Digital Feedback Loop Research Articles

Magnetic Tweezers: Micromanipulation and Force Measurement at the Molecular Level

Cantilevers and optical tweezers are widely used for micromanipulating cells or biomolecules for measuring their mechanical properties. However, they do not allow easy rotary motion and can sometimes damage the handled material. We present here a system of magnetic tweezers that overcomes those drawbacks while retaining most of the previous dynamometers properties. Electromagnets are coupled to a microscope-based particle tracking system through a digital feedback loop. Magnetic beads are first trapped in a potential well of stiffness ∼10 −7 N/m. Thus, they can be manipulated in three dimensions at a speed of ∼10 μm/s and rotated along the optical axis at a frequency of 10 Hz. In addition, our apparatus can work as a dynamometer relying on either usual calibration against the viscous drag or complete calibration using Brownian fluctuations. By stretching a DNA molecule between a magnetic particle and a glass surface, we applied and measured vertical forces ranging from 50 fN to 20 pN. Similarly, nearly horizontal forces up to 5 pN were obtained. From those experiments, we conclude that magnetic tweezers represent a low-cost and biocompatible setup that could become a suitable alternative to the other available micromanipulators.

Implementation of a photonic automatic gain control system for correcting gain variations in the Green Bank Telescope fiber optic system

The application of a photonic automatic gain control system, over a 3 km loop of single-mode fiber for the 100 m Green Bank Telescope in order to correct the effects of polarization state changes, is described. Commercially available photodetector conversion gains are polarization mode dependent, which produce microwave gain changes on the order of 10−2. With the selection of the least polarization sensitive photodetector, the gain changes were reduced to less than the measurement limit of 10−4. The implementation of the control loop with a digital feedback loop is detailed, along with measured results and correction limit explanations. Effects of temperature on the quadrature point and corrections for small temperature deviations with a direct current bias circuit is also described.

A programmable riometer for Earth and Mars

We have developed a miniature radio receiver designed to operate as a relative ionospheric opacity meter (riometer). This project was funded by NASA as an enabling technology for future planetary radio science missions. We sought to reduce the instrument's size, mass, and power so that it would be practical for a Mars lander or rover mission. A recent study by our group indicates that a riometer might work well on Mars and offers a potentially rich science return. The technology also has immediate terrestrial applications. For example, the University of Maryland operates a chain of imaging riometers at the Automatic Geophysical Observatories (AGOs) in Antarctica. Our riometer includes features that are desirable for extended autonomous operation such as those with AGOs: low power consumption, wide dynamic range and linearity, computer command and data interface, and the ability to be remotely reconfigured. The receiver design provides significant improvements over previous implementations used in riometers. The high degree of system linearity, combined with a digital feedback loop (including a low‐duty calibration cycle), allows more time for viewing the radio sky. We implemented several of the receiver subsystems in a field‐programmable gate array, including the receiver detector, the control logic, and the data acquisition and processing blocks. Considerable efforts were made to eliminate or minimize RF noise and spurious emissions generated by the receiver's digital circuitry. Results of laboratory and field tests are presented and discussed.

A current imaging tunneling spectroscopy study of localized states at the Fermi level on circular pit edges in graphite

Thermal oxidation of the (0001) basal plane of highly oriented pyrolitic graphite leads to the removal of monoatomic carbon layers from the surface and to the formation of circular pits and steps on the exposed plane. Scanning tunneling microscopy and current imaging tunneling spectroscopy are used to study a localized surface state at the Fermi level on the circular edges of the pit. The surface state appears as the maximum of the local density of states at 10–50 meV below the Fermi level. The appearance of the state does not depend on the orientation of the step. The explanation for this effect includes the superposition of the two different surface states. The first one is due to the existence of an edge in the graphite sheet. The second one is due to fused disclination centers caused by mechanical deformations on the terrace. The first-principles band calculations performed on the stepped surface of graphite show an existence of a localized state at the Fermi level [1]. This state is due to the cut of the graphite sheet and is localized near the step. However, to the best of our knowledge, the existence of this state has not yet been confirmed experimentally. The experimental observation of this state might be difficult since its appearance depends on the orientation of the step. As has been proved by tight-binding calculations, if the orientation of the step is rotated by 30◦, the localized state does not appear [1] and the electronic structure near the step is similar to that of a bulk sample. Since the detection of the localized state near the step edges can cause some problems, specific surface preparations and techniques are required to give us local spectroscopic information at atomic level. The thermal oxidation of the (0001) basal plane of highly oriented pyrolitic graphite (HOPG) leads to the removal of monoatomic carbon layers from the surface and to the formation of circular pits and steps on the exposed plane. The circular geometry of the pit makes it possible to find the ∗ To whom all correspondence should be addressed. (Fax: +48-42/790030, E-mail: zbklusek@mvii.uni.lodz.pl) proper orientation of the step edges (running around the pit) and to detect the localized state at the Fermi level. The invention of current imaging tunneling spectroscopy techniques (CITS) enables us to obtain spectroscopic information from the tunneling current characteristics showing spectral features in agreement with other surface-sensitive techniques [2]. The purpose of this study is to obtain a detailed understanding of the graphite tunneling spectra recorded near the pit step edges, which can be used further to determine whether the localized state at the Fermi level exists. The results obtained will make it possible to understand better the thermal oxidation process and can be useful in interpreting the spectra obtained from thin metal layers deposited on thermally oxidized graphite, the deposited clusters on thermally oxidized graphite, or graphite in general. 1 Sample preparation and experimental setup In order to create surface pits and steps the thermal oxidation of graphite is applied [3–5]. The oxidation process is carried out in an externally heated quartz tube in ambient air conditions. The heating is started by the direct inserting of the sample into the tube preheated to the desired temperature and is stopped by withdrawing it from the tube. The experiments are performed at the temperature of 800 ◦C. The heating time is 30 min. After the treatment the samples left to cool in ambient air. STM/CITS investigations are performed in the air in the constant-current mode with the use of a digital feedback loop. The typical tunneling current is 1 nA. Tips are obtained by electrochemical etching from the Pt90-Ir10 alloy wires. The method proposed by Feenstra et al. [6] is applied to normalize all the measurements. The divergence problem in the case of (dI/dV)/(I/V) is overcome by applying some amount of broadening (∆V ) to the I/V values [7]. The positions of spectral features are insensitive to the value chosen for ∆V when the normalization of dI/dV is carried out. However, the half width of the peaks decreases when the ∆V tends to larger values. That is why we observe the narrower peaks on the (dI/dV)/(I/V) curves in comparison to the (dI/dV).

A high speed, low power PRML read channel device

A complete read channel device using PR-IV signalling and maximum likelihood detection is described. Descriptions, simulated performance and measured results of the digital adaptive feedback loops (AGC, FIR tap weights, DFE tap weights, frequency and phase) in the read channel are presented. Analog FIR filter and hash A/D converter performance is presented. The weighted averaging servo demodulation technique used in the device is described and compared to integration. This monolithic CMOS device consumes less than 0.75 W when reading at 85 Mbps, and occupies a step-and-repeat area of 24.6 mm/sup 2/.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

The Frankfurt calibration system for relative gravimeters

Artificial accelerations are used for the absolute calibration of relative gravimeters. Such accelerations are generated by the Frankfurt calibration system. The system consists of three stepper-motor-driven spindle blocks which can be mounted on any arbitrary body: here in the experiment a platform where spring gravimeters can be placed. The vertical motions of stepper-motor-driven spindle blocks are controlled by glass gauges. In a digital feedback loop, imperfections of the screw can be compensated. The driving functions for the stepper motors are sinusoidal waves with periods between 200 s and 2400 s. With the stroke of the system (± 10 mm) artificial accelerations up to ± 1000 × 10-8 m s-2 can be generated. During the experiment twelve gravimeters were investigated. The calibration factors as well as the frequency transfer functions of the instruments were determined with accuracies of better than 10-3. The results coincide well with the calibration factors derived from the gravity calibration line in Sèvres for the same instruments.

Optical data synchronization using tunable transmitters and nonzero dispersion links

A new digital feedback loop for synchronizing optical data with a clock at an arbitrary point in a fiber-optic link has been experimentally demonstrated and its physical limitations have been analyzed. The feedback loop consists of a tunable transmitter, a nonzero dispersion link, and standard microwave and digital electronics. The feedback loop was able to suppress thermal fluctuations with an accuracy better than 1 ps using either a temperature tuned DFB laser diode or a current tuned DBR laser diode. No bit-error rate penalty was measured during closed loop operation compared to back-to-back transmission. The physical limitations of the loop stem from noise in the receiver and the actuator increment. Thermally induced phase fluctuations in the fiber at frequencies above the loop bandwidth were found negligible. The maximum experimental delay time for stable operation was 0.86 s, indicating the possibility of closed loop operation for very long fiber links. The feasibility of packet synchronization is discussed. Moreover, a new scheme is proposed to circumvent wavelength reset when the system approaches the operating boundaries. >

Smart feedback loops

It is necessary to find the golden mean in allocating the processing resources of a computer control system. Traditionally, feedback loops operate at the lower levels to ensure safe and stable operation of the accelerator. At present we use analogue and digital feedback loops. Some systems, such as the RF, require more complex algorithms. A possible way of providing these, using digital signal processors is described. The results of tests with the Race-Track Microtron Linac are given and the sources of the main internal and external disturbances have been analysed.

A Fully Computerized Scanning Tunnelling Microscope

A new, fully computerized and inexpensive, scanning tunnelling micro- scope was designed and built. Its key design feature is the application of the high sensitivity bimorph, for the coarse positioning of the bimorph is > 0.03 mm and can be divided up to the 12 bit number of steps. The bi- morph positioning implies that this microscope is fully computer controlled, and using of bimorph instead of inch-worm implies low cost of the unit. The microscope can create images of the surface in the constant current mode and in the imaging current mode. The presented microscope works with the z-axis analog or digital feedback loop optionally. All parameters of the analog and digital feedback loop are computer controlled. Due to the digital z-axis feedback loop benefits, electron tunneling spectroscopy mode is naturally accessible. The performance of the microscope was tested in air by imaging of surface of HOPG (high oriented pyrolytic graphite).

Strip speed measurement using distributed infra-red detectors

A non-contact method for measuring the speed of electrically conducting strip over the range of speeds found in industry is described. The method is thermal in character and relies on the ability of the strip to store heat for short periods of time. A wavelength-sensitive reading head consisting of an array of equally spaced infra-red detector s senses the heat pattern present on the moving strip. The signals from the array elements are combined and the array acts as a spatial filter. The resultant signal is then fed back to a heater, or writing head, placed upstream of the reading head, resulting in a closed loop time-delay feedback system which oscillates at a frequency proportional to the strip speed. Phase shifts within the feedback loop cause a departure from linearity in the speed/frequency relationship. This effect is minimised by a digital feedback loop which performs phase correction. The method has been implemented using inexpensive pyroelectric detectors, an accuracy of 0.4% being obtained with strip of adequate emissivity. Use of high-performance detectors would enhance performance, particularly for low emissivity material.

Thermal noise-limited sensitivity of the pulse-biased SQUID magnetometer

We numerically calculated and obtained an approximate expression for the thermal noise-limited sensitivity of the pulse-biased SQUID sensor used in the integrated magnetometer. We found that the sensitivity is inversely proportional to the bias frequency and inversely proportional to the critical current of the Josephson junction to the 4/3 power. The sensitivity is also proportional to the temperature to the 4/3 power, and is nearly independent of the bias pulse duration. The sensitivity expression is similar to that of the rf SQUID, however, the coefficient is more than one order of magnitude smaller. The best sensitivity has the same order of magnitude as that of the dc SQUID. The quantum limit is attained at a bias frequency of 0.9 GHz at 4.2 K with the typical junction parameters. This sensitivity is available for the SQUID magnetometer operated in the digital feedback loop.

AC power calibrator with a precision digital wattmeter in the feedback loop

This paper describes a new microprocessor-based ac power calibrator. The output stability and accuracy of measurements are achieved using a high-precision electronic digital wattmeter in a digital feedback loop.

Gated base-line restorer without ‘‘droop’’

Different configurations of an analog gated base-line restorer circuit are discussed. An elimination of a ‘‘droop’’ rate is achieved by replacing the analog memory by a digital one. A design employing a digital feedback loop is described.

Digital E-beam testing

A digital measurement of the secondary electron current in E-beam testers is performed by counting the separate electrons that hit the detector. This has advantages for waveform measuements with short beam pulses or low currents. For long pulses and large currents a digital-feedback loop is designed, which in an iterative way adjusts the voltage on the retarding grid of a spectrometer. Both techniques offer a multi-sampling capability, which gives a proportional increase of the measurement speed.

Digital feedback loops for d.c. SQUIDs

Digital feedback loops for d.c. SQUIDs using pulse-rate modulation are proposed and investigated by simulations and measurements. The design of a d.c. SQUID magnetometer yields a simulated energy resolution of ≈ 11 h ( h = Planck's constant) where ≈ 3 h corresponds to additional noise due to the feedback loop. Measurements at Josephson sampler prototype chips showed that the digital feedback loop is very stable and fast enough to obtain a flicker-free display on an oscilloscope. In contrast to conventional feedback loops an integration of sensor and feedback loop on one or several chips with Josephson junctions seems feasible.

Regulated DC Power Supply Employs Digital Control for Firing its SCRs

A regulated DC power supply using full-wave rectification by SCRs’, with digitally-variable firing angle, is described. The output voltage is sampled by a high-gain operational amplifier which controls the frequency of a voltage-controlled oscillator (VCO). The firing angle of the SCRs' is varied to stabilize the output voltage by counting a fixed number of variable-frequency pulses from the VCO, from each zero crossing of the mains sinusoidal voltage. The merit of the circuit is the inherent stability achieved through the digital negative feedback loop. The load regul-tion, line regulation, ripple factor and temperature stability of the circuit are measured. In particular, the circuit has an excellent temperature stability from 12°C to about 45°C.

A computer controlled dielectric time domain spectrometer

A computer controlled dielectric TDS system, based on a dual channel sampling oscilloscope, is described. One channel provides an accurate time reference for the necessary Fourier transformations. A digital feedback loop ensures the relative stability (<or approximately=0.5 ps) of the recorded pulses, allowing long time accumulation and averaging of data. The system allows both reflection and transmission TDS methods to be used.

A low power magnetometer for vehicle detection

A miniature, low power two-axis Brown type ring. core magnetometer is described which can be used for motor vehicle detection. For relatively little power (<1mw), the sensor functions over a large dynamic range ( <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">\pm.8\times10^{-4}</tex> Tesla) with a sensitivity of 7μv/nT (both orthogonal axes) and a self noise at zero field less than 3nT <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">p_p</inf> (.03 to 3 Hz). A low power digital feedback loop has been developed to null the magnetometer signal and compensate for temperature induced offset variations. The sensor is also suitable for a stationary short baseline gradiometer.

Direct digital closed-loop control of h.v. d.c. convertors

As an alternative to present analogue-based control schemes, the basic design characteristics and dynamic performance of a newly proposed direct digital control are described. Tests are carried out in a specially designed scaled-down model provided with the necessary monitoring facilities for direct digital control. Special emphasis is given to the performance of relatively weak a.c. systems, and therefore the model includes suitable representation of the system parameters and of the filters normally used. It is shown that the implementation of equidistant-firing control by means of digital feedback loops provides fast recoveries from convertor and line disturbances.

A Tracking Static State Estimator

A tracking static state estimator is a digital feedback loop (a computer algorithm) which uses real time measurements of voltage magnitudes, watt flows, and var flows to track the static state (voltage at all buses) as it varies during the daily load cycle. The design and performance of simple gain matrices suitable for wide ranges of operating conditions are discussed.