Search results for " Lock-in"
showing 10 items of 13 documents
FPGA based digital lock-in amplifier for fNIRS systems
2018
Lock-In Amplifiers (LIA) represent a powerful technique helping to improve signals detectability when low signal to noise ratios are experienced. Continuous Wave functional Near Infrared Spectroscopy (CW-fNIRS) systems for e-health applications usually suffer of poor detection due to the presence of strong attenuations of the optical recovering path and therefore small signals are severely dipped in a high noise floor. In this work a digital LIA system, implemented on a Zynq® Field Programmable Gate Array (FPGA), has been designed and tested to verify the quality of the developed solution, when applied in fNIRS systems. Experimental results have shown the goodness of the proposed solutions.
Thermal NDE of thick GRP panels by means of a Pulse Modulated Lock-In Thermography technique
2010
This work describes the development and implementation of an infrared thermal NDE procedure for the evaluation of subsurface defects. The approach is called Pulse-Modulated Lock-In Thermography (PMLT) and is based on the analysis of the frequency response of the measured temperature and comparison with the carrier frequencies launched by the external heat delivering source. The heat deposited on the object is in particular modulated as a train of square waves. This is easily achieved by periodically shuttering the heat source. The temperature is then sampled throughout the deposition of a few square waves. A lock-in algorithm is then implemented able to selectively filter out components at …
Thermoelastic stress analysis of a 2D stress field using a single detector infrared scanner and lock-in filtering
2009
A low resolution, low cost, fast infrared scanner is used to acquire the temperature change along a line on the surface of cyclically loaded samples. The temperature signal is sampled versus time by exploiting the raster scanning movement of the thermocamera single detector. The temperature data is then post-processed by a Fast Fourier Transform based lock- in algorithm implemented in MATLAB ® , in order to filter out the thermoelastic effect induced temperature change. A procedure is also implemented in order to extend the data sampling time by opportunely synchronising successively grabbed data frames. The effectiveness of such synchronisation procedure is first demonstrated by performing…
Implementation of active infrared NDT techniques using long square heating pulses
2012
The present work describes the implementation of active IR Thermography techniques for the NDT of thick polymer and glass-fibre reinforced polymer (GRP) composite panels. A low cost Thermal NDT set-up is proposed, comprising a single-detector IR camera with low thermal resolution and low frame rate, and common low-power halogen lamps as external heat source devices. The use of halogen lamps in particular requires several seconds of switch-on time in order to deliver meaningful and effective heat quantities. The influence of such long heat deposition intervals is investigated on the possibility to implement Transient Thermography and Lock-In Thermography techniques. Regarding the Transient T…
The Egan problem on the pull-in range of type 2 PLLs
2021
In 1981, famous engineer William F. Egan conjectured that a higher-order type 2 PLL with an infinite hold-in range also has an infinite pull-in range, and supported his conjecture with some third-order PLL implementations. Although it is known that for the second-order type 2 PLLs the hold-in range and the pull-in range are both infinite, the present paper shows that the Egan conjecture may be not valid in general. We provide an implementation of the third-order type 2 PLL, which has an infinite hold-in range and experiences stable oscillations. This implementation and the Egan conjecture naturally pose a problem, which we will call the Egan problem: to determine a class of type 2 PLLs for …
IMPLEMENTAZIONE DI TECNICHE NDT DI TERMOGRAFIA AD INFRAROSSI SU PANNELLI IN COMPOSITO GRP PER APPLICAZIONI NAUTICHE
2010
Il presente lavoro descrive l’implementazione di due tecniche di termografia attiva ad infrarossi per il controllo non distruttivo (IRNDT) di delaminazioni sub-superficiali su pannelli in composito (GRP) di grosso spessore, tipicamente impiegati per uso navale. Si è in particolare fatto uso di attrezzature a basso costo consistenti in comuni lampade alogene a bassa potenza come sorgenti di calore esterne, ed una termocamera ad infrarossi a singolo sensore a bassa risoluzione termica e basso frame rate. Le due tecniche implementate sono la Pulsed Thermography (PT) ed una nuova variante di Lock-In Thermography denominata Pulse-Modulated Lock-In Thermography (PMLT), basata sull’analisi in freq…
IMPLEMENTAZIONE DI TECNICHE DI TERMOGRAFIA ATTIVA IRNDT SU COMPOSITI GRP MEDIANTE LA DEPOSIZIONE DI IMPULSI TERMICI DI LUNGA DURATA
2011
Nel presente lavoro viene analizzata l’implementazione di tecniche attive di termografia ad infrarossi per analisi NDT su pannelli in composito. In particolare si propone l’uso di lampade alogene come sorgente di calore esterna, le quali richiedono tempi di accensione dell’ordine di diversi secondi al fine di depositare efficaci quantità di calore. Si discutono quindi le problematiche connesse all’implementazione di tecniche di Transient Thermography e di Lock-In Thermography in presenza di tempi lunghi di deposizione di calore. Viene implementata una tecnica di modulazione della deposizione di calore mediante treni di onde quadre di lunga durata (PMLT - Pulse Modulated Lock-In Thermography…
Thermoelastic signal processing using an FFT lock-in based algorithm on extended sampled data
2009
A fast infrared scanner is used to acquire the thermoelastic effect induced temperature changes along a line on the surface of cyclically loaded tensile samples. The raster scanning movement of the single detector allows the sampling of temperature versus time. This data are then post-processed by means of a lock-in algorithm coupled with 1D and 2D FFT analyses in order to filter out the thermoelastic signal from the noisy measured signal. A data extension algorithm is proposed which uses the information from different acquired frames to extend the data sampling window. The whole signal processing setup is evaluated on experimental data with successful results, proposing a potential tool fo…
Rilevamento del segnale termoelastico a campo intero mediante analisi lock-in in modalità off-line
2014
Il presente lavoro propone una procedura di post-processamento del segnale termografico misurato su provini sollecitati in modo ciclico al fine di ricavare il segnale termoelastico. Dopo aver descritto la Correlazione Lock-In, usata tradizionalmente dai sistemi commerciali per ricavare il segnale termoelastico, il lavoro mostra come tale procedura sia numericamente equivalente alla determinazione delle armoniche della Trasformata di Fourier Discreta (TFD) del segnale campionato. Viene quindi proposta ed implementata una procedura di filtraggio del segnale termoelastico basata sulla applicazione diretta della TFD, che ha il vantaggio di poter in alcune circostanze fare a meno di un segnale d…
Lock-In Signal Post-Processing Techniques in Infra-Red Thermography for Materials Structural Evaluation
2013
This paper describes the potential of off-line thermographic signal processing by means of Lock-In Correlation algorithms, in order to implement structural health monitoring and stress analysis techniques. Thermal datasets acquired by infrared thermocameras are locked-in and correlated numerically with opportune reference signals, and amplitude and phase values of various harmonics retrieved by means of a Fast Fourier Transform and time averaging based filtering. This information is then processed for NDT defect probing and for evaluating the Thermoelastic Effect induced temperature changes. Two case studies in particular are discussed, implementing the proposed signal lock-in processing: a…