Digital tachometer from 1 to 2500 RPM with speed alarm HELP!

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giorgia

Joined: 25 Dec 2001
Posts: 32
Helped: 1

26 Mar 2008 17:05 sec help 2500

Hello everyone
i'm going to build a digital tachometer capable to measure revolutions from 1 to 2500 RPM with resolution of 1 rpm and speed alarm on low and high speed.
the shaft generate 3 pulses per revolution.
any help, source code or suggestion are very welcome
Regards
Giorgia

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blueroomelectronics

Joined: 17 Sep 2006
Posts: 1681
Helped: 99
Location: Toronto, Canada

26 Mar 2008 18:07 2500 rpm tachometer

We're still waiting for

Quote:

Dear friends,IdeaQuestion
I'm going to design a pwm controller for a 10 kw 220Vdc motor
the speed setting will be varied by a potentiometer
and the reverse direction by a switch / push button
anyone will help me with schematics source code etc..?

THANKS in advance

Giorgia:D

laktronics

Joined: 06 Jan 2007
Posts: 945
Helped: 156

27 Mar 2008 14:00 build digital tachometer

Hi,
You can use an 89C2051 like micro and use one of its timers as a 16 bit counter and count the pulses for 0.1 sec and get rpm bydividing the count value by 18.
Regards,
Laktronics

FvM

Joined: 22 Jan 2008
Posts: 5161
Helped: 767
Location: Bochum, Germany

27 Mar 2008 17:10 speed alarm

Quote:

count the pulses for 0.1 sec and get rpm bydividing the count value by 18

1 RPM means one pulse each 20 seconds. For a usable measurement, you should evaluate pulse period respectivly multiple period for higher frequencies.

rikie_rizza

Joined: 05 Aug 2006
Posts: 334
Helped: 20
Location: Bikini Bottom, between a rock and a pineapple

28 Mar 2008 3:00 tachometer 2500rpm

Anyway, what is the difficult part? You got the pulses (3 symmetrical pulse per revolution), just buffer the signal up to digital limit, connected to a microcontroller and create a simple program...

Which part you dont understand?

laktronics

Joined: 06 Jan 2007
Posts: 945
Helped: 156

28 Mar 2008 5:06 Re: Digital tachometer from 1 to 2500 RPM with speed alarm H

Hi,
My previous post is incorrect, you have to compute the period of the pulse using a timer and calculate the rpm from there. Sorry for the error. Thanks FVM for your suggestion.
If T is the period, you get one revolution in 3T sec.
So your revolutions per sec = 1÷(3T)
Revolutions per minute = 60÷(3T) RPM
Regards,
Laktronics

laktronics

Joined: 06 Jan 2007
Posts: 945
Helped: 156

28 Mar 2008 13:11 Re: Digital tachometer from 1 to 2500 RPM with speed alarm H

Hi,
By the way your project will be much easier if you can get a shaft encoder with 60 or multiples thereof of pulses per revolution so that a simple counter on micro will give the RPM with one RPM resolution directly. Since your purpose is only speed measurement and annunciation, without any feedback control, the measurement dealy of one sec is also not important. I would even say it is worth trying to make a strobo scope with 60 or 120 markings on it, mounted on the shaft for generating the required pulses.

Regards,
Laktronics

Alan69

Joined: 01 Apr 2005
Posts: 79
Helped: 4

30 Mar 2008 14:12 Re: Digital tachometer from 1 to 2500 RPM with speed alarm H

Don't be so locked in mentally to 60/min. 60/min is a totally arbitrary relationship in the first place, and has little place in a microcontroller. If you don't need RPS you shouldn't be doing it by seconds.

Regardless of 60 or only 1 pulse per revolution, make your counts/timing window have a binary relationship to 1 minute. Timing by 1/64th of a minute and doing a simple shift multiply is easier and gets RPM just as well as accurately timing 1 second or .1 second etc and making some arbitrary 60 relationship to make it work. The timing window should have a direct binary relationship to whatever the output units are, anything else introduces completely unnecessary extra math instead of just shifts..