Hybrid Feedback Loop Research Articles

Output tracking for a class of non-minimum phase nonlinear systems: A two-point boundary value problem formulation with a hybrid regulator

The asymptotic output tracking problem is studied for a class of non-minimum phase nonlinear systems without requiring the a priori knowledge, or even the existence, of a finite-dimensional exosystem that generates the prescribed periodic reference signal. The design strategy is illustrated in two steps. First, it is shown that the knowledge of a solution to a certain two-point boundary value problem involving the underlying zero-dynamics of the plant is instrumental and sufficient to construct a state feedback regulator that achieves boundedness of the trajectories and (exact) asymptotic tracking, hence completely circumventing the need for solving partial differential equations. Then, since the computation of the latter solution may be affected by numerical errors that are particularly detrimental in the presence of unstable zero-dynamics, the above architecture is robustified by means of an additional hybrid feedback loop whose trajectories converge to a solution of the two-point boundary value problem. Once the latter scheme has been established and discussed, the extension to the case of output feedback is presented, firstly in the specially structured case in which the input vector field depends only on the measured output and then extended to the generic case. The theory is then corroborated by means of a physically motivated numerical example involving an inverted pendulum on a cart. Interestingly, it is also shown that the solution provided by the hybrid scheme above coincides with the limiting solution of a suitably defined cheap optimal control problem.

Single Cell Characterization of a Synthetic Bacterial Clock with a Hybrid Feedback Loop Containing dCas9-sgRNA.

Genetic networks that generate oscillations in gene expression activity are found in a wide range of organisms throughout all kingdoms of life. Oscillatory dynamics facilitates the temporal orchestration of metabolic and growth processes inside cells and organisms, as well as the synchronization of such processes with periodically occurring changes in the environment. Synthetic oscillator gene circuits such as the "repressilator" can perform similar functions in bacteria. Until recently, such circuits were mainly based on a relatively small set of well-characterized transcriptional repressors and activators. A promising, sequence-programmable alternative for gene regulation is given by CRISPR interference (CRISPRi), which enables transcriptional repression of nearly arbitrary gene targets directed by short guide RNA molecules. In order to demonstrate the use of CRISPRi in the context of dynamic gene circuits, we here replaced one of the nodes of a repressilator circuit by the RNA-guided dCas9 protein. Using single cell experiments in microfluidic reactors we show that this system displays robust relaxation oscillations over multiple periods and over several days. With a period of ≈14 bacterial generations, our oscillator is similar in speed as previously reported oscillators. Using an information-theoretic approach for the analysis of the single cell data, the potential of the circuit to act as a synthetic pacemaker for cellular processes is evaluated. We also observe that the oscillator appears to affect cellular growth, leading to variations in growth rate with the oscillator's frequency.

Uncoveringthe cellular capacity for intensive and specific feedback self-control of the argonautes and MicroRNA targeting activity.

The miRNA pathway has three segments—biogenesis, targeting and downstream regulatory effectors. We aimed to better understand their cellular control by exploring the miRNA-mRNA-targeting relationships. We first used human evolutionarily conserved sites. Strikingly, AGOs 1–3 are all among the top 14 mRNAs with the highest miRNA site counts, along with ANKRD52, the phosphatase regulatory subunit of the recently identified AGO phosphorylation cycle; and the AGO phosphorylation cycle mRNAs share much more than expected miRNA sites. The mRNAs for TNRC6, which acts with AGOs to channel miRNA-mediated regulatory actions onto specific mRNAs, are also heavily miRNA-targeted. In contrast, upstream miRNA biogenesis mRNAs are not, and neither are downstream regulatory effectors. In short, binding site enrichment in miRNA targeting machinery mRNAs, but neither upstream biogenesis nor downstream effector mRNAs, was observed, endowing a cellular capacity for intensive and specific feedback control of the targeting activity. The pattern was confirmed with experimentally determined miRNA-mRNA target relationships. Moreover, genetic experiments demonstrated cellular utilization of this capacity. Thus, we uncovered a capacity for intensive, and specific, feedback-regulation of miRNA targeting activity directly by miRNAs themselves, i.e. segment-specific feedback auto-regulation of miRNA pathway, complementing miRNAs pairing with transcription factors to form hybrid feedback-loop.

Electro-optic chaotic system based on the reverse-time chaos theory and a nonlinear hybrid feedback loop.

A novel electro-optic chaos source is proposed on the basis of the reverse-time chaos theory and an analog-digital hybrid feedback loop. The analog output of the system can be determined by the numeric states of shift registers, which makes the system robust and easy to control. The dynamical properties as well as the complexity dependence on the feedback parameters are investigated in detail. The correlation characteristics of the system are also studied. Two improving strategies which were established in digital field and analog field are proposed to conceal the time-delay signature. The proposed scheme has the potential to be used in radar and optical secure communication systems.

A control theory perspective on configuration management and Cfengine

Cfengine is an autonomous agent for the configuration of Unix-like operating systems. It works by implementing a hybrid feedback loop, with both disrcete and continuous elements.

Versatile high performance digital SQUID electronics

The digital SQUID Electronics DSE32 has been developed to perform software gradiometry with SQUID magnetometers in multi-channel systems using both High-T/sub c/ and Low-T/sub c/ SQUIDs. A low-noise preamplifier is followed by a fast hybrid feedback-loop, combining the bandwidth of an analog feedback-loop with the accuracy of a digital loop. By resetting and counting of flux quanta in the SQUID, the dynamic range has been substantially increased. All parameters of SQUID and electronics are firmware controlled and allow for auto-setup and tuning. Data are transferred with a rate of 20 Mbit/s by means of noise-minimizing, bi-directional optical fiber. Low power consumption allows the use of a battery, further reducing external noise pick-up. The electronics layout is designed for multi-channel applications, as needed in biomagnetic systems.

Controlled wavelength bistability and multistability of tunable semiconductor lasers

A new type of bistable device is reported: its nonlinearity affects the wavelength, rather than the usual intensity. The device is based on a tunable multielectrode laser diode driven by a hybrid feedback loop which exhibits a wavelength nonlinearity. Bistable and multistable operation is obtained with steady states which are fixed wavelengths.

An optical angular discriminator application to information processing

The authors propose a simple system, based on geometrical optics, which allows the realization of a non-linear response curve. It uses a multiple beam system illuminated by a bundle of rays. Some experimental results are given, together with the possible applications for the optical logical operators. It is also shown that by adding a hybrid feedback loop to the basic system, a multistable operator can be obtained. Several extensions are finally presented.