An analysis of the use of electrical power measurements for process control in large‐scale stirred vessels

The mechanical power that is transferred to a liquid via the shaft of an agitator is obtained from a measurement of the electrical power flowing into the electric motor within an accuracy of less than 5%. The accuracy obtained is limited by the calibration procedure. As the implementation of an electric power measurement is both simple and comparatively inexpensive, the technique is suitable for a continuous monitoring of the power used by agitators in the process industries.     

Drop-in replacement of R22 in heat pumps used for district heating — influence of equipment and property limitations

In Swedish district heating systems several large (25 MW) turbo-compressor driven heat pumps using R22 are installed. The only commercially available alternative is R134a, but its use could decrease the heating capacity by 35%. In this paper a method for finding the best working fluid for a specific heat pump plant is presented, and applied to a district heating plant. First, a screening is made among almost 2000 mixtures, using criteria such as condenser pressure, Mach number and temperature glide. Simulations of the plant are then made to investigate the change in heating capacity and COP when using a mixture instead of R134a. The results show that there are mixtures that can offer a substantially higher heating capacity than R134a, but there is a decrease in COP. The importance of considering the limiting parameters of the heat pump, such as maximum volume flow to each compressor stage and minimum evaporator pressure, is shown.     

Ergonomic procurement guidelines for visual display units as a tool for progressive change

The Swedish Telecommunications Administration (STA) has put to gether a set of requirement specifications for general purpose Visual Display Units (VDUs), and in particular for VDUs to be used in telephone directory services and similar computer supported customer services. For VDUs based on Cathode Ray Tube technology, a rationale is presented for ergonomic requirements such as dark characters on a light background, a minimum refresh rate of 70 Hz, limits for acceptable levels of electric and magnetic field strengths, and full tilt, swivel and height adjustability of the VDU. Standardized measurement methods for e.g. legibility and resolution are needed in further developing the ergonomic procure ment process.     

A discussion of model accuracy in system identification

Model quality and model accuracy are of prime interest in system identification. In this contribution we will review and discuss these concepts. In particular we will split model errors into contributions from a ‘random error’ and a ‘bias error’ and describe and discuss how to assess these two concepts.     

Necessary and sufficient conditions for stability of LMS

Guo and Ljung (1995) established some general results on exponential stability of random linear equations, which can be applied directly to the performance analysis of a wide class of adaptive algorithms, including the basic LMS ones, without requiring stationarity, independency, and boundedness assumptions of the system signals. The current paper attempts to give a complete characterization of the exponential stability of the LMS algorithms by providing a necessary and sufficient condition for such a stability in the case of possibly unbounded, nonstationary, and non-φ-mixing signals. The results of this paper can be applied to a very large class of signals, including those generated from, e.g., a Gaussian process via a time-varying linear filter. As an application, several novel and extended results on convergence and the tracking performance of LMS are derived under various assumptions. Neither stationarity nor Markov-chain assumptions are necessarily required in the paper     

Optimal Estimation of Rational Feedforward Control via Instrumental Variables: With Application to a Wafer Stage

Iterative control enables a significant control performance enhancement by learning feedforward command signals from previous tasks in a batch‐to‐batch fashion. The aim of this paper is to develop an approach to estimate the parameters of rational feedforward controllers that provide high performance and extrapolation capabilities towards varying tasks. An instrumental variable‐based algorithm is developed that leads to unbiased parameter estimates and optimal accuracy in terms of variance. Furthermore, a noncausal implementation of rational feedforward controllers is proposed, aiming to improve performance by means of pre‐actuation. Simulation and experimental results are presented to confirm that optimal accuracy is obtained with the proposed approach, and show the advantages of pre‐actuation in terms of performance.     

Mesh objective continuum damage models for ductile fracture

During machining processes, the work piece material is subjected to high deformation rates, increased temperature, large plastic deformations, damage evolution and fracture. In this context the Johnson‐Cook failure model is often used even though it exhibits pathological mesh size dependence. To remove the mesh size sensitivity, a set of mesh objective damage models is proposed based on a local continuum damage formulation combined with the concept of a scalar damage phase field. The first model represents a mesh objective augmentation of the well‐established element removal model, whereas the second one degrades the continuum stress in a smooth fashion. Plane strain plate and hat specimens are used in the finite element simulations, with the restriction to the temperature and rate independent cases. To investigate the influence of mesh distortion, a structured and an unstructured meshes were used for the respective specimen. For structured meshes, the results clearly show that the pathological mesh size sensitivity is removed for both models. When considering unstructured meshes, the mesh size sensitivity is more complex as revealed by the considered hat‐specimen shear test. Nevertheless, the present work indicates that the proposed models can predict realistic ductile failure behaviors in a mesh objective fashion. Copyright © 2015 John Wiley & Sons, Ltd.     

Prediction of solubility for HFC working fluids in model substances for compressor oils

This study is a first step towards the prediction of the behaviour of working fluids and compressor oils. Different thermodynamic models are investigated and a modified Flory-Huggins-based predictive model, termed MFH, is proposed for solubility of HFCs in long-chained hydrocarbons. From experimental data, 7 general parameters have been fitted for the MFH model to describe 14 binary systems. The UNIQUAC model demands 28 specific parameters without the possibility of extending their validity to other mixtures. A predictive UNIFAC model requires 8 general parameters, but its uncertainty, 17%. is poor compared to the MFH model's 7%.     

Swelling and mechanical properties of cellulose hydrogels. II. The relation between the degree of swelling and the creep compliance

The mechanical properties of swollen cellulose hydrogels have been studied. The degree of swelling of the gels was varied between 0.75 and 6.3 g water/g dry gel (g/g) by partial drying followed by reswelling in water. Creep rate was measured in uniaxial compression in the time interval 15–900 s for gels in equilibrium with water. Isochronous relations between stress and reversible strain were found to be linear, and creep compliance was calculated from the slopes. Both the creep compliance and the creep rate increase with an increased degree of swelling. General observations, such as the high strain limit of linearity in the stress–strain curves and the magnitude of the creep compliance, indicate similarities between swollen cellulose gels and rubbery networks. It is therefore assumed that the statistical theories for swollen networks can describe the amorphous matrix of the gels. In order to obtain creep compliance values representative of the amorphous matrix, the experimental values were corrected for the presence of crystalline regions. It is also suggested that non-load-bearing microvoids are present at high swelling levels. According to calculations based on the theory, the network chains of the amorphous regions in a gel swollen to 2.4 g/g contain about 11 monomer units and the Flory-Huggins interaction parameter χ equals 0.2.     

Colloidal Processing of a Very Fine BaTiO3 Powder—Effect of Particle Interactions on the Suspension Properties, Consolidation, and Sintering Behavior

The relation between the suspension state and the rheological properties, the consolidation, and packing of a very fine (nanosized) BaTiO₃ powder has been investigated. The BaTiO₃ powder was suspended in a nonaqueous medium by adsorbing fatty acids and a polymeric dispersant, poly(12-hydroxy stearic acid), (PHS), at the BaTiO₃/decane interface. Calculated interparticle energies imply that the suspension with PHS adsorbed is colloidally stable, while the suspensions with oleic and octanoic acid can be characterized as weakly and strongly flocculated, respectively. Analysis of settling experiments and rheological measurements at high concentrations confirmed these characteristics. Pressure filtration resulted in nearly identical green body densities in spite of the differences in colloidal properties, but the preliminary sintering experiments and microstructural characterization showed that the strongly flocculated suspension displays a significantly retarded sinterability compared to the colloidally stable and the weakly flocculated suspensions. The absence of a correlation between green density and sintering behavior was explained by considering both the volume taken by the adsorbed fatty acids and the PHS polymer—which can be substantial for nanosized powders—and the state of the suspension. While a decrease in the thickness of adsorbed surfactant or polymer layer will enable a higher particle packing density, such a thin adsorbed layer results in a more strongly flocculated suspension which will resist dense packing. Hence, it is suggested that the green bodies of the colloidally stable and the weakly flocculated suspensions correspond to a relatively homogeneous, but loosely packed, green body microstructure. The strongly flocculated suspension results in a green body with a more inhomogeneous microstructure.