Why batteries.

One burning question I've had for years: in this age of cheap and plentiful flash memory, why are machine tool manufacturers still using dynamic memory and batteries for parameters and servo data storage? (I'm looking at you, Fanuc)

Maybe because flash memory corrupt in few years.. I`m not sure though.

BerzerkaDurk said:

One burning question I've had for years: in this age of cheap and plentiful flash memory, why are machine tool manufacturers still using dynamic memory and batteries for parameters and servo data storage? (I'm looking at you, Fanuc)

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Because when it inevitably dies they can screw you for replacing it.



Gotta make $$$ somehow......

maybe Fanuc are using incremental encoders and the battery is there not just to preserve the memory state, but to keep track of any movement when the machine is powered down, it may move due to vibration from some external source or temperature changes, so that part of the controller has to be awake to keep track of this, so when you power the machine back up, it knows exactly where it is and you don't need to "zero" it every time

as for parameters, no idea, I would have used couple or more copies in a flash memory and read them to dynamic on power up, few copies in flash for error checking purposes

Good question. I have a Fanuc 6t which does not use batteries. I am pretty sure everything is written to bubble memory and no powered needed. I wonder what the discussions were like when fanuc went from that, to a technology that required the battery maintainer. It does seem strange to build in such an obvious weak link into an otherwise extremely reliable and robust system. But maybe it wasn’t as obvious to them as it is to some of us using machines well past there intended lifecycle.

CNC machines from the manufacturer point of view have a lifetime similar to cars[from the same point of view]

So, basically, batteries last the life of the machine

AFAIK all machines with incremental encoders have to home, so none keep track of anything when the machine is off. Even my machine with an absolute encoder reads it on startup.


CNC machines are very conservatively designed. Flash memory and SSD drives have only become cheap in the last 5 years or so, and, as mentioned, have a finite lifespan.

Generally, if you store your parameters and programs, battery loss is a minor nuisance.

I changed my old heidenhain from 3 AA to 3 D cells because the factory holder rotted from old batteries. Shoulda bought lithiums, but ......

Fanuc has used Flash for some memory in their controls starting with the 16/18 series of 20 something years ago. The system software and ladder program is stored on Flash on those controls. Flash was acceptable for that data because it is primarily read and rarely written. This same architecture was used by Mitsu and Yasnac too. Flash has a finite number of write cycles before it begins to fail. Because of that it is not considered reliable enough to use for data that is, or may be, written frequently.

Arguably one could contend that parameters are not altered so frequently the problem of Flash write endurance should not be an issue. Personally I'd say that is true, but if one considers that parameters can be written in a program with the G10 command that may not be the case for some users.

The 6, 11, and 12 series that used bubble memory worked around the main limitation of bubble which is slow read write speed. At power up the stored data in bubble was transferred to DRAM for execution. Older Okuma (OSP3000 and OSP5000) used a similar scheme. Bubble is slow and was expensive. The Fanuc 10 series of the same generation as the 11 and 12 series used SRAM and battery backup because it was a lower cost model and bubble memory would have pushed the price too close to the 11 series for the market.

"Absolute" encoders use the battery to keep them powered up while the machine is off. That is how they keep track of movement when the machine is off. Encoder counts are stored in the encoder while the power is off. One can test this by moving the table while powered down. Small machines you can just push the table while larger machines will need to have the ballscrew turned. When you power back up the encoder sends the new position data to the control.

Even if a Flash technology with infinite write cycles is developed, a CNC control will still have a battery to maintain some basic information in SRAM or CMOS. Pretty much the same as the battery your PC uses to retain the BIOS and RTC.

While Fanuc might like users to consider the lifespan of their controls to be similar to a car, they know that is not the case. Within limits like lack of component availability, they still support their controls of 30+ years ago.

Vancbiker said:

While Fanuc might like users to consider the lifespan of their controls to be similar to a car, they know that is not the case. Within limits like lack of component availability, they still support their controls of 30+ years ago.

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Fanuc actually say they will provide spares for a minimum of 25 years on their own websites too, so the idea that Fanuc consider the life of a machine tool to be like that of a car is a bit off, unless you consider the life of a car to be 25+ years if driven 40 hours a week for 25 years.

In the grand scheme of things replacing a battery is just one of the many preventative maintenance tasks that needs to be done, sure £20 odd for a battery cell isn't cheap but compared to the cost of the machine itself it's neither here nor there. Which reminds me, I need to order a couple more for my Fanuc controls soon.

Real "Absolute" encoders don't need batts.

I have a machine with 8 axi's that was unhooked* for 5 (?) years and there is no HOMEING routine.

Indramat control (1999)

* Litterally - the machine control cables dettached between the machine and the control cabinet.


FANUC "absolutes" are obviously just INCR with batt retain.



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Think Snow Eh!
Ox

Ox said:

.....FANUC "absolutes" are obviously just INCR with batt retain.

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While they are not true absolute encoders, they are a really good "compromise" IMO. Mitsu and Yaskawa "absolutes" are similar in concept.

Older Okuma true absolute encoders were kinda like what you describe the Indramat ones. No battery and position able to determined on power up even if moved while power off. The downside was they were incredibly complex devices. Lots of gears driving little magnet wheels around little coil windings. Expensive and considering the complexity (both mechanical and electrical) pretty reliable. Nowhere near as reliable as the Fanuc flavor or "absolute" though and double the cost. Okuma did have a repair/exchange scheme that basically refunded half your money when you returned a repairable "core" which helped.

Place I retired from at one time had 4 Okuma HMCs and one 4 axis VMC. Each HMC had 9 encoders and the VMC had 4. On average we had to replace one encoder per year. Kept one on the shelf. In comparison we had ~50+ other CNCs with mostly Fanuc, some Mitsu, and a few Yasnacs. So rare was the need for a new encoder, that I can't recall swapping more than a handful of times.

1: there are absolute for the complete axis travel, and absolute within on revolution of the encoder shaft. for example, Inductisyn, Accupin and resolvers are absolute within on revolution of the encoder shaft; they require no power to maintain the angular position info.
2: the bubble memory of the '70s was very slow to read and became too expensive; in 1990 a 2 Mbyte BMU listed at US$43k. The various flash memories are too slow for writing, and the write/erase cycle times are always limited, although wear leveling helps.
3: dram is the fastest memory tech; its need for refresh cycles limits it to 'operational' memory.

I believe the correct answer is that machine control builders in general, and Fanuc in particular, are about 10-20 years behind the technology curve. Pretty much every consumer electronics company on the planet, and all the auto manufacturers have learned how to deal with flash wear at least a decade ago.

Still, batteries are less antiquated than the UI in many of these controls.