Lecture B4 - Hydraulics, Pneumatics, and Actuation Systems PDF

Dr Gianfranco Claudio & Dr Tim Harrison WSC353 INTERFACING FOR MECHATRONIC SYSTEMS Lecture plan Sensors (1) 31st Oct 14:00 Sensors (2) 7th Nov 14:00 Strain Gauges, Charge Amplifiers, 14th Nov 14:00 Sensor Networks & Relays Actuators (1)...

Dr Gianfranco Claudio & Dr Tim Harrison WSC353 INTERFACING FOR MECHATRONIC SYSTEMS Lecture plan Sensors (1) 31st Oct 14:00 Sensors (2) 7th Nov 14:00 Strain Gauges, Charge Amplifiers, 14th Nov 14:00 Sensor Networks & Relays Actuators (1) 21st Nov 14:00 Actuators (2) 28th Nov 14:00 Thermal and Noise considerations 5th Dec 14:00 Some practical examples 12th Dec 14:00 Part B: Practical Considerations Lecture B4: Hydraulics, Pneumatics and other actuation systems Learning Outcomes Understand the range of different actuators available Understand the basics of how actuators work Understand how to interface with actuators Understand their advantages and limitations  Overview Microcontroller Analogue Conversion Filter / De- Glitcher Analogue Electronics Amplifier Actuation Actuator Gearing / Linkage Environment Why? Not just interfacing with sensors, but also actuators Many of the techniques/information also apply to actuators: Voltage vs current sensor/actuator networks DAC / ADC Noise  Overview Power supply - connected to a source of energy Power amplifier - regulates the energy flow Electrical to/from the actuator Fluid Actuator - applies action to the physical system Mechanical … ACTUATOR SYSTEM Energy Power source supply Power Actuator amplifier From controller To physical system Actuator systems Shape Memory Alloys Fluid systems Rheological fluids Hydraulic Electro- Pneumatic Magneto- Electro-magnetic Electro-active polymers Solenoids Electric motors Piezo Electric Taxonomy of actuators: Shape Memory Alloys (SMAs) A group of metallic materials that return to some previously defined shape or size when subjected temperature Nickel-Titanium (Ni-Ti) is most flexible and beneficial for engineering applications SMA actuators possess an extremely high force to weight ratio: 0.25mm Ni-Ti wire contracts approx. 10% with force 5N But … low efficiency: a SMA actuator is effectively a heat engine where the material converts thermal energy directly into work. Microsoft’s ‘muscle wire’  Electro-Rheological (ER) Fluids ER fluids experience dramatic changes in rheological properties in the presence of an electric field Material: polymer particles in fluoro-silicone base oil Can control (resistance to) fluid flow by means of an electric field Very fast-acting and direct Low control power But … High electric fields required ~ 1kV/mm Tends to need convoluted flow path to achieve large enough shear pressure YouTube Electro-Rheological (ER) Fluids  Magneto-Rheological fluids Suspensions of magnetically-polarisable micron-sized particles suspended in a low volatility carrier fluid, usually a synthetic hydrocarbon Yield stress of the fluid when in its 'on' state can be controlled very accurately and quickly by varying the magnetic field intensity But … Fluids are heavy Main application is in vehicle shock absorbers Fluid degrades Magneto-Rheological fluids Audi Magnetic ride Electro-active polymers (EAPs) Polymers whose shape is modified when a voltage is applied to them. 2 types: Dielectric - actuation is caused by electrostatic forces between two electrodes which squeeze the polymer. Characterized by a large actuation voltage (several thousand volts), but very low electrical power consumption Ionic - actuation is caused by the displacement of ions inside the polymer. Only a few volts are needed, but higher electrical power needed for actuation, and energy needed to hold at a given position Festo’s ‘AirRay’ Electro-active polymers (EAPs) Aside: Bio-mimicry EAPs, SMAs often used for ‘biomimetic’ applications like ‘electric muscle’ etc. Biomimicry is basically trying to replicate what happens in nature Broad range of applications from SMAs, to reflective coatings based on butterfly wings, to the way a computer display uses the color gamut for optimal energy usage Piezoelectric Opposite to piezoelectric sensors like accelerometers Applied voltage/charge creates mechanical effect Queensgate Displacement nanopositioning Force 20 micron actuator Large forces & high speeds but, 500Kg load Small movements Materials and actuators widely available Square, rectangular, ring shape Axial, shear or bending effect ThorLabs Kemet Piezoelectric Most common example is quartz used for timekeeping Amplified oscillator circuit Quartz used to regulate oscillation Normally set to 32,768 Hz High enough that people can’t hear (higher than 20kHz) Quartz clocks 32,768 = 215, so can use a 15-bit counter that overflows once per second Solenoids Electromagnetic linear actuation On/off, end to end control Return spring Can have back-to-back 2-coil, 3-position solenoids Force proportional to current Advantages Simple Robust Large variation of size/force available Disadvantages Not fully variable Can cause magnetic interference RS online Fluid Systems Hydraulic Pneumatic Why fluid systems? Fluid systems are a less likely choice in a world becoming increasingly all-electric, but they still have their place. Very high power density Intrinsically safe for working in explosive atmospheres Less susceptible to electrical interference Make more sense where electrical supply is not guaranteed But Leaks Loss of efficiency in conversion - often electrically powered Why fluid systems? Before the days of communal electrical power generation and efficient long-distance distribution… Watermills. Good for continuous low-ish power operations Less good for intermittent, higher power operations Is there a way to store energy for those short duration pulses? Slowly pump water up to a high-level tank Quickly release when a burst of power is needed Working fluid - water Water rarely used today, more likely oil (hydraulics) or gas (pneumatics). Historically water Developed from use of waterwheels Readily available Non-toxic Not a problem to leak water to the environment Easy to store But Low temperature range - no good below 0°C or above 100°C* Dissolved air Slightly compressible Cavitation Working fluid - water Tower Bridge was originally water powered As lifts appeared in buildings they were often water powered Docks and industrial complexes used hydraulics for cranes etc. Hydraulic power supply companies existed; London Hydraulic Power company 800 psi (55 bar, 5.5 MNm-2) 180 miles of pipes Originally steam powered Converted to electric pumps in 1920s Operated from 1883 to 1977 Bristol Docks hydraulic Pipes now used as cable ducts power station and water tower - wikipedia Working fluid - air It is usual to use a higher than atmospheric pressure air supply, but vacuum systems or inert gasses are occasionally used Advantages Readily available Non-toxic Not a problem to leak air to the environment Better temperature range But Compressible Expands and contracts with temperature Leaks difficult to detect Inert gasses are an asphyxiation hazard Working fluid - air Pneumatic systems today Large vehicle brakes, trucks, busses, trains Applied by springs, released by air pressure Wojciech Sawczuk & Grzegorz M. Szymański Vacuum systems previously used Workshops Air tools tend to be more compact than electric versions Safer than mains electric power? Typically 6 to 7 bar (0.6 to 0.7MNm-2,85 to 90psi) systems Bosch Professional Food processing Hydraulic leaks not desirable Rowse Pneumatics Simple pneumatic logic is possible Dental tools Do you really want electric tools or hydraulic oil in you mouth?! Working fluid - oil Advantages Non compressible Excellent temperature range Excellent power transmission particularly at high pressure Typically 3000 psi (205 bar, 20.5MNm-2), occasionally 5000psi Robust But Needs return pipework to form an enclosed system Leaks a big problem Fluid can be toxic and/or corrosive and/or flammable Working fluid - oil Brembo Typically used where high power density is required Vehicle brakes (cars and aircraft) Plant (big yellow machines) Aircraft (flight controls etc.) Chemtronics JCB Working fluid - oil Brembo Typically used where high power density is required Vehicle brakes (cars and aircraft) Plant (big yellow machines) Aircraft (flight controls etc.) Chemtronics JCB Working fluid – Fuel “Fueldraulic” Advantages Not too dissimilar to hydraulic oil There might be a handy local supply of pressurized fuel Easily returned to the fuel system But Leaks a big problem High pressure fuel spraying on your hot engine will not end well… Uses Diesel engine injectors? Aircraft engine mechanical actuation e.g. variable stator vanes  Fluid systems ACTUATOR SYSTEM Energy Power source supply Power Actuator amplifier From controller To physical system  Hydraulic systems - oil Accumulator Valve Pump Cylinder Pressure relief valve Reservoir  Hydraulic systems - oil Accumulator Valve Pump Cylinder Pressure relief valve Reservoir Hydraulic systems Supply Pressure Return Pressure Hydraulic systems Supply Pressure Return Pressure Hydraulic systems Supply Pressure Return Pressure Hydraulic systems - oil  Pneumatic systems High pressure reservoir Valve Compressor Cylinder Pressure Vent relief valve Intake Vent Low pressure reservoir is the atmosphere! Fluid system components – air compressor Typically a positive displacement type pump Think piston engine in reverse Normally single cylinder Small compressors will have an integral reservoir Hyundia Power Products Occasionally a centrifugal compressor Big systems  Fluid system components – hydraulic pump Typically a positive displacement type pump Piston Gear pump Swashplate Mobile hydraulic tips Savree Fluid system components – valves Poppet or spool valve Directly operated with a lever, or Solenoid actuated Poppet valve Supply pressure 2 states - open or closed Held in one position with a spring Return Spool valve Control action Control action more than 2 states Spool moves to open and close different ports Fluid system components – cylinders Converts fluid pressure to Linear motion Can be single or double acting Single or double ended Internal and external seals can be a source of leakage Force proportional to pressure difference* IQSdirectory Fluid system components – motors Converts fluid pressure to Rotary motion Works like a pump in reverse M Can be positive displacement Piston Gear Swashplate Or Centrifugal ETS hydro  Fluid systems - Summary Type Fluid Fluid Fluid Return Power compressibility flammability pipework density Pneumatic Benign High No No, vent to Medium atmosphere Hydraulic Benign Lower No Optional High (water) Hydraulic At best Lowest Depends on Yes High (oil) unpleasant fluid type Additional Resources Wikipedia: Bimorphs, Piezoelectric motor, Electrorheological fluids, magnetorheological fluids, Shape memory alloy, Biomimicry YouTube video on MR fluid, YouTube video on MR dampers, Lecture plan Sensors (1) 31st Oct 13:00 Sensors (2) 7th Nov 13:00 Strain Gauges, Charge Amplifiers, 14th Nov 13:00 Sensor Networks & Relays Actuators (1) 21st Nov 13:00 Actuators (2) 28th Nov 13:00 Thermal and Noise considerations 5th Dec 13:00 Some practical examples 12th Dec 13:00 Lecture plan Sensors (1) 31st Oct 14:00 Sensors (2) 7th Nov 14:00 Strain Gauges, Charge Amplifiers, 14th Nov 14:00 Sensor Networks & Relays Actuators (1) 21st Nov 14:00 Actuators (2) 28th Nov 14:00 Thermal and Noise considerations 5th Dec 14:00 Some practical examples 12th Dec 14:00