Hello all. The 3986 driver was a great solution to steppers and I have quite a few running daily. There is one non-cnc project I am developing that requires smoother motion at very low speeds than the 3986 can deliver at 1/16, so the next step is to build the Allegro 3938 three phase brushless DC controller. Unless I have missed something, which is possible, the 3938 will serve my purposes of closed loop processor controlled motion. I built the 3986 drivers very closely to the 3986 demo board, and only added a shunt for sensing current into the board, plus I used 540's instead of the surface fet version they used. I am still waiting to see if Allegro has the 3938 demo board schematic available, if so, I will model it very closely(unless there are mods we suggested here), making changes more for the fet selection RC values. I will still use through hole fets at first, as I find that in prototyping, they are the first things to need replacing if there are errors, shorts, etc.
The concept of the driver is similar to the 3986. You send it PWM and DIRECTION. There are a few other inputs like BRAKE and REF for example that will receive info from the processor. The 3938 drives the 3 fet stages in sequence based on Hall effect sensors location. Current is managed by sense resistsors, Speed is set by PWM input, direction by DIR input. Quadrature encoders will be read by the procesoor in my case the Parallax Propeller 32 i/o 80mghz 8*32 bit multiprocessor). I will ultimately use a step and dir input. The processor will move the motor to the desired position based on PID. In stepping this will be done at the highest speed pissible, in my first application, accel, speed, decel rates will be programmed for travel between several specific predetermined positions. A simple app will allow user config of parameters.
I have the bulk of the 3938 driver and processor board done(2 boards). I will post more details and ideas if there is any interest. The driver board shown below. A few values undecided. I am using Eagle btw. No large caps have been added yet, if required.
http://www.allegromicro.com/datafile/3938.pdf