×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

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

Jobs

Length measurement using rotary encoder

Length measurement using rotary encoder

Length measurement using rotary encoder

(OP)
We have a sheet metal roll forming machine that performs various operations before finally shearing to length, the component length is required to vary, however the component length should be within +/- 0.5mm max.  The drive is hydraulic and the encoder is driven by a wheel (rubberized) mounted on a common shaft with the encoder, contact between the wheel and metal is good.  We have found that the steps/mm have to be changed regularly to keep the length within tolerance, the encoder has been changed and the system checked as much as possible.

Any suggestions?

RE: Length measurement using rotary encoder

is your part continually moving?, and if so, does the shear travel with the part. Sounds like you may have variable timing from shear signal to actual shearing, allowing variable feed between those two instances. If you don't have a travelling shear, you'll want a controlled decel to stop, shear, accel. If you do have a travelling shear, how does it locate to the part.

A high speed or 'flying' shear requires a fixed relation between the shears inertial speed and that of the part speed.

RE: Length measurement using rotary encoder

(OP)
The part travels at very slow speed (less than 4m/min), and start and stop regularly along the machine to be punched and finally cut to length. The acceleration and deceleration time is almost 0; 10mm before reaching the required position the speed is reduced (less than 0.5m/min) and finally stopped. Every time the part is stopped, the encoder position is logged into file; after comparing the log with the required positions we found that the difference in mm is always less than 0.25mm but the real length varies and some times could be up to 2mm longer or shorter than the required length. When this happen we slightly increase or decrease the steps/mm and then we can produce parts with the correct length for a while.

RE: Length measurement using rotary encoder

Some hypothetical thoughts.

Does it appear that the encoder is dropping steps?  You might want to check data cables or reduce cable length if possible.  Is the encoder experiencing a "reset" periodically (power loss or shut downs"?

Could the process be changed to utilize proximity sensors for establishing fixed positions instead?

Regards,

RE: Length measurement using rotary encoder

Is it possible that the piece reverses direction at any time? Your encoder/counter may not interprete such a condition properly. This could be as small as a punch shifting the piece slightly, which could be enterpreted as a forward movement by the system.

RE: Length measurement using rotary encoder

(OP)
Thanks for your replies,

We have checked the encoder and the rest of the electronics associated and we found nothing wrong.

We tried with a new encoder and encoder module on the plc.

The encoder cable is the original with no extensions (under 800mm), properly shielded, far away from any power cables or any other source of electromagnetic noise, connected in differential mode.

There is no power loss or shut downs

We also tried a second stand alone plc with an encoder connected to the same encoder shaft running at the same time to compare the encoder values and they are almost identical.

It does go backward a fraction of mm every time stops to punch or shear but we can also see the encoder values decreasing, and when using the additional encoder we can see exactly the same encoder values.

Finally, unfortunately we can not use any other sensor for stabilising fixed positions because the distance between punches could be anything from 200mm up to 2500mm.

Regards

RE: Length measurement using rotary encoder

Have you looked at encoder resolution?

RE: Length measurement using rotary encoder

(OP)
The encoder resolution is 10000.  Fortunately the problem seems to have been resolved by making small adjustments to the steps/mm value and finally it settled down, though from the symptom's it was not easy to diagnose.
Thank you to every one for your suggestions.
Mike

RE: Length measurement using rotary encoder

Are you absolutely sure that you do not have a mechanical problem?

Sounds like there could be a slight amount of slippage between the drive roll and the metal sheet.  From the looks of it you have covered most if not all of any electrical issues.

Steve

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!


Resources