Segfault Re/BOOTED Part 3: How Seggfault is Formed

Friday, December 10th, 2010 @ 1:07 | Project Build Reports, SEGFAULT

The first part of this Segfault update, and the second, are linked herein respectively. Whoa… can I actually get back to building things now? The last state that Segfault was left in mechanically consisted of two stripped drive motors and a few banged up bits of sheet metal (well, and the loss of two “frame standoffs” on the right side).  Before I could use it again, I would have to replace those drive motors with something else.

The answer was found hiding in a closet at the MIT Edgerton Center. These are 27:1 Banebots P80 gearmotors, similar to ones formerly provided with FIRST kits of parts. The new P80s are pretty legitimate in terms of construction and materials used, and I rather like them. Only problem is, they don’t share the same mounting pattern as the KOOOOLmorgen pancake gearmotors. That was solved rather quickly with a waterjet-cut adapter plate made from frame scrap.

The hole that passed the Kollmorgen motors didn’t fit around the square diagonal of the Banebots. I got out the killing-head and opened up the circle another quarter inch or so. (That’s actually a boring head, but with that giant slab of tool steel mounted in there, it can only kill things and ruin livelihoods.)

A few selective spacers later and the motors once again drop into place in the subframe. Since the wheel hubs and shaft diameters match, I didn’t have to process the motors any further. And thus Segfault was pitched back together.

back to course VI

Two weeks ago (or thereabouts), I sent out the locked-antiphase/class-D motor driver H bridges to Advanced We’re So Awesome We Send You Free Food Circuits on the quick-and-dirty turnaround option.

They really are quite cute. I did these up all through-hole this time around because of component availability (no guarantees on finding all the SMT values and packages I needed in the rush schedule). I had a few IR21844 DIP chips left over, and they found homes on these boards.

Of course, every rush-sleep-deprivation-designed short-turnaround board is bound to have some kind of boneheaded mistake in it. In my case, it was accidentally switching the high side source (/half bridge midpoint) and the high side gate drive supply connections and not noticing until I wondered why my 15v gate drive supply was directly accessible through said phase midpoint. A few chops with a knife and then some Little Blue(and yellow, and purple) Wire followup fixed that.

I began putting together the new filter board alongside the motor drivers. This time, instead of using my custom 555 negative-5-voltificalator, I purchased some cool 24-to-15 volt DC/DC converter modules that gave isolated +/- 15 volt rails. Unfortunately, I found out the hard way that the LMC6484 quad op amps couldn’t handle a whole 30 volts across the power supply pins, so I ended up having to down-regulate both sides to +/- 5 volts. There is also a 3.3v LDO regulator just to feed the sensors.

Motor drivers temporarily hooked up for testing. Consensus: Yeah, I guess they work. Actually, it wasn’t quite that easy. Because of my isolated power supply on the front end, the gate drive chips couldn’t get a common ground between the input and output sides. I had to jump the “VSS” and “COM” pins on the 21844 to make them actually turn on. But wait… what on earth are those wires on the op amps?

I had to wing-jump +/- 5 volts to the LMC6484 on the motor drivers. This chip serves as the 21kHz triangle wave oscillator for the PWM outputs. Unfortunately, moving the supply rails way closer and changing the “divider” value together reduced the frequency to a paltry 6.2kHz. That’s the constant ear-piercing noise in the video.

Concurrent to the electronics assembly process was the repair of the control panel. Previously, this had been wired up to give voltages out at each of the potentiometers. Since the new controller needed the resistance of the pots to control the PI & K gains, I had to perform some air-wiring on the 10 pin cable.

After bench-testing the motor drivers, it was time to start hooking up the Serious Power wiring. I just reused most of what was already in Segfault for this one. The controllers themselves have already been super-professionally mounted using chunks of velcro.

The 100th build report of Segfault is a picture of me taking everything back out after checking for wire length and bench-testing the controller with all the gain knobs hooked up, on live power.  A small test motor was used initially since the power supplies could only push about 5 amps max, and the Banebots motors don’t get up in the morning for anything less than like 40. You can really see the difference between the AFFC and the old controller. Overall, the layout is much cleaner. Granted I did move alot of the H-bridge control circuitry onto their own PCBs…

With temporary Clockerpaxxx dropped into place. I’ll be making a more permanent 7 or 8S pack array for Segfault later. For now, the two robot packs run it just fine for about half an hourt of net run time.

And we’re ready to roll. We’ve already seen what happened after this. By the end of the day of demoing and test riding, I had taken out 2.1 out of the nominal 2.3 amp-hours in each battery. These things were toast – it was hitting the 18 volt cutoff of the DC/DC converter, so near the end of the day, Segfault would occasionally just fall over dead.

A1234 cells being amazing as always, they charged right back up to take in 2.1 amp hours…. at a rate of 9 amps

.

end of segfault

Well, not truly the end. But now that the project is demonstrably working, I’m ready to move on. I’ll be making the bigger battery pack and possibly making the control mounting more robust, but otherwise, I’m going to consider Segfault a (finally) finished tale after a year of head-scratching.

 

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