sensors & systems - Industrial Technology Magazine

MACHINE BUILDING & AUTOMATION
Hydraulics & Pneumatics
Closing the loop
Ian Whiting of Moog Controls looks at the relative merits of
electronic and mechanical feedback systems for Servovalves, and
explains where electrical feedback can provide the best option
Many may have assumed that the advent of
electrical feedback (EFB) servovalves would
spell the end for their mechanical feedback
(MFB) counterparts that close the spool
control loop mechanically. The actual picture today is
that each offers its own advantages in different
applications.
The decision governing the choice of servovalve for
any given application is ultimately based on the dynamic
performance (in terms of frequency response), static
performance (for example hysteresis and threshold) and
on the flow rate. Other criteria include vibration level,
temperature and the installation package size.
Additionally, environmental factors such as ATEX
approved intrinsic safety have a part to play in the
decision making process.
It is no doubt tempting to assume in these modern
times that electrical feedback will always provide the best
option, but in servovalve technology EFB does not
necessarily replace MFB. Whilst offering obvious benefits
in general, there are some quite commonly occurring
cases where the requirements are out of reach using EFB
technology. MFB continues to fill the needs of these
applications. In particular, the benefits of MFB
technology become apparent in any application where
installation space is at a premium. And MFB technology
is a must in cases where vibration levels (and/or the
general environment) are extremely severe or the unit is
operating at elevated temperature.
So what specific requirements might steer you
towards electrical feedback over the mechanical
alternative? Well, low system pressure is a key
consideration. An MFB valve has specific pilot stage
requirements in terms of supply pressure in order to
achieve acceptable spool response. The EFB counterpart
– typified by the direct drive valve (DDV) – with direct
spool actuation is able to move the spool regardless of
supply pressure, making it an ideal solution in those
cases where the oil supply pressure is low or variable.
Further, the ability of the valve to overcome hysteresis
(Top right) The digital interface direct drive valve is widely used in
modern servo systems. (Bottom left) Servo Jet (SJ) pilot technology
provides a low contamination, single inlet design. (Bottom right)
Direct drive (DDV) spool valve eliminates pilot stage leakage.
Annular
Area
Nozzle
Jet
Pipe
Receiver
and threshold problems and to minimise the drift
effects caused by changes in supply pressure and
temperature gives improved static performance.
Because the spool position is being monitored
electronically and compensated for temperature, the EFB
valve is far superior to its MFB counterpart.
Spool monitor diagnostics is another consideration.
This is where the spool position signal is made available
to the control system, making it possible to spot faults or
the beginnings of deterioration in the valve. In industries
where reducing downtime is critical, the possibility of
building in predictive maintenance programmes can
provide an important competitive edge.
Benefits of reduced wear
Where operating life is an issue, or where the location of
the valve makes maintenance difficult, the EFB
servovalve is an inherently lower wear option than the
MFB alternative. Unlike the feedback mechanism in an
MFB device, the electronic sensor in an EFB valve is noncontacting,
and so offers an almost unlimited lifespan.
The ultimate life of the valve or the periods between
required maintenance are then defined by other operating
and environmental factors.
Reduced internal leakage can be critical in cases
where there is a limited oil supply capacity. Once more
the EFB valve is an improving factor where spool
actuation leakage is reduced via the use of Servo Jet (SJ)
pilot technology or completely eliminated in the case of
the DDV valve. Similarly, reducing contamination may
also be a consideration in valve selection. This is
especially difficult to handle in MFB valves when the
torque motor becomes contaminated. In EFB valves,
contamination effects are reduced via the single inlet
design of the SJ pilot or in the case of DDV valves by
eliminating the pilot stage completely.
The latest state-of-the-art EFB solutions move
conventional valve spool positioning technology forward
into the application solution arena. This is achieved via
the introduction of digital electronics at the heart of the
valve control system. The move to digital technology
gives significant performance improvements. Advanced
Cable
Hole
Permanent
Magnets
Centring
Springs
Bearing Coil Armature Plug
control algorithms
inside the valve
are capable of delivering
improvements in the valve dynamic
performance. In addition, field tuning by the user to
achieve a specific performance result becomes possible.
As standard, the performance characteristics of both
MFB and EFB servovalves are factory set and tuned to
meet general application needs. However in many cases
(especially pressure control) optimisation of the valve
performance may play an important part in achieving a
good result.
Valve configuration parameters allow additional field
configurable settings to be used. Examples might include
user setting of direction dependant valve spool opening
time and peak flow rate to meet the system requirement.
Including these functions as part of the valve itself
encourages best practice in setting up the servo system.
Further user configurable functions are available for
diagnostics and health monitoring.
Other motivating factors in the use of digital
technology include the availability of serial
communications, reducing field cabling and electrical
noise. A single easy to install data ring (such as CANbus)
allows the signalling of a large number of valves over
considerable distances with none of the noise problems
that are often associated with analogue signals.
Multi sensor applications are often required in
hydraulic servo control systems, where as well as
providing position measurement data the valve may also
be required to operate in pressure control mode. In such
cases the digital EFB valve solutions further reduce field
cabling.
As we have seen, it is no exaggeration to say that the
advent of electronic feedback and the subsequent
developments in electro-hydraulic servovalves have
opened up new markets and brought higher levels of
precision control to existing markets. The introduction of
on-board digital electronics has further expanded the
capability of Moog valve solutions in terms of
performance, installation cost, functionality, reliability
and cost effectiveness, making EFB technology a
worthwhile and easily achieved upgrade path in
applications currently using MFB valves.
In new applications, EFB valve technology (analogue
or digital) should, in most cases, be considered as the
first option at the machine design stage. At the same
time, though, the complementary attributes of MFB and
EFB valve technology continues to be the mainstay of the
Moog valve product range, enabling solutions to be
provided in across the widest possible spread of
application areas.
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INDUSTRIAL TECHNOLOGY • May 2008