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Stellaris® Stepper Motor RDK User’s Manual
November 4, 2009 31
Closed-loop PWM mode combines the features of Chopper and Open-loop PWM modes. Closed
loop PWM mode uses a programmed PWM duty cycle to set the current. However, the current is
also measured each time a PWM pulse is applied, and the duty cycle is adjusted to compensate
for variation in the measured current compared to desired drive current.
Decay Mode
This parameter specifies Fast Decay mode or Slow Decay mode. In Slow Decay mode, the
H-bridge low-side switches are closed to remove voltage from the winding. This allows current to
circulate in the winding and decay slowly. In Fast Decay mode, all the switches are opened to
remove voltage from the winding, and the current decays rapidly.
For the unloaded motor, slow decay seems to work best.
Step Mode
This parameter specifies the stepping mode. In Full-Step mode, full positive or negative current is
applied to the windings at each point in the four-step stepping cycle. In Half-Step mode, the
winding current is full negative, positive, or 0 at each point in an eight half-step stepping sequence.
In Micro-Step mode, the current in each winding is varied sinusoidally over a sequence of 8
microsteps per step. In Wave-Step mode, full steps are made as in Full-Step mode, except that
only one winding is energized at a time.
The unloaded motor runs smoother at slower speeds using half stepping, but full stepping may be
appropriate for higher speeds.
For very slow speeds, less than 200 steps per second, Micro-Step mode may be used to smooth
out the motion of the motor.
Off Blanking Time
If Chopper mode is used, this parameter sets the amount of time that the winding remains off after
the winding current reaches the drive current.
Caution – Great care should be taken if decreasing this value. If the off-blanking time is not long
enough, the current may not drop enough between the times when the voltage is applied, and the
current can continue to rise in the winding, above the rated current. Use lab equipment to observe
the winding current when making adjustments to this value.
ID Units Range Default
PARAM_DECAY_MODE choice Fast or Slow Slow
ID Units Range Default
PARAM_STEP_MODE choice Full, Half, Micro,
or Wave
Half
ID Units Range Default
PARAM_BLANK_OFF
μ
S 20–10000 100