Programmable Logic Controllers (PLC) - Part 5

Programmable Logic Controllers (PLC) - Part 5

PLC Timer Functions

Outline

• Introduction

• PLC Timer Functions

• Examples of Timer Function Applications

Objectives

• Describe PLC retentive and delay timer functions.

• List and describe major timing functions that are commonly used in circuits and processes.

• Apply PLC functions and PLC circuitry to process control for timing functions.

• Apply PLC timers in multiple timing problems that combine two or more of the basic timing functions.

• Apply PLC timers for the control of processes.

Introduction

• The most commonly used process control device after coils and contacts is the timer.

• The most common timing function is TIME DELAY-ON, which is the basic function.

• There are also many other timing configurations, all of which can be derived

from one or more of the basic TIME DELAYON functions.

• PLCs have the one basic function timer capability in multiples.

Timer Uses

• The timing block functions are used with various contact arrangements and in multiples to accomplish various timing tasks.

• Typical industrial timing tasks include timing

of the intervals for welding, painting, and heat treating.

• Timers can also predetermine the interval between two operations.

• With a PLC you can utilize as many timer blocks as you need, within the PLC memory limitations.

Basic Timing and Derived Functions

• Here we illustrate the basic PLC TIME DELAYON function and some other derived timing functions.

• Typical of the derived functions are

TIME DELAY-OFF,

 interval pulse timing, and multiple pulse timing of more than one process operation.

Timer Variables

• One major advantage of the PLC timer is that its time may be a programmable variable time as well as a fixed time.

• The variable time interval may be in accordance with a changing register value.

• Another advantage of the PLC timer is that its timer accuracy, repeatability, and reliability are extremely high because it is based on solid-state technology.