mesa + battery

Hello.
In systems with mesa and incremental encoders can put a lead acid battery backup with 5v step down powering mesa card and encoders in case of power loss that will store
encoder position data in ram memory and when linuxcnc restarts read these position data
from mesa?
What changes needs this to mesa firmware  hostmot2 driver and linuxcnc?
Functionality like absolute encoders but without absolute encoders.

Something like this is implemented by kuka robots that have a pcb (rdc)
which drives and reads continuously the resolvers with battery backup and
converts resolvers signals to ssi (serial data) feed to robot controller.
In case of power loss this pcb has supply from battery backup so robot
does not need homing after mains power loss.

Basically no, because you would have to keep LinuxCNC running as well
(since part of the quadrature count is stored in LinuxCNC)

Better off to use absolute encoders

If mesa except from sending encoder data to linuxcnc stores encoder counts
to some ram memory on restarting linuxcnc this can read by linuxcnc.
What changes this needs to mesa firmware and linuxcnc?
It is not easy to use absolute encoders in older systems
for example in a mill with linear scales most times are incremental encoders.
 

Well if you're using a Mesa card I assume you have control over the host PC as well? Why not just use a UPS for the PC and Mesa card logic supply. A low power PC and whatever 5v powers a mesa card could use a very small UPS or run for a very long time.

You can generally find ways to power the Mesa card directly from the PC, even if it's a mini pc

Yes this is an option that works out the box.
But mesa only option i think is nearest to alsolute encoders and more stable.

vre wrote:

Yes this is an option that works out the box.
But mesa only option i think is nearest to alsolute encoders and more stable.
 

lol but "mesa only" is not an option.

Another option is to use an absolute converter, which I suppose is just an external counter. I'm sure there's something out there but all I know of is this:

www.dmm-tech.com/encv_index.html

doesn't mention having a battery but I bet it can be modified for battery backup. Also not sure if DMM encoder protocol works with Mesa SSI but PCW could probably answer that

I suppose a large (say 64 bit counter) could be used
but its a bit awkward compared to a multi-turn
absolute encoder. You would still need to home
(reset the large counter) if you lost power

Note that your backup needs to power the
interface card and all encoder.

I don't think the DYN adapter helps as its likely a single turn
quadrature to absolute encoder converter

PCW wrote:

I suppose a large (say 64 bit counter) could be used
but its a bit awkward compared to a multi-turn
absolute encoder. You would still need to home
(reset the large counter) if you lost power

Note that your backup needs to power the
interface card and all encoder.

I don't think the DYN adapter helps as its likely a single turn
quadrature to absolute encoder converter

 

Not really that awkward. I suppose it could count a big number and do a modulo or something on it to get position and turn count but I don't really see why it cant just reset the count after the motor resolution and increment the turn counter. Much less big number. Either way it's not really a problem for a beefy MCU. I'd think multiturn is just a firmware thing vs singleturn

As for reset and home, that looks like a command that is sent to the encoder ASIC over the serial encoder interface. Pretty sure that's all the DMM drives do which is what this module is meant for. DMM only uses absolute serial encoders, they just brand their standard drives as "incremental" and the firmware ignores all the features of an absolute encoder. I think the absolute encoders are actually physical incremental encoders with an ASIC or MCU that does absolute position on the motor itself. Like the control serial port has a position reset command that doesn't do anything with the incremental drives, it works on the absolute drive by telling the absolute encoder to reset itself.

I'm not terribly familiar with hm2 SSI but is it bidirectional? like can it command a serial encoder to reset itself?

The DYN adapter has a fixed resolution and does seem to be single turn.
(its absolute nature is mainly for commutation which it seems to acquire via Hall inputs)


Multiturn absolute encoders typically do not have any provision to reset except at encoder
installation.  They use internal battery backup, gears,  or some clever scheme (like the Wiegand effect)
that generates enough power through turning the shaft that they can update their non-volatile turn
counter when main power is not available. So basically they never lose position.

Its possible to mimic this but it is awkward as the volatile count will requires homing sometimes
(when your backup fails). This might possibly be done with a different LinuxCNC configuration
(normal home to index) vs the normal absolute. All in all, I'm not sure having this partial absolute
operation just to avoid homing is that big of an advantage.

Absolute encoders are typically not just incremental encoders with processing, as this would not
have the correct position count at startup. This initial position count is also  needed for commutation.
The ones I have seen have either just  absolute tracks or a combination of absolute tracks and a higher
resolution incremental track.



 

kuka robots that have resolvers(resolvers are not multiturn absolute) do that..
The pcb(kuka calls it rdc or rdw) that drives and reads the resolver has battery backup so after power loss robot does not need mastering(homing)
needs only remastering if battery goes off.

mesa card + encoders are usually 5v very low current so lead acid battery will give enough backup time.. (kuka has 2x12v 5ah=120Wh)