ME8843

The George W. Woodruff School of Mechanical EngineeringThe George W. Woodruff School of Mechanical Engineering

ME 8843

Advanced Mechatronics

Instructor: Professor Charles Ume

Introduction to Hydraulic and Pneumatic Systems

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Outline

• Introduction

• Hydraulic system

• Pneumatic system

• Key components

– Valves

– Actuators

• Examples

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Hydraulic/Pneumatic Systems

• Use fluids as working media

• Convert electrical/mechanical energy into potential energy of fluids (pump, compressor)

• Transmit power through distribution lines (pipes, air hoses)

• Convert potential energy of fluids/compressed gas into mechanical energy that turns linear/rotary actuators

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Applications

• Advantages

– adaptable power distribution

– constant force actuators

– power amplification

– inexpensive

• Disadvantages

– difficult to control position

– leaks and contamination of working

fluid

Air Conveyor Impact Wrench Hydraulic Jack

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Pascal’s Law

• Pascal's law states that:

"a change in the pressure of an

enclosed incompressible fluid is

conveyed undiminished to every part

of the fluid and to the surfaces of its

container.“

– Force determined by pressure

– Speed determined by flow rate

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Hydraulic Systems• Move large loads by controlling high-pressure fluid in

distribution lines and pistons with mechanical or

electromechanical valves

• 1000psi – 3000psi

• Closed systems, always recirculating same fluid

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Hydraulic Systems

• Advantage:

– Able to generate extremely large forces from compact actuators

– Easy to control speed

– Easy to implement linear motion

• Disadvantage:

– Large infrastructure (high-pressure pump, tank, distribution lines)

– Potential fluid leaks

– Noisy operation

– Vibration

– Maintenance requirements, expensive

– Characteristics of working fluids change with temperature and moisture

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Pneumatic systems

• Pneumatic systems similar to hydraulic systems

• Use compressed air as working fluid rather than hydraulic liquid

• 70psi - 150psi, much lower than hydraulic system pressures, much lower

forces than hydraulic actuators

• Energy can be stored in high pressure tanks

• Open systems, always processing new air

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Pneumatic systems

• Advantage:

– Constant force

– Clean (food industry)

– No return lines needed

– Adaptable infrastructure

– Possible light, mobile pneumatic systems

– Fast system response

• Disadvantage:

– Difficult to achieve position control (compressible air)

– Noisy

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Key components of Hydraulic and

Pneumatic

• Pump/Compressor

• Pressure regulator

• Valve

• Actuator

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Valves• Infinite position valve as shown in figure on right:

– allows any position between open and closed to modulate flow or pressure

• Finite position valve:

– has discrete positions, usually just open and closed, providing different pressure and flow condition

• Ports: inlet and outlet connections to valve

• Finite position valve usually specified as “x/y valve”

– x: number of ports (sum of inlets and outlets)

– y: number of positions

– 4/3 valve: 4 ports and 3 positions

Pressure regulator

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Types of Valves• Type: Spool, poppet, ball, butterfly valves, etc.

Check valve (One directional flow)

Poppet valve

Spool valve

Ball valveButterfly valve

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Valve symbols

Control methods

Valve connections

Valves with controls indicated

Position with texts

indicates initial position

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4 ports/3 positions Solenoid Spool Valve

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Example:

Pneumatic lift system (analogous to car jack)

Lift load

Lower load

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Hydraulic/Pneumatic actuators

• Cylinders with piston driven by pressurized fluid

• Single acting cylinder (SAC)

• Double acting cylinder (DAC)

• Two well-defined endpoints

• Rotary

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Key parameters in choosing air cylinders

• Stroke length

• Bore size

• Pressure rating

• Mounting style

• Return type (SAC vs. DAC)

– Spring force in SAC

• Loads

• Temperature range

• Lubrication

• Material Compatibility

Force

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Example 1: LEGO house builder

• Weight

• Stroke

• Speed

• Force

• Accurate

positioning

not required

Lead ScrewPneumatic

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Example 2: Anti-Lock Braking System

Regular Automobile Breaking System Includes:

•Hydraulic actuation

•Pneumatic power assist

ABS includes additional features:

– sensors

– valves

– hydraulic pump

– control unit

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Hydraulic System

Supplies the main

braking force to the

pistons at the wheels

•Proportioning Valves – control the pressure provided to the front and rear

• Can change pressure distribution according to vehicle weight distribution

•Metering Valves- engage the rear breaks before the front

Front

circuit

Rear

circuit

actuated by brake pedal

fluid reservoir

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Pneumatic Power AssistBrake Applied

Vacuum from engineBrake Released

• Brakes applied

• Opens check valve to pressurize one side of diaphragm

• Pressure difference assist in applying braking force

• Pushes pistons in master cylinder

• Brakes released

• Check valve closes and engine vacuum is again applied to both chambers

Bi-directional check valve

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Anti-lock Breaking System• Wheel speed sensor

• Electric hydraulic pump

– Stores fluid in pressurized chamber

• Solenoid valves

– Open: braking pressure supplied directly from master cylinder (under normal

conditions)

– Closed: isolate master cylinder pressure line (modulation)

– Release: applies stored pressure to blocked break lines (modulation)

nitrogenpressurized

fluid

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Reference

• Mechatronics, by Sabri Cetinkunt, published by Wiley

• Introduction to Mechatronics and Measurement Systems, Second Edition,

by David G. Alciatore and Michael B. Histand

• Mechatronics: Electronic Control Systems in Mechanical Engineering, by

W. Bolton

• http://en.wikipedia.org/wiki/Pascal%27s_law

• http://en.wikipedia.org/wiki/Pneumatic_cylinder

• http://www.bimba.com

• http://www.tpub.com/content/engine/14105