SENSORLESS FIELD ORIENTED CONTROL OF BRUSHLESS ...

CHAPTER 6 - Sensorless Techniques
It is evident that control of a brushless PM motor requires knowledge of the shaft position. For
six-step 120° commutation, only six pulses per electrical revolution are required (hence three
Hall-effect sensors are often used); for 180° commutation, position feedback of higher resolution
is required (such as that provided by an encoder or resolver). This chapter discusses techniques
that can be used to replace a physical shaft position sensor by indirectly determining the shaft
position. Hence, in BPMS motor control, sensorless refers to the lack of a position sensor; voltage
and/or current sensors are still required.
First, a brief overview of techniques is presented. Then the general structure of a sensorless FOC
system is described and an open-loop estimation scheme is presented. Then the closed-loop
topology (a Luenberger observer) is discussed further.
Overview of Techniques
As with BPMS motor types, PWM methods, and direct torque control schemes, there are a very
large number of sensorless techniques reported in the literature. As before, there are many
different approaches to classifying the sensorless methods. In this report the top-level distinction
is between the techniques applicable when using a 120° inverter and those applicable when using
either a 120° or a 180° inverter.
In the 120° inverter there is a 60° period per half-cycle when both switches in a phase leg are
OFF. This gives a chance at monitoring the terminal voltage to detect the zero-crossing of the
bEMF. These techniques are usually based on voltage measurement but not always. There are a
number of ways to accomplish the task in practice. There are also a large number of practical
issues that must be dealt with to ensure successful operation; the issues differ between methods
and not all methods are applicable to all motors. For a given motor and drive, use of a sensorless
control scheme may limit operation to an operating region that is only a subset of the capability of
the same motor and drive under sensored control. All 120° voltage measurement techniques fail
at low speed, some may fail with increasing speed or load, and none can achievable appreciable
commutation advance unless a neutral connection is used in sensing. As discussed in Appendix
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