On 11/30/2017 12:29 PM, Gene Heskett wrote:
2. and this is for Jon. I have a fixed gain scale setting
that prevents the pwm from exceeding a 98% duty cycle. The
pwmgen is running at 20 kilkohertz, and when the pid is
outputting 46 something it is up against this limit, with
a 1.06 u-sec recharge time. The data that you get from the
Pico site says half a microsecond is needed to recharge
the gate drivers. So this should be enough. The 12 volts
is nominally 11.6. BUT, its sitting out there right now at
that minimum of 1.06 microsecnds off time, with the
spindle turning about .8 rps. It ought to be at 3 grand.
Clicking fwd, no reaction, add a + button click and it
hits that 3 grand in about 1/2 a second. 30 seconds later
it starts to slow, and in another minute its down to about
.8 rps and running steadily. Motor is seeing a noisy 2
volts, and it has almost zero torque. While this is
"percolating", I'm going to reduce that scale, wideing the
off time to see what effect it might have.
Well, I have seen some VERY odd behavior if the gate driver
chips decide the bootstrap capacitors are below safe gate
voltage. This is for the brush motor servo amps? They
always had good strong gate drivers, but I did up the value
of the bootstrap and power decoupling caps to 1 uF on later
units. (These are, unfortunately, on the back of the board.)
If you can get the +12 V supply up to 12 or even a little
above, that may help. I forget the exact under voltage
lockout level, but I think it might be 10.6 V. So, with an
11.6 V supply, minus .7V for the bootstrap diode, there
REALLY isn't much left before the UVLO triggers.
Note that the optos in the servo amps alter the duty cycle
some, and if the LED series resistors are too high a value,
it makes it worse. But, that would NARROW the on-time, and
make the OFF-time wider, so that is not likely the problem.
Jon
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