It's no secret that the power source for machine motion can greatly impact the amount of energy a machine consumes. Conventional wisdom holds that electric drives are more efficient than their fluid-power counterparts. But for certain tasks, hydraulics holds the upper hand. Equipment with a single, controllable hydraulic pump and accumulator can consume less power than electric motors and gearboxes in applications with a lot of back-and-forth motion, that move or hold heavy loads, or that have a large number of motion axes.
Consider a large robot that pours concrete into molds for sewer pipe, underground vaults, and catch basins. The machine builder, Hawkeye Group of Mediapolis, Iowa, switched to hydraulics because the electric motors initially powering the robot's two axes were ill-suited for the heavy and varying loads. Filled with concrete, the feeder mechanism weighs up to 10 tons, and Hawkeye's engineers found such loads would “trip out” and shut down the electromechanical drives. The system wasted electricity and could not always produce the required motion.
Such applications need extremely large electric motors to match the motive force of hydraulic motion arms. And adding to inefficiency in Hawkeye's casting robot, one motion axis moves the other's motion base as the robot arm follows various pouring contours. Thus, the weight of the motor on the upper axis becomes an additional energy-wasting burden on the base axis.
In comparison, a significantly smaller motor powers the hydraulic pump in the redesigned circuit, saving energy, and actuators weigh much less than the electric motors they replace. A similar weight advantage has resulted in hydraulics being the standard on heavy-duty construction and mobile equipment.
The power-saving benefits of hydraulics in the above application stem from the fact that a single electric motor and hydraulic pump can power many motion axes. And each axis, in turn, is a fraction of the weight of an electric motor-driven actuator of equivalent power. In addition, the hydraulic pump's motor can be sized to meet the average load the system must carry. This can be significantly smaller than in electromechanical systems, where the motor driving each axis must be sized for peak loads.
Hydraulic systems smooth out energy requirements with accumulators. These simple devices store energy in the form of fluid under pressure and release it when needed, making them useful tools in developing efficient circuits.
Most hydraulic accumulators use the compressibility of a gas - usually nitrogen - for storing energy. Basically, a hydropneumatic accumulator has a fluid compartment and a gas compartment, with a gas-tight element separating the two. For instance, bladder accumulators consist of a pressure vessel and an internal elastomeric bladder that contains the gas. The bladder is charged through a gas valve at the top of the accumulator, while a poppet valve at the bottom prevents the bladder from being ejected with the outflowing fluid.
When system pressure exceeds nitrogen precharge pressure, the poppet valve opens and hydraulic fluid enters the accumulator. The change in gas volume between minimum and maximum operating pressure determines the useful fluid capacity.
Thus, accumulators store energy when demands are less than average and transmit energy back into the system to satisfy peak requirements. Sufficient accumulator capacity ensures the hydraulic pump need not respond to sudden changes in the demand for oil.
Hydraulics also saves energy when machines must move slowly. That's because it can be inefficient to gear down the motion of electric motors via gearboxes. Overall efficiency depends on the efficiency of the motor and gearbox together. But gearbox efficiency specifications in catalogs are generally based on a single operating point and not entirely accurate for every application.
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Power loss in a gearbox is mostly from friction, which generates heat. And friction depends on the quality of gearing, the number of tooth engagements, and the load torque. A basic rule for spur gears is a 10% loss per engagement. And, in general, the lighter the load and higher the ratio, the less likely a gearbox will reach the manufacturer's stated efficiency. For instance, efficiencies in one 16-mm diameter spur gearbox range from 87% at a 6.3:1 ratio to about 40% at a 10,683:1 ratio.
In many cases, when the motor is at peak efficiency, the gearbox is not. (For more details, see “A second look at gearbox efficiencies,” June 20, 2002, Machine Design magazine.)
An additional factor in hydraulics' favor over gear-driven systems involves inertia and backlash. These can make geared systems hard to precisely control when moving heavy loads at high-speed with a lot of back-and-forth motion. Some machines have gone to linear motors to improve responsiveness but they, too, sacrifice efficiency, as linear motors can be much less efficient than rotary motors with gearing. (See “Gearing up for efficiency,” July/August 2009, Energy Efficiency & Technology.)
Clamping is another area where hydraulics holds the advantage. A motionless, load-holding hydraulic actuator consumes no power, given the proper valving. This is in contrast to holding precise position with an electric motor against a force or weight. To avoid wasting energy, a complex clutch or braking mechanism may be needed to hold the load.
Improving hydraulic efficiency
Of course, hydraulic systems also lose power due to fluid friction in pumps, valves, and piping, and hydraulic systems need to be designed to negate these effects. Today, manufacturers across the board are introducing more energy-efficient hydraulic components ranging from pumps and valves to seals, filters, and even fluids. They're also stressing more-efficient overall designs. For several recent examples, see “Sizing tube lines for efficient hydraulics,” “Hydraulic fluids that improve fuel economy,” and “Fluid-power efficiency on the rise,” respectively from the May 20, Sept. 23, and Nov. 18, 2010 issues of Machine Design.
What can be done to make hydraulics even more energy efficient? One way is by adding controls to run the hydraulic pump only when needed, eliminating energy wasted by idling; and at the precise rate needed, saving the system from working harder than necessary.
Variable-speed hydraulic pump controls being implemented today often rely on variable-frequency motor drives (VFDs). A VFD starts only when the circuit demands flow and precisely controls the pump speed. It also ramps velocity up and down smoothly, avoiding electrical startup transients that are wasteful and can damage nearby electronics. VFDs typically communicate via network interfaces with machine or plant controllers for high performance and efficiency.
One type of application that benefits from this approach involves presses with idle time between compression cycles. If possible, the hydraulic pump can shut off completely. But when idling is necessary (as, for example, to cool the hydraulic fluid), it can take place at a lower speed than needed to support full operation.
In some cases, “intelligent” pump controls are letting machine manufacturers improve hydraulic-system efficiency through valve-less designs. Valves generally control motion or pressure by throttling flow, and this restriction inefficiently generates heat. Systems where a precisely controlled pump sends just the required amount of fluid to an actuator, without intermediary valves, eliminates these losses.
For instance, instead of using conventional servo or proportional valves to control flow and pressure in a hydraulic-press circuit, the PSH drive developed by Voith Turbo H+L Hydraulic in Germany controls the rotational speed and torque of an electric servomotor that drives a hydraulic pump. Closed-loop control of motor speed and torque, in turn, controls flow rate and pressure of the press's hydraulic system.
The servo pump can vary flow, has high dynamic response, and runs quietly. The drive generates high speeds when loads are light, then ramps up pressure as loads increase. This reportedly boosts productivity and results in higher quality finished parts. The drive is well-suited to presses because it controls a single axis. Officials say it saves up to 50% in electricity costs. It also simplifies the press design without limiting performance.
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Hydraulic “transformers” are another innovation. They combine the functions of a pump and hydraulic motor and can convert input flow at one pressure to output flow at another pressure. The conversion is also reversible, save for a small energy loss due to internal friction and leakage. In essence, the product of input pressure and flow equals that of output pressure and flow. The concept is comparable to that of an electric transformer where the product of voltage and current, in principle, remains constant.
According to officials at Innas, Breda, the Netherlands, the high-efficiency devices can control loads with minimal throttling losses. The transformers have high dynamic response, can amplify and continuously vary pressure or flow to control linear cylinders or rotary motors and actuators, and are suitable for both stationary and mobile equipment.
Because they can transform a high flow at a low pressure to a smaller flow at higher pressure, this offers the possibility to recuperate energy. For example when a forklift truck lowers a load, the energy can be recovered and stored in an accumulator. The same holds for recovering braking energy in hybrid vehicles.
Innas officials note that independent measurements on a seven-piston bent-axis transformer showed efficiencies up to 83%. Theoretical efficiency of a nine-plunger version is up to 90%.
The road to better efficiency is not just about strict energy savings among the drive components. Smooth, coordinated motion between multiple actuators also lets machines move quicker and be more productive. With hydraulics, there are some special considerations when it comes to motion control.
Using a general-purpose electromechanical controller for hydraulic axes is usually not recommended, for several reasons. Even with slow-moving actuators, hydraulic motion control typically involves nonlinear relationships between inputs and outputs, so setting up and tuning control loops is much easier using controllers specifically designed for the task. Using a general-purpose controller typically entails much more time-consuming setup and tuning to, hopefully, match performance inherent in an electrohydraulic controller.
Axis controllers for hydraulic drives are also designed to optimize dynamic performance. Advanced controllers can transition smoothly from accurately positioning a hydraulic actuator to controlling the force it applies — something difficult to do with general-purpose computers without jumps or discontinuities in the motion. Hydraulic motion controllers use special preprogrammed functions to smoothly vary accelerations and decelerations, ultimately permitting faster operating speeds while extending machine life.
The latest electrohydraulic motion controllers also provide straightforward interfaces to standard communications networks. Most support a range of industrial fieldbuses, with the most ubiquitous interface being Ethernet. Using EtherNet/IP, a PC can download motion parameters into the controller and read the results of motion steps. It's even possible for production-control personnel to monitor processes remotely via an Internet or intranet connection to the machine.
For example, the conversion of electric motors to hydraulics in the Hawkeye system necessitated a new control system to take full advantage of the hydraulic capabilities. The company opted for a Delta Computer Systems RMC150 electrohydraulic motion controller that gave faster, more precise tracking of the motion. The robot poured molds faster than before, and the faster the concrete delivery system travels around the mold, the thinner a layer it can lay down. The end result is not only faster production, but better quality concrete products.
In another case, the Valley Hay Co. in Harrisburg, Ore., also increased production efficiency when precise electrohydraulic control supplanted an electromechanical motion system.
The RMC150 controls and synchronizes four 13-in.-diameter, 72-in.-stroke hydraulic cylinders that compress bales of organic material for shipment. Better multi-axis synchronization shortened the bale compression cycle from 300 seconds in the case of ryegrass bales to just 53 seconds. And shipping containers that required two hours to load can now be loaded in just 45 minutes. The hydraulic control upgrade has made Valley Hay's entire production operation more efficient.
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As these applications suggest, energy can be wasted in many areas around the machine. Taking a thoughtful approach to decisions like when to run the hydraulic pump and when to shut it off is an example of the broader system-level analysis that can help machine builders save energy. And this analysis ensures all drive-train components are working at their most-efficient speeds.
A designer may be missing the point by focusing on whether an electric-motor-driven actuator is more efficient than a hydraulic actuator or vice versa. Instead, look at the overall system and determine what type of motion power is most efficient in that instance.
But consider hydraulics for any application with one or more of these requirements: holding force or pressure; making precise slow movements; moving heavy loads; one actuator moves another actuator; and where fast, smooth linear movement may reduce overall cycle time, thus reducing energy requirements of related systems.
Researchers focus on fluid-power efficiency
The Center for Compact and Efficient Fluid Power (CCEFP), headquartered in Minneapolis, is a network of researchers, educators, and industry experts aiming to make hydraulic and pneumatic systems more efficient and effective. The CCEFP is a National Science Foundation Engineering Research Center, supported by the NSF as well as seven participating universities and 55 industrial partners.
Until the Center was established in 2006, the U.S. had no major fluid power research center - compared with thirty centers in Europe.
The goals of 25 current research projects include:
Improve fluid-power efficiency to significantly reduce petroleum consumption, energy use, and pollution. And improve transportation efficiency by developing fuel-efficient hydraulic hybrid vehicles.
Investigate new technologies that will make fluid power cleaner, quieter, and safer.
Develop more-compact systems for a new generation of devices such as autonomous rescue and service robots, equipment to increase the mobility of an aging population, and fluid-powered portable hand tools.
Center for Compact and Efficient Fluid Power, www.ccefp.org
Hawkeye Group, www.hawkeyepipe.com
Machine Design, http://machinedesign.com
Valley Hay Co., http://valleyhayco.com
Voith Turbo H+L Hydraulic, www.us.voithturbo.com