Steady State Compressor Model as a Basis for Anti-Surge Control Action

This paper discusses active surge control in variable speed axial compressors. A compression system equipped with a variable area throttle is investigated. Based on a given compressor model, a fuzzy logic controller is designed for surge control and a proportional speed controller for speed control. The fuzzy controller uses measurements of the change of mass flow and the change of pressure rise to determine the control action. The presented approach does not require the knowledge of system equilibrium or the surge line. Simulation studies show promising results that the proposed fuzzy logic controller is capable to suppress surge at different operating points.

Optimizing frequency noise calibration and manipulation in an active feedback control loop

Existing optimal scheduling packages for water distribution systems (WDS) calculate the time schedules for control elements such as pumps, valves and treatment works following the idea of open loop control. The calculations take into account the hydraulic model of WDS, operational constraints and the time dependent electrical tariff. The time horizon is typically one day or one week depending on the network storage capacity and the structure of the electrical tariff. If such a schedule is applied to a physical system the predicted and the physical reservoir levels diverge due to uncertain demands and as a result the calculations need to be repeated at some time using updated demand prediction and the current measured reservoir levels. The aim of this paper is to investigate the feasibility of synthesising continuous feedback from reservoir levels to pump and valve operation, taking into account the operational constraints and the electrical tariff. This attempt can be seen as a generalisation of a simple practical rule where a pump is controlled by the level of an associated reservoir, e.g. the rule `if the tank is full switch the pump OFF, if the tank is empty switch the pump ON'. The proposed methodology relies on preparing a control law in an off-line mode which is subsequently used in a real time situation. The traditional optimal scheduling problem is solved many times for typical initial reservoir levels in the system and a family of reservoir and corresponding control trajectories is generated. These trajectories are approximated by a hyper-surface which represents an optimal control law ready to use in the field. The control action will follow the sequence: log the time, measure the current reservoir levels, evaluate from the hyper-surface the corresponding operation for pumps and valves and finally apply the control action to the pumps and valves. The approach avoids time-consuming online-recalculation of pump schedules, and additionally the feedback control is much more robust with respect to uncertain demands. The limitation of the approach is dictated by the number of reservoirs and number of initial reservoir levels which need to be considered. In the worst case the computational complexity may increase exponentially with the number of reservoirs.^ Initially this approach was applied to a simple system containing a single reservoir fed by one pump station with a time varying tariff and subsequently to a medium-size system with four reservoirs and four pump stations. This paper was presented at the 8th Annual Water Distribution Systems Analysis Symposium which was held with the generous support of Awwa Research Foundation (AwwaRF).

Virtual constraints are functional relations (i.e., constraints) on the state variables of a robot's model that are achieved through the action of actuators and feedback control instead of physical contact forces. They are called virtual because they can be re-programmed on the fly without modifying any physical connections among the links of the robot or its environment. Previous analytical and experimental work has established that vector relative degree two virtual holonomic (i.e., only configuration dependent) constraints are a powerful means to synchronize the links of a bipedal robot so as to achieve walking and running motions over a variety of terrain profiles. This paper introduces a class of virtual nonholonomic constraints that depend on velocity through (generalized) angular momentum while maintaining the property of being relative degree two. This additional freedom is shown to yield control solutions that handle a wider range of gait perturbations arising from terrain variations and exogenous forces. Moreover, including angular momentum in the virtual constraints allows foot placement control to be rigorously designed on the basis of the full dynamic model of the biped, instead of on the basis of an inverted pendulum approximation of its center of mass, as is commonly done in the bipedal robotics literature. This new class of control laws is shown in simulation to be robust to a variety of common gait disturbances.

Event-driven intermittent feedback control is a form of feedback control in which the corrective control action is only initiated intermittently when the variables of interest exceed certain threshold criteria. It has been reported in the literature that the CNS uses an event-driven intermittent control strategy to stabilize the human upright posture. However, whether the threshold criteria may change under different postural task conditions is not yet well understood. We employ a numerical study with inverted pendulum models and an experimental study with 51 young healthy individuals (13 females and 38 males; age: 27.8 ± 6.5 years) with stabilogram-diffusion, temporal and spectral analysis applied to COP (Center of Pressure) trajectories measured from these experiments to examine this aspect. The present study provides compelling evidence that inducing a natural arm swing during quiet stance appears to lead to higher sensory dead zone in neuronal control reflecting higher intermittency thresholds in active feedback control and a corresponding lower sensory dependence. Beyond the obvious scientific interest in understanding this aspect of how CNS controls the standing posture, an investigation of the said control strategy may subsequently help uncover insights about how control of quiet stance degrades with age and in diseased conditions. Additionally, such an understanding will also be of interest to the humanoid robotics community as it may lead to insights leading to improving control strategies for posture control in robots.

An advanced digital control and monitoring system for turbomachinery is introduced. Important aspects for the design of anti-surge control systems, such as the influence of the controller program execution time on the performance, are investigated. The influence of the gas composition (molecular weight) on the compressor surge line is demonstrated. Control laws are discussed. The design basis of an ultra-high speed anti-surge and speed control system is shown and the data-logging features are demonstrated on the log of a compressor surge. The paper closes describing some dynamic process simulation being performed using the controller hardware and software for process simulations.

Pump networks are pipe networks in which most head changes are produced by pumps. When nonmonotoncity exists in one or more pump characteristic curves, possibility of multiple operating points exists. An optimization model for steady state networks of arbitrary topology, complexity, and size is presented. Stationary points of the optimization model define possible operating points. Uniqueness conditions as well as conditions for possible multiple operating points are identified. Operating points can correspond to minimums, maximums, or saddle points of the optimization model objective function. A two pump network example illustrates the different possibilities. It is concluded that the stationary point character defines the relative stability of different steady state operating points.

Unidirectional synchronization of Hodgkin–Huxley neurons

To achieve proper operation of an online static uninterruptible power supply (UPS) system, feedforward and feedback (FF/FB) controls are introduced. Feedforward control is applied to reduce the low order harmonics in the output voltage due to the ripples included in the input DC voltage to the UPS inverter resulting from the controlled rectifier, while feedback control is applied mainly for stabilizing the output voltage. In this paper, a feedforward technique is selected that was found to give free ripple output, and two types of feedback techiques are compared and the most suitable one is considered. The paper gives the modeling and simulation results of the whole system, that compare the performance without and with FF/FB under different steady state and transient conditions.

The operating points of a turbocharger compressor tend to approach or cross its surge line while an engine is accelerating, particularly under low-engine speed conditions, hence the need for an acceptable surge margin under low compressor-speed conditions. A method shifting the stability limit on a compressor low-speed line toward a lower flow rate is expected and inlet recirculation is often observed in a centrifugal compressor with a vaneless diffuser near a surge and under a low compressor-speed condition. First, examples of inlet recirculation were introduced in this paper, whereupon the effect of inlet recirculation on compressor characteristic was discussed by 1-D consideration and the potential shown for growth of inlet recirculation to destabilize compressor operations. Accordingly, this study focused on suppressing the effect of inlet recirculation on compressor characteristics using small fins mounted in a compressor-inlet pipe, and examining whether they enhance the compressor operating range under low-speed conditions. Small fins are known as inlet fins in this paper. According to test results, they showed great promise in enhancing the compressor operating range during inlet recirculation. Besides, attempts were also made to investigate the qualitative effect of inlet fins on flow fields using CFD and the disadvantages of inlet fins were also discussed.

It is well known that compressor surge imposes a significant limit on the flow range of a turbocharged internal combustion engine. The centrifugal compressor is commonly placed upstream of the inlet manifold and hence, it is exposed to the intermittent flow regime of the inlet valves. Surge phenomena has been well studied over the past decades, there still remains limited information with regards to the unsteady impact caused by the inlet valves. This study presents an experimental evaluation of such a situation. Engine representative pulses are created by a downstream system comprising a large volume, two rotating valves, a throttle valve and the corresponding pipe network. Different pulsation levels are characterized by means of their frequency and the corresponding amplitude at the compressor inlet. The stability limit of the system under study is evaluated with reference to the parameter B proposed by Greitzer [7–9]. B describes the dynamics of the compression system in terms of volume, area, equivalent length and compressor tip speed as well as the Helmholtz frequency of the system. For a given compressor, as B goes beyond a critical value, the system will exhibit surge as the result of the flow instability progression. The reduced frequency analysis shows that the scroll-diffuser operates in an unsteady regime, while the impeller is nearly quasi-steady. In the vicinity of the surge point, under a pulsating flow, the instantaneous operation of the compressor showed significant excursions into the unstable side of the surge line. Furthermore, it has been found that the presence of a volume in the system has the greatest effect on the surge margin of the compressor under the unsteady conditions.

Inlet swirl distortion has become a major area of concern in the gas turbine engine community. Gas turbine engines are increasingly installed with more complicated and tortuous inlet systems, like those found on embedded installations on Unmanned Aerial Vehicles (UAVs) and Auxiliary Power Units (APU). These inlet systems can produce complex swirl distortion in addition to total pressure distortion. The effect of swirl distortion on engine or compressor performance and operability must be evaluated. The gas turbine community is developing methodologies to measure and characterize swirl distortion. There is a strong need to develop a database containing the impact of a range of swirl distortion and total pressure distortion on compressor operability. In one aircraft installation of an APU, the compressor experienced a wide range of combined total pressure and swirl distortion levels as a result of the APU inlet geometry. In this unique installation, the inlet system includes a check valve at the entrance of the inlet plenum through which all the airflow to the APU compressor passes. During APU operation, the check valve flaps are pushed open by the force of the airflow entering the plenum. The orientation of the check valve with respect to the compressor centerline introduces a varying amount of combined total pressure and swirl distortion. Consequently, it was necessary to test and map the compressor for a range of check valve positions in order to find an orientation with the lowest surge margin loss. The check valve comprises of two “D” shaped flaps hinged at the diameter of a circular flow entrance to the plenum. The plenum forms a “U’ shaped volume which allows the flow to turn 90 degrees from the check valve into the compressor annulus. A series of compressor tests was conducted where the check valve was rotated on its base such that the check valve had various clocking orientations with respect to the compressor axis. The check valve orientation was varied by lining the flap hinge line parallel to the compressor centerline (defined as zero degrees) then rotating the check valve in 30 degree increments from this reference all the way up to 150 degrees. Compressor speed lines were mapped at each check valve orientation and the surge margin loss/gain from a nominal undistorted surge line was determined. Fully 3D CFD analyses of each of these check valve orientations were run. The CFD models included the full valve and plenum geometry up to the compressor annulus. From the CFD analysis, the ARP1420 (Reference 1) total pressure distortion descriptors and the AIR5686 (Reference 2) swirl distortion descriptors were calculated on the annular plane at the compressor eye. Using these total pressure and swirl distortion descriptors determined for each check valve orientation along with the surge margin change available from the test data, a correlation of operability behavior was successfully obtained. This paper discusses the lessons learned and a recommended methodology todetermine an operability correlation for combined total pressure and high swirl distortion inlet systems.

Electrification is playing a major role in the industrial and energy sector, with heat pump (HP) market expected to grow significantly in the next future in accordance with the current energy transition phase, which aims to reduce the utilization of fossil fuels for heat production sector. It is therefore of crucial importance to find new ways to increase heat pump performance and reliability, containing maintenance costs. The use of dynamic compressors in HPs makes it possible to combine good performance with high compactness and silent operation, but unlike the volumetric compressor, this equipment could undergo dangerous instability during operation, which can occur in closed-cycle configuration, quite unusual for dynamic compressors. The aim of this paper is to present a new test-rig for stable and unstable performance analysis of dynamic compressors for innovative heat pumps. An in-depth description of the plant and instrumentation system is provided. The performance of the compressor is analyzed for different operating points, with a particular focus on near-surge operation. Experimental uncertainties and their reduction through data reconciliation techniques is thoroughly investigated.

Properties of a discrete-time enhanced linear-quadratic controller

Probabilistic load flow (PLF) calculation, as a fundamental tool to analyze transmission system behavior, has been studied for decades. Despite a variety of available methods, existing PLF approaches rarely take system control into account. However, system control, as an automatic buffer between the fluctuations in random variables and the variations in system variables (e.g., nodal voltages), has a significant impact on the final PLF result. To consider control actions' influence, this paper proposes the first analytical PLF method for the transmission grid that takes into account primary and secondary frequency controls. This method is based on a high-precision linear power flow model, whose precision is even further improved in this paper by an original correction approach. This paper also proves that if the joint probability distribution (JPD) of random variables is expressed by a Gaussian mixture model (GMM), then the JPD of system variables is an infinite GMM. By leveraging this proposition, the proposed method generates the joint PLF of the whole system at the quasi-steady and steady states. The high accuracy and satisfactory efficiency of this method are verified on multiple test systems with different percentages of renewables.