me 515 mechatronics - university of...

ME 515 Mechatronics 11/28/2006

1

ME 515 Mechatronics

Introduction to Pneumatic and Hydraulic Actuators

Asanga RatnaweeraDepartment of Mechanical Engineering

Faculty of Engineering University of Peradeniya

Tel: 081239 (3627)Email: [email protected]

28 November 2006 Asanga Ratnaweera, Department of Mechanical Engineering

2

Hydraulics and Pneumatics

Pumps, valves, rods,

fluidic switches

switchboard ("Ventilinsel")


2


3

Pneumatic ActuatorsPneumatics – Systems using air or other gasses.

Hydraulics – Systems involving liquids.

Pneumatic Actuators – applies force or creates motion through the use of air pressure.No Leaking problemsFaster than hydraulicsMain drawback is the compressibility of airOperating pressure: 500 kPa – 1MPa


4

Hydraulic Actuators

linear movementbig forces without gearsactuators are simplein mobile machinesBad efficiencymotor, pump, actuator combination is lighter than motor, generator, battery, motor & gear combination Operating pressure 7 – 20 MPa


3


5

accumulator

Hydraulic Power Supply

Absorbs shocksStores energyMaintains constant pressure

Gear pump


6

Pneumatic Power Supply


4


7

Cylinders

There are two basic types of cylinders:Single acting cylinderDouble acting cylinder


8

CylindersSingle acting cylinder

used when the control pressure is applied to one side of the piston, a spring often being used to provide the return movement of the piston.

Double acting cylinderused when the control pressures are applied to each side of the piston. A difference in pressure between the two sides then results in motion of the piston.


5


9

Directional Control ValvesSpool valves Poppet valves


10

Valve SymbolsThe symbols used for control valves consist of a square for each of its switching positions.

Arrow-headed lines are used to indicate the directions of flow in each of the positions, with blocked-off lines closed flow lines

Two ports four ports1 1

2 2

1 2

3 4


6


11

Valve SymbolsEx:

Flow path closedPort 1

Port 2

1

2

Position 1

1

2

Position 2


12

Valve SymbolsTherefore, for two port (two way) two position valve can be represented in symbolic form as shown below:

2/2 valve


7


13

Valve SymbolsThree way valves

A three-way directional valve consists of three ports connected through passages within a valve body that are shown here as port A, port P and port Ex.

3/2 valve


14

Valve SymbolsSeveral methods are used to actuate valves.

plunger

spring

roller

push button

lever

pedal

solenoid

pilot valve


8


15

Valve SymbolsTherefore, complete symbolic representation of valves are:

Poppet valve

2/2 valvePush button operated spring return valve

lever operated 3/2 valve

Solenoid operated spring return 3/2 valve


16

Valve Symbols: ExampleEx: applications of 2/2 valves to move a load up and down


9


17

Valve Symbols

Solenoid operated spring return 4/2 valve

pilot operated spring return 4/2 valve


18

Valve Symbols: ExampleTime delay circuit


10


19

Valve Symbols: ExampleThe operation of the time-delayed pneumatic circuit is:

When the push-button on the three-port valve on the left is pressed, the air flow is restricted by the unidirectional-flow control valve and slowly enters the reservoir.The pressure builds in the reservoir slowly, causing the time delay.Only when the pressure in the reservoir is high enough will it operate the five-point valve, triggering the release of air and causing the piston to outstroke.When the push button is pressed on the second three-port valve (on the right), the five-point valve will switch, triggering the release of air and causing the piston to instroke.


20

Directional ValvesCheck Valve

Free flow can only occur in one direction through the valve, that which results in the ball being pressed against the spring.


11


21

Directional ValvesShuttle Valves

The inlet port connected to the higher pressure is automatically connected to the outlet port while the other inlet port is closed.


22

Pressure Control Valvespressure sequence valves

used to sense the pressure of an external line and give a signal when it reaches some preset valve

Under normal conditionsWhen working pressure is more than the preset value

Symbolic representation


12


23

Pressure Control Valvespressure limiting valves

used as safety devices to limit the pressure in a circuit to below some safe


24

Pressure Control Valvespressure regulating valves

used to control the operating pressure in a circuit and maintain it at a constant valve

Speed of motion of the piston can be adjusted


13


25

Pressure Control Valvespressure regulating valves: symbolic representation


26

Some Useful SymbolsActuator Valve

2 3

1

3 4

1 2

5


14


27

Some Useful Symbols


28

Some Useful Symbols


15


29

Valve ArrangementsOR control

Valve A Valve B

Shuttle valve


30

Some Applications


16


31

Valve Arrangements

Valve A Valve B

AND control


32

Pneumatic component arrangement


17


33

Some ApplicationsDouble acting cylinder with five port valve


34

Some ApplicationsDouble acting cylinder with five port valve

When the piston is retracted (completes the instroke), the piston operates the roller-trip control, opening the first three-port valve (in the centre of the diagram).The air output from the three-port valve operates the five-port valve (these are connected by the dashed line), releasing air and causing the piston to outstroke. When the piston is fully extended, it operates the push-button control, causing the second three-port valve (to the right of the diagram) to switch. The air released by the three-port valve operates the five-port valve (these are connected by the dashed line).The air released by this section of the five-port valve causes the piston to retract. The speed of the instroke is controlled by the unidirectional-flow control valve.When the piston on the double-acting cylinder is retracted (completes the instroke), the piston operates the roller-trip control and the cycle is then repeated.


18


35

Choice of CylindersThe choice of cylinder is determined by:

the force required to move the load andthe speed required.

Hydraulic cylinders are capable of much larger forces than pneumatic cylinders. However, pneumatic cylinders are capable of greater speeds. The force produced by a cylinder is:

Force = A x PA - cross-sectional area of the cylinder P - working pneumatic pressure;

Effective area (A) = piston area – piston rod area


36

Piston SpeedIf the volume flow rate of hydraulic liquid into a cylinder is Q m3/s, and the velocity of the piston of cross-sectional area A m2 is v m/s then:

Q=AvHowever, the speed of a pneumatic cylinder cannot be calculated in this way

The speed depends on the rate at which air can be vented ahead of the advancing piston.

A valve to adjust this can be used to regulate the speed.


19


37

Control Valve SizingThe equation relating the rate of flow of liquid Q through a wide open valve to its size is :

ρ∆PAQ v=

Av - the valve flow coefficient∆P - the pressure drop across the valve and Ρ - the density of the fluid.

This equation is sometimes written, with the quantities in SI units, as Where Cv is the valve flow coefficient.

ρPCQ v

∆×= −51037.2


38

Control Valve SizingAlternatively it may be found written as shown next.

GPCQ v

∆×= −61075.0

G - the specific gravity or relative density.


20


39

Control Valve SizingSome typical values of Av, Cv and valve size.

47426117811971523319Avx10-5

20011075503022148Cv

2560192016001260960800640480

Valve size (mm)Flow coefficient


40

Cylinder SequencingEx:

Determine the size of a valve that is required to control the flow of water when the maximum flow required is 0.012 m3/s and the permissible pressure drop across the valve at this flow rate 300 kPa.

3v 103001000012.0A×

=

ρ∆PAQ v= 5

v 103.69A −×=

From the table above

Valve size is 960 mm


21


41

Rotary Actuators

Vane Pump Inline axial piston pump


42

Rotary Actuators


22


43

Advantages of Pneumatic System

CleanPneumatic systems are clean because they use compressed air. If a pneumatic system develops a leak, it will be air that escapes and not oil.

SafePneumatic systems are very safe compared to other systems. We cannot, for example, use electronics for paint spraying because many electronic components produce sparks.

ReliablePneumatic systems are very reliable and can keep working for a long time.


44

Advantages of Pneumatic SystemEconomical

If we compare pneumatic systems to other systems, we find that they are cheaper to run. This is because the components last for a long time.

FlexibleOnce you have bought the basic components, you can set them up to carry out different tasks


23


45

Pneumatic Actuators: Main feature

Simple!Easy to useDurableVersatileLight weightEasy to Control


46

Pneumatics actuators: When to useLow cost and easy to installClean and easy to maintainLow power densitiesOnly on/off or inaccurate control necessary

me 515 mechatronics - university of...

Documents