MACHINE BUILDING & AUTOMATION Hydraulics & Pneumatics Closing the loop Ian Whiting of Moog Controls looks at the relative merits of electronic and mechanical feedback <strong>systems</strong> 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 <strong>systems</strong>. (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 <strong>systems</strong>, 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. MORE INFORMATION: Enter D545 on the enquiry card or visit ‘latest issue stories’ at www.industrialtechnology.co.uk for further details. You can also find more stories from Moog Controls and more news on valves 54 INDUSTRIAL TECHNOLOGY • May 2008
More details: Write in 550 on the free information card More details: Write in 551 on the free information card Micro Hydraulic Components ● Lee Plugs ● Restrictors ● Check Valves ● Relief Valves ● Shuttle Valves ● Solenoid Valves ● Flow Controls ● Filter Screens SEND FOR OUR FREE PRODUCT CATALOGUE Lee Products Limited, 3 High Street, Chalfont St Peter, Gerrards Cross, Bucks. SL9 9QE. Innovation in Miniature Tel: 01753 886664 Fax: 01753 889588 www.leeproducts.co.uk e-mail: sales@leeproducts.co.uk More details: Write in 552 on the free information card 55
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