My name is Josh, and I’ve been working in automation since 2016. I am currently employed by Festo as an Application Engineer (feel free to shoot me a PM if you ever need Festo help!), primarily helping automotive customers select the right valves, pneumatic actuators, and electric actuators for their machine designs.
The goal of this post is to provide a basic guide to FRC-focused pneumatics systems. Commonly used terms will be defined, some basic best practices will be covered, along with some common pitfalls and FAQs. Other useful resources I’ve stumbled across or been sent by others can be found at the end.
This is not intended to be a definitive guide on how to implement a pneumatics system from nothing, however, one of the resources linked at the end covers this fairly well if that’s what you’re after. I have not evaluated all information in every resource I linked for accuracy, nor am I enough of an expert on all topics to do so.
Definitions have been sourced from my own working knowledge and research, and are targeted at a beginner’s level of understanding. If you have any feedback on terms I may have missed/poorly defined, best practices you have developed, or other useful resources, please chime in!
- Flow rate - a measure of the volume of air that will pass through a component over an amount of time, generally related to port size and component design; typically described by Cv rating or in Standard Cubic Feet per Minute (SCFM)
- Cv - A coefficient used to describe flow, related to the effective area of the flow path (more info can be found here, but it’s not exactly light reading)
- SCFM - The volume of air at a specific temperature that flows through a components given a specific drop in pressure across the component (see “standard nominal flow rate” in this document)
The internals of a 5/2 (five port, two position) single-acting, pilot-operated, body-ported spool valve, from Festo marketing materials with a few minor edits from me (aka the squiggly-looking lines showing flow)
- Solenoid Valve - a valve that switches the path of air flow using an electromagnetic solenoid
- Solenoid (From Merriam-Webster) - A coil of wire usually in cylindrical form that when carrying a current acts like a magnet so that a movable core is drawn into the coil when a current flows and that is used especially as a switch or control for a mechanical device (such as a valve)
- Pilot-operated valve - a type of valve that utilizes air pressure to assist the electromagnetic solenoid in switching the fluid path; This video illustrates the principle well, but please note that it uses a 3 position valve which aren’t commonly used in FRC
- Position - A working state of a valve; typical FRC valves are two-position, but three-position valves can be used for some interesting functions as well (maybe a post for another time)
- Port - An opening on the valve to atmosphere; valves will generally have a supply port and varying numbers of working and exhaust ports depending on valve specifics; in the above image, port 1 is the supply port, and the path is out of port two, while air is exhausting from port 4 to port 5
- Way - paths that the fluid can take into or out of a valve; sometimes used interchangeably with port, depending on how you define exhaust paths
- Spool - The portion of the valve that is physically moved to change the path of air flow; note that different manufacturers of valves have different approaches to this
- Single-acting/single solenoid valve - A valve that uses a single solenoid to switch from one position to the next, but then has some sort of spring return to its original state; won’t hold an actuator in position upon disable; see the above image for an example of the internals
- Double-acting/double solenoid valve - A valve that uses two solenoids to switch between valve positions; will hold the position of whichever solenoid was last activated, persists on disable
- Body-ported valve - A valve like that in the image above, where working air comes out of the valve body; Convenient for keeping valves as close to actuator as possible, but require additional wiring/plumbing considerations usually
- Manifold-ported valve - A valve that must be mounted to a manifold, where working ports and supply ports will be out of the bottom of the body and through the manifold; Good for reducing footprint of components with common air supply and single location to run wiring to and from
- Manifold - A block designed to interface with the ports of a valve; Capable of supplying pressure or power to multiple valves depending on configuration
- Pneumatic Actuator - blanket term for an air-powered device that turns air pressure and flow into motion
- Cylinder - Most common type of pneumatic actuator used in FRC
- Piston - frequently, and incorrectly, used to refer to a pneumatic cylinder; actually refers to the sliding part inside the cylinder that is pressurized on either side to extend or retract the cylinder
- Piston rod - The part of the actuator that extends out of and retracts into the cylinder body; care should be taken to avoid non-axial loads (strong under compressive loads, weak under bending/side loading)
- Single-acting cylinder - a cylinder that requires pressure to change from its original state (can be normally retracted or normally extended), but then returns by spring force when pressure is no longer applied (similar to a single-acting valve in that it will not persist in its actuated state, but only if pressure is not maintained as opposed to electrical power for the valve)
- Double-acting cylinder - a cylinder that changes states based on which side of the piston has been pressurized
Threads And Fittings
Image: Tapered and parallel thread interfaces, from this page
- Tapered and parallel threads - With different pneumatics components you may encounter different types of these two classes of thread; see above image for examples
- Common tapered threads you may encounter: British Standard Pipe Taper (aka BSPT or R thread), National Pipe Thread (aka NPT), and NPTF (similar to NPT, but more threads-per-inch, frequently found on the end of piston rods); A closer look at the difference between BSPT and NPT can be found here
- Common parallel threads: British Standard Pipe Parallel (aka BSPP or G thread) and metric threads, such as M3, M5, M7, etc., where the number represents the major diameter of the thread
- Please note that numbers used in describing threads (G ½, ⅛” NPT, etc) that are non-metric do not necessarily refer to a nominal dimension of the fitting; This page gives a good look at the actual dimensions of different thread types
- Tapered male threads can be used with female-threaded parallel ports of nominally the same size, i.e. you can use an R 1/2 (BSPT) male fitting in a G 1/2 (BSPP) port, as long as you use an additional sealing element (teflon tape) like you would with any tapered threaded connection
- NPT (not NPTF) and R tapered threads require some sort of addition to the threads to create a seal. This is often achieved with teflon tape. Some caution must be taken to ensure a proper wrap, and therefore a proper seal, and to avoid tape fragments clogging up your system. The video on this page from Swagelock demonstrates the process well.
- Straight/Parallel-thread fittings rely on an o-ring or other sort of element to create a seal between surfaces. This page gives a good overview on different thread types and their sealing methods.
- Tubing ends should be cut as straight as possible, and should be pushed as far into a push-to-connect fitting as they can be (Order transposed per @GeeTwo’s suggestion). You can get a proper hose cutting tool for <$10 from many places.
Air Consumption Reduction
- Single-acting cylinders - By utilizing spring force to extend or retract, you’re approximately halving your air volume requirement per sequence; be careful, as this might not be feasible if your application requires significant force in both extension and retraction, or if uncontrolled movement when the robot gets disabled is dangerous or undesirable
- Use double-acting cylinders without plumbing to the retract port to achieve a similar effect to a single-acting cylinder (depending on the application and desired function, the extend port could also be left open to atmosphere); see this pic from this post and zoom in on the central cylinders for an example
Avoid side-loading your piston rods as much as possible
- Please, if you’ve read through the rest of this, I make this one simple request: your piston rods will thank you (and they’ll remain much straighter, which only helps keep them extending and retracting quickly and with force!)
- Using clevices or other cylinder mounts that allow some play in different directions will help with this
- Look at existing industrial solutions for inspiration
Common Problems and FAQs
Valve Not Switching/Actuator Not Actuating
- Double check the operating voltage for your valves per the manufacturer, and that the PCM is providing said voltage
- Make sure any manual overrides a valve has, on the valve body, are not in a locked position preventing the valve from electronically switching
- Via @Nate_Laverdure: If a brand-new pilot-operated is not behaving as expected, manually actuating the valve may help “unstick” the pilot valve; the valve spool may also have settled into an undesired intermediate position and manually actuating it will help ensure it is put into a correct state
Can I use a single-acting valve with a double-acting cylinder?
- Yes! Internally, your typical single- and double-acting 5/2 valves are virtually identical aside from how the spool moves; see Spectrum’s Advanced Pneumatics Guide linked below for details on when/why this is desirable
Is [particular valve or actuator] legal?
- Highly dependent on a given year’s rules; Restrictions in the past have been based around Cv or flow rating, port size, tube size, etc.
- It should be noted that pneumatics components are generally considered “sacred” by the manual, i.e. they are not allowed to be modified like some other COTs components may be
- Please read the rules from a given year closely
- In 2019, some confusion was caused due to this portion of the manual:
Image: A screenshot of rule R84C from section 10.9 of the 2019 FRC Manual Robot Construction Rules
- The text in parentheses led people to believe that ⅛”, and therefore 3mm, were diameter dimensions to look at in regards to port size. If you were to actually measure the diameter of a ⅛” NPT threaded port, it would measure at approximately .4”, depending on where you measure along the tapered thread. This is because the ⅛” refers to the nominal pipe size of hard pipe that would be used in a fitting, which is based on the pipe’s inner diameter. No, it’s not intuitive. Yes, it is an incredibly mundane topic to try and research. Looking up the dimensions for a given pipe thread size will help you find your answer.
FIRST Pneumatics Manaul 2017 - A good overview of the KoP pneumatics components, I haven’t seen a more updated version, so please be conscious of rule changes/legality of components
Spectrum Advanced Pneumatics Guide - For those looking to take steps in strategic component choice, along with some good advice on special applications of pneumatics
The Compass Alliance’s Pneumatics Pathway - Beginner Building Blocks and other linked resources cover lots of ground, but keep in mind rules have changed since some of the documents were produced
Mead Handbook - Good overview of several pneumatics topics, including useful charts and sample calculations for determining cylinder speed, required air volume, and other things
[CD Post]: A Beginner’s Guide To Pneumatics - A post from 2014, so be wary of any changes in legal components or rules, but a very thorough walkthrough of how to setup a pneumatics system from scratch