Getting the Power to Control
One of the advantages of using machines to control processes for you is that you can harness tremendous amounts of power and energy which humans do not possess.
By a flick of a switch, huge sliding flood gates can move to allow large quantities of water to flow out of a dam. With just a slight twist on a handle, supertankers weighing hundreds of thousands of tons are able to turn because of the corresponding rotation of huge rudders.
Similarly, processes operating valves can be remotely operated to control flow, temperatures, pressures and so on.
All these final control elements - flood gates, rudder, valves - must be operated by actuators. The function of the actuator is to move to control elements in the processes.
In order to move the actuator, there must be a medium for transmitting the required power or action. All the actuators are mechanical in nature. The common means of transmitting the power are: pneumatic (air), hydraulic (fluid), and electrical (motors).
The actuators must be designed to work for that medium. There are several advantages and disadvantages for choosing a particular medium. For example, pneumatic actuators are particularly useful when the atmosphere contains flammable gases. By using pneumatic actuators, there is very little chance that a spark can be generated to cause a fire. However using air for actuating may be more expensive to install and to maintain.
Using electrical motors to drive gearing may be an excellent choice when cost is a consideration. Electrical wires can be laid more easily and electrical systems are relatively clean.
Use of hydraulics may be excellent where power and simplicity is important.
Most of the actuators operate in a linear (straight line) manner. Hydraulic rams, and pneumatic diaphragms work in this manner. There are also some that are constructed to operate in a rotary manner.
Electrical motors normally operate in a rotary manner. However if a linear motion is needed, this can be obtained by means of spur gears, rack and pinion arrangements, crank and other means.
Actuators can provide the power to move final control elements, but do they know when to stop?
A good control system will provide the sensing and feedback loop to tell the actuator when to stop its actions.
Well folks, be in control!
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