Engineers often grapple with the question of whether electric or pneumatic actuators are the better choice for their applications. The inherent advantages and disadvantages of each option relative to the other are well known. But there is more to the story of both technologies than just the basics. To properly evaluate these longtime actuation competitors, engineers must also be aware of developments that have altered the competitive landscape.

One major example is the advent of servo pneumatics, which, although not new, is becoming better known in industry, according to Gilbert Guajardo, product marketing manager for Bimba Manufacturing Co., an actuator manufacturer based in University Park, Ill. A hybrid featuring desirable characteristics of both pneumatic and electric cylinders, servo pneumatics use feedback sent to electric valves to control pneumatic cylinders between the fully extended and fully retracted positions—the only positions that can normally be guaranteed by pneumatics alone, Guajardo says.

Precise Enough?

Although it cannot match the precision of true electric technology in controlling cylinder position, servo pneumatics offer position control that is precise enough for many applications, Guajardo says. In addition, servo pneumatics deliver “very robust" force and speed exceeding that offered by electric cylinders, he says, but fall short of true pneumatics in those areas due to the addition of the electric valves.

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As for cost, Guajardo says, servo pneumatics fall between true electric and pneumatic options, but are closer to the price of more expensive electric actuation.

ET series electrothrust cylinder servo-motor driven actuator. Image source: Parker HannifinET series electrothrust cylinder servo-motor driven actuator. Image source: Parker Hannifin Besides offering a combination of force, speed and control, servo pneumatics also can be a good fit for space-constrained applications, says Tom Worsnopp, product manager for servo pneumatics and PLCs at another actuator manufacturer Festo Corp., Hauppauge, N.Y. Like conventional pneumatics, servo pneumatics do not require the presence of a motor in close proximity to the actuator. Instead, the actuation process runs on air supplied via a line from a compressor that can be located well away from the actuator.

While servo pneumatics has enhanced the appeal of pneumatic actuation technology, other developments have made electric actuation more attractive to potential users. These include the increased availability of electric actuators with built-in Ethernet connectivity. This allows the actuators to be hooked up to any controller or human-machine interface, says Aaron Dietrich, director of marketing for actuator manufacturer Tolomatic Inc., Hamel, Minn. These connections, along with extras such as built-in Ethernet-enabled sensors, allow improved control of electric actuators and increase the amount of information that can be obtained from them, Dietrich says.

He adds that some pneumatic actuators also have valves with Ethernet connectivity.

“Over the Ethernet, you can turn the valves on and off to extend and retract the actuators, and read whether the valves are on or off," he says. That is generally the extent of the control or information, however. “You can't get speed information or force readings from a basic pneumatic actuator, whereas you can with an electrical counterpart."

Cost Calculations

Basic pneumatic actuators often shine in applications requiring fast movements and/or actuation of heavier loads. Electric actuators with roughly similar capabilities in these areas can be two to five times as expensive as their pneumatic counterparts, says Dietrich. He says, however, that the cost of the most expensive components in an electric actuation system—such as the ball screw and the servo motor, for example—have been coming down.

Electric valve actuators use a single-phase or three-phase AC or DC motor to drive a combination of gears to generate the desired torque level. Image source: Valve WorldElectric valve actuators use a single-phase or three-phase AC or DC motor to drive a combination of gears to generate the desired torque level. Image source: Valve World “Cost pressures [on actuator users] are always huge," he says. “So the closer you can bring the prices of electric and pneumatic technology, the easier it is for someone to make the case to go to electric."

Another cost-related development benefitting electric actuators is the increased availability of stepper motors with encoder feedback. Although they are still not cost-competitive with pneumatics, electric actuation systems with stepper motors and encoders can be less expensive than their counterparts equipped with servo motor/encoder combinations. As a result, Festo's Worsnopp says, “when people are transitioning to an electromechanical solution, the first thing they ask is, 'Can I get away with a stepper motor with encoder feedback?'"

The question is asked because using this combination often means making performance sacrifices. Servo motors can go faster and provide smoother motion than steppers. In addition, the absolute encoders that are usually paired with servo motors often are superior to those that work with stepper motors. But these advantages come at a price. The cost of a servo motor and drive capable of handling an absolute encoder can be as much as three times as high as that of a stepper motor and drive, says Worsnopp.

Less Complexity

Cost and capability are two of the key factors engineers consider when choosing between electric and pneumatic actuators. A third factor—one that has traditionally worked in favor of pneumatics—is complexity. “With electric actuators, one of the barriers has been the fact that it tends to be a more complex solution, at least initially for new users," says Bimba's Guajardo.

Jeremy Miller, linear mechanics product manager for the Electromechanical Division of Cleveland, Ohio-based Parker Hannifin Corp., puts it another way. Electromechanical motion, Miller says, has always involved a certain amount of “black magic" that can scare people off. So Parker is focused on the development of low-cost rod-style actuators that will simplify integrating an electromechanical actuator into what might normally be considered a pneumatic application.

One way the company hopes to do this is with a simplified user interface similar to what is offered by some relatively new electric actuators on the market. Such an interface can make both setup and operation of electric technology accessible to the non-expert.

“Historically, an electrical engineer has been responsible for programming that whole system, but it doesn't have to be that way today," Miller says.

Another way to simplify electric actuation technology is to combine normally separate components into a single package. According to Miller, Parker and some its competitors now have integrated packages on the market that include an electric actuator, motor and drive.

“Normally all that equipment has to be wired together," Miller says. “But if people can buy something that, out of the box, is ready to be plugged into the wall and used, that goes a long way toward taking the fear and mystery out of electromechanical motion."

Bimba's Guajardo says that one thing that has been preventing such integrated packages from becoming more widely available is a patent on the integrated servo motor. This discouraged companies other than the patent holder from marketing a similar product. But now, Guajardo reports that patent has expired, making it likely that more integrated servo motors will hit the market.

Although they do not include the actuator itself, such products combine a motor, encoder, drive and controller, thereby eliminating the need for users to wire the components together.

“It's a nice, neat little package," Guajardo says of the integrated servo motor. “If I had my choice, that's the way I would go because of its convenience and ease of use."