Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here


Counting pulses to display rotation speed

Counting pulses to display rotation speed

Counting pulses to display rotation speed

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.


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
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!

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close