instructor: professor charles ume introduction to...

The George W. Woodruff School of Mechanical Engineering

ME 8843 Advanced Mechatronics

Instructor: Professor Charles Ume

Introduction to Hydraulic and Pneumatic Systems


Outline

•  Introduction •  Hydraulic system •  Pneumatic system •  Key components

– Valves – Actuators

•  Examples


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 the potential energy of fluids/compressed gas into mechanical energy by linear/rotary actuators


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


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


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


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


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


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


Key components

•  Pump/Compressor •  Pressure regulator •  Valve •  Actuator


Valves •  Infinite position valve:

–  allows any position between open and closed to modulate flow or pressure. Example: faucet, pressure regulator, ball valve.

•  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 –  y: number of positions –  4/3 valve: 4 ports and 3 positions

Pressure regulator: Infinite Position


Valves (II) •  Type: Spool, poppet, ball, butterfly valves, etc.

Check valve

Poppet valve

Spool valve

Ball valve Butterfly valve


Valve symbols

Control methods

Valve connections

Valves with controls indicated


4 ports/3 positions Solenoid Spool Valve


Example: Pneumatic lift system


Hydraulic/Pneumatic actuators

•  Cylinders with piston driven by pressurized fluid •  Single acting cylinder (SAC) •  Double acting cylinder (DAC) •  Two well-defined endpoints •  Rotary


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


Example 1: LEGO house builder

•  Weight •  Stroke •  Speed •  Force •  Accurate

positioning not required

Lead Screw Pneumatic


Example 2: Anti-Lock Braking System

•  Hydraulic actuation •  Pneumatic power assist •  ABS:

–  sensors –  valves –  hydraulic pump –  control unit


Hydraulic System

Supplies the main braking force to the pistons at the wheels

• Metering Valves- engage the rear breaks before the front

• Proportioning Valves – control the pressure provided to the front and rear and can change pressure distribution according to vehicle weight distribution

Front circuit

Rear circuit

actuated by brake pedal

fluid reservoir


Pneumatic Power Assist Brake Applied

Vacuum from engine Brake Released

•  Brakes applied •  pushes the pistons in the master cylinder •  opens check valve to pressurize one side of the diaphragm •  pressure difference assist in applying baking force

•  Brakes released •  check valve closes and engine vacuum is again applied to both chambers

Bi-directional check valve


Anti-lock Breaking System •  Wheel speed sensor

•  Electric hydraulic pump –  Stores fluid in a pressurized chamber

•  Solenoid valves –  Open: braking pressure supplied directly from the master cylinder (under normal

conditions) –  Closed: isolate master cylinder pressure line (modulation) –  Release: applies stored pressure to blocked break lines (modulation)

nitrogen pressurized

fluid


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

instructor: professor charles ume introduction to...

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