Using these tools, the HI and H₂O amounts in the vapor are determined from least square fits of the experimental spectra in both regions. It is first shown that the measured and the calculated transmissions are in good agreement. Furthermore, the species amounts determined independently in the two spectral regions are very consistent, confirming the quality of the optical soundings.     

Optimal metric for condition rating of existing buildings: is five the right number?

In the context of the built environment in the recent years, the concept of maintenance has changed from corrective to preventive maintenance. There is evidence that preventive maintenance is much more efficient than corrective maintenance, since severe deteriorations that may represent danger to people are avoided, and also money is saved. To make periodic inspections of the buildings is useful to quantify, the extent to which deteriorations are severe or not, in order to facilitate decision making and prioritise interventions. To this purpose, many scales have been used and are used to assess the severity of damage and degradation of the building components. But it appears evident that there is not consensus among users and these scales are different between them, with different number of degrees and metrics for the measurement of the condition state. The main goal of this paper is to calculate which is the optimal metric (which is the optimal number of degrees) of a severity scale of damages in buildings, so the corresponding scale could be of widespread and of common use among professionals, avoiding the problems of comparison between different evaluators. The proposed methodology to calculate the optimal metric of a scale can be also extended to other scopes.     

Data-driven and tool-supported elicitation of quality requirements in agile companies

Software Quality Journal - Quality requirements (QRs) are a key artifact needed to ensure the quality and success of a software system. Despite their importance, QRs rarely get the same degree of...     

A robust hierarchical production planning for a capacitated two-stage production system

In this paper, we propose a robust hierarchical production planning approach for a two-stage real world capacitated production system operating in an uncertain environment. The first stage of the system produces a set of semi-finished products having relatively stable annual demands, and the second finishing stage produces finished products having highly variable weekly demands. The fixed production setup costs incurred at the first stage are considerably high. Fixed production setup costs incurred at the second stage are fairly small compared to those of the first stage. We propose an integrated hierarchical planning model, where semi-finished products from the first stage (i.e. the aggregate level) are disaggregated into finished products to be produced in the second stage (i.e. the operational level). As a result of the relatively stable demands and the high setup costs experienced at the first stage, a cyclical aggregate planning model is proposed for production planning at the upper level of the hierarchical plan. Based on this aggregate plan, a modified periodic review policy is then proposed for production planning at the lower level. Finally, a coupling plan, linking the two planning levels, is proposed to ensure the feasibility of the disaggregation process at every period.     

Contributions of empirical and quantile processes to the asymptotic theory of goodness-of-fit tests

This paper analyzes the evolution of the asymptotic theory of goodness-of-fit tests. We emphasize the parallel development of this theory and the theory of empirical and quantile processes. Our study includes the analysis of the main tests of fit based on the empirical distribution function, that is, tests of the Cramér-von Mises or Kolmogorov-Smirnov type. We pay special attention to the problem of testing fit to a location scale family. We provide a new approach, based on the Wasserstein distance, to correlation and regression tests, outlining some of their properties and explaining their limitations. Dedicated to Miguel Martín Díaz whose scientific criticism has greatly inspirated our research by years. Research partially supported by DGICYT, grants PB98-0369-C02-01 and 02. E. del barrio and C. Matrán have also been supproted by PAPIJCL grant VA08/97.     

LCA of poplar bioenergy system compared with Brassica carinata energy crop and natural gas in regional scenario

The poplar bioenergy system has been analysed applying life cycle assessment (LCA) to compare its environmental performance to: Ethiopian mustard bioenergy system and natural gas. The life cycle impact assessment (LCIA) shows that the use of fertilizers is the highest impact in four of the 10 environmental categories, representing between 39% and 67% of the impact in them. The diesel used in transport vehicles and agricultural tractors also has a significant impact in another five of the 10 analysed categories 40–85%. The poplar bioenergy system contributes to global warming with 1.90–1.98 g CO₂ eq MJ^?1 biomass produced. The production and transport as far as the thermoelectric plant of the poplar biomass consumes 0.02 MJ of primary energy per 1 MJ of biomass stored. In comparison with Ethiopian mustard and natural gas, it reduces primary energy consumption by 83% and 89% and the greenhouse gas emission by 84% and 89%, respectively. The results of the analysis support that the poplar bioenergy system is viable from an energy balance and environmental perspective for producing energy in southern Europe, as long as it is cultivated in areas where water is available. This latter point and the better environmental performance of both crops in comparison to natural gas allows us to affirm that the combination of several crops adapted to the local agro-climatic conditions of the territory will be the most suitable strategy in Mediterranean areas that wish to reach the global energy production targets in terms of biomass established by the European Union (EU).     

Feasibility assessment of Brassica carinata bioenergy systems in Southern Europe

A detailed reliability assessment was made of electricity generation systems in Spain that are based on Brassica carinata cultivation. The assessment considers the following chain of energy generation: biomass cultivation and harvesting, transportation and electricity generation in biomass power plants (10, 25 and 50 MW). Flue gas desulphurisation systems have been included for larger plants following the criteria of the Spanish legislative framework. Six scenarios were analysed in accordance with the following aspects: two crop distributions around the power plant and three power plant sizes. The results show that the cost of biomass delivered at the power plants ranges from 107.81 to 112.54 € Mg⁻¹ dry basis.Sensibility analysis shows that variation in biomass production in the field demonstrates that biomass cost delivered at the plant is notably affected and consequently so is the system's feasibility.Furthermore, the increase of the price of CO₂ emission credits, also considered in sensibility analysis, can help to improve the reliability of systems because of the increase of gross profit for each scenario.This study clears up the Economic uncertainty of B. carinata biomass energy systems based on the single use of this renewable energy resource. Higher crop productivities are needed to ensure an economic reliability of the analysed systems. On the other hand biomass mix can solve SO₂ emission cleaning cost for large power plants, improving the reliability of B. carinata application as fuel.