User's Manual

PMAC2 User Manual
18 Setting Up PMAC2 for Direct PWM Control
I
kHz
PWMFreq kHz
900
117 8
4
1=−






int
,964.
*()
The frequency should be set within the specified range for the drives controlled by PMAC2. Too high a
frequency can lead to excessive drive heating due to switching losses; too low a frequency can lead to
lack of responsiveness, excess acoustic noise, possible physical vibration, and excessive motor heating
due to high current ripple.
I906 controls the PWM frequency of machine interface channels 5-8 by the same equation. This
frequency does not have to be the same as the frequency for channels 1-4, but it does have to have a
synchronous relationship with the phase clock derived from the PWM 1-4 frequency with I901. The
following relationship must hold for proper operation of channels 5-8:
}Integer{
PhaseFreq
Freq]85[PWM
*2 =
−
I900 also sets the frequency of the MaxPhase clock to twice the PWM frequency. The MaxPhase clock is
the highest frequency at which PMAC2's phase update tasks, which include phase commutation and
digital current loop closure, can operate. Note that any change to I900 automatically changes the phase
and servo clock frequencies.
Phase Clock Frequency Control: I901
I901 determines how the actual PHASE clock is generated from the MaxPhase clock, using the equation
1901I
)kHz(eqMaxPhaseFr
)kHz(PhaseFreq
+
=
Note:
Any change to the value of I901 also changes the servo clock frequency
automatically.
Every phase clock cycle, PMAC closes the digital current loop and performs the commutation for motors
that have been configured for these algorithms.
I901 is an integer value with a range of 0 to 15, permitting a division range of 1 to 16. Typically, the
PHASE clock frequency is in the range of 8 kHz to 12 kHz.. The 60 MHz PMAC2 is capable of
performing commutation and digital current loop for eight axes at up to 12 kHz; the 40 MHz is capable of
doing this for eight axes at up to 9 kHz, or for 6 axes at up to 12 kHz.
Generally, the phase clock frequency that fits within the 8 to 12 kHz range within PMAC2’s computation
capabilities is chosen. Sometimes the actual PWM frequency is adjusted slightly to provide a suitable
phase clock frequency.
For example, on large drives set up for 5 kHz PWM, the MaxPhase frequency is 10 kHz, so an I901 value
of 0 sets the phase clock frequency to 10 kHz. On smaller drives set up for 18 kHz PWM, the MaxPhase
frequency is 36 kHz, so an I901 value of 2 sets the phase clock frequency to 12 kHz, or a value of 3 sets 9
kHz.
Servo Clock Frequency Control: I902
I902 determines how the SERVO clock is generataed from the PHASE clock, using the equation
1902I
)kHz(PhaseFreq
)kHz(ServoFreq
+
=
I902 is an integer value with a range of 0 to 15, permitting a division range of 1 to 16. On the servo
update, which occurs once per SERVO clock cycle, PMAC2 updates commanded position (interpolates)
and closes the position/velocity servo loop for all active motors, whether or not commutation and/or a
digital current loop is closed by PMAC2. Typical servo clock frequencies are 1 to 4 kHz.