In machine and industrial systems design, there are numerous
areas in which design engineers have to make a decision about whether to use
pneumatic or electro-mechanical devices. In this "Design Decisions" viewpoint from
Exlar, a provider of electro-mechanical devices, the advantages of
electro-mechanical actuators are explored in comparison to pneumatic options.
Exlar believes that system design requirements for handling multiple
positions, higher levels of positioning accuracy and flexibility in changing
positions are among the key elements offered by an electro-mechanical solution
that make them preferred to pneumatic solutions for many design applications.
Another significant factor in favor of electro-mechanical devices is the higher
cost of operation for pneumatic products. The cost savings provided by an
electro-mechanical actuator system in place of pneumatics becomes very apparent
when comparing direct energy costs. To
illustrate, following is a comparison of a horizontal point-to-point move with
15 inches of stroke and 35 lbs of tooling weight to be cycled 30 times per
minute with a duty cycle of 50 percent.
With an electro-mechanical cylinder, the move time of 0.5 seconds
can be achieved using a motion profile with 0.1 second acceleration, 0.3
seconds at constant velocity and 0.1 second deceleration. The maximum velocity of this profile is 37.5
inches per second. Based on this
information, the power required for the electro-mechanical actuator to perform
this application is less than 100W with an energy cost at $0.07 per kwh, less
than $145 per year.
Using a pneumatic cylinder with a load of 35 lbs and the
required maximum speed of 37.5 inch/sec, a pneumatic cylinder with a diameter
of 2 inches is used with an assumed air pressure of 85 psi. The cylinder volume
in combination with the cycle rate gives annual air consumption of 847 thousand
cubic feet of compressed air at 85 psi.
This equates to an annual energy cost of more than $4,300.
When comparing installed system cost over time, the
component cost differences between a pneumatic and electric system are
recovered through energy savings in as little as five months in typical
systems. The energy cost savings in subsequent
months are approximated in the chart below.
Other Considerations
In addition to operational costs, another advantage electro-mechanical cylinders have over pneumatic cylinders is the energy cost wasted by air leaks. The chart below
shows up to $9,000 lost per year with a simple air leak of .25 inch in
diameter.
From 2004 to 2007 the price per kWh for industrial large-scale
consumers increased approximately 40 percent and indicators point to a doubling
of energy costs by 2014. This is another
major driving force behind machine builders' preferences to move to
electro-mechanical solutions
Electro-mechanical devices can also be considered greener
when it comes to overall CO2 reduction. With 85 percent of energy production in
the United States coming from fossil fuels, studies by groups like the Fraunhofer
Institute indicate that the CO2 emissions from fossil fuel power plants can be
33 times less when powered by an electro-mechanical solution in place of a
pneumatic solution.
John Walker is vice
president of marketing/customer service, for Exlar.
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