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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
ME8843
The George W. Woodruff School of Mechanical EngineeringThe George W. Woodruff School of Mechanical Engineering
Outline
• Introduction
• Hydraulic system
• Pneumatic system
• Key components
– Valves
– Actuators
• Examples
ME8843
The George W. Woodruff School of Mechanical EngineeringThe George W. Woodruff School of Mechanical Engineering
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
ME8843
The George W. Woodruff School of Mechanical EngineeringThe George W. Woodruff School of Mechanical Engineering
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
ME8843
The George W. Woodruff School of Mechanical EngineeringThe George W. Woodruff School of Mechanical Engineering
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
ME8843
The George W. Woodruff School of Mechanical EngineeringThe George W. Woodruff School of Mechanical Engineering
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
ME8843
The George W. Woodruff School of Mechanical EngineeringThe George W. Woodruff School of Mechanical Engineering
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
ME8843
The George W. Woodruff School of Mechanical EngineeringThe George W. Woodruff School of Mechanical Engineering
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
ME8843
The George W. Woodruff School of Mechanical EngineeringThe George W. Woodruff School of Mechanical Engineering
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
ME8843
The George W. Woodruff School of Mechanical EngineeringThe George W. Woodruff School of Mechanical Engineering
Key components of Hydraulic and
Pneumatic
• Pump/Compressor
• Pressure regulator
• Valve
• Actuator
ME8843
The George W. Woodruff School of Mechanical EngineeringThe George W. Woodruff School of Mechanical Engineering
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
ME8843
The George W. Woodruff School of Mechanical EngineeringThe George W. Woodruff School of Mechanical Engineering
Types of Valves• Type: Spool, poppet, ball, butterfly valves, etc.
Check valve (One directional flow)
Poppet valve
Spool valve
Ball valveButterfly valve
ME8843
The George W. Woodruff School of Mechanical EngineeringThe George W. Woodruff School of Mechanical Engineering
Valve symbols
Control methods
Valve connections
Valves with controls indicated
Position with texts
indicates initial position
ME8843
The George W. Woodruff School of Mechanical EngineeringThe George W. Woodruff School of Mechanical Engineering
4 ports/3 positions Solenoid Spool Valve
ME8843
The George W. Woodruff School of Mechanical EngineeringThe George W. Woodruff School of Mechanical Engineering
Example:
Pneumatic lift system (analogous to car jack)
Lift load
Lower load
ME8843
The George W. Woodruff School of Mechanical EngineeringThe George W. Woodruff School of Mechanical Engineering
Hydraulic/Pneumatic actuators
• Cylinders with piston driven by pressurized fluid
• Single acting cylinder (SAC)
• Double acting cylinder (DAC)
• Two well-defined endpoints
• Rotary
ME8843
The George W. Woodruff School of Mechanical EngineeringThe George W. Woodruff School of Mechanical Engineering
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
ME8843
The George W. Woodruff School of Mechanical EngineeringThe George W. Woodruff School of Mechanical Engineering
Example 1: LEGO house builder
• Weight
• Stroke
• Speed
• Force
• Accurate
positioning
not required
Lead ScrewPneumatic
ME8843
The George W. Woodruff School of Mechanical EngineeringThe George W. Woodruff School of Mechanical Engineering
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
ME8843
The George W. Woodruff School of Mechanical EngineeringThe George W. Woodruff School of Mechanical Engineering
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
ME8843
The George W. Woodruff School of Mechanical EngineeringThe George W. Woodruff School of Mechanical Engineering
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
ME8843
The George W. Woodruff School of Mechanical EngineeringThe George W. Woodruff School of Mechanical Engineering
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
ME8843
The George W. Woodruff School of Mechanical EngineeringThe George W. Woodruff School of Mechanical Engineering
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