Pneumatics

From Team 449 Wiki

Pneumatics use compressed air to power actuators. The actuators are usually pneumatic cylinders (commonly called "pistons" on the team) or rotary actuators, although these are rare. The primary advantage of pneumatics over motors is that pneumatics provide fast, linear motion. Generally, pneumatics cylinders are binary actuators: they are either in or out. Three-position cylinders are possible, but expensive and take weeks to ship. The pneumatics system of the robot is controlled by the pneumatics control module.

Theory

Air is a compressible fluid. Compressed air is a method of storing energy (E=P \Delta V). and expanding air can be used to do work. In a pneumatic cylinder, the pressurized air is diverted into a sealed chamber, but one end of it is allowed to move. As the air expands, it does work on that end, and whatever is attached to it.
Sometimes, we need the pressure in one part of the system to be different from pressure in another part of the system. For example, the stored pressure in the tanks is allowed to go up to 120 psi, but the pressure going to any devices (the working pressure) may only go up to 60 psi. To do this, we use a device called a regulator, which controls the flow rate of air passing through it. Regulators can also be used to control the pressure going in to one end of a cylinder, as well as other uses. Note that everything in between two regulators, or in the high- or low-pressure sections (see Regulators), will be at the same pressure.

Terminonogy

We use the words upstream and downstream to refer to the relative positions of components of the pneumatic systems. If we imagine air flowing from the tanks to the pneumatic devices, the farther upstream a location is, the less distance an air molecule would travel to get there from the tanks.

Pneumatic Cylinders

Main article: Pneumatic cylinder

Double-action cylinder operation.

Pneumatic cylinders or pistons have a rod in them that moves in and out when air is put in. Single-action cylinders have only one air input, which drives the cylinder one direction. They also have a spring on on the inside to return them to their original position. They can be either spring-extended (rod is naturally out) or spring-returned (rod is naturally in). Double-action cylinders are more commonly used, and are generally preferable to single-action cylinders. They have two air inputs, generally one at each end. When air is put into the bottom, the rod extends; into the top, the rod retracts.

Dimensions

A cylinder is primarily characterized by its bore diameter and stroke length. The bore diameter is the inner diameter of the cylinder. The stroke length is the difference in length between the extended and retracted positions.

Pneumatic controls

In addition to cylinders, a pneumatics system requires various other devices to control the flow of air.

Solenoids

Pneumatics are controlled by solenoid valves, usually referred to simply as solenoids. Single-action solenoids switch air from being either on or off. Generally, these are used with single-action cylinders, although they can be used with double-action ones, such as on the Ballbasaur (2014) catapult. Double-action solenoids are much more widely used, and switch air between two different outputs. They are used to control double-action cylinders. They are designed so that when the air input is connected to one output, the other output is allowed to vent to the atmosphere. This prevents both sides of the cylinder from being pressurized, allowing the rod to move.

Regulators

A regulator imposes a maximum pressure on all locations downstream of it. "The regulator", in systems with only the FIRST-required regulator, refers to the regulator that reduces the working pressure to 60 psi, from 120 psi. Everywhere upstream of it is the high-pressure secton, and everywhere downstream is the low-pressure section. The high pressure section should not have any additional regulators, meaning that the entire section will be at the same pressure. Additional regulators should not be used unless necessary, which is not often.

Gauges

Pressure gauges are used to see what the pressure is in a section of the system. There must be a gauge in both the high- and low-pressure sections. The one in the high-pressure section is the upstream gauge, and the one in the low-pressure section is the downstream gauge. The downstream gauge should be upstream of any additional regulators.

Release valve

The release valve (generally a ball valve) should be in the high-pressure section, and be easily accessible. Its purpose is to allow the system to be quickly depressurized.

Connectors

Pneumatics systems require various connectors to connect tubing together.

Design Considerations

The most important thing to know about using pneumatic cylinders is that loads must be applied parallel to the shaft. Otherwise, the shaft will bend and render the cylinder unusable. Another thing to think about it how the cylinder will be mounted. When used for rotating a mechanism, they are often (though not always) mounted so that both ends can pivot. When two sides of a mechanism both need to be moved, it is possible to do this by mounting a single cylinder in between them. Hard stops are used to restrict the motion of both sides to the desired positions. See the active gear mechanism on Calcifer and the elevator on Totedile.

Force Output

The output force of a cylinder is F=PA where P is the working air pressure and A is the inner cross-sectional area of the cylinder. This should be taken into account when deciding cylinder dimensions. Another important thing to note is that the purpose of a cylinder is to change length. This means that if a cylinder is shorter than it needs to be, an extension can be added to end to make it the right length. Conversely, a cylinder does not have to extend along its entire stroke, if a hard stop is used.

Pneumatics vs. Motors

Pneumatics and motors both have their uses. Since the point of pneumatics is to provide fast, linear motion, they are ideal for actuating something that needs to be in one of 2 or 3 colinear positions. However, pneumatics can also be used when a mechanism rotates between positions. By placing cylinders at and angle, as done on the catapult in 2014 and the wedge in 2016, a mechanism can be rotated by cylinders. This has the advantage that if the mechanism is forced to rotate in the other direction (against the cylinders), it is much less risky than backdriving a motor. However, when using cylinders to rotate a mechanism, one needs to remember \tau=rFsin\theta: the torque provided by a cylinder is weaker the closer it is mounted to the pivot and the smaller the angle between the shaft and the mechanism.