how to make an incremental encoder "absolute"

Hello,
I have build a rotary axis with a direct drive motor. To control this motor I use a high resolution TTL encoder (629760 ppr after quadrature
and interpolation). For the application it would be beneficial to have an absolute encoder, because homing is not always possible and I don't want to make a 360 degrees mistake, in the present situation this is possible. I solved it for now by performing a defined homing procedure at machine startup and after that never home again, even after emergency stop. This is possible because the controller/encoder remains powered. Only when the power is down, homing is necessary.
I was thinking of the following situation: Is there a counter on the market that reads the encoder TTL signal and counts the absolute position. This device should be battery packed and when I startup the machine, I want to ask the counter the absolute position (eg RS232) and than I am able to sent this offset to the controller.
Mark
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Do you use the once per revolution HOME pulse setup, "C" in a A Quad B setup?
Check into how sytem works, might be able to periodically transfer counters used to or use non-volital registers in the first place, then reload from these at startup.

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counters
I use an optical ring encoder without C pulse, I created one myself by adding a proximity sensor on the axis. This sensor I use for homing but it is not very accurate (+-10 pulses). Furthermore I don't want to home anymore, so I have to count the A qaud B pulses too. Maximum rotation is 360 degrees and speed is 2 rps, this means more than 1 Mpps.
Mark
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mindspringnews wrote:

That doesn't work if your system has any chance of moving while it's powered down. You'll remember the last position, but you won't capture the changes.
--

Tim Wescott
Wescott Design Services
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That is a problem, but if he used an incoder with HOME pulse supplied, then sync's during first rotation after TOTAL power loss w/ movement during. Need HOME and TRUE UPS on the control system encoder section, else if power loss MUST go past TRUE HOME first.

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mindspringnews wrote:

-- snip --
Quote from the OP: "because homing isn't always possible".
An incremental encoder + home is obviously more than sufficient if you can home regularly.
--

Tim Wescott
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Are ANY absolute encoder systems, with that resolution available and are they in sync at startup at those resolutions?
Does it really need to be done and if so what is the cost of doing? If you keep system always powered and use a HOME pulse and REQUIRE going past HOME if UPS power lost I bet lowest cost solution, IF can tolerate in the application?

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mindspringnews wrote:

You can get dual-speed resolver systems that are accurate down to 20 arc-seconds (about 1:65000), with 16-bit DACs you can have resolutions down to sub-microradian levels. They don't require homing, but I haven't priced them compared to high-res encoders.
The user will have to answer your other questions.
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Tim Wescott
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Powering the controller by an UPS could solve my problem, I will take a look at that.
Mark
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I want to be able to keep power on the encoder and still want to count every pulse, so there is no need for storing the position.
Mark
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snipped-for-privacy@tue.nl (Oldchatterman) wrote:

You still have to keep the power to the encoder, haven't you? Without a living encoder, your battery driven device will be useless in front of manual fiddling.
As I assume, you controller powers the encoder - have you thought about a UPS or a battery for your controller?
Andreas
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(Oldchatterman) wrote:

I will investigate this option!
Mark
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Oldchatterman wrote:

You can buy such encoders ready made, for example from Heidenhein - as I know. But they are much more expensive than incremental encoders. There are also special chips, that are made for counting encoder signals (we have used two of them, but there are more). If you couple such chip with memory and some additional circuits - it can surely be used for this purposes. I haven't found such ready made solution, but this can be surely developed for a relatively small amount of money.
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Wieslaw Bicz

---------------========== OPTEL sp. z o.o. ===========---------------
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-=- http://www.optel.pl ====-------

I was looking for such a chip, but I am not able to design the complete part. Can you provide some more information? The reason for not using an absolute encoder from heidenhain was the limited space I got for building the system.
Mark
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Oldchatterman wrote:

If you want, we can design such part for you and produce it in the amount, that you want. If you are interested in it, send me please a more detailed specification.
Best regards
Wieslaw Bicz
---------=== mailto: snipped-for-privacy@optel.pl -=- http://www.optel.pl ====-------
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Oldchatterman wrote:

You may want to change to a multispeed resolver for a sensor. They make absolute measurements and are nearly as easy to deal with as interpolated optical encoders.
--

Tim Wescott
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Are there resolvers with a 629760 ppr? I haven't seen any. Even if there were there would be servere speed limitations. I would have suggested SSI rotary absolute encoders if it weren't for the high PPR requirement. I think the battery idea is best for now.
This may work if your controller has a SinCos interface. http://www.stegmann.com/product/servo/index.html
Peter Nachtwey
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there
I don't want to use another encoder, the encoder I use perfectly matches the size constraints I have.
Mark
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What is your controller? A Quad B hi speed input sections on even some micro PLC's, PLC's normally can be battery backed up, and some continue to work even when the program is stopped. Some have system variables that can be checked to determine if power, battery or UPS was lost and then used in top of ladder logic to REQUIRE homing if this happens. Whatever your using, a similar operation should be possible using a power loss relay / logic input that requires resetting if power, battery or UPS is lost.
If critical, use a TRUE UPS on your controller and loss of UPS detection logic / relay.

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