Brushless actuator control board, r2

The first revision of the brushless servo control board for SMMB was successful in getting a leg to jump.  I ended up doing a small-run second revision that addressed a few minor problems and added a couple more capabilities.

  • RS422 Debug/Link Port: I had a 3.3V serial port exposed previously for debugging, however it caused my USB-serial converter to dislike itself due to common mode ground shifts and it wasn’t reliable at high baud rates (>3Mbps).  I also wanted to support “linked” modes, where two servos would perform control in the actuator space at full rate.
  • Debug through holes: r1 had a number of debug connections, all of which were unpopulated SMD pads.  I decided that through holes were easier to connect debug wires to.
  • Vertical SWD connector: I had initially thought I would hide the SWD connector within an enclosure.  However, the initial enclosure prototypes made that seem less desirable, so I switched it to vertical.
  • More debugging points: When bringing up the first board, I ended up doing a lot of carefully balancing scope probes on various pins, when there was plenty of board room to just have through hole debug points.  Lesson learned.
  • FET temperature sensing: r1 just had an external temperature sensor port, r2 additionally has a thermistor next to the FETS.

Macrofab’s current pricing scheme provides a great incentive to keep your BOM below 20 parts, as that is the only way to get quick turn service.  Otherwise you pay an extra 2 or 3 weeks of calendar time.  In r1, I went to some lengths to stay under 20, however, it just wasn’t going to work with r2, so I left a few easy-ish or non-critical parts unpopulated to do them myself: the connectors, LEDs, and one really big diode.

Control board r2
PCB as received from Macrofab
Installing LEDs
Installing 0603 LEDs under the microscope
Finished board
Board finished with all parts and leads attached


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