PBM 3960/1
This is the ideal solution for a system
where there is an available micropro-
cessor with extra capacity and low cost
is more essential than simplicity. See
typical application, figure 14.
increase on the positive edge of the sine-
cosine curves. Fast current decay is
used at higher speeds to avoid current
dragging with lost positions and incorrect
step angles as a result.
Ramping
Every drive system has inertia which
must be considered in the drive system.
The rotor and load inertia play a big role
at higher speeds. Unlike the DC motor,
the stepper motor is a synchronous
motor and does not change its speed
due to load variations. Examining a
typical stepper motor’s torque-versus-
speed curve indicates a sharp torque
drop-off for the “start-stop without error”
curve. The reason for this is that the
torque requirements increase by the
cube of the speed change. For good
motor performance, controlled accelera-
tion and deceleration should be conside-
red even though microstepping will
improve overall performance.
Double-pulse Programming
The normal way is to send two write
pulses to the device, with the correct
addressing in between, keeping the
delay between the pulses as short as
possible. Write signals will look as
illustrated in figure12. The advantages
are:
• low torque ripple
• correct step angles between each set
of double pulses
• short compromise position between
the two step pulses
• normal microstep resolution
Single-pulse Programming
A different approach is to send one
pulse at a time with an equally-spaced
duty cycle. This can easily be accom-
plished and any two adjacent data will
make up a microstep position. Write
signals will look as in figure 13. The
advantages are:
• higher microstep resolution
• smoother motion
The disadvantages are:
• higher torque ripple
• compromise positions with almost-
correct step angles
User Hints
Never disconnect ICs or PC Boards
when power is supplied.
Choose a motor that is rated for the
current you need to establish desired
torque. A high supply voltage will gain
better stepping performance even if the
motor is not rated for the V
MM
voltage,
the current regulation in PBL 3771/1
will take care of it. A normal stepper
motor might give satisfactory result, but
while microstepping, a “microstepping-
adapted” motor is recommended. This
type of motor has smoother motion due
to two major differences, the stator /
rotor teeth relationship is non-equal
and the static torque is lower.
The PBM 3960/1 can handle
programs which generate microsteps at
a desired resolution as well as quarter
stepping, half stepping, full stepping,
and wave drive.
Fast or Slow Current Decay?
There is a difference between static
and dynamic operation of which the
actual application must decide upon
when to use fast or slow current decay.
Generally slow decay is used when
stepping at slow speeds. This will give
the benefits of low current ripple in the
drive stage, a precise and high overall
average current, and normal current
Programming PBM 3960/1
There are basically two different ways of
programming the PBM 3960/1. They are
called “single-pulse programming” and
“double-pulse programming.” Writing to
the device can only be accomplished by
addressing one register at a time. When
taking one step, at least two registers are
normally updated. Accordingly there
must be a certain time delay between
writing to the first and the second
register. This programming necessity
gives some special stepping advantages.
9
ERICSSON [ ERICSSON ]
