Experiment No :05 Following experiments to be done on pneumatic trainer:

 Expt No :05

 Following experiments to be done on pneumatic trainer:

 a. Automatic reciprocating circuit    b. Speed control circuit        c. Electro pneumatic circuits                           

d. Pneumatic circuit involving Shuttle valve/ Quick exhaust valve / Two pressure valve

Aim: To study various pneumatic circuits to control single-acting and double-acting cylinders

Apparatus:   Hydraulic trainer

Precautions:    

ü  Always keep the sufficient air supply pressure to the Electro-Pneumatic Trainer

ü  Keep the air filter regulator knob (AFR) in locked position at the pressure required for  particular experiment.

ü  Always maintain the pressure in the compressor more than 2.5kg/cm².

ü  Maintain oil level in the compressor.                                                                  

Theory :

A pneumatic circuit is formed by various pneumatic components, such as cylinders, directional control valves, flow control valves, pressure regulator, signal processing elements such as shuttle valve, two pressure valve etc. 

Pneumatic circuits have the following functions

·         To control the entry and exit of compressed air in the cylinders.

·         To use one valve to control another valve

·         To control actuators or any other pneumatic devices  

A pneumatic circuit diagram uses pneumatic symbols to describe its design.  Some basic rules must be followed when drawing pneumatic diagrams.

a)Automatic reciprocating circuit  (Electro pneumatic circuits) : 

 

Fig :Automatic double acting cylinder  (forward motion) 

Introduction : Electro pneumatics is now commonly used in many areas of Industrial low cost automation. They are also used extensively in production, assembly, pharmaceutical, chemical and packaging systems. There is a significant change in controls systems .

Electro-pneumatic control consists of electrical control systems operating pneumatic power systems. In this solenoid valves are used as interface between the electrical and pneumatic systems. Devices like limit switches and proximity sensors are used as feedback elements.

 In Electro Pneumatics, the signal medium is the electrical signal either AC or DC source is used.  Working medium is compressed air.  Operating voltages from around 12 V to 220 Volts are often used. The final control valve is activated by solenoid actuation.

In Electro pneumatic controls, mainly three important steps are involved:

a.       Signal input devices -Signal generation such as switches and contactor, Various types of contact and proximity sensors

b.      Signal Processing – Use of combination of Contactors of Relay or using Programmable Logic Controllers

c.       Signal Out puts – Out puts obtained after processing are used for  activation of  solenoids, indicators or audible alarms  

 

b. Speed control circuit :

Speed control of Pneumatic Cylinders can be conveniently achieved by regulating the flow rate supply or exhaust air. The volume flow rate of air can be controlled by using flow control valves which can be either two way flow control valve or one way flow control valve There are two types of throttling circuits for double acting cylinders:

i)                    Supply air throttling (meter –in)

ii)                  Exhaust air throttling (meter –out)

Supply air throttling (meter –in) :

This method of speed control of double acting cylinders is also called meter –in circuit (Figure a ). For supply air throttling, one way flow control valves are installed so that air entering the cylinder is throttled.  The exhaust air can escape freely through the check valve of the throttle valve on the outlet side of the cylinder. There is no air cushion on the exhaust side of the cylinder piston with this throttling arrangement. As a result, considerable differences in stroking velocity may be obtained even with very small variations of load on the piston rod. Any load in the direction of operating motion will accelerate the piston above the set velocity. Therefore supply air throttling can be used for single acting and small volume cylinders. 

 

 

Fig : Throttling Circuits (Speed Control )

Exhaust air throttling (meter –out)

This method of speed control of double acting cylinders is also called meter-out (Figure  b). In exhaust air throttling throttle relief valves are installed between the cylinder and the main valve in such a way that the exhaust air leaving the cylinder is throttled in both directions of the motion of the cylinder. The supply air can pass freely through the corresponding check valves in each case. Exhaust air throttling is practically used for the speed control of double acting cylinders

C. Control Of Single Acting Cylinder Using OR / Shuttle  Valve

Fig: Shuttle valve

 

Fig: Control of a single acting cylinder using OR valve

d.     Quick Exhaust Valves :

A quick exhaust valve is a typical shuttle valve. The quick exhaust valve is used to exhaust the cylinder air quickly to atmosphere. Schematic diagram of quick exhaust valve is shown in Fig. In many applications especially with single acting cylinders, it is a common practice to increase the piston speed during retraction of the cylinder to save the cycle time. The higher speed of the piston is possible by reducing the resistance to flow of the exhausting air during the motion of cylinder. The resistance can be reduced by expelling the exhausting air to the atmosphere quickly by using Quick exhaust valve. 

Fig : quick exhaust valve

Fig: Application  of quick exhaust valve

e.       Two Pressure /AND  valve :

This valve is the pneumatic AND valve. It is also derivate of Non Return Valve. A two pressure valve requires two pressurised inputs to allow an output from itself. The cross sectional views of two pressure valve in two positions are given in Figure. This valve has two inputs  and  one output . If the compressed air is applied to either one input  the spool moves to block the flow, and no signal appears at output . If signals are applied to both the inputs , the compressed air flows through the valve, and the signal appears at output.

 

Fig : Two Pressure /AND  valve

 

Fig: Application  of AND valve 

Conclusion  : Studied various pneumatic circuits such as  Automatic reciprocating circuit , Speed control circuit , Electro pneumatic circuits  ,Pneumatic circuit involving Shuttle valve/ Quick exhaust valve / Two pressure valve  successfully .