Trust! But, Verify!
"Trust, But Verify!"
That was a popular phrase not too many years ago.
I have to take exception to the comment that...
If using a plc there is no need for aux contacts, the timer can be initiated when the output is energized.
It is certainly true that a PLC can simply turn ON an Output and then
ASSUME that the particular function controlled by that Output is, in fact, operating as expected.
While, in the most strict sense, AUX contacts are not really absolutely required, it is foolish not to use them if they are available.
Building a Control System without the
"Trust, But Verify" concept is
"Minimalist Engineering". A system designed in the
"Minimalist Engineering" style can and will work properly only as long as
everything is OK. If
anything goes wrong, then this system has the potential to Forkulate itself right up against one of the more unforgiving Laws of Physics...
Two (or three, or four,...) bodies CAN NOT occupy the same space at the same time!
In addition to
causing bodies to be in particular spaces at particular times, a control system is also supposed to
prevent multiple bodies from trying to occupy the same space at the same time.
While I am sure that RS simply meant that,
electrically, the AUX contact is not required to run the timer, which is true, I suggest that,
logically, the AUX contact
is required!
The first reason being that the PLC should
KNOW that the particular device is operating as expected. (at least, to the extent that the Mag or Drive is responding to the command). There is always the possibility that the motor is running, but a coupler, chain or whatever, has broken. Other inputs are required to monitor those conditions.
The second reason is that, if the timer is supposed to be measuring the "ON-TIME" of a particular device, then the question becomes...
Are we looking for "COMMANDED ON-TIME" or "ACTUAL ON-TIME"?
If the answer is
"COMMANDED ON-TIME", then the AUX contact
is not required for the timer - although it is still required (or, at least, HIGHLY recommended) for the process.
If the answer is
"ACTUAL ON-TIME", then the AUX contact
is required for the timer as well as the process.
Now, with respect to the accumulation of time...
This depends on how your particular PLC works. If you have a 1-sec pulse that is automatically produced by the PLC, then use that pulse, as Pierre suggested, to ADD 1 to a retentive register (memory word or Double-Word).
If you don't have the automatic pulse, then do as Tom suggested to get the pulse (1-sec, 1-min, 1-hour... whatever) and then use that pulse, as Pierre suggested, to ADD 1 to your retentive "time-register".
As I recall, GE timers are "count-down" as are TI timers (S7-200 timers are "count-up"). Also, there are .1-sec, .01-sec and .001-sec timers. Decide which one you need for the particular precision you need. If you are essentially building an Hour-Meter, don't bother with the .01 or .001 timers (save them for other more critical issues); use the .1-sec timer.
TIMER PRESET * PULSE (Approx**)
.001 1001 Once per Second (+.001/-.000 Sec)
.01 101 Once per Second (+.01/-.00 Sec)
.1 11 Once per Second (+.1/-.0 Sec)
* The extra "1" at the end is to ensure that a full second has passed.
** These timers are also subject to error caused by scan time.