Machine Design

Electropneumatic Cylinders

Pneumatics is generally preferred for applications such as material handling, transporting parts, pick-and-place operations, and small-parts assembly. But, until recently, pneumatic systems were almost entirely limited to on/off control -- for example, pressure applied or not applied, or a cylinder extended or retracted. However, the need for increased capabilities in automated systems has spurred development of programmable pneumatic equipment, able to produce freely determined rod positions. Advances in closed-loop control theory, development of fast-acting precision pneumatic valves, and the availability of high-speed electronics and user-friendly interfaces now make proportional pneumatics a reality, offering performance that matches closed-loop electrical and hydraulic systems.

One such system features a pneumatic cylinder with a linear position sensor built into the rod, a pneumatic brake, and an electronic controller. Continuous monitoring of rod position and velocity, combined with positive stopping action of the brake, ensures accurate positioning. The system allows up to 32 user-defined set points for each of eight programs. It interfaces to I/O devices such as computers or programmable controllers, or to solid-state relays for timing and output. Typical applications include material-handling, spot welding, and pick-and-place systems. A multiaxis positioning configuration is also offered for control of up to eight axes.

Another type of pneumatic, closed-loop control system features a continuous-acting valve as the control element, along with position sensor and electronic controller. The system contains no mechanical brake. Controlling cylinder movement requires quick and precise valve response. In a positioning application, for example, as the rod approaches the set point, the valve shifts overcenter to build up pressure that opposes cylinder movement. Internal algorithms control rapid shifting of the valve from one side to the other, giving smooth deceleration to the desired position. Variables other than proportional feedback come into play. Thus, the system requires a more powerful microprocessor than typically found in hydraulic servosystems.

As the control element, valve precision and speed must exceed that of the control loop as a whole. Valve speed determines how rapidly the system compensates for unwanted deviations. A rule of thumb that governs the stability of closed-loop control requires the valve natural frequency to be at least three times that of the controlled system, for example, the cylinder and load. Large-diameter, short-stroke cylinders could pose control difficulties. But for most material-handling applications, cylinder natural frequencies in the 1 to 5-Hz range are well within the capabilities of pneumatic control valves.

When free programmability and multiposition capabilities are not important, lower-cost options are available. Most pneumatic cylinder manufacturers offer piston or rod position sensing capability with their product, enabling the user to interface with electronic controls. One common method is the mechanical limit switch, although this device seems to have fallen into disfavor. The drawbacks most often cited are the susceptibility of mechanical parts to damage or wear, and that the switch gets in the way of machine operation. More often, proximity switches mounted to the cylinder OD are used. These provide noncontact indication of cylinder piston position.

One type of proximity switch is an enclosed reed switch that is activated by a permanent magnet mounted to the piston. As the piston approaches, the magnetic field closes the switch, completing an electric circuit and producing an electric signal. Others prefer to use Hall-effect sensors in combination with a piston-mounted magnet. The main difference between these two devices is that a reed switch is a mechanical device, while a Hall-effect switch is electronic, with no moving parts to wear out. Reed switches have the advantage of operating on ac or dc current; Hall-effect switches are limited to dc only. Also, reed switches are about half the price of their electronic counterpart, although the price differential is decreasing. On the other hand, Hall-effect devices react much faster than reed switches, on the order of 100,000 versus 500 Hz. It is a waste of money to install microprocessor-based logic, and then be hampered by a slow switch. Overall, the trend seems to be towards these solid-state devices.

Both switches have a repeatability of 0.005 in. or less. But often rod velocity, not switch accuracy, determines the overall repeatability of rod position. Because air is a compressible fluid, precise positioning of the cylinder rod -- especially in midstroke -- is difficult, regardless of the switch used. In many applications, even if the switch is very fast and accurate, getting the valves to react fast enough may be the real problem. In such cases, mounting fast-acting valves as close to the cylinder as possible is important. This eliminates excess compressible air in the lines between cylinder and flow control, giving more precise control.

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