Thursday, May 30, 2013

FRC Pneumatics

I setup an FRC compressor with the basic pneumatic kit from Andy Mark.   This was my first foray into pneumatics with the exception of a failed attempt with an educational kit from Clippard. The theory of pneumatics is well documented and the FRC resources out there are extensive.  The links below are for two different primers on pneumatics.

FIRST 2013 FRC PneumaticsInfo_RevC
FIRST Pneumatics Explained 

The setup I used involved a 12V compressor with a max rating of around 120psi.  A pressure switch increases from a short to high resistance when the pressure  reaches 110psi, allowing feedback control to be implemented if the compressor is driven through a microprocessor which also reads the pressure switch.   While the master circuit is highly pressurized, a regulator can be used to create a lower pressure circuit (60psi) which is used to drive pneumatic cylinders.

Learning involved here included how to use teflon pipe tape (1-2 wraps max, wrap in the same direction that you will turn the female so that the tape is locked in, leave thread off the last 2-3 threads), to turn carefully when using NPT fittings in order to not strip threads (which leads to pressure leaks), to wire up the regulator correctly (the arrow indicates the OUTLET, not the INLET), and to use a correctly rated relay for the compressor (which can draw up to 10A at startup... the first relay I used overheated drastically because of the current running through it).

I also learned a great deal about solenoids and about pistons/cylinders.  Solenoids come in two types, double and single.  A double solenoid has two separate electromagnets which must each be separately controlled by your microprocessor.  Switching one on and other off diverts air pressure into one of the two outlet ports.  Reversing both electromagnets switches the port, causing the cylinder to pull in or extend, depending on how it is hooked up.  A single solenoid has one electromagnet and a spring to return to a default position.  Energizing the electromagnetic cause the air pressure to switch from the default port to the other port.  When the electromagnetic is released, air pressure switches back to the default.
Single Solenoid
Double Solenoid
For the single solenoid shown above, the inlet port (labelled #1 on the bottom) is open by default to the top right port (labelled #2).  When 12V is applied, port #2 is now connected to a release port (open to atmosphere) labelled R2 or #5 in the bottom right.  Simultaneously, the inlet #1 is now connected to the top left port (labelled #4).  So if ports 2 and 4 are connected to two ends of a cylinder, the pressure will switch, causing the piston in the cylinder to switch locations (out if in, in if out).  When the 12V is removed, port #4 vents through R1 (labelled as #5, bottom left port).  This is shown below.


The double solenoid is similar, although a little simpler because of the symmetry.  In this case, applying 12V to either side of the solenoid connects the inlet (#1) to the port on that side (#2 on the right, #4 on the left).  The other outlet port then vents (#4 vents to #5, or #2 vents to #3).  If neither or both of the electronics see 12V, behavior is unpredictable; there is no default behavior to this type of double solenoid.


I learned a bunch about pneumatic cylinders.  I knew the basics already, namely about pressure versus force.  Cylinders have a bore which is the diameter of the pressure chamber and of the piston.  The bigger the bore, the larger the cross sectional area upon which pressure is being exerted.  This leads to a larger force, since F = P x A.  You have several other choices with cylinders, including the initial length, the stroke (how far the piston extends), the methods of mounted on each end, and double acting vs. single acting cylinders.  When using a solenoid of the types listed above, double acting cylinders, which have two air inlet ports, are used.  This allows active motion of the cylinder with positive air pressure to cause motion in both directions.  Conversely, single acting cylinders have only one port.  The cylinder is extended by air pressure, and then has a strong spring on the inside.  When the air pressure is relaxed, the spring retracts the cylinder.

I used Bimba cylinders, the type used by FRC this year.  They have a website which allows you to customize the cylinder you want and then display it.  You can even export CAD files of the cylinder:  http://www.bimba.com/Products-and-Cad/Actuators/Inch/Round-Line/Non-Repairable/Original-Line-Cylinder/ .  In my case, I used a Bimba 096-DX which has a 1.09 “ bore, 6” stroke, is double acting and has a rear pivot hole for mounting. I bought a D-166-1 rod clevis for the front.  The cylinder is 10.78” in length from front to back pivot points.

Simple arduino code was written which fires the cylinder in either direction based on a keyboard serial input.  The code is available here.  A video of the setup in action is shown below.




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