Environmental Engineering Reference
In-Depth Information
architectures of the ISG variety use VRM designs that are based on 6/4 saliency
pattern repeated two, three or more times about the periphery of the machine. To
such machines, 0.1
resolution or higher are necessary for proper timing of the
signals. Such position sensors are more likely laboratory grade and, sometimes of
precision instrument quality, not the rugged sensor demanded in an automotive
environment.
Rajashekara
et al.
[4] provide a comprehensive summary of position sensorless
techniques employed on the five major electric machine types. In Table 7.1, a
summary of the common types of sensorless methods investigated, or under
investigation, are given referenced by ac machine type. In all position sensorless
techniques there is strong reliance on accurate measurement of machine currents,
voltages and temperature environment.
Table 7.1 Summary of sensorless control methods
Electric machine type Sensorless control method
Reference
Asynchronous
Slip frequency calculator
Slot harmonics (signal injection and heterodyning)
Flux estimation
[5]
Observer based
[6]
Model reference
[7]
Permanent magnet
Back-emf sensing
[8]
Stator third harmonic voltage
[9]
Phase current sensing
[10]
Synchronous reluctance Torque angle calculator
[11]
Stator third harmonic voltage
[12]
Switched reluctance
Incremental inductance measurement
[13]
Flux current method
[14]
Mutual induced voltage
A common form of position sensorless control for induction machines has been
to measure the stator currents and voltages and from these measurements plus
motor parametric data, compute the slip, then subtract this slip frequency from the
excitation frequency to arrive at rotor speed. These methods do work but unfortu-
nately require pure integration of the rotor induced voltages, which is prone to
contain dc offsets that corrupt the integrator output. State observers have been used
to estimate rotor flux but are plagued by tracking error of stator currents and rotor
flux. Yoo and Ha [5] introduced a technique wherein a motor speed estimator is
constructed using a main estimator and a complimentary estimator. This method
is currently being pursued by others who are interested in minimizing the impact
that differentiation of the stator currents has on estimating rotor flux and from
it, motor speed. In particular, Khalil
et al.
[6] show that by estimation of the
quadrature axis current and its derivative it is possible to then compute rotor flux
and rotor speed. In this scheme the voltage reference signals to the induction
machine are developed using a sliding mode controller. Differentiation of stator
