Negative Feedback Connection Research Articles

A Low Mismatch Current Charge Pump Applied to Phase-Locked Loops.

This paper presents a charge pump circuit with a wide output range and low current mismatch applied to phase-locked loops. In this designed structure, T-shaped analog switches are adopted to suppress the non-ideal effects of clock feedthrough, switching time mismatch, and charge injection. A source follower and current splitting circuits are proposed to improve the matching accuracy of the charging and discharging currents and reduce the current mismatch rate. A rail-to-rail high-gain amplifier with a negative feedback connection is introduced to suppress the charge-sharing effect of the charge pump. A cascode current mirror with a high output impedance is used to provide the charge and discharge currents for the charge pump, which not only improves the current accuracy of the charge pump but also increases the output voltage range. The proposed charge pump is designed and simulated based on a 65 nm CMOS process. The results show that when the power supply voltage is 1.2 V, the output current of the charge pump is 100 μA, the output voltage is in the range of 0.2~1 V, and the maximum current mismatch rate and current variation rate are only 0.21% and 1.4%, respectively.

Passivity-based control of underactuated mechanical systems with Coulomb friction: Application to earthquake prevention

Passivity property gives a sense of energy balance. The classical definitions and theorems of passivity in dynamical systems require time invariance and locally Lipschitz functions. However, these conditions are not met in many systems. Characteristic examples are nonautonomous and discontinuous systems due to the presence of Coulomb friction. This paper presents an extended result for the negative feedback connection of two passive nonautonomous systems with set-valued right-hand side based on an invariance-like principle. Such extension is the base of a structural passivity-based control synthesis for underactuated mechanical systems with Coulomb friction. The first step consists of designing the control able to restore the passivity in the considered friction law, achieving stabilization of the system trajectories to a domain with zero velocities. Then, an integral action is included to improve the latter result and perform a tracking over a constant reference (regulation). Finally, the control is designed considering dynamics in the actuation. These control objectives are obtained using fewer control inputs than degrees of freedom, as a result of the underactuated nature of the plant. The presented control strategy is implemented in an earthquake prevention scenario, where a mature seismogenic fault represents the considered frictional underactuated mechanical system. Simulations are performed to show how the seismic energy can be slowly dissipated by tracking a slow reference, thanks to fluid injection far from the fault, accounting also for the slow dynamics of the fluid’s diffusion.

A SYSTEM FOR REGULATING THE COMMON COMPONENT OF THE INPUT VOLTAGE IN A RECEIVER

A method of building a system for adjusting the common mode variation of the input signal of the receiver in SERDES systems is presented. A negative feedback connection between the current source with constant conductivity and the matrix of series resistors is introduced, which allows to decrease the variation of common mode voltage level of the continious time linear equalizer circuit. By providing a high voltage level, the current source transistors operate in the saturation mode and provide a low gain in the low frequency range. The simulation results show that in the case of using the proposed method, the amplification of the system at low frequencies decrease by approximately 76% due to the reduction of the amplification factor at working frequencies by about 4.8%.

Investigation on the elimination of geyser in a cryogenic pipe by a recirculation method

• A recirculation structure is designed to successfully eliminate the geyser in a cryogenic pipe. • The geyser-elimination physics and the mechanism of recirculation formation are revealed. • A sine-wave-like oscillation process is found to exist in the initial process of the recirculation. • An improved approach is proposed to enhance the recirculation ability and reduce the oscillation amplitude by as much as 55%. A geyser elimination approach using recirculation methods is investigated in the present work, which is essential to guarantee the integrity of facilities. Both experimental and numerical methods are adopted in the investigation using LN 2 as working fluids. A recirculation pipe is designed to eliminate geysers in a cryogenic vertical pipe. The performance of the recirculation method is comprehensively explored involving its formation mechanism and oscillation physics. A deeper understanding of the liquid recirculation is achieved. The physics of the ambiguous oscillation phenomenon is revealed. It is concluded that by breaking down the energy storage pattern inside the cryogenic pipe, geysers can be perfectly eliminated by the recirculation method. Liquid recirculation in the pipeline is found to be initiated firstly by the liquid density difference between the vertical pipe and recirculation pipe and dominated by liquid flashing in the recirculation pipe. A negative feedback connection between void fraction and mass flow rate is the reason for the oscillation in its initial process. Besides, an effective method is proposed to enhance the recirculation ability as well as to attenuate the unstable oscillation by imposing a gradually increased heat flux. It is found that the oscillation amplitude could be reduced by as much as 55%.

Solving matrix equations in one step with cross-point resistive arrays

Conventional digital computers can execute advanced operations by a sequence of elementary Boolean functions of 2 or more bits. As a result, complicated tasks such as solving a linear system or solving a differential equation require a large number of computing steps and an extensive use of memory units to store individual bits. To accelerate the execution of such advanced tasks, in-memory computing with resistive memories provides a promising avenue, thanks to analog data storage and physical computation in the memory. Here, we show that a cross-point array of resistive memory devices can directly solve a system of linear equations, or find the matrix eigenvectors. These operations are completed in just one single step, thanks to the physical computing with Ohm's and Kirchhoff's laws, and thanks to the negative feedback connection in the cross-point circuit. Algebraic problems are demonstrated in hardware and applied to classical computing tasks, such as ranking webpages and solving the Schrödinger equation in one step.

Motion Control Design for an Omnidirectional Mobile Robot Subject to Velocity Constraints

A solution to achieve global asymptotic tracking with bounded velocities in an omnidirectional mobile robot is proposed in this paper. It is motivated by the need of having a useful in-practice motion control scheme, which takes into account the physical limits of the velocities. To this end, a passive nonlinear controller is designed and combined with a tracking controller in a negative feedback connection structure. By using Lyapunov theory and passivity tools, global asymptotic tracking with desired bounded velocities is proved. Simulations and experimental results are provided to show the effectiveness of the proposal.

PASSIVITY-BASED PARAMETER ESTIMATION AND COMPOSITE ADAPTIVE POSITION CONTROL OF INDUCTION MOTORS

In the rotor reference frame, the input-output linearization theory was adopted to decouple the rotor position and rotor flux. We then designed two adaptation laws to estimate the rotor resistance and mechanical parameters of the motor. The passive properties of the negative feedback connection from the rotor flux observer to the rotor resistance estimator, and the position controller were analyzed according to the passivity theorem. The overall control system was proved to be globally stable. Finally, experimental results show that the proposed scheme is robust to the variations of the rotor resistance and load torque disturbances. Furthermore, the estimated parameters can converge to the actual values.

After 40 Years, Retina Reveals It Uses Positive Feedback, as Well as Negative

​TheThe eye is not a camera, and the retina is not a piece of film. Indeed, the retina might be better likened to a computer running Photoshop, given the extent of image processing that it performs before passing visual information along to the brain. A central aspect of that processing is called center-surround inhibition, in which illumination stimulates the firing of a small number of retinal cells, accompanied by inhibition of surrounding cells. This phenomenon increases spatial contrast and sharpens perception of edges. Horizontal cells in the retina have a positive feedback synapse onto photoreceptor cells that locally offsets a previously known negative feedback connection. The mechanism of center-surround inhibition is complex, but one key part is played by interactions between photoreceptor cells and horizontal cells. Photoreceptors—the rods and cones that absorb light to begin the visual process—form synapses with horizontal cells, which, as their name implies, spread out horizontally across the retinal surface. Each horizontal cell receives input from dozens of photoreceptor cells, integrating and processing their input before passing it further along the visual chain. While photoreceptor cells activate horizontal cells, horizontal cells also subsequently influence photoreceptor cells, inhibiting their activation, an inverse influence called a negative feedback loop. But in this issue of PLoS Biology, Skyler Jackman, Richard Kramer, and colleagues show that the two are also linked by a positive feedback loop, which may help offset the loss of signal strength inherent in negative feedback, helping to maintain strong signal output without degrading spatial contrast. The authors began by isolating whole retinas from the anole lizard. Unlike most other sensory cells, photoreceptors continually release neurotransmitter in the unstimulated condition, that is, in the dark. The neurotransmitter glutamate is released from small sacs called synaptic vesicles that fuse with photoreceptor membrane. After their contents are released, bits of the membrane pinch off to generate new vesicles that gradually refill with glutamate for another round of release. The authors tapped into this recycling process to stain the vesicles with a dye as they were forming, allowing them later to monitor the rate of exocytosis of the labeled vesicles, and thus, the degree of stimulation of the photoreceptor. Because the negative feedback effect of horizontal cells activated by glutamate is to inhibit photoreceptor exocytosis, the authors expected that addition of exogenous glutamate into the synapse would reduce the rate of exocytosis. Instead, they found the opposite—an increase in glutamate release with addition of glutamate, the essence of a positive feedback loop. Through a series of experiments that blocked or stimulated different receptors, they established that the effect was mediated by a certain kind of receptor, called an ionotropic glutamate receptor. Since such receptors are found on the surface of the horizontal cells, but not on photoreceptor cells, and since ablating horizontal cells eliminated the effect, the authors concluded that positive feedback originated in these horizontal-cell receptors. Activation of the receptor allows calcium ions to flow into the horizontal cells, and when the authors released calcium directly into the horizontal cells, the photoreceptors were stimulated even in absence of excess glutamate, clinching the case. Further experiments will be needed to identify the messenger released by the horizontal cell that stimulates the photoreceptor cell. More experiments will also be needed to confirm these results in rods, since the anole lizard's retina contains only cones. How has this positive feedback mechanism remained hidden in plain sight through four decades or research on the retina? One reason, the authors suggest, is that it is only robustly observed in whole retina preparations, while retinal slices have been used for the majority of work elucidating retinal function. The authors propose that the function of the positive feedback loop is to offset the effect of negative feedback, which reduces overall output from a given visual stimulus. By tightly controlling the spatial spread of the positive stimulatory signal, they suggest that the system can maintain high signal strength without losing the sharp contrast enhancement of the center-surround system. Jackman SL, Babai N, Chambers JJ, Thoreson WB, Kramer RH (2011) A Positive Feedback Synapse from Retinal Horizontal Cells to Cone Photoreceptors. doi:10.1371/journal.pbio.1001057

Generalization of Impedance Matching to Nonlinear Dynamics of Robot Tasks

This paper aims at generalizing the concept of impedance matching to nonlinear circuits that express nonlinear dynamics of robot tasks, where a tool endpoint (or fingertip) with soft material presses a rigid and fixed object or environment. Under such a situation that characteristics of reproducing force of the elastic material with respect to displacement is nonlinear and unknown, impedance control is established by constructing a negative feedback connection of two hyperstable blocks. Then, the minimum upper-bound of the average power of force error under the existence of measurement noises of both displacement and force is derived in a sense of H-infinity disturbance attenuation, which corresponds to a generalization of the well-known theorem of maximum power transfer for impedance matching of linear electric circuits. Choice of design parameters is discussed from the viewpoint of better matching of two passive blocks to attain optimal transient response and attenuation of the effect of disturbances.