User's Manual

Turbo PMAC User Manual
124 Setting Up Turbo PMAC-Based Commutation and/or Current Loop
The two-guess phasing search is quick and requires little movement. It works well provided that external
loads such as gravity and friction are low. However, if there are significant external loads, it may not
prove to be a reliable phasing-search method (and unreliable phasing search methods can be dangerous);
if this is the case, another method such as the stepper-motor method described below should be used.
The two-guess method is selected by setting Ixx80 to 0 or 1. With Ixx80 at 0, the phasing search is not
executed automatically during the power-on/ reset cycle; a $ command must be used to execute the
phasing search. With Ixx80 at 1, the phasing search will automatically be executed during the power-on
reset cycle; also, it can be subsequently executed with a $ command.
Two parameters must be specified to tell Turbo PMAC how to do this phasing search. Ixx73 specifies the
magnitude of the torque command during each guess, with units of 16-bit DAC bits. Typical values are
2000 to 6000; 4000 (about 1/8 of full range) is a usual starting point. Ixx74 sets the duration of each
torque command and the evaluation of its response, with units of servo cycles. Typical values are 3 to 10;
5 (about 2 msec at the default servo update) is a usual starting point.
Stepper-Motor Phasing Search
The other automatic method of phasing search for a synchronous motor is the stepper-motor method.
This method forces current through particular phases of the motor, as a stepper-motor controller would,
and waits for it to settle. With proper operation, this will be at a known position in the commutation
cycle. This method is equivalent to two steps of the current-loop six-step test described above.
The stepper-motor phasing search requires more movement and more time than the two-guess method,
but it is more reliable in finding the phase accurately in the presence of large external loads.
The stepper-motor method is selected by setting Ixx80 to 2 or 3. With Ixx80 at 2, the phasing search is
not executed automatically during the power-on/reset cycle; a $ command must be used to execute the
phasing search. With Ixx80 at 3, the phasing search will be executed automatically during the power-on
reset cycle (this is not recommended); also, it can be subsequently executed with a $ command.
In this method, Ixx73 controls the magnitude of the current through the phases, with 32,767 representing
full range. Typically a value near 3000, about 1/10 of full range, will be used, although the actual value
will depend on the loads.
Ixx74 controls the settling time for each of the two steps used in the search. In this mode, the units of
Ixx74 are servo cycles*256, about 1/10 sec with the default servo cycle time. Typically a settling time of
1-2 seconds is used.
In the stepper-motor phasing search, Turbo PMAC first forces current to put the motor at the +/-60
o
point
in the phasing cycle and then waits for the settling time. Then it forces current to put the motor at the 0
o
point in the phasing cycle and again waits for the settling time. It checks to see that there has been at least
1/16 cycle (22.5
o
) movement between the two steps. If there has been, it forces the phase position register
to 0, clears the phasing-search-error motor status bit, and closes the servo loop. If it has detected less
movement than this, it sets the phasing-search-error bit, and disables (kills) the servo loop.
If the stepper motor phasing search is done outside of the power-on/reset cycle, the phasing search
algorithm will fail on detection of an amplifier fault or overtravel limit condition. Turbo PMAC will set
the phasing-search-error bit and disable the servo loop. If done inside the power-on/reset cycle, Turbo
PMAC cannot detect these errors automatically, but the search will likely fail due to lack of movement.
Custom Phasing Search Methods
It may be necessary or desirable to write a custom phasing-search algorithm. Usually these are executed
as Turbo PMAC PLC programs, but often they can be tried and debugged using on-line commands. The
on-line commands are particularly useful if the phasing search is done only in development to establish a
reference for an absolute sensor.