Actuators

Title: Actuators

Authour: Rich Schuster

Date Presented 2008.12.12

Introduction
In a chemical process, actuators are needed. The classic need is switching a valve from open to closed, and depending on the valve type, the motion needed to open it changes. Actuators can provide motion in either a linear or circular fashion, and can derive their power from a number of sources.

Pneumatic
Compressed air is regularly available in a facility, which makes for an effective actuator. Air can be used for both a rotational or linear actuator, however a rotational actuator commonly rotates only to 180 degrees. Pneumatic motors, on the other hand, are found in many tools, and can turn continuously, however discharge air at a rapid rate, and inherently inefficient.

A pneumatic cylinder is used to achieve linear motion, and consists of a piston encased in a cylinder. When there is more pressure on one side of the piston, the piston moves until the forces are equal or it has finished it's travel. A pneumatic cylinder can be either single acting or double acting, where a single acting cylinder can be driven only in one direction (returned either by outside force or an internal spring), while a double acting can be driven in each direction. The force exerted by a pneumatic cylinder is a function of the area of the piston, so a cylinder is stronger in one direction than the other (the linkage takes up area on one side). This is normally not a problem, however can cause the cylinder to crawl when both sides have equal pressure.

Hydraulic
Using liquid fluids to actuate is similar to using compressed air, and most hydraulic actuators have a pneumatic counterpart. Hydraulics can be difficult to use because they require a hydraulic circuit to carry the fluid to and from the actuator. Pneumatics on the other hand, can vent to atmosphere. Hydraulics are also prone to mess if a leak is to occur, and oil can be a fire hazard. However hydraulics are advantageous over pneumatics largely for their inability to compress. This can be an advantage or disadvantage; pneumatics add a level of dampening to a system to protect the equipment, while hydraulics can supply more force faster. This is why brake lines are hydraulic, and why it is imperative to bleed brakes of air.

Mechanical
Mechanical can be a reference to two types of actuator, either manual or a linkage, although it is commonly thought to be manual. You are familiar with a manual actuator, turning on a spigot is an example. These are cheap and easy to install, however automation is impossible.

A mechanical linkage is used to link two elements of a process directly together. An excellent example of this is a toilet tank. The float is connected to the flush valve at the bottom of the tank mechanically, and the valve is automatically closed when the float is at level. A linkage is advantageous because of it's simplicity and reliability, however it is very difficult to optimize once constructed.

Electric
Electric actuators are very diverse, and electricity is readily available, even outside industrial settings. Potential problems with electric actuators is that they can be potentially overcomplicated, and less reliable than their counterparts.

Most people are familiar with rotary electric motors. Electric motors run on AC or DC current, and run on one phase or three phase electricity, depending on the environment (if you ever need to run a three phase motor on one phase and you don't have a converter, just wrap a cord around the shaft and give it a spin to start... be careful). Electric motors are used in locations where many rotations are needed, such as a stirrer in a tank.

A stepper motor is a specific kind of electric motor. For most electric motors a voltage is applied across the terminals, causing the rotor to spin. A stepper motor is controlled by a microcontroler instead, so the position of the motor can be controlled directly, without feedback. A stepper motor is rated by the resolution, or how many steps it has per revolution, such as 100 or 1000. This makes them more accurate to drive, and are frequently used in positioning systems, such as CNC machines. Similar results on other motors can be achieved through the use of a shaft encoder.

Electricity can also be used to perform linear motion. Solenoids are light load electric linear actuators, used to control pneumatic and hydraulic actuators. Electro-mechanical actuators are achieved by replacing a manual knob with a motor. These are easy to implement, however to achieve respectable force significant gearing is needed, increasing complexity and decreasing reliability.

Integrating actuators with valves
The most basic method of actuating a valve is to replace the manual interface, either a knob or a handle, with a rotary actuator. This is effectively what an electro-mechanical actuator achieves. However, more elegant approaches may be taken. Often, a manual version of a valve has a screw in order to achieve linear motion, such as a globe or gate valve. A linear actuator could replace the screw in order to increase simplicity.

A diaphragm valve commonly is actuated by either hydraulics or pneumatics. Above the diaphragm is isolated from the flow of the fluid, and can be pressurized with a fluid to open or close the valve.

Links
ValveTypesSelection Wikipedia Linear Actuator

Wikipedia Electric Actuator