Pneumatics

2022-07-24 flower-machine pneumatics industrial-automation 22 min read

This is the next article in the series about our "Flower Machine".

Pneumatics means using compressed air to perform mechanical work. Our Flower Machine actually works using pneumatics only! But why would you choose pneumatics? And what components are needed? In this blog post, we will give an overview of pneumatics and their relationship with automating manufacturing.

Image of some pneumatics

Pneumatics in Manufacturing

In automated manufacturing, the most basic requirement is something that moves. There are many ways to achieve movement of course, with different trade-offs each. Pneumatics is one of them and has some key features that make it a great fit for manufacturing:

As a demonstration, take a look at this video. Almost all the movements you can see are driven by pneumatics. This is the kind of application where pneumatics fits the requirements perfectly.

Giving an Overview

The hardest part for us when building our Flower Machine was that we did not know what there is to know. Sadly, information about pneumatics is sparse and there are few places to find an overview over all the pieces to the puzzle. Thus, instead of writing yet another article explaining the basics, we want to focus instead on giving an overview and showing where the detailed information can be found.

Between valves, pipe threads, and vacuum - we have a lot of ground to cover:

  1. The Basics
  2. Compressed Air Source
  3. Safety
  4. Types of Cylinders
  5. Directional Control Valves
  6. Connections: Tubing, Pipe Threads, and Fittings
  7. Controlling Speed
  8. Electro-Pneumatics
  9. Vacuum Suction Technology
  10. Pneumatic Schematics

1. The Basics

Nonetheless, we need to take a moment for the minimum basics to get started. Let's begin with arguably the most important part of a pneumatic system: the actuators, the parts that actually move.

These are called air cylinders. A cylinder has one or two chambers that can be filled with compressed air to make the cylinder move. For example, here is a double-acting cylinder. Double-acting means it has two chambers: One to extend and one to retract.

Image of cylinder mechanism

To control the air going into the cylinder, we need valves. More specifically, we need directional control valves. Theses valves can open to let air into one chamber of the cylinder and at the same time exhausting the air from the other chamber. Here is a schematic to visualize this concept. Red is the pressurized side and blue is the exhausting side. We will go into more detail about this later, when talking about the valves again.

Image of valve in retracting position
Image of valve in extending position

And that's all you need to know for now. Of course the devil is in the details. But in the end, almost all pneumatic systems are just a more complicated version of this.

If you want a more thorough explanation of the basics, have a look at these resources:

2. Compressed Air Source

Before doing anything with pneumatics, compressed air is needed. Compressed air is usually generated by a compressor. From there, it passes through a filter to remove moisture and particles. Valves and cylinders will wear out quickly if the air is not clean.

Next, it enters a regulator where it is reduced to a stable operating pressure. As the pressure has direct impact on the speed and force of the air cylinders, it is important that supply pressure is well regulated and does not fluctuate too much.

Commonly, the filter and regulator can be combined into a single FR-unit or filter-regulator. A manometer/pressure gauge is attached so you can keep an eye on the actual pressure in the system. Here is an example of such a unit:

Image of an FR-unit

Typical operating pressure of pneumatic systems (i.e. after the regulator) is around 4 to 6 bar. The general rule is to keep the pressure as low as possible as air consumption is directly proportional to it. Compressed air is a quite expensive source of energy so keeping consumption low is important.

Maintenance Unit

For any real application, the filter-regulator is just one part of the so-called maintenance unit. Other componenents which are usually found in a maintenance unit might be:

Image of a maintenance unit

More details about compressors, filters, and regulators are explained in the "How a Industrial Pneumatic Systems Works" video on YouTube.

3. Safety

Before diving in, we have to talk about safety. Pneumatic systems have a number of dangers that you need to be well aware of:

The whole topic of safety is quite complex. Ideally, you should make yourself familiar with the local regulations and standards for machine safety and adhere to their requirements. Don't take this blog post as any kind of safety advice.

4. Types of Cylinders

Let's start by going back to the air cylinders. There are many variants available.

First of all, the example above just showed a double-acting cylinder. These have two chambers and air is needed for movement in either direction.

There are also single-acting cylinders with only one air chamber. In place of the other chamber, a spring is used which automatically pushes the cylinder back as soon as the air chamber is no longer pressurized.

Image of single-acting cylinder mechanism

Beyond the method of control, there are also many different ways of how cylinders actuate, for different applications. We're going to give a short overview first and then leave you with some links to more detailed articles about air cylinders.

Basics Cylinders

So far, we have only looked at the most basic type: A linearly moving cylinder with a piston rod. In the real world, these cylinders come in many shapes and sizes. Here are a few:

Image of single-acting cylinder mechanism

Such cylinders are essentially identified by two key parameters:

  1. The stroke length describes how far the piston can travel. This can be single-digit millimeters or up to a meter or two. Some cylinders even allow adjusting the stroke length.
  2. The piston diameter, in combination with the supply air pressure controls the force of the cylinder.

The bigger the diameter, the harder the cylinder can push or pull. The relationship is the following:

P is the supply pressure in pascals and D the diameter in meters. So a cylinder with 32mm diameter at 4 bars of pressure will have a force of

So about the equivalent force of 33 kg. As you can see, the force scales quadratically with the diameter. It gets to dangerous amounts really fast.

Beyond this, there are usually several bells and whistles that you can choose from for your cylinder. For example, some cylinders have built-in cushioning at the end for a smooth stop. Without it, the movement can be very abrupt when going fast (which also decreases lifespan).

Guided Cylinders

One thing the cylinders above all have in common is that their rod/piston can rotate freely. For some applications this is not a problem but for many it is. That's why guided cylinders exist. These have additional rods parallel to the piston to prevent it from rotating.

Image of guided cylinders

Slide Tables

Even guided cylinders are sometimes not precise enough. They usually resist rotation quite well but have play laterally. For applications where this is not acceptable, slide tables are an answer. These are, in the end, just a standard cylinder combined with linear rails for zero-play linear movement.

Image of a slide-table

Rotating "Cylinders"

Even though quite a lot of movement is linear in factory automation, sometimes things must rotate as well of course. There are two types of rotating pneumatic actuators:

Here you can see a typical rack-and-pinion type cylinder:

Image of a rack-and-pinion cylinder

Grippers

Finally, because gripping parts is such a common task, special gripper cylinders are available. They have two fingers moving in opposite directions to grip onto something.

Image of gripper

More about cylinders/actuators

This was a brief overview. For more information, maybe look at the following links:

5. Directional Control Valves

To make an air cylinder do anything useful you need to control air flow into its chambers. In the end, any valve would do but specialized directional control valves are best for the job.

Image of single-acting cylinder mechanism

The valves itself also need to be controlled and in modern applications this is almost always done electrically, through use of a solenoid. Thus, these valves are often referred to as solenoid valves.

A DCV has a number of ports, through which air can flow in and out. Typically, there is the pressure port where supply pressure is connected, at least one working port where, for example, a cylinder is connected, and at least one exhaust port where air can exhaust when the valve deactivates.

The function of a valve is encoded in its schematic symbol. You can see all the ports and, in each of the rectangles, how they are connected to each other in each of the positions the valve can take. Further, you can see how each "side" of the valve is actuated. Here, there is a solenoid (-MB1) on the left side and a spring on the right side. This means the valve can be activated using the solenoid and will return back to the original position by spring force as soon as the solenoid deactivates.

Image of valve in retracting position

The port-numbering/naming is standardized and works like this:

Silencers

One component that is visible in the picture and schematic above but wasn't talked about so far are silencers or mufflers. These reduce the noise from the exhausting air which would make quite a loud and sharp sound otherwise.

In the schematic, they are connected to the exhaust ports (3 and 5) and in the image, they are the gold-colored components on the bottom of the valve.

Finally, here are some further resources talking about the working principle of directional control valves in much more detail:

Valve Terminals

In complex machinery, many of these valves will be needed. Because this requirement is so common, special equipment exists to organize a large amount of valves. This is called a valve terminal or valve manifold.

Image of a festo valve terminal

These valve terminals allow organizing a large amount of valves neatly, supplying each one with the supply pressure, and routing away all the exhaust air. Usually, they also bring an electrical terminal to control all the solenoids using a single connection to an industrial bus network (e.g. ProfiBus, Ethernet/IP, IO-Link).

And if you're curious, the large component sticking out the left side is the silencer for the combined exhaust air from all the valves.

6. Connections: Tubing, Pipe Threads, and Fittings

Cylinders, valves, and all other components don't have much use on their own. To make them useful, they need to be connected to each other, to form a pneumatic circuit.

Pneumatic Tubing

This is done using pneumatic tubing, which directs air from one component's port to another's.

Image of a pneumatic tubing

This tubing comes in many different materials and diameters. Importantly, the tubing is dimensioned by it's outside diameter. Common and cheapest is PU tubing. The most common diameters (outside US) are 4mm, 6mm, 8mm, 10mm, 12mm.

Pipe Threads

But even that is not enough. How do you connect the raw tubing ends to a valve or cylinder? The answer is complicated... But generally, the component's ports have an inner thread where an "adapter piece", called a fitting, can be screwed in. The threads here are a science in themselves. These are not e.g. standard metric threads but so-called pipe threads. They come in many variations. We'll try to clear things up a bit:

First of all, there are two main variants: parallel (straight) threads and tapered threads

Image of thread types

Image from www.thehopegroup.com/blog

As you can see, the important difference is the way the seal is accomplished. For tapered threads, it is common to have an additional sealing agent on the threads - often times this means wrapping thin PTFE-tape around it.

With the diameters and thread pitch, things start to get complicated: The Americans have, as always, their own system. Fortunately, the rest of the world has mostly come to a common standard based on the originally British BSP thread types. For all the gory details, there's existing resources you can have a look at:

For us, there are three important standards:

For R and G, the diameters are based on fractions of inches. For example R1/8, R1/4, R3/8, etc. Again, the links above have more details about them all. Your takeaway should be to double-check the thread type for all equipment.

Fittings

Into the threaded ports, a fitting is inserted which has a mechanism to secure a pneumatic tube on the other side. By far the most common (and most convenient) type are push-connect fittings. To insert the tubing, you just push it into the fitting. That's it. And to release it again, you push on the plastic (or metal) ring and pull out the tube at the same time.

Image of straight and angled push-connect fittings

As you can see, these fittings come in straight or angled variants. Usually, you can get them connecting any of the thread-sizes to any of the common tubing diameters. As an example, a PC fitting for M5x0.8 to 4mm OD (outer diameter) tubing.

7. Controlling Speed

Equipped with all this knowledge so far, you'll be able to make a cylinder move. Except, it will move fast. So fast, that you can't even watch it travel. This is not terribly useful...

To slow the cylinder down to a more manageable speed, the air flow must be restricted. This is where throttle valves come in. The most common form is a valve that is a combination of a flow-restrictor and a check valve in parallel. This way, airflow is restricted in one direction, but the air can flow freely the other way.

Image of throttle check valves

The most intuitive idea might be to restrict air flowing into the cylinder's chambers and let air escape from the other chamber freely (through the check-valve, note its direction in the image).

Image of meter-in configuration

The cylinder can then only move as fast as the chamber can fill with air. While this works, it has a big disadvantage. As the chamber is slowly filling with air, the force the cylinder is pushing with will be quite low. Ideally, the cylinder should constantly push with its full force while traveling. This is especially important for achieving smooth motion due to the stick-slip effect.

To do this, we instead restrict the flow out of the opposite chamber while letting air flow freely into the first chamber. This means the piston is immediately experiencing the force of the full pressure and thus will be pushed forward with constant force.

Image of meter-out configuration

In general, this is the configuration that is used most often. The only important thing to remember here is that it depends on one of the chambers starting out pressurized. If you're not careful, the first movement could still end up going at full speed (the solution is a soft-start valve which slowly ramps up the supply pressure when turning on a machine).

Now, going back to the images of throttle check valves from above, you might have noticed how some of them have push-connectors on both sides, but some have a threaded port. That's by design - these are intended to be screwed directly into the ports of a cylinder. This is a very common sight in pneumatic machinery: A cylinder with two throttle valves for controlling its speed:

Image of cylinder mechanism

8. Electro-Pneumatics

When talking about the DCVs (directional control valves) we've already talked about the electrically actuated variant, called a solenoid valve. As most pneumatic applications these days are electrically controlled, the whole field is often referred to as electro-pneumatics.

Controlling valves is only half of the story, though: The electrical brain of the machine also needs to have a way to sense the positions of each actuator to know when movements have completed.

This is done using cylinder sensors which magnetically sense when the cylinder piston head has reached a certain position. A lot of cylinders come with small grooves along their side where the cylinder sensors can be slid in.

Image of a cylinder sensor

9. Vacuum Suction Technology

A seemingly completely different technology which is also a part of pneumatics is vacuum suction. Vacuum suction is heavily used in the industry to grip parts. That's because it is super simple to use and often much easier to implement than mechanical gripping.

Image of cylinder mechanism

Vacuum Ejectors

The first component you need is a vacuum ejector which produces the "negative" pressure. In fact, these actually work by using compressed air as well! The physical principle behind this is the venturi effect.

When compressed air is applied to the pressure port, the ejector sucks in air from the vacuum port.

Suction Cups

On the vacuum port of the ejector, you'll need to connect a suction cup. These come in many different shapes and diameters, each specialized for a certain application.

Vacuum Filters

Finally, there are also special filter which may be connected between ejector and suction cup. Their purpose is to filter out large particles so they do not enter the vacuum ejector. This helps increase the lifespan of the device.

For more information about vacuum suction, here are some links:

10. Pneumatic Schematics

The last topic we're going to talk about is schematics of pneumatic circuits. As everywhere else, it is vital to document the connections and inner workings of a system. And thus, there exist standards for documenting pneumatic circuits as well.

Example of a pneumatic circuit

In general, air flows from the bottom to the top - the compressor is the element at the very bottom and the actuators are at the very top.

To draw such circuits, a number of tools are available. Here are some free options if you want to play with them a bit:

We're going to leave you with a page from the Flower Machine pneumatics documentation:

Example of a pneumatic circuit

That's all for now. We hope this article was insightful and provided a good introduction into pneumatics. If you have any questions or you want to suggest changes/improvements to this page, feel free to contact us!

If you want, you can read the next blog-post in the Flower Machine series here: Poppy Logic Controller. Or head to the overview of all flower-machine posts.