Power up with hydraulics

ControlDesign.com

Once a declining power source, new initiatives in hydraulics are fueling their revival. University of Minnesota Professor, Kim Stelson, outlines work done on campus test beds and by progressive machine builders.

By Kim A. Stelson, Director of Engineering Research for the Center for Compact and Efficient Fluid Power

FLUID POWER has unexcelled capabilities that make it the only viable choice for some industrial machine power applications. Compared to an electric motor, a hydraulic actuator has 12 times more torque for the same volume, 100 times more power for the same volume, 500 times more power for the same weight, and 50 times the bandwidth for the same power.

Consequently, fluid power is a growing industry (See Figure 1 below) with component sales of $12 billion for the U.S. and $33 billion worldwide. Systems sales are one to two orders of magnitude higher.

Despite its intrinsic advantages, hydraulic power has been losing market share to electric-powered devices for injection molding machines. Although hydraulic machines still dominate the larger sizes, electric machines are displacing hydraulic machines in smaller sizes, where the higher power and force capabilities of hydraulics aren’t as important. There are key factors contributing to this trend. Hydraulic machines can be noisy, inefficient, and more difficult to maintain.

So, is the trend toward electric machines inevitable? Maybe not. There is considerable work underway to improve hydraulic power’s efficiency enough that it reemerges as a preferred power source.

FIGURE 1: U.S. FLUID POWER COMPONENT SALES

U.S. Census Bureau’s Industrial Report for Fluid Power is based on an annual survey of U.S. manufacturers. Results for 2005 include the National Fluid Power Association’s (NFPA) estimates for November and December. Estimated growth for 2006 is a composite based on several independent forecasts.
Source: U.S. Census Bureau (Click image to enlarge).

Big Energy Saving Opportunities
In proportional valve or servo valve control, the flow to a hydraulic cylinder or other actuator is modulated by opening or closing an orifice in the valve. The power lost in the valve, P, is the product of the pressure drop, ∆ p, and the flow, Q, or P= ∆ p*Q. When the valve is fully open,  ∆ p is very small and little power is lost in the valve. At the other extreme, when the valve is fully closed, Q is zero so no power is lost. It’s at intermediate values that the power loss is a maximum, so it’s important to avoid partial load for efficient operation.

Pulse-width modulation (PWM) is one control approach that can almost entirely eliminate throttling losses in valves. The idea of PWM control is to either have the valve fully open or fully closed, but never partially open. The valves are opened and closed at a fixed frequency, and the fraction of time that the valve is opened regulates the average flow. This approach is commonplace in electronic systems, where SCR-controlled PWM systems have entirely replaced rheostats in everyday devices such as light dimmers.

Migrating PWM control to hydraulic systems is a research challenge. A high switching frequency is desirable, so ripple in the flow is more easily smoothed out. However, rapidly opening and closing a valve is difficult, particularly for high flow rates. Also, the opening and closing times for the valve must be as short as possible because throttling losses occur during the transition interval. If the opening and closing times are too long, the energy savings are reduced significantly.

	FIGURE 2: HYDRAULIC INJECTION MOLDING MACHINE
	Energy savings of 80% could be realized with energy-saving hydraulics.

To understand the potential energy savings of efficient hydraulics, consider a 180-ton hydraulic injection molding machine (See Figure 2). Such a machine consumes 32.1 kW of power, including 3.2 kW for the electric heaters and 28.9 for the hydraulics. The vast majority of the power for the hydraulics, some 23.9 kW, is lost through valve throttling. If throttling losses could be eliminated by using PWM control, the required power could be reduced to 8.2 kW, producing a 74% savings. If 20% of machines in use adopted a throttle-less approach, the resulting savings would be approximately $150 million per year for U.S. users and manufacturers.

Though not a pure machine application, excavators also show the energy-saving potential of hydraulics. Research shows that, with conventional controls, only 60% of the input results in useful work, while 18% of the energy is lost in pump inefficiencies, 18% is lost in valve throttling, and 4% is lost in the pistons and lines. If these losses could be reduced by more-efficient pumps and PWM control, and if regeneration could be employed to capture energy in one phase of the work cycle and reused in another phase, an estimated 30% energy savings would be produced. If this approach were adopted for all earthmoving equipment, estimated annual energy savings would be $1.15 billion.