User manual

QSH4218 Manual (V1.11 / Feb 8th, 2006) 10
Copyright © 2006, TRINAMIC Motion Control GmbH & Co. KG
Hints:
Q: How to determine if the given driver voltage is sufficient?
A1: Just listen to the motor at different velocities. Does the “sound” of the motor get raucous or
harsh when exceeding some velocity? Then the motor gets into a resonance area. The reason is,
that the motor back-EMF voltage reaches the supply voltage. Thus, the driver can not bring the full
current into the motor any more. This is typically a sign, that the motor velocity should not be further
increased, because resonances and reduced current affect motor torque.
A2: Measure the motor coil current at maximum desired velocity.
For microstepping: If the waveform is still basically sinusoidal, the motor driver supply voltage is
sufficient.
For Fullstepping: If the motor current still reaches a constant plateau, the driver voltage is sufficient.
If you determine, that the voltage is not sufficient, you could either increase the voltage or reduce
the current (and thus torque).
5.4 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 me-
dium 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-
stepping and full-
stepping for high
velocities
200 * (number
of microsteps)
per rotation
Low to very high. Full torque At high velocities,
there is no audible
difference for full-
stepping
Table 5: 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.
5.4.1 Fullstepping
When operating the motor in fullstep, resonances may occur. The resonance frequencies depend on
the motor load. When the motor gets into a resonance area, it even might not turn any more! Thus you
should avoid resonance frequencies.
Hints:
Q: How to avoid motor resonance in fullstep operation?
A1: Do not operate the motor at resonance velocities for extended periods of time. Use a reasonably
high acceleration in order to accelerate to a resonance-free velocity. This avoids the build-up of
resonances. When resonances occur at very high velocities, try reducing the current setting.
A2: A resonance dampener might be required, if the resonance frequencies can not be skipped.