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Twang Commutation Initialization
Twang commutation initialization is a method that moves the motor twice to find and set the initial com-
mutation angle of a brushless motor. When a fixed set of motor currents is applied to the three phases
of the motor, the rotor will rotate to an electrical angle with a known relationship to the applied current.
Since there are N/2 (N is the number of motor poles) electrical cycles per mechanical revolution of the
motor, the actual rotor position can be 1 of N/2 mechanical locations. In "Twang" initialization, at power
up, the servo drive applies current to all three motor phases in such a way that the motor rotor is forced
to move to a known electrical position with respect to the stator. The rotor position can actually end up
in one of two locations: The first is a stable equilibrium position and the second is an unstable equilib-
rium position. Since the servo drive cannot be certain at which of these two points the motor has
stopped, it applies a second set of currents which will rotate the motor to another known location. This
location will always be a stable equilibrium position and therefore results in one unique initial commuta-
tion angle.
The first set of currents applied will cause the rotor shaft to rotate up to +180 electrical degrees
(+180*2/N mechanical degrees). The second set of currents will cause the motor to rotate either 60 or
120 electrical degrees, depending upon which of the two locations the motor stopped at when the first
set of currents was applied.
TECHNOLOGY
Selection of a commutation initialization method
The first step in selecting a commutation initialization method is to determine whether motion can be
tolerated upon power up. If motion is not acceptable, then the motor must be equipped with Hall sen-
sors or commutation tracks and Hall initialization should be used. If motion is acceptable at power up,
then the second item which will prevent Twang initialization and Dither initialization from working prop-
erly is the presence of large external torque applied on the motor rotor. If large external torque exist
which either resist rotor motion (such as a break or excessive friction), or cause the rotor to rotate
(such as a gravity), then Twang and Dither can result in a non-optimal commutation angle. This occurs
because these modes both rely upon finding equilibrium between the applied motor current and the ro-
tor position; an external torque will alter this equilibrium position. If a large enough current is applied
during initialization, this external torque can be overcome and an acceptable commutation angle can
be achieved.
If motion is acceptable on power up and external torque is minimal, then either Twang or Dither initiali-
zation can be used. Dither is the quickest and requires the least motion. As the motion is very differ-
ent between these two modes, the operator should try each one and determine which works best for
his application.
If Twang or Dither initialization is selected, the amount of current applied to the motor during initializa-
tion must be set. This value can be set from MotionMaestro's commutation dialog. Typical values
range from 2.5 amps to 5.0 amps. This value should be set as small as possible while still being large
enough to overcome external torque. If too small a value is used, the motor will not be optimally com-
mutated, and this will result in rough motion and larger than normal current required to move the motor.
Appendix H
Summary of Contents for Omega SMC9930
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