Time constant | EPFL Graph Search

In physics and engineering, the time constant, usually denoted by the Greek letter τ (tau), is the parameter characterizing the response to a step input of a first-order, linear time-invariant (LTI) system. The time constant is the main characteristic unit of a first-order LTI system. In the time domain, the usual choice to explore the time response is through the step response to a step input, or the impulse response to a Dirac delta function input. In the frequency domain (for example, looking at the Fourier transform of the step response, or using an input that is a simple sinusoidal function of time) the time constant also determines the bandwidth of a first-order time-invariant system, that is, the frequency at which the output signal power drops to half the value it has at low frequencies. The time constant is also used to characterize the frequency response of various signal processing systems – magnetic tapes, radio transmitters and receivers, record cutt

Improved methodology and set-point design for diagnosis of model-plant mismatch in control loops using plant-model ratio

Alireza Karimi

Performance of any model-based control scheme depends on the quality of model. When these schemes deliver poor loop performance due to model-plant mismatch (MPM), a detection of the same needs to be in place. A recently introduced plant model ratio (PMR) not only detects MPM but also facilitates a unique identification of the source of mismatch, namely, gain, dynamics (time constant) and delay mismatches. The prime objective of this work is to improve the PMR approach in a few key aspects, namely, estimation and experimental effort, and assessment procedure by taking a fresh perspective of PMR and conducting a detailed theoretical study of its signatures. A rigorous assessment procedure based on the theoretical properties of PMR is devised. Three threshold-based hypotheses tests are proposed for significance testing of PMR. A key contribution of this work is the design of set-point with minimal excitation for diagnosis of MPM, based on the features of PMR. The revised methodology is demonstrated and compared with the existing method through simulation examples. The study also demonstrates the potential of the proposed method in serving as a prelude to full/partial model re-identification.

Elsevier2014

Timescales of N-H bond dissociation in pyrrole: a nonadiabatic dynamics study

Momir Malis, Antonio Prlj

The excitation wavelength dependent photodynamics of pyrrole are investigated by nonadiabatic trajectory-surface-hopping dynamics simulations based on time dependent density functional theory (TDDFT) and the algebraic diagrammatic construction method to the second order (ADC(2)). The ADC(2) results confirm that the N-H bond dissociation occurring upon excitation at the origin of the first excited state, S-1(pi sigma*), is driven by tunnelling [Roberts et al., Faraday Discuss., 2013, 163, 95] as a barrier of DE = 1780 cm(-1) traps the population in a quasi-bound minimum. Upon excitation to S-1(pi sigma*) in the wavelength range of 236-240 nm, direct dissociation of the N-H bond takes place with a time constant of 28 fs. The computed time constant is in very good agreement with recently reported measurements. Excitation to the lowest B-2 state in the 198-202 nm range returns a time constant for N-H fission of 48 fs at the B3LYP/def2-TZVPD level, in perfect agreement with the experiment [Roberts et al. Faraday Discuss., 2013, 163, 95]. For the same wavelength range the ADC(2)/aug-cc-pVDZ decay constant is more than three times longer than the experimentally reported one. The accuracy of the B3LYP/def2-TZVPD dynamics is checked against reference complete-active-space second-order perturbation theory (CASPT2) calculations and explained in terms of correct topography of the pi pi* surface and the lack of mixing between the pi pi* and the 3p(x) Rydberg states which occurs in the ADC(2) method.

Royal Soc Chemistry2015

Prototype mitre bends of the ex-vessel waveguide system for the ITER upper launcher: Thermal hydraulic simulations and experiments with off-center mm-wave beams

René Chavan, Timothy Goodman, Humberto Torreblanca Quiroz, Matteo Vagnoni, Anastasia Xydou

On ITER, long pulse gyrotrons are required as a power source for electron cyclotron heating (ECH) and current drive (CD). The microwaves are guided from the gyrotrons, which are placed far from the Tokamak, into the plasma by transmission lines (TLs) and a launching antenna (launcher). Each of the four ECH Upper launchers features eight waveguide (WG) TLs, with at least 95% of the power from the gyrotrons coupled into in the main HE mode of the TLs. In the ex-vessel portion of the system between the port-plug closure plate and the isolation valve and diamond window, there are miter bends (MBs) that change the direction of the TL by reflecting the millimeter waves (mm-waves) using mirrors; the mirrors must handle 1.31 MW, 170 GHz, 3600 s pulses. Various MBs perform these reflections at an angle of 90 degrees, or nearly 100 degrees. As a result of the ohmic dissipation, an intensive peaked heat flux appears near the center of the MB mirror and thus, a dedicated cooling system is present to ensure the temperature control of the mirror and housing e.g. [1]. The power that is not found in the HE mode can cause the beam to be not perfectly centered, resulting in an off-centered heat flux on the mirror surface; as is the case for the experiments described here. This study presents new finite element modeling of such beams, created in CFX of ANSYS Workbench [2], compared to the experimental findings for pulses of 170 GHz, 0.5 MW and 240 s. Monitor points are placed in the same positions as TCs that have been fitted in the mirror close to the heated surface and direct comparison of the temperature values is performed. Through transient simulation the time constants are calculated and compared with those of the experiments.

2021

Prototype mitre bends of the ex-vessel waveguide system for the ITER upper launcher: Thermal hydraulic simulations and experiments with off-center mm-wave beams

René Chavan, Timothy Goodman, Humberto Torreblanca Quiroz, Matteo Vagnoni, Anastasia Xydou

2021