moteus controller r4.1

Another step in my plan for the next revision of the moteus servo mk2, is an updated controller board.  As mentioned in my roadmap, I wanted to revise this board to make improvements in a number of domains:

  • Communications: Now instead of RS485, the primary communications interface is FD-CAN.  This supports data rates of up to 8 Mbit and packet lengths up to 64 bytes.  The header is nominally at the original CAN bit rate, but I have no need to be standards compliant and am running very short busses so I may run everything at the higher rate.
  • Connectors: Now there exist power connectors, in the form of XT30 right angle connectors and they are also daisy chainable like the data connectors.  Additionally, all the connectors exit from the bottom of the board to make routing easier in configurations like the full rotation leg.
  • Controller: This uses the relatively new STM32G4 controller series.  It is lower power than the STM32F4, supports FD-CAN, and also supports closely coupled memory, which may allow me to improve the speed of the primary control loop execution by 3 times.
  • Voltage range: This board now has 40V main FETS, with all other components at 50V rating or higher.  Thus it should be safe with inputs up to 8S (34V or so).
moteus r4.1 rendering

It still maintains a number of the capabilities of the moteus r3.1 controller:

  • Integrated FOC encoder: An AS5048 encoder is mounted in the center of the back, which allows direct mounting above the rotor for FOC control.
  • Form factor: The entire board is 45x54mm, with M2.5 mounting holes.  It is smaller than a 60mm BLDC motor and much smaller than an 80mm one.
  • Integrated current sensing: It uses a DRV8323 to drive the FETS, which includes current sensing for closed loop current control.

My first attempt at this, “r4”, came back from fabrication in an nonredeemable state.  I used the digikey supplied footprint for the STM32G4 UQFN part, which looked mostly correct on the surface.  However, while the footprint was good, the pinout was for the TQFP variant!  This resulted in me shorting out several power pins to ground right next to the exposed pad in a way I couldn’t easily rework.

r4.1 seems to be in better shape so far.  It powers up, and I now have blinking lights!

moteus r4.1 as built

Next up is actually porting the control software to the new controller and communications interface.