Application of fuzzy-sliding mode control and electronic load emulation to the robust control of motor drives

This thesis is concerned with the experimental investigation of robust speed control strategies for the industrial motor drive systems. The first objective of the thesis is to implement a high performance programmable dynamometer which can provide desired linear and non-linear mechanical loads for the experimental validation of the robust control methods. The discrete time implementation of the conventional dynamometer control strategy (the inverse model approach) is analysed and it is shown that this method suffers from the stability and noise problems. A new dynamometer control strategy, based on speed tracking and torque feedforward compensation, is developed and successfully implemented in the experimental system. The emulation is placed in a closed loop speed control system and the experimental results are compared with the corresponding ideal simulated results for the validation of the dynamometer control strategy. The comparisons show excellent agreement for a variety of linear and nonlinear mechanical load models and such a high performance experimental load emulation results are reported for the first time in research literature. The second objective of the project is to investigate the Fuzzy Logic Control (FLC) and the Sliding Mode Control (SMC) approaches in order to develop a simple,• algorithmic and practical robust control design procedure for industrial speed drive control systems. The Reaching Law Control (RLC) method, which is an approach to SMC design, and the FLC are used together in order to develop a practical robust speed control strategy. The robustness of the proposed control approach is tested for a variety of linear and non-linear mechanical loads provided by the dynamometer. Using the new robust control method, good output responses are obtained for large parameter variations and external disturbances

Projektiranje upravljanja mehatroničkog sustava zasnovano na dinamičkom oponašanju mehaničkog tereta

The paper presents and analyses rapid prototyping methods for the dynamic emulation of mechanical loads. Approaches can be applied in design, testing and validation of the mechatronic systems propelled by electric drives. Actual system (prototype) is replaced by the torque controlled electromechanical load, for which the required torque is calculated through the closed-loop control algorithm. The active load is connected mechanically to the drive shaft, using the clutch. Also possible applications in the control design for variable speed and torque drives are described. For the illustration of the method emulation of the pump mechanism is given.U članku se opisuje i analizira brza metoda dinamičkog oponašanja mehaničkog tereta. Pristup se može primijeniti za projektiranje, testiranje i provjeru valjanosti mehatroničkog sustava upravljanog elektromotornim pogonom, te posebno za prototipna ispitivanja. Aktualni sustav (prototip) je zamijenjen elektromehaničkim teretom, koji je upravljan momentom. Potrebni moment se računa iz zatvorene regulacijske petlje. Aktivni teret je spojkom mehanički povezan s pogonskom osovinom. Prikazane su također moguće primjene u projektiranju sustava promjenjive brzine i momenta. Za ilustraciju metode oponašanja tereta prikazan je pumpni mehanizam

Nonlinear optimal control of interior permanent magnet synchronous motors for electric vehicles

At present time, research in the field of Electric Vehicles (EV) is significantly intensifying around the world due to the ambitious goals of many countries, including the UK, to prohibit the sale of new gasoline and diesel vehicles, as well as hybrid vehicles, in the near future around 2030-35. The primary goal of this Ph.D. research is to improve the propulsion system of electric vehicles' powertrains through improvements in the control of Interior Permanent Magnet Synchronous Motors (IPMSM), which are commonly used in EV applications. The proposed approaches are supported by simulations in Matlab, Matlab-Simulink and laboratory-based experiments. The research initially proposes an analytical solution in implicit view for a combined Maximum Torque per Ampere (MTPA) and Maximum Efficiency (ME) control, allowing to determine the optimal d-axis current, based on the concept of minimisation of the fictitious electric power loss. With the exception of two parameters, the equation is identical to that of the ME control. Therefore, upgrading the ME control to the combined MTPA/ME control is relatively easy and doesn't require any change in hardware beyond a few minors of controller code in the software. The presented research demonstrates an easy-to-apply combined MTPA/ME control leading to the ‘Transients Optimal and Energy-Efficient IPMSM Drive’ providing smooth transitions to the MTPA control during transients and to the ME control during steady states. A concept of ‘Nonlinear Optimal Control of IPMSM Drives’ is also introduced in this Ph.D. research. The velocity control loop develops nonlinearities when energy consumption optimisation methods like MTPA, ME, or combined MTPA/ME are added. In addition, the control system's parameters can be inaccurate and fluctuate depending on the operating point or possible uncertainties in real-time operation. In the proposed method, the control structure is the same as in the Field Oriented II Control (FOC), with the close velocity and two current loops, but the Proportional-Integral (PI) controllers are replaced by Nonlinear Optimal (NO) Controllers. The linear part of the controller is designed as a Linear Quadratic Regulator (LQR) with integral action for each loop separately. This is, in fact, a PI controller with optimal gain parameters for a specific operating point. The nonlinear part takes the required fluctuations of the control system’s optimal gain parameters in real-time operation as new control actions to improve a robust control structure. The design procedure for the nonlinear part is similar to that of the LQR, but the criterion of A. Krasovsky's generalised work is used, and the analytical derivations lead to an explicit control solution for the nonlinear optimal part. The nonlinear part emulates the adjustments for updating the linear part’s optimal LQR gains based on operating conditions, instead of employing extensive look-up tables or complicated estimation algorithms. The proposed control is robust in the allowed range of the system’s parameters. In conclusion, upgrading existing industrial IPMSM drives into a robust and optimal energy-efficient version that can be used for electric vehicle applications is the main advantage of the novel control concept described in this Ph.D. research. For this upgrade, only a small portion of the software that is related to the PI controllers needs to be changed; no new hardware is needed. Therefore, it is cost-effective and simple to transform existing industrial IPMSM drives into a better version with the proposed method. This feature also leads to the design of more adequate IPMSM drives to meet the demands of Electric Vehicle (EV) operating cycles

Modeling and Emulation of Induction Machines for Renewable Energy Systems

Electric motors with their drive systems utilize most of the generated power. They account for about two-thirds of industrial energy utilization and about 45% of the global energy utilization. Therefore, optimizing a motor and its corresponding drive system with their control can save energy and improve system efficiency. It can be risky and difficult however to test large prototype electrical machines and study their dynamic characteristics; or to test electric drives at high power levels with different machine ratings and operating conditions. One way to effectively evaluate these systems is to emulate the electric machine using a power electronic converter with the help of a real-time simulation system. Power electronic converters and their control systems are increasingly being used in industries with different power ratings at high switching frequencies in the kHz range. In this thesis, an induction motor emulator based on a power electronic converter is developed to allow detailed testing the converter and controller. A proportional-resonant current controller in the abc-frame and pulse-width modulation are employed. The conventional model of the induction machine (IM) with constant parameters does not represent accurately the machine’s performance for severe transients specifically during starting and loading conditions. Magnetic saturation effects should be considered. Hence, experimental procedures to determine the flux saturation characteristics in the main and both stator and rotor leakage flux paths are achieved. Machine models that consider or neglect the main and leakage flux saturation are compared with experimental results. The model which considers the magnetic saturation effect in both flux paths results in more accurate transient responses. Likewise, the dynamic response of the induction motor emulator during startup and loading transients show the effectiveness of using the developed emulator to resemble closely a real motor. The relationship between the stator and rotor leakage reactance of the induction machine according to IEEE Std. 112™ is assumed to be constant under all operating conditions. However, this is not accurate during severe transients such as the direct online startup and loading conditions of a three-phase induction motor. The leakage reactance of the machine can vary widely during severe conditions. Hence, using constant parameters in the machine model will result in an inaccurate dynamic performance prediction. Moreover, considering a constant ratio between the stator and rotor leakage reactance is no longer valid for all current levels. In this thesis, a direct and precise method is proposed to estimate and separate the stator and rotor leakage reactance parameters under normal operating conditions and when the core is deeply saturated. The method exploits the 2D time-stepping finite element method (FEM) with a coupled circuit. The obtained current-dependent reactance functions in both leakage flux paths are included in the dq-model of the IM. Other machine parameters are determined by implementing the standard tests in FEM. To verify the effectiveness of the proposed method, the predicted results are compared to the dynamic responses obtained experimentally from a three-phase, 5-hp squirrel-cage IM. A power electronic converter-based self-excited induction generator (SEIG) emulator is developed. The testbed replaces a wind- or microhydro-turbine driven squirrel-cage induction generator that works within an isolated power system to feed power in remote areas. It supports testing and analyzing the dynamic performance of islanded generation systems which comprise numerous kinds of parallel-operated renewable energy sources. The risk and cost associated with the testing, analysis and development of novel control topologies and electrical machine prototypes are reduced considerably. The dq-model of the SEIG in the rotor reference frame is implemented in a real-time controller. Saturation in the main and leakage flux paths are included in the machine model. The generator model with modified parameters is verified and used in the emulator. The cascaded voltage and current loops utilizing the proportional-integral controllers in dq-frame are employed. A voltage-type ideal transformer model is used as a power interface for the emulator whereas an excitation capacitance is added to the power-hardware-in-the-loop block diagram. Likewise, the dynamic response of the induction generator emulator during voltage buildup and loading conditions validates the effectiveness of using the developed emulator to resemble closely a real generator

Inverter Design for SiC-based Electric Drive Systems with Optimal Redundant States Control of Space Vector Modulation

The need for inverters with ever increasing power density and efficiency has recently become the driving factor for research in various fields. Increasing the operating voltage of the whole drive system and utilizing newly developed SiC power switches can contribute towards this goal. Higher operating voltage allows the design of drives with lower current, which leads to lower copper losses in cables and machine, while SiC switches can drastically increase the inverter efficiency. Offshore renewable power generation, such as tidal power, is a typical application where the increase of operating voltage can be highly beneficial. The ongoing electrification of transportation calls also for high power electric powertrains with high power density,where SiC technology has key advantages.In the first part of the thesis, suitable control schemes for inverters in synchronous machine drive systems are derived. A properly designed Maximum Power Point Tracking algorithm for kite-based tidal power systems is presented. The speed and torque of this new tidal power generation system varies periodically and the inverter control needs to be able to handle this variable power profile. Experimental verification of the developed control is conducted on a 35 kVA laboratory emulator of the tidal power generation unit.Electric drives using multilevel inverters are studied afterwards. Multilevel inverters use multiple low-voltage-rated switches and can operate at higher voltage than standard two-level inverters. The Neutral Point Clamped (NPC) converter is a commonly used multilevel inverter topology for medium voltage machine drives. However, the voltage balancing of its dc-side capacitors and the complexity of its control are still issues that have not been effectively solved. A new method for the optimal utilization of the redundant states in Space Vector pulse-width-Modulation (SVM) is proposed in this thesis in order to control its dc-link voltages. Experimental verification on a 4-kV-rated prototype medium-voltage PMSM drive with 5-level NPC converters is conducted in order to validate the effectiveness of the proposed control technique.Low switching and conduction losses are typical characteristics of SiC switches that can be utilized to build inverters with high power density, due to the increased efficiency and smaller form-factor. Due to the above, SiC power modules have been particularly attractive for the automotive industry. The design approach of 2-level automotive inverters has been studied in this project. Moreover, a new design approach for the cooling system of automotive inverters has been developed in this thesis, which fine-tunes the inverter heatsink utilizing standard legislated test routines for electric vehicles. Multiple conjugate-heat-transfer (CHT) computation results showcase the iterative optimization procedure on a test-case 250 kW (450 A) automotive SiC inverter.Finally, the experimental testing of high power machine drives in order to verify the control and the hardware design is an important step of the development process. Thus, the performance of the prototype 450 A SiC 2-level inverter has been been experimentally validated in a power hardware-in-the-loop (P-HIL) set-up that emulates an automotive drive system. Several challenges have been addressed with respect to the accurate modelling of the motor and the control of the circulating power in the system. A new control technique utilizing the redundant states of the SVM has been developed for this set-up to effectively suppress the zero-sequence current to 3.3 % of the line current at rated power

Design of an Elastic Actuation System for a Gait-Assistive Active Orthosis for Incomplete Spinal Cord Injured Subjects

A spinal cord injury severely reduces the quality of life of affected people. Following the injury, limitations of the ability to move may occur due to the disruption of the motor and sensory functions of the nervous system depending on the severity of the lesion. An active stance-control knee-ankle-foot orthosis was developed and tested in earlier works to aid incomplete SCI subjects by increasing their mobility and independence. This thesis aims at the incorporation of elastic actuation into the active orthosis to utilise advantages of the compliant system regarding efficiency and human-robot interaction as well as the reproduction of the phyisological compliance of the human joints. Therefore, a model-based procedure is adapted to the design of an elastic actuation system for a gait-assisitve active orthosis. A determination of the optimal structure and parameters is undertaken via optimisation of models representing compliant actuators with increasing level of detail. The minimisation of the energy calculated from the positive amount of power or from the absolute power of the actuator generating one human-like gait cycle yields an optimal series stiffness, which is similar to the physiological stiffness of the human knee during the stance phase. Including efficiency factors for components, especially the consideration of the electric model of an electric motor yields additional information. A human-like gait cycle contains high torque and low velocities in the stance phase and lower torque combined with high velocities during the swing. Hence, the efficiency of an electric motor with a gear unit is only high in one of the phases. This yields a conceptual design of a series elastic actuator with locking of the actuator position during the stance phase. The locked position combined with the series compliance allows a reproduction of the characteristics of the human gait cycle during the stance phase. Unlocking the actuator position for the swing phase enables the selection of an optimal gear ratio to maximise the recuperable energy. To evaluate the developed concept, a laboratory specimen based on an electric motor, a harmonic drive gearbox, a torsional series spring and an electromagnetic brake is designed and appropriate components are selected. A control strategy, based on impedance control, is investigated and extended with a finite state machine to activate the locking mechanism. The control scheme and the laboratory specimen are implemented at a test bench, modelling the foot and shank as a pendulum articulated at the knee. An identification of parameters yields high and nonlinear friction as a problem of the system, which reduces the energy efficiency of the system and requires appropriate compensation. A comparison between direct and elastic actuation shows similar results for both systems at the test bench, showing that the increased complexity due to the second degree of freedom and the elastic behaviour of the actuator is treated properly. The final proof of concept requires the implementation at the active orthosis to emulate uncertainties and variations occurring during the human gait