[Feedback Control Systems Phillips And Harbor 4th Edition.rar

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Saija Grzegorek

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Jun 13, 2024, 1:55:46 AM6/13/24

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Every valve has a part that physically moves to open and close it.This part's shape and motion varies greatly with the valve's design and, unfortunately, does not have a generic collective name.For conciseness, the term flapper is used below.

When discussing different valve actuators, confusion sometimes arises when the word solenoid is used in association with a pneumatic actuator.Isn't the solenoid (which might also be called electro-magnetic actuator) a different actuation mechanism than pneumatic actuation?Yes, it is, but to control the compressed air causing the valve to shut off, vent, and switch ends requires another level of actuation.The solenoid valve attached to the pneumatic cylinder is entirely dedicated to switching compressed air.In one sense, it may trigger the valve's actuation but isn't really part of the actuator.

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With the flapper in the closed position, a seal forms between it and the valve body.Depending on the application, pressure range, and maximum operating temperature, several types of seals are used:rubber o-rings, molded rubber rings, rubber diaphragms or cups, soft metal/hard metal seals, hard metal/hard metal seals, and soft metal/polished sapphire flat.

or rubber cup seal is trapped around its rim by the valve body.The diaphragm's center is deflected into a saddle or port to block gas flow through the valve.This seal type is used in valves designed for foreline applications where the operating temperatures are not high and pressures vary from rough to high vacuum.

seals have a sealing action similar to the CF flange.A metal knife-edge, machined into the body, mechanically deforms a copper pad mounted on the flapper.The valve is closed with a torque wrench and each pad has a relatively short life.However, the pad is replaceable and the valve's life span is not limited by the seal.Such valves enable baking to high temperature (300 C, perhaps 450 C), making them UHV compatible.A version of this mechanism is also found in all metal leak valves.Hard Metal/Hard Metalseals are shaped like a large diameter, thin cupped washer (Belleville washer) around the flapper's seal face.As the valve closes, the mechanism causes this washer to flatten (increasing its O.D. and decreasing its I.D.) forcing its inner/outer edges to seal by elastic deformation to the flapper/body, respectively.The valves are bakeable to 300 C and UHV compatible.

seals have a flapper-mounted optically flat sapphire pad mating to a copper gasket surrounding the valve's exit port.The closing force plastically deforms the copper to match the sapphire surface profile exactly.The valve's design prevents rotation of either component so the valve always re-seats in the same place.These valves, which are bakeable to 450 C, are used to control exceptionally low gas flows into UHV chambers.

To move the flapper, a rotary or linear motion is transmitted from air-side to vacuum-side through a seal.Both the part that moves the flapper and the mechanism that causes movement are called the actuator.Actuators are driven by manual hand-wheel or levers, electro-magnetic actuators, motors, or pneumatic cylinders.

A rotating actuator is often sealed by a dynamic o-ring seal compressed around the shaft.Occasionally, they are used for linear actuators but the greater risk of gas burst makes them less popular than other linear seals.Most dynamic o-ring actuator seals are satisfactory for rough and the (mid) high vacuum pressure range.

The most common linear actuators are sealed in metal bellows.The bellows are often welded to the plate in the actuator shaft but are sealed to the valve body by welding, static o-ring seal, or metal gasket seal.Depending on the valve's design, the seals enable operation well into high vacuum or UHV ranges.

The rubber diaphragm described earlier is its own seal since the air-side actuator does not penetrate into the vacuum but simply pushes the diaphragm.In a similar way, the corrugated metal diaphragms, sealing extremely fine control leak valves, stretch to enable small travel distances required by the actuator.

The conductance of a gate valve can either be determined experimentally or, since the fully open valve's shape is not unlike a tube, calculated using Dushman's table.However, some valves have stated conductance numbers that far exceed those determined from Dushman's table.

These various formulas and factors, plus the question "how can a long tube have a greater conductance than a short valve?", create difficulties for users unfamiliar with some deep vacuum technology concepts.Attempting to compare conductances for gate valves from different manufacturers can cause confusion.However, two points can be made:(a) if two gate valves have identical dimensions for thicknesses and clear bore, their conductances are equal;(b) when making conductance calculations, using the minimum calculated conductance is always the safe policy.

The choice of actuator is determined by the power needed to seal the flapper, convenience, automatic control, or remote operation.In general, small bore valves that can be easily reached may have manual actuators.For remote or automatic operation, solenoid or pneumatic actuation is used.Large bore valves with heavy flappers often need pneumatic actuation even if they can be reached.Both small and large valves used as conductance controllers (to give a desired gas flow or pressure control), require the flapper in intermediate, variable positions between fully open/closed.Manual actuation is occasionally used, but most often motor actuation is appropriate.

Gate valve applications include isolation between vacuum volume and pump, isolation between chamber and loadlock during sample introduction to the latter,access between chamber and loadlock during sample transfer, and isolation between synchrotron beam lines and experimental stations.

Placing a valve between the pump and chamber necessarily reduces the pumping speed from that chamber.To minimize this effect, the valve must have a high gas conductance meaning it must have a large diameter, unobstructed, straight-through bore, and a short distance between flange faces.While not a question of conductance, transferring objects from one chamber to another through a valve also demands a wide unobstructed path, which gate valves supply.

The gate valve's plate-like flapper, in its open position, completely retracts from the bore.The flapper's seal is most often an o-ring held in a groove cut into its surface.In well-designed valves, as the valve closes, the flapper remains some distance from the valve body so the opening's edges do not chop the o-ring.As it reaches the end of its linear travel, an over-center mechanism forces the flapper against the valve body, compressing the o-ring to make the seal.

While most gate valves will not leak with a 15 psi overpressure on either side of the flapper, typical installations have the o-ring facing the volume where vacuum is retained during pressure cycling or venting.That is, the increased pressure assists in forcing the flapper into its closed position and maintaining the seal.

The angle valve's applications include rough pumping shut-off, foreline switching between pumps, cryo-sorption pump shut-off, isolating foreline traps, UHV shut-off, sorption trap isolation, isolating sections baked to high temperature, and more.

As the name implies, the valve's ports are at 90 with the flapper's motion along the axis of one port.Although large-diameter angle valves (often called poppet valves) exist, the most popular sizes have ports between 3/4" and 3".Their right angle construction reduces the gas conductance for a given bore-size to somewhat less than the equivalent length straight tube.

The flapper is sealed by o-ring, elastomeric disk, or soft metal/hard metal.The last named can be baked to high temperature and are extensively used on UHV systems.Although other actuator seals may exist, by far the most common is the flex metal bellows seal.The bellows' "outer" end is usually welded to a plate attached to the actuator shaft.The "inner" end is sealed to the body by using either an o-ring or knife-edge copper gasket seal similar to the CF flange.

Angle valves are actuated manually, by pneumatic cylinder, or by electro-magnetics.Valves using the last mechanism are sometimes called 'solenoid valves,' but the similarity of name to valves that divert compressed air in pneumatic valve actuation (noted earlier) causes confusion.Here, the 'solenoid' directly actuates the valve's flapper.This is a vacuum valve (not a compressed air valve) and is attractive for automating some parts of a system without the need for compressed air.

These valves are essentially on-off devices and are rarely used for conductance control (see below)except in the most primitive, manual way.Some advantages are construction simplicity and ease of mounting/demounting.Positioning a valve between two (rigid) tubes with a common centerline, or parallel/offset centerlines, is more difficult than using an angle valve and tubes at 90.

Although well able to withstand 15 psi overpressure on either side of the flapper, the valve is often mounted with the flapper o-ring facing the normally evacuated space.A popular version of this valve is known as the block valve because it is manufactured from a block of aluminum.

These valves share most features and applications noted for angle valves.The difference is one port is turned 90 so its axis is parallel with the other port's axis.This means the valve's conductance is reduced by the additional right angle flow path.In-line metal sealed versions are not available.

Ball valves are popular in many gas and water applications but less frequently found in vacuum applications.They do, however, provide low-cost, rugged performance at rough vacuum pressures and are found in foreline and trap applications.

The ball (flapper) is held in two PTFE rings that surround the connecting ports and seal the valve body to the ball.In the open position, a through-hole in the ball aligns with the ports.Given the limited vacuum applications, this valve's overall conductance is reasonable and rarely an issue.The ball's actuator, usually a handle rotating a shaft, is sealed by a dynamic o-ring.