Full closed loop 7i76E+7i85S+closed loop steppers and drivers

Hello all,

First excuse me for my bad English. I'm from Bulgaria, Europe.

I bought 4 sets closed loop stepper LC86H3114 and closed loop stepper drivers LCDA2260H from Lichuan for my diy CNC router (2 for Y, 1 for X, 1 for Z)
I'm going to buy 7i76e + 7i85s to to realize true closed loop from motors to linuxcnc using the 4 encoders in 7i85s.

Аfter much reading in the forum I saw comments that it is difficult to do this with closed loop drivers (like a LCDA2260H)
www.lichuan.pl/images/produkty/LCDA2260H...20of%20LCDA2260H.pdf

I have a questions:

1.
Can I do feedback from motor encoders to 7i85s? Maybe then I have to set LCDA2260H to open loop control (this driver can be set to a open or closed loop control mode) ?

Or feedback from motor encoders need be returned to the drivers and 7i85s together.

Or is possible only local closed loop to the drivers. Then if for some reason I lose steps between linuxcnc and drivers (noise, etc.) the work is failing

2.
I have 2 motors for Y.
Do I need 2 encoders inputs (in 7i85s ) for both motors or one it is sufficient?
I ask this because with 7i76e+7i85s I have 5 encoders (4 for motors and 1 for spindle).
If I add the 4th axis in the future i will need another one encoder input.
In this case I need from 7i76e and 7i52s. But 7i52s has 50pin connector !
What are the options in this case?


Thanks in advance !

Ivan

Hello,

If I were you, probably I would start by using what you have: the closed loop on the drives. This is for what the hardware that you bought was designed to. In this case you just need to have sure that you have a good noise immunity between the 7i76E and the drives. A good wiring scheme with a good cable should be enough, and cheap.
Test it and if it fails then go to a full closed system.

Regarding the full closed system, I only have experience with servos and controlling them with velocity mode.
If the case of steppers I do not know what is the best solution. Hopefully someone here can provide you the correct direction.

Thanks emcPT.

But anyway can anyone please tell me is this configuration working in linuxcnc? :
7i76e+7i85s - where the my motor encoders wired to the 7i85s (without using the drivers encoders inputs)

thank you!

That will not work, the motor driver needs encoder feedback to work.
fancier servo drives usually have local encoder feedback _and_
can provide simulated encoder outputs to the controller.

I think you are better off using closed loop step drives with a basic step/dir (no feedback)
interface for a couple of reasons: 1. Tapping into the encoder signals is likely to be
troublesome EMI wise since they are not designed for this use. 2. None of the closed loop step
drives I have seen have encoders with index, so one of the big advantages of encoder feedback
(accurate homing) is lost.

If you used a separate encoder you could work around these issues, but this seems like a lot of
trouble just to avoid lost steps, and a properly configured system should never lose any steps.

Thanks PCW for joining the discussion. I read a lot of your advice on different topics and appreciate your opinion.

That will not work, the motor driver needs encoder feedback to work.

Maybe this driver can work without encoder feedback. In my first post there is a link to driver specifications.
On last page (8) there is a table with Specific parameter description.
Parameter 7 (Open closed loop control mode setting) can be set 0 or 1.
0: open close control
1: closed loop control
Maybe this is a way to get the driver to work without feedback from motors. And then the feedback from the motors returns to 7i85s.
Or I'm wrong ?

If i'm wrong than I do not need 7i85s. Only 7i76e --> drivers -->motors

I found the following post in cnczone:
"....The encoder on the motor goes to the driver which will track the position. The driver will send the breakout board (7i76e in your case) a fault signal if it does not perform the move commanded. To get it all working you will have 3 sets of step/dir signals out of the 7i76e, one for each driver. And the drivers will each send a fault and move complete input signal back to the 7i76e that will go into one of the input slots of the card. There is no need to read the actual encoder values in Linuxcnc, you just need to know if the motor did not do what it was supposed to, the driver will use the encoder to achieve the move on its own. ..."

I do not need another solution if the driver really alerts 7i76e to an error.
Probably this is happening with Alarm+ and Alarm+ connections.
Is this right?

Yes if the drives support open loop mode you can use external feedback, but:

You lose 2 of the advantages of the closed loop motor control:

1. ~0 motor current at 0 load torque (because the drives in close loop mode are basically servos
and do not supply motor current unless the motor is out of position)

2. Stalling in the step motor sense (loss of rotor/stator field sync) is not possible
I think you are much better of using the drives as designed (local closed loop, external open loop)

EDIT

And yes, you should wire the alarm signals to LinuxCNC to warn of drive faults,
move_complete is probably not of much use...

@PCW
Thank you very much for your explanations !
I will use only local closed loop and will buy only 7i76e.
Good luck !
Ivan