Delta Tau GEO BRICK LV User Manual
Turbo PMAC User Manual
108
Setting Up Turbo PMAC-Based Commutation and/or Current Loop
However, if the command from the position/velocity servo loop is noisy, as can be the case with a low-
resolution position sensor, this filtering effect can be desirable, and Ixx76 can provide better performance
than Ixx62.
resolution position sensor, this filtering effect can be desirable, and Ixx76 can provide better performance
than Ixx62.
Analytic Calculation of Current-Loop Gains
With some basic knowledge of motor and amplifier parameters, it is possible to calculate the current-loop
gains directly. It is strongly advised that these computed gains be checked against the values determined
through the auto-tuning or interactive tuning of the Turbo Setup program.
gains directly. It is strongly advised that these computed gains be checked against the values determined
through the auto-tuning or interactive tuning of the Turbo Setup program.
The motor parameters needed are:
•
R
pn
Motor phase-to-neutral resistance (Ohms)
= R
pp
/
√3 (Motor phase-to-phase resistance / √3)
•
L
pn
Motor phase-to-neutral inductance (Henries)
= L
pp
/
√3 (Motor phase-to-phase inductance / √3)
The amplifier parameters needed are:
•
I
sat
Maximum (saturated) current reading from phase-current A/D converter (Amps).
This is a DC value, not an RMS AC value. This value can be derived from the current-sensor gain K
c
(volts/amp) and the maximum voltage in volts that the A/D-converter can read V
cmax
: I
sat
= V
cmax
/K
c
.
•
V
DC
DC bus voltage for the amplifier.
This can be derived from the AC RMS supply voltage V
AC
: V
DC
=V
AC
*
√2.
Finally, the Turbo PMAC parameter needed is:
•
T
P
Phase-update period (sec)
This can be derived from the phase update frequency f
P
in kHz: T
P
=1/(1000*f
P
)
Next, the following performance specifications for the current loop are required:
•
ω
n
Desired natural frequency of the closed current loop in radians/sec
This can be derived from the desired natural frequency f
n
in Hz:
ω
n
(rad/s) = 2
πf
n
(Hz).
If the damping ratio (see below) is in the range 0.7 to 1.0, which it should be in most cases, the
desired bandwidth of the current loop basically is equal to the natural frequency. Usually values of
200 Hz to 400 Hz are used.
desired bandwidth of the current loop basically is equal to the natural frequency. Usually values of
200 Hz to 400 Hz are used.
•
ζ Desired damping ratio (dimensionless). A value of 0.7 here yields a step-response overshoot of
about 5%; a value of 1.0 here yields no overshoot.
about 5%; a value of 1.0 here yields no overshoot.
Now we can compute the proportional current-loop gain K
cp
and the integral current-loop gain K
ci
according to the formulas:
(
)
DC
V
pn
R
pn
L
n
2
sat
I
cp
K
−
=
ζω
DC
V
pn
L
2
n
P
T
sat
I
ci
K
ω
=
Finally, to compute the I-variables to represent these gains, we use the formulas:
66
Ixx
4
cnt
max
PWM
cp
K
76
Ixx
62
Ixx
∗
∗
=
+
66
Ixx
*
8
cnt
max
PWM
ci
K
61
Ixx
∗
=
Here, PWMmaxcnt is I7m00 for a channel directly driven by the Turbo PMAC; it is MI900, MI906, or
MI992 for a channel on a MACRO Station.
MI992 for a channel on a MACRO Station.