Contactless linear encoder for 5069 CompactLogix PLCs?

PreLC

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Hello PLCs.net!

I have a sliding table controlled by a servo. The coupling on this servo is not reliable and we need to reteach the positions the table stops on really often. This is currently done by checking the final part and using metrology lab feedback to adjust the table.

I wanted to put a linear encoder, the sensor would be fixed to the base, and the scale to the sliding table.
I was looking at these linear absolute encoders:
www.heidenhain.com/products/linear-encoders/sealed

But they seem to not have an AB interface.
Has anyone worked with similar tech? I tried to use the edge of a prox sensor, but homing using that sensor isn't a long term solution... I would prefer EIP or IOLink, but anything that works, works.

Regards,
-PreLC
 
Last edited:
If your going with absolute, you’ll probably need SSI as drbitboy suggests.

Another option I’ve seen from Heidenhain is an incremental linear encoder with the z channel on unique spacings throughout the length. This allows a minimal travel across two z flags to get the absolute reference on power cycles.
 
I have used Banner Time Of Flight laser encoders for controlling hoists up to 20 meters of travel, they are listed for 12 and 24 meter ranges. These proved to be very accurate getting the hoist to within 1mm over the range.



The ones I used were analog output, but I think other communication outputs were available, and now maybe even IOLink since this was a few years ago.



Plus there is a line I have worked on with German laser encoders that are DeviceNet, and also came Ethernet. The Banner's were about $750, the German ones about $3500. They also had an incredible range and accuracy (1/4mm over 1/4km I think) I don't remember the name or have any photos of the German brand.
 
drbitboy said:
This

https://literature.rockwellautomation.com/idc/groups/literature/documents/um/1734-um007_-en-p.pdf

i.e. SSI, maybe?


[Update: corrected link]
Click to expand...

Hmm. I was looking at the manual, and the sensor resolution was 1...65535. For the heidenhain, the accuracies are represented in um. Is there a way to find out if they are compatible? I've never selected a non-rotary encoder before...

Peter Nachtwey said:
Go with the SSI or more modern technology.
EIP and IOLink have have to much phase delay.
Magnetic tape might be an option.
https://blog.pepperl-fuchs.us/pgv-absolute-positioning-system-vs.-magnetic-tape-guidance-systems


A lot depends on the required update rate.
Click to expand...

I can move down to 1mm/s and need an accuracy of +/-0.1mm at least. With this requirement, would that require lets say less than 25ms in sampling delay with EIP/IOLink?
The magnetic tape idea is interesting, I'll check it out!
 
For fully specifying an encoder you need to know

  • Linear/Rotary (although you have already specified linear)
  • Maximum travel
  • Required resolution (I believe you said +/-0.1mm)
  • Maximum Speed of Travel
  • Electrical Interface Required (SSI is a good choice)
  • Measurement Type - Relative / Absolute? (SSI is absolute)
  • For non-contact: what gap is desired
  • Environmental factors i.e. Temperature, humidity, etc
  • And as is true for all things, how much $ can you spend.
 
ndzied1 said:
For fully specifying an encoder you need to know

  • Linear/Rotary (although you have already specified linear)
  • Maximum travel
  • Required resolution (I believe you said +/-0.1mm)
  • Maximum Speed of Travel
  • Electrical Interface Required (SSI is a good choice)
  • Measurement Type - Relative / Absolute? (SSI is absolute)
  • For non-contact: what gap is desired
  • Environmental factors i.e. Temperature, humidity, etc
  • And as is true for all things, how much $ can you spend.
Click to expand...

Hi nOrM,

I think I have all those answers:

  • Linear/Rotary (although you have already specified linear)
    -Linear
  • Maximum travel
    -1800mm
  • Required resolution (I believe you said +/-0.1mm)
    -+/-0.1mm-0.05mm would be acceptable.
  • Maximum Speed of Travel
    -300mm/s usually, but using this secondary encoder is only required when the motor is uncoupled with the ball screw. So I can go as low as 1mm/s
  • Electrical Interface Required (SSI is a good choice)
    -Seems like AB has an SSI interface, so that works. EIP and IOLink would work too.
  • Measurement Type - Relative / Absolute? (SSI is absolute)
    -Absolute only.
  • For non-contact: what gap is desired
    -Maximum possible, but low gap is okay too. This area is protected from chips.
  • Environmental factors i.e. Temperature, humidity, etc
    -It's in a machining center, so coolant and aluminum chips, room temp coolant.
  • And as is true for all things, how much $ can you spend.
    -Big bux.
 
I didn't ask about update rate required. SSI would be the fastest of the ones you have mentioned.

You mention that the table is controlled by a servo. It sounds like a machine tool so the servo controller is almost certainly doing a closed loop control. You really need to have a feedback that goes directly to the servo controller to get good control on the table position. So I wonder if the mention of an SSI adapter and EIP and IOLink are general AB devices or something purpose built for the servo controller?

Even though you have "Big bux" you don't want to waste them on something the controller cannot read.
 
ndzied1 said:
I didn't ask about update rate required. SSI would be the fastest of the ones you have mentioned.
Click to expand...

My acceleration/deceleration is 100mm/s^2, speeds in 1mm/s. I need to be within 0.1mm precision, so s=ut+1/2at^2, I need to have a reaction time of 0.035825756949558 seconds, using 10x to keep Nyquist's corpse from rolling in his grave, I guess we can be okay with 0,0036s or 3.6ms :D

You mention that the table is controlled by a servo. It sounds like a machine tool so the servo controller is almost certainly doing a closed loop control. You really need to have a feedback that goes directly to the servo controller to get good control on the table position. So I wonder if the mention of an SSI adapter and EIP and IOLink are general AB devices or something purpose built for the servo controller?
Click to expand...
It's closed loop, but sometime the motor(which has a built-in rotary encoder) loses position because we decouple it from the ball screw for maintenance. This encoder should be okay just going to the PLC and working as a sanity check/home measurement device.

Even though you have "Big bux" you don't want to waste them on something the controller cannot read.
Click to expand...
True, Need to remind myself of this so future me doesn't blame current me.
 
PreLC said:
I can move down to 1mm/s and need an accuracy of +/-0.1mm at least. With this requirement, would that require lets say less than 25ms in sampling delay with EIP/IOLink?
The magnetic tape idea is interesting, I'll check it out!
Click to expand...


Our customers sometimes use magnetic tape for die casting machines. Magnetic tape is more rugged that MDT rods


A lot depends on the load. Heavy loads will require a derivative gain. That means the position must be sampled a consistent intervals so the velocity can be calculated using the difference in two position divided by the time. How do you know when a sampled position wast taken? Also resolution is important. The finer the resolution, the finer the velocity estimates are.
 

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