Pneumatics (broken)

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.

Pneumatic Cylinders

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

Pneumatics are controlled by solenoid valves. 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.

Pneumatics board

The pneumatics board is the term for all of the equipment except the solenoids needed to run a pneumatic system. It consists of storage tanks, the upstream pressure gauge, the regulator, the downstream pressure gauge, and the pressure relief valve.

Design Considerations

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.

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.