ELECTRO PNEUMATIC COMPONENTS

 ELECTRO PNEUMATIC COMPONENTS

As the industrial automation control system is more and more complicated it is necessary to make the pneumatic component electrically interact with the control system

Frequently used electro pneumatic components are

• Cylinder switches
• Pressure switches
• Solenoid valves

Cylinder switches

      These switches detect the magnetic ring of a cylinder piston. and make an electrical contact signal for switching

Reed switch: A reed switch consists of two tiny leaf springs (reeds), hermitically sealed into a glass tube. The outer ends provide the electrical connection of the switch. The two ends inside the tube are silver or gold plated for a better electrical contact. They overlap with a very small clearance between them. If the switch comes into a magnetic field and the two blades pick up different polarity they attract each other and make an electrical contact (Switch closed)

REED SWITCH

Static switch: In high frequency operation where millions of cycles are reached in a relatively short time, a static switch should be used. One can compare the working principle of such a switch roughly with e.g. a light sensing resistor. Which has an extremely high resistance in the dark (open contact),but relatively low one (Closed contact) if exposed to light. The difference is that the switching part of a static switch is varied by magnetic field instead of light. In using such a switch we must be aware that it will be a part of the controlled electronic circuit and therefore bound to a fixed specified voltage and current

STATIC SWITCH

                               

Cylinder switches advantages

• Quiet in operation
• Longer life time
• Fast response time
• Maintenance free
• Reed contacts free from corrosion and contamination

Disadvantages

• Current or voltage that can be switched is limited
• High contact wear (arcing) if the inductance of the connected circuit is relatively high
• For a inductive load contact protection circuit is required

Pressure switches

These are designed to control the pressure level in a pneumatic circuit. The switch transforms the pressure change into an electrical signal. when a preset pressure is reached the electrical control changes state

Pressure switches

                    

 Solenoid valves

A valve is a device, which receives an external command to release ,stop or redirect the fluid that flows through it.

Directional control valves in particular as their name implies control fluid flow direction. They are used in pneumatic circuits to provide control functions which

• Control direction of actuator motion (power valve)
• Perform logic control function(AND,OR,NOT function)
• Stop and start fluid flow (on off valve)
• Sense machine and actuator position (Limit valves)

DC valves are classified according to their specific design characteristic

1. Internal valve mechanism which directs the flow of the fluid. Such mechanisms can be either be a poppet disc, poppet ball, sliding spool, a rotary plug or a combination of poppet and spool
2. Number of switching positions/ways. These are usually two or three. Some valves may provide more than three and in exceptional cases up to six switching positions(selector valves)
3. Number of connection ports. These ports connect the pressure lines to the internal flow channels of the valve mechanism and often also determine flow rate through it
4. Method of valve actuation. Which causes the valve mechanism to move into an alternative switching position

2/2 NC solenoid valve  (Direct operated)

This is basically a two position valve of the poppet type which is normally in the closed configuration. It is directly actuated on one side and returned by spring pressure. Compressed air is present at connector P other passage of air to the output (A)  is blocked by the armature .If an electrical signal is applied  to the coil of solenoid the armature is energized  by the coil as a result of the magnetic field .The compressed air flows from (P) to (A).The return spring causes the valve to adopt its idle position again as soon as the electrical signal is  switched off.

3/2 Solenoid valve (Pilot operated)

                           

Valves with pilot control are employed in order that the operating coils need not be excessively large. The device shown operates similarly to the previous 2/2 way solenoid valve .The difference lies in indirect actuation of the pilot position. An electrical signal causes the armature in the coil of the solenoid to open a passage. The compressed air flowing through the air channel from inlet port 1(P) to the armature and from there to the valve piston causing the valve to change over. This actuated position 1(P) to 2(A) is maintained until the solenoid is Deenergised. 

5/2 solenoid pilot operated

Valve shown below performs the same function as the 4/2 way solenoid valve. The design principle is different in this case. The 4/2 way solenoid valve is purely a poppet valve and the 5/2 way solenoid valve is a piston spool valve. An electrical signal triggers a change over of the armature. The compressed air flows through the air passage to the valve pilot piston and switches it. In the middle of the valve a sealing disc is used to open the passage from 1(P) to 4(A) or 2(B). Exhausting takes place from 2(B) to 3(S)

When the electrical signal is removed the return pilot pressure brings about the normal position

1(P) connected to 2 (B). Exhausting takes place from 4(A) to 5(R).