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ME 8843

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ME 8843

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  1. ME 8843 Advanced Mechatronics Instructor: Professor Charles Ume Introduction to Hydraulic and Pneumatic Systems

  2. Introduction Hydraulic system Pneumatic system Key components Valves Actuators Examples Outline

  3. 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 Hydraulic/Pneumatic Systems

  4. Applications • Advantages • adaptable power distribution • constant force actuators • power amplification • inexpensive Air Conveyor Impact Wrench Hydraulic Jack • Disadvantages • difficult to control position • leaks and contamination of working fluid

  5. 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 Pascal’s Law

  6. 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

  7. 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 Hydraulic Systems

  8. 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

  9. 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 Pneumatic systems

  10. Pump/Compressor Pressure regulator Valve Actuator Key components of Hydraulic and Pneumatic

  11. 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 Valves Pressure regulator

  12. Type: Spool, poppet, ball, butterfly valves, etc. Types of Valves Poppet valve Spool valve Check valve (One directional flow) Ball valve Butterfly valve

  13. Control methods Valve symbols Position with texts indicates initial position Valve connections Valves with controls indicated

  14. 4 ports/3 positions Solenoid Spool Valve

  15. Example: Pneumatic lift system (analogous to car jack) Lift load Lower load

  16. Cylinders with piston driven by pressurized fluid Single acting cylinder (SAC) Double acting cylinder (DAC) Two well-defined endpoints Rotary Hydraulic/Pneumatic actuators

  17. Stroke length Bore size Pressure rating Mounting style Return type (SAC vs. DAC) Spring force in SAC Loads Temperature range Lubrication Material Compatibility Key parameters in choosing air cylinders Force

  18. Example 1: LEGO house builder • Weight • Stroke • Speed • Force • Accurate positioning not required Pneumatic Lead Screw

  19. 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

  20. Hydraulic System fluid reservoir Supplies the main braking force to the pistons at the wheels actuated by brake pedal Front circuit Rear circuit • 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

  21. Pneumatic Power Assist Brake Released Brake Applied Vacuum from engine Bi-directional check valve • 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

  22. 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) pressurized fluid nitrogen

  23. 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 Reference