Fast Dynamic Behaviour Research Articles

Generalised look‐up table concept for direct torque control in induction drives with multilevel inverters

This study deals with direct torque control (DTC) of induction motor drives when a multilevel inverter is involved. The proposed control technique is finalised to reduce the amplitude of torque oscillations and to improve the motor dynamic response regardless of operating speed conditions while assuring a satisfactory input current distortion. This is achieved by using a special look-up-table built with a novel approach. The control itself is based on the application in each sampling interval of one or two voltage vectors suitably selected, in order to ensure low electromagnetic torque ripple in steady-state operations and fast dynamic behaviour in transient operations. In the algorithm, the converter voltage vectors selection is carried out by using a novel look-up table whose entry points are computed by means of a specialised relation, starting from the outputs of multilevel hysteresis controllers. The suggested method, suitable for a generic multilevel inverter, is detailed for a neutral point clamped three-level inverter. The proposed algorithm is validated by several numerical results, and compared with a DTC algorithm based on a traditional three-speed range look-up tables. Finally, some experimental results, obtained with a scale laboratory setup, are shown, discussed and compared with the numerical ones.

Hysteresis current controller for a general n ‐level inverter fed drive with online current error boundary computation and nearly constant switching frequency

A space vector-based hysteresis current controller for any general n-level three phase inverter fed induction motor drive is proposed in this study. It offers fast dynamics, inherent overload protection and low harmonic distortion for the phase voltages and currents. The controller performs online current error boundary calculations and a nearly constant switching frequency is obtained throughout the linear modulation range. The proposed scheme uses only the adjacent voltage vectors of the present sector, similar to space vector pulse-width modulation and exhibits fast dynamic behaviour under different transient conditions. The steps involved in the boundary calculation include the estimation of phase voltages from the current ripple, computation of switching time and voltage error vectors. Experimental results are given to show the performance of the drive at various speeds, effect of sudden change of the load, acceleration, speed reversal and validate the proposed advantages.

Feasibility study for a new high resolution Thomson scattering system for the ASDEX Upgrade pedestal

A new Thomson scattering diagnostic is proposed for the study of fast plasma dynamics in the pedestal of ASDEX Upgrade. The diagnostic will measure electron temperature and density profiles over a ∼ 3 cm wide area in the edge transport barrier region, with ∼ 1–2 mm spatial resolution and ∼ 10 kHz sampling rate. A challenging goal of the project is the study of the bootstrap current in the plasma pedestal by measuring the distortion and shift of the electron distribution along the toroidal direction. Expected spatial and time resolutions of the current density measurements are ∼ 3 mm and ∼ 1 ms correspondingly. The new diagnostic will be used to study the fast dynamic behaviour of the pedestal bootstrap current, where models indicate that it plays a key role in regulating edge stability, e.g. during ELMs. The diagnostic design is based on the intra-cavity multi-pass system currently in operation in TEXTOR, which uses a probing ruby laser, a grating spectrometer and two fast CMOS cameras for scattered light detection, and has achieved measuring accuracies of the order of ∼ 1% for ne and ∼ 2% for Te. Parts of that system will be reused in ASDEX Upgrade (some with significant modifications), but the laser multi-pass and light collection systems are entirely redesigned. Restrictions in space and line-of-sight availability have led to the adoption of a design which uses in-vessel multi-pass mirrors and light collection optics, requiring a number of innovative technical solutions to permit remote laser alignment and light collection. We give an overview of the project, discuss the underlying physics basis and present a number of technical solutions employed.

Developments in industrial applications of self-tuning control

Control strategies based on auto-tuning and self-tuning principles are now emerging to provide a realistic option for a wide range of industrial processes and plant. This is particularly the case when considering batch and fed-batch process control systems in which assumptions on slowly varying process dynamics permit the use of such adaptive schemes. However, there are many industrial systems where commercial ‘off the shelf’ controllers do not provide an adequate solution. Consequently there is much on-going research activity in the refining and tailoring of algorithms in order to provide effective implementation on specific industrial applications. With consideration given to two industrial case-study applications, the paper describes extensions and refinements that have been introduced in order to accommodate for improved estimation and control law design algorithms when dealing with systems exhibiting fast dynamic behaviour and non-linear response characteristics respectively. The requirement for improved estimation schemes arises when one is compelled to make use of low-order linear model structures whereas when dealing with non-linear systems, which exhibit slowly varying dynamic characteristics, one can afford to invest in improvements in control law design.

Design of Self-Tuning Controllers for Industrial Plant

Self-tuning control is now rapidly maturing to provide a realistic option for the control of a wide range of industrial systems. This is particularly evident in the field of proccss control where system lime constants arc relatively slow and assumptions on slowly varying plant dynamics permits the use of such adaptive schemes. However, the application of self-tuning techniques to systems exhibiting fast dynamic behaviour and/or severe non-linear characteristics appears to be somewhat limited and it is this, coupled with recent advances in computer technology, that has provided the impetus for much interest and on-going research. By adopting a ease study approach, the paper gives detailed consideration to the application of self-tuning control to an engine test cell which exhibits fast dynamic behaviour and to a high temperature gas fired heat treatment proccss which exhibits severe plant non-linearities. Drawing from these two industrial applications, some of the problems encountered when implementing standard self-tuning control schemes arc highlighted and the need for the tailoring and refining of the algorithms for specific applications is emphasised.

A retrospective self‐tuning controller

AbstractA new self‐tuning control algorithm appropriate for industrial systems which exhibit non‐linear characteristics and fast dynamic behaviour is proposed. The resulting self‐tuning controller makes use of a weighted sum of the control input generated from a standard self‐tuning control algorithm combined with a set of control input suggestions evaluated retrospectively. Use is made of a past sequence of steady state gains evaluated over a prespecified retrospective horizon together with a filtered prediction of the current system output which is obtained from a steady state Kalman filter. Adopting such an approach provides an element of corrective action based on information which would otherwise be disregarded by a standard self‐tuning scheme.Simulation studies involving a number of identified linear models of an industrial hydraulic servo system are presented. In terms of reduced control input variance and increased accuracy of set‐point following, the effectiveness of the new retrospective self‐tuning controller is compared to that of a standard self‐tuning pole placement controller and results indicate that significant improvements in overall performance are possible.

Logical covariance matrix reset in self-tuning control

The paper investigates the application of a state-space self-tuning pole-placement strategy to a non-linear hydraulic test rig designed specifically for evaluating the effectiveness of self-tuning control schemes. The test rig is representative of a wide range of industrial systems exhibiting both non-linear characteristics and fast dynamic behaviour, such factors, until recently, prohibiting the effective use of linear self-tuning schemes. In recognition of this shortfall, a novel technique for enhancing the covariance matrix update stage within the parameter estimation algorithm of a self-tuning controller is proposed. The technique, termed logical covariance matrix reset, requires only a minimum of computational overhead and is shown to give rise to a significantly improved parameter tracking ability by making more effective use of potential information available to the estimation algorithm at set-point change. Results of simulation studies involving the application of the logical resetting technique to the hydraulic test rig are presented when the rig is subjected to slow variation in feed flow and sudden variation in load.

Industrial Applications of Self-tuning Control

The use of self-tuning control is now becoming well established in U.K. industry as witnessed by the availability of commercial self and auto-tuning controllers. As a consequence there is much on-going research activity in the refining and tailoring of algorithms in order to accommodate effective implementation on specific industrial applications. The paper considers three potential industrial systems, each exhibiting non-linear and/or fast dynamic behaviour. These are: hydraulic servo system, dynamometer torque loop and a heater-fan system. Drawing from the results of the three investigations, relative advantages of adopting features such as logical reset action, switched controller structure and bilinear model structures are discussed.