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.
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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
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Proportional valves can fulfill the needs of precise positioning
applications
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Servo solenoids are less susceptible to contamination damage than
traditional servos
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Servo solenoid valves are less costly than servos
Application tip
Valve sizing: Think flow!
By Colin Ingram, Swagelok, Solon, OH
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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
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° 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° from the farthest front position to the nearest rear position.
Forward and backward tilt parameters are 20° 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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