PneumaticsPneumatics

Control Valve Symbols

2/2 Way Directional Control Valve (Flow

Switch)

• The ports are indicated (on the initial status) :-

Output Port ( Top )Inlet Port ( Bottom )

• Flow is indicated by an arrow ( No flow by lines at right angles )

• For every control valve status a square is drawn

Actuation of Control Valves :-1. Mechanical

• General

• Pushbutton

• Lever Operated

• Foot Pedal

• Spring Return

• Spring Centered

• Roller

• Idle Return Roller

Actuation of Control Valves :-2. Electrical / Pneumatic

• Direct Pneumatic

• Indirect Pneumatic

• Pressure Release

• Single Solenoid

• Double Solenoid

• Electro-Pneumatic

Describe this Control Valve ?

4/2 Way Directional Pushbutton Control Valve

with Spring Return

• The control valve has four ports

• The control valve is operated by a Pushbutton and returned by a Spring

• The control valve has two positions

• The control valve changes the flow direction at the output ports

Linear Actuators

• Single Acting Cylinder

• Double Acting Cylinder

• Double Acting Cylinder with double ended piston

• Double Acting Cylinders with non-adjustable and adjustable cushioning on one or both ends

3/2 Pneumatic Control Valve

2Actuate Spring Return

Actuate Spring Return2

3/2 Pneumatic Control Valve driving a Linear Actuator

What Happens if the Pressure Supply is lost ?

Open 22

• Non-Actuated • Actuated

Open2 2

5/2 Pneumatic Control Valve

4 2Actuate Spring Return

4 2Actuate Spring Return

5/2 Pneumatic Control Valve driving a Linear Actuator

What Happens if the Pressure Supply is lost ?

• Actuated

4 2Open

• Non-Actuated

4 2Open

Cascade Valves for higher flows

How does this improve cylinder flow rates ?

4 2

2 2

Flow Control Elements - 1

• Check Valve

• Spring Loaded Check Valve

• Quick Exhaust Valve

• Adjustable Flow Control

• One-Way Flow Control

Improved Speed of Actuator Return

How does this improve cylinder performance ?

2

Actuator Flow Rate Control

What is restricted by each flow valve ?

4 2

Flow Control Elements - 2

A B Z

0 0 0 0 1 1 1 0 1 1 1 1

A B Z

0 0 0 0 1 0 1 0 0 1 1 1

A

Z

B

Two Pressure Valve (AND Gate)

Shuttle Valve (Or Gate )

A

Z

B

Logical Switching Operation

What Happens if the Pressure Supply is lost ?

Flow Control Elements - 3

Sequence Valve

Time Delay Valve

Timing Actuator Operation

Pneumatic Switches to indicate Actuator Position

How does this change the circuit operation ?

Roller Limit

Switch

Design Exercise

1 Extend the previous circuit so that the actuator holds for the time delays in continuous cyclic operation

2 Change the above circuit so that it cycles continuously without delay

Design Exercise Solutions

To Remove delays simply remove delay units ?

A+A-

A+

A-

Circuit Diagram

Sequential OperationFrom the previous Circuit note :

• If the sequence is to commence on a start signal a manual start valve is needed

• An Actuator needs an individual pneumatically driven control valve

• Each identified sequence position needs a switch driven valve

• Connections between valves programme the desired sequence, i.e.

A- A+a-

a+A+ Actuator direct.a+ Valve

Where :-

Two Actuator Operation

For the following sequence

then we require :• a start valve, and• two cylinders driven by pneumatically operated 5/2 control

valves, and• four switch driven 3/2 directional valves

A+

B+a+ b+

A-a-

B-b-Start

Solution :

A+

a+

a-

A-

A-A+

B+B-

B-B+

b-

b+

Start

Actuator ‘Trapping’Consider the following sequence :

• All is well until we try to retract actuator B via b+ as it will still be actuated by a+ and hence remain in the initial activated position.

• A conflict for actuator A occurs for b- and a-

• This ‘trapping’ always occurs when a sequence calls for a reversing of an actuator without any intervening motion of another actuator.

A+

B+a+ b+

B-b-

A-a-Start

Solution : Isolate Pressure SourcesA+

a+b-

A-

A-A+

B+B-

B-B+

a- b+

Start

Electro-Pneumatics

Advantages

• Electrical transmission time quicker

• Programming Flexibility

• Greater Sensor availability

Disadvantages

• small actuation forces

• physically large and power consuming for large force applications

How do solenoid actuators work ?

• An electrical conductor creates a magnetic field around it whose density ( B ) is proportional to the size of the current ( I ) passing through it

• The greater the number of conductor turns ( N ) the stronger the magnetic field

• A former ( r ) is used to align and concentrate the resulting magnetic field

Ampere’s Law : NIB r0

Electrical Symbols 1 :- Switches • Normally Open

• Normally Closed

• Mechanically Linked

• General manual

• Press activated

• Pull activated

• Rotate activated

• Roller activated

Electrical Symbols 2 :- Indicators and Instruments

• Horn

• Lamp

• Audio

• LED

• General Instrument

• Ammeter

• Voltmeter

A

V

Electrical Circuit Diagrams

Signal Flow

+24V

A

0V

Energy Supplies

Input Elements

Processing and Final Control

Elements

Logical Switching Circuits

NOT Gate

AND Gate

OR GateA B Z

0 0 0 0 1 1 1 0 1 1 1 1

A B Z

0 0 0 0 1 0 1 0 0 1 1 1

A Z

0 1 1 0

A

A

A B

B

Problem Exercises

1 Design and build an electric circuit that will activate a single acting linear actuator and an indicator light whenever a switch is operated.

2 Repeat the above but using a double acting linear actuator and a spring return valve

3 Repeat 2 above but use a double solenoid activated valve

Solutions :

Y1

Start

+24V

0V

Y1

Exercise 1

Y1

4 2

Exercise 2

?Y1

4 2

Exercise 3

Design Exercise 1Comment upon possible problems with the following circuits :

0V

Start

+24V

0V

Y1 Y2

Circuit 1 Circuit 2

Y1

Start

+24V

Design Exercise 2

For failsafe operation spring return valves/actuators are often used.

Design an electro-pneumatic circuit to extend an actuator on application of a manual set switch and hold it extended via an electrical limit switch. Retracting the actuator only on application of a reset switch.

Exercise 2 Solution - Part 1

Open when retracted

Y1

OUT

Set

+24V

0V

Reset

OUT

Y1

What Happens if the Set switch is released prior to the actuator becoming fully extended ?

Exercise 2 Solution - Part 2

Set

+24V

0V

Reset

IN

Y1 Y1

IN

Open when retracted

To overcome this problem then move the limit switch to the retract position and use the normally closed contacts, i.e :

Relays

Electromagnetic devices which use a solenoid to control a number of contacts mechanically. They offer the following advantages :-

• Switch large current systems ( via contacts ) from small currents ( to coil )

• Switch multiple contacts

• Provide ‘Memory’

Electrical Symbols 3 :- Relays

• Electro-mechanical

• Delay energised

• Delay de-energised

• Single active winding

• AC relay

• Thermal relay

• Indicating DC resistance

• Driving a Shut-off valve

500

How may we overcome the memory of the electro-pneumatic valve seen

in exercise 3 previously ?

Y2Y1

4 2K1

Start

+24V

0V

Y1 Y2

K1K1K1

Relay Latching for ‘Memory’

Set Dominant Reset Dominant

K1

Start

+24V

0V

Reset

K1 K1

K1

Start

+24V

0V

Reset

K1

K1

‘Failsafe’ Latching Design Exercise

For failsafe operation spring return valves/actuators are often used.

Using a relay design an electro-pneumatic circuit to extend and hold extended an actuator on application of a manual set switch, and retract the actuator on application of a reset switch.

‘Failsafe’ Latching Solution

Y1K1

Set

+24V

0V

Reset

K1

K1

Y1