Pulse Density Research Articles

Mapping forest structural heterogeneity of tropical montane forest remnants from airborne laser scanning and Landsat time series

Abstract Tropical montane forests are important reservoirs of carbon and biodiversity and have a central role in the hydrological cycle. They are, however, very fragmented and degraded, leaving isolated remnants across the landscape. These montane forest remnants have considerable differences in forest structure, depending on factors such as tree species composition and degree of forest degradation. Our objectives were (1) to analyse the reliability of airborne laser scanning (ALS) in modelling forest structural heterogeneity, as described by the Gini coefficient (GC) of tree size inequality; (2) to determine whether models are improved by including tree species-sensitive spectral-temporal metrics from the Landsat time series (LTS); and (3) to evaluate differences between three forest remnants and different forest types using the resulting maps of predicted GC. The study area was situated in Taita Hills, Kenya, where indigenous montane forests have been partly replaced by single-species plantations. The data included field measurements from 85 sample plots and two ALS data sets with different pulse densities (9.6 and 3.1 pulses m−2). GC was modeled using beta regression. We found that GC was predicted more accurately by the ALS data set with a higher point density (a cross-validated relative root mean squared error (rRMSECV) 13.9%) compared to ALS data set with lower point density (rRMSECV 15.1%). Furthermore, important synergies exist between ALS and LTS metrics. When combining ALS and LTS metrics, rRMSECV was improved to 12.5% and 13.0%, respectively. Therefore, if the LTS metrics are included in models, ALS data with lower pulse density are sufficient to yield similar accuracy to more expensive, higher pulse density data acquired from the lower altitude. In Ngangao and Yale, forest canopy has multiple layers of variable tree sizes, whereas elfin forests in Vuria are of more equal tree size, and the GC value ranges of the indigenous forests are 0.42–0.71, 0.20–0.74, and 0.17–0.76, respectively. The single-species plantations of cypress and pine showed lower values of GC than indigenous forests located in the same remnants in Yale, whereas Eucalyptus plantations showed GC values more similar to the indigenous forests. These results show the usefulness of GC maps for identifying and separating forest types as well as for assessing their distinctive ecologies.

Quantification of impact of lime on mechanical behaviour of lime cement blended mortars for bedding joints in masonry systems

In the case of blended lime-cement mortars used for bedding joints in masonry systems, substitution of cement with lime in the binder involves changes in strength and stiffness. However, extensive quantification and correlation of these changes in mechanical properties appear to be scarcely explored in existing literature. This work aims at providing a methodical experimental campaign, targeting 14 different lime-cement mixes with the quantity of lime in the binder varying from 10% to 75% (by volume), binder-aggregate (B/Ag) ratios – of 1:3, 1:4, 1:5 and 1:6 at 6 different curing ages from 7 to 180 days. Changes in compressive strength and flexural strength were expressed as functions (equations) of lime content in the binder, B/Ag ratio and curing age. Every 1% increment in the quantity of lime in the binder led to approximately 1.4% decrease in mechanical strength of the mortar with respect to the reference (10% lime in the binder). Furthermore, correlation(s) between ultrasound pulse velocity (UPV), density, compressive and flexural strength have also been explored. Compressive strength divided by flexural strength provided an almost constant value for all lime-cement compositions at all ages (ratio ∼ 3), decreasing as a function of B/Ag ratio in the mix. The work has been concluded with a discussion on trends in E-modulus (4–18 GPa) and open porosity (23%–27%) as a function of lime content in the binder of the mix and the age of the mortar.

Influence of Sampling Design Parameters on Biomass Predictions Derived from Airborne LiDAR Data

This study investigated the influence of sampling design parameters on biomass prediction accuracy obtained from airborne lidar data. A one-factor-at-a-time and a global sensitivity analyses were applied to identify the parameters most impacting model accuracy. We focused on several lidar and field survey parameters that can be easily controlled by users. In this pine plantations study site, a decrease in pulse density (4 to 0.5 pulse/m2) led to a small decrease in prediction accuracy (−3%). However, variability in the number of field plots, positioning accuracy, and plot size, significantly impacted model performance. To obtain a robust model, a minimum of 40 field plots, along with field plot position accuracy of 5 m or lower, and field plot radius exceeding 13 m are recommended. The minimum diameter at breast height (DBH) threshold and the choice of the allometric biomass equation were found to have lesser impacts on model accuracy. In addition, accuracies of DBH and tree height measurements were respectively shown to have a minor and negligible contribution to the prediction error. Significant field measurement costs will still be needed to ensure good-quality models for biomass mapping. However, by reducing pulse density, cost savings can be made on lidar acquisition.

A Low-Subharmonic, Full-Range, and Rapid Pulse Density Modulation Strategy for ZVS Full-Bridge Converters

The pulse-density-modulation (PDM) zero-voltage-switching (ZVS) full-bridge converter is a promising power converter for wireless power transfer (WPT) systems. The converter has the advantages of both direct conversion ratio control and load-independent soft switching. These advantages reduce the overall system complexity and power loss. However, the converter suffers from the limitations of large low-frequency subharmonics, a narrowed modulation range, and a large modulation delay. These limitations are caused by the existing PDM strategy, which was designed to generate a symmetric ZVS current to ensure the ideal ZVS for minimizing the switching loss. This paper finds that even with an asymmetric ZVS current, the ideal ZVS can still be ensured by the negative feedback effect of the dead-time voltage. Based on this finding, a PDM strategy that allows asymmetric ZVS currents is proposed to overcome the aforementioned limitations. In experiments, a ZVS full-bridge converter was modulated by the existing and the proposed PDM strategies, respectively. The converter performances and responses were compared and the results showed that the proposed PDM strategy can overcome the limitations while achieving ZVS. The proposed PDM strategy was also tested in a WPT system for verification.

Voltage Control of Intermittent Pulse Density Modulation for Two Battery-operated HEECS Choppers and its Application for Range Extension of Electric Vehicles

Voltage Control of Intermittent Pulse Density Modulation for Two Battery-operated HEECS Choppers and its Application for Range Extension of Electric Vehicles

Low-power ASIC suitable for miniaturized wireless EMG systems

Abstract Nowadays, the technology advancements of signal processing, low-voltage low-power circuits and miniaturized circuits have enabled the design of compact, battery-powered, high performance solutions for a wide range of, particularly, biomedical applications. Novel sensors for human biomedical signals are creating new opportunities for low weight wearable devices which allow continuous monitoring together with freedom of movement of the users. This paper presents the design and implementation of a novel miniaturized low-power sensor in integrated circuit (IC) form suitable for wireless electromyogram (EMG) systems. Signal inputs (electrodes) are connected to this application-specific integrated circuit (ASIC). The ASIC consists of several consecutive parts. Signals from electrodes are fed to an instrumentation amplifier (INA) with fixed gain of 50 and filtered by two filters (a low-pass and high-pass filter), which remove useless signals and noise with frequencies below 20 Hz and above 500 Hz. Then signal is amplified by a variable gain amplifier. The INA together with the reconfigurable amplifier provide overall gain of 50, 200, 500 or 1250. The amplified signal is then converted to pulse density modulated (PDM) signal using a 12-bit delta-sigma modulator. The ASIC is fabricated in TSMC0.18 mixed-signal CMOS technology.

An AC-AC High Frequency Pulse Density Modulated Full Bridge Series Resonant Converter for Induction Heating Application

<p class="Abstract">This paper deals with a new AC – AC single stage full bridge series resonant converter for induction heating application. The proposed system is simulated with high power density modulation technique to produce a output power of 1.8Kw.The main feature of the proposed converter is to reduce the total harmonic distortion produced in the output current for the switching frequency of 29Khz.Asymmetrical pulse density modulation technique is proposed to achieve maximum output power with less harmonic content. It is found that the THD of the proposed high power density modulated converter is 13.43%. Here, the validity of the proposed converter for domestic induction heating application is compared and summarized.</p>

Improved Pulse Density Modulation for Semi-bridgeless Active Rectifier in Inductive Power Transfer System

This paper proposes an improved pulse density modulation (IPDM) power control method for semi-bridgeless active rectifier (SBAR) in inductive power transfer (IPT) system. In this method, the input voltage pulse sequences of the SBAR can be distributed more evenly compared with the traditional PDM, so the output voltage ripple can be reduced effectively. Besides, for multiple paralleled pickups, the interleaved IPDM is developed to further suppress the output voltage ripple. In addition, the characteristics of zero-voltage switching and zero current-switching for the SBAR's switches can also be achieved in the proposed IPDM method by synchronization control for the gate signals of the SBARs. The experimental results on a 30 kW IPT prototype with four paralleled pickups verified the proposed method.

A comparative study on surface strengthening characterisation and residual stresses of dental alloys using laser shock peening

ABSTRACT In this work, Ni–Cr and Co–Cr alloys at untreated and treated conditions by laser are studied. Optical and scanning electron microscope, an X-ray diffractometer with an energy dispersive X-ray spectrometer, micro-hardness and digital surface roughness measuring testers are used to analyse the characterisation of both the unirradiated and irradiated specimens. The high surface micro-hardness is formed in the laser irradiated region of the sample during laser shock peening (LSP). In this case, high dislocation densities can produce. The LSP method involves with recoil pressure loading at the sample surface with minimum heating effects in the laser treated region. Hence in this work, the LSP of Ni–Cr and Co–Cr alloys is investigated to understand the influence of a power density, pulse density, pulse duration and irradiated laser spot diameter on the treated layer characteristics. The micro-hardness data is compared using ANOVA and post-hoc Tukey test. It is observed that the selected laser parameters have significant influence on the resulting surface characteristics. The experimental results of LSP with the WCM process show the microstructure considerably refined grains via arrays of dislocation and thereby impart a favourable compressive residual stress filed for improving mechanical properties and mastication strength.

Design of power control for ultrasonic cleaning systems based on hybrid PID/fuzzy methodology

The high-power ultrasonic cleaning system is used in the manufacturing of clean product contaminants. The pulse density modulation (PDM) switching strategy is more efficient for cleaning the rough surface of equipment. This paper proposes a novel pulse density modulation duty cycle stepping (PDMDCS) method that incorporates a proportional–integral–derivative controller and fuzzy logic for the mechanical resonance frequency tracking and PDM duty cycle stepping for ultrasonic cleaning system power control. The experimental results show that based on the same hardware architecture, the proposed PDMDCS method increases the power control range and resolution of power control without adding more hardware that compares to mechanical resonance frequency control method.

Method to Reduce the Bias on Digital Terrain Model and Canopy Height Model from LiDAR Data

Underestimation of LiDAR heights is widely known but has never been evaluated for several sensors and for diverse types of ecological conditions. This underestimation is mainly linked to the probability of the pulse to reach the ground and the top of vegetation. Main causes of this underestimation are pulse density, pattern of scan (sensors), scan angles, specific contract parameters (flying altitude, pulse repetition frequency) and characteristics of the territory (slopes, stand density and species composition). This study, carried out at a resolution of 1 × 1 m, first assessed the possibility of making an adjustment model to correct the bias of the digital terrain model (DTM), and then proposed a global adjustment model to correct the bias on the canopy height model (CHM). For this study, the bias of both DTM and CHM were calculated by subtracting two LiDAR datasets: high-density pixels with 21 pulses/m² (first return) and more (DTM or CHM reference value pixels) and low-density pixels (DTM or CHM value to correct). After preliminary analyses, it was concluded that the DTM did not need specific adjustment. In contrast, the CHM needed adjustments. Among the variables studied, three were selected for the final CHM adjustment model: the maximum height of the pixel (H2Corr); the density of first returns by m2 (D_first); and the standard deviation of nine maximum heights of the neighborhood cells (H_STD9). The modeling occurred in three steps. The first two steps enabled the determination of significant variables and the shape of the equation to be defined (linear mixed model and non-linear model). The third step made it possible to propose an empirical equation using a non-linear mixed model that can be applied to a 1 × 1 m CHM. The CHM underestimation correction could be used for a preliminary step to several uses of the CHM such as volume calculation, forest growth models or multi-temporal analysis.

Technical study on the production of blocks with composites of cement-wooden wastes from pallets of Pinus sp

A composite between particles obtained residual pallets of Pinus sp and cement was manufactured. Also, three methods were performance for wood particles: hot water washed, unwashed and unwashed with calcium carbonate (CaCO3). Different concrete-wood particle ratios were tested. Curing time was evaluated with ultrasonic pulse velocity (USV) and density of composites. Water absorption, module of rupture (MOR) in flexion, abrasion strength and resistance to decay by fungi attack were too determined. Results showed that using USV and density, curing occurs within 8 days. Moisture absorption ranges15 and 22% and decreases with decreasing of the amount of particles. MOR ranged from 0.40 a 1.23 MPa and the blocks with unwashed wooden particles and with CaCO3 addition had the greatest significant differences. In abrasion strength, blocks with unwashed wooden particles reported the best results. Finally, it was concluded that mixture with proportion of 60:40 (concrete:wood) and CaCO3 had the best performance and were not attacked by fungi decay.

Residual stress, phase, microstructure and mechanical property studies of ultrafine bainitic steel through laser shock peening

The aimed study proposes laser shock peening without a coating of high strength ultrafine bainitic steel to mitigating the fatigue failures for automotive and structural engineering applications. Laser pulse density of 2500 pulses/cm2 (75% overlapping) was optimised based on the induced residual stresses for employing the wide range of characterisations. The roughness and topographic results showed that surface roughening was controlled by tuning the laser pulse density. The High-Resolution X-ray Diffraction analysis confirmed the lattice misorientation resulting peak shift and the trend towards martensite phase transformations. The electron microscopic micro/nanostructure analyses revealed the grain refinement features such as nano-twins, micro shear bands and shear cells. The work hardening depth analysis indicates the significant enhancement in the mechanical properties. Completely reversed (R = −1) high-cycle fatigue tests extended the lifespan by an average of five times than the untreated. Also, it has potential to repair the structural components effectively.

Investigation of the performance of a pilot-scale barrel atmospheric plasma system for plasma activation of polymer particles

This study reports the development and performance of a pilot-scale barrel atmospheric plasma reactor for the atmospheric plasma activation treatment of polymer particles. The polymer particles treated included acrylonitrile butadiene styrene (ABS) and polypropylene (PP). These particles had diameters in the range of 3–5 mm. The initial studies were carried out using a laboratory-scale barrel reactor designed to treat polymer particle batch sizes of 20 g. A pilot-scale reactor that could treat 500 g particle batch sizes was then developed to facilitate pre-industrial-scale treatments. The effect of operating pulse density modulation (PDM) in the range 10%–100% and plasma treatment time on the level of activation of the treated polymers were then investigated. ABS revealed a larger decrease in water contact angle compared with PP after plasma treatment under the same conditions. The optimal treatment time of ABS (400 g of polymer particles) in the pilot-scale reactor was 15 min. The plasma-activated polymer particles were used to fabricate dog-bone polymer parts through injection molding. Mechanical testing of the resulting dog-bone polymer parts revealed a 10.5% increase in tensile strength compared with those fabricated using non-activated polymer particles.

The GEDI Simulator: A Large-Footprint Waveform Lidar Simulator for Calibration and Validation of Spaceborne Missions.

NASA's Global Ecosystem Dynamics Investigation (GEDI) is a spaceborne lidar mission which will produce near global (51.6°S to 51.6°N) maps of forest structure and above‐ground biomass density during its 2‐year mission. GEDI uses a waveform simulator for calibration of algorithms and assessing mission accuracy. This paper implements a waveform simulator, using the method proposed in Blair and Hofton (1999; https://doi.org/10.1029/1999GL010484), and builds upon that work by adding instrument noise and by validating simulated waveforms across a range of forest types, airborne laser scanning (ALS) instruments, and survey configurations. The simulator was validated by comparing waveform metrics derived from simulated waveforms against those derived from observed large‐footprint, full‐waveform lidar data from NASA's airborne Land, Vegetation, and Ice Sensor (LVIS). The simulator was found to produce waveform metrics with a mean bias of less than 0.22 m and a root‐mean‐square error of less than 5.7 m, as long as the ALS data had sufficient pulse density. The minimum pulse density required depended upon the instrument. Measurement errors due to instrument noise predicted by the simulator were within 1.5 m of those from observed waveforms and 70–85% of variance in measurement error was explained. Changing the ALS survey configuration had no significant impact on simulated metrics, suggesting that the ALS pulse density is a sufficient metric of simulator accuracy across the range of conditions and instruments tested. These results give confidence in the use of the simulator for the pre‐launch calibration and performance assessment of the GEDI mission.

Crispness, speech intelligibility, and coloration of reverberant recordings played back in another reverberant room (Room-In-Room).

This work examines the acoustical and perceptual consequences that can be found in a transfer chain consisting of a sound recorded in one room which is played back over a loudspeaker in another room. The total resulting "Room-In-Room" (RinR) response can be modelled as a convolution of the Room Impulse Response of the first and second room. Due to the convolution an increase in the reverberation time, pulse density, and a change of the temporal envelope of the early reflections can be observed, compared to a single room. In the spectral domain, the convolution results in an increase in spectral modulation strength, responsible for coloration. The listening test investigating the perceptual consequences of RinR found a decrease in perceived crispness due to reproduction in a playback room, especially for highly reverberant conditions. When within normal sized rooms the reverberation time and total source-receiver distance were kept constant, RinR and a single room condition showed no reduction in crispness. On the other hand, a strong increase in the perceived coloration was measured. Furthermore, a decrease in speech intelligibility has been found for RinR conditions, compared to single rooms (Speech Reception Threshold of 2-3 dB).

Optimizing the Remote Detection of Tropical Rainforest Structure with Airborne Lidar: Leaf Area Profile Sensitivity to Pulse Density and Spatial Sampling

Airborne Laser Scanning (ALS) has been considered as a primary source to model the structure and function of a forest canopy through the indicators leaf area index (LAI) and vertical canopy profiles of leaf area density (LAD). However, little is known about the effects of the laser pulse density and the grain size (horizontal binning resolution) of the laser point cloud on the estimation of LAD profiles and their associated LAIs. Our objective was to determine the optimal values for reliable and stable estimates of LAD profiles from ALS data obtained over a dense tropical forest. Profiles were compared using three methods: Destructive field sampling, Portable Canopy profiling Lidar (PCL) and ALS. Stable LAD profiles from ALS, concordant with the other two analytical methods, were obtained when the grain size was less than 10 m and pulse density was high (>15 pulses m−2). Lower pulse densities also provided stable and reliable LAD profiles when using an appropriate adjustment (coefficient K). We also discuss how LAD profiles might be corrected throughout the landscape when using ALS surveys of lower density, by calibrating with LAI measurements in the field or from PCL. Appropriate choices of grain size, pulse density and K provide reliable estimates of LAD and associated tree plot demography and biomass in dense forest ecosystems.

Pulse Density Modulated ZVS Full-Bridge Converters for Wireless Power Transfer Systems

Pulse density modulation (PDM) is an advanced technique for maximum efficiency point tracking of wireless power transfer (WPT) systems. By using PDM, both voltage regulation and efficiency maximization can be achieved without dc/dc converters. PDM is also compatible with the dual-side soft switching technique that utilizes resonant tanks and synchronous rectification. However, this soft switching technique depends on coupling and load conditions. Hard switching may occur when the coupling of coils gets stronger or the equivalent load is not properly controlled. To eliminate the dependence and ensure the soft switching under various operating conditions, this paper proposes a PDM zero-voltage-switching (ZVS) full-bridge converter for WPT systems. The converter employs a ZVS branch between switching nodes to provide a ZVS current, and uses a specially designed modulator to obtain the valid ZVS current waveforms. Experimental results verified the proposed operating principles and showed that the additional power loss caused by the ZVS current is insignificant. The overall efficiency of the WPT prototype was 93 ∼ 73% when the power transfer distance was 0.1 ∼ 0.4 m, among which up to 85% efficiency was observed when the distance equaled the coil diameter.

Atmospheric Plasma-Assisted Ammonia Synthesis Enhanced via Synergistic Catalytic Absorption

Plasma-assisted ammonia synthesis has been one of the promising alternatives for building a sustainable ammonia production infrastructure. This study improved this process by introducing an in situ catalytic absorption mechanism using magnesium chloride. Other than its catalytic functions, the absorption mechanism involved two pathways of forming Mg3N2 and Mg(NH3)6Cl2. Meanwhile, the pulse density modulation (PDM) was introduced to improve the energy performance of the system. The series of efforts improved the plasma energy efficiency of the system and achieved the highest value of 20.5 g/kwh.

Airborne laser scanning for terrain modeling in the Amazon forest

ABSTRACT Very few studies have been devoted to understanding the digital terrain model (DTM) creation for Amazon forests. DTM has a special and important role when airborne laser scanning is used to estimate vegetation biomass. We examined the influence of pulse density, spatial resolution, filter algorithms, vegetation density and slope on the DTM quality. Three Amazonian forested areas were surveyed with airborne laser scanning, and each original point cloud was reduced targeting to 20, 15, 10, 8, 6, 4, 2, 1, 0.75, 0.5 and 0.25 pulses per square meter based on a random resampling process. The DTM from resampled clouds was compared with the reference DTM produced from the original LiDAR data by calculating the deviation pixel by pixel and summarizing it through the root mean square error (RMSE). The DTM from resampled clouds were also evaluated considering the level of agreement with the reference DTM. Our study showed a clear trade-off between the return density and the horizontal resolution. Higher forest canopy density demanded higher return density or lower DTM resolution.