Reference Manual

PMAC 2 Software Reference
PMAC I-Variable Specification 105
Remarks
WARNING:
An unreliable phasing search method can lead to a runaway
condition. Test your phasing search method carefully to make sure
it works properly under all conceivable conditions. Make sure your
Ix11 fatal following error limit is active and as tight as possible so
the motor will be killed quickly in the event of a serious phasing
search error.
Ix74 defines the time that an open-loop output is to be used if a power-on phasing search is
done for a PMAC-commutated motor (Ix01=1). A phasing search is required for a
synchronous motor (Ix78=0) such as a permanent-magnet brushless motor with no
absolute position sensor (Ix81=0). The phasing search is done automatically as part of the
power-on phasing search if Ix80 is 1 or 3; if Ix80 is 0 or 2, the on-line $ command must be
used must be used to initiate the phasing search.
If Ix80 is 0 or 1, the “two-guess” phasing search is used; Ix74 has units of servo cycles and
controls the time for the open-loop command at each “guess” of the phase angle. Typical
values are 3 to 10 servo cycles; 5 is a good starting point.
If Ix80 is 2 or 3, the “stepper-motor” phasing search is used; Ix74 has units of (servo
cycles*256) and controls the time current is forced into each phase and PMAC waits for
the motor to settle into the “step” position. With the default servo cycle rate of 2.25 kHz,
each unit of Ix74 represents about 0.1 seconds in this mode; typical values are 10 to 20.
See Also
Power-Up Phasing Search (Setting Up PMAC Commutation)
I-Variables Ix01, Ix73, Ix78, Ix80, Ix81
Ix75 Motor x Power-On Phase Position Offset
Range
-8,388,608 .. 8,388,607
Units
Encoder counts * Ix70
Default
0
Remarks
Ix75 tells PMAC the distance between the zero position of an absolute sensor used for
power-on phase position (specified by Ix81) and the zero position of PMAC’s
commutation cycle. It is used to reference the phasing algorithm for a PMAC-commutated
motor with an absolute sensor (Ix81 > 0). If Ix80 is 1, this is done automatically during
the power-up/reset cycle. It will also be done in response to a $ command to the motor.
The SETPHASE command also uses Ix75, copying the Ix75 value directly into the phase
position register. This mode is typically used to correct the phasing at a known position
(usually at the index pulse of the encoder) after a rough phasing (hall-sensor read or
phasing search) gives you enough torque for basic motion to the known position.
The proper value for this parameter can be found with a simple procedure that should be
done with an unloaded motor, after satisfactory operation has been achieved using a
power-on phasing search.
For use with the Ix81 absolute read, define an M-variable to the absolute sensor (TWR
form for a resolver, Y form for an absolute encoder). Next, drive the motor to the zero
position in the commutation cycle, either by issuing a $ command with the motor set up for
the “stepper motor” phasing search (Ix80 = 1 or 3), or by manually setting the phase
offsets for the motor.