Observed Field Oriented Control of Induction Motors

Background:

The technique of field oriented control (FOC) was first proposed by Blashke in 1972. Since then it has been the most widely used technique of speed control for induction motors. Roughly speaking, the FOC approach utilizes a nonlinear coordinate transformation to rewrite the rotor flux and stator current dynamics into: i) one second-order, linear system with a transformed voltage as an input and the magnitude of the rotor flux as the output, and ii) a second-order, "almost linear" system with a transformed voltage as an input and the rotor velocity as the output. The linear and the almost linear structure of these two transformed systems are then used to motivate the construction of two linear rotor velocity/rotor flux magnitude tracking controllers.

The Approach:

Motivated by the fact that the FOC scheme neglects the nonlinear coupling between the rotor velocity and the the rotor flux dynamics, we have designed an adaptive observed field oriented control (OFOC) scheme which is free of singularities, does not require rotor flux measurements, and decouples the rotor flux tracking/rotor position (or velocity) tracking objectives. That is, provided the rotor position, rotor velocity, and the stator current measurements are available, the controller ensures global asymptotic rotor position and rotor flux tracking despite the uncertainty associated with the mechanical subsystem parameters and the stator circuit electrical parameters. These enhancements to the standard FOC control scheme are achieved by utilizing observed rotor flux in the stator current/voltage transformation in lieu of the actual rotor flux and by systematically constructing a more sophisticated rotor flux observer.

The Setup:

Experiments were conducted in the Clemson University's Union Camp Motor Laboratory to test the performance of the OFOC scheme on standard off-the-shelf induction motors. The first algorithm implemented was an adaptive position tracking controller.
Another experiment was conducted to demonstrate the effect of field-weakening on the power loss in the machine. Field weakening refers to the technique of reducing the flux reference above some base speed. That is, the desired rotor flux magnitude function is initially maintained at a constant value which is enough to generate the required torque and beyond some base speed, the desired rotor flux magnitude function is decreased to a lower value, which denotes the steady-state desired rotor flux magnitude.

The Mechatronics Workstation - Union Camp Laboratory

Some Experimental Results:

Select and click to view some of the experimental plots:



Publication:

For more details on this research, please refer the following publication:

P. Vedagarbha, D. Dawson, W. Rhodes and T. Burg, "Nonlinear Control of Induction Motors: The Observed Field Oriented Control Scheme", Proceedings of the IEEE Conference on Decision and Control, Kobe, Japan, Dec. 1996, pp. 4707-4712.