On Wednesday 17 April 2019 01:48:06 Chris Albertson wrote:

> 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

Ahh, Chris, this is not a servo, I am talking about the XY motors, which 
are steppers and not particularly strong ones.  They are effectively 
always stalled and the current used is set by the dipswitches on the 
driver.  So generally speaking, measureing the current is meaningless to 
a stepper that has remained locked to the steps issued.
> 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.

That, with some monkey business incorporating the switch into the sliding 
wrench channel that is pushed out to engage the double flats on the 
chuck shank, might be doable, but the spring to absorb the torque would 
have to be 50x stronger, and if not restrained properly would prevent 
the wrench from being moved out to grab the flats, or retracting it to 
free the spindle. This wrench must be free to move in and out quickly as 
its used only the break the nut loose, then retracted and the motor spun 
yo finish the loosen and drop the tool or tighten to about finger tight, 
prior to using the xy motors, driven in an arc to match the radius the 
sockets are mounted on the carousel disk. This power from the xy motors 
is what breaks the nut loose or cinches it up to hold the tool while 
cutting. Spindle power is not applied during that circular move as the 
retractable wrench has it immobilized anyway, so the the nut is driven 
tight, or backed off depending on which direction the nut and carousel 
is being rotated by the xy motors.  I'd be forced to use an air cylinder 
to get the needed force to move the wrench. Could be done, but not in 
the physical space available since it would take at least a 1"bore x 
3/4"stroke double action cylinder. I expect there will need to be a 
pause, and the spindle motor pulsed to turn until the wrench engages the 
flats, then pulsed the other way to make sure its fully engaged. How to 
do that while the nut is still tight, is another problem yet to be 
solved if the gator style socket isn't used. I don't know why, but I've 
an aversion to that as I have a set of those and they work but are a 
pain in the ass to place over the hex properly. The set I have do not 
have retractable pins like the gators either.  Different designs maybe? 

If you've not watched the videos on hackaday, that would explain it at 
least as well as I can.

> On Tue, Apr 16, 2019 at 9:57 PM Gene Heskett <ghesk...@shentel.net> 
> > 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>
> >
> >
> >
> > _______________________________________________
> > Emc-users mailing list
> > Emc-users@lists.sourceforge.net
> > https://lists.sourceforge.net/lists/listinfo/emc-users

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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