Linear motors provide high speed, acceleration, and precision with minimal backlash following error and settling times. This is often
one of the most costly technologies, largely
because of the cost of magnets and linear
feedback devices needed. Force density can
be difficult because of the lack of a mechanical drive train, which also makes using a linear motor in a vertical application difficult.
Focus on Precision
Recall that the key performance characteristics for precision are resolution, repeatability,
accuracy, and velocity control. Always start
with an understanding of needs relative to
resolution. Starting with the proper minimum
incremental step is the foundation for all
other measurements regarding precision.
For accuracy, higher levels are a system cost
driver both from a material and implementation standpoint. First, determine if you really
need accuracy. Techniques to avoid requiring
a system with true accuracy are either teaching or mapping system positions.
Velocity control is typically only required for
scanning applications with high resolutions
or high-speed "on-the-fly" dispensing applications. This is another characteristic that will
drive up the cost of the system.
When it comes to precision, linear motors
and ball screws are typically far superior. The
majority of motion applications do not require
high levels of precision, which is why the
timing belt remains the commonly applied
Focus on Expected Life
The key performance characteristics for
expected life are mechanical efficiency,
Wear resistance is typically a
function of the type of friction
involved in the mechanism. The
lower the friction of the device, the
lower the wear. The one exception
is for the linear motor. Although
this drive train is typically consid-
ered "frictionless," there are mov-
ing cables in many configurations
that create a wear point and may
limit system life if not properly
Contamination resistance or dirt
resistance must be a high priority in harsh
environments. However, for most environments, it has a low priority.
Maintenance is closely related to the
mechanical efficiency and wear resistance.
The ideal scenario is that no field
maintenance is required for the
intended life of the machine. That
said, all mechanical devices have
a life limitation. In design, it is
critical to understand how wear
or relaxation will impact the lon-ger-term "precision"
characteristics of the
When it comes to
expected life, timing
belts are the most suitable technology because
of their high efficiency
and limited maintenance
Focus on Throughput
The key performance characteristics for throughput are speed
capacity, maximum acceleration,
frequency response and duty cycle.
For speed capacity, depending on
the length of travel required, either speed or
accel/decel is most important. For applications with a longer travel where more of the
cycle time is spent at the top velocity, speed
is most critical. On the other hand, for applications with shorter moves where more time
is spent accelerating and decelerating, accel/
decel is most important.
Frequency response is a key attribute for
applications with tight position requirements
(high levels of precision) that are time sensitive. Most pick-and-place applications don’t
require high levels of frequency response. The
attribute can be a cost driver.
Duty cycle is typically a characteristic
that should be checked and validated to
ensure the selected design will work in the
When it comes to throughput, linear motors
are the best choice because they can provide
high speeds and high accelerations. That they
have no mechanical compliance means they
can also provide a high frequency response.
The key performance characteristics for special considerations are force density, material
cost, implementation cost, and travel length.
Force density is an issue of packaging. As
machine designs continue to miniaturize, the
characteristic becomes more important, especially when you are specifying end effectors
or tooling mounted to an axis.
Both material and implementation costs
are always key considerations because it is
important to get the right combination of
other characteristics at the minimum cost.
Lastly, for most applications, travel length
restrictions are not an issue.
No doubt, selecting the best drive train is a
balancing act. When you optimize on one key
performance characteristic, you are likely to
give up a benefit relative to another performance characteristic. Of utmost importance is
to understand the performance requirements
of your machine. First, understand your
requirements relative to precision, expected
life, throughput, and special considerations.
The rest is a matter of picking a few attributes
from each category and leveraging the guidelines provided.
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