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Old September 6th, 2018, 04:30 AM   #16
NetNathan
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Maybe IFM-Efector
BUT..you will have to add a disk with trigger points on it to the shaft.

DI0101 (Compact evaluation unit for speed monitoring)
https://www.ifm.com/us/en/product/DI0101

Manual
https://www.ifm.com/mounting/80235738UK.pdf
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Last edited by NetNathan; September 6th, 2018 at 04:33 AM.
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Old November 19th, 2018, 04:45 AM   #17
Aleksandra
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Quote:
Originally Posted by JesperMP View Post
Basically use the time from 1st to last impulse within a fixed update period, and count the number of pulses within that period. That will give greater precision.
It will be slightly more complex to program, but not by much.

If I understood, what you are saying is to take for example 10s fixed period, count the number of pulses, divide that number with this fixed period, and multiply by 6 to get RPM. Please correct me if I am wrong.
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Old November 19th, 2018, 05:30 AM   #18
JesperMP
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Quote:
Originally Posted by Aleksandra View Post
If I understood, what you are saying is to take for example 10s fixed period, count the number of pulses, divide that number with this fixed period, and multiply by 6 to get RPM. Please correct me if I am wrong.
No, I said "use the time from 1st to last impulse within a fixed update period"

If you have an update "window" between r and r (reset), you count the no of impulses by the rising edges.
And you start another timer at the 1st rising edge.
For every subsequent rising edge you indrement the counter, and you save the time value.
When the window closes, you calculate the flow based on the no of counted impulses and the saved time.

Pulses:
__-__-__-__-__-__-__-__-__
window:
r--------------------------r
count:
--0--1--2--3--4--5--6--7--8
timer:
--t(start)----------------t(end)
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Old November 19th, 2018, 09:13 AM   #19
Aleksandra
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Quote:
Originally Posted by JesperMP View Post
No, I said "use the time from 1st to last impulse within a fixed update period"

If you have an update "window" between r and r (reset), you count the no of impulses by the rising edges.
And you start another timer at the 1st rising edge.
For every subsequent rising edge you indrement the counter, and you save the time value.
When the window closes, you calculate the flow based on the no of counted impulses and the saved time.

Pulses:
__-__-__-__-__-__-__-__-__
window:
r--------------------------r
count:
--0--1--2--3--4--5--6--7--8
timer:
--t(start)----------------t(end)

Sorry for bothering you, but I don't understand. What do you mean by "update window"? When you say "start another timer", does it mean that I should use two timers?


Anyway, I understand how to count impulses by rising edges, but I am on fire when it comes to implementing RPM. The problem is that I have to measure the drum speed per minute (which is by the way very slow, approximately 7 RPM), but I have to update HMI screen for example every 20 seconds. So I am confused on should I set the timer to 20 seconds and then multiply the calculated value by 3.


I don't think this is good idea because 60/7 = 1 impulse per every 8.5 seconds. Which means I could loose information. But I don't come up with another idea, since I am a beginner.

Last edited by Aleksandra; November 19th, 2018 at 09:25 AM.
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Old November 19th, 2018, 09:33 AM   #20
Aleksandra
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P.S. This is my first time to hear about this "window" term, I don't even know how to google it
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Old November 19th, 2018, 09:55 AM   #21
scott.lawrence
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window as used here is a period of time, as in the window for measuring your signal or the time between signal processing
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Old November 19th, 2018, 10:02 AM   #22
Mad_Poet
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Correct me if I'm wrong, but couldn't one add a gear to the
slow moving shaft and a prox to count the gear teeth as
the shaft turns? (Generating a number of pulses for each
partial revolution.)
Poet.
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Old November 19th, 2018, 11:06 AM   #23
JesperMP
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Quote:
Originally Posted by Aleksandra View Post
When you say "start another timer", does it mean that I should use two timers?
Yes.
Let one timer be the one that updates your calculation every 20 seconds.
The other timer is for measuring the time from the 1st rising edge to the last rising edge of the impulses you count.

Quote:
Originally Posted by Aleksandra View Post
The problem is that I have to measure the drum speed per minute (which is by the way very slow, approximately 7 RPM), but I have to update HMI screen for example every 20 seconds.
You have approx 8.5 seconds per revolution. So you would get 2 or 3 impulses per 20 seconds. That is just barely enough. You need at least 2 impulses within the 20 seconds window.
I would consider adding some measure to get more impulses per revolution.

Quote:
Originally Posted by Aleksandra View Post
So I am confused on should I set the timer to 20 seconds and then multiply the calculated value by 3.
As you argue yourself it is not good enough to assume a fixed time of 20 seconds.

You have to measure the time between the first rising edge and the last rising edge. And then calculate the RPM = ((no of impulses -1) * (60 sec per min) / ((time between 1st edge and last edge) * (impulses per revolution)).
For example if you measure 3 rising edges within 16 seconds then
(3 - 1 impulses) * (60 sec per min) / (16 sec) * (1 impulse per revolution)
= 7.5 revolutions per minute
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