Unit outcome 5.2 Pneu Speed Control and various functional valves W2020 PDF

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Speed control methods for pneumatics

Unit Outcome 5.2

 Main factors influence speed - Air flow rate - Resistance

Pneumatic Speed Control, Logic Control Valves various functional valves and components

When the resistance is constant, the maximum speed of an air cylinder is determined by

Learning Objective

Speed ( in / min) 

Speed control methods for pneumatics

Q(in 3 / min) A (in 2 )

Because air is compressible, the speed control or speed calculation for pneumatic cylinder is complicated. The speed of an air cylinder depends not only on air flow rate, but also on loads, seals, and other many factors. Load, piston speed Load, piston speed Load > pAp, piston speed=0

Quick Exhaust Valves Introduce OR gate shuttle valve Introduce AND gate shuttle Valve

In engineering it is seldom to accurately control speed of air cylinders.

The speed equation V 

Pressure Sequence Valve

Q is easy to use in hydraulics, but A

difficult to use in pneumatic, as Q varies with flow rate, resistance, seals and other factors.

Pressure Switch Time Delay Valve Pneumatic Impulse Ejector

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 Methods

- Meter-out

- Meter-in Flow control valve (FCV) will adjust air flow rate to control speed.

1) A flow control valve is installed in

(The air flow rate to cylinder is not constant, it depends on resistance. In hydraulics the flow rate is constant provided by fixed displacement pump, thus a PRV is necessary to diverge fluid to get different speed)

2) Force balance in working direction

the outlet line of a cylinder

is: p 1A1  p 2 A2  R ( p2  0 , p2 could > p1, pressure intensified)

 A flow control valve is installed in the inlet line of a cylinder  Force balance in working direction p1 A1  R ( p2  0 , no pressure intensification)

Characteristics:

Characteristics

a) Most speed control uses meter-in

a) Suited for small power system and low speed. Speed stability is better than that controlled by meter in.

b) Suited for low power system and low speed. Best suited for a resistive load.

b) Be able to resist impact. For example tool breaking, finish broaching operation (In this case the working direction is retracting direction.)

c) Not be able to resist impact. Such as tool breaking, finish broaching operation

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 Quick Exhaust Valves

 Application for Single-Acting Cylinder

Example Exhaust air has to flow through long lines to the exhaust ports of 5/2/DCV to be released. The fiction resistance decreases the cylinder moving speed

To solve the problem the quick exhaust valve is used.

o Quick exhaust by a OR Gate shuttle valve

Purpose To increase cylinder moving speed. Only used for compressed air Method Directly venting exhaust air to atmosphere at cylinder ports rather than through a long connecting line back to the control valve. Operating principle A: Non-Actuated position. A rubber disc, about 1” diameter, floats across and touches all three ports. B:

Directional valve actuated. The rubber disc seals exhaust port. Air flows into cylinder. The cylinder extends.

C:

Directional valve reversed, the rubber disc seals inlet port, the air from cylinder vents to atmosphere directly. The cylinder retracts fast.

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Application for Double-Acting Cylinder

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Introduce OR gate

OR Gate Shuttle Valve

When Event A or Event B (Any of them) occurs, Event C happens. We see this logic state is OR gate.

(double check valve with cross bleed) (3-way valves with no normal position)

For example button A or button B (any of them) is pressed, the clamping activity (extension) occurs

Characteristics: - Two inlet ports, one outlet port - When p1 > p2, the ball is pushed to right; inlet p2 is blocked, air flows though inlet p1 to outlet A. Air can reversely exhausts from outlet A to p1 port, when p2=0.

- When p2 > p1, the ball is pushed to left; inlet p1 is blocked, air flows though inlet p2 to outlet A. Air can reversely exhausts from outlet A to p2 port, when p1=0. 9

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- Difference between a OR Gate shuttle valve and two standard check valve placed backback Outlet A

An Application Use an OR gate shuttle valve and two buttons (3/2 DCV) to build up a pneumatic circuit. The circuit has the following function: When manual button A or foot button B (any of them) is pressed, the clamping activity (extension) occurs. Solution:

p1

p2

Air cannot exhaust from the cylinder, when two standard check valves are placed backback.

Generally speaking we do not use an OR gate shuttle valve and control buttons (3/2 DCVs) to directly control actuators (cylinders), because 1) These valves usually have small size and low flow capacity. Actuators usually need high flow rate. 2) These valves do not have logic memory function. A pulse control signal cannot control cylinder continuously moving.

A

B

C

We can use a 5/2 DCV with logic memory function and high flow capacity to directly control actuators, use the OR-gate shuttle valve and buttons to control the pilot ports of the 5/2 DCV to realize small signal to control big signal. The circuit above is the solution for the question.

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Introduce AND gate

 AND gate shuttle Valve

Only when Event A and Event B (both of them) occur, Event C can happen. We see this logic state is AND gate.

For safety reason many A machines require operators p p to start machines by pressing two push buttons at G1 G2 a time. It guarantees operators’ hands are not in dangerous area at machine starting. AND gate shuttle valve is designed for this purpose.

For example only button A and button B are pressed at same time, the clamping activity (extension) can occurs.

Only when G1 and G2 have the pressure p, outlet A can supply pressurized air.

A

B

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When the air pressure p is applied to only one of inlet ports G1 or G2, the outlet port A cannot get air pressure; In this case the poppet will seal the air path.

An Application: Example 1: Requirements: 1) Only button A and button B are pressed at a time, the clamping activity (extension) can occurs. 2) Press button C unclamp (retraction) (Explain why use a 5/2 DCV: 1) flow rate of valves 2) A pulse signal can control cylinder motion. It is easily to realize automatic control)

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 In-direct PSV (Pressure Sequence Valve)

Pressure Sequence Valve

A

A pressure sequence valve is classified to the pressure control valve. Its work principle is similar to the pressure relief valve. When the system pressure reaches a setting value, the pressure sequence valve is opened to transfer fluid power to the next actuator.

A Z

Z

PSV R p

 Direct-acting sequence valve and symbol Pressurized oil comes from port 1. When p < pc (cracking pressure), the poppet seals port 1 and port 2. The oil flows through port 1 to cylinder 1; When p > pc, PSV starts opening, port 1 and port 2 is connected. When p=pf (full relief pressure), all the oil flows from port 1 to port 2; to cylinder 2.

p

When the pressure at pilot port Z reaches set value, 3/2 valve changes position.

Z

A A

When p=pf, PSV is fully opened, and the pressure is maintained to constant, oil flows through slender channel 3 to tank.

Z

PSV R p

C2 2

C1

p

1

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Pressure Switch

A pressure switch is an instrument that automatically senses a change in pressure and opens or closes an electrical switching element when a predetermined pressure is reached. The image below show the operate principle of a piston type of pressure switch

http://www.wisconline.com/Objects/ViewObject.aspx?ID=HYP10010

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Pressure switch can be high pressure setting or low pressure setting

Time delay valve (TDV) The time delay valve is used to delay an operation in certain period of time.

Pressure switch can be wired either Normally opened (N.O.) Normally closed (N.C.)

Pressure Switch –N.O.

A When the pressurized air is applied to pilot port Z, the internal accumulator of TDV is being charged; when the pressure at the accumulator reaches the setting value (It takes some time), TDV is opened or closed to change work state of actuators

Pressure Switch –N.C.

 Application of pressure switch

 When Z receives signal, timer starts

Operational principle:  Start button on →  left solenoid energized→

R

Z Signal input

p

 5/2 DCV left position

activated →

 When timer is done, 3/2 DCV changes position

 cylinder extending →  air pressure p increased

when cylinder contacts the workpiece, p reaches setting valve of the pressure switch →

 Pressure is supplied to outlet port “A”.

 the pressure switch on→  right solenoid energized →  5/2 DCV right position

activated →  cylinder retracting.

When thickness of workpiece changed, a pressure switch still works. However a limit switch will not work in this case. For holding workpieces or forging the thickness of the workpieces varies with the product process, thus a pressure switch in this case is better than a limit switch. 21

 The delay time is determined by the FCV

Simplified symbol

A Z

TDV

p 22

Pneumatic Impulse Ejector Some pneumatic systems consume air with peak values. That means a large amount air is needed in a short time; in remainder cycle time only few air is needed. Q

Cycle time

Pneumatic impulse ejector is used to solve this peak demand problem. It is used to store energy and mounted close to work station for quick blasts of air. Allow smaller supply sizing, as ejector can slowly fill between cycles. Otherwise rapid loss of pressure would result.

 Work principle and Symbol Work principle: - When the 3/2 DCV in normal position, air is filled to air chamber in cycle time. - The ball seals the right port of quick exhaust valve - When the peak demand required, the 3/2 DCV changes position by electrical or mechanical.

Air chamber

IN

Out

p

A

- The ball seals the left port of quick exhaust valve.

3/2 DCV is N.O. p

- large amount of air quickly blasts to the pneumatic system.

R

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