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Browse Prior Art Database

Lead Angle Control in a Step Motor

IP.com Disclosure Number: IPCOM000099500D
Original Publication Date: 1990-Jan-01
Included in the Prior Art Database: 2005-Mar-15
Document File: 2 page(s) / 97K

Publishing Venue

IBM

Related People

Heink, PJ: AUTHOR

Abstract

Disclosed is a technique for inexpensively monitoring and controlling the lead angle in a step motor. This technique does not require any additional sensors or encoders other than the step motor itself and allows the step motor system to operate reliably despite wide fluctuations in system loads and motor/driver tolerances. It also results in a more quiet and efficient operation of the step motor system and does all of this at only a minimal increase in cost. The following description assumes that the step motor is being driven in full steps with two phases on by a bipolar chopping driver.

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This is the abbreviated version, containing approximately 52% of the total text.

Lead Angle Control in a Step Motor

       Disclosed is a technique for inexpensively monitoring and
controlling the lead angle in a step motor.  This technique does not
require any additional sensors or encoders other than the step motor
itself and allows the step motor system to operate reliably despite
wide fluctuations in system loads and motor/driver tolerances.  It
also results in a more quiet and efficient operation of the step
motor system and does all of this at only a minimal increase in cost.
The following description assumes that the step motor is being driven
in full steps with two phases on by a bipolar chopping driver.

      The technique is based on the reconstruction of the back
electromotive force (back EMF) signal in a motor coil while the motor
is operating.  The zero crossing of this signal is detected and it is
compared with the phase commutation signal.  The time delay between
these signals gives an accurate indication of the average lead angle
of the motor.  With the lead angle thus resolved, it can be
controlled by adjusting the chopping current to the step motor.

      As an alternative, it is also very easy to use the zero
crossing in one coil of the motor to trigger the phase commutation in
the other phase, thus always maintaining a constant lead angle.
Speed can be controlled by adjusting the chopping current and/or the
driver voltage.

      The back EMF is a voltage generated in the windings of a motor
which results from the motion of the rotor in the motor.  Its
amplitude is proportional to the velocity of the rotor and it
generally has a polarity of opposing the applied voltage which runs
the motor.  The back EMF in a motor winding can be expressed as:

                            (Image Omitted)

Ub=V-iRm-Ldi/dt where
    -    Ub is the back EMF voltage,
    -    V is the voltage applied to the winding, it can be measured
as V2-V1 where V2 and V1 are the voltages, relative to ground, sensed
on each of the two leads of the motor winding.
    -    i is the current through the motor winding
    -    Rm is the motor winding resistance
    -    L is the motor winding inductance
    -    di/dt is the change in motor current with respect to time.
    We construct the back EMF signal by measuring:
    V2-V1#aVref'
    where
    Vref is a reference voltage used by the motor driver to set the
constant current level (usually by chopping the voltage on and off
with some duty cycle),

      -_ a is chosen as IcRm/Vref where Ic is the chopping current
re...