Pneumatics Some basics for FIRST Robotics. ROBOTICS ACADEMY: FRC PNEUMATICS Safety First The rapid expansion of air in a ruptured air tank or line is.

Pneumatics

Some basics for FIRST Robotics

ROBOTICS ACADEMY: FRC PNEUMATICS

Safety FirstThe 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 aircylinder

ROBOTICS ACADEMY: FRC PNEUMATICS

Pressure-Force-Stress Calculations

P2P1F

Rcyl

Rrod

Dual action cylinders can have air pressure on both sides of the piston

€

F = P2πRcyl2 − P1π Rcyl

2 − Rrod2

( )

Force on piston rod

Material stress in cylinder walls

€

σhoop = PRcylindertwall thickness

€

σlongitudinal = PRcylinder

2twall thickness

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

Typically, either P1 or P2 is zero

A gauge measures the pressure difference between the supplied pressure and the surrounding air (about 14.7 psi)

psia (absolute)=psig (gauge)+14.7

Gauge pressures

ROBOTICS ACADEMY: FRC PNEUMATICS

Basic Thermodynamics: P, r, V, T relationsNeed 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

€

ρ =ρ0

PP0

€

ρ =ρ0

PP0

⎛

⎝ ⎜

⎞

⎠ ⎟

1k

Τ = Τ0

P

P0

⎛

⎝ ⎜

⎞

⎠ ⎟

k−1k

k ≈ 1.4 for air

Absolute pressures

ROBOTICS ACADEMY: FRC PNEUMATICS

Simplified Capacity Calculations

€

PV = PTVT + PCVC + PVVV + PRVR + PV...

Tank Cylinder #1

Cylinder #2

Rotary #1Each use of a cylinder is a separate drain on the tank

Conserve air by using the lowest pressure and smallest cylinder volume that will do the job

Cylinders use a working pressure that is lower than the tank pressure

Absolute pressures

ROBOTICS ACADEMY: FRC PNEUMATICS

High Pressure (115 psig) Components

Compressor

Distribution block

Pressure Switch (compressor on/off at 115 psig)

Relief Valve(pops if pressure exceeds 120 psig)

Air Storage Tank(rated to 250 psig)

Pressure Gauge

Valve

Pressure Regulator(drop pressure from 115 psig to 5-50 psig )

ROBOTICS ACADEMY: FRC PNEUMATICS

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

ROBOTICS ACADEMY: FRC PNEUMATICS

Application Pressure (5-50 psig) Components

Switch/Valve Bank for Double-Action Cylinder

Double-Action Cylinder (pushes and pulls)

“T” for Tubing

Flow Control Valve (limit cylinder extension rate)

Valve (use lots of these) Pressure Gauge

Pressure Regulator(drop pressure from 50 psig to 5-50 psig )

ROBOTICS ACADEMY: FRC PNEUMATICS

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.

ROBOTICS ACADEMY: FRC PNEUMATICS

Questions?

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