Getting the Power to Control

By: Thomas Yoon

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 folksScience Articles, be in control!

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