|
Why pneumatic actuators occasionally operate jerky, bumpy, or even
just move to one end?
A general overview
CAUTION
Technical Notes presented here are for general
reference only. Information contained in these notes may not be
applicable to your specific situations or these notes are unlikely to present all the relevant
issues or constraints associated with your specific situations. Hence
caution should be exercised before using any information contained
in these notes.
Send your comments on this article to
info@kanair
- Background
- Typical Jerky Behavior
- Effects of Jerky Motion
- Identifying the Source
- Alternate Solutions
- Conclusion
Air pressure is widely used in motion control applications primarily because
of favorable economics and its ease of generation and control. In valve
actuators, air pressure is used in cylinders and air motors. Most pneumatic
actuators use some form of piston cylinders or diaphragms with great success.
In rare cases, a pneumatic actuator is seen to provide jerky output motion.
This note is an A general short overview of this particular problem.
Some commonly observed actuator (mis)behavior patterns are shown below:
- When air pressure is applied to open the actuator, a "unusual" delay is
observed to build up air pressure. As the pressure builds up, the actuator
opens with a "thud" and accelerates towards open position. Air pressure
drops considerably until stabilized, based on flow control valve setting.
This behavior may be attributed to stuck or wedged gate
or plug. Once the gate is released from the seat, load and hence the pressure
required is very low. Air expands resulting in an accelerated motion in
opening direction. In rare cases, an internally rusted cylinder (from water in
air supply) may also create similar effect. If exhausting air on closing side
is metered with a speed control valve ( called meter-out control), the amount
of acceleration can be limited.
- When the ball or disk or gate of the valve is of heavy weight (having
large moment of inertia), the opening motion may be superimposed by
oscillatory motion of the actuator. ( The actuator seems to open at different
speeds- faster and slower in repeated manner). The oscillation frequency may
be slower when cylinder piston is at around the middle.
This behavior is because of the "spring-mass" effect
created by the springiness of air and the inertia of the gate along with poor
damping in the system.
- The actuator moves fast, and then slow in repeatedly but random fashion,
without any identifiable pattern - simply stated actuator moves in jerky
motion.
This behavior may be attributed to stick-slip motion
with predominantly coulomb friction. The origin for the friction could
be from the valve or cylinder because of misalignment, poor surface finish,
overlay, rubber coat, galling, loss of friction-reducing coating, boundary
layer effects, contaminants and slurries, gouging, heat related expansion,
pressure locks, and other specific factors.
- The actuator moves in a single frequency or in beats with apparent
constant amplitude. Normal hunting or limit cycle oscillations come into this
category. This behavior can be seen more in actuators with pneumatic
positioners or "bang-bang" servos.
This behavior can be attributed to what is known as
"non-linearities" in the actuator system. These non-lineraities can include
deadband, loose fittings such as in keyways and mounting bolts. This behavior
can be seen in electrical and hydraulic actuators as well with its own unique
characteristics.
- Another type of unwanted actuator motion could be from hydrodynamic or
Bernoulli forces created in the valve.
Essentially in these circumstances, the fluid medium
generates a (negative or over running) torque in the same direction as the
actuator. In these cases, the actuator, as it moves, will suddenly "zip" to an
end. Even in modulating application, the actuator may simply move to one end,
when it is supposed to be at some intermediate position based on the input
command signal.
Depending on the severity of these unwanted actuator motion as well as on a
host of other factors such as how often the actuator is operated, the
jerky motion may have the following deleterious effects:
- Safety of personnel.
- Increased wear and tear in plug and actuator internals.
- Potential fatigue life issues for valve, actuator and associated
appurtenance.
- Transfer of vibration to pipeline and other location.
- Noise related issues.
- Performance degradation.
- Heat generation.
Once the actuator is in place, if the unit is accessible, then a
'non-intrusive' and logical approach may be used to find a solution.
Non-intrusive here implies that the valve is not removed from the line or the
actuator itself is not disassembled.
Measurements and observations of actuator parameters and behavior may give
key information to the nature of the actuator problem. such information
may include:
- Pressure readings in static and dynamic conditions.
Since pressure readings provide the load behavior,
this information can be a key indicator of the problem.
- Effect of different settings of flow or speed control valves.
Tightening the flow increases damping which
results in lower amplitude oscillations when problem is associated with
high inertia of the plug.
- Frequency of oscillation
Fixed amplitude oscillations can generally
attributed to loose mechanical connections or deadband.
- Type of noise, vibration, or jerky motion in conjunction with
actuator position.
If the actuator "jumps" at frequent intervals it
could be due to stick-slip or dry friction.
- Effect of inactivity of actuator for long time on jerky motion.
When a plug or a gate stays in contact with
another surface for long time with very fine clearance, as in the case
of a closed valve, depending on fluid medium and impurities, the
boundary layer in these clearances may slowly grow and cause an apparent
solidification of the clearances. In other words, if we assume a
constant pressure between two ends of a capillary tube, given
enough time, the flow through the capillary will decrease or stop as
if the capillary is plugged. Observation of this phenomena,
sometimes called 'obliteration', was reported to have been made
initially by a team of MIT scientists in 1922. The boundary layer
solidification can be broken by disturbing the surface, but will start
all over again once left undisturbed. One of the techniques used in
hydraulics industry to alleviate this problem is use of a dither signal,
a high frequency, low amplitude oscillation in the area affected by this
phenomena.
- Unique motion such as Stick-slip (saw tooth motion) indicating heavy
coulomb friction.
Stick-slip is the classical coulomb friction where
it takes more force to move a stationary part as opposed to a reduced
force for a moving unit. In other words, static friction is larger
than dynamic friction. In an air actuator, once the valve moves,
friction is reduced which results in an expansion of air and higher
actuator speed. When higher load is agin encountered by the actuator, it
slows down or temporarily stops, again repeating the "saw-tooth" motion.
The motion is somewhat analogous to the "squealing" of chalk on a black
board.
If intrusive or non-intrusive evaluations cannot solve the specific
problem, alternate means can be sought. Any alternate solution, again,
depends on the particular behavior of the valve.
For example, basic problems such as misalignment,
lack of required lubricants, loose mechanical connections or worn out
internals have to be fixed when they are the source for the problem.
Certain problems can be fixed by alternate techniques. For example,
Kanair has supplied modulating hydraulic actuators to replace pneumatic
actuators used in a power plant damper application. In this case,
under certain load, a pneumatic positioning damper/louver (actuator) will
simply move to one end without faithfully following the input position
command. It was not possible to predict when this happened because of the
multitude of flow factors involved. The load changes along with the
compressibility of the air simply overran the actuator.
Kanair hydraulic actuator was designed to account
for any negative aerodynamic forces and keep the damper position as
commanded from the control room.
In another case of a pneumatic actuator on a butterfly valve, a cost
effective method was to use an air over oil system. In an air over oil
system, air pressure is applied on top of the oil in tanks and the
actuator cylinder is operated with hydraulic oil. The relative
stiffness provided by the oil eliminates the problems associated with the
compressibility of air.
When properly designed, hydraulic actuator by virtue of high oil
stiffness (represented by bulk modulus) can handle heavy inertial load
without undesirable oscillations or jerky motion. It is also easier
to dampen a misbehaving hydraulic actuator using techniques such as
orifice controls
This note provided an overview on the jerky motions sometimes observed in
pneumatic actuators. These notes are unlikely to present all the relevant issues
or constraints associated with your specific situation. Hence caution should be
exercised before using any information contained in these notes.
A kanair technical note : KTN-104.
Copyright © 2003 Kanair, Inc. All rights reserved.
Revision 0: 11/20/03.
|
