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.
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.