Rockwell-automation 1772-LV Mini-PLC - 2/15 Programmable Controller (S Manual de usuario descargar pdf (Pagina 8)

Chapter

21

An Introduction to Programmable Controllers

This chapter reviews general fundamentals common to our programmable

controllers (PC’s). When you are finished, you will have read several important

concepts that will help you understand this manual. You’ll be able to:

Describe what a programmable controller does.

List and describe the functions of the four major sections of a programmable

controller.

Describe how the four major sections of a programmable controller interact.

Give an example of a simple program.

If you can do all this now, then turn directly to chapter three.

You are probably familiar with the traditional methods of machine control

(Figure 2.1). Sensing devices located on the machine detect changes in the

machine’s condition. For instance, a part arriving at a work station would

contact and close a limit switch, the sensing device. As a result, an electrical

circuit is completed and a signal is sent to the control panel.

Figure 2.1

Traditional

Methods of Machine Control

Relays

Control Panel

Output

Sensing

Devices

Machine

Devices

10152I

At the control panel, the electrical signal enters a bank of relays or other

devices, such as solid state modules. Circuits within the control panel open or

close causing additional electrical signals to be sent to output devices at the

machine. For example, a relay energized by the limit switch may complete

another circuit energizing the output device, a clamp, which secures the part at

the work station.

Objectives

Traditional Controls

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Rockwell-automation 1772-LV Mini-PLC - 2/15 Programmable Controller (S Manual de usuario Pagina 8