Mesa and SSI absolute encoders

Hi group!

I have a gantry mill that uses Mesa 8I68+3X20 and 7I33s for the servo interface.
The axis setup is XXYYZZBC.

I use Bosch Indramat ECODRIVE servodrives with incremental encoder interface.

These drives can also operate in absolute encoder interface mode with SSI.

I have not been able to make a reliable homing procedure for the gantry setup since there are two servos on each linear axis.
Having absolute encoder signals would eliminate the homing problem.

I read that SSI was implemented in hostmot2.

Does anybody know if it is possible to implement absolute encoders for this setup, and if so can I use the 7I33 boards or do I need other boards for SSI signals?

Best Regards!
Lars Levin

SSI requires differential inputs and outputs so while its possible to use the
7I33 encoder inputs in differential mode as the inputs, you still need some
differential outputs.

A 7I44 daughtercard is probably a good choice so get the SSI wiring all on one card,
but there are many options

Ok, so maybe I can take away one 7I37 card and put one 7I44 instead.

But is this setup doable, or is it a lot of reprogramming and coding to get a system up and running?

What are the many options?

The SSI interface is supported in linuxcnc 2.6 and later

Are your encoders multiturn? (so absolute for the whole range of motion)

Homing with absolute encoders is fairly uncharted territory on the
linuxcnc and hal side so will possiblly be "interesting"

Yes the encoders are multiturn, so absolute for the whole range.

I can set value 1 to 65536 steps/revolution. Now in incremental mode it is set to 2000 steps/rev.

Value is stored in servomotors by a battery.

Homing will probably be a minor problem, since I know exactly every motors absolute position.

I also have a 3 axis retrofitted machine with same servosystem, maybe I should try doing this on that machine first to see that it works.

So hardware vice I just need a 7I44?

I think you will find the homing part more difficult then you think.
As Peter said, linuxcnc hasn't been adapted to absolute encoder homing.

Linuxcnc assumes that it homes to a specific point not to align it's self to the current position.
I think this may take a fundamental change in linuxcnc to accomplish.
It would be great to add. I have a linear encoder with absolute distance-coded indexs - that would be along the same lines.

Chris M

Linuxcnc can be set up to 'remember' its last position when it starts up by reading a file.
this is used on machines that are difficult tohome and don't move when turned off.

Maybe you could use this mechanism to do what you wish.

look here:
www.linuxcnc.org/docs/2.6/html/config/in...tml#sub:TRAJ-section

POSITION_FILE=

If the encoders are truly absolute then homing is not needed and things may just work

I will try a absolute encoder setup sometime next week and see how it behaves

Looking forward to hopefully getting some positive results from your test setup next week!

If you setup a system, do you need all axes to be either absolute or incremental, or can you mix?

I attach two pdfs with the drives SSI interface data. Can you please check that this looks to be right ?

Best Regards!

Lars

As far as I know each axis can be set up independently

The SSI interface looks standard and should be fine