Delta Tau PMAC2 PCI User Manual download pdf (Page 57)

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PMAC2 User Manual

Setting Up PMAC2 Commutation (Direct PWM or Sine Wave) 49

If the stepper motor phasing search is done outside of the power-on/reset cycle, the phasing search

algorithm will also fail if an amplifier fault or overtravel limit condition is detected. PMAC2 will set the

phasing-search-error bit and disable the servo loop. If done inside the power-on/reset cycle, PMAC2

cannot detect these errors automatically, but the search will likely fail due to lack of movement.

Custom Phasing Search Methods

Warning:

Make sure an algorithm of this type can be executed reliably. Do not attempt this

algorithm if the position sensor or drive is unpowered or faulted. PMAC2 does not

permit the open-loop enabled state required for this PLC if it is into overtravel

limits. The automatic overtravel limit functions may have to be disabled with

Ix13, Ix14, and Ix25. Special logic may be required to step out of a limit before

the full phasing can be done. Remember that improper phase referencing can lead

to runaway conditions. Make sure that both PMAC2 fatal following error limit

Ix11 and amplifier overcurrent fault protections are active and working.

It may be necessary or desirable to write a custom phasing-search algorithm. Usually these are executed

as PMAC2 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.

Most custom algorithms are variations on the stepper-motor phasing search method. They use the phase-

current offset values Ix29 and Ix79 with an O0 command to force current into particular phases so the

motor will lock at a certain physical position in its phasing cycle. The following table shows the positions

in the phasing cycle created by different combinations of Ix29 and Ix79 for 3-phase motors. Usually the

magnitudes of the non-zero values are 2000 to 3000:

Ix29

=0 <0 <0 =0 >0 >0

Ix79

>0 >0 =0 <0 <0 =0

A. Phase Pos.

e -60

e -120

e+180

e120

e 60

B. Phase Pos.

+180

e 120

e 60

e 0

e -60

e -120

Case A shows the resulting positions for all direct PWM systems, and for sine-wave output systems with

Ix72<128.

Case B shows the resulting phase positions for sine-wave output systems with Ix72>128.

For example, the following set of on-line commands typed into the terminal window of the PMAC2

Executive program could be used to force a motor to the zero position in its phasing cycle, set the phase

position register as zero, and enable the motor.

#1O0 ; Enable the motor with open-loop zero magnitude

I129=0 ; No offset on Phase A

I179=3000 ; Positive offset on Phase B to force to 0 deg

M171=0 ; Write zero into phase position register

I179=0 ; No offset on Phase B

J/ ; Close servo loop

Note:

On preliminary firmware versions dated before October 95 (use the DATE

command), these commands produce angles rotated -30

from the results shown

here. With preliminary firmware of dates 10/5/95 and 10/16/95 the phase

references are 180

off from what is shown in this example. Using this example,

unchanged with firmware of those dates will likely lead to dangerous runaway

conditions. If using firmware of these dates, contact the factory for free upgrades.

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