Motors: expansible chamber type – With motive fluid valve – Relatively movable inlet and exhaust valves for single...
Reexamination Certificate
2003-03-17
2004-10-12
Lazo, Thomas E. (Department: 3745)
Motors: expansible chamber type
With motive fluid valve
Relatively movable inlet and exhaust valves for single...
C091S464000
Reexamination Certificate
active
06802242
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
(Not Applicable)
STATEMENT RE: FEDERALLY SPONSORED RESEARCH/DEVELOPMENT
(Not Applicable)
BACKGROUND OF THE INVENTION
The present invention pertains generally to fluid flow control and, more particularly, to a control system for a pneumatic circuit with improved cylinder sensitivity effected through the use of check valves for selectively isolating components on either end of the cylinder.
Pneumatic systems typically involve a source of compressed air that is routed through a network of pipes. The compressed air is typically obtained from a compressor which is usually driven by an electric motor or an internal combustion engine. The compressed air is routed to a positioner which ultimately controls the flow of compressed air to and from a cylinder in order to move a piston sealed within the cylinder. The piston may have a shaft extending out of the cylinder and connected to the component to be moved. The positioner provides pneumatic signals in the form of compressed air which is routed to control valves or boosters. The boosters are selectively opened and closed to regulate the flow of the compressed air to and from the cylinder. The boosters receive the pneumatic signals and may be opened and closed by pneumatic pilots connected on either end of each booster. The pneumatic pilots of the boosters are connected to the positioner through signal lines. The boosters are also connected to the source of compressed air through feed lines. The signal lines are typically of a smaller diameter than feed lines because they supply and exhaust compressed air into and out of the cylinder at relatively low flow rates. However, at higher flow rates, the positioner provides a greater flow of compressed air into the signal lines with a pressure sufficient to actuate the pneumatic pilots of the volume boosters. The actuated boosters allow compressed air to flow from the larger diameter feed lines into and out of the cylinder at the higher rate.
The pneumatic system moves the piston by forcing air into a first end of the cylinder while simultaneously withdrawing or exhausting air out of a second end of the cylinder in order to advance the piston along the length of the cylinder. Conversely, the pneumatic system may also force air into the second end of the cylinder while simultaneously exhausting air out of the first end of the cylinder in order to retract the piston in the opposite direction. By driving the air into alternate ends of the cylinder, the piston is moved such that the shaft can be displaced in any position for doing useful work. The compressed air may pass through a regulator to control the amount of pressure available in the pneumatic circuit. The compressed air may also pass through a filter to clean the air to prevent damage to components thereby ensuring that the components have a long and reliable working life.
Pneumatic systems are commonly used in large scale applications such as in power plants and refineries for controlling system components such as a working valve. In such applications, it may be desirable to quickly and repeatedly position the piston to within thousandths of an inch. In order to quickly and precisely position the piston, a pair of boosters may be connected to the first end of the cylinder arid another pair of boosters may be connected to the second end of the cylinder. The first pair of boosters may include one booster configured for “pushing” air into the first end of the cylinder with the other of the pair configured to “pull” air from the first end. Likewise, the second pair may include one “pusher” and one “puller” booster. Quick exhaust valves may also be installed between the puller boosters and the respective first and second ends of the cylinder. Operating in conjunction with the puller boosters, the quick exhaust valves exhaust air out of the cylinder at high flow rates. Although configured to respectively supply and exhaust air into and out of the cylinder at high flow rates, the pusher and puller boosters also have the individual capability to respectively exhaust and supply air out of and into the cylinder, although at significantly lower flow rates. At low flow rates, the pusher boosters on the first and second ends of the cylinder supply compressed air to the cylinder solely through the smaller diameter signal lines. However, at higher flow rates, the positioner provides sufficient pressure of compressed air to the pneumatic pilots through the signal lines such that the pusher boosters are actuated. Depending on whether the compressed air is to be supplied to the first end or to the second end of the cylinder, the first or second actuated pusher booster will allow compressed air to flow to the first or second end through the larger diameter feed lines. For example, if compressed air is to be supplied to the first end at a high flow rate, then the pusher booster connected to the first end provides the majority of compressed air to the first end while the puller booster connected to the first end provides a negligible amount of compressed air. Simultaneously, the puller booster connected to the second end exhausts the majority of compressed air from the second end while the pusher booster connected to the second end exhausts a negligible amount of compressed air.
The sensitivity of the boosters in responding to pneumatic signals is controlled by adjustable restrictions or needle valves which are incorporated into the boosters. The needle valves are connected in parallel across the boosters at the pneumatic pilots. When the pressure of compressed air acting on the pneumatic pilots reaches a preset level, the booster toggles from a “closed” or null position to a supply or exhaust position. In either the supply or exhaust position, a greater flow of compressed air from feed lines may pass through the boosters and enter or exit alternate ends of the cylinder. Thus, the adjustable restrictions provide a means for setting the point at which the booster are activated by the pneumatic pilots so that the booster toggles from the null position to either the supply or the exhaust position.
As mentioned above, the positioner adjusts the position of the piston by forcing air into alternate ends of the cylinder. However, due to the compressible nature of air, dynamic instability may result within the pneumatic circuit such that the piston is difficult to precisely and rapidly position. For example, within typical pneumatic circuits, when there are active components such as a pusher and a puller booster connected to a first end of the cylinder, the adjustment of the sensitivity of the pusher booster may affect the total capacity of the compressed air into the cylinder on that same first end of the cylinder. More specifically, in the example, if the sensitivity of the pusher booster in responding to pneumatic signals is increased, the pusher booster will toggle to the supply position in response to relatively small pneumatic signal changes. However, the non-activated puller booster will simultaneously provide a small flow of compressed air to the cylinder through the signal lines. Because of the compressibility of air, the piston will not start to move toward the second end until both the pusher and puller booster on the first end have sufficiently pressurized. Thus, the overall speed of the piston in responding to signal changes is reduced. In addition, the position of the piston within the cylinder may fluctuate as the boosters respond to small signal changes, resulting in dynamic instability. In addition, because the non-activated booster on either side of the cylinder must supply compressed air through signal lines each time the piston moves, the total requirement of compressed air that must be provided by the positioner to regulate the piston position is increased.
The prior art discloses several pneumatic circuits with control systems designed to improve the accuracy and response time with which the piston may be positioned within the cylinder. One such prior art device includes an actuator system which modulates
Sherikar Sanjay V.
Steinke Joseph Henry
Control Components Inc.
Lazo Thomas E.
Stetina Brunda Garred & Brucker
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