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I have been learning about absolute encoders, and am testing a multi-turn absolute encoder (link to datasheet is here). During testing, I realized that after shutting down the power, although the encoder position is saved, the number of turns is not, which means the encoder position is non-volatile, but the number of turns is volatile.

So, forgive me for a lack of fundamental knowledge, but: does there exist a type of absolute encoder that saves the number of turns, even after power is turned off and on again?

I have tried searching for this on stackexchange and on Google, using the search terms "non-volatile" and "multi turn absolute encoder", but I could not find an answer.

EDIT: while searching more about different types of multi-turn encoders, I found a type of encoder that uses what is called the "Wiegand Effect". Wikipedia states "As the encoder revolves, the Wiegand wire core coil generates a pulse of electricity sufficient to power the encoder and write the turns count to non-volatile memory". I have also found an example of this encoder, and this example's datasheet is here. So, I guess I should modify my question as, "Are there any other types of absolute encoders that saves the number of turns, even after power is turned off and on again (other than Wiegand-based encoders)?" I am interested in learning all the different types, if there are any others.

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  • \$\begingroup\$ Do read motioncontroltips.com/… \$\endgroup\$
    – Huisman
    Commented Sep 26, 2019 at 6:40
  • \$\begingroup\$ Place the position variable in your PLC's retention area. \$\endgroup\$
    – Jeroen3
    Commented Sep 26, 2019 at 7:20

2 Answers 2

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Manufacturers choose different strategies about absolute encoders. Asian and Japan manufacturers tend to use kind of incremental encoder that needs an additional lithium battery at driver side to power the encoder at power outage.

European manufacturers usually use hall sensors with magnet gears, similarly like a water counter for counting revolutions, while the disc is made of glass or it's a resolver - this is so called multiturn absolute encoder. Disc or resolver gives the position within one revolution, while the gearbox measures numbers of revolutions. Such encoders don't need power for counting position. However an EEPROM inside is a good to have, used to store reference or null position.

enter image description here

Encoder Gears

I had used Sick, Kuebler, Indramat,...I can confirm it does not need power. While Fanuc has an additional battery in motor driver.

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  • \$\begingroup\$ Such encoders don't need power for counting position. Do you mean, there are self-powered or not using power at all? \$\endgroup\$
    – Huisman
    Commented Sep 26, 2019 at 6:44
  • \$\begingroup\$ They are not self powered, they just do nothing when powered off, but once powered on they send the true position even if they were moved while powered off. \$\endgroup\$
    – Marko Buršič
    Commented Sep 26, 2019 at 6:56
  • \$\begingroup\$ @Marko Buršič Just to confirm for both the Asian-style (with battery) and European-style encoders (with gears): if the encoder's position AND number of revolutions was changed while the encoder was powered off, will the encoder be able to remember these two values once it is turned on again? \$\endgroup\$
    – Cherry
    Commented Sep 26, 2019 at 7:25
  • \$\begingroup\$ As OEM integrator I can confirm that Sick encoder would do this. As a CNC service, I can only say that disconnecting a Fanuc encoder or battery drain it will loose the position. Those battery powered are rather proprietary, never used as OEM. What is your project? \$\endgroup\$
    – Marko Buršič
    Commented Sep 26, 2019 at 7:45
  • \$\begingroup\$ @Huisman - There are two different functions being performed - measurement and readout. The measurement function is self-powered, in that a very small amount of power is required to move the gears and this is provided by the shaft connection. The readout function is electronic/optical, and this requires external power to operate. So with no power the unit functions "normally", recording the current position of the shaft. When power is applied, the operation of the gears is unaffected, but their position is read out and reported. \$\endgroup\$
    – WhatRoughBeast
    Commented Sep 26, 2019 at 14:24
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Multi-turn absolute encoders.

Example, this on is up to 4096 rotations of 4096 counts each.
http://www.beisensors.com/pdfs/hmt25-optical-absolute-encoder.pdf

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  • \$\begingroup\$ I read your link, and I am confused with this sentence: "It keeps track of the exact position even during periods of power loss without the need for a battery backup". Does this quote mean that the encoder will also be able to keep track of the exact number of revolutions, even during periods of power loss? The encoder I tested (which I linked in my question) uses a similar quote as the encoder you linked, but nevertheless the encoder I tested does not keep track of number of revolutions during power loss. \$\endgroup\$
    – Cherry
    Commented Sep 26, 2019 at 7:35
  • \$\begingroup\$ @Cherry Yours does not specifiy it saves anything, just that it can keep track of turns separately. The HMT25 explicitly writes that is keeps track of the exact position, of which there are 16,777,216. \$\endgroup\$
    – Jeroen3
    Commented Sep 26, 2019 at 8:27

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