[alan_3301]

Hi I am trying to decide between mesa 7i77 or 7i76 for interface with 2 delta asd-a2 drives and dmm dyn3h drive.
7i77 is analog interface with encoder counters, 7i76 is step/direction interface with no encoders.
The machine is a lathe, around 12x12" X Z travel, with 5mm pitch ballscrews. driven by delta 400w servos
I plan to have spindle C axis , which is a 750w servo driven 2 to 1.
Here is where I am unsure about which to choose.
Both brands of drive take position or analog speed/torque input.

The delta drives have 1,280,000 pulse encoders going to the drive, but the drive will output to linuxcnc between 20 and 320k pulses per rev,
If I use position input, it seems the drive would have better control, having access to finer resolution encoder signals.
The electronics gear ratio is adjustable from 2 to 500k pulse per rev (what a range)
If my math is right, a ratio of 508:100 or 1968.5 pulses per rev, would give me 1 pulse per 0.0001" travel on a 5mm pitch screw. (1:1 = 10k pulses per rev)
the 7i76 doesnt have encoder inputs, so I wouldn't be able to/ or need to run the encoders output back to linuxcnc.
That all seems fine, either mode should work, but analog would give me DRO's that reflect actual position.

However for the DMM drive, it has a 16,384 pulse encoder on the motor, but will only output to linuxcnc 500 - 2048 pulses per rev.
which would be 2000 - 8056 quadratures per revolution. I don't know if linuxcnc will see a max of 2048 pulses, or 8056 quadratures,
but even assuming 8056 positions, I would only have 0.044 degrees per step. On a 6" diameter part, that would be like 0.0015" per step. (2:1 at the chuck)
Or would it be the worst case of that scenario, 2048 positions, 0.087 chuck degrees per step, or 0.0045" per step on a 6" diameter part?
The first case seems fair, the second case seems too coarse.
Now if I chose position mode for the spindle servo drive, I would have a max of 16,384 steps per revolution,\
which would give me 0.0219 servp degrees per step, 0.0109 chuck degrees per step which would be 0.00057" per step on a 6" diam part.

So with position mode, I can double my resolution on the C axis.

Does this all sound right? or am I missing something?
I will admit, that I will probably never need that much resolution for any parts I will be making, and the machine construction is probably not stiff enough to realize that resolution either, but I understand that with C axis positioning, The higher the resolution, the better.

Here is what I can understand for pros/cons of analog and position

position
pros - ease of setup, positioning accuracy as fine as highest resolution of input.
cons - max speed limited by max pulse frequency

analog
pros - ability to tune on linuxcnc side, DRO has encoder feedback.
cons - positional resolution limited to simulated encoder feedback to linuxcnc.

Thanks for any replies, I am really excited to get this thing going, just dont want to make any great mistakes in doing so.

[mactec54]

alan_3301

Are you going to use live tooling for milling drilling with the spindle???

What is your spindle belt drive ratio 1:1 or 2:1???

Analog is better if you can do it, with closing the loop in the control, Step/Dir will work ok as well, the loop will be closed in the Drive

[alan_3301]

Live tool would be a wish in the far future, but I would like to be able to broach tooth splines with the C axis.
The headstock bearings are 1600rpm max, so I would be running 2:1 servo:spindle
That dmm is 3000 rpm continuous

thanks.

[alan_3301]

I guess the only reason I am unsure about analog or position, is it seems I will have nearly twice the resolution with positioning mode, because of the max of 8056 counts from the dyn3 going back to the control, if I were to do analog. I would be able to have 16384 pulses per rev with position mode right?

[mactec54]

alan_3301

With running 2:1 you then will have double the Torque which you will need, you will also need a Brake to hold the spindle to do the broaching, what you get out of the drive is just a ratio of the 16384 just like using electronic gearing, you still have 16384 at the motor, with the 2:1 this also doubles your resolution you will have at the spindle