I - 44
© 1998 IXYS All rights reserved
IXMS 150
Fig. 12 Complete Microstepping System
Q1, Q2: IRF9530
Q3, Q4: IRF532
D1, D2: MUR161OCT
D3, D4: MUR410
U1: 40498
U2: 40508
All Diodes are 1N4148 unless
otherwise specified
Fig. 11b Simplified laplace transform for stability analysis
phase shift left (i.e., 180 - (loop phase))
at the gain crossover. This number
gives the most intuitive feeling for how
the loop will respond to perturbations
and variations in system parameters.
Theoretically, a system with 1 degree of
phase margin is stable. However, a
step input to a system with small phase
margin will cause an underdamped,
ringly response or an oscillation that
dies out after a long time.
In a step motor, this overshoot and ring
in the current waveform is unaccep-
table. As the phase margin of a system
is increased, the response to a step
input slows down and the ringing is de-
creased. The response becomes more
damped. In a practical system, the
minimum acceptable phase margin is
about 30 degrees. More than 90
degrees slows the system response
with no significant improvement in sta-
bility. 60 degrees is usually considered
optimal, if no other constraints exist.
In a PWM motor drive amplifier, there
are several additional constraints that
apply. Because the levels of voltage
and current being switched are so high,
synchronous noise appears every-
where and can degrade system perfor-
mance. It is common to see apparent
instabilities that are simply loop ampli-
fication of subharmonic switching
transient noise. It is important to main-
tain at least 60 degrees of phase
margin and to maintain as much gain
margin as is practical. The PWM
comparator delays, power stage gate
drive delays, and the sampling tech-
nique used to generate the current
feedback signal also account for signifi-
cant phase delays when the switching
frequency is high, or when the excitation
approaches the switching frequency.
For these reasons it is usually advis-
able to design for a calculated 60 to 90
degree phase margin because of the
importance of the effects not accounted
for in the linearized circuit model.
