Length measurement using rotary encoder

[splitter]

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?

 

[automatic2]

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.

 

[splitter]

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.

 

[PSE]

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,

 

[automatic2]

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.

 

[splitter]

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

 

[tacm]

Have you looked at encoder resolution?

 

[splitter]

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

 

[SteveMeisel]

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