Multi-loop Structure Research Articles

Segmented surface coil resonator for in vivo EPR applications at 1.1 GHz

A four-loop segmented surface coil resonator (SSCR) with electronic frequency and coupling adjustments was constructed with 18 mm aperture and loading capability suitable for in vivo Electron Paramagnetic Resonance (EPR) spectroscopy and imaging applications at L-band. Increased sample volume and loading capability were achieved by employing a multi-loop three-dimensional surface coil structure. Symmetrical design of the resonator with coupling to each loop resulted in high homogeneity of RF magnetic field. Parallel loops were coupled to the feeder cable via balancing circuitry containing varactor diodes for electronic coupling and tuning over a wide range of loading conditions. Manually adjusted high Q trimmer capacitors were used for initial tuning with subsequent tuning electronically controlled using varactor diodes. This design provides transparency and homogeneity of magnetic field modulation in the sample volume, while matching components are shielded to minimize interference with modulation and ambient RF fields. It can accommodate lossy samples up to 90% of its aperture with high homogeneity of RF and modulation magnetic fields and can function as a surface loop or a slice volume resonator. Along with an outer coaxial NMR surface coil, the SSCR enabled EPR/NMR co-imaging of paramagnetic probes in living rats to a depth of 20 mm.

Design and control of a brushless DC limited-angle torque motor with its application to fuel control of small-scale gas turbine engines

This paper presents techniques for design and control of a brushless direct-current (DC) limited-angle torque motor (LATM) with its application to fuel control of gas turbine engines. Given the desired specifications, a two-pole brushless DC LATM with a toroidally wound armature is designed using selected ferromagnetic material and rare-earth permanent magnets; its electromagnetic characteristics is then computationally found and well tuned using the finite element method (FEM) in order to ensure whether the design meets the performance specifications. To achieve the simple and inexpensive semi-closed-loop fuel control, a robust position controller (including a proportional-integral-derivative (PID) controller with a prefilter) is synthesized for providing the required positioning performance for the developed motor, thereby achieving an inexpensive semi-closed-loop fuel control. A closed-loop fuel controller associated with the proposed position controller and a flow meter is then proposed based on multi-loop control structure in order to achieve required linear input–output relationship. All the proposed fuel control laws were implemented using a stand-alone single-chip digital signal processor (DSP). Experimental results are conducted to show the efficacy and usefulness of the developed limited-angle torque motor with its application to an experimental gas turbine fuel control test platform.

Duty Phase Control for Single-Phase Boost-Type SMR

In this paper, a novel duty phase control (DPC) for single-phase boost-type switching-mode-rectifier (SMR) is developed and digitally implemented in DSP-based system. Compared to the conventional multiloop control structure with inner current loop and outer voltage loop, it is noted that there is only one voltage loop tuning the phase of predefined duty pattern (i.e. duty phase) in the proposed DPC. Due to no current loop, sampling inductor current and tracking current are unnecessary when SMRs are operated to shape the current waveform and regulate the output voltage. It implies that the single-loop DPC is simple and adaptable to the implementation with digital and analog integrated circuits. In this paper, first, the effect of the duty phase on the input current is analyzed and modeled. It shows that the sinusoidal current waveform can be naturally generated by the predefined duty pattern and the current amplitude is roughly proportional to the controllable duty phase. Then, a voltage controller is designed to regulate the dc output voltage by tuning this duty phase. Finally, some simulated and experimental results have been given to illustrate the performances of the proposed DPC.

Network Structure and Robustness of Intracellular Oscillators

Sustained oscillations play a key role in many intracellular functions, such as circadian time keeping, cell cycle control and calcium signalling. The oscillations are in all cases driven by feedback interactions taking place in biochemical reaction networks. While a single feedback loop in principle is sufficient to generate such oscillations, experimental evidence reveal that more complex network structures, involving multiple feedback loops, underly intracellular oscillations. One hypothesis frequently set forth is that a multi-loop structure is motivated by the need for robustness to internal and external perturbations. We here consider robustness analysis of several recently published models of circadian clocks to determine the role of the underlying network structure in providing robust stability of the oscillators. The robustness analysis is based on adding dynamic perturbations to the network interactions, similar to that used in robust control theory. To elucidate the role of various interactions in providing robust oscillations, we consider blocking specific interactions. Biologically, this contrasts the often considered gene knockouts and implies that genes are persistently expressed. We find that different models have highly different active structures and also differ significantly in their robustness. While some models essentially rely on a single loop in generating robust oscillations, other models have more intricate structures in which some loops provide oscillations and other serve to increase the robustness. Other models again have redundant loops that provide failure tolerance in the face of large perturbations, such as gene knockouts.

Modeling and Robust Control of a Three-Phase Buck+Boost PWM Rectifier (VRX-4)

The modeling and control of a three-phase three-switch buck-type rectifier system with integrated boost output stage (voltage range extended 4-switch rectifier) is analyzed in this paper. A cascaded multiloop control structure is presented that has the following features: constant output voltage for a wide input voltage range and stepwise changes of load; sinusoidal input currents in phase with the mains voltages that also remain sinusoidal in case of unbalanced mains conditions; and active damping of the input filter resonance. For the control design, an equivalent dc-dc small-signal model of the converter is derived and verified by simulations and measurements. Based on this and on the identification of the critical operating point, the controllers for the inner dc current and outer voltage control loops are selected. The stability of the closed loop system is discussed and the robust operation of the system is verified by measurements on a 5-kW prototype.

Microwave Diagnostics of Dynamic Processes and Oscillations in Groups of Solar Coronal Magnetic Loops

Low-frequency (LF) modulations of the solar microwave radiation (37 GHz) recorded at the Metsahovi Radio Observatory, are analyzed. Since the intensity of solar microwave radiation, produced by the electron gyrosynchrotron mechanism, is dependent on a value of the background magnetic field [Dulk, G. A.: 1985, Ann. Rev. Astron. Astrophys. 23, 169–224], slow variations of the magnetic field associated with disturbances of the electric current in a radiating source, can modulate the intensity of the microwave radiation. The observed multi-track features of the LF spectra are interpreted as a signature of a complex multi-loop structure of the radiating source. Application of the equivalent electric circuit models of interacting loops allows to explain and reproduce the main dynamical features of the observed LF modulation dynamic spectra.

Multi-loop model-based control structure for robot manipulators

Most nonlinear control concepts used in robotics are based on a more or less accurate inverse model of the robot. In contrast to this, the design and properties of a general $n$-loop control structure based on a divided forward model of the robot, the so-called multi-loop Model Following Control Structure ($n$-MFC), is presented in this paper. Its theoretical basics and its concept are explained. The stability and robustness of the proposed control structure is analyzed. The theoretical assumptions are verified in many experiments with a two-joint robot manipulator. Qualitative as well as quantitative results of the experiments are presented and discussed.

Position Analysis of the RRP-3(SS) Multi-Loop Spatial Structure

The paper presents the position analysis of a spatial structure composed of two platforms mutually connected by one RRP and three SS serial kinematic chains, where R, P, and S stand for revolute, prismatic, and spherical kinematic pair, respectively. A set of three compatibility equations is laid down that, following algebraic elimination, results in a 28th-order univariate algebraic equation, which in turn provides the addressed problem with 28 solutions in the complex domain. Among the applications of the results presented in this paper is the solution to the forward kinematics of the Tricept, a well-known in-parallel-actuated spatial manipulator. Numerical examples show adoption of the proposed method in dealing with two case studies.

Design and implementation of a cost-effective position control system for an ironless linear motor

Due to attractive features including a high reliability, low maintenance, and a low cost, stepping motors with an open-loop drive have found wide application in motion control systems. However, the performance of a stepping motor that is under open-loop control deteriorates when either the frequency of the input pulse train is increased or an external load is unexpectedly added. A high-performance, cost-effective direct-drive position-control system with an ironless linear permanent-magnet synchronous motor (ILPMSM) is proposed to overcome these difficulties. Instead of using a multi-loop cascaded-feedback control structure, the position is controlled by regulating the orientation and magnitude of the excited space current vector of the moving coil of the ILPMSM. Also, the effect of the dead time on the positioning accuracy is investigated and a cost-effective position-error compensation approach in the d-q plane is introduced. Compared with general hybrid stepping motors several advantages are observed when using the proposed control strategy, including: (i) the resolution of the stepping increment and the holding force are software programmable; and (ii) limited losing steps can be automatically eliminated during the travelling process. A high-performance digital signal processor is used to implement the proposed control algorithm. Experimental results verify that the proposed approach exhibits a satisfactory performance.

Predictive Digital Control of Power Factor Preregulators With Input Voltage Estimation Using Disturbance Observers

The paper presents a fully digital control of single-phase boost power factor preregulators (PFPs) based on inductor (or switch) current and output voltage measurements. Input voltage sensing is avoided using a disturbance observer, which provides a waveform proportional to the rectified input voltage. The proposed solution is based on a multiloop structure for PFPs with an internal deadbeat current control and a conventional outer voltage control, possibly with fast dynamic response. The resulting control algorithm is simple, accurate, and robust with respect to parameter mismatch. The digital control has been implemented both in a field programmable gate array and in a digital signal processor (TMS320F2812), to test the proposed algorithm with different control delays. Experimental results on a single-phase boost PFPs show the effectiveness of the proposed solution.

Modeling, simulation and control of a scheibel liquid–liquid contactor: Part 2. Model-based control synthesis and design

This study describes an application of model-based control techniques to a liquid–liquid extraction process. The control design is based on a reduced order model of the process obtained by system identification. A multi-loop decentralized control system, in the framework of internal model control (IMC), is initially selected to control the 2 × 2 control loops of the extraction column. The interaction of loops is then investigated to explore the feasibility of such multi-loop control structure. A centralized multivariable control system is synthesized with a model predictive control (MPC) technique. The simulation results demonstrate good servo and regulatory characteristics for both control system designs. However, the MPC control of extraction proved to be superior to the IMC one in terms of speed of response, stability, robustness, and loops interaction. MPC has also shown ability to handle control operation under input variables constraints, which has significant practical importance.

Re-configurable control system for autonomous unmanned aircraft

A new control system which can utilise various control methods and algorithms is presented. A re-configuration controller is based on a multi-loop structure of the basic controller. The supervisory part of the reconfiguration controller changes the basic controller depending on the actual state. The state classifier is based on the fuzzy logic. The control system thus obtained can potentially cover full aircraft state range from typical to tricky operations.

Robust performance of the multiloop perturbation compensator

A novel perturbation attenuation method is proposed for robust performance of mechanical systems. First, we give a unified view on a class of existing perturbation observers and define the residual perturbation. In terms of the view and the definition, a new perturbation compensator with multiloop structure is developed. It effectively compensates the perturbation (i.e., model uncertainty and external disturbance) to the plant in a hierarchical and recursive fashion. In the multiloop perturbation compensator (MPEC) proposed, as the number of loops increases, the external disturbance condition for system stability is greatly relaxed and the perturbation attenuation performance is gradually enhanced but the robust stability margin on the modeling error becomes more strict. A recursive algorithm for general n-loop case of the MPEC is derived. By combining the developed robust perturbation compensator with a nominal feedback controller, a robust motion controller is synthesized. Experimental results for XY positioner and 2-DOF robot arms demonstrate the excellent robust tracking performance in spite of arbitrary large perturbation inputs.

Low-Cost INS/GPS Integration: Concepts and Testing

The high cost of inertial units is the main obstacle for their inclusion in precision navigation systems to support a variety of application areas. Standard inertial navigation systems (INS) use precise gyro and accelerometer sensors; however, newer inertial devices with compact, lower precision sensors have become available in recent years. This group of instruments, called motion sensors, is six to eight times less costly than a standard INS. Given their weak stand-alone accuracy and poor run-to-run stability, such devices are not usable as sole navigation systems. Even the integration of a motion sensor into a navigation system as a supporting device requires the development of non-traditional approaches and algorithms. The objective of this paper is to assess the feasibility of using a motion sensor, specifically the MotionPak™, integrated with DGPS and DGLONASS information, to provide accurate position and attitude information, and to assess its capability to bridge satellite outages for up to 20 seconds. The motion sensor has three orthogonally mounted ‘solid-state’ micro- machined quartz angular rate sensors, and three high performance linear servo accelerometers mounted in a compact, rugged package. Advanced algorithms are used to integrate the GPS and motion sensor data. These include INS error damping, calculated platform corrections using DGPS (or DGPS/DGLONASS) output, velocity correction, attitude correction and error model estimation for prediction. This multi-loop algorithm structure is very robust, which guarantees a high level of software reliability. Vehicular and aircraft test trials were conducted with the system in land vehicle mode and the results are discussed. Simulated outages in GPS availability were made to assess the bridging accuracy of the system. Results show that a bridging accuracy of up to 3 m after 10 seconds in vehicular mode and a corresponding accuracy of 6 m after 20 seconds in aircraft mode can be obtained, depending on vehicle dynamics and the specific MotionPak™ unit used. The attitude accuracy was on the order of 22 to 25 arcmin for roll and pitch, and about 44 arcmin for heading.

Uninterruptible power supply multiloop control employing digital predictive voltage and current regulators

A digital control technique for the inverter stage of uninterruptible power supplies (UPSs) is described, which is based on voltage and current predictive regulators. Its aim is to achieve a deadbeat dynamic response for the controlled variables (output voltage and inverter current). The controller maintains the advantageous conventional multiloop structure and is capable of guaranteeing a high-quality dynamic performance. Moreover, its design is extremely simple and requires only a reasonably accurate knowledge of the output filter parameters. Finally, the only sensed variables are the output voltage and the converter output current. The validity of the proposed strategy is demonstrated by means of simulation and experimental results referring to a single-phase UPS laboratory prototype (1 kVA).

Development of gain scheduling for trajectory tracking problems in input-output linearizable batch processes

In this paper, a design methodology has been developed for the control of non-autonomous batch processes which typically require good tracking performance. The design, based on the framework of differential geometric methods and linear gain scheduling theory, involves a multi-loop structure. The inner loop is an input-output linearizing state feedback law based on a nominal, time-invariant plant model. The outer loop includes a gain scheduling controller which provides the desired stability and tracking performance in the face of time varying parameters. The procedure is applied to a semi-batch polymethylmethacrylate synthesis problem. The results show excellent tracking performance of the gain scheduling controller.

Decentralized control of wafer temperature for multizone rapid thermal processing systems

Decentralized control is shown through analysis and experimentation to be an appropriate strategy for wafer temperature control in certain multizone rapid thermal processing (RTP) systems. An input-output controllability analysis is conducted to illustrate that the direction associated with the reference command (set-point) corresponding to a spatially uniform temperature trajectory specification is nearly in alignment with the "most" controllable direction associated with the maximum singular value for a multiple concentric lamp configuration. Consequently, the control structure need not alter the directionality of the plant and, thus, can be achieved by a simple decentralized controller where the lamps are paired individually to sensors to achieve a multiloop structure where all interactions are not taken explicitly into account. This result is shown to produce acceptable performance even for an ill-conditioned plant since the directions corresponding to the smaller singular values are irrelevant to the uniform temperature control criteria. Moreover, straightforward nonmodel-based tuning of the controller is enabled due to the simplicity of the decentralized control structure.

A Study of Interference among Parts of a Machine in Motion. 3rd Report. Graph-Based Judgment of Interferential Characteristics of Multi-Loop Structure.

This study deals with the problem of analyzing the phenomena of interference among parts of a machine in motion. Some machines cause interference because of their basic structure as well as of their superficial form. A previous report demonstrated that a machine causes interference in a motion when its basic structure realizes a certain relationship among the torsion numbers, that is, the numbers of relative revolutions to occur between links in kinematic pairs in the motion. This report presents a graph-based method of judging the realization of the relationship for multi loop structures. First, the basic structure and the torsion numbers are represented in a graph. Next, this graph is transformed through a series of several graphs into a graph, called the fourth pair graph, each arc of which corresponds to a set of kinematic pairs in the structure and has a weight: the sum of all the torsion numbers that are each related to a kinematic pair in the Set. It is graph theoretically shown that when the weight of an arc in the fourth pair graph is not zero, the above-mentioned relationship is realized. Hence, by calculating the weights, one can easily judge the interferential characteristics of the structure.

Supervisor Control of Heat Exchanger Networks

This work presents a simple modeling technique for commanding the optimal operation of heat exchanger networks. The proposed representation provides all the necessary relationships to define an optimizing supervisory system aimed to determine the most convenient set points of a multiloop control structure at any instant. The modeling approach used for the optimization emphasizes the computation of heat exchanger duties rather than net work intermediate temperatures, this simplifies the modeling step since all the conditions can be written by simple inspection of the network structure, and reduces the computational work.

Power System Stability Control with Robust Multi-Loop Automatic Voltage Regulators

The primary voltage regulation for synchronous generator control is addressed in the context of modern interconnected power system problems. Voltage regulation in the synchronous generators is maintained by the Automatic Voltage Regulators (AVR). A brief review of the role of AVR in the overall power system problem is given. The use of a multi-loop AVR control structure involving advanced optimal control theories is also presented to improve the performance of generator terminal voltage control in primary regulation and system stability