discrete control elements

7/30/2019 Discrete Control Elements

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Discrete Control ElementsA discrete control element is one that has a limited

number of states (usually two: on and off ).

Example In the case of valves, this means a valve designed to

operate either in open mode or closed mode, notin-between.


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On/off valvesAn on/off valve is the fluid equivalent of an electrical

switch: a device that either allows unimpeded flow oracts to prevent flow altogether.

Valves commonly used for on/off service include ball,plug, butterfly,check and gate valve.


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Different Valves


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Fluid power systems Fluid Power System have given the ability of

pressurized fluids to transmit force over long distancesusing fluid as a mechanical power-conducting media.

Fluid systems may be broadly grouped intopneumatic (gas, usually air) and hydraulic (liquid,usually oil).


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SYMBOLS OF VARIOUS COMPONENTS


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SYMBOLS OF VARIOUS COMPONENTS


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SPOOL VALVE Spool valve is a special

type of flow-directing valveused in pneumatic andhydraulic systems to direct

the pressurized fluid todifferent locations.

The symbology for a spoolvalve is a set of boxes, each

box containing arrows orother symbols showing theintended direction(s) forthe fluids travel.


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INTERIOR VIEW(2-way SPOOL VALVE) Cleanliness is very

important.

The spool has wide areas called lands that

act to cover and uncoverports.


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Piston Holds Position when the spool valve is in its

center position, the outerboxes in the symbol areinactive. This is representedin the following diagram byshowing the outer boxes inthe color grey. In thisposition, the spool valveneither admits compressed

air to the cylinder nor ventsany air from the cylinder. As aresult, the piston within thecylinder holds its position.


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PISTON MOVEMENT IN RIGHT DIRECTION If the spool valve is

actuated in onedirection, the spool pieceinside the valve assemblyshifts, directingcompressed air to oneside of the cylinder while

venting air from theother side.


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PISTON MOVEMENT IN LEFT DIRECTION If the spool valve is

actuated in the otherdirection, the spool pieceinside the valve assemblyshifts again, switchingthe directions of air flow.


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Spool Valve:


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HYDRAULIC SYSTEMs


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UNI-DIRECTIONAL HYDRAULIC

MOTOR CONTROL SYSTEM


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SHUNT REGULATOR METHOD


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VARIABLE DISPLACEMENT PUMP METHOD


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FILTER Placement of a filter at the inlet of the pump in all

hydraulic systems. Filtration is an absolute essentialfor any hydraulic system, given the extremely tightdimensional tolerances of components insidehydraulic pumps, motors, valves, and cylinders.

Even very small concentrations of particulateimpurities in hydraulic oil may drastically shorten thelife of these precision-machined components.


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Fluid power systems advantages

over electric power Fluid power systems present little hazard of accidently

igniting flammable atmospheres (no sparks produced)

Fluid power systems present no hazard of electricshock or arc flash.

Fluid power systems are often easier to understand andtroubleshoot than electric systems.

Fluid power systems may be safely used in submerged(underwater) environments.

Pneumatic systems are self-purging.


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INSTRUCTIONS FOR USING

PNEUMATIC SYSTEMS Pneumatic (instrument air) systems must be free of water

vapor and particulate contamination for much the samereason. Water is perhaps the most common contaminant in

instrument air systems, causing corrosion of metalcomponents and subsequent clogging of orifices.

Special devices called air dryers installed in instrument airsystems use solid materials called desiccants to absorb

water entrained in the compressed air. The desiccantmaterial is regenerated by the dryer mechanism on aregular cycle, but must be periodically replaced when its

water-absorbing ability wanes.


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Solenoid valvesA valve is a mechanical device for controlling the

flow of a fluid or a gas.

Asolenoid valve is an electromagnetic valve foruse with liquid or gas controlled by running orstopping an electrical current through a solenoid,

which is a coil of wire, thus changing the state ofthe valve


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Operation of a Solenoid Valve

The operation of a solenoid valve is similar to thatof a light switch, but typically controls the flow ofair or water, whereas a light switch typicallycontrols the flow of electricity.

A solenoid valve has two main parts: the solenoid

and the valve. The solenoid converts electricalenergy into mechanical energy which, in turn,opens or closes the valve mechanically.


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. .2. Inlet Port 5. Coil Windings 8. Spring

3. Outlet Port 6. Lead Wires 9. Orifice


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Solenoid Valves

Solenoid valves may use metal seals orrubber seals, and may also have electrical

interfaces to allow for easy control.

A spring may be used to hold the valve

opened or closed while the valve is notactivated.


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Solenoid ValvesWhen current flows in the coil, magnetic field is

generated and it reduces the air gap. Generallyspring is used to create gap when coil is not

energized.

Small signal can operate solenoids. Large back emfis generated when coil is switched off.

Diode is used to suppress the back emf and itprevents the control electronics from damage


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Solenoid Valves In switched on mode diode is reverse biased

During back emf diode is forward biased and

conducts and creates the alternative path for thecurrent.


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Classifications of Solenoid ValvesSolenoid-actuated valves are usually

classified according to the number of ports

(ways).1. 2-Way Solenoid valve2. 3-Way Solenoid valve3. 4-Way Solenoid valve

.


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Types of Solenoid ValvesSolenoid valves may have two or more ports:

in the case of a two-port valve the flow is

switched on or off; in the case of a three-port valve, the outflow is switched betweenthe two outlet ports. Multiple solenoid

valves can be placed together on a manifold


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2-Way solenoid valve2-way solenoid valves operate in a manner

analogous to single-pole single-throw

(SPST) electrical switches: with only onepath for flow.

A two-way valve has two ports.

When power is applied, the pressure flowsfrom one port to the other.

With pressure off, the flow ceases.


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2-Way solenoid valve


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2-Way solenoid valveSolenoid valve symbols often appear

identical to fluid power valve symbols, with

boxes representing flow paths anddirections between ports in each of the

valves states. Like electrical switches, these

valve symbols are always drawn in theirnormal (resting) state, where the return

springs action determines the valve position


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2-way Solenoid Valve


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3-way solenoid valves 3-way solenoid valves operate in a manner analogous

to single-pole double-throw (SPDT) electricalswitches: with two paths for flow sharing one common

terminal.


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Working principleA 3-way valve will pass fluid between ports 1

and 3 in its normal (resting) state, and pass fluidbetween ports 1 and 2 in its energized state.


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4-way solenoid valvesWhen a pneumatic actuator requires air pressure

applied to two different ports in order to move twodifferent directions (such as the case for cylinders

lacking a return spring), the solenoid valve supplyingair to that actuator must have four ports: one for airsupply (P), one for exhaust (E),and two for the cylinderports (typically labeled A and B)


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The following diagram shows a 4-waysolenoid valve connected to the

piston actuator of a larger (process)ball valve:


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Cut away diagram


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Application of solenoid valveSolenoid valves are also used in industry to control theflow of all sorts of fluids.

A common use for 2 way solenoid valves is in central

heating. These are controlled by an electrical signalfrom the thermostat to regulate the flow of heated waterfrom a heat pump to the in room radiators. Such valvesare particularly useful when multiple heating zones are

driven by a single heat pump.


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discrete control elements

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