Igor Chudov <ichudov@...> writes:
>
> I have the W axis working.
>
> http://www.chudov.com/projects/Bridgeport-Series-II-Interact-2-CNC-Mill/34-
Adding-Servo-Control-to-Knee/
>
> When commanded, the W axis moves up and down, roughly as instructed,
> according to my G0 W... commands.
>
> It is not super precise or well tuned, yet, but really not so bad and
> compares favorably to cranking the knee handle. I will try to evaluate
> backlash and such, when I get more time to tune this system.
>
> For those too lazy to look at pictures, the W axis is a 1,000+ chunk of
> metal, moved up and down by a 15:1 DC gearmotor, that turns a shaft that, in
> turn, turns a ACME nut on the knee screw. There is a pneumatic assist on the
> knee, which helps a lot.
>
> There is one problem, however: after the motion stops, ppmc.0.DAC.3 does not
> go down to zero, and instead stays high, while the system is not moving. I
> explain this by remaining under power to move "one more hair distance"
> against friction, nothing is moving, and nothing is there to decrease the
> output.
>
> If it decreases a little bit, no disaster should happen, the knee will not
> drop, the system is self braking etc.
>
> Instead, current is provided to the motor, the not enough to move the rotor,
> but enough to heat it up too much.
>
> So. How would I somehow configure EMC to stop providing any current to "W",
> when not really moving it?
>
> ------------------------------------------------------------------------------
Hello Igor,
I have been following your knee-to-W-axis progress with great interest, since I
have been planning to convert the powered knee on my Kasuga mill the same way.
What is your ppmc.0.DAC.3 output value while the knee servo is stalled??
My first thought is that the W axis servo needs to hit the target position
within a reasonable ferror value for W. What is your W ferror value set to
right
now?
You said: "...the system is self braking etc." If by this, you mean stiction,
then realize that this is a "feature" on the knee, since it isn't supposed to
move when downward force is applied through the Z axis quill. But this same
feature is working against you with a true servo system. My first guess is that
the motor & gearbox need to be big enough to overcome this without drawing a
lot
of amps and a large DAC output. If you can reduce the stiction in the knee
ways,
then you will need to counterbalance the knee more.
Can the air pressure in the counterbalance cylinder be modulated? Could you for
example, install a variable brake master air valve from a truck or bus that is
actuated in such a manner as to offset combined downward (gravitational) force
of the x/y tables, the vises, setup and workpiece(s)? This could be adjusted
once during the job setup procedure in order to reduce ppmc.0.DAC.3 to an
acceptable level, and would not change until the weight on the table is changed.
Good luck!
Tom
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