Applications engineers in the food and pharmaceutical industries may now have a new alternative in their search for an actuator that's clean and precise.
A Washington-based company has introduced a hybrid fluid power actuator that can achieve positioning accuracies down to 0.003 inches without concern over leakage. The device, which is being marketed toward automation and test equipment, pulp and paper processing systems, and packaging machinery, as well as pharmaceutical and food processing, takes an unusual tack in its approach to load actuation. Instead of using hydraulics for power, it employs a conventional pneumatic actuator for power and uses hydraulics only for control. "We've taken the best of those two technologies and eliminated the negative aspects of both," says David Foster, marketing research analyst, RB Hybrodynamics, Inc.
(Mukilteo, WA; http://rbi.ims.ca/3845-523).
Engineers achieve that convergence in a variety of configurations, but the common element is the use of air for power and hydraulics for damping, and therefore, control. A typical (but by no means only) configuration includes two parallel cylinder bores, one pneumatic and one hydraulic, rigidly connected between the cylinder rods. "You essentially have two mobile elements that cannot move independent of each other," noted Mark Shteynberg, consultant for RB.
As a result, the pneumatic actuator can be used for the simple task of pushing/pulling (or rotary motion), without regard to positioning. The hydraulic side provides position control by applying force in the opposite direction. RB engineers say that hydraulic control can be embodied in several forms. It can be as simple as a permanent orifice between two hydraulic chambers or an adjustable orifice in the form of a needle valve. The engineers have also placed a servovalve between the hydraulic chambers and have achieved closed loop control of the hybrid system.
RB Hybrodynamics executives say that their technology addresses the problems caused by the compressibility of air. By using hydraulics for control, they say, they eliminate dealing with the inherent mathematical non-linearities of compressible fluids. Furthermore, because the hydraulics are used for control instead of power, they say there are no concerns over fluid leakage, and the technology can therefore be used in food processing and other industries where cleanliness is critical. "Air is tough to control," Foster says. "This way, you get the control aspects of hydraulics but you don't have to deal with leakage."
Lock Step: One hybrid actuator
configuration involves a rigid mechanical connection between pneumatic and
hydraulic cylinder rods.
The company's engineers have recently employed their hybrid actuators to control drilling depth in an application for a major aerospace company. The units are also replacing hydraulic actuators in a tomato packaging plant. There, they reportedly eliminate the need for frequent inspections of existing hydraulic actuators.
Ultimately, RB engineers say there is no reason the technology could not be employed in applications requiring complex motion control. "We have already used a programmable motion controller for sophisticated motion profiles," Shteynberg says.