Delta Tau PMAC2 PCI User Manual download pdf (Page 56)

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PMAC2 User Manual

48 Setting Up PMAC2 Commutation (Direct PWM or Sine Wave)

When properly phased, a positive O-command should cause movement in the positive direction; a

negative O-command should cause movement in the negative direction. If the opposite results are

received, there will be a dangerous runaway condition when the servo loop is closed.

Two-Guess Phasing Search

PMAC2’s 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, PMAC2 calculates the two responses, PMAC2 calculates 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 be executed automatically during the power-on

reset cycle; it can also be subsequently executed with a $ command.

Two parameters must be specified to tell PMAC2 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. 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 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 will automatically be executed 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.

In the stepper-motor phasing search, PMAC2 first forces current to put the motor at the +/-60

point in the

phasing cycle and waits for the settling time. Then it forces current to put the motor at the 0

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

) 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.

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Delta Tau PMAC2 PCI User Manual Page 56

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