7i76 +/-10V spindle output (-10v to +10v spindle output)

I have a lathe which uses steppers, but has a servo for the spindle.

I have a 7i43 + 7i76 (see forum.linuxcnc.org/27-driver-boards/3333...fier-driver-h-bridge for a bit more info).

I have a Copley 422 servo amplifier (manual attached), which takes +/-10V analog input (that is -10V to +10V).

I want to drive the Copley 422 from the 7i76. I'm aware of Mesa's dedicated servo analog output cards, but I can't utilise these due to my setup.

I see from the 7i76 manual, that is it possible to set it up in 'bipolar' mode for spindle output, to give out -5 to +5v. Is there any way to get +/-10v?

Thanks

The 7I76 analog output cannot do +-10 bipolar directly
(a 20V supply will damage the 7I76) but you can do it with a external relay
and a isolated power supply (like a 12V wall adapter)

The easiest way is to use a DPDT relay drivern by SPINDLE DIR to reverse the SPINDLE- and
SPINOUT pins connected to the drive

Another way is to use a SPDT to select SPINDLE- or SPINDLE+ as the common signal going to the
spindle drive but this has the disadvantage that negative speed is commanded if off
(so the spindle enable better work...) and the speed control number is backwards for the reverse direction

PCW, thanks for the reply.

The Copley 422 takes differential input (ref+ and ref-) +/-10V. It does not output a +10 and -10V supply for use.

I'm aiming to use just two wires to drive it (i.e. ref+ and ref- differential).

I'm currently thinking:
Set up 7i76 in bipolar with -5 and +5v
Use a ADA4922-1 Differential 18-Bit ADC Driver - which takes a single ended input and outputs a differential signal, with a fixed gain of 2. (Datasheet attached). I believe this would take the +/-5V signal from the 7i76 and output +/-10V (Although, I may need a gain of 4?)
SPST relay driven by SPINENA switching SPINDLEOUT output

Thoughts?

You dont need a differential driver (a single general purpose OP amp can do the scaling
and you just connect REF+ or REF- to common)
But you still need matched +-5V supplies and +-15V supplies for this so
I think the DPDT relay scheme, is much easier
(you still need a floating 10-12V supply for this but a standard wall wart will do for this)

Peter,

Thanks for your help.

I now have the servo spindle working in unidirectional mode. There is a PID loop - it is closed loop spindle speed control.
Attached are my config files for anyone who may be interesting.

The Copley 422 wiring is:
Enable /POS and /NEG are grounded
REF+ is grounded
REF- is connected to 7i76 SPINDLEOUT

I'm using my computer power supply for the TB4 +ve and -ve.
12V on TB4 +ve and ground on TB4 -ve.

Peter, is there any chance you could do a diagram or give more explanation of the DPDT circuit? I don't understand how it will generate negative voltage.
From what I understand of what you've posted:
When clockwise (forward) direction is desired, SPIN+ is 12V, SPINOUT is connected to servo amp REF input, and SPIN- is connected to ground. The SPINDIR pin does not change this configuration
When reverse (counterclockwise) direction is desired, SPINDIR activates a DPDT switch, which swaps SPINOUT and SPIN-, meaning SPIN+ is 12V, SPINOUT is now ground, and SPIN- is now connected to servo amp REF input.

Thanks

Looking into other options:
What about a 'Charge pump DC-to-DC voltage converter' like TC7660, which generates a negative voltage from a positive one.
(The TC7660 inverts 1.5V to 12V, so not actually suitable given we need 0V to 10+V inverted).

Use a SPST relay to switch to the charge pump when negative required?

My long term goal is to be able to use the servo spindle for positioning (C Axis)

More googling reveals that the 7i76 spindle output will be too slow for C axis positioning use.

So at this stage, I think I'll settle on a goal of forward and reverse spindle speed control.

Yes, the 7I76 spindle analog output is too slow for real position mode servo control.
(but fast enough for closed loop spindle speed control)

Since you have both REF+ and REF- inputs you can simply use the DPDT relay to reverse the connections
from the 7I76 SPIN- and SPINDLE OUT to REF+ and REF-. Having the differential inputs also means you can
use a non floating power supply (like your PC power supply)

For wiring, look up "reversing switch" or "reversing relay"

If you want full closed loop analog controlled positioning you could add a 7I83 later (though it is really overkill)

Thanks Peter.

I'm looking at using a small relay such as: Omron G6S-2F 12DC - 12V DC, DPDT, 140mWatt coil power
With a flyback diode across the coil

From the manual the 7i76 spindle output can switch 50mA - at 12V this is 600mW, which gives plenty of safety margin.

Yes, that should be fine but a non SMT relay might be easier to mount
(dont use a 5V relay as the drop across the OPTO will lower the coil voltage significantly)