Pulse Modulation Technique Research Articles

Application of Bragg grating sensors in the characterization of scaled marine vehicle models

Experiments using two different fiber-optic Bragg grating strain sensor systems to measure dynamic loads, slamming forces, and bending moments on a scaled catamaran model are reported. The system designed to measure slamming forces is based on dynamic locking of the laser diode wavelength to the peak wavelength of a Bragg grating mounted in a force cell. The sensor system has a dynamic range of 27 dB (1-550 N) and a bandwidth in excess of 1 kHz. For the measurement of bending moments, a time-domain multiplexing system, which used a unique pulsed rf modulation technique, of two Bragg grating sensors mounted on a steel beam was designed. The dynamic range of this system was also 27 dB [17 Nm (Newton meter) with a 0.03-Nm resolution]. The bandwidth was limited to approximately 50 Hz.

New pulse-modulation technique for guidance and control of automated spacecraft

A comprehensive approach to problems such as automated on-orbit rendezvous and soft landing on a planetary surface is presented for spacecraft employing pulse-operated (on-off) propulsion systems. Using a technique derived from robust control theory, a new class of guidance algorithms that modulate the duration and frequency of thruster firings is developed. These algorithms allow analytical characterization of transient errors, limit cycle deadband, and the set of possible terminal conditions in the design process, without the use of dynamical approximations such as linearization. With this approach, the desired performance is ensured in the presence of dynamical modeling errors with known bounds; the effects of navigational errors can be minimized to the extent that they can be bounded. A realistic application is illustrated via computer simulation of a hypothetical mission scenario, in which a robotic spacecraft equipped with an aided-inertial guidance system soft lands on the planet Mars. N missions carried out with robotic spacecraft, self-contained guidance and control are often desirable and in some cases a necessity. Although a wealth of theory exists for applications such as automated orbital transfer, rendezvous, station keeping, and soft landing, among others, much of this knowledge base presumes the use of continuous throttleable or fixed-thrust propulsion, whereas in practice many spacecraft are equipped with on-off gas jets or thrusters because of their relative simplicity and low cost. This paper describes a new technique for pulse-modulated operation of these devices intended for use in the aforementioned problems. A representative , but not exhaustive, review of previous work emphasizing mission applications is as follows. Much of the ren- dezvous literature is devoted to the determination of optimal trajec- tories and their associated thrust profiles. Both theoretical and op- erational aspects of rendezvous are discussed in the survey papers by Jezewski et al.1 and Leonard and Bergmann.2 Parten and Mayer3 and Young and Alexander4 describe the rendezvous and docking procedures developed for the Gemini and Apollo piloted missions, respectively; these procedures have also been used in Space Shut- tle missions. Automated rendezvous schemes have been proposed for the Space Shuttle5'6 but not implemented. Previous missions in- volving soft landing have all been accomplished with throttleable propulsion systems. The early work leading to the guidance algo- rithms used in the Apollo missions was performed by Cherry7 and Battin.8 The operational Apollo lunar module guidance system is described by Klumpp.9 The automated guidance schemes used by the Surveyor and Viking spacecraft were similar: the terminal de- scent system developed for the Surveyor lunar lander is described by Cheng et al.,10 and Ingoldby11 discusses the corresponding system for the Viking Mars landers. Another prevalent feature of the literature is the use of linear approximations of the spacecraft's dynamics to obtain an analyt- ically tractable problem. Given the nonlinear behavior of on-off thrusters, the closed-loop dynamics of systems employing these

Inhibition by ethoxyzolamide of a photoacoustic uptake signal in leaves: Evidence for carbonic anhydrase catalyzed CO2-solubilisation

A photoacoustic pulse-modulation technique is applied for the study of a CO2-stimulated gas uptake signal in leaves (Reising and Schreiber, Photosynthe Res 31: 227-238, 1992). It is shown that this uptake signal can be substantially suppressed by application of the carbonic anhydrase inhibitor, ethoxyzolamide, to leaf discs. This inhibitor does not affect the O2-evolution signal in air or the chlorophyll fluorescence induction pattern at high CO2, when non-saturating light intensities are used. On the basis of these findings it is concluded that at least a major part of the CO2-stimulated photoacoustic uptake signal results from light-modulated CO2-solubilisation catalysed by carbonic anhydrase. Modulated CO2-solubilisation appears likely to be induced by light driven H(+)-translocation from the stroma into the thylakoid lumen. Comparison of the induction patterns of chlorophyll fluorescence quenching and the uptake signal suggests a correlation between membrane energisation and CO2-uptake. The importance of O2-dependent electron flow as a major cause of membrane energisation is discussed. It is proposed that in the absence of CO2 the combination of Mehler- and ascorbate peroxidase reactions does not result in a photobaric signal, as O2-uptake and O2-evolution components cancel each other. Two main conclusions, which are of considerable importance for future practical applications of the photoacoustic method, are drawn from these findings: (1) When high CO2 is applied to leaves, the photobaric uptake component may provide a unique means of monitoring the function of stromal carbonic anhydrase in vivo. (2) Brief flushing of the photoacoustic cell with air may prevent the occurrence of an uptake signal, thus allowing a straight-forward deconvolution into photothermal and O2-evolution components.

A generalized analysis of binary halftone representation of images

A halftone representation of continuous tone images is a useful method for displaying images on devices that can display only two gray levels, “black” and “white”. We present a detailed and generalized analysis of a binary representation of continuous tone functions, develop analytic expressions of the Fourier spectrum, and discuss the reconstruction error. The discussion makes distinction between spatially continuous and discrete functions. This analysis is applicable to pulse modulation techniques such as pulse width (PWM), pulse density (PDM) or pulse position (PPM) modulation, which are generated in all types of dither and carrier procedures.

Studies of long-pulse responses in four-wave mixing using a step-modulated probe

An experimental method for examining temporal responses of nonlinear materials has been studied using four-wave mixing with pulses having widths on the order of 100 ns. By using a special probe pulse modulation technique, fast resonant nonlinear mechanisms can be detected in the presence of dominant thermal mechanisms. In addition, the relative strengths of the fast and slow nonlinearities as they appear in the experiment can be estimated by comparing the experimental results to those of a simple theoretical model. Results for several known materials are presented that demonstrate the method and its sensitivity for detecting fast mechanisms. Methods of increasing the sensitivity are discussed. >

Review of resonant link topologies for inverters

Resonant link inverter configurations have been proposed as a means of obtaining significantly lower switching losses, thereby allowing operating frequencies and switch power ratings to be raised. In the paper, resonant link topologies are analysed by means of time-domain simulation, and the frequency spectrum typical of a resonant link inverter is compared with one typical of a hard-switched inverter. The resonant link system results in an increase in circuit complexity, and the frequency spectrum is limited by the need to employ integral pulse-modulation techniques.

Limitations of the Pulse-Modulated Technique for Measuring the Fluorescence Characteristics of Algae

Precise measurements of the minimal fluorescence yield (F(o)) and maximal fluorescence yield (F(m)) of a dark-adapted sample are prerequisites for the quantification of other fluorescence parameters. The pulse amplitude-modulated chlorophyll fluorometer (PAM 101 Chlorophyll Fluorometer, Heinz Walz, Effeltrich, Germany) and saturating pulse technique have frequently been used in measuring F(o) and F(m) and in resolving the contributions of photochemical and nonphotochemical quenching to the total fluorescence yield. The extent to which instrument-dependent factors may affect the accurate measurement of F(o) and F(m) is addressed. It is shown that the increase in pulse amplitude-modulated measuring beam intensity at 1.6 and 100 kHz was nonlinear at higher light intensity settings. The implications of this for measurements of F(o) (1.6 kHz) and F(m) (100 kHz) are discussed. It is also demonstrated that underestimation of F(m) may result due to saturation of the PAM 101 photodiode by scattered infrared light associated with intense light pulses. In addition, it is shown how sample-dependent factors may affect measurements of F(o) and F(m) in samples with low chlorophyll concentrations, in particular, dilute algal suspensions of Phaeodactylum tricornutum and Chlamydomonas reinhardtii. A technique is presented for the accurate measurement of F(o) in algal suspensions (<8 mug chlorophyll a mL(-1)). The importance of examining the saturating pulse transient and F(m) level as a function of the damping setting, pulse width, and pulse intensity, and in the presence of 3-(3,4-dichlorophenyl)-1, 1-dimethylurea is discussed.

Comparison of sinusoidal and pulse modulation techniques for generating optical frequency reference combs

The optical frequency combs generated by pulse and sinusoidal modulation of an LiNbO3 Mach–Zehnder device arecompared andcontrasted. It is shown that the former operates with lower drive, although being restricted to narrower tooth spacings than offered by the sinusoidal technique.

Reduction State of Q and Nonradiative Energy Dissipation during Photosynthesis in Leaves of a Crassulacean Acid Metabolism Plant, Kalanchoë daigremontiana Hamet et Perr

Fluorescence was measured in leaves of the CAM plant Kalanchoë daigremontiana using a pulse modulation technique at room temperature. During a 12-h light period at 500 micromole photons per square meter per second (400-700 nanometers) in air containing 350 microbar CO(2), the component of fluorescence quenching related to the reduction state of Q, the primary electron transport acceptor of PSII, remained fairly constant and showed that only 20% of Q were in the reduced form. The reduction state was slightly increased at the onset and at the end of the light period. By contrast, the nonphotochemical component of fluorescence quenching which is a measure of the fraction of nonradiative deexcitation underwent marked diurnal changes. Nonradiative energy conversion was low during the phase of most active malic acid decarboxylation in the middle of the light period when uptake of atmospheric CO(2) was negligible, and when internal CO(2) partial pressures were higher than in air; this allowed for high rates of CO(2) reduction in the chloroplasts. Nonradiative energy conversion was high during the early and the late light period when atmospheric CO(2) was taken up and internal CO(2) partial pressures were below air level. Manipulation of the internal CO(2) partial pressure during the late light period by increasing or decreasing the external CO(2) partial pressure to 1710 and 105 microbar, respectively, led to changes in the magnitude of energy dependent fluorescence quenching which were consistent with the relationship between nonradiative energy dissipation and internal CO(2) partial pressure observed during the diurnal cycle. Again, the reduction state of Q was hardly affected by these treatments. Thus, changes in electron transport rate during the diurnal CAM cycle at a given photon flux density lead primarily to alterations in the rate of nonradiative energy dissipation, with the reduction state of Q being maintained at a relatively low and constant level. Conditions are described under which nonphotochemical dissipation of excitation energy reaches a maximum value and the reduction state of Q is increased.

Measurement of microwave field strength in ESR by a pulsed modulation technique

It is shown that conventional ESR equipment can be used to measure the microwave field strength B, in the cavity. The method requires no calibration procedure, but yields B 1 values directly in a wide range of microwave power levels. It is based on a pulsed modulation technique and dc signal detection.

Advanced sounding: 1. The FMCW alternative

HF radar employing linear frequency‐modulated continuous wave or “chirp” modulation provides an alternative to the more conventional pulsed amplitude modulation technique in advanced ionosphere sounding. An analysis is presented which shows that the phases of the discrete Fourier components of the receiver output can be used in the same fashion as the phases of pulsed, fixed frequency ionospheric echoes to evaluate the standard digital parameters, including group path by the method of stationary phase. This can be used to overcome the group path/Doppler velocity ambiguity of linear frequency modulation. A description is given of a sounding structure from which unambiguous group range, angle of arrival, O/X mode, and Doppler velocity can be evaluated. A simple error analysis is also given.

Modulation Schemes in Low-Cost Microwave Field Sensors

The use of modulation schemes in short-range microwave field sensors are discussed and in particular the ideas are applied to low-cost self-detecting doppler sensors based on a two-terminal negative resistance diode oscillator which acts as a load variation detector. FM-CW and pulse modulation techniques are described which can lead to improved performance in these sensors, and experimental results of bias modulation, varactor modulation, and pulsed operation of transferred-electron oscillators are presented. Modulation techniques can be applied in order to eliminate false alarms due to unwanted targets by enabling the doppler sensor to measure or discriminate target range, and the potential merits of different schemes based on frequency modulation with linear and sinusoidal modulation patterns are explored. A novel environmental profiling technique is proposed which exploits the response of frequency-modulated sensors to multiple stationary targets for use in a versatile short-range surveillance system employing digital processing techniques. The mechanism of pulsed operation in self-detecting sensors is described, and the effects of temperature variations in the active device, which result in frequency chirp, are considered. The technique of extracting the doppler signal from the pulsed bias current using a pulse cancellation circuit is demonstrated, and the effects of bias current changes during the pulse (due to temperature variations) on the use of this type of circuit are discussed.

Exciton Absorption in GaSe under Intense Excitation

High excitation absorption was investigated in GaSe at 2 K and 77 K, using a N 2 laser. Change in absorption coefficient was measured directly by a pulse-modulation technique. A structure was found at an excitonic transition around 2.1 eV. The structure shows peak shift and broadening of an exciton absorption line. Shift and broadening are due to an elastic collision between two excitons. Two kinds of induced absorption were also observed. One is possibly ascribed to exciton excitation associated with collision with the second exciton and the other to excitation of a trapped electron to a conduction band.

Miniaturization of Thyristorized Power Supplies Using Pulse Modulation Techniques

In case of data handling and data transmission system, the reduction in size of the equipment to consume less customer floor space is as important as the low cost and high efficiency. Although the use of thin film and integrated circuits has sufficiently reduced the size of logic portions of the circuitry yet the miniaturization of dc power supplies is also sometimes necessary when commercial ac power source is used.This paper discusses the circuitry and possible application of a miniature power supply using pulse modulation technique.

Analysis of a DC Power Supply to Regulate on an Open-Loop Basis

Telephone equipment being used for data transmission, and housed on telephone customer premises, requires miniature ac to dc power supplies. The miniaturization of a dc power supply associated with this equipment has been facilitated by the use of pulse modulation techniques. An analysis is presented of an open-loop regulated dc power supply operating from a commercial ac source. The analysis illustrates how open-loop regulation has been achieved through the use of pulse modulation techniques. The pulse modulation is achieved with a parallel inverter operating into a 2-core transformer at a 20-kHz switching rate. The analysis includes the cases where 1) under ideal conditions there is no error in the pulse duration, 2) the error in pulse duration is a constant finite amount, and 3) pulse duration error varies on a periodic basis. Both calculated and measured results are presented based on tests made on a breadboard model of the open-loop inverter regulator.

A New Pulse Modulator for Accurate DC Amplification with Linear or Nonlinear Devices

A new pulse-modulation technique and modulator circuit has been developed, offering several advantages over conventional methods. The technique and circuit are applicable in a broad range of control system and data transmission problems. Use of this method is appropriately considered in any application where dc or low-frequency information is to be amplified or transduced. In addition to both an intrinsically high degree of accuracy and the numerous potential advantages of sampled-data and digital techniques provided by use of this modulator, the modulator offers a solution to the bandwidth problems encountered in conventional pulse modulation methods. Pulse width modulation and pulse frequency modulation both require unrealizable infinite device bandwidth in order to obtain full control of the pulse train duty ratio between zero and unity. The modulator described in this article requires only a finite device bandwidth to obtain full control. The logical development of the modulator principles, based upon the requirements of limited bandwidth and good accuracy, are presented in this article. The mechanization of the modulator circuit in transistorized form is given, with the most accurate configuration discussed at length. Test results of the circuit are summarized. Application of the modulator is discussed, including both theory related to the application, such as modulator dynamic response characteristics and pulse demodulation, and specific applications in space vehicle autopilots, dc amplifiers and servo systems.