Quote:
Originally Posted by Al Skierkiewicz
Chris,
During the time that valid drive signals are being converted to output, the brake and coast mode is not enabled. The selected mode only takes place when the input PWM goes to 127 or neutral value
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This is why we need you in these forums Al.. Very good point. My memory of the hbridge.c was a little fuzzy... they only go to the BRAKECOAST when voltage in is zero or when the limit switches are tripped.
Darn... I was hoping to be done with this. I might drink Erik's beers anyway just to help me forget this problem.
So where does this leave us? Let me think outloud with the new ground rules:
It all hinges on the "off" PWM current. We have a low FET "on" and the rest "off". Any current flow must now pass through a protection diode. As long as the diode can conduct, the current is heading toward a steady state value of -Vemf/R (assuming positive current during the "on" PWM drive). If the L/R is short with respect to the PWM period, the current decays quickly and is zero for most of the off phase. So the average current for the whole pulse is determined by the area under the "on" pulse which = (12-Vemf)/R*duty. This would be the case for the slow Victors and we would expect a nonlinear response equal to my old coast formula.
For the JAG, the L/R is large relative to the PWM period. If the current cannot decay to zero during the "off" phase, it will continue to accumulate until the average current stabilizes. Since the current never gets below zero we can consider the problem as the superposition of a steady state Vemf and a pulsed 12v. And this leads us to the linear equation = (12*duty -Vemf)/R .
SOOOO...I think the two formulas are ok but now it would be due to the 15khz JAG vs the slow Victors. I didn't see this in my previous excel because I assumed the steady state current was heading towards zero in the "off" state instead of -Vemf/R.
TODO: rerun excel with revised off current and see if I can verify the above
Stay tuned again. I'm determined to collect my beers
