Some basics for FIRST Robotics Pneumatics Some basics for FIRST Robotics

Safety First The rapid expansion of air in a ruptured air tank or line is an explosion A blown oxygen cylinder wrecked this lab FRC teams use much lower pressures FIRST limits tank pressure to 115 psi(Pounds per Square Inch), similar to common workplace systems and big truck tires. FIRST limits working pressure to 60 psi, about the same as a narrow bicycle tire. Treat pressurized air with respect! 2000+ psi air cylinder

Pressure-Force-Stress Calculations A gauge measures the pressure difference between the supplied pressure and the surrounding air (about 14.7 psi) psia (absolute)= psig (gauge)+14.7 Dual action cylinders can have air pressure on both sides of the piston Force on piston rod P2 P1 F Rcyl Rrod Typically, either P1 or P2 is zero Gauge pressures Cylinder rupture can occur if the stress in the cylinder wall approaches the yield strength of the material. Safest with: Lower pressure Smaller diameter cylinders Thicker tube walls Material stress in cylinder walls

Basic Thermodynamics: P, r, V, T relations Need to know how much air is in the tank and how many times cylinders can be extended on a tank full. The ideal gas law P=rRT is a good approximation, but both r and T are unknown in the pressurized tank. Real state of gas is between: isothermal—constant temperature isentropic—reversible with no energy loss In practice, heat conducts to the hardware and constant temperature is a reasonable approximation Absolute pressures

Simplified Capacity Calculations Absolute pressures Cylinders use a working pressure that is lower than the tank pressure Rotary #1 Each use of a cylinder is a separate drain on the tank Cylinder #2 Tank Cylinder #1 Conserve air by using the lowest pressure and smallest cylinder volume that will do the job

High Pressure (115 psig) Components Air Storage Tank (rated to 250 psig) Valve Compressor Pressure Switch (compressor on/off at 115 psig) Relief Valve (pops if pressure exceeds 120 psig) Pressure Regulator (drop pressure from 115 psig to 5-50 psig ) Distribution block Pressure Gauge

Some Basic Information on High Pressure Side Pressure switch, pressure relief valve and a gauge must be located down stream from the compressor and ahead of any valves. Pressure switch is electrically in line with the control circuit for the compressor motor controller. (Not in the compressor motor circuit) Liberal use of valves allows components to be isolated for leak testing, function testing, disabling and venting the system. Gauge and pressure regulator should be mounted for easy access. Leaks are likely to be the greatest challenge. Push-in tube connectors may be a big source Use PTFE (Teflon®) tape around threaded joints Swab soapy water around all joints and watch for bubbles

Application Pressure (5-50 psig) Components “T” for Tubing Flow Control Valve (limit cylinder extension rate) Double-Action Cylinder (pushes and pulls) Switch/Valve Bank for Double-Action Cylinder Pressure Regulator (drop pressure from 50 psig to 5-50 psig ) Valve (use lots of these) Pressure Gauge

Tips for the Low Pressure Components Use the lowest possible working pressure to minimize air usage, abuse of hardware and potential for injury. Multiple regulators allows several working pressures. Use a gauge after every regulator to set pressure on that leg Use flow control valves to limit the extension/retraction rate. Also, safer due to reduced velocity of components. Again, leaks may be the biggest challenge.

Questions? ???