What happens when absolute encoders roll around ? Drives spit the dummy ?

[nick mulder]

Hello,

I'm looking at a second hand Allen Bradley AC servo and an Ultra3000 drive

The servo uses a high res SinCos absolute encoder that covers a full 4096 turns of the shaft. Thats full turns of the shaft, not just 4096 ppr...

https://www.mysick.com/saqqara/get.a...ration+english

So what happens if I were to ask it to go further than 4096 turns ?

1111111111111111 >> 0000000000000000

(! ?)

I understand the absolute encoder position would be out by a full phase of 4096 turns but would the drive stop or fault or otherwise cause issues ?

I just want it to run continuously at the moment

[keebler303]

Absolute encoder rollover will throw an error E33. I only looked briefly at the manual so I am not sure if you can just let it rollover without a problem but generally an error means the drive will fault.

Matt

[nick mulder]

ah,

ok - interesting

so my question is now:

can I disable 'position rollover' in an Ultra3000 drive

error E33 would indicate I cant but error but error E02 (I think where the drive has the foresight to recognise the demanded velocity profile would drive the encoder past its range) mentions:

(applies only when the machine cycle position rollover is enabled).

but searching for 'machine cycle position rollover' yeilds nothing in the manual

[nick mulder]

in the ultraware manual it mentions that you can disable the position rollover:

[QUOTE]Disabled: to cause the position variable to
rollover only when it exceeds 2147483647 countsQUOTE]

so with a high res encoder with 32768 ppr running at 2800 rpm it'll still error at:

(2147483647 / 32768) / 2800 = 23 minutes

that is of course if it started at 0

?!?!?

[keebler303]

I found this in the help for Ultraware:

Machine Cycle: 1 Single Turn Absolute 1 Select: Enabled: Causes the SRS encoder to be used as an absolute feedback device Disabled: Causes the SRS encoder not to be used as an absolute feedback device. (Default) Note: This parameter is visible only if the off-line motor selection or online motor has an SRS/SRM encoder. Position Rollover 1 Select: Enabled: Causes the position variable to rollover to zero when it exceeds the specified size. Disabled: Causes the position variable to rollover only when it exceeds 2147483647 counts, or its equivalent in other units. Note: This parameter is disabled when Single-Turn Absolute is enabled. Size Select a rollover value in counts, an integer at which the next increment of a position variable will be the value of "0": Note: This parameter is disabled when Single-Turn Absolute is enabled, and is automatically set to one revolution.

It looks like you should be able to get it to work like you want if you play around with the parameters a bit.

Matt

[nick mulder]

Thing is it seems you cant set the counts to infinity, just numbers less than the maximum of 2147483647 - which you'd eventually reach

The manual is ambiguous as to what happens at this point - I suspect another E33 error ...

There is the possibility of dividing/multiplying the counts in the drive but I don't know if this is pre or post the absolute counter - and in any case you're just delaying the inevitable error.

What I need is certainty there is a setting that allows arbitrarily long run counts. Ah well...

[keebler303]

I think you want position rollover enabled where it is shown as disabled in my screenshot. The size tells the drive when to expect the rollover, not how many counts to make before throwing an error. For instance if your encoder counted to 255 and then rolled over, you would set the size to 256 (0+255) so that the drive knew to expect a rollover from 11111111 to 00000000 every 256 counts. I think this will let you run until the cows come home without error. Truth is you will have to try it out to be sure.

Matt

[unterhaus]

seems to me that 4096 revolutions should be enough for any linear application.

[keebler303]

Originally Posted by unterhaus seems to me that 4096 revolutions should be enough for any linear application.

No one said it was a linear application, maybe its for a spindle. I think it is more about trying to make do with what you were able to find on ebay. If you are running the motor continuously, you probably don't want this type of encoder, but if its what you have, you do what you can to make it work in your application.

Matt

[nick mulder]

Originally Posted by nick mulder I just want it to run continuously at the moment

[mike_Kilroy]

Originally Posted by keebler303 ...... If you are running the motor continuously, you probably don't want this type of encoder, but if its what you have, you do what you can to make it work in your application. Matt

We put similar encoder, among others, in our Kollmorgen motors. We use Heidenhien Endat encoders that can come as 4096ppr and 4096 revs for absolute feedback. If we are running in velocity mode we roll over transparently as our pos counters also go to like same 2bill number. No problem.

The AB is not Endat but Stegmann and it has almost identical output (in fact most of our Kollmorgen drives can accept them too.

I assume since we can do it they can too.

Did you try it yet if you have the motor and drive?

[unterhaus]

I was just reading the manual of my servo amp and there was a note that if you have had too many writes of the memory for the absolute encoder, you have to send the drive back to the factory. This is a different brand, but AB sells the same drive with a different number. Something to check into. Too many bad experiences on ebay have gotten me to the point where I don't buy anything until I read the manual. Lots of "good deals" get away, but I have a basement full of "good deals" already.