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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.