QSH2818_11 Datasheet, PDF (11/14 Pages) TRINAMIC Motion Control GmbH & Co. KG. – motor controllers and drivers

QSH2818 Manual (V1.04 / 2010-OCT-19)
11
6.4 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.
The Back EMF constant can be calculated as:
U _BEMF
V
 _{rad /}

s ^

MotorHoldi ngTorque Nm
2  I _NOM A
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.
6.5 Choosing the commutation scheme
While the motor performance curves are depicted for fullstepping and halfstepping, 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 behavior as well as an increased
position resolution. The amplitude of the waves is 1.41 times the nominal motor current, while the
RMS values equal 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
Fullstepping 200 steps per
rotation
Halfstepping
200 steps per
rotation * 2
Microstepping 200 * (number of
microsteps) per
rotation
Mixed: Micro- 200 * (number of
stepping and microsteps) per
fullstepping for rotation
high velocities
Velocity range
Low to very high.
Skip resonance
areas in low to
medium velocity
range.
Low to very high.
Skip resonance
areas in low to me-
dium velocity
range.
Low to high.
Low to very high.
Torque
Full torque if dam-
pener used,
otherwise reduced
torque in resonance
area
Full torque if dam-
pener used,
otherwise reduced
torque in resonance
area
Reduced torque at
very high velocity
Full torque
Comments
Audible noise
especially at low
velocities
Audible noise
especially at low
velocities
Low noise, smooth
motor behavior
At high velocities,
there is no audible
difference for full-
stepping
Table 6.3: Comparing microstepping and fullstepping
Copyright © 2010, TRINAMIC Motion Control GmbH & Co. KG