NJM3777
Full-step mode
Both windings are energized at all the time with the same current, I
M1
= I
M2
. To make the motor take one step, the
current direction (and the magnetic field direction) in one phase is reversed. The next step is then taken when the
other phase current reverses. The current changes go through a sequence of four different states which equal four
full steps until the initial state is reached again.
Half-step mode
In the half-step mode, the current in one winding is brought to zero before a complete current reversal is made. The
motor will then have taken two half steps equalling one full step in rotary movement. The cycle is repeated, but on
the other phase. A total of eight states are sequenced until the initial state is reached again.
Half-step mode can overcome potential resonance problems. Resonances appear as a sudden loss of torque at
one or more distinct stepping rates and must be avoided so as not to loose control of the motor´s shaft position.
One disadvantage with the half-step mode is the reduced torque in the half step positions, in which current flows
through one winding only. The torque in this position is approximately 70 % of the full step position torque.
Modified half-step mode
The torque variations in half step mode will be elimi-nated if the current is increased about 1.4 times in the halfstep
position. A constant torque will further reduce resonances and mechanical noise, resulting in better performance,
life expectancy and reliability of the mechanical system.
Modifying the current levels must be done by bringing the reference voltage up (or down) from its nominal value
correspondingly. This can be done by using DACs or simple resistor divider networks. The NJM3777 is designed to
handle about 1.4 times higher current in one channel on mode, for example 2 x 600 mA in the full-step position,
and 1 x 800 mA in the half-step position.
NJRC [ NEW JAPAN RADIO ]
