PNEUMATIC PNEUMA TIC CIRCU CIRCUITS ITS
A J AY AY C H A C K O PR11ME1002
PNEUMATICS Pneumatics system uses pressurised air to transmit and control power. (or) Pneumatics is a type of power transmission that uses a gas and pressure differential differe ntial to create movement. Commonly Air is used as fluid which is freely and safely available.
COMPONE COMP ONENTS NTS OF A PNEUMATIC PNEUMATIC SYSTEM •
•
•
•
•
•
•
Compressor Regulators and Gauges Check Valve Buffer Tank or Accumulator Feed Lines Directional Valves Actuators
Compressor A compressor is a pump powered by an electric motor. This pump compresses the air to a higher Pressure. Compressors have a tank to store the air. As a rule pneumatic components are designed for a maximum operating pressure of 8-10 bar but in practice it is recommended to operate at between 5-6 bar. Due to the pressure losses in the system the compressor should deliver between 6.5-7 bar. There are mainly 4 types of compressor 1) Centrifugal compressor 2) Rotary vane compressor 3) Rotary screw compressor 4) Reciprocating air compressor •
•
Pressure Regulator
SYMBOLIC REPRESENTATION
Pressure Gauge 0-150 psi 0-1 MPa
Air Flow Directional Arrow
Controlled Pressure Side
Pressure Adjusting Knob *Pull out before turning *Push in to lock *Turn Clockwise to Raise Pressure.
High Pressure Inlet Lower Pressure Settings Means Less Air Consumption
FRL UNIT (FILTER REGULATOR LUBRICATOR) Sometimes in a pneumatic system there is a need to filter the air, monitor the air pressure or to lubricate the air for smooth running of the system. For this purpose FRL unit is used. SYMBOLIC REPRESENTATION
Check Valve Check valves are one-way valves that are installed to the hose connecting the compressor or compressor tank to the buffer tank. They allow the compressed air to accumulate in the buffer tanks, but do not allow backflow into the compressor or compressor tank. •
SYMBOLIC REPRESENTATION
Buffer Tank or Accumulator Buffer tanks are secondary storage units for the compressed air originating from the compressor. They store the high-Pressure compressed air for eventual use with the pneumatic actuators. These tanks help to prevent uneven airflow surges in the actuators; allow the compressor cycle to maximize its shutoff timing; and allow the compressor to be kept at far place from the actuators. •
•
Feed Lines
Feed lines are hoses that transfer pressurized air through the pneumatic system. Large diameter hoses allow the pressurized air to travel quickly, eliminating airflow backups. The pipe diameter of the air distribution system should be selected in such a way that the pressure loss from the pressurised reservoir to the consuming device ideally does not exceed approx. 10 kPa (0.1 bar). The selection of the pipe diameter is governed by: 1. Flow rate 2. Line length 3. Permissible pressure loss
4. Operating pressure 5. Number of flow control points in the line
Directional Valves Directional valves are placed before actuators. Multiple-valve systems are installed on projects with multiple actuators to power. Directional valves receive input from mechanical or electrical control sources. They re-direct, stop or release the pressurized air to its appropriate actuators at the times desired. Directional valves can be triggered by the action of a button, spring, lever, pedal, solenoid or other device. The different types of DVC are spool type, poppet type, seat type etc. •
Spool Valve: A spool moves horizontally with in the valve body to control the flow of fluid.
POSITIONS OF A 5/2 DCV
A Solenoid Controlled 5 Ported, 4 Way 2 Position Valve
WORKING OF A SINGLE ACTING AND DOUBLE ACTING CYLINDER USING DCV SINGLE ACTING CYLINDER
DOUBLE ACTING CYLINDER
DIRECTIONAL CONTROL VALVES SYMBOL DEVELOPMENT •
•
•
•
•
Valve switching positions are represented as squares The number of squares shows how many switching positions the valve has Lines 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 & outlet ports) are shown by lines on the outside of the box
LIMIT SWITCHES •
Limit switches or valves are a type of position sensors. Limit valves uses a roller-lever actuator. This types of valves are available as a 3-way or a 5-way valve. This type of valve are normally actuated by a cylinder piston rod at the ends or limits of its extension of retraction stroke through physical contact. 3/2 valve
VALVE SYMBOL STRUCTURE
VALVE SYMBOL STRUCTURE •
•
•
The function of a valve is given by a pair of numerals separated by a stroke, e.g. 3/2.. The first numeral indicates the number of main ports. These are inlets, outlets and exhausts but excludes signal ports and external pilot feeds. The second numeral indicates the number of states the valve can achieve.
VALVE SYMBOL STRUCTURE •
•
A 3/2 valve therefore has 3 ports (normally these are inlet, outlet and exhaust) and 2 states (the normal state and the operated state) The boxes are two pictures of the same valve operated
normal
VALVE SYMBOL STRUCTURE •
A valve symbol shows the pictures for each of the valve states joined end to end
operated
normal
VALVE SYMBOL STRUCTURE •
A valve symbol shows the pictures for each of the valve states joined end to end
operated
normal
VALVE SYMBOL STRUCTURE •
The port connections are shown to only one of the diagrams to indicate the prevailing state
normal
VALVE SYMBOL STRUCTURE •
The operator for a particular state is illustrated against that state
Operated state produced by pushing a button
VALVE SYMBOL STRUCTURE •
The operator for a particular state is illustrated against that state
Operated state produced by pushing a button
Normal state produced by a spring
VALVE SYMBOL STRUCTURE •
The operator for a particular state is illustrated against that state
Operated state produced by pushing a button
Normal state produced by a spring
VALVE SYMBOL STRUCTURE •
The valve symbol can be visualised as moving to align one state or another with the port connections
VALVE SYMBOL STRUCTURE •
The valve symbol can be visualised as moving to align one state or another with the port connections
VALVE SYMBOL STRUCTURE •
The valve symbol can be visualised as moving to align one state or another with the port connections
VALVE SYMBOL STRUCTURE •
A 5/2 valve symbol is constructed in a similar way. A picture of the valve flow paths for each of the two states is shown by the two boxes. The 5 ports are normally an inlet, 2 outlets and 2 exhausts
VALVE SYMBOL STRUCTURE •
The full symbol is then made by joining the two boxes and adding operators. The connections are shown against only the prevailing state
VALVE SYMBOL STRUCTURE •
The full symbol is then made by joining the two boxes and adding operators. The connections are shown against only the prevailing state
VALVE SYMBOL STRUCTURE •
The full symbol is then made by joining the two boxes and adding operators. The connections are shown against only the prevailing state
VALVE SYMBOL STRUCTURE •
•
The boxes can be joined at either end but the operator must be drawn against the state that it produces. The boxes can also be flipped A variety of symbol patterns are possible normally closed
normally open
VALVE SYMBOL STRUCTURE •
•
The boxes can be joined at either end but the operator must be drawn against the state that it produces. The boxes can also be flipped A variety of symbol patterns can be produced Reverse connected
VALVE FUNCTIONS
VALVE FUNCTIONS Basic valves before operators are added
Examples, push button operated with spring return Normal position
Function 3/2
Function 5/2
Operated position
OPERATORS
OPERATORS Manual General manual
Lever
Push button
Pedal
Pull button
Treadle
Push/pull button
Rotary knob
OPERATORS Mechanical
Plunger
Pressure
Spring normally as a return
Pilot pressure
Roller
Differential pressure
Uni-direction or one way trip
Detent in 3 positions
OPERATORS Electrical Solenoid direct Solenoid pilot
Solenoid pilot with manual override and integral pilot supply
Solenoid pilot with manual override and external pilot supply
When no integral or external pilot supply is shown it is assumed to be integral
PORT MARKINGS
PORT MARKINGS
2
Push Button 3/2 Valve Spring Return 3
1
4
2
Push Button 5/2 Valve Spring Return 1 5
3
Actuators An actuator is the component in a pneumatic system that does the work. There are numerous types of actuators, powered by pressurized air. Plunge and cylinder actuators are used frequently. The pressurized air is released into the cylinder to move a piston forward as the air is forced into the chamber. Examples of actuators at work are air-powered tools in construction and dentistry. Types of Actuator
Actuators may be grouped in a number of ways: Electromechanical - Electromagnetic - The device has one coil which provides the field energy and the energy to be transformed. The attractive force is unidirectional so a return device of some type is needed, often a spring. Relays or solenoids are used to switch - fans, head lights, horn, wipers. - Electrodynamic - based on the (Lorenz) force generated when a current carrying conductor (coil) is held in a magnetic field. - DC motors Fluid mechanical - pneumatic. A common device is the pneumatic cylinder - hydraulic. A common device is the hydraulic cylinder •
•
The pneumatic cylinder has a significant role as a linear drive unit, due to its 1. relatively low cost, 2. ease of installation, 3. simple and robust construction and 4. ready availability in various sizes and stroke lengths. The pneumatic cylinder has the following general characteristics: 1. Diameters 2.5 to 320 mm 2. Stroke lengths 1 to 2000 mm 3. Available forces 2 to 45000 N at 6 bar 4. Piston speed 0.1 to 1.5 m/s 5. Single acting with return spring or Double acting
IMPORTANCE/ADVANTAGES OF PNEUMATIC CIRCUIT •
•
•
•
•
Pneumatics can be used for low cost industrial applications – simple or complex. Welding and fabricating, press tools, automatic machine tools, assembling, material handling, etc. Work function and control function is obtained by the same power medium throughout the system Pneumatic circuits can be designed using number of methods. Easy to expand once installed
DISADVANTAGES •
Initial weight and cost is high
•
Requires fine tuning for optimum use
•
Limited uses with larger actuators
•
It produces noise due to the exhaust of air from the system but can be eliminated by using mufflers
CONSTRUCTION OF PNEUMATIC CIRCUITS
SIMPLE PNEUMATIC CIRCUIT PARTS OF PNEUMATIC SYSTE •
Compressor
•
FRL Unit
•
Direction control valve
•
Actuator
•
Cylinder
SPEED CONTROL PNEUMATIC CIRCUIT •
The motion of a double acting cylinder can be controlled by using flow control valves.
TIME DELAY CIRCUIT
In certain engineering applications, a predetermined time delay may be required to actuate a material handling platform. At the end of the extension stroke, the platform has to wait for a predetermined time for material removal and then it has to return back.
PNEUMATICS CIRCUIT DESIGN METHODS USED BY ENGINEERS 1. Classic method – the method is based on the knowledge of pneumatic logic elements and application of the logics. (AND, NAND, OR, NOR, etc.) 2. Cascade method – the method is simple to apply and results in reliable and easily understood circuits. 3. Step-Counter method – A step counter is a digital modular counter, constructed from stepping units which serve as sequence steppers for asynchronous sequential fluid power controls 4. Logic design method (with Karnaugh- Vetch Maps) – a graphical design method using Karnaugh – Vetch maps is used to simplify intricate control problems and it produces the simplest possible result. 5. Combinational circuit design – combinational circuit output signals depend solely on the momentary state of their input signals or input variables. Its design tools are truth tables and Karnaugh Vetch maps.
CLASSIC METHOD •
•
Each pneumatic element is a logic element performing various logic functions like AND, NAND, OR, NOR etc. The principle of Boolean algebra and De Morgan theorem are employed to solve problems in the analysis of control logics
THE FORWARD MOTION OF A SINGLE ACTING CYLINDER COULD BE OPERATED FROM EITHER TWO LOCATIONS • •
•
The pre condition is that the cylinder should operated from any of the two sources. Therefore A & B are signalling Elements and Y is the output. As per precondition output signal is at Y so either A is NOT AND & B is YES OR or A is YES AND B IS NOT. Then the control equation can be written as A . B + A . B = Y. The logic diagram for this equation is as below
And type/ 2-pressure valve
CASCADE METHOD TO DESIGN PNEUMATIC CIRCUIT OF A MATERIAL HANDLING EQUIPMENT •
•
•
•
Designing a pneumatic circuit for material handling using 3 cylinders. Step 1 – Extension and Retraction of the system is denoted by + & - signs, therefore A+, A-, B+, B-, C+ & CStep 2 – the sequence should be split in minimum no. of groups without repeating. So C-,A+,B+ & B-,A-,C+ Step 3 – Number of pressure lines are equal to the number of groups, so 2. number of group valve is number of groups minus one, so 1 group valve. Cylinder actuating valve are equal to the no. of cylinders.
ARRANGE THE PARTS OF PNEUMATIC CIRCUIT AS SHOWN
THE GROUP VALVE IS IN ITS LEFT-HAND POSITION, GROUP 1 IS PRESSURISED. LINE 1 IS CONNECTED DIRECTLY TO THE PILOT LINE (-) OF CYLINDER ACTUATING VALVE VC
CYLINDER C RETRACTS AND THE LIMIT VALVE C0 IS ACTUATED AND THE PRESSURE PASSES FROM MANIFOLD LINE 1 THROUGH C0 TO THE PILOT LINE(+) OF CYLINDER ACTUATING VALVE VA
CYLIND ER A EXTENDS CYLINDER EXTEN DS AND ACTUA A CTUATES TES LIMIT VAL ALVE VE A1 AND AN D PRESSURE PASSES PASSES FROM LINE 1 THROUGH A1 TO THE PILOT LINE (+) OF THE CYLINDER VALVE VB.
B EXTENDS EX TENDS AND AN D ACTUATES ACTUATES LIMIT VAL ALVE VE B1. AS B+ IS THE TH E LAST STEP IN GROUP 1 NOW THE CONNECTION IS SHIFTED BY CONNECTING TO THE PILOT LINE II OF THE GROUP VALVE
LINE II IS CONNECTED TO THE (-) OF CYLINDER ACTUATING VALVE VB, SO B RETRACTS AND VALVE B0 IS ACTUATED
PRESSURE PASSES THROUGH THE MANIFOLD LINE II TO THE PILOT LINE (-) OF THE CYLINDER ACTUATING VALVE VA, SO A RETRACTS AND ACTUATES LIMIT VALVE A0
THE PRESSURE PASSES FROM LINE II THROUGH A0 TO PILOT LINE (+) OF CYLINDER ACTUATING VALVE VC, SO THE CYLINDER C EXTENDS AND ACTUATES LIMIT VALVE C1 AND THE PROCESS CONTINUES TILL THE SUPPLY IS STOPPED
CASCADE CIRCUIT
MATERIAL HANDLING EQUIPMENT
PNEUMATIC CIRCUIT FOR THE ABOVE MATERIAL HANDLING EQUIPMENT USING CASCSDE METHOD