02 May 2001
Fluid Power Basics
by Peter Nachtwey
"Fluid power" refers to energy transmitted via a fluid under pressure. In hydraulic systems, that fluid is a liquid (e.g., oil or water). Pneumatic fluids are typically compressed air or inert gas. Fluid power's motive force results from the principle that pressure applied to a confined fluid is transferred uniformly and undiminished to every portion of that fluid and to the walls of its container. A surface (e.g., a cylinder piston) will move if the difference in force across the surface is larger than the total load plus frictional forces. The resulting net force can then accelerate the load proportionately to the ratio of the force divided by the mass.
Fluid Power Advantages
Fluid power is used in diverse applications ranging from mobile construction and aerospace equipment to powering industrial machinery and offers several advantages over other types of motive force.
In fluid power systems, a single source of fluid pressure (e.g., a compressor or pump) can power many motion axes or fluid power devices. The power source can be located where space isn't critical. Because much of the system's size and weight is off-loaded onto the power unit, the individual actuators can be small compared with the power they produce. In addition, they're often quieter and generate less heat than electric actuators. Fluid power actuators can also be used in hazardous environments where electric sparks must be avoided.
By using accumulators to store energy, a hydraulic power unit needs to provide only slightly more than the average demand, increasing efficiencies for machines with varying load cycles.
In applications requiring a constant holding pressure or torque (e.g., presses), hydraulic actuators have a big advantage because they use no energy while they're stationary. In contrast, electric motors draw considerable current to maintain torque, even while stopped. Most motors will overheat and fail under these conditions.
Hydraulic cylinders are very smooth and efficient for linear movement. There are no poles that cause cogging, and there's no need for backlash compensation.
Hydraulic vs. Pneumatic Power
Although hydraulic and pneumatic power share many common characteristics, there are some key differences. For example, because hydraulic fluid is much less compressible than gas, hydraulics are preferable to pneumatics when precise position control is required. Conversely, pneumatic power has an edge in applications where hydraulic oil's presence could cause problems (e.g., food processing machines). Pneumatic systems are also typically less expensive to build.
Designing a Fluid Power System
Designing the power circuit, as well as selecting and physically placing system components, is critical to maximizing the system's performance. Figure 1 shows a typical hydraulic system with an electronic motion controller. Following is a discussion of key system elements.
Pumps supply the oil or air under pressure required to move the system's cylinder pistons, converting mechanical power to fluid power. Usually only one pump is required to power all the system's cylinders, and it needs to be only large enough to supply the average amount of oil required by the application per machine cycle. (This assumes that properly sized accumulators are precharged so they can store fluid under pressure when the system isn't moving.) Undersized pumps lack sufficient controlling pressure, resulting in a system that can't move at desired speed or decelerate to a target position quickly. Oversized hydraulic pumps are wasteful, as they cost more and require that oil be bypassed to the storage tank when the system isn't moving.
Cylinders convert fluid power to linear mechanical power (in rotary applications, a motor replaces the cylinder). As with pumps, correctly sized cylinders are critical. Increasing a cylinder's size boosts the natural frequency of operation and allows faster acceleration but at the cost of requiring larger valves and pumps than those used by smaller cylinders in non-speed-critical applications.
Accumulators are energy storage devices, or reservoirs, for air or oil pressure. For best results, use accumulators of adequate size and place them close to the valves. Accumulators serve two important functions. First, they store energy so pumps don't have to be sized for peak loads. Second, when sized correctly, they keep the system pressure relatively constant. This is important when using a motion controller because the proportional-integral-derivative gains should change as a function of the system pressure. Thus, maintaining constant system pressure reduces the need to change gains as a function of pressure in the motion controller. This also provides for smoother motion when moving slowly. Oversized pumps don't replace accumulators. If there's more than one actuator (valve/cylinder combination) in the system, consider peak loading when sizing the accumulator.
Solid piping, rather than hose, should be used between the valve and the cylinder, as hoses contract and change shape, and any change of area affects controllability. Keep pipes as short and straight as possible because pressure drops occur in bends. Place the valves on the cylinders, or as close to them as possible, to keep the fluid volume between the valve and cylinder as small as possible. This helps keep the system's natural frequency as high as possible.
For high-performance motion, use linear valves with zero overlap. They can be either servo valves or servo-quality proportional valves. A valve that uses most of its range is generally easier to control, so match response time and flow rates to the application, avoiding gross oversizing. The valves are called "zero overlap" because there's no "dead zone" between active control ranges (ranges that increase or decrease fluid pressure). Valves with overlap may be advantageous for manually controlled systems, but not for high-performance, high-precision position/pressure systems.
The oldest and least sophisticated hydraulic or pneumatic systems employed an on/off pressure valve control, sometimes called "bang-bang" because of the "jerkiness" that discrete control causes. New systems designed for smooth, accurate motion use electronically controlled variable valves. These devices can employ sophisticated predictive control algorithms, using inputs from position and pressure sensors to provide tighter control and more flexibility than was possible with hydraulic control elements. By including special smart functions, controller manufacturers can offer higher productivity to machine builders.
Another selection criterion to remember when choosing a motion controller is to ensure that it interfaces easily to other system computing elements (e.g., programmable logic controllers, industrial computers, or human-machine interfaces). New controllers have fieldbus interfaces: standardized, high-performance connections such as Ethernet and Profibus. Ensure the motion controller not only supports the proper electrical connections but is also certified specifically for compatibility with the modules to which it connects.
The motion controller gets its position and pressure inputs either from transducers mounted in the cylinders (linearly actuated systems) or via encoders mounted on axes (rotational systems). Choose those that interface directly with the transducers. Pressure transducers in linear activation systems are typically mounted on either side of the piston (Figure 1) to measure differential pressure. To measure linear position, many systems use magnetostrictive displacement transducers (MDTs), which don't require a homing step. Figure 2 shows a typical MDT-equipped motion controller.
Performance, Precision, and Price
Hydraulic and pneumatic power offer many advantages over electric motors, especially for systems that require high-speed linear travel, moving or holding heavy loads, or very smooth position or pressure control. In comparison, hydraulic and pneumatic actuators are smaller and quieter. They also produce less heat and electromagnetic interference at the machine than electric actuators do. In many casesparticularly in high-performance hydraulic or pneumatic systemsthey offer machine builders a considerable cost savings compared with similar machines employing purely electrical or mechanical motion.
Selecting appropriate system components and programming the appropriate motion control algorithms can result in a system that offers high performance and precise motion at a reasonable cost. For more information, go to the National Fluid Power Association Web site (www.nfpa.com).
Figures and Graphics
- Figure 1: Fluid power control system (hydraulic).
- Figure 2: A motion controller may use an MDT position transducer to measure linear position.
- Fluid Power Basics (PDF)
Peter Nachtwey, president of Delta Computer Systems, has more than 19 years of experience developing hydraulic, pneumatic, and vision systems for industrial applications. He graduated from Oregon State University in 1975 with a BSEE and served in the U.S. Navy until 1980. He has worked for IECC and Applied Theory Inc. as a systems engineer. He became president of DCS in 1992. Contact him at 11719 NE 95th Street, Suite D, Vancouver, WA 98682-2444; tel: (360) 254-8688; fax: (360) 254-5435; www.deltacompsys.com.