Electric motion controls challenge hydraulics and pneumatics

Electrics challenge hydraulics and pneumatics

By Theresa Houck, Associate Editor

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When Thomas Edison invented a practical, safe and economical incandescent light bulb, he couldn’t have foreseen the proliferation of electrical systems that would eventually affect every human on a daily basis. One derivative of that invention is electric motion control for manufacturing. In an increasing number of applications, it provides lower costs, easier maintenance, improved energy efficiency, cleaner working environments, better safety and more precise control than traditional pneumatic and hydraulic motion control systems.

“I’m a machine design guy, and I avoid pneumatics if I can,” says Dale L. Henson, president and owner of equipment builder Engineering by Design (www.ebdesign.com). “I try to get them out of my machines because of all the troubles associated with them, like noise, maintenance issues and lubrication splashing. In general, you can make things work cheaper with pneumatics, and they definitely have their place, but they’re not as reliable and they’re more objectionable to have in a manufacturing environment.”

The forces driving a switch to electric motion control include the machine’s cost of ownership and maintainability, requirements for absolute movement accuracy, safety, and maintaining data for legal reasons if someone were to get hurt to show that the machine did what it was supposed to do.

“Over the lifetime of a machine, an electric solution can offer a lower cost because there aren’t as many components that need to be maintained on a regular basis,” says Rodney Rusk, automotive industry manager, Bosch Rexroth Electric Drives and Controls Group (www.boschrexroth-us.com). For example, with pneumatic and hydraulic solutions, filters, line air or fluid quality, and seals have to be checked regularly. “Combined with labor hours and machine downtime, that adds up to lost production time costs,” Rusk says.

Getting down and dirty

Electric servo systems have long been used in applications such as semiconductor cleanrooms, cutting/contouring systems, printing mechanisms and robotics. However, they now are gaining ground in applications such as automated inspection systems and packaging machines that traditionally relied on pneumatics for positioning tasks.

Servo electric solutions traditionally are used for applications demanding high precision, and pneumatics are used for applications requiring a high power density and speed or applications that don’t require multiposition movements. “However, with the emergence of linear motor technology, both requirements are fulfilled,” says Nuzha Yakoob, product manager, positioning, Festo Corp. (www.festo.com/us). “The cost of a linear motor system is still higher than that of an equivalent pneumatic or conventional electric servo solution. But as with all new technologies, the component costs will gradually drop over time, so it could subsequently replace pneumatic positioning systems.”

Rusk says electric actuators are getting a foothold in gantry applications, transfer lines and standalone machining centers. “However, you’ll find a mix of all three technologies on a regular basis. Each provides their benefit and their specialty to the overall function of a machine.”

Dominant technology

Typical manufacturing plants have pneumatic, hydraulic and electric actuation and control systems, resulting in a need for maintenance competencies in all three disciplines. “As plant operations seek to simplify maintenance and reduce costs, one option that’s often considered is consolidating to fewer technologies,” says Tom Strigel, product marketing manager, servo motors, Rockwell Automation (www.rockwellautomation.com). “Since electrical systems will inevitably be present in any facility, the focus tends to be on expanding their use and eliminating pneumatic and hydraulic technologies.”

He says electric motion control is becoming an alternative of choice because hydraulics are more prone to leaks and require managing fluids and filtration, along with space and enclosures for hydraulic reservoirs and pumps.

One company that made the switch is a major automotive OEM that was experiencing problems with its hydraulically actuated rack-and-pinion system on a transfer device. The system exhibited poor motion control, problems associated with leaking hydraulic fluid (environmental, clean-up and associated safety issues), and resulting machine repair and downtime.

The OEM retrofitted the device with an electric servo drive and a 4-in. lead screw and polymer nut assembly. The servo-controlled linear motion reduced vibration and associated wear, and gives more accurate control.

“The choice of the electric servo was based mainly on maintenance,” explains Fred Leishman, senior sales engineer, TranTek Drive Systems (www.trantekautomation.com). “The combination of the screw drive system and the servo control eliminated a large quantity of high-maintenance wear items that were costly and time-consuming to replace.” Leishman says the system needed to be very fast, and they wanted to be able to control the acceleration and deceleration to make the motion as smooth as possible. “The electric servo combined with the DriTran drive provided speed and control,” he says. Significantly reducing machine maintenance and eliminating hydraulic fluid additions along with leaked fluid clean-up and disposal costs led to a two-month payback.

Covers more ground

Electric actuation supports safety, accuracy and advanced control. “The advent of safe motion in the past two years has really become a driving force for electric-based control,” Rusk explains. Mechanical and electrical lockout/tagout devices for preventing physical injury can be bypassed. Safe motion integrated into the drive technology allows you to lock out machine cell drives so when maintenance needs to be performed, you can put the machine into a safe mode that can’t be bypassed. The motors will not allow any force to be applied or any movement to take place, “or it may allow only minimal movement but at such a slow rate of speed that some maintenance could take place without endangering personnel,” Rusk says.

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