Uncooled Sensor Research Articles

High-resolution debris-cover mapping using UAV-derived thermal imagery: limits and opportunities

Abstract. Debris-covered glaciers are widespread in high mountain ranges on earth. However, the dynamic evolution of debris-covered glacier surfaces is not well understood, in part due to difficulties in mapping debris-cover thickness in high spatiotemporal resolution. In this study, we present land surface temperatures (LSTs) of supraglacial debris cover and their diurnal variability measured from an unpiloted aerial vehicle (UAV) at a high (15 cm) spatial resolution. We test two common approaches to derive debris-thickness maps by (1) solving a surface energy balance model (SEBM) in conjunction with meteorological reanalysis data and (2) least squares regression of a rational curve using debris-thickness field measurements. In addition, we take advantage of the measured diurnal temperature cycle and estimate the rate of change of heat storage within the debris cover. Both approaches resulted in debris-thickness estimates with an RMSE of 6 to 8 cm between observed and modeled debris thicknesses, depending on the time of the day. Although the rational curve approach requires in situ field measurements, the approach is less sensitive to uncertainties in LST measurements compared to the SEBM approach. However, the requirement of debris-thickness measurements can be an inhibiting factor that supports the SEB approach. Because LST varies throughout the day, the success of a rational function to express the relationship between LST and debris thickness also varies predictably with the time of day. During the period when the debris cover is warming, LST is heavily influenced by the aspect of the terrain. As a result, clear-sky morning flights that do not consider the aspect effects can be problematic. Our sensitivity analysis of various parameters in the SEBM highlights the relevance of the effective thermal conductivity when LST is high. The residual and variable bias of UAV-derived LSTs during a flight requires calibration, which we achieve with bare-ice surfaces. The model performance would benefit from more accurate LST measurements, which are challenging to achieve with uncooled sensors in high mountain landscapes.

Monitoring Metal Wear Particles of Friction Pairs in the Oil Systems of Gas Turbine Power Plants

In the example of the aviation gas turbine engine the problem of monitoring metal wear particles of friction pairs in the oil systems of gas turbine power plants is considered. The solution based on using the multi-channel cluster single-coil eddy current sensor (CSCECS) with sensitive elements in the form of single circuits is proposed. The CSCECS provides the detection of ferromagnetic and non-ferromagnetic particles and their ranking by several size groups. The sensor is invariant to the size (inner diameter) of the monitored oil pipeline and has high throughput and identical sensitivity across all channels. Two variants of the hardware structure of the debris continuous monitoring system (DCMS) prototype implementing the proposed approach are suggested. The first variant is intended for engine bench tests and contains the CSCECS with integrated preamplifiers and forced air cooling of the electronic modules. The second variant of the DCMS prototype involves the use of the uncooled sensors without built-in electronics and it focuses on operation in autonomous mode not only in bench tests but also during the engine normal operation. A brief description of the DCMS operational algorithm is given. The algorithm is the same for both hardware versions but differs at the software implementation level. The correctness of the algorithm for the detection and size identification of the wear metal particles was verified during the laboratory experiments with a total duration of 5 h and 30 min. The DCMS prototype was also examined during the full-scale engine bench tests. The experiments indicated that the number, size, and magnetic properties of the particles detected by DCMS generally corresponded to the number, size, and magnetic properties of the particles fixed by the MetalSCAN oil debris monitoring system which was used for verification of the DCMS functional capability. The results were also confirmed through laboratory analysis of the wipe samples on the debris filters. However, unlike the existing approaches, the design of the CSCECS additionally made it possible to evaluate the oil flow features in the pipeline of the engine lubrication system.

Factors Influencing Temperature Measurements from Miniaturized Thermal Infrared (TIR) Cameras: A Laboratory-Based Approach.

The workflow for estimating the temperature in agricultural fields from multiple sensors needs to be optimized upon testing each type of sensor’s actual user performance. In this sense, readily available miniaturized UAV-based thermal infrared (TIR) cameras can be combined with proximal sensors in measuring the surface temperature. Before the two types of cameras can be operationally used in the field, laboratory experiments are needed to fully understand their capabilities and all the influencing factors. We present the measurement results of laboratory experiments of UAV-borne WIRIS 2nd GEN and handheld FLIR E8-XT cameras. For these uncooled sensors, it took 30 to 60 min for the measured signal to stabilize and the sensor temperature drifted continuously. The drifting sensor temperature was strongly correlated to the measured signal. Specifically for WIRIS, the automated non-uniformity correction (NUC) contributed to extra uncertainty in measurements. Another problem was the temperature measurement dependency on various ambient environmental parameters. An increase in the measuring distance resulted in the underestimation of surface temperature, though the degree of change may also come from reflected radiation from neighboring objects, water vapor absorption, and the object size in the field of view (FOV). Wind and radiation tests suggested that these factors can contribute to the uncertainty of several Celsius degrees in measured results. Based on these indoor experiment results, we provide a list of suggestions on the potential practices for deriving accurate temperature data from radiometric miniaturized TIR cameras in actual field practices for (agro-)environmental research.

A Practical Validation of Uncooled Thermal Imagers for Small RPAS

Uncooled thermal imaging sensors in the LWIR (7.5 μm to 14 μm) have recently been developed for use with small RPAS. This study derives a new thermal imaging validation methodology via the use of a blackbody source (indoors) and real-world field conditions (outdoors). We have demonstrated this method with three popular LWIR cameras by DJI (Zenmuse XT-R, Zenmuse XT2 and, the M2EA) operated by three different popular DJI RPAS platforms (Matrice 600 Pro, M300 RTK and, the Mavic 2 Enterprise Advanced). Results from the blackbody work show that each camera has a highly linearized response (R2 > 0.99) in the temperature range 5–40 °C as well as a small (<2 °C) temperature bias that is less than the stated accuracy of the cameras. Field validation was accomplished by imaging vegetation and concrete targets (outdoors and at night), that were instrumented with surface temperature sensors. Environmental parameters (air temperature, humidity, pressure and, wind and gusting) were measured for several hours prior to imaging data collection and found to either not be a factor, or were constant, during the ~30 min data collection period. In-field results from imagery at five heights between 10 m and 50 m show absolute temperature retrievals of the concrete and two vegetation sites were within the specifications of the cameras. The methodology has been developed with consideration of active RPAS operational requirements.

Overcoming the Challenges of Thermal Infrared Orthomosaics Using a Swath-Based Approach to Correct for Dynamic Temperature and Wind Effects

The miniaturization of thermal infrared sensors suitable for integration with unmanned aerial vehicles (UAVs) has provided new opportunities to observe surface temperature at ultra-high spatial and temporal resolutions. In parallel, there has been a rapid development of software capable of streamlining the generation of orthomosaics. However, these approaches were developed to process optical and multi-spectral image data and were not designed to account for the often rapidly changing surface characteristics inherent in the collection and processing of thermal data. Although radiometric calibration and shutter correction of uncooled sensors have improved, the processing of thermal image data remains difficult due to (1) vignetting effects on the uncooled microbolometer focal plane array; (2) inconsistencies between images relative to in-flight effects (wind-speed and direction); (3) unsuitable methods for thermal infrared orthomosaic generation. Here, we use thermal infrared UAV data collected with a FLIR-based TeAx camera over an agricultural field at different times of the day to assess inconsistencies in orthophotos and their impact on UAV-based thermal infrared orthomosaics. Depending on the wind direction and speed, we found a significant difference in UAV-based surface temperature (up to 2 °C) within overlapping areas of neighboring flight lines, with orthophotos collected with tail wind being systematically cooler than those with head wind. To address these issues, we introduce a new swath-based mosaicking approach, which was compared to three standard blending modes for orthomosaic generation. The swath-based mosaicking approach improves the ability to identify rapid changes of surface temperature during data acquisition, corrects for the influence of flight direction relative to the wind orientation, and provides uncertainty (pixel-based standard deviation) maps to accompany the orthomosaic of surface temperature. It also produced more accurate temperature retrievals than the other three standard orthomosaicking methods, with a root mean square error of 1.2 °C when assessed against in situ measurements. As importantly, our findings demonstrate that thermal infrared data require appropriate processing to reduce inconsistencies between observations, and thus, improve the accuracy and utility of orthomosaics.

Performance of an uncooled imaging interferometric spectrometer with intrinsic background radiation

A signal-to-noise performance of an imaging Michelson interferometer based on corner cubes is modeled and experimental results are presented. The influence from the background radiation of the uncooled sensor is explained. The noise equivalent spectral radiance is determined in the 700 to 1300 cm − 1 range at two spectral resolutions using blackbody sources. The performance of the sensor system is tested on a scene at an average temperature of 19°C. From these results, the performance is given in terms of number of samples and spectral resolution. The performance model is used to predict the sensor performance at different scene temperatures.

Real-Time Biomagnetic Signal Processing for Uncooled Magnetometers in Cardiology

Despite obvious advantages over their electric counterparts, magnetic measurements are still performed very rarely for medical diagnosis. The high operating costs of established systems with a sufficient limit of detection, mainly based on superconducting quantum interference devices, prevent the spread of magnetic diagnostic procedures. Consequently, the demand for alternative low-cost magnetic sensor systems is large. Several suitable, uncooled sensor concepts exist, but their usability-especially, when operated outside magnetically shielded chambers–requires improvement. This paper highlights basic, real-time signal processing concepts, and implementations to improve the signal quality for different biomedical applications with a focus on cardiology in unshielded measurement environments. Several processing steps in the digital domain are described, such as noise reduction (by means of cancellation and suppression), adaptive sensor signal combination, and adaptive signal averaging. Finally, basic cardiologic feature extraction methods are performed using uncooled magnetometers in combination with the described signal enhancement stages.

Mid- infrared uncooled sensor for the identification of pure fuel, additives and adulterants in gasoline

The aim of the present study is to test the ability of a low-cost and portable middle infrared spectrometer based on a linear array of 1×128 of PbSe, coupled with a linear variable optical filter in the wavelength range of 3–4.5μm, for the differentiation of pure chemical substances and quality control of fuels. Potential additives and adulterants for gasoline were tested, considering the alcohols ethanol, n-butanol, n-propanol and n-hexanol as potential additives and methanol and diesel oils as adulterants. Multivariate analysis of variance (MANOVA) applied to the scores obtained in the Principal Component Analysis (PCA) was conducted to analyze the spectral data and distinguish between the individual components. For the purposes of classifying anonymous samples, the centroid of each pure substance in the canonical variables was calculated, followed by the distance calculated between new samples to such centroids, assigning the individual to the most proximate category. The results demonstrated that the technique was able to discriminate between gasoline, diesel oils and the alcohols methanol, ethanol, n-propanol, n-butanol and n-hexanol and that it had the potential to be applied in the fuel industry.

A novel uncooled thermal imaging sensor using MEMS transistor

In this paper a new uncooled thermal imaging sensor using MEMS transistor with movable gate is presented. The novelty of this work is making moveable polysilicon gate by two minder bi-materials arms on silicon substrate using MEMS technology. The arms are deflected with absorbed infrared temperature from objects, and move the gate plate. By changing air gap between gate and Si channel, the drain current of transistor will be changed. The geometrical parameters are obtained using analysis of the structure. A FEM simulation was performed using COMSOL Multiphysics software to verify the validity. This structure has many advantages; including compatibility with CMOS fabrication process, high sensitivity, low power consumption, no need signal amplification, ability to miniaturization of pixel size, much more than other techniques that used until now.

The investigation of heterogeneous flow generated by the direct current plasma torch

In the article, the two-phase flow of electric arc gas heater of the linear scheme is studied. The power of the plasma torch can be varied from 200 to 1500 kW. For stabilization of the electric arc a magnetic coil is used. The operation of the plasma torch took place at overpressure in the discharge chamber. Injection of the powder was made near the exit of the nozzle. A powder of SiO2 was used as a disperse phase. The size of the particles was not more than 50 microns. The dispensing device was used for the powder injection. The technique of velocity measurement in high-temperature heterogeneous flow from the registration of flow by the high-speed camera is presented. The results of measurements indicate that the speed of the particles much lower than the speed of the gas. The results of measuring the heat flux along the axis of the plasma torch are presented. The heat flux was measured by means of regular mode uncooled sensors with tablet type calorimeters.

室温動作の高感度THzアレイセンサとカメラの開発およびTHzカメラの応用

室温動作の高感度THzアレイセンサとカメラの開発およびTHzカメラの応用

Design and Analysis of Novel Micromachined Thermocouples with Vertical Free-Standing High-Aspect-Ratio Geometry

This paper describes the design and analysis of free-standing three-dimensional (3D) thermocouples (TCs) for uncooled high-resolution infrared imaging. In contrast to the conventional horizontal thermocouple design, our 3D TCs consist of vertically oriented coaxial thermoelectric legs covered by an infrared absorber plate. The high aspect ratio of the 3D TCs of up to 100:1 leads to increased thermal resistance and, thus, higher electrical responsivity. Furthermore, reduced lateral dimensions of the device are achieved by coaxial mounting. This allows a closely packed arrangement of 3D TCs, which results in a high-resolution microarray sensor setup. These features represent the 3D TC’s innovative uniqueness and open up new possibilities for further minimized uncooled thermal sensors and low-cost high-resolution imagers. We developed a fabrication process for 3D TCs with doped polysilicon as thermoelectric legs and a geometry with diameter of \(5 \,\mu \)m and length of \(515 \,\mu \)m. Performance analysis has been carried out using the lumped-heat-capacity method, resulting in an electrical responsivity and specific detectivity of a 3D TC of 383 V/W and \(1.7\times 10^8\) cm \(\sqrt{\rm{Hz}}\)/W, respectively.

Simulation method for LWIR radiation distribution using a visual ray-tracer

Infrared cameras with passive, uncooled sensor chips utilize the longwave infrared (LWIR) range of the electromagnetic spectrum with wavelengths between 8 and 14 μm for image generation. The reason for this is that every object at room temperature is self-luminous at that wavelength. Therefore, every surface acts as a source of radiation in a LWIR scenario. To gain an impression and to model the effects and circumstances in an infrared scenario, a simulation method is required. In the visual domain this task is accomplished by ray-tracing software, which allows the generation of synthetic images as well as the analysis of irradiance distribution in a given scene. In this paper a way to apply one of such ray-tracers to a LWIR scenario is demonstrated. A possible application of the proposed simulation method in test-system modelling and uncertainty analysis is also presented.

Plasmonic Band-Pass Microfilters for LWIR Absorption Spectroscopy

Absorption spectroscopy in the long wave infrared provides an effective method for identification of various hazardous chemicals. We present a theoretical design for plasmonic band-pass filters that can be used to provide wavelength selectivity for uncooled microbolometer sensors. The microfilters consist of a pair of input reflection gratings that couple light into a plasmonic waveguide with a central resonant waveguide cavity. An output transmission grating on the other side of the structure pulls light out of the waveguide where it is detected by a closely spaced sensor. Fabrication of the filters can be performed using standard photolithography procedures. A spectral bandpass with a full-width at half-maximum (FWHM) of 100 nm can be obtained with a center wavelength spanning the entire 8–12 μm atmospheric transmission window by simple geometric scaling of only the lateral dimensions. This allows the simultaneous fabrication of all the wavelength filters needed for a full spectrometer on a chip.

High performance read-out IC design for IR image sensor applications

In this paper, high performance read-out integrated circuits (ROIC) for infrared (IR) image sensor applications is proposed. Because an uncooled microbolometer image sensor, used in an IR image sensor, is made by a micro electro mechanical systems process, the resistance of bolometer by each process does not appear same value under same temperature condition. This resistance variation generates a different output signal for same input by each chip. In order to improve on these drawbacks, a ROIC, which compensates for the error described above, was designed. Instead of a single input mode, a differential input mode ROIC was proposed and thus, a new circuit structure was proposed. Using results from a computer simulation, improvements such as that the effect of the process error was decreased 10---12% without an additional compensation circuit was found. Based on the simulation results, a prototype device including a ROIC and a micro bolometer with 16 × 16 cell arrays was fabricated and characterized.

CMOS-SOI-MEMS Transistor for Uncooled IR Imaging

This paper reports the design, fabrication technology, post-CMOS micromachining and characterization of CMOS-silicon-on-insulator (SOI)-microelectromechanical system (MEMS) transistors. The thermally isolated micromachined CMOS-SOI-MEMS transistor reported here is designed for uncooled infrared (IR) sensing and is dubbed here as ldquoTMOS.rdquo The measured dc and noise electrical characteristics of the as-processed (virgin) transistor as well as those of the post-CMOS-MEMS-processed transistor (TMOS) are reported and compared. In particular, the threshold voltage temperature dependence and the temperature coefficient of current (TCC) at subthreshold are reported. The results indicate that the post-CMOS-MEMS processing does not degrade the performance of the transistors. The electrooptical performance of the TMOS is characterized and reported. With TCC on the order of 4%-10%, depending on the gate voltage, responsivity of 40 mA/W, noise equivalent power on the order of several tens of picowatts, and calculated noise equivalent temperature difference on the order of 64 mK, this uncooled IR sensor in standard CMOS-SOI technology may provide a high performance at a lower cost compared to state-of-the-art uncooled sensors based on bolometers implemented in non-CMOS materials like vanadium oxide or amorphous silicon.

A thermoacoustic sensor for recording high-power nanosecond microwave pulses

The operating principle and design of an uncooled noise-immune high-power microwave pulse sensor is described. The sensor operation is based on the effect of acoustic signal generation when microwave pulses are absorbed in a layer structure, in which a thin metal nanometer-thick film is used as an absorber. The sensor is placed in a free space and intended to detect microwave pulses with ∼10- to 100-ns durations in a 10- to 300-GHz frequency band with a pulse repetition rate of up to 5 kHz. For 10-ns-long pulses, the sensor sensitivity is 0.5 V/mJ.

A New CMOS Read-out IC for Uncooled Microbolometer Infrared Image Sensor

An uncooled microbolometer image sensor, used in an IR image sensor, is made by a micro electro mechanical systems (MEMS) process, so the value of the microbolometer resistor has a process variation. Also, the reference resistor, which is used to connect to the microbolometer, is fabricated by a standard CMOS process, and the difference between the values of the microbolometer resistor and the reference resistor generates an unwanted output signal for the same input from the sensor array. In order to minimize this problem, a new CMOS read-out integrated circuit (ROIC) was designed. Instead of a single input mode, a differential input mode scheme and a simple method to compensate the resistor value are proposed. Using results from a computer simulation, it is observed that the output characteristic of the ROIC was improved and the effect of the process variation was decreased without using complex compensation circuits. Based on the simulation results, a prototype device including an ROIC that was fabricated by a standard 0.25um CMOS process and a microbolometer with a 16 x 16 sensor array was fabricated and characterized.

Miniaturized InSb mid‐IR photovoltaic sensor for room temperature operation

AbstractWe have developed a miniaturized InSb middle‐infrared photovoltaic sensor (InSb PVS) that operates at room temperature. A single InSb photodiode of the InSb PVS consists of an InSb p+‐p–‐n+ photodiode structure grown on a semi‐insulating GaAs (100) substrate, with a p+‐Al0.17In 0.83 Sb barrier layer between p+ and p– layers to reduce diffusion of photo‐excited electrons. The n‐ and p‐type dopants used are Sn and Zn, respectively. To avoid 1/f noise, we operated our InSb photodiode in photovoltaic mode to output an open‐circuit voltage. Moreover, to improve the signal‐to‐noise ratio, 700 photodiodes arranged on a 600 × 600 μm chip were serially connected to each other. We found that the output signal voltage of the sensor depends on the Zn doping concentration in the p– layer. The peak output voltage was obtained at about 6 × 1016 cm–3, which corresponded to the minimum value of the carrier concentration in the p– layer. After packaging, the final external size of the sensor was 2.2 × 2.7 × 0.7 mm. To our knowledge, our sensor is the smallest uncooled sensor in the mid‐IR range reported up to now. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Growth, annealing and thermo-electrical properties of Cd1−xZnxS thin films for microbolometers

Cd1−xZnxS (0<x<0.14) nanocrystalline thin films were grown and tested for use as sensing elements in microbolometer-type uncooled infrared detectors instead of vanadium oxides. Films of thickness 80–700 nm were deposited on SixNy/silicon as well as on glass and SiO2/silicon wafers using both thermal evaporation and MOCVD. The films were characterized by AFM, SIMS and electrical measurements. CdS thin films showed good parameters for use in microbolometer-type infrared uncooled sensors. The thermal coefficient of resistance (TCR) was in the range of 1.1–3.6% for the different film thicknesses. The TCR and electrical resistance of CdS films were optimized using annealing in different environments (vacuum, nitrogen, hydrogen and NF3) and by using a capping silicon nitride layer. The correlations between grain size, grain boundaries structure and TCR were established. Owing to the nanoscale film structure and thickness, significant grain growth and grain-boundary reconstruction occurred during minute long annealing time at temperatures as low as 200 °C.