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Electric motion controls challenge hydraulics and pneumatics

March 1, 2006
Electrics challenge hydraulics and pneumatics

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.

Feed-forward control is simpler with drive-based motion systems. “A servo drive solution is so much more responsive than a hydraulic solution, and it’s extremely easy to implement feed-forward control effectively,” says Brian Boulter, president of consultant ApICS LLC (www.apicsllc.com). “The response of the servo positioning and motion control solution can be implemented optimally with little or no knowledge of control system stability theory. One can simply follow a recipe provided with the motion control canned software package, and get almost optimum performance from the drive.”

Joe Kimbrell, drives, motors, and motion control product manager with Automation Direct (www.automationdirect.com), agrees: “For high accuracy and low-tolerance applications, it’s almost critical. One example is CNC machine tool applications.”

Friendly software

Boulter says users often are making the switch from old hydraulic to drive-based systems because the “canned” software available for implementing motion control solutions is geared toward drives.

“This is not always the case, however, as some vendors, such as Delta Computer Systems, provide motion control software geared toward hydraulic and pneumatic solutions,” Boulter says. “But these are few in number and market share compared to the dominance of drive-based motion control solutions.

“The most popular canned motion control software packages are from vendors such as Rockwell Automation, Siemens, Baldor, Emerson Electric and other brand-name drive/motor supplier companies,” he says. Most vendors that sell hydraulic and pneumatic systems don’t provide software packages for motion control solutions. In the past, these solutions were implemented with controllers using analog electronics. “Some vendors have automated their control solutions, but generally these solutions are geared toward heavy industrial projects such as presses, rolling mill gauge/flatness control, and other highly specialized heavy-industry processes.”

Using analog electronics is a disadvantage for controlling pneumatics and hydraulics. “For the applications in which they are currently used, analog control is not a problem,” Boulter adds. “It’s just that there are fewer and fewer people who know how to set up and tune analog controllers.”

Tough decision

When deciding whether to use electric, hydraulic or pneumatic control systems, machine builders determine both the equipment’s technical requirements and running costs (see sidebar, “Critical decision points”). The benefits of technologies must be compared.

“For example, for a multi-position application that doesn’t require the precision offered by an electric system but needs to move large loads from one point to another at high speeds, where space and cleanliness are not constraints, a servo-hydraulic solution will outperform an electric solution of approximately the same form factor,” Yakoob explains. “To match the performance of the servo-hydraulic solution, the electric servo system would need to be up-sized significantly, which in turn may offset the future running costs.”

In addition, factors such as brown-field versus green-field sites come into the equation. When dealing with a brown-field site, often there are limitations on what type of technologies can be implemented due to structural and technology issues within the infrastructure of the site. “With green-field sites, the machine builder has more flexibility because infrastructure tends to be state-of-the art and allows for more flexibility when implementing technology solutions,” Rusk says.

Other site requirements or the product itself may influence the decision. For example, “Medical devices often are made in clean rooms,” says Henson. “If you’ve got pneumatics in there, that means you’ve got lubricants and moisture that can’t exhaust into the clean room.”

Standards also can affect the decision to avoid pneumatics. In the current landscape of CE requirements, NFPA 79 and other standards, “The world does not want to see pneumatics anywhere near electronics,” Henson says. If pneumatics are associated with electronics, they have to be completely isolated because dripped fluid can get in the electronics, and hoses can break. “So in general, I try very hard to get all pneumatic-type functions into electric in my machines,” he says. “That drives up the cost, but it also makes the machines easier to control, safer and easier to get qualified regarding the industry standards.”

Power reigns supreme

In the past few years, rising energy costs have hit manufacturers hard on the bottom line. “A growing advantage for electric servos is energy efficiency,” says Yakoob. While a comparable electric servo system is relatively more costly than a typical pneumatic system, the associated cost of creating and maintaining the energy supply — in this case, compressed air — is quite high. “Over the life of the machine, the running costs of an electrical system are substantially lower,” she says.

Whether or not electric control is more energy efficient depends on the application. “In a facility with an installed infrastructure for managing hydraulic equipment like hydraulic power units and such, then the hydraulic solution would be the solution,” says Dan Halloran, engineering manager, electronics and controls with Moog Inc. (www.moog.com/industrial). “Yet one has to understand that a complete electric system will be smaller in terms of its footprint. If the application deems an electric solution, it usually saves money and space.”

One Moog customer in the tube end forming business reduced power consumption 75% and noise 50% by switching the forming axis from hydraulic to electromechanical. The new system uses an actuator capable of up to 100 kN with real-time diagnostics and a proprietary integrated toggle mechanism. The system delivers machine cycles in 1.3 sec. and position accuracy of 0.0004 in. within a 4-in. stroke.

Hydraulics, pneumatics hold their own

But don’t write off hydraulics just yet. ”Comparisons illustrating the superiority of electrics are often comparing modern electric control with older-generation hydraulic control,” says Bill Savela, P.E., motion products marketing manager with Delta Computer Systems (www.deltamotion.com). “With the advent of the new generation of motion controllers for one- and two-axis applications that have capabilities for precision hydraulic and pneumatic control, accuracy and controllability advantages usually disappear.”

Some new fluid power motion controllers work with very precise position and pressure transducers and generate outputs to servo-quality valves. They use control algorithms that allow closed-loop position and pressure control to accuracies better than 0.001-in. range, with loop times of 1 ms or less.

“This capability allows hydraulics and pneumatics to serve all but the fastest, most precise motion applications,” Savela says. For example, hydraulic actuators can lift and hold heavy loads without separate brakes, can move heavy objects at slow speeds or apply torque without gearing, and consume less space and produce less heat at the actuator than electric motors.

“Servo-pneumatic systems offer certain benefits of electric servo systems — multi-position with high speed, high force and reasonable repeatability of 0.2 mm,” Yakoob says. “In the recent past, there have been more manufacturers developing such systems.”

As a result, in many cases hydraulics and pneumatics are staying competitive with electric motion control. “Both technologies are finding ways of innovating and making sure they bring value to the table for the customer — specifically in the area of network communications,” says Rusk. “A lot of them now have plug-in modules for their connections, whether it’s Ethernet, Profibus or whatever. They’ve begun to almost convert into a semi-electric provider.”

Pneumatic and hydraulic controls manufacturers have reduced maintenance requirements, provide better seals to prevent contamination and leakage, and are controlling raw material costs to provide stable or lower per-part pricing. The combination of force control and speed control with limited position control makes their solutions attractive in appropriate applications.

One of the key advantages of pneumatic systems is that, unlike electric servo systems, overload issues are not as drastic, making pneumatics/servo-pneumatics more desirable in large-load-handling and dispensing/filling applications. Also, when comparing pneumatic and electric products for food industry wash-down requirements, pneumatic controls still have a cost advantage.

Bar Mill 1 at Nucor Steel, Darlington, S.C. switched from hydraulic to pneumatic controls when it upgraded a Danieli Morgårdshammar automatic stacker to replace the cooling bed, continuous stacker, shear and other finishing equipment. The upgraded machine now produces angles, flats, rounds, rebar and round corner squares at speeds as high as 3,000 ft./min.

“The equipment used hydraulic finger controls for the layers on top of the machine, and we moved those to pneumatic,” says Ray Evans, Nucor’s Bar Mill 1 maintenance supervisor. “One of our main customers builds joists for facilities, and the joists have to be painted. If the hydraulics spring a leak, it sprays onto the steel and you can’t paint it. That material has to be thrown away. So we converted it to air. Also, the mag rotation was hydraulic. We converted it to a motor and gearbox so we could control the speed, rotation and precise pick-up of material.”

Evans says the conversion from hydraulic to pneumatic has paid off on Nucor’s bottom line. Downtime has decreased from 6% or 7% to 1.8%. The upgraded machine also has increased efficiency in tons per hour. But, “The biggest benefit is keeping hydraulic fluid off the joists so our customers don’t have to throw away steel,” he says.

Best as a team

Hybrid control solutions are the new frontier of motion control. They’re designed to harness the best of both hydraulic and electric control technologies, incorporating flexibility, increased efficiency and cleanliness into a single unit. Several companies including Moog and Delta Computer Systems offer hybrid solutions.

“Electromechanical controls and the field of mechatronics is an important and growing aspect of motion control,” says Savela. “Electrohydraulic motion controllers have an advantage because they can control both hydraulic and electric motion very well from the same controller. The same cannot easily be done with an electric-oriented motion controller.”

One company that benefits from hybrid motion control is NorskeCanada, North America's third largest uncoated groundwood paper company and one of the world’s few producers of sawdust-based pulps. The company replaced a hard-to-maintain proprietary controller in its thermomechanical pulp refiners at its Powell River, British Columbia facility with programmable electrohydraulic controllers.

As a result, it gained fast and stable response from the machine, and power consumption transients that used to occur as the older control system compensated for the changing wood chip load were smoothed out by quicker response and more accurate loading.

Kimbrell believes manufacturers will get even more excited about using electric motion control as the ease of use and flexibility of the newer generation of drives continues to reach the market. “It’s brought high-performance functionality down to the commodity level. Anyone involved with maintaining the plant can easily integrate some of the advanced technology themselves.”

Actuators for our photographs were provided by:

Bosch Rexroth Corporation’s Electric Drives & Controls Group
Festo Corporation
Moog Inc.

Critical decision points

  • Technical capabilities. What is the user trying to accomplish?
  • Ease of use. How easy it to commission the equipment? This includes PLC or CNC programming and technical training of on-site technicians.
  • Production time. How fast can the equipment run and for how long before efficiencies or other factors must be checked and adjusted?
  • Initial system cost and simplicity of design. Electric servo solutions tend to be more costly than pneumatics, but the cost of electric servos continues to drop.
  • Long- and short-term ability of manufacturers to maintain the equipment. Machine builders are starting to question whether manufacturers have enough trained personnel to maintain machines when they’ve fulfilled their contract and left the site.
  • Energy consumption.
  • Maintenance and parts availability.
  • Precision and repeatability.
  • Reliability.
  • Flexibility/changeover. Higher flexibility is typically possible with servo pneumatics or electric servo controls.
  • Cleanliness. Pneumatic and electrical systems typically are used instead of hydraulics.
  • Safety. Hydraulics typically are avoided in hazardous environments.
  • Acceleration requirements.
  • Speed.
  • Work piece size and weight.
  • Component size and space restrictions.

Battlefront: injection molding

The high power demand of plastic injection molding often calls for hydraulic motion controls. “Very large injection molding machines with high tonnage certainly mandate hydraulic because it is cost-effective to use big cylinders,” says Dan Halloran, engineering manager, electronics and controls with Moog Inc. (www.moog.com/industrial). “And at the highest end of performance solutions, we see areas where innovative designs like hybrid solutions can be used as well.”

However, for one OEM of high-performance optical disk injection molding machines, Moog facilitated the move from hydraulic to electric controls. The two companies partnered to develop a motion control/process control system with a 2.5-sec. production cycle time and less than 1% weight variation of the optical disk. The servo drive includes a controller for significantly increased bandwidth. The OEM achieves higher productivity and stability, better accuracy, improved energy efficiency, reduced cycle times and the flexibility to produce all current optical disk formats, plus new ones in the future.

Flight simulation has incredibly precise requirements. While most large motion control simulation solutions in the past were hydraulic, recent control applications are moving to electric to saved facility space, create an oil-free environment and reduce maintenance. FlightSafety International (FSI), Tulsa, Okla., uses Moog electric motion control systems for its flight simulation training facilities. FSI has 21 60-inch simulators like those shown here, one 36-inch simulator, and more electric-controlled simulators on order.

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