Datasheet

QSH2818 Manual (V1.02 / 13-November-2007) 10
Copyright © 2007, TRINAMIC Motion Control GmbH & Co. KG
5.4 Back EMF (BEMF)
Within SI units, the numeric value of the BEMF constant has the same numeric value as the numeric
value of the torque constant. For example, a motor with a torque constant of 1 Nm/A would have a
BEMF constant of 1V/rad/s. Turning such a motor with 1 rps (1 rps = 1 revolution per second =
6.28 rad/s) generates a BEMF voltage of 6.28V.
Thus, the Back EMF constant can be calculated as:
[
]
[]
AI2
NmngTorqueMotorHoldi
s/rad
V
U
NOM
BEMF
⋅
=






The voltage is valid as RMS voltage per coil, thus the nominal current I
NOM
is multiplied by 2 in this
formula, since the nominal current assumes a full step position, with two coils switched on. The
torque is in unit [Nm] where 1Nm = 100cNm = 1000mNm.
One can easily measure the BEMF constant of a two phase stepper motor with a (digital) scope. One
just has to measure the voltage of one coil (one phase) when turning the axis of the motor manually.
With this, one gets a voltage (amplitude) and a frequency of a periodic voltage signal (sine wave).
The full step frequency is 4 times the frequency the measured sine wave.
5.5 Choosing the Commutation Scheme
While the motor performance curves are depicted for fullstepping, most modern drivers provide a
microstepping scheme. Microstepping uses a discrete sine and a cosine wave to drive both coils of
the motor, and gives a very smooth motor behaviour as well as an increased position resolution. The
amplitude of the waves is 1.41 times the nominal motor current, while the RMS values equals the
nominal motor current. The stepper motor does not make loud steps any more – it turns smoothly!
Therefore, 16 microsteps or more are recommended for a smooth operation and the avoidance of
resonances. To operate the motor at fullstepping, some considerations should be taken into account.
Driver Scheme Resolution Velocity range Torque Comments
Fullstepping 200 steps per
rotation
Low to very high.
Skip resonance areas
in low to medium
velocity range.
Full torque if dampener
used, otherwise reduced
torque in resonance area
Audible noise
especially at low
velocities
Microstepping 200 * (number
of microsteps)
per rotation
Low to high. Reduced torque at very
high velocity
Low noise,
smooth motor
behaviour
Mixed: Micro-
steppin
g
and
fullsteppin
g
for
high velocities
200 * (number
of microsteps)
per rotation
Low to very high. Full torque At high velocities,
there is no
audible difference
for fullstepping
Table 5.3: Comparing microstepping and fullstepping
Microstepping gives the best performance for most applications and can be considered as state-of-the
art. However, fullstepping allows some ten percent higher motor velocities, when compared to
microstepping. A combination of microstepping at low and medium velocities and fullstepping at
high velocities gives best performance at all velocities and is most universal. Most Trinamic driver
modules support all three modes.