User manual

PMAC User Manual
90 Setting Up PMAC Commutation
If a non-absolute sensor is used for commutation, PMAC must perform a search move for the proper
phasing reference every time it powers up (with an absolute sensor, this only needs to be done once in the
development of the system). There are several ways to do this phasing search. PMAC has two automatic
methods executed by firmware; other methods or enhancements of these methods can be executed with
PLC programs.
A power-on phasing search permits commutation of permanent-magnet brushless motors without the need
for a more expensive and possibly less accurate absolute sensor. However, a phasing search may not be
dependable in some applications; in these cases, an absolute sensor will be required.
Two-Guess Phasing Search
The PMAC first automatic phasing search method is called the two-guess phasing search, because it
makes two arbitrary guesses as to the phase position, briefly applies a torque command using each guess,
and observes the response of the motor to each command. Based on the magnitude and direction of the
two responses, PMAC calculates the two responses and the proper phasing reference point. It then starts
the commutation based on this reference, and closes the servo loop to hold position.
The two-guess phasing search is very 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 Ix80 to 0 or 1. With Ix80 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 Ix80 at 1, the phasing search will automatically be executed during the power-on
reset cycle; it can also be subsequently executed with a $ command.
Two parameters must be specified to tell PMAC how to do this phasing search. Ix73 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. Ix74 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.
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 Ix80 to 2 or 3. With Ix80 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 Ix80 at 3, the phasing search automatically executes during the power-on reset
cycle (this is not recommended); it can also be subsequently executed with a $ command.
In this method, Ix73 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.
Ix74 controls the settling time for each of the two steps used in the search. In this mode, the units of Ix74
are servo cycles*256, about 1/10 sec with the default servo cycle time. Typically a settling time of 1-2
seconds is used.