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Servo Pumps Give Hydraulics End-Users An Edge
by Kristin Lewotsky, Contributing Editor
Motion Control & Motor Association Posted 03/15/2015
Replacing fixed-speed pumps with servo precision reduces noise, increases responsiveness, and can 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 need every advantage you can get. In response to the recession, you’ve probably already stripped down your operations, rescaled your inventory and wrung every bit of margin possible from your supply chain. Now, you need your equipment to deliver top performance at lowest possible cost. If you work with hydraulic systems, for example in metal forming or injection molding, you’re in luck. By simply switching from fixed-speed pumps to servo-motor-driven pumps, you can decrease operating costs and increase product quality while reducing noise (see figure 1). Let’s take a closer look at how.
In a conventional hydraulic system, a fixed-speed motor drives the pump that circulates the hydraulic oil from the reservoir to the hydraulic cylinder. To keep the oil circulating, the pump needs to run constantly, consuming energy even when the actuator is stationary.
Pumps in hydraulic systems are commonly driven by induction motors, which are inherently less efficient than the permanent-magnet motors used in servo designs. Induction motors have to draw current in order to generate the magnetic field required to get the rotor to turn. Their efficiency also decreases 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 using excessive magnetizing current and creating heat,” says Ken Kerns, marketing manager for low voltage drives, Siemens Industry Inc. (Norcross, Georgia). Excess current draw costs money, and excess heat at minimum introduces cost and complexity in the form of thermal management and at worst can lead to early failure.
Of course, if you’re reading this article, you probably know about these problems firsthand. What you need is a solution. Hybrid electrohydraulic systems leverage the power density of hydraulics with the responsiveness and energy savings of electromechanical technology. Unlike the induction motors used in fixed-speed pumps, the permanent magnet motors used in servo pumps operate synchronously and without any latency. Even better, they run efficiently across a range of speeds. “With a servo pump, you don’t have to have continuously circulating hydraulic fluid,” says Kerns. “Basically, the motor comes on and 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 is stationary. Indeed, analysis shows that a conventional hydraulic system will cost considerably more over time 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 Monte Swinford, regional factory automation sales manager for Bosch-Rexroth (Hoffman Estates, Illinois). “If they run the same part, same cycle, they will see 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 and deterministically, making it a good fit for applications like punch presses. The solutions also involve less complex systems with smaller footprints while incorporating value-added devices like smart drives with real-time diagnostic capabilities (see figure 3). In the case of applications that would normally place the reservoir at a distance from the hydraulic cylinder, the use of a servo pump at the actuator can eliminate the need for long hydraulic hoses that are prone to leaks or breakage.
Examples of hybrid systems in action
Applications like injection molding are perfectly positioned to benefit from the technology. Rubber injection molding processes, for example, require the pressure to be applied and then maintained at a set level for an extended period of time while the rubber fills all nooks and crannies of the mold. In the case of a system run by a fixed-speed pump, the motor would be running the entire time, consuming power and generating heat, even though it is only maintaining pressure. With a servo pump, the holding torque 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 of application engineering, Baumueller Nuermont Corp. (Windsor, CT). ”As a result, even though you might have to maintain a holding current, the actual power that you’re using is minimal.”
Indeed, a calculation of energy consumption and cost for this application based on the duty cycle of the motor showed a clear advantage for the servo-driven design. “The customer was happy with the prototype 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 of the servo-motor-driven pump technology. The approach allows bi-directional pumping of the hydraulic fluid, enabling the customer to reduce the size of the existing hydraulic press reservoir to nothing more than an accumulator in the hydraulic circuit. Fewer components and less oil means a smaller footprint, lower cost for the system, better products, less chance of leakage, and less money spent on both purchase and disposal of the hydraulic oil.
Of course, shrinking the volume of oil does increase the importance of thermal management. The smaller volume of oil means that it heats up more quickly. When the temperatures get too high, the pump can fail. To guard against this, the addition of auxiliary cooling in the hydraulic circuit may be necessary to ensure temperatures remain under control.
Making better products
Servo pumps deliver benefits beyond just cost savings. The level of control provided by closed-loop feedback leads to more precise motion. In contrast, conventional hydraulic systems may not even use a drive, let alone feedback. They may simply use a motor with a simple on/off switch that runs at constant speed. The closed loop feedback of a servo pump equips the system to deliver the exact torque or speed 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 getting that system up to a certain pressure,” says Kerns. “More often than not, you’ll see a hydraulic system that doesn’t even have a digital feedback loop, just a regular analog loop. The big benefits of the servo motor are the response time and the fact that you’re able to run those motors up to really, really high RPMs 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 a shape, 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 more accurate control of the motion of the hydraulic actuator, creating better quality parts.
The servo sounds of (near) silence
Although system cost and performance are important, the modern industrial environment imposes other challenges to machine design and operation. Particularly if you are involved in industries like automotive manufacturing, you face growing regulation for working conditions. Here, too, servo motors provide superior solution to conventional fixed-speed pump designs by virtue of their whisper-quiet performance. “When customers take the risk and make the change, their jaws just drop with how quiet their new machine is,” says Swinford. That performance is not just good for end-users but for OEMs, he notes. “I have one client who has kept their factory full for the last three years because their end users are so excited by the noise reduction that they keep ordering new machines just to replace the ones in the field. Most of them wind up having flashing lights on the machine to let customers know that the power’s on.”
As intriguing as the technology may be, it’s important to remember that the performance of the system is 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. “The best thing to do is to start with a clean piece of paper. Sure, you can just add something onto what you have and make it work, but it’s not going to be as competitive in the global marketplaces as starting over.”
Everyone interviewed for this article agreed that the adoption of the technology is still in the early stages. Far from tempting manufacturers to wait until later to consider the technology, that should spur OEMs and end-users alike to explore the use of servo pumps. Taking action at the beginning of a market evolution is the way to derive maximum benefit from the conversion. Designing servo pumps into your next hydraulic system will position your organization to gain a significant competitive advantage over organizations working with the incumbent technology.