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Method to Detect the Rotor Position of a Sensorless Brushless DC Motor

IP.com Disclosure Number: IPCOM000120129D
Original Publication Date: 1991-Mar-01
Included in the Prior Art Database: 2005-Apr-02
Document File: 3 page(s) / 122K

Publishing Venue

IBM

Related People

Erickson, KJ: AUTHOR [+2]

Abstract

A method is described that can be used to detect the rotor position of a sensorless brushless DC motor for the purpose of reliably starting such motors with the correct drive phase and sequence in order to minimize false starts and movement in the wrong direction, which may be detrimental in certain motor-driven systems.

This text was extracted from an ASCII text file.
This is the abbreviated version, containing approximately 52% of the total text.

Method to Detect the Rotor Position of a Sensorless Brushless DC Motor

      A method is described that can be used to detect the
rotor position of a sensorless brushless DC motor for the purpose of
reliably starting such motors with the correct drive phase and
sequence in order to minimize false starts and movement in the wrong
direction, which may be detrimental in certain motor-driven systems.

      Fig. 1 contains a block diagram of the spindle motor drive
logic used to implement the method depicted herein for detecting the
rotor position of a sensorless brushless DC motor.  This method is
based on a relative inductance measurement technique, which
determines the lowest inductance motor phase for the existing rotor
position, and is used to resolve the correct phase of motor current
to apply to operate the motor in the desired direction from an
existing position.

      The drive circuits contained in this diagram are used to effect
the rotor position detection method by driving all three windings of
the motor simultaneously.  This is done by selectively switching the
motor centertap to either a high or low drive level while at the same
time switching all of the low or high phase drivers on, respectively,
and all of the high or low phase drivers off, respectively.  In this
manner, inductance information can be obtained from all three motor
phases simultaneously for each of the two current drive directions.

      The current sense values that are used to determine the current
rise time of each phase, and hence the inductance of each phase, can
be digitized either simultaneously by three A/D converters at the
sample time, or S/H circuits can be used to save the current sense
values at the sample time, which can then be multiplexed to a single
A/D converter sequentially to be digitized.  When the rotor position
detection is not being performed, the centertap switches are both
turned off, and the motor can be driven normally by the phase
switches.

      NOTE:  There are a number of other possible ways to architect
multiplexers, S/H circuits, and A/D converters to acquire the six
current sense values.

      The diagram in Fig. 2 shows how the motor drive is enabled and
disabled simultaneously for all three motor windings, and how each
winding has a different current rise time, representing different
values of inductances for each winding.  One of the benefits of
applying the motor current to all three phases at the same time is
that the combined torques of the phases tends to cancel such that it
virtually eliminates any problem of the mot...