First-order Compensator Research Articles

Simultaneous use of polarization microscopy and dark field techniques in the study of microorganisms.

Simultaneous application of polarization microscopy and dark field techniques has been used to study the internal structure of microbial cells. The dark field technique displays subtle cell structures like glowing objects on a dark background. In the polarizing microscope, cross polarizing filters along with the first-order quartz compensator and a rotary table show the maximum birefringence of the individual structures. The material containing microorganisms was collected in the villages of Sýkořice and Zbečno (Křivoklátsko Protected Landscape Area). The objects were studied in a laboratory microscope Carl Zeiss Jena type NfpK equipped with In Ph 160 basic body with variable dark field, special condenser with interchangeable diaphragm apertures, a rotary table, Meopta Praha polarizer, analyzer, first-order quartz compensator from LOMO Sankt Petersburg, and a digital Nikon D 70 DSLR camera. Three orders of microorganisms were studied: Siphonocladales, Chlorococcales, and Peritricha. Anisotropic structures in different amounts and sizes (e.g., granules and microfibrils) or in different configurations (e.g., cell walls or pellicle) have been found in all Protista organisms under study. Filamentous algae of the genus Cladophora (Cladophoraceae, Siphonocladales, Ulvophyceae) featured a strongly birefringent cell wall (shape birefringence) surrounded by less birefringent periphyton (microbial biofilm), at the edges of which cyanobacterial fibers could be recognized-a very important finding. The coccal algae of the genus Scenedesmus (Scenedesmataceae, Chlorococcales, Chlorophyceae) exhibited not only strongly birefringent granules, but also strongly birefringent microfibrils in the cytoplasm outside the strongly birefringent cell walls-very important finding. Of all the studied microorganisms, the weakest birefringence was shown in the surface membrane (pellicle) of the Vorticella (Vorticellidae, Peritricha, Ciliata). On the other hand, the ring of cilia on the top of the body had a somewhat stronger birefringence-an important finding. In conclusion, the dark field technique provides a high contrast image in the microscope and, if supplemented simultaneously by polarization microscopy, will allow us to partially infer the composition of the examined structures.

Simultaneous use of Interphako interference contrast and polarization microscopy in the study of microorganisms.

Simultaneous application of polarization microscopy and Interphako interference contrast has been used to study the internal structure of algal cells. The interference contrast technique showed fine cell structures (important is the selection of interference colors according to the Mach-Zehnder interferometer setting). In a polarization microscope, the crossed polarization filters together with the first-order quartz compensator mounted turntable showed the maximum birefringence of the individual structures. Material containing green algae was collected in the villages Sýkořice and Zbečno, Protected Landscape Area (PLA) Křivoklátsko. The objects were studied in a Carl Zeiss Jena NfpK laboratory microscope equipped with an In 160 base body with an Interphako In contrast interference module including a Mach-Zehnder interferometer with variable phase contrast, a special condenser with interchangeable aperture plates, a turntable, a Meopta Praha polarizer, a LOMO Sankt Petersburg analyzer, and a quartz compensator with first-order red and the digital camera DSLR Nikon D 70. Green algae of three orders were studied: Siphonocladales, Zygnematales, and Desmidiales. Anisotropic structures were found in all studied representatives of the green algae of the phylum Chlorophyta. Especially their cell walls showed strong birefringence (in all representatives of these orders). On the other hand, a representative of the order Siphonocladales (the genus Cladophora, Cladophoraceae, Ulvophyceae) was rarely found to display weak birefringent granules of storage substances due to the setting of the Mach-Zehnder interferometer and the use of the first-order compensator (interference colors are intensified). In addition, a very weak birefringence of periphyton cells (microbial biofilm) was found. In the study of the second algae of the genus Spirogyra (Zygnemataceae, Zygnematales, Conjugatophyceae), a strongly birefringent connecting wall between algal cells was found in contrast to the weaker birefringence of the peripheral wall. It was the use of Interphako interference contrast together with polarization filters and a first-order quartz compensator that particularly emphasized the central part of the connecting wall. In the study of the twinned Pleurotaenium algae (Desmidiaceae, Desmidiales, Conjugatophyceae), a strongly birefringent wall was found along the periphery of the cell with a nucleus in the middle part (isthmus). In this narrowing in the center of the cell, a sharply delimited birefringent edge of the cell wall is visible, especially when using Interphako interference contrast along with crossed polarization filters and a first-order quartz compensator. In conclusion, Interphako interference contrast provides a high degree of image contrast in a microscope and, if suitably simultaneously complemented by polarization microscopy (including a first-order quartz compensator), it will allow us to infer some of the composition of the investigated structures. However, working with Interphako interference contrast is considerably more difficult (setting Mach-Zehnder interferometer) than using other contrast techniques (positive and negative phase contrast, color contrast, relief contrast, and dark field).

Concomitant use of polarization and negative phase contrast microscopy for the study of microorganisms.

A simultaneous application of negative phase contrast and polarization microscopy was used to study the internal structure of microbial cells. Negative phase contrast allowed us to display the fine cell structures with a refractive index of light approaching that of the environment, e.g., the cytoplasm, and converted an invisible phase image to a visible amplitude one. In the polarizing microscope, cross-polarizing filters, together with first-order quartz compensator and a turntable, showed maximum birefringence of individual structures. Material containing algae was collected in ponds in the villages Sýkořice and Zbečno (Protected Landscape Area Křivoklátsko). Objects were studied in a laboratory microscope (Carl Zeiss Jena, type NfpK), equipped with a basic body In Ph 160 with an exchangeable module Ph, LOMO St. Petersburg turntable mounted on a centering holder of our own construction and a Nikon D 70 digital SLR camera. Anisotropic granules were found only in the members of two orders of algae (Euglenales, Euglenophyceae and Chlorococcales, Chlorophyceae). They always showed strong birefringence and differed in both number and size. An important finding concerned thin pellicles in genus Euglena (Euglenales, Euglenophyceae) which exhibited weak birefringence. In genus Pediastrum (Chlorococcales, Chlorophyceae), these granules were found only in living coenobium cells. In contrast, dead coenobium cells contained many granules without birefringence-an important finding. Another important finding included birefringent lamellar structure of the transverse cell wall and weak birefringence of pyrenoids in filamentous algae of genus Spirogyra (Zygnematales, Conjugatophyceae). It was clearly displayed by the negative phase contrast and has not been documented by other methods. This method can also record the very weak birefringence of the frustule of a diatom of genus Pinnularia (Naviculales, Bacillariophyceae), which was further reinforced by the use of quartz compensator-an important finding. Simultaneous use of negative phase contrast and polarization microscopy allowed us to study not only birefringent granules of storage substances in microorganisms, but also the individual lamellae of the cell walls of filamentous algae and very thin frustule walls in diatoms. These can be visualized only by this contrast method, which provides a higher resolution (subjective opinion only) than other methods such as positive phase contrast or relief contrast.

Trajectory tracking control for robot manipulators using only position measurements

ABSTRACTIn the paper, the trajectory tracking control problem is investigated for robotic manipulators which are not equipped with the tachometers. Our contribution consists in establishing uniform asymptotic stability in closed-loop system by using the dynamic position-feedback controller with feedforward. Using Lyapunov vector function and comparison principle, we construct the non-linear controller with variable gain matrices and first-order linear dynamic compensator such that the origin of the closed-loop system is uniformly asymptotically stable. The controller is shown to be robust with respect to parameters incertainties. We illustrate the utility of our result by simulation tests with reference to a two-link planar elbow robot manipulator.

Discrete‐time speed servo system design – a comparative study: proportional–integral versus integral sliding mode control

This work presents a comparative study of two approaches for design of high performance discrete-time velocity servo systems with induction motors (IMs). The first approach uses the conventional proportional–integral controller with one-step delayed disturbance compensator (OSDC). The OSDC is based on measurements of the control input and plant states. The second approach uses the discrete-time integral sliding mode (DISM) controller with a first-order disturbance compensator, which is based on switching function signal measurement only. The controlled plant, consisting of IM in the commonly used indirect field-oriented control (IFOC) scheme, is approximated by first-order dynamic model for control design purpose. Comparisons are conducted through simulations considering an ideal case (without unmodelled dynamics) and a regular one, using MATLAB/Simulink models of real IFOC structure elements. The experimental investigation is conducted as well. The performed tests show that both controllers enhanced with the corresponding disturbance compensators are suitable for high-performance control systems design if speed measurement is near noise-free case, with better performance from DISM. If Euler backward difference is applied for speed detection, both control systems need some retuning in order to make a compromise between chattering and tracking accuracy.

Hybrid Positive Feedback Control for Active Vibration Attenuation of Flexible Structures

A new controller is introduced in this paper as a novel method for active vibration suppression in flexible structures. The hybrid positive feedback (HPF) uses a second- and a first-order compensator that are fed by the displacement and velocity feedbacks, respectively. Parallel pairs of the HPF controller are implemented when suppression in multimode condition is issued. Since the controller uses two gains for each pair of the actuator/sensor patch for each mode, a suitable gain optimization method has to be used to ensure the optimum performance. To this end, ${\cal H}_2$ and ${\cal H}_{\infty}$ optimization approaches are utilized. For validation purposes, the controller is verified numerically and experimentally for vibration control of a cantilever beam. System identification is performed, then closed-loop system responses to simultaneous multimode and swept frequency excitations are obtained. According to the results, the HPF controller has a superior performance compared to the conventional method of positive position feedback. Vibration displacement amplitudes were reduced by more than 85% relative to the uncontrolled state. The best performance was achieved by the ${\cal H}_2$ -optimized HPF, as the net value for vibration displacement amplitude reduction in the multimode condition was 90% of the uncontrolled amplitude.

English

Lag-lead compensators are well known in automatic control engineering. They have 3 or 4 parameters to be adjusted (tuned) for proper operation depending on the compensator order. The frequency response of the control system or the root locus plot are traditionally used to tune the compensator in a lengthy procedure. A selected very slow second-order process has a time response to a unit step input of 150 seconds settling time is controlled using a first-order lag-lead compensator (through simulation). The lag-lead compensator is tuned by minimizing the sum of square of errors of the control system using MATLAB. Four functional constrains are used to control the performance of the lag-lead compensated control system. The result was reducing the settling time to only 0.666 seconds. The steady-state characteristics of closed-loop control system using the first-order lag-lead compensator were excellent. It was possible reduce the steady-state error to less than 0.01 using the tuned first-order lag-lead compensator.

Application of colour metallography in the examination of ancient metals

The use of colour in the metallographic examination of ancient metals is useful both for research and teaching purposes. Application of different optical methods of examination include: polarized light, with or without a first-order red compensator, and differential interference contrast microscopy (DIC). The use of DIC can be combined with the other techniques mentioned in the text for the examination of etched or unetched metal surfaces, which are best observed microscopically using a metallograph. Colour techniques are now used routinely in industrial metallography, and are available for a variety of metals and alloys. Colour tint etching can be used to enhance grain structure or different phases present in the metallic artefact which may not be visible in conventional etched samples, or which lack optical contrast. Many more applications of these techniques in the examination of ancient metals will be published in the literature as they become better known and second-hand equipment with DIC lenses enter the market at a price suitable for conservation laboratories to purchase them.

Design of the robust synchronous generator stator voltage regulator based on the interval plant model

In this paper a novel method for the stator voltage regulator of a synchronous generator based on the interval plant mode, is presented. Namely, it is shown in the literature that, in order to design a controller for the first-order compensator, the limited number of interval plants needs to be examined. Consequently, the intervals of the plant model parameter variations used to calculate the four extreme interval plants required for the sequential PI controller design are determined. The controller is designed using frequency-domain-based techniques, while its robust performance is examined using simulation tests.

First-order polarization mode dispersion compensator using two independent feedback signals enabling separation of principal states of polarization and differential group delay controls

We propose and demonstrate a novel first-order polarization mode dispersion (PMD) compensator separating principal-states-of-polarization (PSP) control from differential-group-delay (DGD) control by using two independent feedback monitoring signals. To verify the proposed operating principle, we fabricated an automatically adaptive module-type 40-Gb/s PMD compensator on printed circuit boards. The results show that unlike previous typical compensation method of controlling alternately PSP and DGD, the proposed scheme provides independent continuous PSP tracking regardless of DGD control by delay line under rapidly varying PSP condition, thus resulting in stable compensated signal with reduction in compensation time.

Acceleration feedback control of human-induced floor vibrations

Active vibration control (AVC) via a proof-mass actuator is considered to be a suitable technique for the mitigation of vibrations caused by human motions in floor structures. It has been observed that actuator dynamics strongly influence structure dynamics despite considering collocated actuator/sensor control. The well-known property of the interlacing of poles and zeros of a collocated control system is no longer accomplished. Therefore, velocity-based feedback control, which has been previously used by other researchers, might not be a good solution. This work presents a design process for a control scheme based on acceleration feedback control with a phase-lag compensator, which will generally be different from an integrator circuit. This first-order compensator is applied to the output (acceleration) in such a way that the relative stability and potential damping to be introduced are significantly increased accounting for the interaction between floor and actuator dynamics. Additionally, a high-pass filter designed to avoid stroke saturation is applied to the control signal. The AVC system designed according to this procedure has been assessed in simulation and successfully implemented in an in-service open-plan office floor. The actual vibration reductions achieved have been approximately 60% for walking tests and over 90% for a whole-day vibration monitoring.

Active Vibration Control With Modified Positive Position Feedback

A novel active vibration control technique based on positive position feedback method is developed. This method, which is a modified version of positive position feedback, employs a first-order compensator that provides damping control and a second-order compensator for vibration suppression. In contrast, conventional positive position feedback uses a single second-order compensator. The technique is useful for strain-based sensors and can be applied to piezoelectrically controlled systems. After introducing the concept of modified positive position feedback, this paper investigates the stability of the new method for locating gain limits. Stability conditions are global and independent of the dynamical characteristics of the open-loop system. Using root locus plots, proper compensator frequency is identified and damping of the closed-loop system is studied. The performance of the modified positive position feedback for both steady-state and transient dynamic control is studied. The experimental and numerical results show that the proposed method is significantly more effective in controlling steady-state response and slightly advantageous for transient dynamics control, as compared with conventional positive position feedback.

Performance of a single-input multiple-output decision feedback equaliser for polarisation mode dispersion compensation

To compensate the performance degradation induced by polarisation mode dispersion (PMD) in high-speed optical transmission, a single-input multiple-output (SIMO) decision feedback equaliser (DFE) technique is proposed to combat all orders of PMD-induced distortion. The scheme is based on a new SIMO PMD channel model which utilises information embedded in both polarisation states. The performance analysis of the proposed PMD SIMO DFE scheme is provided along with explicit expressions for the filter coefficients. The analysis results show that the new scheme provides a significant improvement over using a first-order optical compensator and conventional DFE.

Comments on "Compact first-order polarization mode dispersion compensator based on birefringent crystals"

In 2004, with the title quoted above, Yang Chang-Xi, Li Shi-Guang, Miao Hou-Xun, Tian Yu, Zhang Er-Yao, and Jin Guo-Fang published an article in Chinese Physics Letters (CPL), vol.21 p.326 (2004). Regrettably, the technical contents of this paper are substantially the same as those of a paper published in the Journal of Lightwave Technology (JLT) in 2003 (Yan et al., vol.21, no.7, p.1676) and an OFC postdeadline paper, which was presented and published in 2002. However, the authors (Yang et al.) did not reference any of the earlier publications. Yao and Yan compares the journal papers in this commentary.

Pulse broadening due to polarization mode dispersion with first-order compensation

We give the exact analytical expression for the autocorrelation function of the polarization mode dispersion (PMD) vector of a fiber link in which first-order PMD is compensated for at the output. We use the result to obtain the mean-square width of a Gaussian pulse in the presence of a first-order PMD compensator.

Comparison of different feedback signals for one-stage polarization-mode dispersion compensators

The performance of a one-stage polarization-mode dispersion (PMD) compensator with different feedback signals is studied and compared with that of a pure first-order PMD compensator. We show that the performance of the one-stage PMD compensator has a strong dependence on feedback signals, and that the efficiency of some feedback signals depends on modulation formats. With proper choice of feedback signals, the one-stage PMD compensator can achieve better performance than the first-order PMD compensator.

Compensation of higher order polarization-mode dispersion using phase modulation and polarization control in the transmitter

We demonstrate a higher order polarization-mode dispersion (PMD) compensation scheme using a polarization controller and a phase modulator in the transmitter, as well as a traditional first-order compensator in the receiver. The effectiveness of this approach is experimentally demonstrated for a 10-Gb/s nonreturn-to-zero (NRZ) transmission over a link with an average PMD of 50 ps. Under this large PMD condition, the system power penalty can be reduced from /spl sim/4 dB that is achieved with a first-order compensator to 1 dB using the proposed technique. According to a reasonable outage criterion, simulation results show that the tolerance to average PMD for the 40-Gb/s NRZ system is improved to /spl sim/10 ps, as compared to the 7-ps value using only first-order PMD compensation. Such improvement may double the PMD-limited transmission distance.

Simple all-fiber-optic component for first-order polarization mode dispersion compensation

A simple and reliable all-fiber-optic component for a first-order polarization mode dispersion compensator is presented and tested in a 10-Gbit/s transmission system. The proposed component incorporates tunable differential group delay (DGD) provided by several pieces of polarization maintaining (PM) fibers of different DGD that were spliced together with short lengths of single-mode fibers between them. The total DGD of the compensator is determined by the alignment of fast and slow axes of the PM fiber sections. Applying this component as a first-order polarization mode dispersion (PMD) compensator, we demonstrate that a component with seven pieces of PM fiber segments could possibly compensate the first-order PMD up to 63.5 ps with 1-ps steps and a switching time of 300 ms. We also optimize the number of PM fiber sections in this component for a first-order PMD compensation in a 10Gbit/s system based on experimental results.

Impacts of polarization mode dispersion on the pulse-width with considering initial pulse chirp and fiber GVD

The impacts of polarization mode dispersion (PMD) on the pulse-width in linear systems have been investigated. The analytical solutions, including the effects of initial frequency chirp and group velocity dispersion (GVD), are derived. Analyses show that the pulse broadening effects induced by the second-order PMD depend on GVD and chirp parameter, which are different from those induced by the first-order PMD. An initially chirped Gaussian pulse is taken as an example, upon which analytical solutions of rms pulse-width are derived before and after the first-order PMD compensation. The first-order PMD compensator is also evaluated based on these solutions. The results show that the pulse broadening effects will be resisted efficiently by choosing appropriate GVD and chirp parameter; in general, the post-transmission compensation method will be less efficient than the PSP-transmission method.

Robust control of a magnetic-bearing flywheel using dynamical compensators

This paper considers the robust control of a current-controlled magnetic bearing with a pair of opposing electromagnets and a rigid rotor. A first-order dynamical compensator which uses only position feedback information is applied for control. The dynamical compensator design is based on a linearized one-dimensional second-order linear model, which is treated as an interval system in order to cope with parameter uncertainties. Through robust stability analysis, a parameterization of all the first-order robust stabilizing dynamical compensators for the interval magnetic-bearing system is first obtained. Then, by appropriately selecting the free parameters in the robust stabilizing controller, the H-two norm of the disturbance-output transfer function is made arbitrarily small over the system parameter intervals, and the H-infinity norm of the output-input transfer function is made arbitrarily close to a lower bound. The proposed approach is applied to control a flywheel supported by active magnetic bearings. Simulation and experimental results show that this approach is simple, effective and very robust.