Introduction to Pneumatics

What are Pneumatics?

• Pneumatics is the use of pressurized gas (we use compressed air) to create motion.
• Similar to Hydraulics, which uses fluids instead of gases.

Why use Pneumatics?

• Pneumatic Cylinders are much lighter than motors. Although there is a large base weight (compressor, accumulators, etc), as long as several cylinders are used, it usually balances out to be lighter than motors overall.
• Pneumatics are strong. Anywhere from 0 to 188 lbs of force per cylinder, depending on bore size and pressure.  Strength is easily adjustable with flow control fittings.
• Pneumatics are simpler than motors. Once the base system is setup, it is easier to mount a cylinder than a motor.  Furthermore, pneumatics do not require complex chain, cable or rack-and-pinion systems to achieve linear motion.
• Pneumatics are more durable than motors. When motors burn up, pneumatic cylinders simply stall if they do not have enough force to complete the task.  Stalling a pneumatic cylinder causes no damage to the cylinder.

Disadvantages of Pneumatics

• High initial weight cost.
• No intermediate positioning. The cylinder is either all the way in or all the way out.  It is hard and typically impractical to construct a pneumatic system that allows the cylinder to stop mid-stroke.

Applications of Pneumatics

• Pneumatics are great for straight movement.
• Linkages can be created if rotation is needed.
• Great for grabbers, as shown in 254's 2004 robot.

Pneumatic Components

Compressor

• Converts electricity to compressed air.
• Has attached relief valve to protect system from too much pressure.  Relief valve bleeds air at 120psi
• Controlled by a Relay Module using a 20amp breaker, not a fuse.  Always run in forward.  Do not attempt to create a vacuum by running it in reverse.
• Can produce significant vibration.  Rubber vibration isolation mounts are provided in the kit and are recommended.

Accumulator(s)

• Store compressed air at 120psi
• Up to four can be used on the robot.

Pressure Switch

• Digital switch detects pressure.  Switches to "on" if pressure drops below 95psi.  Switches "off" after pressure reaches 115psi
• Connects to digital input on digital sidecar.

Regulator

• 2 Regulators are provided in the kit.
• Primary regulator regulates pressure to an adjustable value (maximum of 60psi).  It must be used with a pneumatic system on the robot.
• Secondary regulator allows presence of a reduced pressure leg.  Can be optionally used after the primary regulator on a robot.

Solenoid Valves

• Used to control pneumatic cylinders and actuators.
• Single solenoids always default to one position if power is cut.
• Double solenoids remain in active position when power is cut.

Dump Valve

• Used to release all air in the pneumatic system.

Fittings

• Pipe threads require teflon tape.  Do not wrap on first two threads, as threads are tapered and tape tends to loosen, clogging airflow.
• On quick-release connectors, tubing is attached by pushing into connector.  End of tubing must be clean to ensure proper seal.

Flow Control Fittings

• Regulate flow of air into and out of a cylinder.  Used to control cylinder extension and retraction rate.
• If used to regulate flow, attach directly on only one end of the cylinder.

Cylinders

• Linear actuators.
• Force = Pressure x Area
• Maximum force of 188lbs with 2" diameter cylinder (largest available in FIRST) at 60psi
• Due to rod, area of face when retracting is smaller than area of face when extending.  Therefore force is greatest when extending.

Additional Resources

• 2009 FIRST Pneumatics Manual

• Types of Gears
• Gear Terminology
• Involute Tooth Profile
• Gear Ratios

Types of Gears

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There are many types of gears. Gears mesh with other gears of the same type.

Spur Gears

• Straight teeth
• Meshing gears are on paralell shafts.
• The most common gears used in FIRST.

Helical Gears

• Angular teeth cause each tooth to engage with the matching tooth gradually, resulting in smoother engagement.
• Meshing gears are on paralell shafts.
• Produce side loads

Bevel Gears / Miter Gears

• Gears have teeth on an angle.
• Meshing gears are on perpendicular shafts
• Bevel gears - 2 meshing gears have different tooth count.
  Miter gears - 2 meshing gears have same tooth count.

Worm Gear

Gear Terminology

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Tooth Count: The number of teeth on the gear

Pitch Circle: The circular profile of the meshing surface of the gears

Pitch Diameter: The diameter of the pitch circle. Can be derived using formula (Tooth Count / Diametrical Pitch).

Diametrical Pitch (DP)

• The number of teeth of a gear per inch of its pitch diameter.
• Measure of how big the teeth are.
• Common sizes are 12 (big teeth), 20, 24, 32 (small teeth)
• Meshing gears must have the same diametrical pitch

Pressure Angle

• The angle at a pitch point between the line of pressure which is normal to the tooth surface, and the plane tangent to the pitch surface.
• Most commonly used pitch angles (for spur gears) are 14.5° and 20
• What you need to know: Meshing gears must have the same pressure angle.

The Involute Tooth Profile

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• If gears had straight teeth, the distance from the center of the gear to the point of contact would be continuously changing, resulting at speed being transferred inconsistently as shown in this animation.
• The involute tooth is curved so that as the gear turns, the tooth engages with the meshing tooth in such a way that the speed is transferred inconsistently as shown in this animation.

Gear Ratios

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• Gear ratios allow gears to change the speed output and torque of a transmission.
• Power is fixed (with a given motor).  The faster a gearbox is geared, the less torque it has.
• To find the ratio, divide the number of teeth on the output gear by the number of teeth on the input gear.
• The teeth on the meshing gears will go by at the same rate.  Because the circumference on a smaller gear is smaller, it will rotate more times for every rotation of a large meshed gear.
• Multiply ratios together to get total gearbox ratio.

Part One: Components

Below are all basic components of the 2009 FIRST Robotics Compeition Control System.  No sensors are included.  This document provides an overall wiring reference.

Part Two: Power Wiring

This section will discuss proper power wiring of an FRC robot.  This guide may be missing small details.  Working with electricity is dangerous!  Do not attempt any wiring, crimping or soldering without proper training from a NASA Lab Mentor.  Always have an experienced team member or mentor check over your wiring before power-on.  This section will reference this diagram.

Battery to Power Distribution Board

Both leads of the battery pass through the Anderson Quick-Disconnect connector.  The negative wire passes straight from the joining quick-disconnect connector to the negative terminal on the power distribution board.  The positive wire passes from the quick-disconnect connector to the "BAT" terminal of the main power breaker.  The "AUX" terminal of the main power connector connects directly to the positive terminal of the power distribution board.

cRio Mobile Device Controller

Wire from the 24v out on the end of the Power Distribution Panel to the 24v in on the cRio Mobile Device Controller.

Analog Breakout Board(s) and Digital Sidecar(s)

Wire from any 12v output on the Power Distribution Panel to the 12v input on each digital sidecar or breakout board.

Robot Signal Light

Wire from the "RSL" output on the Digital Sidecar to the input on the Robot Signal Light.

Wireless Bridge

Wire from the regulated 12v output on the end of the Power distribution panel to the 12v input on the Wireless Brige.

Speed Controllers and Relays

Wire from any 12v output on the Power Distribution Panel to the 12V input on each speed controller or relay.  Note that some motors have minimum fuse ampage ratings.

Part Three: Signal Wiring

This section will use reference this diagram.

Digital Sidecar(s)

Wire one DB-37 Cable from the output of a cRio Digital Module to the input on the digital sidecar.

Wireless Bridge

Connect RJ-45 Ethernet Cable from Port 1 on cRio to input on Wireless Bridge.

Speed Controllers

Connect a PWM cable from one of the ten PWM outputs on a digital sidecar to the PWM input on a motor speed controller.

Relays

Connect a PWM cable from one of the 8 relay outputs on the digital sidecar to the input on the relay.