TIMERS
:- All PLC’s have timer instructions. Timers are output
instructions that are internal to the programmable logic
controller. Timers provide timed control of the devices that they
activate or de-activate.
Basic functions
of timer :-
- Timers are used to delay an action.
- Timers are used to run an operation for a predetermined period
of time.
- Timers are also used to record the total accumulated time of
continuous or intermediate events.
Timer’s
instructions :-
Timers consists of following parts: timer address, preset value,
timer base, and accumulated value, as shown in figure below.
fig(a)
In the above figure , term instruction name is, timer on delay (
TON ), timer base is 1.0 seconds, timer address is T4:0,
accumulated value of zero(0) and a preset value of 200.
Each timer instruction has three very useful status bits. These
bits are timer enable (E N), timer timing (TT), and timer
done(DN).
There are 3 types of
timers: -
- On- delay timer
- Off-delay timer
- retentive timer.
On delay
timer :-
- Use this instruction to program a time delay after instructions
become true.
- On – delay timers are used when an action is to begin a
specified time after the input becomes true. For example, a certain
step in the manufacturing is to begin 45 seconds after a signal is
received from a limit switch. The 45- seconds delay is the on-delay
timers preset value.
Off- delay
timer :-
- Off- delay timer instructions is used to program a time delay
to begin after rung input goes false.
- As an example, when an external
cooling fan on a motor is provided, the fan has to run all the time
the motor is running and also for certain time (say 10min) after
the motor is turned off. This is a ten minute off- delay timer. The
ten-minute timing period begins as soon as the motor is turned
off.
Retentive timer :-
- Retentive timer is a timer which
retains the accumulated value in case of power loss, change of
processor mode or rung state going from true to false (rung state
transition).
- Retentive timer can be used to track the running time of a
motor for its maintenance purpose. Each time the motor is turned
off, the timer will remember the motor’s elapsed running time. The
next time the motor is turned on, the time will increase from
there. This timer can be reset by using a reset instruction.
Reset
:-
- This instruction is used to reset the accumulated value of
counter or timer.
- It is used to reset a retentive timer’s accumulated value to
zero.
A typical timer
element :-
A timer element is made up of three 16 bit words:
- Word 0 → 3 status bits (EN, TT, DN).
- Word 1 → Preset values.
- Word 2 → Accumulated value.
fig(b)
Addressing a
timer is as follows:-
- The address format in order to address the timer element is
T4:3
Where, T = T identifies this as a timer file.
4 = represents the default timer file 4.
Note: SLC timer can be assigned any unused file
from 9 to 255.
:3 = This is timer three in file 4. There are 256 timers available
in each file. Timers 0 through 255 are available.
- Preset value of the timer can be addressed in the following
way:T4:3.PRE
Where, T= identifies this as a timer file.
4= represents the default timer file 4.
:3= timer 3 in file 4.
. = this point is the word delimiter. It separates the timer
number, called the structure, from the subelements. Here the
subelement is PRE, for preset value.
- Accumulated value of the timer can be addressed as shown
below:T4:3.ACC
Where, T= identifies this as a timer file.
4= represents the default timer file 4.
:3= timer 3 in file 4.
. = this point is the word delimiter. It separates the timer
number, called the structure, from the subelements. Here the
subelement is ACC, for accumulated value.
The
status bits of the timer can be addressed in the following way:
- Word zero, bit 13, is the done bit. It is identified as DN.
This bit is set when the timer’s accumulated value is equal to the
timer’s preset value. It can be addressed as T4:3/DN.
- Word zero, bit 14, is the timer timing bit. It is identified as
TT. TT is set when the timer is timing. It is addressed as
T4:3/TT.
- Word zero, bit 15, is the timer enable bit. It is identified as
EN. EN bit is set whenever the timer is enabled. It is addressed as
T4:3/EN.
The on-delay
timer instruction :-
Fig(c)
The above figure is used to explain the on-delay timer
instruction.
Here, T4:2 represents timer file 4, timer element 2, preset value
is 50, accumulated value is 0 and timer base is 1 second. Input
module is in slot 1 and output module is in slot 2.
- As long as the instruction I:1/0 is true, the on-delay timer
T4:2 will increment every one second toward its preset value of 50
seconds. The accumulated value displays the current number of
seconds that passed. When the accumulated value is equal to the
preset value, the timer’s done bit will get energized or set. So
when the timer’s done bit gets energized, the rung 003, instruction
T4:3/DN becomes true and logical continuity is passed and the
output O:2/2 gets energized.
- As long as the I:1/4 is true, the timer instruction is enabled.
Hence, rung 000 becomes true and logical continuity is passed and
the output O:2/0 is energized.
Note: An on-delay timer is not retentive in nature.so any loss of
continuity to the timer instruction on rung 000 will cause the
timer to reset itself to an accumulated value of 0.
- When the timer is timing i.e rung 000 is true and accumulated
value is less than preset value, timer timing bit(TT) is true. So
the rung 002 becomes true i.e instruction T4:2/TT is true and
output instruction O:2/1 is energized.
Note: As long as the rung 000 is true i.e instruction I:1/0 is
true, the timer instruction is considered enabled. The enable bit
will be true when the timer timing bit is true. Timer enable bit
will be set through the transition from the timer-timing bit to the
timer-done bit. Timer enable bit is set as long as there is logical
continuity through all input instructions to the timer instruction,
no matter the relationship between the preset value and accumulated
value. When the rung goes false, the enable bit is reset.
The off-delay timer
instruction:-
Fig(d)
The above figure is used to explain the off-delay timer
instruction. Here, T4:2 represents timer file 4, timer element 2,
preset value is 200, accumulated value is 0 and timer base is 1
second. Input module is in slot 1 and output module is in slot
2.
- As an example, consider an external cooling fan on a motor
which has to run all the time when the motor is running and also
for 200 seconds after the motor is turned off. For this purpose, we
use 200- second off-delay timer. The 200-second timing cycle begins
when the motor is turned off.
- When the instruction I:1/0 is true, the motor is turned on i.e
instruction O:2/0 becomes true. In other words, rung 000 becomes
true. When the instruction I:1/0 is true, rung 001 becomes true,
which will make the off-delay timer T4:2 enable. So as long as the
instruction I:1/0 is true, the timer enable bit, EN, is true and
hence, rung 002 become true, which inturn makes the output
instruction O:2/1 true. The done bit is set as long as the rung 001
is true i.e the done bit is set when the enable bit is set. So the
rung 004 is true. Hence, the external cooling fan is energized i.e
instruction O:2/3 is true. So at this point, both motor and
external cooling fan are energized.
- When the motor is turned off, i.e the instruction I:1/0 becomes
false, the output instruction O:2/0( motor) becomes false and motor
is turned off. When rung 001 transitions from true to false, the
TOF( off-delay timer) instruction begins timing. The done bit and
the external cooling fan( O:2/3) will still remains on, or true,
for the preset value of 200 seconds. The time period between the
point when the rung becomes false and the point when the 200
seconds preset time expires for T4:2 is called delay after the
input goes false, or the off-delay. The timers done bit and its
associated output stay true until the off-delay of 200- seconds
expires. The time expires when the accumulated value reaches the
preset value. When the input instruction I:1/0 goes from true to
false, the timer enable bit (EN) is reset and timer timing bit(TT)
is set. The timer will start timing at this point. The timer timing
bit(EN) becomes true whenever the rung transitions from true to
false and the accumulated value is less than the preset
value.
Note: The timer done bit, bit 13, is set when the rung 000 becomes
true. It will remain set through the true to false transition and
until the accumulated value is equal to the preset value. This bit
is commonly used to control the other logic when an output needs to
be turned-on or turned-off after its rung has been off for the
preset time interval.
The retentive timer
instruction:-
Retentive timer instruction is used when we want to retain the
accumulated value through power loss, processor mode change, or
change in the rung state from true to false. The retentive timer
instruction will measure the cumulative time period for which its
rung is true. One of the example of retentive timer is that, it can
be used to track the running time of a motor for maintenance
purpose. The retentive timer is used to track the accumulated time
the motor has run. In our example , our motor needs maintenance
after 3600 seconds or, one hour of running time. Each time the
motor is turned off, the timer needs to remember the motors total
elapsed running time. The next time the motor is turned on, the
timer will increase the accumulated running time from where it is
left off. When the total accumulated motor running time has been
reached, a maintenance remainder pilot light will be lit. A
retentive timer is used in this application.
Fig(e)
Here, T4:2 represents timer file 4, timer element 2, preset
value is 3600, accumulated value is 0 and timer base is 1 second.
Input module is in slot 1 and output module is in slot 2.
The retentive timer on, RTO instruction, behaves similar to the
timer-on delay instruction, with exception that when the RTO
instruction goes false, it will retain its accumulated value.
The retentive timer will retain its accumulated value during the
following conditions:
- When its rung goes false.
- When processor losses power. But, the battery for memory back
up must be in good condition.
- When the processor faults.
- When the processor operating mode is changed from remote run or
remote test to remote program mode.