Infrequently Asked Questions about Linear Actuators
By Patrick Lehr, Product Manager –
Precision Mechanics, Parker Hannifin
getting ready to choose a linear actuator for a specific device or machine
should have a list of questions ready to ask suppliers and manufacturers of
those devices. These lists usually contain FAQs (frequently asked questions),
and most firms that sell actuators are prepared for them. But those suppliers,
in many instances, expect potential buyers to ask other, perhaps more probing
and revealing questions: the so-called infrequently asked questions (iFAQs).
Here are a
pair of questions engineers should ask when considering specifying linear
need speed and accuracy over a long length. What type of actuator should I use?
A. That’s a
smart question to ask. Many design engineers overestimate how accurate
traditional motors and actuators are over long travel runs. They mistakenly
believe that if the actuator works well for short runs, it will work equally
well on long ones. Although many types of linear systems meet two of the three
requirements engineers typically want (long travel lengths, high speed, and
high positioning accuracy), linear motor actuators are the only ones that
provide all three without compromise. They are often used in semiconductor
manufacturing, consumer electronics inspection, medical and life science
applications, machine tools, printing, and packaging applications.
To provide a
little background, let’s define linear motors. Essentially, a linear motor is a
rotary motor that has been unwound and laid out flat. It lets the motor couple
directly to the linear load. In contrast, other designs use a rotary motor and
couple it through mechanics, which can introduce backlash, efficiency losses,
and other inaccuracies. Linear motors also tend to have higher maximum
velocities compared to ball screws of the same travel length.
types of linear motors are used today. The first is ironcore, which has coils
wound around teeth made of ferrous materials and wrapped in laminate. These
motors have the highest force per size and good heat transfer, and are
generally the least expensive. However, iron in the motor leads to increased
cogging (torque due to interactions between the motor’s magnets), so they are
often somewhat less precise than the second type, ironless linear motors.
As the name implies, ironless linear motors don’t have any
iron inside. The forcer is essentially an epoxy plate in which tightly wounded
copper coils have been inserted. It slides between two rows of magnets facing
each other. (This is also known as a U-channel magnetic way.) A spacer bar down
one side of the magnets links them together. The main advantages of ironless
motors are lower attractive forces and no cogging. This makes them more precise
than ironcore motors. However, two rows of magnets make ironless units more
expensive than ironcore versions. Managing heat transfer can also be difficult,
so it’s important to understand early whether a particular application will run
the risk of overheating. The newest ironless motors feature overlapped coils
that provide more surface contact for heat dissipation. This design also lets
the motor have a higher force density.
The third and final type are slotless linear motors, which
are basically hybrids of the first two types. A slotless motor has a single row
of magnets like the ironcore, which helps keep its price lower. A laminated
backiron ensures good heat transfer, as well as lower attractive forces and
cogging than ironcore motors. Slotless motors also offer the advantage of a
lower height profile than ironless in addition to their lower price. For
designers who prioritize keeping components in their machines as small as
possible, every millimeter of space saved can be crucial.
can I know if a given actuator is suitable for use in a specific environment?
A. All too
often, design engineers choose actuators in isolation and don’t consider where
they will be used. Linear actuators have critical moving parts that only work
properly within environments for which they were designed and manufactured.
Using an inappropriate linear actuator can cause problems ranging from improper
operation to irreparable damage to the actuator itself. For “dirty”
applications, such as a cutting tool that throws off particles and scrap, the
actuator will require sealing and shielding to protect it from contaminants.
From the opposite
perspective, an actuator without the proper protection can introduce
contamination into a clean environment, compromising the application. Normal
wear and tear will cause linear stages to generate particulates over time.
Cleanrooms or vacuum environments are often restricted to using equipment that
doesn’t release any particulates, so it is critical for actuators used in these
environments that they are equipped with seals and shields to prevent
particulates from entering the environment. Some mechanical devices that
provide linear motion, such as in semiconductor processing, move only microns
at a time, so even the least amount of contamination can compromise and ruin an
shields protect critical components from exposure to harsh environments,
letting linear actuators run as they were designed to perform. For clean
environments, seals and shields protect the application’s environment from
possible contaminants created by the actuator—not the actuator itself. In
addition to seals and shields, custom linear actuators can be designed with
positive pressure ports that purge contaminants inside the unit, keeping the
performance and life cycle at a maximum.
A variety of
environmental factors must be considered when choosing linear actuators. These
include ambient temperatures, the presence of moisture, exposure to chemicals
and gasses (other than room air), radiation, the level of air pressure (for
applications that are performed in vacuum), cleanliness, and nearby equipment.
For example, is there a piece of equipment in the vicinity that could transfer
vibrations that would affect the linear stage’s performance?
stage’s Ingress Protection (IP) rating, which is typically provided in its
specifications, indicates whether it has the proper protection from specific
environments. IP ratings are defined levels of the effectiveness of an
enclosure’s seals against intrusion from foreign bodies (dust and dirt) and
various levels of moisture.
Enclosure ratings takes the form of
“IP-“ followed by two digits. The first digit indicates the degree of
protection from moving parts and foreign bodies. The second digit identifies
the level of protection against exposure to different levels of moisture (from
drips to sprays to total submersion). The chart below explains the IP
Taking the time to check an
actuator’s IP rating early in the selection process offers a quick and easy way
to eliminate units unsuitable for the environment. For example, an actuator
with an IP30 rating offers no protection against moisture, but it will keep out
finger-sized objects. If moisture protection is essential, look for an actuator
with a higher rating, such as IP54, which protects dust and water splash.
Actuators without intrusion or moisture protection, however, can offer
economical alternatives for environments where contaminants are not a concern.
To learn more about Parker's linear motor and high-precision capabilities, visit our dedicated linear motor stage site.