Which Daughterboard is suitable for expanding Encoders and I/O of a 7i96S?

Hi everybody,

I'm planning on building a 3 axis cnc router with closed loop steppers. As far as I can tell, the 7i96S which I already have does not come with enough pins to connect all of the encoders from the closed loop steppers. Would the 7I89 be a reasonable way to expand? As a newcomer I am a bit overwhelmed with making sense of the capability and connectors between mesa cards.
Another route would be to put the 7i96S to the side and just buy a new card such as the 7I95T.

Any recommendations welcome, thanks in advance!

Best,

Florian 

A 7I89, 7I85S, or 7I85 can add more encoders to a 7I96, but
typically you would not use the encoder feedback to LinuxCNC
in a step/dir system unless  you had separate linear encoder feedback.


 
 

Thanks for your response! So you would connect the feedback through the driver of the stepper, and then the driver would handle correcting missing steps etc.?

Yes.

LinuxCNC tells driver to move (step and direction)
Driver moves motor, and checks it has moved commanded amount (encoder feedback to driver).
Driver will have a fault output pin that can be fed back to LinuxCNC.
Cheap closed loop steppers may have a fixed fault window of a certain number of steps.

Most closed loop steppers are not practically much different than open loop steppers.
They are not stronger than open loop steppers. But if there is a stepper stall, they can detect it and notify LinuxCNC. This might save parts being destroyed, or might not.
If a stepper 'loses steps' it has stalled - it has exceeded its maximum torque. It can only 'make up lost steps' by moving extra at a later time. So the part geometry will always be affected.

There is truth to the saying that if a stepper system loses steps, it has not been designed or implemented correctly.
If steppers lose steps, something is wrong (speeds to high, acceleration too high, mechanical binding, etc)

Closed loop steppers are not the same as servos.

Thanks a lot for the explanation! As I'm building the machine myself, the probability of designing or implementing something incorrectly is non-zero, and I was under the impression that using closed loop steppers would provide some form of breaking point which would shut the machine down before crashing it. And being able to access encoder data directly through LCNC seemed much more purposeful than trying to get the configuration software for the drivers working on Linux.

The manual for the drivers(HBS86H) provides the following:

"Alarm signal: OC output signal, active when one of the following protection is activated: over-voltage, over current and position following error. This port can sink or source 20mA current at 24V. In default, the resistance between ALM+ and ALM- is low impedance in normal operation and become high when ES-D1008 goes into error."

How would I go about connecting this signal to the 7i96S?

The fault outputs are basically normally closed switches
So for example if the 7I96S input common is at +24V
you would wire ALM+ to a 7I96S input and ALM- to ground.
 
The corresponding 7I96S input hal pin would then be connected
with a net command to say joint.0.amp-fault-in:

net drive_fault hm2_7i96s.0.inm.00.input-04-not joint.0.amp-fault-in

Because the ALM outputs are normally closed, you could save
7I96S inputs by wiring them all in series so you end up with one
alarm signal for all drives, using only a single 7I96S input.
 

[PCW, as usual, beat me to it]

Depends on whether you set up your 7i96s inputs with a common ground or common voltage.  Are your other inputs (limit switches, proximity sensors, etc.) NPN or PNP?

i.e. what is connected to terminal "I-COM"?

If you aren't sure, what other input components are you using?

Many thanks PCW and spumco! If I understand PCWs scenario correctly, I would need to supply common voltage to the inputs. As we haven't committed to any other input components yet, we would be able to adapt to this requirement. That would be PNP if I understand it correctly?

Either NPN or PNP type sensors will work but since the 7I96S only
has one common pin, all sensors/switch wiring must be the same.

The PNP sensor wiring scheme (7I96S Input common = GND) does
have a couple of advantages:

1. The inputs will work with low voltage sources such as 5V MPG encoders.
That is, you can mix input voltages so for example 24V for mechanical
switches and proximity sensors, and 5V for MPG A/B signals.

2. With the suggested Normally Closed (NC) switch setup,
a shorted (to GND) switch wire will show up as a fault.

Thanks a lot for spelling it out for me, having the advantages of one scheme listed like that is very helpful. Do I understand correctly that the instructions you specified above about the wiring of the ALM+ was for the NPN scheme?