Time Constant Of System Research Articles

Design and analysis of thermo-structure for cryogenic radiometer

Absolute cryogenic radiometer is built based on a new theory of electrical-substitution measurement, which is for measuring the radiant power by using the equivalent electrical power and has recently served as a primary standard for radiant power measurements. This study aims to design and implement a cryogenic radiometers to measure the optical power in a range from <inline-formula><tex-math id="M1">\begin{document}$0.1\;{\text{μ}}{\rm{W}}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="6-20181880_M1.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="6-20181880_M1.png"/></alternatives></inline-formula> to 2 mW, which can substitute for the imported products. Intensive experiments are performed to study the thermal circuit of cryogenic radiometer, and systematically analyze the influences of cavity assembly and heat link materials on the responsivity and thermal time constant of cryogenic radiometer. On this basis, the thermo-structure mechanical parts are developed, which are comprised of a blackbody cavity, heat link and heat sink. Both the heat sink and the blackbody cavity are made of OFHC copper that is plated with gold. All surfaces are highly polished and reflective to reduce any radiative effects. The absorptance of the cavity can reach up to 0.999995 at 633 nm. And then, a characteristic parameters’ test system of cryogenic radiometer is built. Through optimizing the temperature control system and improving the design of the heat sink, the standard deviation of the heat sink can be kept under 0.2 mK for 30 min. By using that test system, the responsivity and thermal time constant of cryogenic radiometer with four different kinds of heat link materials (OHFC copper, 6061 Al, SS304 stainless steel, and polyimide) are tested experimentally. The experimental results show that the responsivity and thermal time constant are 35.5 K/W and 23 s for OHFC copper, 318.9 K/W and 106 s for 6061 Al, 434.8 K/W and 297 s for SS304 stainless steel, 714.8 K/W and 506 s for polyimide. As the thermal conductivity of heat link material changes, the two parameters of responsivity and thermal time constant will simultaneously change significantly. The responsivity and thermal time constant are a pair of mutually constrained parameters, and temperature stability is an important parameter for designing the thermo-structure. After increasing the responsivity, it will not only significantly increase the measurement time and resource consumption, but also affect the temperature control stability, and hence limiting the measurement accuracy. All the test data indicate that the characteristic parameter of cryogenic radiometer can be adjusted by changing the material and structure of heat link. The obtained results will have a certain reference value for the index distribution of cryogenic radiometer characteristic parameters and designing the next generation of absolute cryogenic radiometers.

Experimental Investigation and Mathematical Modelling of Pressure Response for Steam Generator

Cold startup of boiler is the process of boiler operation with water at ambient temperature and pressure with all intake and discharge valves are fully closed to permit fast development of pressure. A mathematical model is developed to estimate the pressure response during cold startup of a perfectly insulated steam generator unit. A commercial type pressure switch is used in this unit to control and maintain the desired set point of the steam operating pressure. This mathematical model assume that the thermal properties of the supplied liquid water are temperature dependent. It is based on a novel Pressure Marching Technique that is coded using a FORTRAN language computer program. The maximum percentage error of (8.24 %) was obtained when comparing the predicted results of the mathematical model with the measured values obtained from the experimental test that was done using a (2 kW) electric steam generator unit with a volume of (30 litter) and maximum operating pressure of (8 bar). In addition, the same behavior of the predicted results was obtained when compared with results of a previously published article. It was found that the time constant of the pressure control system is directly proportional with its operating pressure set point and with the volume of the steam generator and its void fraction. A (50%) increase in the pressure set point will increase the time constant by (66.16%). Increasing the boiler volume by (166.667%) will increase the time constant by (166.677%) and increasing the boiler void fraction by (150%) will increase the time constant by (23.634%). The time constant is inversely proportional with the heating power of the steam generator. A (100%) increase in the heating power will decrease the time constant by (50%). The time constant is independent of the initial water temperature. Also, it was found that the time delay to start water evaporation is directly proportional with the volume of the steam generator. A (166.667%) increase in boiler volume will increase the time delay by (166.65%). The time delay is inversely proportional with the initial water temperature and with the heating power and void fraction of the steam generator. A (38.889%) increase in the initial water temperature will decrease the time delay by (8.882%). Increasing the heating power by (100%) will decrease the time delay by (50%) and increasing the boiler void fraction by (150%) will decrease the time delay by (16.665%). The time delay is independent on the operating pressure set point.

Experimental Investigation and Mathematical Modelling of Pressure Transfer Function for Air Compressor

A mathematical model is developed to estimate the pressure response of an insulated electric air compressor. A pressure switch is modeled as a comparator and the electric motor as an amplifier. It is assumed that the compressed air is a perfect gas when applying the isentropic process. In addition, the effect of a step, ramp and sinusoidal functions of disturbance signals on the air pressure has been studied. A good agreement was obtained when comparing the predicted results with the measured values obtained from the experimental test that was done using a (1.32 kW, 23 litter and 8 bar) electric reciprocating air compressor. In addition, the same behavior of the predicted results was obtained when compared with results of a previously published article. It was found that the time constant of this control system is directly proportional with the value of the spring constant that is inserted inside the pressure switch and with the volume of air storage vessel, and it is inversely proportional with the gain of the amplifier and with the effective cross-sectional area of the pressure switch diaphragm and it is independent of the value of operating pressure set point. In addition, when the value of disturbance signal is positive, it will increase the output pressure response and when it is negative, it will decrease it.

Real‐time parameter estimation based intelligent controllers for AGC operation under varying power system dynamic conditions

In this paper, real-time parameter estimation based intelligent controller has been proposed for the optimum automatic generation control (AGC) operation of multi-area interconnected power system. In earlier AGC studies, supplementary controllers were designed for presumed power system dynamic conditions, such as constant system loadings, fixed values of power system dynamic model parameters. However in practical AGC system, these power system model parameters, namely frequency bias parameter B, steam chest and reheater time constants & , power system time and gain constants & , load sensitivity factor D, etc., continuously varies depending upon the consumer's load demand and number of power generating units participating in AGC. The controller's operation no longer remains optimum as these system parameters changes from their initial values. In view of the above, parameter estimation based real-time intelligent controller has been designed which automatically adjusts its controller gain settings to its optimum conditions in real-time after sensing changes in power system model parameters. At first, different methodologies are presented to estimate dominant power system model parameters. Then, presented methodologies are utilised in the design of intelligent controller for AGC system. The proposed controller is successfully tested upon IEEE 39 bus system with various case studies.

Development of FOPDT and SOPDT model from arbitrary process identification data using the properties of orthonormal basis function

Most of industrial process can be approximately represented as first-order plus delay time (FOPDT) model or second-order plus delay time (FOPDT) model. From a control point of view, it is important to estimate the FOPDT or SOPDT model parameters from arbitrary process input as groomed test like step test is not always feasible. Orthonormal basis function (OBF) are class of model structure having many advantages, and its parameters can be estimated from arbitrary input data. The OBF model filters are functions of poles and hence accuracy of the model depends on the accuracy of the poles. In this paper, a simple and standard particle swarm optimisation technique is first employed to estimate the dominant discrete poles from arbitrary input and corresponding process output. Time constant of first order system or period of oscillation and damping ratio of second order system is calculated from the dominant poles. From the step response of the developed OBF model, time delay and steady state gain are estimated. The parameter accuracy is improved by employing an iterative scheme. Numerical examples are provided to show the accuracy of the proposed method.

Homogeneous Time Constants Promote Oscillations in Negative Feedback Loops.

Biological oscillators are present in nearly all self-regulating systems, from individual cells to entire organisms. In any oscillator structure, a negative feedback loop is necessary, but not sufficient to guarantee the emergence of periodic behaviors. The likelihood of oscillations can be improved by careful tuning of the system time constants and by increasing the loop gain, yet it is unclear whether there is any general relationship between optimal time constants and loop gain. This issue is particularly relevant in genetic oscillators resulting from a chain of different subsequent biochemical events, each with distinct (and uncertain) kinetics. Using two families of genetic oscillators as model examples, we show that the loop gain required for oscillations is minimum when all elements in the loop have the same time constant. On the contrary, we show that homeostasis is ensured if a single element is considerably slower than the others.

Real-time velocity scaling and obstacle avoidance for industrial robots using fuzzy dynamic movement primitives and virtual impedances

PurposeThis paper aims to increase the safety of the robots’ operation by developing a novel method for real-time implementation of velocity scaling and obstacle avoidance as the two widely accepted safety increasing concepts.Design/methodology/approachA fuzzy version of dynamic movement primitive (DMP) framework is proposed as a real-time trajectory generator with imbedded velocity scaling capability. Time constant of the DMP system is determined by a fuzzy system which makes decisions based on the distance from obstacle to the robot’s workspace and its velocity projection toward the workspace. Moreover, a combination of the DMP framework with a human-like steering mechanism and a novel configuration of virtual impedances is proposed for real-time obstacle avoidance.FindingsThe results confirm the effectiveness of the proposed method in real-time implementation of the velocity scaling and obstacle avoidance concepts in different cases of single and multiple stationary obstacles as well as moving obstacles.Practical implicationsAs the provided experiments indicate, the proposed method can effectively increase the real-time safety of the robots’ operations. This is achieved by developing a simple method with low computational loads.Originality/valueThis paper proposes a novel method for real-time implementation of velocity scaling and obstacle avoidance concepts. This method eliminates the need for modification of original DMP formulation. The velocity scaling concept is implemented by using a fuzzy system to adjust the DMP’s time constant. Furthermore, the novel impedance configuration makes it possible to obtain a non-oscillatory convergence to the desired path, in all degrees of freedom.

Preliminary dynamic tests of a CO2 transcritical power cycle for waste heat recovery from diesel engine

Due to environmental safety and miniaturization requirement along with better temperature matching with heat source, CO2 transcritical power cycle (CTPC) has recently been gaining a lot of attention for application to waste heat recovery from internal combustion engines. In this paper, a constructed kW-scale CTPC system was dynamically tested to extract energy from exhaust gas of a heavy-duty diesel engine. The preliminary experiments were conducted under engine condition (1100 rpm, 50% load) with two operating modes: the constant valve opening and the constant pump speed. In order to characterise the dynamic process, the measuring system recorded data every second and also time constant and settling time are employed in data processing. The results show that valve inlet pressure follows the external changes timely, while temperatures respond a little slowly. The time constant of the current system is rather small showing from 20 to 25 seconds, which indicates an excellent capability of CTPC system of dealing with highly transient heat sources. In addition, predicted net power output and thermal efficiency show an upward trend with the increase of pressure ratio. Within the scope of these tests, CTPC system expects a maximum net power output of 1.34 kW and a maximum thermal efficiency of 3.54%.

Development of the visible light communication device for swarm using nonlinear synchronization

This paper describes development of a visible light 4 Pulse position modulation (4PPM) data communication device for underwater swarms using a nonlinear synchronizing system. SWARM—a group robot that uses intelligence as a group—do as ants and bees, is expected to be useful for marine resource exploration. Specifically, numerous SWARMs will be grouped to explore marine resources efficiently. An important difficulty is the change in electronic circuit characteristics because of high water pressure in the deep sea. Development of small and simple underwater SWARM communication devices must eliminate this problem without using large pressure-proof containers. As described herein, we considered nonlinear synchronizing systems as effective because they can be synchronized even if system time constants differ. Such systems resemble the glow mechanism used by fireflies. They have individuality, but they flash synchronously when in a group. We developed a visible light communication device using this nonlinear synchronization system based on the firefly concept. Test results confirmed that they are synchronized even if a time constant difference exists between them. Furthermore, we achieved 4PPM data communication using the nonlinear synchronization signal as a communication clock.

Development of a commercially viable piezoelectric force sensor system for static force measurement

A compensation method for measuring static force with a commercial piezoelectric force sensor is proposed to disprove the theory that piezoelectric sensors and generators can only operate under dynamic force. After studying the model of the piezoelectric force sensor measurement system, the principle of static force measurement using a piezoelectric material or piezoelectric force sensor is analyzed. Then, the distribution law of the decay time constant of the measurement system and the variation law of the measurement system’s output are studied, and a compensation method based on the time interval threshold and attenuation threshold is proposed. By calibrating the system and considering the influences of the environment and the hardware, a suitable value is determined, and the system’s output attenuation is compensated based on the value to realize the measurement. Finally, a static force measurement system with a piezoelectric force sensor is developed based on the compensation method. The experimental results confirm the successful development of a simple compensation method for static force measurement with a commercial piezoelectric force sensor. In addition, it is established that, contrary to the current perception, a piezoelectric force sensor system can be used to measure static force through further calibration.

Enhancing Power Transformer Differential Protection to Improve Security and Dependability

Current differential principle is a well-known principle used for protection of transformers, motors, generators, buses, and any other type of power equipment with input and output current measurements. Further the principle is used in developing percent differential protection which can be programmed to the desired sensitivity for detecting in-zone faults, and security during external faults. This protection dependability is usually achieved by modeling a differential-retraining characteristic with two regions, operating and non-operating, and tracking the real differential restraint ratio during faults. Some external faults with high DC offset and high X/R system time constant would easily saturate the installed current transformers (CTs), which in return would cause high differential/restraint ratio above the preset characteristic into the operating region. In such cases the differential protection would operate and cause unwanted transformer trip. This paper focuses on a number of enhancements applied to the differential principle of the main differential protection, and defines guidance on how to setup the protection for better sensitivity and security. The paper is supported by fault cases, showing the improved security and dependability during internal/external faults with and without CT saturation.

Advanced myopia, prevalence and incidence analysis

Various high-percentage high-incidence medical conditions, acute or chronic, start at a particular age of onset t1 (years), accumulate or progress rapidly, with a system time constant t0 (years), typically from 1week to 5years, and then level off at a plateau level [Formula: see text], ultimately affecting 10-95% of the population. This report investigates the prevalence and incidence functions for myopia and high myopia as a function of age. Fundamental prevalence versus time and incidence versus time results allow continuous prediction of myopia and high myopia population fractions as a function of age. This is a retrospective study. Nine reports are calculated with N=444,600 subjects. There were no interventions other than usual regular eye examinations and subsequent indicated refraction change. The main result is continuous prediction of myopia prevalence-time data along with incidence rate data (%/year), age of onset (years), system plateau level, and system time constant (years). These parameters apply to progressive myopia and high myopia (R<-6D), useful over several decades. The primary finding of this research is that the prevalence ratio of high myopes (R<-6.0D) to common myopes is expected to increase from 15% entering college to 45% or more after college and graduate school. These statistics are particularly relevant to the many years of study required by M.D., Ph.D., and M.D./Ph.D. programs.

Transient processes in welding inverters and their influence on the stability of welding current

Inverter converters used for dc-arc electric welding have been manufactured by many foreign and domestic companies and showed good results for decades. Until recently, ac welding was carried out only at an industrial frequency of 50 Hz. Earlier, we suggested carrying out arc welding at an increased frequency within the range of 25–75 kHz, proving it to be efficient taking into account welding physical processes. Results of investigations of “slow” electrical processes in a high-frequency inverter for ac welding at different stages of the welding cycle are given in this article. In particular, the transition processes in the inverter are studied in detail taking into account a whole variety of factors. We conducted a complex of investigations concerning the study of the character and duration of the transient processes in an alternative sign welding inverter with increased frequency, as well as the level of influence of the transition coefficient and time constant of the inverter-control system on these processes. The influence of design and parasitic data of the scheme on the nature of the “fast” and “slow” transient processes and changes in the parameters of the electric arc (loading) and parameters of the actuating signals were assessed. All this allowed determining the effect on the stability of the welding current and, therefore, on the welding quality.

Optimization of a Sensible Thermal Storage System by a Lumped Approach

This study is focused on the optimization of a sensible thermal storage system subjected to a cyclic energy source. The objective is to minimize the system heat storage mass and its geometry to guarantee its operation within an admissible temperature range. The storage medium is modeled as a Lumped system and the working fluid by a non-capacitive energy balance. The storage medium is composed of an array of parallel flat plates submitted to an air stream. The optimization is based on an exhaustive search method. It is observed that the minimum heat storage mass is proportional to the mass airflow rate. An optimal relationship between the superficial heat transfer rate and the fluid flow inertia, given by the dimensionless parameter NTU, is found to be constant (4.03) and independent in respect to the mass flow rate. The system time constant was invariable (3,230 s) for the optimal relationship between the interface heat transfer rate and the system inertia.

First-Order Optimal Line-of-Sight Guidance for Stationary Targets

� In this paper, an explicit optimal line-of-sight guidance law for second-order binomial control systems is derived in closed-loop without acceleration limit. The problem geometry is assumed in one dimension and the final time and final position are fixed. The formulation is normalized in three forms to give more insight into the design and performance analysis of the guidance law. The computational burden of the guidance law is reasonable for today's microprocessors; however curve fitting or look-up table may be used for the implementation of the second-order optimal guidance law. The performance of the second-order optimal guidance law is compared in normalized forms with zero-lag and first-order optimal guidance laws using third-, fourth-, and sixth-order binomial control systems with/without acceleration limit. Moreover, the effect of the final time, the equivalent time constant of the vehicle control system, the vehicle-to-target line-of-sight weighting factor in cost function, and acceleration limit are investigated. Normalized miss distance analysis shows that the miss distance of the second- order guidance law is smaller than the two mentioned schemes for small total flight times, especially with large maneuvering capability.

Estimation of Directed Effective Connectivity from fMRI Functional Connectivity Hints at Asymmetries of Cortical Connectome.

The brain exhibits complex spatio-temporal patterns of activity. This phenomenon is governed by an interplay between the internal neural dynamics of cortical areas and their connectivity. Uncovering this complex relationship has raised much interest, both for theory and the interpretation of experimental data (e.g., fMRI recordings) using dynamical models. Here we focus on the so-called inverse problem: the inference of network parameters in a cortical model to reproduce empirically observed activity. Although it has received a lot of interest, recovering directed connectivity for large networks has been rather unsuccessful so far. The present study specifically addresses this point for a noise-diffusion network model. We develop a Lyapunov optimization that iteratively tunes the network connectivity in order to reproduce second-order moments of the node activity, or functional connectivity. We show theoretically and numerically that the use of covariances with both zero and non-zero time shifts is the key to infer directed connectivity. The first main theoretical finding is that an accurate estimation of the underlying network connectivity requires that the time shift for covariances is matched with the time constant of the dynamical system. In addition to the network connectivity, we also adjust the intrinsic noise received by each network node. The framework is applied to experimental fMRI data recorded for subjects at rest. Diffusion-weighted MRI data provide an estimate of anatomical connections, which is incorporated to constrain the cortical model. The empirical covariance structure is reproduced faithfully, especially its temporal component (i.e., time-shifted covariances) in addition to the spatial component that is usually the focus of studies. We find that the cortical interactions, referred to as effective connectivity, in the tuned model are not reciprocal. In particular, hubs are either receptors or feeders: they do not exhibit both strong incoming and outgoing connections. Our results sets a quantitative ground to explore the propagation of activity in the cortex.

ANALYSIS ON GENERALIZED IMPACT ANGLE CONTROL GUIDANCE LAW

In this paper, a generalized guidance law with an arbitrary pair of guidance coefficients for impact angle control is proposed. Under the assumptions of a stationary target and a lag-free missile with constant speed, necessary conditions for the guidance coefficients to satisfy the required terminal constraints are obtained by deriving an explicit closed-form solution. Moreover, optimality of the generalized impact-angle control guidance law is discussed. By solving an inverse optimal control problem for the guidance law, it is found that the generalized guidance law can minimize a certain quadratic performance index. Finally, analytic solutions of the generalized guidance law for a first-order lag system are investigated. By solving a third-order linear time-varying ordinary differential equation, the blowing-up phenomenon of the guidance loop as the missile approaches the target is mathematically proved. Moreover, it is found that terminal misses due to the system lag are expressed in terms of the guidance coefficients, homing geometry, and the ratio of time-to-go to system time constant.

Role of Introduction of a Resistor in Series with the Working Electrode in Impedance Related Electrochemical Measurements

AbstractIntentional introduction of a resistor in series with the working electrode during electrochemical measurements can apparently change the constituents of the electrochemical system. The introduced resistor behaves like the uncompensated solution resistance, which was often regarded as an unwanted factor. However, the introduction of the resistor brings substantial benefits in some circumstances such as (a) compressing the impedance dynamic range of an electrochemical system and making the time domain EIS measurement more reliable; (b) tuning the RC time constant of an electrochemical system to a larger or even desired value and making the potential step based capacitive biosensing more flexible.

Modeling and measurement study on an intermittent heating system of a residence in Cambridgeshire

The intermittent heating is considered to be able to achieve a high energy saving rate compared with the continuous heating. This paper builds a two-stage lumped parameter model to study the thermal dynamic process of the intermittent heating. Meanwhile, the on-site measurement was carried out on a residence in Cambridgeshire to verify the correctness of the model. On-site measurement validates the model we build and shows that the intermittent heating system we measured is able to provide comfort indoor thermal environment. However, the associated energy saving rate is only about 5% compared with the continuous heating system, which is not significant. Both the model and the on-site measurement show that the temperature change of the indoor air after the heating was recovered follows the exponent law and accordingly the thermal dynamic process of intermittent heating can be characterized by the thermal time constant. The thermal time constant is found to be a key index of intermittent heating. Low thermal capacity and high heat transfer coefficient lead to a small thermal time constant. The intermittent heating system with small thermal constant time is able to elevate indoor temperature quickly and has large energy saving potential. The thermal time constant of the intermittent heating system we surveyed has been calculated. Lastly, suggestions for improving the energy saving rate of intermittent heating systems have been provided in this paper.

Exponential prevalence and incidence equations for myopia

BackgroundThis project relates prevalence‐time data, incidence rate data, age of onset, system plateau level and system time constant, using exponential equations, as they apply to progressive myopia, useful over several decades.MethodsCross‐sectional refractive data is analysed for nine studies with a total number of subjects at 444.6-K (345, 981, 7.6-K, 39, 421-K, 383, 2-K, 12-K, 255), with ages ranging from five to 39 years. Basic exponential equations allow calculation of the prevalence versus time function Pr(t) as a percentage and the incidence rate function In(t) (percentage per year), system time constant t0 (years), onset age t1 (years) and saturation plateau level <S> (percentage).ResultsThe prevalence of myopia as a function of time Pr(t) (years) and incidence of myopia as a function of time In(t) (percentage per year) are continuously generated and compared with prevalence/incidence data from various reports investigating student populations. For a general medical condition, typical values for time constant t0 may range from one week to five years, depending on the health condition. Typical plateau levels for myopia may range from 35 to 95 per cent. Herein, data from nine demographic studies of myopia are analysed for prevalence Pr(t) with an accuracy within 14 per cent and incidence In(t) within 2.6 per cent per year, onset t1 = 1.5 years, time constant t0 = 4.5 year. By comparison, linear regression can predict the prevalence of myopia Pr(t) within 11 per cent and estimates a constant incidence rate for myopia In(t) of 4.7 per cent per year (95 per cent CI: 2.1 to 7.3 per cent per year].ConclusionsThe initial incidence rate at onset age In(t1) and system time constant t0 are inversely related. For myopia, onset age, time constant and saturation plateau level are fundamental system parameters derived from age‐specific prevalence and incidence data.