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Counting pulses to display rotation speed

Counting pulses to display rotation speed

(OP)
Hi Chaps
I have a small test rig consisting of an SLC-500 rack, a small three phase motor driven by a VFD, and an optical sensor reading a slot in a disc mounted to the motor shaft. I have written a small ladder program to count the pulses for a period of time using a free running timer then after the time period has expired shift the value into an integer file and multiply it up to get an RPM value.

This works ok but has some draw backs: the updated value takes time to display, when the motor is stopped I get strange values. I am using a fast input card but at full motor speed the values jump around a lot. Can anyone point me to a more stable robust solution.

Thanks.

RE: Counting pulses to display rotation speed

This was discussed quite recently, see thread240-361296: How to design a speed sensor circuit with a monitor

There are several techniques where you can build a MV (moving average) or LPF (low pass filter) to increase resolution. To avoid getting strange values, which you do get when period time is longer than your counter's length, you can use a timer that interrupts the counting process when a certain time is reached. You should then output a message instead of data.

Sounds a bit complex for ladder programming. I would use instruction programming or block diagram instead.

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Counting pulses to display rotation speed

you can also buy a fairly cheap digital to analog converter and receive a 4-20mA signal.

RE: Counting pulses to display rotation speed

Along with what Taff said i have used a product made my moore industries call an FDY, its a frequency to current loop converter. Works great!

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