Racine, WI-Although servo valves provide hydraulic applications with the repeatability of control once made available only through electromechanical means, servo valves add cost that manufacturers must either absorb or pass on to customers. Engineers at Bosch have figured out a way of removing some of that cost in applications that require less stringent positioning accuracy than servo control systems typically deliver.
Until now, if you wanted pressure and speed control from your hydraulic system, there were only two main options. One option is a traditional pressure/speed control system using a proportional direction valve and a proportional reducing or relieving valve. The disadvantages with this approach are (a) the pressure spikes that occur while waiting for the response from the reducing valve, and (b) the extra plumbing required by using two separate control valves. The pressure spikes cause the inaccuracy in the production equipment.
Bosch's 10% DO3 valve directs oil back to the tank in about half the time required of a standard proportional reducing valve. Pressure control accuracy is approximately 90% that of a servo control valve.
The other option is closed-loop servo control systems. They have faster response times than reducing valves and minimize the occurrence of pressure spikes, but require the addition of a vent valve that adds plumbing and cost to the system.
While some production lines can't afford less than 99.99% repeatability and must have servo control, other production lines enabled by hydraulics could do with less repeatability, especially if it lowers the line's plumbing cost. It is for these less-demanding applications that engineers at Bosch developed the 10% proportional valve series.
Bosch's 10% valve eliminates the need for proportional reducing and relieving valves in traditional pressure speed control systems. It also eliminates the need for vent valves in closed-loop servo control systems.
Unlike proportional valves that have spools with 20% overlap in the centered position, Bosch made its valves with only 10% overlap. "The reduction to 10% reduces the distance the spool travels. It allows the valve to direct oil back to the tank faster," says Chris Kolbe, an engineer and product manager at Bosch Automation Technology. "Whereas our regular DO3 valve might have a 9-millisecond response time, a 10% valve might take about 5 milliseconds."
Bosch engineers designed the valve with the A and B ports going to the tank in the center position, until the valve receives input of more than 1V. "Beyond 1V, the 10% valve acts like a directional valve and connects either the pressure port to the A port or the pressure port to the B port, depending on the polarity of the input signal," says Kolbe.
"With the 10% valve, we also have a larger diameter orifice leading back to the tank, so you can get 4 gpm versus 1 gpm with a comparable standard proportional valves," adds Kolbe. Standard proportional valves have smaller paths to vent oil to the tank, so pressure inside the hydraulic system spikes before it can be relieved.
"The 10% valve is a modified proportional valve for applications up to 230 gpm. The response time is slightly slower than a servo valve, but it costs approximately 20% less," Kolbe notes.
Contact Chris Kolbe, Bosch Automation Technology, 7505 Durand Ave., Racine, WI 53406 Tel: (262)598-2412; FAX: (262)554-8100; E-mail: [email protected]; or Enter 506
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