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Servo Pumps Give Hydraulics End-Users An Edgeby Kristin Lewotsky, Contributing EditorMotion Control Association (http://www.motioncontrolonline.org/company-profile-detail.cfm/Internal/Motion-Control-Association/company/1183)

POSTED 03/15/2015

Replacing fixed-speed pumps with servo precision reduces noise, increases responsiveness, andcan cut energy costs by as much as 90%.

In today’s market, just getting the job done is no longer enough. If you’re a manufacturer or OEM, you

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Página 1 de 7Motion Control Technical Features - Servo Pumps Give Hydraulics End-Use...

Motion Control Technical Features - Servo Pumps Give Hydraulics End-Use...

Now, you need your equipment to deliver topperformance at lowest possible cost. If you workwith hydraulic systems, for example in metalforming or injection molding, you’re in luck. Bysimply switching from fixed-speed pumps toservo-motor-driven pumps, you can decreaseoperating costs and increase product qualitywhile reducing noise (see figure 1). Let’s take acloser look at how.

Energy savingsIn a conventional hydraulic system, a fixed-speedmotor drives the pump that circulates thehydraulic oil from the reservoir to the hydrauliccylinder. To keep the oil circulating, the pumpneeds to run constantly, consuming energy evenwhen the actuator is stationary.

Pumps in hydraulic systems are commonly driven by induction motors, which are inherently lessefficient than the permanent-magnet motors used in servo designs. Induction motors have to drawcurrent in order to generate the magnetic field required to get the rotor to turn. Their efficiency alsodecreases when they are run below their rated speed.

“An induction motor has a sweet spot and if you’re not running at that sweet spot, then you’re usingexcessive magnetizing current and creating heat,” says Ken Kerns, marketing manager for low voltagedrives, Siemens Industry Inc. (http://www.motioncontrolonline.org/company-profile-detail.cfm/Supplier/Siemens-Industry-Inc/company/1087) (Norcross, Georgia). Excess current draw costsmoney, and excess heat at minimum introduces cost and complexity in the form of thermalmanagement and at worst can lead to early failure.

Of course, if you’re reading this article, you probably know about these problems firsthand. What youneed is a solution. Hybrid electrohydraulic systems leverage the power density of hydraulics with theresponsiveness and energy savings of electromechanical technology. Unlike the induction motors usedin fixed-speed pumps, the permanent magnet motors used in servo pumps operate synchronously andwithout any latency. Even better, they run efficiently across a range of speeds. “With a servo pump, youdon’t have to have continuously circulating hydraulic fluid,” says Kerns. “Basically, the motor comes onand runs when you need pressure, and with a much faster response time.”

This means that you’re not paying to operate the pump full speed when the hydraulic actuator isstationary. Indeed, analysis shows that a conventional hydraulic system will cost considerably more overtime than a hybrid version using a servo pump (see figure 2).



“The end-user gets energy savings very similar to that of an all-electric driven machine,” says MonteSwinford, regional factory automation sales manager for Bosch-Rexroth(http://www.motioncontrolonline.org/company-profile-detail.cfm/Supplier/Bosch-Rexroth-Corporation/company/1080) (Hoffman Estates, Illinois). “If they run the same part, same cycle, they willsee significant savings. I had one client who saw a 90% reduction in energy usage.”

The instant response delivered by servo motors enables the actuator to operate much more quickly anddeterministically, making it a good fit for applications like punch presses. The solutions also involve lesscomplex systems with smaller footprints while incorporating value-added devices like smart drives withreal-time diagnostic capabilities (see figure 3). In the case of applications that would normally place thereservoir at a distance from the hydraulic cylinder, the use of a servo pump at the actuator can eliminatethe need for long hydraulic hoses that are prone to leaks or breakage.



Examples of hybrid systems in actionApplications like injection molding are perfectly positioned to benefit from the technology. Rubberinjection molding processes, for example, require the pressure to be applied and then maintained at aset level for an extended period of time while the rubber fills all nooks and crannies of the mold. In thecase of a system run by a fixed-speed pump, the motor would be running the entire time, consumingpower and generating heat, even though it is only maintaining pressure. With a servo pump, the holdingtorque of the motor is sufficient to generate the required pressure. “Because you’re using permanent-magnet servo motors, there’s virtually no voltage on the motor,” says Raymond Seifert, director ofapplication engineering, Baumueller Nuermont Corp. (http://www.motioncontrolonline.org/company-profile-detail.cfm/Supplier/Baumuller-Nuermont-Corporation/company/1093) (Windsor, CT). ”As a result,even though you might have to maintain a holding current, the actual power that you’re using isminimal.”

Indeed, a calculation of energy consumption and cost for this application based on the duty cycle of themotor showed a clear advantage for the servo-driven design. “The customer was happy with theprototype and could justify the cost,” Seifert says. “They are now looking at retrofitting more machines.”

A new hybrid electrohydraulic press using a dual-pump design has been developed as an off-shoot ofthe servo-motor-driven pump technology. The approach allows bi-directional pumping of the hydraulicfluid, enabling the customer to reduce the size of the existing hydraulic press reservoir to nothing morethan an accumulator in the hydraulic circuit. Fewer components and less oil means a smaller footprint,



purchase and disposal of the hydraulic oil.

Of course, shrinking the volume of oil does increase the importance of thermal management. Thesmaller volume of oil means that it heats up more quickly. When the temperatures get too high, thepump can fail. To guard against this, the addition of auxiliary cooling in the hydraulic circuit may benecessary to ensure temperatures remain under control.

Making better productsServo pumps deliver benefits beyond just cost savings. The level of control provided by closed-loopfeedback leads to more precise motion. In contrast, conventional hydraulic systems may not even use adrive, let alone feedback. They may simply use a motor with a simple on/off switch that runs at constantspeed. The closed loop feedback of a servo pump equips the system to deliver the exact torque orspeed required for the application, and do it instantly.

“Even if you use a drive with a standard induction motor, you still have an element of delay in gettingthat system up to a certain pressure,” says Kerns. “More often than not, you’ll see a hydraulic systemthat doesn’t even have a digital feedback loop, just a regular analog loop. The big benefits of the servomotor are the response time and the fact that you’re able to run those motors up to really, really highRPMs to get to where you need to be much quicker.”

This level of fine control can benefit applications like metalforming. When metal is deformed into ashape, it has to be slightly over bent so that it will assume the correct form after recoil. For best results,full force should only be applied in the last millimeter or two of motion. Servo pumps enable much moreaccurate control of the motion of the hydraulic actuator, creating better quality parts.

The servo sounds of (near) silenceAlthough system cost and performance are important, the modern industrial environment imposesother challenges to machine design and operation. Particularly if you are involved in industries likeautomotive manufacturing, you face growing regulation for working conditions. Here, too, servo motorsprovide superior solution to conventional fixed-speed pump designs by virtue of their whisper-quietperformance. “When customers take the risk and make the change, their jaws just drop with how quiettheir new machine is,” says Swinford. That performance is not just good for end-users but for OEMs, henotes. “I have one client who has kept their factory full for the last three years because their end usersare so excited by the noise reduction that they keep ordering new machines just to replace the ones inthe field. Most of them wind up having flashing lights on the machine to let customers know that thepower’s on.”

As intriguing as the technology may be, it’s important to remember that the performance of the systemis only as good as the design approach. “It’s not the kind of thing that you just kind of slap together,”Swinford says, citing limiting factors like minimum and maximum pump speed, pressure rises, etc. “Thebest thing to do is to start with a clean piece of paper. Sure, you can just add something onto what youhave and make it work, but it’s not going to be as competitive in the global marketplaces as starting



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Everyone interviewed for this article agreed that the adoption of the technology is still in the earlystages. Far from tempting manufacturers to wait until later to consider the technology, that should spurOEMs and end-users alike to explore the use of servo pumps. Taking action at the beginning of a marketevolution is the way to derive maximum benefit from the conversion. Designing servo pumps into yournext hydraulic system will position your organization to gain a significant competitive advantage overorganizations working with the incumbent technology.

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