Pneumatic Circuit

Speed control circuits.

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
ii) Exhaust air throttling

Supply air throttling.

 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.

Exhaust air throttling. 

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. In this case, the piston is loaded between 

two cushions of air while the cylinder is in motion and hence 

a smooth motion of the cylinder can be obtained. The first 

cushion effect is due to supply air entering the cylinder through 

check valve, and second cushion effect is due to the exhaust air 

leaving the cylinder through the throttle valve at a slower rate. 

Therefore, exhaust air throttling is practically used for the 

speed control of double acting cylinders. Arranging throttle 

valves in this way contributes substantially to the improvement 

of feed behavior.

Sequencing Circuits

In process control applications such as sequencing, the Pneumatics systems are generally employed.
Electrical components such as relays, programmable logic controllers are used to control the operations of
Pneumatic systems. A simple example of a pneumatic sequencing is shown in Figure 1

Components used 

The components used in the circuit are: double acting cylinder, 3/2 roller lever valve, 5/3 pilot operated
direction control valve and a 3/2 push button valve. By using this circuit, a continuous to and fro motion of
the actuator is obtained.

Working
When the 3/2 push button is actuated, the air flows from the source through the push button valve to the 3/2
roller valve (S1). The roller valve is already actuated by the cylinder when the piston rod hits the lever of
S1. Therefore, there is continuous flow to the 5/3 pilot operated direction control valve (DCV). The flow
given to the pilot line 14 actuates the first position of DCV. The air flows from port 1-4 pushes the piston
head which causes the extension of the cylinder. As the cylinder fully extends it actuates the 3/2 roller lever
valve (S2). The roller valve is actuated and air flows through the valve to the 5/3 DCV. The air enters the
DCV through pilot port 12 actuating the second position. Hence the air flows from port 1-2 to the actuator
rod end, causing its retraction. The cylinder reciprocates till the supply is stopped. In this way, we can
achieve the sequencing operation by controlled actuation of various valves in a pneumatic system.

Pictures

Hydraulic Symbol