Comoon input >60Hz

[dandrade]

Question: Some PLC with capacity we read common inputs above of 60Hz ?

60Hz=16ms 100Hz=10ms 150Hz=6.66ms

I need measure to frequency 100-150Hz. of 10 points.

Do not I want to use, input and modules of high speed

 

[Steve Bailey]

It's all a matter of scan time. 150 Hz = 6.6 mS = 3.33 mS ON, 3.33 mS OFF.

If your PLC scan time is less than 3 mS, and your input filter time is fast enough, you have a chance. Otherwise, you need high speed counter inputs.

 

[randylud]

I think I understand your question!

Depending on the processor you pick, you can read inputs that are changing in the 100 to 150 Hz range without a high speed counter input module. An example would be if the processor scan rate is 5 ms, then it scans the inputs about 200 times/second. However, the closer you get to that scan rate with your input signal, the more chances you take on loosing data. It really depends on what you intend to do with the data too. So, check out the processor max. scan time in the manufacturers literature and judge accordingly.

 

[elevmike]

Are you counting something, or are you talking about your power supply?

Many AC input modules will read varying frequencies of the rated voltage. Consult your PLC manual.

Mike

 

[randylud]

Nice to know most of us sing from the same sheet of music! :site:

 

[dandrade]

Edited

There is not problem, SCAN 1-2ms. I write the program, to execute separate program from the reading

Scan 1,3,5,7 = READ INPUTS
Scan 2,4,6,8 = processing program

or not captured two bordes processing 2 SCAN consecutive.

Think now, sthech pulse circuit eletronic. It add + 14ms

100Hz= 5ms + 14ms = 19 ms 150Hz = 3.3 ms + 14= 17.3ms

Im my country no have modules industrial this it. I know logic circuit, not good idea make.

Have others idea? The project not select brand PLC

Edited: It not WORK, Mine first, thought they are bad.

It stayed recorded, for others remember that is not possible, in that case need to a SCAN of microsegundes.

It can be userfull others cases.

 

[elevmike]

Are you talking about using a power supply with a 100hz sine wave, or a counter pulse with a 100hz square wave???

Are you counting a pulse, or attempting to use a normal ac input for a signal from a button or switch ??

Please try again. I dont under stand the question.

Mike

 

[dandrade]

DC INPUT

The inputs are DC.

I consulted my papers, says that PLCs they have circuits of Strech pulse in inputs.
It is possible disable this circuit? Would stay depends on the SCANTIME.

This it is not the certain rote. I know that the resolution would depend on the SCANTIME. But we go defined possibilities. ...true and wrong,bads

Note: Samples capture every 6/min. (60s/6=10Hz)

 

[Gerry]

In order to count pulses reliably, you need to sample the input at a frequency at least twice the frequency of the input. That assumes that the input has a 50% duty cycle. For anything other than 50%, you need to consider the minimum pulse width of the input, and sample at a rate equal the inverse of the pulse width.
e.g.
input freq. = 100Hz with 50% duty cycle
pulse width = 5 msec
sample freq. = 1/5x10^-3 = 200 Hz
If the duty cycle is say 20%, then
pulse width = 2 msec
sample freq. = 500 Hz

While modern PLC's with short programs can achieve low average scan times, the standard input modules have filters to minimise the effects of noise and contact bounce. These filters limit the input frequency response, and most likely will be just as significant in your considerations as scan time (program plus I/O).

 

[dandrade]

Yes. Yours sugest PLC's possible disable Filter input? List...

Sometime limit hardware input frequency, this program have solution for medium precision.