The George W. Woodruff School of Mechanical Engineering
ME 8843 Advanced Mechatronics
Instructor: Professor Charles Ume
Introduction to Hydraulic and Pneumatic Systems
The George W. Woodruff School of Mechanical Engineering
Outline
• Introduction • Hydraulic system • Pneumatic system • Key components
– Valves – Actuators
• Examples
The 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 the potential energy of fluids/compressed gas into mechanical energy by linear/rotary actuators
The 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
The 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
The 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
The 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
The 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
The 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
The George W. Woodruff School of Mechanical Engineering
Key components
• Pump/Compressor • Pressure regulator • Valve • Actuator
The George W. Woodruff School of Mechanical Engineering
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
The George W. Woodruff School of Mechanical Engineering
Valves (II) • Type: Spool, poppet, ball, butterfly valves, etc.
Check valve
Poppet valve
Spool valve
Ball valve Butterfly valve
The George W. Woodruff School of Mechanical Engineering
Valve symbols
Control methods
Valve connections
Valves with controls indicated
The 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
The 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
The George W. Woodruff School of Mechanical Engineering
Example 1: LEGO house builder
• Weight • Stroke • Speed • Force • Accurate
positioning not required
Lead Screw Pneumatic
The George W. Woodruff School of Mechanical Engineering
Example 2: Anti-Lock Braking System
• Hydraulic actuation • Pneumatic power assist • ABS:
– sensors – valves – hydraulic pump – control unit
The George W. Woodruff School of Mechanical Engineering
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
The George W. Woodruff School of Mechanical Engineering
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
The George W. Woodruff School of Mechanical Engineering
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
The 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