Friday, June 24, 2011

DirectDrive: A Simple MechE Solution

The trivial solution to decoupling my Hall effect phase current sensors:

With the three phase outputs coming off the bottom of the board (top in this picture) rather than the side, they don't all pass over the Phase A current sensor, messing with its reading. With this new arrangement, the coupling between phases is much less. Here's the before and after:
This quick fix brings the coupling down to tolerable levels for moving on to other testing. I'm amazed at how close the actual gains are (~15LSB/A) to what the first coupling matrix came up with. I guess linear algebra does work. With these gains, the maximum phase current that can be sensed is about 100A, so if I make it past that mark I will have to reconfigure the traces for a less sensitive current measurement. At that point, I will need bigger wires anyway.

I'm still leaning towards a return to the ACS714 integrated-conductor Hall effect current sensors, but before I give up on the through-the-board sensors, I will also try shielding the wires and/or sensors in such a way that they can only pick up the field from the traces passing under them. Then, I may be able to re-route the wires the way I want, which is off the side of the board.

The next milestone was a light load test on a single phase with a current-limited bench supply. Using the SepEx motor field as a giant thermal sink, I ran one phase for 10 minutes at about 350W (48V/7.5A in, 24V/15A out) with passive heat sinking only. No problems passing this test. The temperature reading was clean and the rate of temperature rise on the sink was a mere 0.5ºC/min. Ballpark thermal estimates (the only type I can do) suggest this is reasonable for such a light load.

At this point I'm past the bench supply stage and the next thing to do is run off a battery. If destruction is to occur, this is the point at which it will happen. Shoot-through on a power supply is survivable, but with a large battery pack ready to shove in hundreds of amps, almost any failure is a catastrophic one for the controller. I will also need a suitable load, since the SepEx was already pretty unhappy after sinking 350W for 10 minutes. The next target to hit will be about 2.5kW, in full three-phase operation. Perhaps I need something that cools itself..........


  1. How has the IR toaster been treating you?

    We've got a Zephyr board heater and PCB-holding rack thing. It works OK with a hot-air gun (the board heater acts as the pre-heat stage in the reflow cycle), but you wouldn't want to produce more than two boards with it. Same with our somewhat unique solder paste applicator; it's neat and precise, but not very efficient.

    What do you use for applying solder paste? Is the oven noticeably better than a hot plate or well, an actual toaster oven? I'm always looking out for procurement... :)

  2. To be honest, this is my first time doing anything other than hot air rework. I normally solder everything by hand with a really nice iron. So I don't have very much experience to draw from. The toaster seems convenient for doing an entire batch of components in one shot.

    The DirectFETs seem like one of the easiest things to reflow, since the pads are huge and, except for the gate and source, it doesn't matter if you bridge adjacent pads. So I applied the paste with a needle directly (no stencil or scraper) in large beads.