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
secondorder, linear system with a transformed voltage as an input
and the magnitude of the rotor flux as the output, and ii) a
secondorder, "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 offtheshelf induction motors. The first algorithm
implemented was an adaptive position tracking controller.
Another experiment was conducted to demonstrate the effect of
fieldweakening 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 steadystate 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. 47074712.
