Volunteer Position - Worcester Polytechnic Institute

Volunteer Position - Worcester Polytechnic Institute

Pneumatic Power Presented by: Raul Olivera, WildStang - 111 2007 FIRST Robotics Conference Outline Some Basics of Pneumatics and Associated Physics Pressure - Absolute & Gage Force, Pressure & Area Air Properties Flow Rates Electrical Analogy Mechanical Power & Work Pneumatic Energy & Power Managing Pneumatic Energy Capacity Power Experiment Pneumatics vs. Motors

2008 FIRST Robotics Conference Pressure - Absolute & Gage Pressure = matter pushing against matter Object pushing against another object Absolute (psia) => True matter based pressure 0 psia => no matter present to press against objects Not too important in our designs Gage (psig) => Relative to Atmosphere 0 psig => pressure in equilibrium with atmosphere All regulators and gauges based on this 2008 FIRST Robotics Conference Force, Pressure & Area Pressure = Force / Area Force = Pressure X Area Example: 30 psig in 2 diameter cylinder 30 psig Area = r2 = (1)2 = 3.14sq-in 2.0 dia. Force = 30 psi X 3.14 sq-in = 94.2 lbs 2008 FIRST Robotics Conference

94.2 lbs Some Basic Properties of Air Compressible Higher Pressure = Higher Friction Ideal Gas Law: PV = nRT Pressure is proportional to Temperature Pressure is inversely proportional to Volume 2008 FIRST Robotics Conference Pressure & Volume 2008 FIRST Robotics Conference Flow Rates Flow rate = Volume / time i.e. CFM (L/min, cu-in/sec) Flow Controls - Valves Solenoid Value Check Valve Relief Valve Flow Control Valve Unintended Flow Restrictions: Narrow Passages Flow Friction Pressure drops while it is flowing due to restrictions

2008 FIRST Robotics Conference Electrical Analogy Pressure = Voltage Volume = Capacitance Flow rate = Current Flow Restrictions = Resistance HOWEVER: Air is compressible => Some unique non-linearities when compared to electrical systems 2008 FIRST Robotics Conference Mechanical Power & Work Work = Force x Distance (in-lb) Also Work = Torque x Revolutions Mechanical Energy is always involved in doing work It is transferred or converted Power = Work / Time (in-lb/s) or Energy / Time

Power Concept How far a resisting object can be moved in a given time The power rating of motors is what allows us to determine which ones can be used for a given job Power rating for pneumatic actuators? Depends greatly on the rest of the pneumatic system 2008 FIRST Robotics Conference Pneumatic Energy & Power Energy = Force x Distance PEU = Force = Pressure x Area Distance = Volume / Area Pneumatic Energy = Pressure x Volume Energy ( psig x cu-in => in-lbs ) Units Power = Energy / Time Power = Pressure x Volume / Time ( Units = in-lbs ) Flow rate = Volume / Time Power = Pressure x Flow rate ( Psig x cu-in/sec => in-lbs/sec )

2008 FIRST Robotics Conference Managing Pneumatic Energy Capacity Pneumatic Energy Capacity = Volume of Pressurized Air Managing the loss and addition of pressurized air is very important WHY - the volume of air used in large cylinders could deplete your supply very quickly if not managed 2008 FIRST Robotics Conference Managing Pneumatic Energy Capacity Store Pneumatic Energy Storage Tanks Tubing, Fittings & Valves Compressor Consume Pneumatic Energy Exhaust of actuators Leakage Add Pneumatic Energy Activate compressor 2008 FIRST Robotics Conference Energy Capacity Example

120 PSI Side 60 PSI Side PEU P V P V PEU Tot PEU 2400.0 120.0 20.0 60.0 10.0

600.0 3000.0 1800.0 90.0 20.0 60.0 10.0 600.0 2400.0 1200.0 60.0 20.0 60.0 10.0

600.0 1800.0 800.0 40.0 20.0 40.0 10.0 400.0 1200.0 533.3 26.7 20.0 26.7 10.0

266.7 800.0 355.6 17.8 20.0 17.8 10.0 177.8 533.3 120 psig 60 psig 2008 FIRST Robotics Conference The Compressor - Published Performance 40.00 50.00

60.00 70.00 80.00 90.00 100.00 110.00 120.00 0.56 0.41 0.38 0.36 0.33 0.27 0.24 0.21 0.18 cu-in / sec PEU/s 16.13 11.81 10.94 10.37 9.50

7.78 6.91 6.05 5.18 645.12 590.40 656.64 725.76 760.32 699.84 691.20 665.28 622.08 Compressor Power Curve 800 700 600 500 PEU's Pressure Flow Rate (PSI) (CFM) 400 300

200 100 0 0 50 100 Pressure Averages about 660 PEU/s in the cut out range (90 to 120 psig) 2008 FIRST Robotics Conference 150 The Compressor - My experiment Pressure (PSI) PEU/s 20 30 40 50

60 70 80 90 100 110 120 341.3 259.4 213.5 183.5 162.2 146.2 133.5 123.3 114.8 107.6 101.5 Compressor Power Curve 400 350 300 PEU/s 250

200 150 100 50 0 20 30 40 50 60 70 Pressure Averages about 110 PEU/s in the cut out range (90 to 120 psig) 2008 FIRST Robotics Conference 80 90 100

110 120 Managing Pneumatic Energy Capacity Energy Capacity Example - AGAIN: Storage Tanks => Energy Capacity = 4524 (2 tanks) Cylinder - 2 dia x 24 stroke Energy Capacity used = 4524 Compressor can replace 160 per second at 60 psig Conclusion: It will take 6.85 to 28.3 seconds to replace the energy used by one activation 2008 FIRST Robotics Conference Managing Pneumatic Energy Capacity Managing the Loss of Energy Use only the amount of energy required, not too much more - WHY? Minimize Volume: tubing length - valve to cylinder cylinder stroke

cylinder diameter Minimize regulated pressure But, keep above valve pilot pressure requirement 2008 FIRST Robotics Conference Optimize Cylinder Stroke, Diameter and Pressure Stroke Shorter stroke => less leverage for angled movement Shorter stroke => less weight for cylinder Diameter Smaller diameter => more pressure required for same force Smaller diameter => less weight for cylinder Pressure Less pressure => need a bigger, heavier cylinder Less pressure => less likely to leak 2008 FIRST Robotics Conference Power Experiment Purpose: Determine Force and Power curves for a pneumatic cylinder Set-up:

8 stroke by 1.5 diameter cylinder All data taken at 60 psig Time recorded to fully extend or contract (8.0) Electronic sensor used at both ends of stroke for timing accuracy Pull Configuration Pulley Push Configuration Cylinder Table Weight 2008 FIRST Robotics Conference Force Values 2008 FIRST Robotics Conference Cylinder / System Hysteresis Actuation hysteresis is very pronounced due to: Internal cylinder and flow friction

Resisting force Force Exerting force This can be bad, cannot move objects at rated force - design for this This could be good, if leakage occurs and pressure drops slightly, the cylinder will still hold Regulated Pressure 2008 FIRST Robotics Conference Pneumatic Power 3.5 3 2.5 Time (Sec) Force versus time curve was non-linear as expected Experimental setup was not perfect, some variation in

data expected Time Vs Weight (Pull) Some friction in cable system Ran several times for each weight and took average 1.5 1 0.5 0 0 10 20 30 40 50 60 70

80 W eight (lbs) Power Vs Weight (Pull) 60 50 Power (Watt) Max force that could move was typically less than 85% of theoretical max force 2 40 30 20 10 0 0 10 20 2008 FIRST Robotics Conference

30 40 50 W eight (lbs) 60 70 80 Pneumatic Power 2.5 Time (Sec) 2 1.5 1 0.5 0 0 20 40

60 80 100 80 100 W eight (lbs) Power Vs Weight (Push) Power (Watt) This pneumatic cylinder systems is not as powerful as better motors in our KOP 1.5 cylinder ~= 80 watts FP motor ~= 171 watts CIM motor (small) ~= 337 watts How do we deal with nonlinear behavior? Design for the max

force to occur before the knee in the curve Time Vs Weight (Push) 90 80 70 60 50 40 30 20 10 0 0 20 2008 FIRST Robotics Conference 40 60 Weight (lbs) Pneumatic Power I said 1.5 cylinder ~= 80 watts Is this True?

What determines the power capacity of a cylinder? Is it limited by: 1) Length of the cylinder No 2) Diameter of the cylinder No 3) Max flow rate of the system Yes 4) The max pressure set on the regulator Yes It is determined by the amount of energy that can be injected into the cylinder - NOT the cylinder parameters 2008 FIRST Robotics Conference Summary Pneumatics systems are analogous to electrical systems except for the extra non-linear behavior Force capacity is determine solely by pressure and cylinder diameter The theoretical rated force can be held but not moved Energy usage is determine by pressure and cylinder volume The entire system design determines the power capacity of any cylinder

It is easy to run out of pneumatic energy when large cylinder volumes are used Your compressor most likely does not perform to published specs - do some experiments 2008 FIRST Robotics Conference Pneumatics vs. DC Motors Some, but not all important differences You are allowed to use as many cylinders as you like However, you are limited in the types and sizes of cylinders allowed You are limited to the KOP Motors Most of what you need for the pneumatic system is provided in the KOP or easily ordered Motors have to be geared to produce the desired forces Cylinder size can just be picked for the forces you need Pneumatics are best suited for linear motion Motors are best suited for angular motion 2008 FIRST Robotics Conference Pneumatics vs. DC Motors Some, but not all important differences Our ability to control the position of mechanisms actuated

by cylinders is very limited We are not given integrated, dynamic airflow or pressure controls We are given much more versatile electronic controls for motors Cylinders can be stalled without damage to the pneumatic system Motors will draw large current and let out the magic smoke Cylinders absorb shock loads rather well and bounce back However, be careful of over pressure conditions caused by flow control valves Motors have to be actively held with feedback controls or locked 2008 FIRST Robotics Conference Pneumatics vs. DC Motors Some, but not all important differences Cylinders use up their power source rather quickly The 4 air tanks we are allowed do not hold much work capacity Motors use up very little of the total capacity of the battery The decision to use Pneumatics

The initial investment in weight is great - mostly due to compressor Otherwise, very limited air capacity if leave compressor off robot Once invested use for as many applications as feasible Easy to add more functionality Cylinders used with single solenoid valves are great for Armageddon devices - stuff happens when power is shut off This could be good or bad - use wisely 2008 FIRST Robotics Conference

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