Pneumatics technology

4. lesson

Institute of Engeneering Sciences

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2/2 Valve actuator control

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2OUT IN

- A pair of the most basic of all valve types the 2/2 can be used to control a single acting cylinder- The normally closed position of the valve is produced by the spring- The operated position is produced by the push button- One valve admits air the other valve exhausts it

2/2 Valve actuator control

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2OUT IN

- The button marked OUT is pushed to operate the valve- Air is connected to the cylinder and it outstrokes- Air cannot escape to atmosphere through the valve marked IN as this is closed- The air at atmospheric pressure in the front of the cylinder vents through the breather port

2/2 Valve actuator control

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2OUT IN

- The push button of the valve marked OUT is released and it returns to a normal closed position- Air is now trapped in the system and provided there are no leaks the piston rod will stay in the outstroked position- If the load increases beyond the force exerted by the air the piston rod will start to move in

2/2 Valve actuator control

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OUT IN

- The button marked IN is pushed to operate the valve- Air escapes and the piston rod moves to the instroked position- The push button must be held operated until the piston rod is fully in- Atmospheric air will be drawn in to the front of the cylinder through the vent port

2/2 Valve actuator control

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2OUT IN

- If the button marked IN is released the piston rod will remain in the instroked position- Any leaks in the installation can cause the piston rod to creep

Direct Control of a Single-acting cylinder

- A 3 port valve provides the inlet and exhaust path and is the normal choice for the control of a single acting cylinder- In the normal position produced by the spring, the valve is closed- In the operated position produced by the push button the valve is open- The push button must be held down for as long as the cylinder is outstroked

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Direct Control of a Single-acting cylinder

- A 3 port valve provides the inlet and exhaust path and is the normal choice for the control of a single acting cylinder- In the normal position produced by the spring, the valve is closed- In the operated position produced by the push button the valve is open- The push button must be held down for as long as the cylinder is outstroked

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Indirect Control of a Single-acting cylinder

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The piston of a large-volume single-acting cylinder (large diameter, large stroke lenght, large distance from valve to cylinder) is to move out after operating a valve and return to its end position after this valve has been released.

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Push

1.1

1.2

1.1

1.2

Operation of valve 1.2 opens the passage for the compressed air from P to A, and hence signal at Z on valve 1.1. Owing to switchover of vave 1.1, air flows from P to A and thus the forward stroke is effected on the single-acting cylinder.

Double acting cylinder They are available in different constructions

such as:ConventionalDouble ended piston rod typeRod less typeTandem typeMulti-position typeRotary type

Conventional cylindersDuble acting cylinders are equipped with two working ports – one on the piston side and the other on the rod side. To achieve forward motion of the cylinder, compressed air is admitted on the piston side and the rod side is connected to exhaust. During return motion supply air admitted at the rod side while the piston side volume is connected to the exthaust. Force is exerted by the piston both during forward and return motion of cylinder.

Duble acting cylinders are available in diameters from few mm to around 300 mm and stroke lengths of few mm up to 2 meters.

Double ended piston rod typePneumatic cylinders operates at much higher speeds than

hydraulic cylinder. Due to this, there is a tendency of the piston to ram against the end covers as the piston approaches the ends at high velocity especially in cylinder with large mass. This impact force can damage the cylinder as well as the piston due to repetitive action.

All duble acting cylinders excepting for small sizes, are provided with end position cushioning arrangement.

This arrangement decelerates the piston motion as it approaches the end of the stroke

Rod less typeDifferent operational principals are used for the construction of rod less cylinders:

Cable cylinderSealing band cylinder with slotted cylinder barrelCylinder with magnetically coupled slide

Rod less cylinder have the following advantages:Available in long lengths – up to 4 m or even

higher (as there is no buckling)Most ideally suited for stopping and fixing

(robotic application)Occupies less space as the extension rod is not

present

Rod less cylinder with magnetic coupling

This cylinder has a hermetically sealed arrangement where piston is housed inside a sealed cylinder barrel. The piston is provided with number of annular ring magnets, radially polarised. An external sleeve which slides over the cylinder, is also provided with similar arrangment of ring magnets.

Thus a magnetic coupling is establised between the piston and slider. As the piston reciprocates due to supply of compressed air, the slider also reciprocates oven the clinder.

Rod less cylinder mechanically coupled

The cylinder barrel is provided with a slot across the entire length. The force is transmitter through a slide permanently connected to the piston. The connection from piston to slide is directed outwards through the slotted cylinder barrel. The slot is sealed by means of a sealing band, which seals the inside of the slot.

The sealing band is guided between the piston seals and passed under the slide. A second metallic cover srtip, covers the slot from the outside to prevent the ingress of dirt.

Vane type rotary actuators

A rotating vane connected to a shaft divides cylindrical chamber in to two compartments. Compressed air is alternately admitted and exhausted from the chambers. The compressed air pressure acting on the vane surface results in a torque. Hence rotary motion is obtained.

The magitude of the torque produced, depends on the surface area of the vane, air pressure and mean radius of the vane.

Direct Control of a double acting cylinder

The only difference between a single acting cylinder and a double acting cylinder is that a double acting cylinder often uses a 5/2 directional control valve instead of a 3/2 directional control valve.

The double acting cylinder can be reversed either by a 4/2 directional or 5/2 directional control valve.

In the normal position of the 5/2 directional valve, „1” is connected with „2” and „4” with „5”. By operating the pushbutton the valve reverses. „1” is connected with „4” and „2” with „3”. The piston of the cylinder travels from the rear to the front end position.

Push

Direct Control of a double acting cylinder

In the normal position of the 4/2 directional valve, „1” connected with „2” and „4” is connected with „3”. By operating the pushbotton, the valve reverses. „1” is connected with „4” and „2” with „3”. The cylinder piston travels from the rear to the front end position.

Push

Memory functionMemory is a common basic function. It can keep a component at a certain state permanently until there is a change of signals. The drawn shows a memory function circuit. When control valve (1) is operated momentarily (that is, pressed for a short time), the output signal of the 5/2 directional control valve (3) will be set to ON.

The signal will stay that way until control valve (2) is operated momentarily and generates another signal to replace it, causing it to stay permanently at OFF.

Indirect control of a double acting cylinder

A double acting cylinder is to be controlled by two valves (1.2 and 1.3) such that the piston travels out when valve 1.2 is operated, and also remain stationary in the forward end position when valve 1.2 is released until the reverse signal is input through valve 1.3 for the return movement.

I. step

II. step

III. step

PUSH

PUSH

Indirect control of a double acting cylinder

A double acting cylinder is to be controlled by 5/2 directional control valve.

The 5/2 double pilot valve is bi-stable therefore the push button valves only need to be pulsed.

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II. step

III. step

PUSH

PUSH

Automatic return of a double acting cylinder

The following methods are commonly used to interrogate the end positions of piston in the cylinder:•Mechanically operated limit switches (roller lever or idle return roller type)•Reed sensor, either with electrical or pneumatic out put (the piston is incorporated with ring magnet)•Electrical proximity switches•Pneumatic signal switches

3/2 way roller lever valveAs a signaling element the directional control valve is operated

for example, by a roller lever to detect the piston rod position of a cylinder.

Reduced actuation force through pilot controlConversion from normally closed position to normally open

position by exchanging ports 1 and 3 and repositioning the upper part of the housing by 180 B.

Semi-automatic controlManual remote start of a double acting cylinder with

automatic returnCylinder identified as “C1”Trip valve operated at the completion of the plus stroke

identified as “S11”

C1 C

1C1

Mechanic push

Manual push

I. step

II. step

III. step

Fully-automatic controlContinuous automatic cycling from roller operated trip valves.Manual run and end of the automatic cycling.Cylinder will come to rest in the instroked position regardless of

when the valve is put to end.Tags for the roller feedback valves S10 and S11 show their relative

positions.

Mechanic pushManual

push

I. step

II. step

III. step

Mechanic push

Thank you for your attention!