Pneumatic Notes

CHAPTER 2.0 PNEUMATICS __________________________________________________________________ 2.1

Learning Outcomes At the end of this course, the students would be able to apply and appreciate the knowledge to: (1) Perform the safety and regulations apply in engineering workshop and/or laboratory. (2) Describe briefly about pneumatic control system. (3) Design the pneumatic circuit diagram and elements. (4) Correctly install components in the pneumatic, electro pneumatic, hydraulic and control system according to the given circuit diagrams. (5) Test runs the assembled system components base on the given circuit diagrams.

2.2

Pneumatics Theory A fluid power system is one that transmits and control energy through the use of pressurized liquid or gas. In Pneumatics, this power is air. This of course from the atmosphere and is reduced in volume by compression, thus increasing its pressure. Compressed air mainly used to do work by acting on a piston or vane. While this energy can be used in many facets of industry, the field of industrial pneumatics is considered here. The correct use of pneumatic control requires an adequate knowledge of pneumatic components and their function to ensure their integration into an efficient working system. Although electronic control using a programmable sequencer or other logic controller is currently specified, it is still necessary to know the function of the pneumatic components in this type of system. 2.2.1

What Can Pneumatics do?

•Operation of heavy or hot doors. •Unloading of hoppers in building, steel making, mining and chemical industries. •Forming operations of bending, drawing and flattening. •Bottling and filling machines. •Pneumatic robots. •Component and material conveyor transfer. •Air separation and vacuum lifting of thin sheets. Figure 2.1 show the application of pneumatic component at chip bonding process which consists of a vacuum unit.

Figure 2.1: Chip Bonding. 2.2.2

The Advantages and Limitations of Pneumatic

•It has several operation modes; can be fully automated, manually operated and semi automated mode. •It is clean, suitable for several processes which are very sensitive, especially for hazardous chemicals and electronic components. •The equipments are very cheap compared to other methods. •The accuracy of the end results is moderate. •Reduce production time and cost – mass production. •There is no other waste produced from the operating system except air. •Compress is relatively insensitive to temperature fluctuation. This ensures reliable operation, even under extreme condition (depend on protection material). However, pneumatic technology also has their own limitations. Listed below are the limitations of pneumatic technology: • • • • •

Only clean air with lowest humidity is allowed to be in pneumatic system. So, compress air need good preparation to filter any kind of dirt and condensate. It is seldom to get uniform and constant piston speed with compressed air. It has the limitation in producing the output force. It is suitable for non-heavy duty purpose. The exhaust air is very noisy. Compressed air is very expensive in terms of conveying the power but the high price is remunerated by the cheap pneumatic equipments, fast and efficient production.

2.2.3

The Basic of Pneumatic System

Pneumatic cylinders, rotary actuators and air motors provide the force and movement of most pneumatic control systems to hold, move, form and process material. To operate and control these actuators, other pneumatic components are required i.e. air service units to prepare the compressed air and valves to control the pressure, flow and direction of movement of the actuators. A basic pneumatic system, shown in Fig 2.2, consists of two main sections:

Figure 2.2: Basic Pneumatic System •

The Air Production and Distribution System (1) Compressor: Air taken in at atmospheric pressure is compressed and delivered at higher pressure to the pneumatic system. It thus transforms mechanical energy into pneumatic energy. (2) Electric Motor: Transforms electrical energy into mechanical energy (3) Pressure Switch: Controls the electric motor by sensing the pressure in the tank. (4) Check valve: Lets the compressed air from the compressor into the tank. It is set to a maximum pressure at which it stops the motor and a minimum pressure at which it restarts it. (5) Tank: Stores the compressed air. (6) Pressure Gauge: Indicates the Tank Pressure. (7) Auto Drain: Drains all the water condensing in the tank without supervision. (8) Safety Valve: Blows compressed air off if the pressure in the tank should rise above the allowed pressure. (9) Refrigerated Air Dryer: Cools the compressed air to a few degrees above freezing point and condenses most of the air humidity. (10) Line Filter: It helps to keep the line free from dust, water and oil.

•

The Air Consuming System (1) Air Take Off. (2) Auto Drain. (3) Air Service Unit. (4) Directional Valve: Alternatively pressurizes and exhaust the cylinder connections to control the direction of movement. (5) Actuator. (6) Speed Controllers: Allows easy speed adjustment of the actuator movement. 2.2.4

Components and Symbol of Pneumatic System

The primary levels in a pneumatic system are: • Energy supply/Air generation and distribution. • Input elements (sensors). • Processing elements (processors). • Actuating devices (actuators) The elements in the system are represented by symbols which indicate the function of the element. The symbols can be combined to represent a solution for a particular control task using the circuit diagram. The circuit is drawn with the same structure as the signal flow diagram above. At the actuator level the addition of the control element completes the structure. The control element controls the action of the actuator after receiving signals sent by the processor elements.

Figure 2.3: Pneumatic elements

The directional control valve (DCV) may have a sensing, a processing or an actuating control function. It the DCV is used to control a cylinder motion, then it is a control element for the actuator group. If it is used in the function of processing signals, then it is defined as a processor element. If it is used to sense motions, then it is defined as a sensor. The distinguishing feature between each of these roles is normally the method of operating the valve and where the valve is situated in the circuit diagram. The development of pneumatic systems is assisted by a uniform approach to the representation of the elements and the circuits. The symbols used for the individual elements must display the following characteristics: • Function • Actuation and return actuation methods • Number of connections (all labeled for identification) • Number of switching positions • General operating principle • Simplified representation of the flow path A symbol does not represent the following characteristics: • Size or dimensions of the component • Particular manufacturer, methods of construction or costs • Orientation of the ports • Any physical details of the element • Any unions or connections other than junctions The symbols used in pneumatics are detailed in the standard DIN ISO 1219, "Circuit symbols for fluidic equipment and systems. (a) Air generation and distribution The air supply for a particular pneumatic application should be sufficient and of adequate quality. Air is compressed to approximately 1/7th of its volume by the air compressor and delivered to an air distribution system in the factory. To ensure the quality of the air is acceptable, air service equipment is utilized to prepare the air before being applied to the control system. Malfunctions can be considerably reduced in the system if the compressed air Is Correctly prepared. A number of aspects must be considered in the preparation of the service air: • Quantity of air required to meet the demands of the system • Type of compressor to be used to produce the quantity required • Storage required • Requirements for air cleanliness • Acceptable humidity levels to reduce corrosion and sticky operation

• • • • • • •

Lubrication requirements, if necessary Low temperature of the air and effects on the system Pressure requirements Line sizes and valve sizes to meet demand Material selection and system requirements for the environment Drainage points and exhaust outlets in the distribution system Layout of the distribution system to meet demand.

As a rule pneumatic component is designed for a maximum operating pressure of 8- 10 bar (800-7000 kPa) but in practice it is recommended to operate at between 5 and 6 bar (500-600 kPa) for economic use. Due to the pressure losses in the distribution system the compressor should deliver between 6.5 and 7 bar (650-700 kPa) to attain these figures. An air receiver should be fitted to reduce pressure fluctuations. In normal operation the compressor fills the receiver when required and the receiver is available as a reserve at all times. This reduces the switching cycles of the compressor.

Figure 2.4: Air supply system The symbols for the energy supply system can be represented as individual elements or as combined elements. The choice between using simplified or detailed symbols is dependent upon the purpose of the circuit and its complexity. In general where specific technical details are to be given such as requirements for non-lubricated air or micro-filtering, then the complete detailed symbol should be used. If a standard and common air supply is used for all components, then the simplified symbols can be used. Due to the high demand at certain stages of the air distribution system, a ring main with cross-feed connections is recommended. In this way the fluctuations are reduced. The ring main should be laid out with a 1-2% gradient to allow drainage points for condensate from the compressor. If there is a relatively high condensate level, then air drying equipment should be fitted specifically to dry the air to the required quality. Condensate is a common cause of failure in pneumatic controls.

The air service unit is a combination of the following • Compressed air filter, Compressed air regulator and Compressed air lubricator. The correct combination, size and type of these elements are determined by the application and the control system demand. An air service unit is fitted at each control system in the network to ensure the quality of air for each individual task. ITEM

SYMBOL

MEANING Compressor

SUPPLY

Pressure Source Pneumatic Pressure Source Air Reservoir Filter : separation and filtration of particles

SERVICE EQUIPMENT

Filter and separator (automatic) Lubricator

Pressure regulator

COMBINED SYMBOLS

Air Service Unit : Filter, Regulator, Gauge, Lubricator Simplified air service unit with lubricator

Figure 2.5: Symbols use in energy conversion and preparation The compressed air filter has the job of removing all contaminants from the compressed air flowing through it as well as water which has already condensed. The compressed air enters the filter bowl through guide slots.

Liquid particles and larger particles of dirt are separated centrifugally collecting in the lower part of the filter bowl. The collected condensate must be drained before the level exceeds the maximum condensate mark, as it will otherwise be re-entrained in the air stream. The purpose of the regulator is to keep the operating pressure (secondary pressure) virtually constant regardless of fluctuations in the line pressure (primary pressure) and the air consumption. The purpose of the lubricator is to deliver a metered quantity of oil mist into a leg of the air distribution system when necessary for the use by pneumatic control and working components. (b) Input Elements Valves can be divided into a number of groups according to their function in relation to signal type, actuation method and construction. The primary function of the valve is to alter, generate or cancel signals for the purpose of sensing, processing and controlling. Additionally the valve is used as a power valve for the supply of working air to the actuator. Therefore the following categories are relevant: • Directional control valves: Signaling elements, Processing elements, Power elements • Non-return valves • Flow control valves • Pressure control valves • Combinational valves b.1) Directional control valves The directional control valve controls the passage of air signals by generating, canceling or redirecting signals. In the field of control technology, the size and construction of the valve is of less importance than the signal generation and the actuation method. Directional control valves can be of the poppet or slide type, with the poppet utilized for small flow rates and for the generation of input and process signals. The slide valve is able to carry larger flow rates and hence lends itself to the power and actuator control role. The way valve is described by: • Number of ports or openings (ways): 2 way, 3 way, 4 way, 5 way, etc. • Number of positions: 2 positions, 3 positions, etc. • Methods of actuation of the valve: Manual, air pilot, solenoid, etc. • Methods of return actuation: Spring return, air return, etc. • Special features of operation: Manual overrides, etc. Explanation Of Symbol

Symbol Development

Valve switching positions are represented as squares The number of squares shows how many switching positions the valve has Line indicate flow paths, arrows shows the direction of flow Shut off positions are identified in the boxes by lines drawn at right angles The connections ( inlet and outlet ports ) are shown by lines on the outside of the box Figure 2.6: Directional Control Valve Symbol Development The directional control valve is represented by the number of controlled connections, the number of positions and the flow path. In order to avoid faulty connections, all the inputs and outputs of a valve are identified. A numbering system is used to designate directional control valves and is in accordance with ISO 5599 (Draft). Prior to this a lettering system was utilized and both systems of designation are presented here: Port or Connection Pressure port Exhaust port Exhaust ports Signal outputs Pilot line opens flow 1 to 2 Pilot line opens flow 1 to 2 Pilot line opens flow 1 to 4 Pilot line flow closed Auxiliary pilot air

SYMBOL

ISO 5599 1 3 5,3 2,4 12 12 14 10 81,91

Lettering System P R (3/2 way valve) R,S (3/2 way valve) B,A Z (single pilot 3/2 way) Y (5/2 way valve) Z (5/2 way valve) Z,Y Pz

EXPLANATION

2/2 - way directional control valve, normally open 3/2 – way directional control valve normally closed 3/2 – way directional control valve, normally open 4/2 – way directional control valve Flow from 1 to 2 and from 4 to 3 5/2 – way directional control valve Flow from 1 to 2 and 4 to 5 5/3 – way directional control valve Mid position closed Figure 2.7: Directional Control Valve, Ports and Positions (ways) The methods of actuation of pneumatic directional control valves are dependent upon the requirements of the task. The symbols for the methods of actuation are detailed in ISO 1219. The types of actuation may vary e.g. manually actuated, mechanically actuated, pneumatically actuated, electrical, combined actuation.

Figure 2.8: Methods of Actuation For example for manual actuation operated is generally obtained by attaching an operator head, suitable for manual control, onto a mechanical operated valve as indicated in Figure 2.9.

Figure 2.9: Manually of Actuation b.2) Flow Control Valve The flow control valve restricts or throttles the air in a particular direction to reduce the flow rate of the air and hence control the signal flow. If the flow control valve is left wide open then the flow should be almost the same as if the restrictor is not fitted. In some cases it is possible to infinitely vary the restrictor from fully open to completely close. If the flow control valve is fitted with a non-return valve then the function of flow-control is unidirectional with full free flow in one direction. A two way restrictor restricts the air in both directions of flow and is not fitted with the non-return valve. The flow control valve should be fitted as close to the working element as is possible and must be adjusted to match the requirements of the application. SYMBOL

EXPLANATION Flow control valve, adjustable One-way flow control valve

Figure 2.10: Flow Control Valves

b.3) Pressure control valves Pressure control valves are utilized in pneumatic systems. There are three main groups: • Pressure regulating valves without relief port • Pressure limiting valves with relief port • Pressure sequence valves The pressure regulating valve controls the operating pressure in a control circuit and keeps the pressure constant irrespective of any pressure fluctuations in the system. The pressure limiting valves are utilized on the up-stream side of the compressor to ensure the receiver pressure is limited, for safety, and that the supply pressure to the system is set to the correct pressure. The sequence valve senses the pressure of any external line and compares the pressure of the line against a preset adjustable value, creating a signal when the preset limit is reached.

SYMBOL

EXPLANATION Sequence valve-in line

Adjustable pressure regulating valve, relieving type Figure 2.11: Pressure Valve b.4) Auxiliary Symbols There are a number of important symbols for accessories which are utilized in conjunction with pneumatics. These include the exhaust air symbols, visual indicators and the methods of connection of components. SYMBOL

EXPLANATION

0 Bar

Pressure gauge

Silencer

Exhaust Plug 0 Bar

Visual indicator

Figure 2.12: Auxiliary Symbol c) Processing Elements As a processing element the directional control valve redirects, generates or cancels signals depending on the signal inputs received. The processing element can be supplemented with additional elements, such as the ANDfunction and OR-function valves to create the desired control conditions. The non-return valve allows a signal to flow through the device in one direction Non-return valves and in the other direction blocks the flow. There are many variations in construction and size derived from the basic non-return valve. Other derived valves utilize features of the non-return valve by the incorporation of non-return elements. The non-return valve can be found as an element of the one way flow control valve, quick exhaust valve. Shuttle valve and the two-pressure valve. SYMBOL

EXPLANATION Check valve

Shuttle valve

AND Valve

Quick exhaust valve

Figure 2.13: Non-return valves and derivatives (d) Actuator (Output) As a power element the directional control valve must deliver the required quantity of air to match the actuator requirements and hence there is a need for larger volume flow rates and therefore larger sizes. This may result in a larger supply port or manifold being used to deliver the air to the actuator. The actuator group includes various types of linear and rotary actuators of. The actuators are complemented by the final control element, which transfers the required quantity of air to drive the actuator. Normally this valve will be directly connected to the main air supply and fitted close to the actuator to minimize losses due to resistance. Actuators can be further broken down into groups: • Linear actuators: Single acting cylinder, Double acting cylinder. • Rotary actuators: Vane type, Air motors.

SYMBOL

EXPLANATION Single acting cylinder with spring return Double acting cylinder Double acting cylinder with double ended piston rod

Double acting 2 cushion cylinder

Air motor rotation in one direction

Rotary actuator

Figure 2.14: Actuators 2.2.5

Development of single actuator circuit

The simplest level of control for the single or double acting cylinder involves direct control signals. With this, the cylinder is actuated directly via a manually or mechanically actuated valve, without any intermediate switching of additional directional control valves. If the port sizes and the flow values of the valve are too large, the operating forces required may be too great for direct manual operation.

Example 1: Direct control of a single-acting cylinder A single acting cylinder of 25mm diameter is to advance a component when a push button is pressed. As long as the push button is activated the cylinder is to remain in the clamped position. If the push button released, the clamp is to retract.

Positional Sketch

Circuit Diagram

Example 2: Indirect control of a double acting cylinder A double acting cylinder is to extend when a push button is operated. Upon release of the push button the cylinder is to retract.

Circuit Diagram Example 3: The logic AND function The piston rod of a double acting cylinder is to advance when both push button of the 3/2 way valve is actuated. If either of these is released, then the cylinder is to return to the initial position.

Circuit Diagram

Example 4: The logic OR function A double-acting cylinder is to advance if one of two push buttons is operated. If the push button is then released, the cylinder is to retract.

Circuit Diagram

Example 5: Memory circuit and speed control of a cylinder The piston rod of a double acting cylinder is to advance when a 3/2 way push button valve is actuated manually. The cylinder is to remain advanced until a second valve is actuated. The signal of the second valve can only

take effect after the first valve has been released . The cylinder is to then return to the initial position. Then cylinder is to remain in the initial position until a new start signal is given. The speed of the cylinder is to be adjustable in both directions.

Circuit Diagram

2.2.6

Combinational valves

The combined functions of various elements can produce a new function. The new component can be constructed by the combination of individual elements or manufactured in a combined configuration to reduce size and complexity.

An example is the timer which is the combination of a one way flow control valve, a reservoir and a 3/2 way directional control valve.

Figure 2.15: Time delay valve Example 1: The timer delay valve A double-acting cylinder is used to press together glued component. Upon operation of a push button, the clamping cylinder extends. Once the fully advanced position is reached, the cylinder is to remain a time of T= 6 seconds and then immediately retract to the initial position. The cylinder retraction is to be adjustable. A new start cycle is only possible after the cylinder has fully retracted.

Circuit Diagram

2.2.7

Development of multiple actuators circuits

In case of multiple cylinder circuits, a clear definition of the problem is important. The representation of the desired motion of all actuators described using the displacement-step diagram. The special condition for the start of the sequence must also be defined. Example 1: Coordinated motion Two cylinders are used to transfer parts from a magazine onto a chute. When a push button is pressed, the first cylinder extends, pushing part from the magazine and positions it in preparation for transfer by the second cylinder onto the out feed chute. Once the part is transfer, the first cylinder retracts, followed by the second. Confirmation of all extended and retracted positions are required.

Displacement Step Diagram

Circuit Diagram 2.3

Equipments

The important elements of pneumatic equipment: • Laboratory trolley or fixed workbench with drawers. • Compressor • Set of devices or individual components (e.g. cylinders, directional control valves, logic elements, linear drive, pneumatic switch) Table 1.0 shown the list of equipment which will be used for assembly at the pneumatic trainer. Table 1.0: List of Equipments Example 2.2.5 Exp. 1 2.2.5 Exp. 2

2.2.5 Exp. 3

2.2.5 Exp. 4 2.2.5 Exp. 5

Energy Compressed air production / Distribution Compressed air production / Distribution Compressed air production / Distribution Compressed air production / Distribution Compressed air production / Distribution

Signal Input Switch / Push button etc. Switch / Push button etc.

Processing Elements

Actuators

-

3/2way DCV

Single acting cylinder

-

5/2way DCV

Double acting cylinder

5/2way DCV

Double acting cylinder

5/2way DCV

Double acting cylinder

5/2way DCV Flow

Double acting cylinder

Dual Switch / Pressure Push button valve Limit switch (AND) Switch / Shuttle Push button valve(OR) Switch / Push button

Control Elements

-

2.2.6 Exp. 1

Compressed air production / Distribution

2.2.6 Exp. 1

Compressed air production / Distribution

2.4

Dual Pressure valve (AND) -

5/2way DCV

Double acting cylinder Double acting cylinder

Working Procedure

•

All components are to be fully plugged in the assembly board

•

Limit switches should be placed so that they contact only the side of the trip cam never at the front. Permissible operating pressure should not be exceeded Pneumatics circuits are to be constructed with plastic tubes. Fully insert the plastic tubes into the plug-in coupling or quick push connectors and pull the locking ring over the tube connection Before the circuit is disconnected turn off the pressure supply In order to avoid damage to the locating pegs carefully unplug the components from the assembly board by pulling them upwards.

• • • • 2.5

Switch / Push button Limit switch Time delay valve Switch / Push button Limit switch

control valve 5/2way DCV Flow control valve

Tasks

a) A pneumatic press is to be operated by two push buttons PB1 and PB2. In order to meet safety requirements, both of the push buttons must be pressed together to start the press. The pneumatic press is retracted immediately when one or both push buttons are released. Draw the circuit diagram for the problem. Designate the valves and indicate the numbering system for the connections.

a.1) a single acting cylinder is to be used by 3/2way pilot valve. a.2) a single acting cylinder is to be used and connected in series. a.3) a double acting cylinder is to be used with used ‘and gate’. a.4) a double acting cylinder is to be used by 3/2way pilot valve. b) Operation of two identical valves by two push buttons causes the forming tool of an edge folding device to thrust downwards and fold over the edge of a flat sheet of sectional area. If both or even just one push button is released, double acting cylinder slowly returns to the initial position.

b.1) a double pilot valve should be fitted for the control of the cylinder. If the cylinder is to retract on reaching its fully extended position, roller lever valves should be used as limit valves to confirm that this position has been reached. b.2) One of the push buttons need only be operated for a short duration and the cylinder will fully extend, since the effect of the signal at input at the 5/2way double pilot valve is maintained until a signal is applied. As soon as the piston rod has reached the forward end position, the limit switch generated a signal to 5/2way double pilot valve and the valve is reversed. The retracted end position of the piston rod can also be sense. These require an additional limit switch.

c) A double acting cylinder is used to press together glued components. Upon operation of a push button, the clamping cylinder slowly advances. Once the fully extended position is reached, the cylinder is to remain for a time of 6 sec. and then immediately retract to the initial position. A new start cycle is only possible after the cylinder has fully retracted and after a delay of 5 sec. During the delay the finished part is manually removed and replaced with new parts for gluing. The retracting speed is to be rapid, but adjustable. Draw the displacement step diagram and circuit diagram for the problem. Designate the valves and indicate the numbering system for the connections.

d) Turned parts for spark plugs are fed in pairs on a rail to multi spindle machining stations. In order to achieve separation, two double acting cylinders are triggered by one actuator in alternating push-pull rhythm. In the initial position, the upper cylinder is retracted, the lower cylinder in the forward position. Turned parts are resting against the second cylinder. A starting signal causes cylinder A to advance and cylinder B to retract. Two sparking plug blanks roll onto the machining station. After and adjustable time of 1 sec, cylinder A returns and cylinder B advances at the same time. A further cycle can be started only when time interval 2 sec. has elapsed. Draw the displacement step diagram and circuit diagram for the problem. Designate the valves and indicate the numbering system for the connections.