Pneumatics

2/2 Way Directional Control Valve (Flow Switch) Control Valve Symbols For every control valve status a square is drawn The ports are indicated (on the initial status) :- Output Port ( Top ) Inlet Port ( Bottom ) 2/2 Way Directional Control Valve (Flow Switch) Flow is indicated by an arrow ( No flow by lines at right angles )

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 ? The control valve has two positions The control valve has four ports The control valve is operated by a Pushbutton and returned by a Spring 4/2 Way Directional Pushbutton Control Valve with Spring Return 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 Actuate Spring Return Actuate Spring Return 2

3/2 Pneumatic Control Valve driving a Linear Actuator Open 2 Non-Actuated Actuated Open 2 What Happens if the Pressure Supply is lost ?

5/2 Pneumatic Control Valve 4 2 Actuate Spring Return 4 2 Actuate Spring Return

5/2 Pneumatic Control Valve driving a Linear Actuator Non-Actuated 4 2 Open Actuated 4 2 Open What Happens if the Pressure Supply is lost ?

Cascade Valves for higher flows 4 2 2 How does this improve cylinder flow rates ?

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 2 How does this improve cylinder performance ?

Actuator Flow Rate Control 4 2 What is restricted by each flow valve ?

Flow Control Elements - 2 Shuttle Valve (Or Gate ) A Z B A B Z 0 0 0 0 1 1 1 0 1 1 1 1 A Z B Two Pressure Valve (AND Gate) A B Z 0 0 0 0 1 0 1 0 0 1 1 1

Logical Switching Operation What Happens if the Pressure Supply is lost ?

Flow Control Elements - 3 Time Delay Valve Sequence Valve

Timing Actuator Operation

Pneumatic Switches to indicate Actuator Position Roller Limit Switch How does this change the circuit operation ?

Design Exercise Extend the previous circuit so that the actuator holds for the time delays in continuous cyclic operation Change the above circuit so that it cycles continuously without delay

Design Exercise Solutions A+ A- Circuit Diagram To Remove delays simply remove delay units ?

Sequential Operation From 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- 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 Sources b- A- 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 ( mr ) is used to align and concentrate the resulting magnetic field Ampere’s Law :

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 Gate A B Z 0 0 0 0 1 1 1 0 1 1 1 1 0 0 0 0 1 0 1 0 0 1 1 1 A Z 0 1 1 0 A B

Problem Exercises Design and build an electric circuit that will activate a single acting linear actuator and an indicator light whenever a switch is operated. Repeat the above but using a double acting linear actuator and a spring return valve Repeat 2 above but use a double solenoid activated valve

Solutions : Y1 Exercise 1 4 2 Exercise 2 ? Exercise 3 Y1 Start +24V 0V

Design Exercise 1 Comment upon possible problems with the following circuits : Circuit 1 Circuit 2 Start +24V 0V Y1 Y2 Y1 Start +24V 0V

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 Y1 OUT Set +24V 0V Reset Open when retracted What Happens if the Set switch is released prior to the actuator becoming fully extended ?

Exercise 2 Solution - Part 2 To overcome this problem then move the limit switch to the retract position and use the normally closed contacts, i.e : Set +24V 0V Reset IN Y1 Open when retracted

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 ? K1 Start +24V 0V Y1 Y2 Y2 Y1 4 2

Relay Latching for ‘Memory’ K1 Start +24V 0V Reset Set Dominant Reset Dominant

‘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 Y1 K1 Set +24V 0V Reset