Fixed Time Constant Research Articles

Analysis of the ionic and dielectric properties of perovskites by impedance spectroscopy

For understanding the operation of perovskite solar cells and light-emitting diodes, knowledge of the dielectric properties is indispensable. The dielectric properties of perovskites are frequency dependent due to the presence of moving ions, which complicates the interpretation of impedance spectra. Using Au/CsPbI2Br/Au capacitors with varied layer thickness as a model system, we demonstrate that in the dark, an extended Maxwell circuit consistently describes the impedance data. From the thickness dependence of the resistivities, both the electronic and ionic conductivities are obtained, whereas the combination of electronic and ionic capacitances with the characteristic frequencies for space-charge formation determines the ion diffusion coefficient and ion density. At low frequencies, a slow transient process with a fixed time constant of ∼0.1 s occurs, governed by the electronic conductivity, being independent of illumination strength and sample thickness. As a possible mechanism, we propose the spatial reorganization of ions within the ion accumulation layer at the electrode/perovskite interface.

GBAS Ionospheric Anomaly Monitoring Based on a Two-Step Approach.

As one significant component of space environmental weather, the ionosphere has to be monitored using Global Positioning System (GPS) receivers for the Ground-Based Augmentation System (GBAS). This is because an ionospheric anomaly can pose a potential threat for GBAS to support safety-critical services. The traditional code-carrier divergence (CCD) methods, which have been widely used to detect the variants of the ionospheric gradient for GBAS, adopt a linear time-invariant low-pass filter to suppress the effect of high frequency noise on the detection of the ionospheric anomaly. However, there is a counterbalance between response time and estimation accuracy due to the fixed time constants. In order to release the limitation, a two-step approach (TSA) is proposed by integrating the cascaded linear time-invariant low-pass filters with the adaptive Kalman filter to detect the ionospheric gradient anomaly. The performance of the proposed method is tested by using simulated and real-world data, respectively. The simulation results show that the TSA can detect ionospheric gradient anomalies quickly, even when the noise is severer. Compared to the traditional CCD methods, the experiments from real-world GPS data indicate that the average estimation accuracy of the ionospheric gradient improves by more than 31.3%, and the average response time to the ionospheric gradient at a rate of 0.018 m/s improves by more than 59.3%, which demonstrates the ability of TSA to detect a small ionospheric gradient more rapidly.

Potential of residential buildings as thermal energy storage in district heating systems – Results from a pilot test

Heat demand in a district heating system can have a significant variation within 1day, which produces problematic conditions for efficient heat generation. Short-term thermal energy storage can decrease this daily variation and make the conditions for generating heat more favorable. By periodically overheating and underheating buildings, causing small variations in indoor temperature, building thermal inertia can be utilized for thermal energy storage. This study presents the results from a pilot test where the potential to function as thermal energy storage was tested for five multifamily residential buildings in Gothenburg, Sweden. The signals from the outdoor temperature sensors were adjusted in different cycles during a total of 52weeks. The delivered heat and indoor temperature were measured during the test. The results indicate that heavy buildings, with a structural core of concrete, can tolerate relatively large variations in heat deliveries while still maintaining a good indoor climate. The study also demonstrated that a fixed time constant is not accurate enough to describe the variations in indoor temperature caused by the utilization of the buildings as short-term thermal energy storage. Degree hours is instead proposed as a simple yet adequate measurement for the thermal energy storage capacity in buildings. Storing 0.1kWh/m2floor area of heat will very rarely cause variations in indoor temperature larger than ±0.5°C in a heavy building.

The time constant of the somatogravic illusion

Without visual feedback, humans perceive tilt when experiencing a sustained linear acceleration. This tilt illusion is commonly referred to as the somatogravic illusion. Although the physiological basis of the illusion seems to be well understood, the dynamic behavior is still subject to discussion. In this study, the dynamic behavior of the illusion was measured experimentally for three motion profiles with different frequency content. Subjects were exposed to pure centripetal accelerations in the lateral direction and were asked to indicate their tilt percept by means of a joystick. Variable-radius centrifugation during constant angular rotation was used to generate these motion profiles. Two self-motion perception models were fitted to the experimental data and were used to obtain the time constant of the somatogravic illusion. Results showed that the time constant of the somatogravic illusion was on the order of two seconds, in contrast to the higher time constant found in fixed-radius centrifugation studies. Furthermore, the time constant was significantly affected by the frequency content of the motion profiles. Motion profiles with higher frequency content revealed shorter time constants which cannot be explained by self-motion perception models that assume a fixed time constant. Therefore, these models need to be improved with a mechanism that deals with this variable time constant. Apart from the fundamental importance, these results also have practical consequences for the simulation of sustained accelerations in motion simulators.

Experimental study of the coupled thermo-hydraulic–neutronic stability of a natural circulation HPLWR

Abstract The HPLWR (high performance light water reactor) is the European concept design for a SCWR (supercritical water reactor). This unique reactor design consists of a three pass core with intermediate mixing plena. As the supercritical water passes through the core, it experiences a significant density reduction. This large change in density could be used as the driving force for natural circulation of the coolant, adding an inherent safety feature to this concept design. The idea of natural circulation has been explored in the past for boiling water reactors (BWR). From those studies, it is known that the different feedback mechanisms can trigger flow instabilities. These can be purely thermo-hydraulic (driven by the friction – mass flow rate or gravity – mass flow rate feedback of the system), or they can be coupled thermo-hydraulic–neutronic (driven by the coupling between friction, mass flow rate and power production). The goal of this study is to explore the stability of a natural circulation HPLWR considering the thermo-hydraulic–neutronic feedback. This was done through a unique experimental facility, DeLight, which is a scaled model of the HPLWR using Freon R23 as a scaling fluid. An artificial neutronic feedback was incorporated into the system based on the average measured density. To model the heat transfer dynamics in the rods, a simple first order model was used with a fixed time constant of 6 s. The results include the measurements of the varying decay ratio (DR) and frequency over a wide range of operating conditions. A clear instability zone was found within the stability plane, which seems to be similar to that of a BWR. Experimental data on the stability of a supercritical loop is rare in open literature, and these data could serve as an important benchmark tool for existing codes and models.

Virtual Five-Axis Flank Milling of Jet Engine Impellers—Part II: Feed Rate Optimization of Five-Axis Flank Milling

This paper presents process optimization for the five-axis flank milling of jet engine impellers based on the mechanics model explained in Part I. The process is optimized by varying the feed automatically as the tool-workpiece engagements, i.e., the process, vary along the tool path. The feed is adjusted by limiting feed-dependent peak outputs to a set of user-defined constraints. The constraints are the tool shank bending stress, tool deflection, maximum chip load (to avoid edge chipping), and the torque limit of the machine. The linear and angular feeds of the tool are optimized by two different methods—a multiconstraint based virtual adaptive control of the process and a nonlinear root-finding algorithm. The five-axis milling process is simulated in a virtual environment, and the resulting process outputs are stored at each position along the tool path. The process is recursively fitted to a first-order process with a time-varying gain and a fixed time constant, and a simple proportional-integral controller is adaptively tuned to operate the machine at threshold levels by manipulating the feed rate. As an alternative to the virtual adaptive process control, the feed rate is optimized by a nonlinear root-finding algorithm. The virtual cutting process is modeled as a black box function of feed and the optimum feed is solved for iteratively, respecting tool stress, tool deflection, torque, and chip load constraints. Both methods are shown to produce almost identical optimized feed rate profiles for the roughing tool path discussed in Paper I. The new feed rate profiles are shown to considerably reduce the cycle time of the impeller while avoiding process faults that may damage the part or the machine.

A CMOS Readout Circuit for Silicon Drift Detectors With on-Clip JFET and Feedback Capacitor

We propose a CMOS circuit designed to be used with silicon drift detectors (SDDs) for high-resolution and high peak stability X-ray spectroscopy. The circuit is developed in the framework of a project for researches on exotic atoms (e.g., kaonic hydrogen) at e-/e+ colliders. The circuit is composed by a low-noise charge preamplifier, by a sixth-order semi-Gaussian shaping amplifier with four selectable peaking times from 0.7 mus up to 3 mus, and by a bipolar shaping amplifier to provide a timing signal required by the experiment. The preamplifier operates with the input JFET directly integrated on the detector itself. A low-frequency current-mode feedback loop allows to stabilize the operating point of the input JFET with respect to background and leakage current variations. The feedback capacitor is also integrated on the detector. Because its value is not known precisely a priori, the preamplifier is designed with the possibility to adjust externally its decay time to match the fixed time constant of the pole/zero network. A baseline holder senses the baseline voltage shifts at the output of the circuits due to the dc changes of the drain voltage of the input JFET in correspondence of background variations and provides a feedback loop back to the preamplifier to stabilize the output baseline. A first prototype has been realized in the 0.35-mum AMS technology. The energy resolution measured using the chip with a SDD of 5 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> is of 137 eV at 6 keV (ENC=8 e-rms)

Are rate constants constant?

Are rate constants constant?

Constant Voltage Anemometer Practice in Supersonic Flows

It is shown that the constant voltage anemometer (CVA) is well adapted for measurements in supersonic flows. Small hot-wire time constant, large bandwidth, high signal-to-noise ratio, variable hot-wire overheat, and independence of the cable capacitance effects of the hot wire were all observed with the CVA. Turbulence data were acquired using two methods. In the first method, signals were acquired with the CVA compensated with a fixed time constant of 0.10 ms at all test points. In situ hot-wire time constant and overheat were also acquired at each test point, which were then used in postprocessing of the turbulence signals. In the second method, full hardware compensation with the actual in situ time constant was set at each test point. The former method shows a larger bandwidth. The noise level was approximately the same in both methods. Theoretical estimates of the relative output levels of CVA, constant current anemometer, and constant temperature anemometer at different overheat ratios show that the CVA has larger output voltage level

Actual state of high-rate digital gamma-spectrometry

Two commercially available digital filters with selectable, fixed time constants and triangular pulse response are discussed to outline their potential advantages over traditional analog filters with semi-Gaussian pulse shape. A solution for the “resolution or throughput rate” dilemma is offered by the preloaded digital filter fulfilling the postulate for the ideal adaptive filter with optimum resolution at any counting rate. Throughput rates of >100 kc/s are demonstrated for the preloded digital filter at resolutions superior to those of fixed shaping time filters.

Analog VLSI architectures for motion processing: from fundamental limits to system applications

This paper discusses some of the fundamental issues in the design of highly parallel, dense, low-power motion sensors in analog VLSI. Since photoreceptor circuits are an integral part of all visual motion sensors, we discuss how the sizing of photosensitive areas can affect the performance of such systems. We review the classic gradient and correlation algorithms and give a survey of analog motion-sensing architectures inspired by them. We calculate how the measurable speed range scales with signal-to-noise ratio (SNR) for a classic Reichardt sensor with a fixed time constant. We show how this speed range may be improved using a nonlinear filter with an adaptive time constant, constructed out of a diode and a capacitor, and present data from a velocity sensor based on such a filter. Finally, we describe how arrays of such velocity sensors call be employed to compute the heading direction of a moving subject and to estimate the time-to-contact between the sensor and a moving object.

A novel eddy current compensation scheme for pulsed gradient systems

We describe a modified pre-emphasis network that uses a number of fixed time constants, only whose amplitudes are adjustable. In comparison with variable time constant pre-emphasis networks we have used, the fixed time constant network is more effective and easier to set on the fly. We have further developed a method to set the fixed time constant network mathematically using a linear least squares technique.

Variable time-constant differentiation in chromatography

The mathematical process of differentiation is useful for enhancing the resolution of chromatographic peaks and reducing interferences caused by a slowly changing detector response. Conventional differentiators use fixed time constants and this necessarily means that, when the chromatographic conditions are held constant, later eluting peaks are increasingly attenuated. By using a microcomputer to store chromatograms and calculate derivatives, the time constant of the derivative can be increased through the chromatogram so that the time constant is optimum for the whole of the derivative. Examples from gas and liquid chromatography are given to demonstrate the value of the technique for qualitative and quantitative analysis.

Improved Feedback Loop for Adaptive Arrays

This paper is concerned with the problem of time constants in adaptive arrays. The paper presents a improved form of an adaptive array feedback loop, which has the property that its time constants are fixed. This property is an advantage over the well-known least mean square (LMS) loop, for which time constants depend on received signal power. Fixed time constants are of interest because they simplify dynamic range problems for adaptive arrays in communication and radar systems.

Reply to R. H. Abbott

IN our article in Nature last year1 we showed that, after a step change of length imposed during tetanic contraction, the early tension recovery varies in speed according to the size and direction of the step, being slowest in large stretches and fastest in large releases. Abbott2 now proposes that this variation is due not to a change of time scale of a single process but to a change in the relative amplitudes of two exponentially decaying components each of which has a fixed time constant. He shows that the records in Fig. 2 of ref. 1 can be approximately fitted in this way, and concludes that “either the results are not typical or the mathematical theory put forward by Huxley and Simmons is not correct”.

Impulse functions for human rod vision.

1. This paper presents accurate increment threshold data for human rod vision for a small number of experimental parameters. The test is small and brief and the large background is either steady or transient.2. The linear threshold disturbance due to an impulse background consists of an input dependent exponential growth phase and an exponential recovery phase of more or less fixed time constant (ca. 0.08 sec).3. The data are treated by applying signal/noise decision theory to a hypothetical filter with two shot noise inputs, viz. the testing signal and the background. The gain and time course of the impulse function of the filter are slightly affected by the magnitude of the input.4. A linear approach is useful since the impulse functions for dark or light-adapted rod vision yield independent information about quantities which have previously only been used to describe the increment thresholds for small tests on steady backgrounds, viz. the integration time and dark light of the fully dark-adapted eye and the gain changes (or changes in the signal/background ratio) which occur on progressive light adaptation.