That’s pretty. MyroPCB sent this panel to me sandwiched between two scraps of MDF which seems to have been the disposable bed of their PCB drills. I spent quite a while looking at the drill patterns from the other orders and wondering what they were (at least one seemed to be a user-generated breadboard…)
I’m fairly amazed with how well the thing turned out. After the designs posted last time, I made a few more minor layout and trace routing changes in order to be more compatible with Myro’s 2oz design rules. They seemed to want 8 mils trace/12 mils space for 2oz – which seems a little loose, as Advanced Circuits could go as low as 6/6 last time I checked. The trace widths weren’t a problem, but several places had problems with 12 mils spacing between elements.
I don’t think I did a very good job with the space thing, since that would have involved routing almost all the board over again (the gate drive area is a tight squeeze), but Myro ran them anyway and they seemed to all have etched properly.
I decided to make 2 boards out of the panel just to make sure that the routing worked. After all, no use in populating all 10 only to discover that I made a truly phenomenal routing derp (and I didn’t have parts for any more than 2 at that time anyway…)
Now with more FETs.
I’m going to try out several different FETs between these ten boards in order to gauge what the effect on continuous operating current without airflow is. I have the following types:
- Infineon’s 034N06, 60v and 3.4mohm. These are pretty much the “worst” ones I have – I bought these originally when I was wanting to not blow up 5 dollars every time a FET goes (like I was with the IRFS3107s).
- Infineon 017N06 60v, 1.7mohm. These are the current choice for Ragebridge and are in all the robots. Conduction losses halved from the 034s. These, along with the 034s, are both compatible with a 36v mode for these controllers. The only thing that limits the hardware to a certain voltage is the choice of FET and capacitor – for 36v operation, I’d want at least 60V FETs and 63v capacitors. 75v FETs would be even better, but they get pricy in low-resistance models.
- IRF3004-7 type, 0.9mohm, 40V. These will be the best choice, I think, for the standard 24v type RB, but IRF is pricier than Infineon (whose nearest competition is the 011N04of 1.1mohm Rds-on and somewhat lower gate charge).
Once I prove the boards working or otherwise, I’d like to snag some of the new Infineon 010N06s that just came out. Basically, the lower the on-state resistance of the FET, the more current I can push through the controller, so long as the switching time is much shorter than the on-conduction time. Right now, with the 1.7mohm FETs, I can push approximately 30 amps continuous in still air while adhering to a 100 degree celsius junction temperature. If I wanted to edge closer to the maximum temperature of the junction, at 175C, then I can push about 40 amps. Again, this is with absolutely no airflow and no heatsinking effect from the board bus traces or large-gauge copper wiring. Any airflow at all would be sufficient to push the current rating up.
The two test boards are loaded one with 034N06 and the other with 017N06. I only have a limited quantity of the 60v types, so the majority of the boards will be limited to 24v nominal (~30v peak).
Fully populated but not yet programmed. I didn’t even leave myself an ISP header on this thing – that 6 pin header is an FTDI cable connector. So how am I supposed to program the blank ATMega chip? How will I even turn it into an Arduino?!
The answer, as usual, is provided by Hobbyking. Seemingly on a war path to be the next DIY electronics dealer, Hobbyking has recently been peddling everything from said Arduini to robot kits. Aided by the hobby multirotor craze (Yes! There are such things as crazy quadrotor guys just like there are crazy 3D printer guys and crazy electric rideables guys), HK now sells quite a few different flashing tools for microcontrollers. This cute spring-loaded programing socket is a steal at $20 – a similar device from a more “legit” manufacturer would certainly run you triple digits.
And as far as I can tell, it actually works, very well. The springs are stiff, so the pins dig into the chip legs tightly and allow some wiggle room too. Only downside, it seems, is that there’s no orientation guide (that I can tell), so it took me a minute of staring at my Eagle files to discern which orientation to use it in (Hint: Tight group of 3 pins faces away from the dot on the micro).
Seriously. Get that right now, especially if you think you might ever be flashing anything 32-TQFP shaped.
I used it in conjunction with an AVRISP mkII programmer, since I couldn’t get that damned “USBASP” thing to be recognized by my computer or to function at all. A 6 to 10 pin ISP crossover cable was made with some quick breadboarding wire to bridge the gap.
After the Arduino buttloader is loaded into flash, the rest of the program uploads are taken care of via FTDI cable.
I had some fun discovering that “Upload via Programmer” in Arduino means “Erase everything and upload just the compiled sketch” – I spent quite a while probing my RESET circuit to see why my FTDI communications weren’t working, and then it hit me that the damn thing wasn’t even blinking once at startup.
The only changes from the Ragebridge version 1 firmware were some pin definitions, since I had moved several connections around to better suit the chip orientation. Furthermore, the now fully-symmetric current sensor layout meant that one of the current control loops had to be inverted.
What I want to do next with the firmware is to make a “mix” mode for the two input channels, which means you don’t need a transmitter with onboard mixing (or to set it up) in order to use the the controller. Eventually I also want to make it take commands in the form of serial packets (over the FTDI TTL serial connector). However, it will not have any more complications than that – no fancy encoder inputs or selectable analog inputs or kitchen sink interfaces. Ragebridge was designed as a rebellion against those modern ‘universal’ ESCs anyway.
Right now, though, it’s just bone simple R/C servo PPM input.
The one thing which prevents a ‘MIX’ mode from being immediately useful on this board is the fact that I neglected to put any hardware selector on the board. One of the digital inputs can be tied to ground or 5V (e.g. with a PC jumper) to activate mixing, but I just.. forgot to put one on there. Instead, you’d need to solder a wire to a random place with GND, which isn’t very useful.
These, and other random layout shortcomings, will be addressed in a 3rd board revision.
In anticipation of possibly having to test 10 boards, I went ahead and designed myself a jig. Similar to a bed-of-nails tester, it just holds a bunch of 3.5mm bullet connectors in place to make all the important power connections (bed-of-bullets tester?!) so I don’t have to solder 8 wires every time.
Here are the connections completed. Inputs and outputs are both my standard 4mm bullet connector interface. A 3.5mm bullet just happens to squish enough to fit into the wire holes on the board, so…
Board #1 under test. Conclusion: it works.
Whoa, I assembled a board, totally untested in the middle of the process, and it works on the first try. That’s actually pretty damn new and exciting.
The second board had an interesting problem where one PWM out was seemingly underflowing repeatedly, very quickly. A bit of probing led me to discover that one of the current sensors had died (or was dead to begin with) and so was pegging at 5 volts. Constantly triggering the overcurrent procedure, it continually decremented the output PWM command, went past its valid ranges (+/- 255), then underflowed its signed int holder and kept decrementing from the top again. This caused the motor to occasionally freak out as the output command entered then exited the valid range.
The spazzing also damaged one side because it was while the board was hooked up to a non-regeneration-friendly power supply. I fixed it by replacing some gate drive components, but I guess now I know which board I am hanging onto!
The test motors were an EV Warrior and a derpy 24 volt scooter motor. Basically the test involved alot of slamming the motors back and forth (on a battery, of course) after validating controller functionality on a current-limited power supply. This was to test if the controller was sensitive to pulsed current-induced noise, and if the smart current limiter was working.
Alright, time to start on the next set of boards. This is like 5 hours of solid work, and in the end, I totally ran out of parts! I’ll have to wait until the next Digikey order comes in before starting the soldering process again. First to run out were my gate drive zener diodes, and oddly enough, 1uF capacitors. I also ran out of ATMega chips and gate drivers.
The development process for this thing is starting to get reeeeally expensive. I have yet to total up the cost per board in this quantity, but the chances of me giving away free samples to alpha testers is very, very slim.
Once I get my refill orders in, I’ll populate and test the rest of these boards, and then set up the structure for an alpha testing run. I intend on releasing these “into the wild” as much as possible in order to find their limits and how people are going to mess up on using them. The guidelines below aren’t set in stone, but are what I have in mind as to what would make a good alpha tester:
- You have a device, whether it be a robot or vehicle, that uses 2 DC motors in some way. Right now. Not “will in a few weeks or months”.
- Your system runs 24 to 36v. Up to 48v is possible, but I would need to specially configure a board for it. HV testing is something I’m interested in doing.
- Your system usually pulls 30 to 60 amps, and can live with 60 amps at most (This means some high-current systems like Magmotor-powered go-karts will experience constant-current mode for longer).