I think you could eliminate the load cell and simply measure motor
current.  Given that the motor is locked at zero RPM, the torque would be a
function of current.  Once you calibrate current to torque it will not
change.   You still need to get this data into a computer but at least you
don't need to build a mechanical widget, just a hall effect sensor on more
lead wire.

One thing is to be VERY careful when powering a stalled motor s you can
burn it up very easy.   Even if you do add the load cell be sure to
 measure and limit the current and keep it will under the limit.  Then you
need to cut the power quickly

Coincides or not but just a second before reading your post I had typed the
line below to define where I'm connecting a current sensor to an SMT32F411
chip pin PA4.  So I thought this could apply to your project too.

#define MM_CS1  PA4   // A2 Current Sense MUST LIMIT VOLTS TO 3V3

One more idea that is even more simple.  I assume you are measuring torque
so you know the nut is on hard enough that it will not come loose.
 Rather then a load cell why not just a simple coil spring and a
microswitch.  It will always require the same amount of force to compress
the spring and then the switch is tripped and yu can back off   This is
basically a load cell with one-bit resolution.

On Tue, Apr 16, 2019 at 9:57 PM Gene Heskett <ghesk...@shentel.net> wrote:

> On Tuesday 16 April 2019 14:34:15 Gene Heskett wrote:
> > Greetings all;
> >
> > While adding a couple timedelays and a couple or2's to my hal file,
> > basically to prove that I can pulse the spindle for the nominally 200
> > msecs used to spin the nut on/off to release the collet and supposedly
> > drop the tool, or just drop it all, I came up against a designed in
> > problem.  This vfd has a pid in it that will no doubt wind up if the
> > speed command is present for any great amount of time, so its possible
> > I might have to cobble up a set of PID params to essentially disable
> > that. Because I think I am going to need to have the speed present at
> > the VI1 input before I enable the fwd or reverse commands for the
> > timed 200 millisecs to drop or pick up the tool during the change. The
> > wind up in that case might be advantageous in that it might jerk the
> > nut loose, or tighten it tighter before the carousel puts the real
> > tighten on it. But likely difficult to control too.
> >
> > But I find its not possible to send this speed request before the
> > run/dir signals. I can mux2 the spindle speed inputs to the 7i76, and
> > have the as yet unwritten tool changer code issue the speed and
> > timings.
> >
> > Is that the choice?
> >
> > A secondary problem if I do the 3d printed tool holders inside the
> > collet sockets, like the orange pieces in one of the videos on
> > hack-a-day, depending on the tool to drop free when the nut backs out
> > the collet, is that the tool does not always come free, and this looks
> > like a showstopper problem that puts me back to buying a handfull of
> > nuts and collets so I can drop it all and then reposition the carousel
> > to pick up the next tool/nut/collet as an assembly. Either way, it
> > seems like I'll unavoidably need an automatic TLO setting to complete
> > the change.
> Some more along the lines of finding out how much force I can figure on
> as being available to tighten or loosen the collet nut. I can get, for
> under a tenner, 4 ea 50Kg load cells and a processor board that converts
> the very low level diffs of a wheatstone bridge made out the these,
> things, and which give a serial output to 12 or more bits.  So all I
> have to do is place the cell kit between a frame and the carriage and
> give it a squeeze to get a measurement.  These are the same as used in
> digital bathroom scales.
> Has someone made up a serial protocol for a Mesa board that could collect
> this data, and show me in pounds or kg, how much force I can get out of
> this machine before a motor slips s "cog", which in turn determines how
> big around the carousel has to be to adequately tighten, and of course
> loosen it too.  The module needs a clock, one output on a 7i76D, and one
> data input, so all we need to do is toggle the output line with the
> servo-thread, and read the bit with an input line. A bit bang receiver
> IOW.  Do we already have such a blob of code?
> Thanks for any comments on my hair brained ideas.
> > I figure on a bigger
> > carousel than shown in the videos. That thought is driven by the
> > difficulties I've had with the hand tightened (with a pair of 12 to
> > 16" wrenches) on the ER32's on the G0704.
> >
> > I've wrecked parts and broken tools because they'll walk out of the
> > collet, dig too deep and finally break off.  That bit of history has
> > cost me several hundred in tools so far. :(
> >
> Thanks for any wisdom shared. URL's etc.
> Cheers, Gene Heskett
> --
> "There are four boxes to be used in defense of liberty:
>  soap, ballot, jury, and ammo. Please use in that order."
> -Ed Howdershelt (Author)
> Genes Web page <http://geneslinuxbox.net:6309/gene>
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Chris Albertson
Redondo Beach, California

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