Advanced control techniques, both robust and nonlinear, are investigated for the design of fast dynamics motor drives and power converters. Several different electric motors are considered in a unified conceptual framework almost independent of the kind of considered motor. Particular emphasis is given to direct drive connections of the load typical of robot applications. The problem of supply line interfacing of motor drives is also addressed. Different power converters structure and related nonlinear control are designed in order to provide near unity power factor and low harmonic distorsion on drive AC supply line. Experimental validation of such motion control algorithm are efficiently carried out using the Rapid Prototyping facility (FastProt), available on LAR. This facility allows simple and fast switching between real and simulated experiment, speeding-up the control validation and tuning, and is especially useful for control system researcher without particular knowledge on digital control and power converter hardware.
The control of electric motors
and more generally the motion control problem has been addressed within
a general framework. In particular, a multi-stage cascade control structure
(velocity, flux, current or torque) is considered allowing for an easy
standardization for the different motors. At each control level, the most
powerful methodologies and the most suitable technologies can be freely
used. An interesting results is reported in "C. Rossi and A. Tonielli.
Robust control of permanent magnet motors: VSS techniques lead to simple
hardware implementations. IEEE Trans. on Industrial Electronics, August
1994", where it is shown that, for permanent magnet motors, VSS techniques
lead to the design of fast dynamics robust regulators easily implementable
with very simple "custom" hardware. Considering the motion control system
as AC line supplied electrical to mechanical energy transformer, is ideally
possible to include on it also the AC to DC converter. This is suitable
mainly when restrictive specifications on supply line current quality
are required and/or motor braking energy must be recovered on line. In
"R. Morici, C. Rossi, and A. Tonielli. Variable structure controller
for ac/dc boost converter. In IEEE Int. Conf. on Ind. Electr. Contr. and
Instr., IECON '94, Bologna, Italy, September 1994", this problem
is addressed using a bidirectional single-phase AC/DC boost converter,
which ensure both robustness to load variation and near-unity power factor.
Also in this case adoption VSS control techniques results in very simple
hardware implementation. Digital control of three-phase AC/DC converter
is reported in " R. Morici. Rapid Prototyping of non-linear Algorithm
for Motion Control Systems. PhD thesis, University of Bologna - DEIS,
Bologna, ITALY, February 1995. in italian", where output feedback
linearization algorithms are adopted in converter control to achieve the
same unity-power factor goal.
Both synchronous and asynchronous machines generate torque as a function of the current. In particular, in a suitable reference frame the quadrature current is responsible for the torque generation. Available power amplifiers are voltage source switching devices, with strong limitations on the maximum admissible switching frequency. Besides, very-fast dynamics links the voltage to the current. An almost ideal (at least in the bandwidth of the other state variables) current source inverter can be obtained using VSS techniques for the design of a closed-loop current controller. The design of a current controller for Y-connected machines has been studied in previous activities, while for DC and Variable Reluctance machines (independently connected windings) is reported in "C. Rossi and A. Tonielli. Feedback linearizing and sliding mode control of a variable reluctance motor. International Journal of Control, 60(4):543--568, 1994" and "C. Rossi and A. Tonielli. Robust control of permanent magnet motors: VSS techniques lead to simple hardware implementations. IEEE Trans. on Industrial Electronics, August 1994". Further developments are reported in "C. Rossi and A. Tonielli. Variable structure current controller for a three-phase inverter using finite-state automaton. IEEE Transaction on Industrial Electronics, 42(2):169--178, April 1995", where a novel power converter feedback current controller is presented. This converter control architecture is suitable for imposing currents on different kind of AC motors and loads.
This kind of motor is particularly suitable for direct-drive robotic applications since motors with a very large number of poles can be easily designed. All the phases of the controller design were considered, i.e., modeling, optimization and control. An original model structure, strongly simplifying optimization and control problems, has been defined and identified for a real torque-motor. The proposed model is based on the use of flux as the selected state variable. This leads to separation of magnetical and positional nonlinearities, which enters additively in the model. Based on the proposed model and using nonlinear optimization techniques, the problem of sharing the motor torque among the three phases has been solved designing a static torque compensator. The torque control problem has been addressed using feedback linearization techniques or VSS techniques and also comparing the two approaches where some singularities arising at zero torque are solved modifying the design of the torque compensator.
The control of high-performance electrical drives requires knowledge of all the machine state variables, some of which are often not directly measurable. In the induction motor control, the knowledge of rotor flux is necessary in order to design a torque and magnetization dynamics decoupling control. The state observation of nonlinear systems, such as induction motor, must be carefully applied: the closed loop system may be unstable also if both control and observer dynamics are stable. It was shown that the best results are obtained designing the observer directly in discrete time domain in order to overcome stability degradation effect, due to discretization process, inherent to digital implementation.
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