Dynamic flow phenomena in a foam application unit

If, during the operation of a foam application unit, a change in the foam production is introduced, the change in the flow becomes noticeable at the point of application onlyafter a certain response delay time. During the operation of the unit it is necessary to take this response delay into consideration. By varying the flow of foam in the application unit, the mass content of foam in the unit is changed as a result of the changed pressure inside the unit. The response delay has been mathematically derived for the flow of agiven type of foam through pipes, as well as through a foam printing unit as a whole. A deviation in the yield of foam on the substrate can be avoided by considering the respons delay caused by a change in the foam production process.     

Sliding mode closed-loop control of FES: controlling the shank movement

Functional electrical stimulation (FES) enables restoration of movement in individuals with spinal cord injury. FES-based devices use electric current pulses to stimulate and excite the intact peripheral nerves. They produce muscle contractions, generate joint torques, and thus, joint movements. Since the underlying neuromuscular-skeletal system is highly nonlinear and time-varying, feedback control is necessary for accurate control of the generated movement. However, classical feedback/closed-loop control algorithms have so far failed to provide satisfactory performance and were not able to guarantee stability of the closed-loop system. Because of this, only open-loop controlled FES devices are in clinical use in spite of their limitations. The purpose of the reported research was to design a novel closed-loop FES controller that achieves good tracking performance and guarantees closed-loop stability. Such a controller was designed based on a mathematical neuromuscular-skeletal model and is founded on a sliding mode control theory. The controller was used to control shank movement and was tested in computer simulations as well as in actual experiments on healthy and spinal cord injured subjects. It demonstrated good robustness, stability, and tracking performance properties.     

Foam application from a closed system – a study of machine and foam parameters

An attempt has been made to gain a greater insight into the interaction between foam and a moving textile substrate. The effects of changing wet pick–up, fabric velocity, liquid viscosity, foam density and mode of application on penetration have been studied. Application from a closed system makes it possible to apply an exact amount of liquid, and reflectance measurements revealed that between 10 and 60% wet pick–up the actual level of wet pick–up is of major importance. The amount of liquid available determines the likelihood of capillary transport. The results obtained in terms of fabric speed and foam density still leave questions to be answered on the mechanisms of foam flow and degradation in the textile.     

The flow properties of foam

The rheological behaviour of foam, prepared from an aqueous solution with lauryl sulphate as surfactant, has been measured with a viscometer as a function of the blow ratio, bubble diameter distribution and concentration of the thickener used. The influence of wall slip and of hysteresis effects on the rheological behaviour have been determined. Calculations and measurements pertaining to the pressure drop in pipes have been performed, in which the foam was considered as a compressible pseudoplastic whose rheology depended on blow ratio. The distribution of the foam passing through a closed distribution unit has been studied. This distribution unit consisted of a pipe and a cross-head distribution slit. Complementary measurements of the flow of foam through narrow slits have been made. The design of a closed foam distribution unit is discussed. The consequences of the choice of several design parameters are examined.     

Logic gate characterization through ringoscillators

Ringoscillators are in frequent use to characterize the dynamic operation of logic gates. Customary the per-gate behaviour is reduced by wiring to a not-function. Here a design method and measurement technique is discussed, that eliminates this restriction. Hence a more comprehensive gate model can be derived from experimental measurements only.     

Bubble size distribution and energy dissipation in foam mixers

The bubble size distribution of a foam produced in a rotor-stator mixer has been determined as a function of several mixing parameters such as the rotor speed, residence time, gas/liquid ratio and the viscosity of the liquid used. A Newton-Reynolds expression for a foam mixer has been determined using energy consumption measurements. Two types of shear fields have been distinguished in the foam mixer, laminar and turbulent, the type of shear field depending on the properties of the foam mixer. After a certain mixing time the bubble size distribution characterised by the mean bubble size was found to reach a stationary value. The stationary bubble size distribution has been correlated with the mixing conditions by a critical Weber number. Both in the laminar and in the turbulent shear field bubble size distributions have been determined as a function of several mixing parameters.     

Stresses occurring during one-sided die compaction of powders

Die compaction of powders is related to the principles of powder mechanics. Measurements are reported of boundary normal stresses occuring during one-sided die compaction of a fine ferric oxide powder. One series of experiments is carried out with carefully cleaned die walls, another series is performed with lubricated dies. The powder yield locus and wall yield loci are determined respectively with the help of a triaxial cell and a powder/wall friction apparatus. For both series of experiments the powder yield locus lies above the wall yield locus. The experimental data give support to the assumption that the stresses at the die wall obey the wall yield locus. With respect to the internal stresses of the powder, evidence is obtained that the powder yield criterion is not fulfilled, at least not necessarily.