User's Manual

PMAC User Manual
Setting Up PMAC Commutation 95
Hall Diagram
This diagram shows the hall-effect waveforms with zero offset, defined such that the V-signal transition
when the U-signal is low (defined as the zero point in the hall-effect cycle) represents the zero point in the
PMAC commutation cycle.
If the hall-effect sensors do not have this orientation, bits 16 to 21 of Ix81 can be used to specify the
offset between The PMAC zero point and the hall effect zero point. These bits can take a value of 0 to 63
with units of 1/64 of a commutation cycle (5.625
o
e). The offset can be computed by doing a phasing
search move to establish a phase reference without the hall sensors, then reading the 24-bit phase position
register (suggested M-variable Mx71 for the high 24 bits) at the V-signal transition with U low — the
hall-effect zero point. This is best done by killing the motor and rotating it by hand while watching the U
and V signals on an oscilloscope or matching M-variables in the PMAC Executive program Watch
Window, and Mx71 in the Watch Window. The offset value can then be calculated as:
64*
71
I
x
71Ix%71Mx
Offset =
The offset computed here should be rounded to the nearest integer.
The description of Ix81 in the Software Reference manual shows the common values of offsets used, for
all the cases where the zero point in the hall-effect cycle is at a 0
o
, 60
o
, 120
o
, 180
o
, -120
o
, or -60
o
point –
where manufacturers generally align the sensors.
To find the precise phase position value on the motor index pulse, perform a power-on phasing search
move to establish a precise phase reference, then read the value of the phase position register Mx71 at the
index pulse, monitoring the pulse either on an oscilloscope or the matching M-variable on PMAC. This
value can be stored in the Ix75 phase position offset variable, which is not used by the automatic
firmware in this mode. In actual operation, this value can then be copied into the phase position register
Mx71 after homing to the index pulse.
The full phase reference then consists of the following steps:
• Do a rough phase reference using the hall-effect sensors as specified by Ix81, either automatically on
power-up/reset if Ix80=1, or on the $ command if Ix80=0.
• Do a homing search move on the motor, using the index pulse as part of the home trigger.
• Wait for the motor to settle in-position (following error less than Ix27) at the home position using the
motor in-position status bit – suggested M-variable Mx40 – [WHILE(M140=0)...]
• Force the motor phase position register to the pre-determined value at this point with a command like
Mx71=Ix75.
Phase Advance
A velocity phase advance gain term (Ix76) allows the phasing sequence to be advanced in the direction of
motion by an amount proportional to the velocity to counteract computational delays, and current lags in
the motor and amplifier. This feature can significantly increase the top speed of the motor, and greatly
increase the energy efficiency of the system. It is not possible when commutating off hall-effect sensors.
Usually, this parameter is set interactively by running the motor at high speed, and finding the setting that
minimizes current draw.