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Engineering productivity kit -- fluid power

Engineering productivity kit -- fluid power

Lines blur between servo and proportional technology

Not long ago, control engineers used servo valves for positioning and proportional valves for velocity control. But it's not that simple anymore

By Charles J. Murray, Senior Regional Technical Editor

Park Ridge, IL--Time was when a mechanical monster like Godzilla used servo valves. But when engineers built the creature for the 1998 film of the same name, they chose a different technology for movement of the head, torso, arms, fingers, and jaws. Their choice: a type of proportional hydraulic valve sometimes called a servo solenoid. The servo solenoid valves supplied the proper fluid flow to each of the monster's hydraulic actuators, enabling it to mimic the movements of a live creature.

Parker Hannifin proportional hydraulic valves position car bodies for welding.

And Godzilla's not alone. Hundreds of other former-servo-type applications now employ proportional technology. Those include: machine tools; assembly machines; welding systems, cutting and lapping equipment, and more. "'More and more customers are choosing that path," notes Phillip Black, a sales engineer for Neff Engineering, a Fort Wayne, IN-based fluid power distributor. "And once they've done it, they continue to choose proportional valves more readily than servos."

Matching servo artistry. That wasn't always the case, however. In the past, control engineers were far more likely to select servo valves for applications requiring precise positioning. The reason: Servo valves offered exceptionally fast response and a form of closed-loop feedback that couldn't be matched by proportional valves. "A servo valve is a precise, critically machined component," notes Don Caputo, marketing manager for Parker Hannifin's Hydraulic Valve Div. (Elyria, OH). "It's a work of art."

Still, not every potential user was able to afford servo artistry. What's more, the finely machined ports inside servo valve bodies were subject to contamination. "Servos are contamination-sensitive," says Dan Macejkovic, manager of product management for Bosch Automation Technology (Racine, WI), which supplied the servo-solenoid valves for Godzilla. "Because of the small orifices, any contamination whatsoever destroys the valves."

That's why manufacturers have continued to work furiously on the development of a valve that could match servo performance, but eliminate the cost and contamination. And many now believe the servo solenoid is the answer.

Named for its ability to operate within a servo loop, the servo solenoid bears almost no resemblance to its conventional brethren. Gone are the finely machined ports and orifices that characterize traditional servos. Instead, the servo solenoid uses a coil, spool, and sleeve, making it significantly larger than traditional servos. Like the servo, however, it offers high frequency response and closed-loop feedback.

Equally important, servo solenoids offer a significant cost break--in some cases, 33% or more. A 10-gal/min servo valve, for example, can cost from $4,800 to $6,000.In contrast, a comparable servo solenoid valve runs about $3,100.

Such cost disparities, of course, have always existed between servo and proportional valves. But in the past, proportional valves couldn't match servos, especially in precise positioning applications, so cost was a moot point.

A small group of valve manufacturers, however, has changed that. During the past decade, they've produced a succession of servo solenoid valves that offer so-called "zero lap" technology.

Eliminating dead zone. Zero-lap is critical to achieving servo-type performance because it enables the valve to respond quickly to the changing needs of the servo loop. In the past, proportional valves couldn't do that because they exhibited about 15-20% overlap between the valve's spool and port. As a result, the spool needed to move as much as 15-20% on each usage before fluid could flow through the port. Therefore, the valve didn't offer the high-frequency response needed for servo loop applications. "When you do closed loop positioning, you need instantaneous response," Caputo explains. "And you can't do that if you've got that much overlap."

The key to the recent success of servo solenoids has been a series of concurrent developments in electronics and coil technology. Because the valve works by shifting its spool with an electrical coil, engineers have been able to improve it by developing faster-acting coils and using electronic feedback to position the spool. Most servo solenoids employ a linear variable displacement transducer (LVDT) on the spool as a feedback device. Then, by sending a proportional signal to the coil, they can shift the spool as much or as little as needed, while using the LVDT for position feedback. "We've learned so much in the past 10 years that now we can now drive the proportional spool almost as fast as a servo spool," Caputo says.

Indeed, the frequency response of proportional valves is closing in on those of servos, experts say. Whereas a conventional proportional valve exhibits a frequency response of about 80 Hz, a servo solenoid operates closer to 140 Hz. Conventional servos typically approach 200 Hz.

For most applications, servo solenoid performance is good enough to warrant replacement of servo valves. Black of Neff Engineering cites one hostile application where a customer who was making Teflon valve seats needed exact pressure and velocity. The servo solenoid stood up to the hostile environment and provided the necessary performance, he says. "We've had no performance issues that made us wish we had gone with a servo," he states.

Similarly, engineers from Newcore Inc., a Bay City, MI-based system integrator, say they've used Vickers zero-lap proportional valves for applications requiring reliability and precise positioning. On a recently completed programmable weld ram, for example, they chose zero-lap proportional valves over servos. "They're more forgiving and less expensive," notes Steve Lamson, a fluid power designer for Newcore. "We wouldn't necessarily use a proportional valve for every application, but servos are just too sensitive for applications like ours."

Servo niche. Valve manufacturers and users agree, however, that servo valves still are viable in many applications. Among them: situations requiring compactness; low power; or explosion-proof designs.

Even on the set of Godzilla, servo valves still had a niche, Bosch engineers say. The baby monster, much smaller than the main monster used in the movie, employed servos instead of proportional valves because of tighter packaging constraints. "The servo solenoids were just too large to fit in the baby," Macejkovic reports.

Still, most experts agree that the range of applications for proportional valves continues to grow, often at the expense of traditional servo technology. "There's been significant advancement in proportional technology over the last ten years," notes Fred Phillips, director of advanced engineering for Vickers Industrial and Mobile Divs. "And it has allowed engineers to use proportional valves for a fairly broad range of closed-loop control applications."


What it means to you

  • Proportional valves can fulfill the needs of precise positioning applications

  • Servo solenoids are less susceptible to contamination damage than traditional servos

  • Servo solenoid valves are less costly than servos


Application tip

Valve sizing: Think flow!

By Colin Ingram, Swagelok, Solon, OH
Straight-through flow paths, such as the kind used on this Swagelok 40 Series ball valve, deliver full flow with a quarter-turn actuation.

What's the best way to choose the correct valve size for your system? Many engineers tend to select valves based on the nominal size of the end connection. For most fluid systems, however, more important measures are valve flow and footprint.

Calculating flow through a valve is more complex than calculating flow through a fixed orifice. The valve flow coefficient, or Cv rating, combines the effects of all flow restrictions related to valve design--including pipe diameter, orifice size, valve passage dimensions, and changes in size and flow direction. Most valve manufacturers use standard, reliable Instrument Society of America (ISA) test methods for determining flow and valve coefficient. You can obtain the ratings from most manufacturers' literature.

In general, valves with straight-through flow paths, such as ball or plug valves, have higher flow coefficients as compared to valves with tortuous flow paths, such as globe-style valves. Therefore, if your system requires reliable on/off service and high flow in a compact size, a valve with a straight-through flow path and quarter-turn actuation is a good option.

Choosing a valve with high flow capability for on/off applications allows you to reduce valve size and typically, cost. (For example, a 1-inch globe-style valve can be replaced with a 3/4-inch, or even a 1/2-inch ball valve.) Both ball and plug valves are effective in high flow applications; ball valves can handle higher pressures, as well.

An additional advantage of high-flow ball and plug valves is that many offer compact footprints and are ideal for use in systems with size constraints.

To speak with an applications engineer, call Swagelok at (440) 349-5934.


Exploding the myths on air cylinder selection

By Clayton Fryer, Training Resources Manager IMI Norgren, Inc. Littleton, CO

There's a widening gap between the engineer's need to complete basic pneumatic calculations and his or her ability to properly apply pneumatic components. There's even a misunderstanding on how to determine the proper cylinder bore for a specific load. A great deal of that misunderstanding has evolved due to the blind faith application of several pneumatic myths.

Those pneumatic myths include:

If a little bit's good, a whole lot is better

  • Pipe size is right size

  • Compressed air is free

To provide a logical, step-by-step method to clear up the myths of cylinder flow calculations, we propose a rather unusual approach to the proper selection of all components in a pneumatic system: We start at the end and complete the required calculations, moving from the actuator, through the connectors and conduit, to the directional control valve, and on to the airline preparation components. This approach provides a pneumatic system that performs reliably, operates efficiently, is properly sized, has the lowest initial installation cost, and offers the lowest cost of operation. These calculations are required if the OEM of the machine expects to reduce the initial cost of the machine. These calculations are also required if the machine user is to receive the benefit of a lower-costing machine and the longer-lasting benefit of lower compressed air operating costs.


Products to watch

Metric rod couplers

Self-aligning piston rod couplers, now available in metric versions, eliminate misalignment, simplify tooling, and increase cylinder life. The couplers compensate for 2 degrees angular error and 1/32nd of an inch (0.08 mm) lateral misalignment on push and pull stroke. They provide greater reliability and reduce cylinder and component wear, simplifying alignment problems in the field.

PHD Inc.,
Box 9070, Fort Wayne, IN 46899; FAX (219) 747-6754.

Metric cylinders

IP Series pneumatic ISO 6431 metric cylinders offer bore sizes ranging from 32 to 200 mm, and nine mounting styles in pressures up to 10 Bar. The IP Series features hard-coat aluminum tubes, aluminum pistons, heads, and caps, as well as radial seat cushions on each end.

Miller Fluid Power Corp.,
800 N. York Road, Bensenville, IL 60106; FAX (630) 766-3012.

Air control valve

The F5 Series valve features a sealing system that extends the life of the valve and provides customers with easy connection, thus reducing wiring time and simplifying labor. The valve directs air pressure under the spool seals, forcing them outward to seal against the valve bore. As the spool shifts with extended operation, pressure under the seals continue to force them outward, compensating for wear and greatly extending the life of the valve. The plug-in, base-mounted air control valve is targeted for machine tools, material handling, part assembly, packaging, and general automation equipment applications.

Parker Hannifin, Pneumatic Div.,
8676 East M-89, Richland, MI 49083; FAX (616) 629-5385.

Pump

The AA Pump is less than 1 inch3 in size and features free flow of 2.7l/min and a maximum pressure of 9 psi. Maximum vacuum is 16 inches Hg.

Sensidyne,
a16333 Bay Vista Dr., Clearwater, FL33760; FAX(813) 532-6930.

Pump

The Elima-Matic sanitary pump is constructed of 316 stainless steel, with a surface finish of 32 microinches or better. The sanitary pump features tri-clamp connections on both the inlet and discharge ports. The pump does not use any valve seats or O-rings. The fewer parts and smooth, flow-through design will not damage the product being pumped, and will allow the passage of solids up to 5/8 inch in diameter. The Versa-Sense leak detection system protects the pumped product from contamination.

Versa-Matic Pump,
6017 Enterprise Dr., Export, PA 15632; FAX(724) 327-4300.

Compact valve manifolds

Lightweight polymer CPA Series valve manifolds, available in 10 and 14 mm valve widths, offer a choice of fieldbus connection, multipin, individual connections, or ASI connections. When used with a fieldbus or device-level network, up to 16 solenoids can be connected up to eight valve positions. Manifolds with multipin connections can incorporate up to 22 coils per manifold. Manifolds with individual solenoid connections accommodate up to 44 coils on a maximum of 22 valve positions per manifold. Festo Corp., Box 18023, Hauppauge, NY 11788; FAX (516) 435-8026.

Air amplifiers

Super air amplifiers move smoke, fumes, and other materials. The amplifiers minimize compressed air consumption and noise, while producing high-volume outlet airflows up to 250 miles/hour. The aluminum die-cast construction incorporates a shim that releases precise amounts of compressed air at exact intervals through the amplifier's center. These jets of air create a constant, high-velocity outlet flow across the entire cross-sectional area. Additional free air is pulled through the unit, producing outlet flows up to 25 times that of the compressed air consumed.

EXAIR Corp.,
1250 Century Circle N., Cincinnati, OH45246; FAX(513) 671-3363.

Tilt column steering option

A tilt column steering option increases operator comfort and contributes to worker productivity. Designed to enhance operator comfort, the column can be tilted 40 degrees from the farthest front position to the nearest rear position. Forward and backward tilt parameters are 20 degrees each. The column features five positions: two back; one center; two forward. It is engaged with a spring release lever that can be mounted on either side of the column for additional flexibility. Typical applications include wheel loaders, backhoe loaders, lift trucks, tractors, sweepers, and other utility vehicles.

Eaton, Hydraulics Div.,
5151 Hwy. 5, Eden Prairie, MN 55344.


The '2X' rule in action


Area (sq. in.)

Push bore Dia. (in.)

0.44

3/4

0.79

1

0.99

1-1/8

1.23

1-1/4

1.77

1-1/2

2.41

1-3/4

3.14

2

4.91

2-1/2

8.30

3-1/4

12.57

4

15.90

4-1/2

19.64

5

28.27

6

38.48

7

50.27

8

78.54

10

113.10

12

153.94

14

Example: Suppose there's a requirement to move a 500-lb load in a vertical direction, using 80 psig. Using the "2X" rule, 500 lb becomes 1,000 lb. Plugging the known values into the F = PA formula, A = 12.5 inches2. Here, the designer must resort to a chart or another calculation that will help determine the nearest standard bore size for a cylinder. By referring to the accompanying table, we see that the 4-inch-bore cylinder has the nearly identical push bore area of 12.57inches2.

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