Hiwin D1 - Gain Margin and Phase Margin; Nyquist Plot

D1 Servo Drive User Manual Operation Principles
HIWIN MIKROSYSTEM CORP. 3-9
MD20UE01-2109
(2) Servo gains
D1 servo drive uses one high-speed digital signal processor (DSP) to control motor. Normally if
servo loops are controlled via digital method, user needs to adjust several servo gains. But for easy
operation, D1 servo drive provides one common gain (CG) for user to adjust all servo gains at the
same time.
Figure3.5.2
3.6 Gain margin and phase margin
3.6.1 Nyquist plot
Gain margin (GM) (Unit: db) is the allowable loop gain which can be increased before closed-loop system
becomes unstable. Phase margin (PM) is the allowable phase delay which can be increased before
closed-loop system becomes unstable.
(1) Gain margin
)(
p
jG

is the relative distance from where the Nyquist plot intersects with the negative real axis to
(-1, j0).
p

is the phase crossover frequency. In figure 3.6.1.1,
= 180)(
p
jG

. For the transfer
function
)(sG
in a loop system, its gain margin =
GM =
)(log
)(
log
p
p
jG
jG


1010
20
1
20 −=
dB.
From figure 3.6.1.1 and the Nyquist plot, the following conclusion is known.
A. If
)(

jG
does not intersect with the negative real axis,
0=)(
p
jG

and
dBGM =
.
When the Nyquist plot does not intersect with the negative real axis at any non-zero finite
frequency,
dBGM =
. Theoretically, loop gain can be infinite before instability occurs.
0.1…………………………………0.5………………………………………1
Stiffness
Response
High
Low