Unit -6 INTRODUCTION TO PNEUMATIC CONTROL

If gas is used as a working medium in a system then it is referred as Penumatic system. The most common gas is simply compressed air, although nitrogen is occasionally used.

Choice of working medium :a) Air is recommend for medium pressure and moderate workb) Combination of air and oil is recommended for greater precision and accuracy of motion of actuatorc) Oil is recommended for high pressure and heavy duty workd) Oil is also suitable for extreme temperature variation.

Characteristics of compressed air (Also advantages)

1. Air is practically available in abundant every where. Air can be transported easily in pipelines and even over long distances.

2. Compressed air can be stored in reservoirs and these reservoirs themselves can be transported easily.

3. Compressed air is relatively insensitive to temperature fluctuations. This ensures reliable operations under extreme conditions.

4. Compressed air does not offer the risk of explosion.5. Unlubricated exhaust air is clean and also leakage of lubricated air if any is not harmful.

Pneumatic System : Pneumatic system uses high compressed air as the working medium. Pneumatic systems are open system because it always processes new air. Fig shows the block diagram of a pneumatic system with their basic elements.

Fig 6.1 Block Diagram of a Pneumatic System

The basic elements of a pneumatic systems are a) Air-Inletb) Filterc) Compressord) Electric Motore) Air Cooler and Treatmentf) Storage receiverg) Control Valve

h) Actuatori) Exhaust

Disadvantages of air as a operating mediuma) Limited to low pressure and hence low force applications.b) Generation of compressed air is expensive compared to electricity.c) Exhaust air noise is unpleasant, hence the necessity of silencer.

Working: The electric motor drives the compressor. The compressor creates suction pressure and draws in air through a filter. The filter arrests suspended particles like dirt, dust and allows clean air into the compressor. The compressor compresses the air from atmospheric pressure to higher pressure depending on the capacity of the compressor.The air gets heated up due to compression. The hot air is then passed through a air reatment unit where the hot air is cooled and moisture and other gases present in the air is removed by the treatment unit. The treated high pressure air is then stored in the reservoir via pressure relief valve. Once the designed pressure is attained by the air in the reservoir the pressure relief valve diverts the air to the atmosphere.A pressure sensor mounted on the receiver senses the pressure and at the maximum signed pressure of the receiver it puts off the motor and hence the compressor.The high pressure from the receiver is used to run an actuators with the help of control vlalves.

Differences between penumatic system and hydraulic systemPenumatic System Hydraulic System

a Uses air as the working medium uses oil as the working medium

b It is an open system ft is a dosed system

c Every time fresh air is drawn Same oil is used again and againd Cannot provide high volume Can provide high volume of

pressurised air on demand pressure oil on demande Suitable for low pressure Suitable for high pressure

Application applicationf Economical Expensive

PNEUMATIC ACTUATORSActuators are those which has motion or to which motion can be imparted. If the motion is imparted by energy contained in air or gas then it is called pneumatic actuator

Pneumatic actuators are broadly classified into three types. They area. Linear cylinder b. Rotary actuators c. Air motors

Linear cylinders: Linear cylinder in pneumatic systems is also called air cylinders. Air cylinders are generally made of either aluminium or non-ferrous materials including plastics to reduce weight and improve heat transfer characteristics and minimize the corrosive action of air.An air cylinder essentially consists of a tube sealed at both ends by covers and fitted with a piston and piston rod. Compressed air is admitted through a port at one end which produces linear motion of the piston. Displaced air is exhausted through secondary port at the other end.

According to operating principle air cylinders are further classified intoa. Single acting cylinderb. Double acting cylinder

Single acting cylinder : In the case of a single cylinder compressed air is applied on one side of the cylinder which extend the cylinder (piston).

The method of retraction depends on the application of the cylinder. If the cylinder is required to produce high force during extension as in clamping or fon-ning operation and very small force is required to retract, then a spring used to retract But the length of the cylinder has to be increased for a given stroke to accommodate the spring.Construction of single acting cylinderPneumatic and hydraulic cylinders are constructed in a similar manner, the major difference arising out of differences in operating pressure, approximately around 10 MPa for hydraulics and I MPa. For pneumatics. The cylindrical tube is held in position by the two end covers with the help of the tie rods. The piston-rod assembly consisting of a cup seal reciprocates inside the cylinder the retraction being effected by the spring. The compressed air enter the inlet port of the cylinder and extends it and low pressure air is expelled through the exhaust port during retraction. Both inlet and outlet ports are provided in the piston end covers

The location of the spring is either on piston side or on rod side and depends on the application of the cylinder.In spring on piston rod side the piston is extended by compressed air and the 'spring' retracts .In the case of spring on piston side the piston is extended by spring

The other types of single acting cylinders area. Diaphragm cylinderb. Rolling diaphragm cylinder

Diaphragm Cylinder: Diaphragm cylinder consists of built-in diaphragm of rubber, plastic or even metal.

Diaphragm cylinders are capable of generating very high forces with very short strokes and are commonly called thrusters. Theoretical thrust is equal to the product of air pressure and effective area of diaphragm. Stroke is normally limited to a maximum of about one third the cylinder diameter. NorTnal positioning is either vei tical or horizontal.

Double acting cylinder: In double acting cylinder each end is pressurized by compressed air alternately thus developing force on either direction. Since compressed air acts on either sice of the piston for extending and retracting of the piston the cylinder is called double acting cyiinder. Fig. 6.6 shows the basic principle of double acting cylinder.

Construction: A double acting cylinder consists of two end covers with one port on each, a cylindrical tube, a piston and piston rod. End caps can be secured to be to the ends of the cylindrical tube by welding, tie rods or threaded connections. To prevent any leakage through the cylinder seamless drawn steel tubes are used.

Working: In the nonnal position the cylinder will be in the retracted position. High compressed is allowed into the piston through port A. The piston extends and the air on the, rod side of the escapes through port B. Delivery of high compresses air in to the cylinder continues till the extension is complete. Similarly high compressed air is allowed into the cylinder through port B and the piston retracts, The air on the piston side escapes through port A during retraction. Compressed air continues to enter the cylinder through B till retraction is completed fig.6.6(a). Cup seals

Cushioning

Air cylinders are fast-acting and the piston will tend to accelerate over the whole length of stroke and develops shock loads on the caps. To avoid this decelerating device or cushion is generally provided in the cylinder geometry. This normally takes the form of a cushion chamber in the end of the cylinder, into which nose part of the piston enters

The cushion boss or piston nose is usually tapered to provide a smooth, progressive entry, but provides a plunger type seal once it has fully entered the cushion chamber

Working.: When the piston approaches the end of the stroke, the nose part enter the cushion chamber from which air can escape only through a controlled bleed. This provides adjustable clamping or braking over the length of cushion travel with the kinetic energy of the piston mass dissipated in the form of heat. There are some drawbacks of integral cushion described above which is effective above a certain speed, unless cushion less is impractically long. This problem of integral cushioning can overcome using a buffer cylinder.

Through-rod cylinderThe force exerted on a piston of a given size cylinder, and air pressure depends on the effective surface area of the piston.

Such a cylinder is called through-rod cylinder or non-differential cylinder. The main advantage of such type of cylinder is stability of the operation since the piston rods on either side will be supported on bearings. In addition to this both the stroke can be used to carryout work, thereby improving the efficiency of the system.

Telescopic Cylinder : Sometime necessity arises to have a long stroke length of the piston but available space will be a limitation for placing a single cylinder. In such situation telescopic

cylinders may be used. Where in the fully retracted position the length of the cylinder will be equal to the length of the conventional cylinder.

Figshows a telescopic cylinder. It consists of a series of anunlar, nested tubes, placed one inside the other. The total stroke length of such a cylinder will be the sum of the effective extension of each cylinder, the force being equal to the p roduct of the pressure and the effective area of the smallest cylinder.

Impact cylinder:

Pneumatic Actuators are used for metal forming . Pressure of air in pneumatic cylinders is less , but large imact loads can be generated by accelerating a hammer connected to a high velocity piston rod of cylinder.Working: . Initially the cylinder is retracted by applying high pressure through port R. Now compressed air is applied to both ports E and R. The cylinder remains in the retracted position because the force exerted on piston rod side is more than the piston side because of effective area is less on piston side x than the effective area y on the rod end side (F = p x a) as shown in fig. Suddenly the piston area 'Y' experiencesport E pressure. With a large volume of air accumulated it is accelerated (extends) rapidly to a very high velocity of the order of 10 m/s.

Rodless cylinders: Rodless cylinders are those cylinder having piston without piston rods. A rodless cylinder consists of a cylindrical tube with a piston head housed inside. The piston moves to and fro inside the cylinder, due to the compress air.

These rodless cylinders are best suited where space is a problem to accommodate the piston rod during extension. Fig. 6.16 illustrates the principle of rodless cylinder.

Types of rodless cylinders

a. C-shaped housing rodless cylinderb. Magnetic type rodless cylinderc. Cable operated rodless cylinder

C-shaped housing type:

It is called C-shaped housing type because the cylinder instead of circular will be of C shape the assembly of the cylinder. In this case the piston head is directly connected to a carriage riding along the outside of the cylinder. An ingenious multipart sealing strip prevents air from leaking out of the open part of the C-section.

Magnetic type rodless cylinder:

It Consists of a cylinder of conventional type with a piston head of strong magnet. A circular carriage surrounds the cylinder and follows the magnet. In this type of cylinder the coupling between the piston head and carriage is remarkably strong. Fig. shows the two views of a magnet type rodless cylinder.

Cable type rod less cylinder

This consists of a cylinder and piston head- On either side of the piston head are fastened a cable which pass around pulleys mounted on the ends of the cylinder. When the piston head moves to and fro due to compressed air, the- cable also moves.

Cylinder Mountings:

The main function of a cylinder mount is to provide a means of anchoring or securing the cylinder. The method of mounting can affect the maximum rating or maximum thrust loading of the cylinder.

The basic types of cylinder mountings area. Foot Mountingb. Flange Mountingc. Swivel MountingCylinder mountings may bea. Rod end cylinder mountingb. Cylinder body mounting

Rod end mountings may bea. Plainb. Threadedc. Flanged. Clevis

Cylinder body mountings may bea. Plain mountingb. Footc. Flanged. Thinione. Clevisd. Tounge or bracket

Vane Motors: (Sliding vane type):

Vane motors are generally lighter, more compact and cheaper than piston motors. Available in power range from fractional capacity to 20 Kw power. Their power to weight ratio is outstanding. Fig illustrates the main parts and operating principle of reversible vane motor. It consists of radial slotted rotor mounted on a shaft eccentric to the power chambers. The vane will slide radially and seal against power chamber wall or cam ring.

The vanes are usually sprung out either mechanically using spring or pneumatically using compressed air against the cam ring to provide a sealing force in the initial starting condition. As the speed of the motor increases the vanes are swung out by centrifugal action.

Air is supplied to the pressure chamber either through ports A or B. If the air is supplied through port A the motor rotate in clockwise direction and if supplied through B, it rotates in the counter clockwise direction.

By varying the axis of rotation of the rotor with reference to the axis of the pressure chamber, the speed of the motor can be varied. The distance between the axes being called eccentricity. Such a motor is called variable speed vane motor.

Applications of vane motorLower capacity motors - Portable air tools, multiple light duty tools etc.Higher capacity motors - Heavy air tools, pump drives, and general driver duties where light starting loads are involved.

Air motors

The radial piston motor in Figure 15-22 does not require reduction gearing for most low-speed/high-torque applications. Force from half the pistons is driving the crankshaft to turn it while the remaining pistons are exhausting. Inlet-outlet ports connected to a rotary valve driven by the crankshaft send 40 to 100 psi air through cored holes in the body and cylinder bores to and from the pistons. Most radial piston air motors run at less than 500 rpm due to air energy waste at faster speeds. These motors can be physically large and take a lot of mounting space. Some manufacturers make axial or in-line piston motors while a few have experimented with gerotor designs.