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20/02/2004
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High performance in hydraulic control has become significantly cheaper and more reliable. Tom Shelley reports
A proportional valve has been developed that uses voice coil technology to give it a response curve up to 400Hz at what is expected to be substantially less than the price of its traditional servo valve competitors.
Servo type control, normally considered essential for moving aircraft control surfaces and in industrial high power robotics, has thus just got cheaper, at a time when it is coming increasingly under threat from more efficient high power electrical servo systems.
The Parker DFplus has a spool directly coupled to a coil able to move back and forth on the outside of a cylindrical permanent magnet. Spool position feedback is by a non contact device. The coil is driven under power in both directions so that spool position does not depend on spring return force. The valve spool is fitted with a weak spring packet, but this is there to drive the spool into a preferred position in the event of power failure or intentional switch off of the enabling signal.
The voice coil drive has a useful force of more than 100N in each direction, far more than is normally found in proportional valves, including dual stroke valves with a solenoid on each end of the spool. The VCD also shows true linearity between force and coil current, unlike solenoids. No particularly fine filtering is required, as is the case with traditional servo valves using a nozzles and flapper arrangement. Despite use of expensive coated components, wear occurs on flappers because of the permanent oil stream through the nozzles.
The initial product is an NG6 (CETOP 3) size rated at 40 litres/minute. According to Martin Lattimer of Parker Hannifin, it is planned to sell the valve for around 2000 euros each, as compared with a typical price of around 3000 euros for a comparable traditional valve with similar performance. Test results show that the new valve achieves a frequency characteristic in the small signal region of 400Hz at -3dB amplitude ratio of a -90 deg phase of greater than 350Hz. The company claims that by comparison, a standard size 04 servo valve achieved 420Hz at -3dB and 250Hz at 90 deg while a highly dynamic NG6 dual stroke solenoid control valve also tested only managed well below 180Hz at -3dB and 212Hz at 90 deg phase shift. Both the servo valve could control flow forces on the spool over the entire pressure drop of 350 bar but the proportional solenoid valve was limited in this respect to a maximum pressure drop of 250bar. Parker claims that because of the high flow rate, an NG6 CETOP 3 DFplus can replace a standard NG10 CETOP 5 standard control valve, "For a few applications."
Plans are afoot to develop larger, two stage valves using the DFplus as a pilot.
Parker Hannifin
Pointers
* Signal response is 400Hz at -3dB amplitude and more than 350Hz at -90 deg phase lag
* Working lives are expected to be better than for servo valves with nozzle and flapper arrangements
* Sale price is set at under half that of competing top end traditional servo valves
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Author
Tom Shelley
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