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Danaher Motion
•
Precision Systems Group
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7C Raymond Avenue, Salem, NH 03079
tel:
603.893.0588
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toll free:
800.227.1066
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fax:
603.893.8280
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url:
www.danaherprecision.com
Microstepping
The MDM-7 drive implements microstepping at the divide by 10 level;
that is, it electronically subdivides each full step into 10 microsteps.
This provides two distinct benefits: the system resolution is increased
ten-fold, and the noise and vibration associated with step rates at or
below the fundamental resonance is greatly reduced. In leadscrew
based systems, microstepping allows fine resolutions to be achieved
with relatively coarse leadscrews; this permits high linear velocities
which would be unattainable with fine pitch leadscrews in full or half
step systems. The benefits of microstepping are best realized at rela-
tively modest division ratios; popular systems which divide full steps
into as many as 256 microsteps provide “empty resolution,” with limit-
ed practical advantages. The step rates required to operate such sys-
tems at high shaft rotation rates are usually beyond the capability of
typical pulse sources. Our divide by 10 level of microstepping has
been chosen as optimal for most real world applications.
Midrange
Resonance
Midrange resonance is a parasitic oscillation of the stepping motor
rotor from its intended position, which occurs while the motor is
rotating. It generally sets in at shaft rotation rates between 5 and 15
revs/sec. (1000 to 3000 full stepstsec. for standard 1.8 degree step-
pers). Depending on the frictional and inertial aspects of the load,
midrange resonance may or may not occur in any given application.
When viewed across a broad range of applications, midrange reso-
nance constitutes a serious problem for stepping motor based sys-
tems. In many examples where the system is incapable of exceeding
5 –10 revolutions per second, the actual cause is stalling due to
midrange resonance. The effect usually manifests itself as a 50–150
Hz vibration which builds in amplitude over 1/2 to 2 seconds, termi-
nating in a stalled motor and loss of position. Most commercially
available stepping motor drives ignore this problem, and may call for
bizarre mechanical fixes such as “hang a drill chuck on the motor
rear shaft,” Lancester dampers, etc. The DPS MDM-7 drive incorpo-
rates circuitry which senses the onset of midrange resonance and
introduces the electronic equivalent of viscous damping to eliminate
the problem. As a result, all motor torque is available for accelerating
the load, as opposed to being wasted driving spurious resonances.
One minor requirement of this circuitry is that the incoming step
pulse train should be smooth - errative variations of more than 15%
between successive step pulses (which can be encountered in
unsmoothed binary-rate multiplier pulse generators) can confuse the
compensation circuitry. This problem is rarely encounter in practice.
Pulse width modulated microstepping drives typically
produce some broad-band audible noise when at a
standstill. The physical basis for these phenomena is
intrinsic to the products’ design, and occurs as follows:
Two free-running oscillators generate the pulse width
modulated sine and cosine coil currents needed for
microstepping. While nominally operating at 20 KHz,
their actual frequency is second order dependent on
coil current and load, and may free-run from 18 to 24
KHz. At the 45 and 225 electrical degree points, the sine
and cosine values are equal, and the PWM frequencies
become equal. When the frequencies are nearly equal,
the two oscillators may “lock” and begin oscillating in
phase. Despite the design objective of decoupling the
oscillators, small currents due to reverse recovery, out-
put switching, and MOSFET gate charging may provoke
a phase lock between them. This phase lock leads to a
small but growing error, and when the servo of the error
amp detects the deviation, it “breaks” the phase lock. If,
for example, the oscillators are in lock for 9 cycles and
break for 1, the result will be an audible 2 KHz beat
product of the 20 KHz nominal frequency.
The effect has been a largely unavoidable effect of
generic microstepping driver designs; a new design
utilizing fixed frequency oscillators is now available
for divide by 1-2-10-20-25-50-100-200 microstepping
resolutions.
Summary
The fixed frequency oscillator design significantly
reduces stepping motor shaft vibration and audible
noise while substantially reducing motor heating due
to iron losses.
The MDM-7 module is the best possible drive module which we cur-
rently provide. Its combination of compact size, high speed perfor-
mance, ease of mounting, wiring, microstepping cabability, midrange
resonance suppression, opto-isolated inputs, and 3.5/7.0 Amp/phase
drive capability is unmatched in any other commercially available
Microstepping
Induced Motor
Shaft Vibration
& Audible Noise
Summary
stepping motor drive. This allows users to simply “drop-in” a small,
high performance drive which will extract the maximum torque
stepping motors can provide. We are interested in your reactions
and comments, as welll as suggestions for future improvement.
