PLC Timer

Introduction :

The most commonly used process control device after coils and contacts is timer. The controller has an inbuilt clock for its functioning for executing its instruction cycle or scan cycle or in that case any of the action, which is very important.
Timer has an internal counter which counts the pulse of this internal clock and generates the required delay. Moreover there are three types of delay

• Logical delay
• Process delay
• Execution delay 

This internal clock is used as a reference when we want to have a logical delay in our program. Depending on number of cycles of the clock the calculation of the delay is calculated. The base time i.e. based on the crystal oscillator present in the controller whose time is taken as the reference for the calculation of the delay can vary for different manufacturers of PLC eg: 1ms, 10ms or 1s. In present scenario our Schneider PLC has time base of 1ms.

Function of Timers:

·        Used with various contact arrangements and in multiples to accomplish various timing tasks. E.g.: Wielding, painting and heat treatment.

·        Predetermine the interval between two operations.

·        Replace the use of industrial timers for driving motor, RC time constant or dash pot it can be easily simulated.

Advantages of Timers:

·        Versatile and flexible than either the industrial or digital electronic timers.

·        Its time may be a programmable variable time as well as a fixed time.

·        Timer accuracy, Repeatability and reliability are extremely high because it is based on solid state technology.

There are four types of timers which are described as follows:

1. On Delay Timer (OND): On delay timers are the timers which are used in most of the applications. This timer comes on after a particular time delay. Thus as the input goes from 0 to 1, the elapsed time starts to increase and when it reaches the time specified by the input preset, the output goes to 1

2. Off Delay Timer: This timer is on for a fixed period of time before turning off. The timer starts when the input signal changes from 1 to 0

3. Pulse Timer: This timer gives an output of 1 for a fixed period of time, starting when the input foes from 0 to 1 and switching back to 0 when the set time of preset has elapsed