14-06-2013, 12:21 PM
Issue of Design Trends
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A closer look at inertia matching
When designing servo mechanisms
there is a rule of thumb that stipulates
that the load-to-motor inertia ratio
should stay within 10:1 and should be
as close as possible to 1:1. Where
exactly does this rule come from and
how does it apply to your specific
application?
In this article we will discuss the concept
of inertia matching. In addition, we will
highlight the associated topic of coupling
stiffness and its effect. Lastly, we will
discuss some potential alternative
solutions in case inertia ratio and
coupling stiffness are less then ideal.
System Dynamics
One obvious observation one can make
is that if a motor and load are tightly
coupled together, then one can not
distinguish motor from load inertia, since
both are just a single lumped inertia. In
that case, what is the meaning of inertia
ratio?
Practical considerations and
alternatives
During the system design phase and
motor sizing, one should calculate the
resonance frequency based on motor
and load inertia, as well as coupling
stiffness. If due to size and cost
constraints one ends up with a
resonance point within the active
operating range, the following guidelines
can be considered (from least to most
drastic and depending on the real open
loop system response):
• Avoid beam (helical) couplings with
low stiffness values.
• Consider a controller with filtering
(e.g. a notch filter) in addition to the
PID loop. This may help attenuate
the resonance but will also affect
overall performance.