Friday, January 13, 2012

MKI: Motor Controller

At some ill-defined point in time routing becomes art, I think. I've spent the greater part of the last week transforming that schematic from my last post into the pretty pictures you will see below. It's my first motor controller so I don't want it to suck. Call it MKI if you like (i.e. motor kontroller one or MKI like the VW GTI). The point is I've hung around Shane too long and am beginning the motor controller phase of my life; which is sure to see many iterations.

This first - shield - controller is being produced for TOBL2 and is therefore stupid compact. I know this because routing was a beach. It's contained within a 1.65" x 1.85" form and will plug directly into the headers on XPWMShield I created last year.

MKI has some things that I'm proud of:

There are isolated ground planes on the bottom layer of the board. This technique was suggested by Shane, of course. The logic pins are tied to a ground plane (SGND) which is only connected to the main power ground plane (PGND) by an 0805 resistor. This resistor is sized such that it will explode when too much current is drawn, acting like a fuse to protect the current-sensitive pins on the logic end. In this case running at 6V and fearing 2A gives a 3ohm resistor.

Power ground plane.

Signal ground plane.

Each motor has three diagnostic LEDs. One to indicate forward, one for reverse, and another for over-current. The latter I cannot tout until I write the software for the controller. This LED is not connected to the actually current sensing pin but will use feedback from it to illuminate. More on that once I have a board to test.

It's not visible now but originally several traces wove through the seven-pin header; which the DRC did not like. Moving these traces basically meant ripping up half the wires and starting fresh...#firstworldproblems. (<-- first and last time I will hash tag...yuck...Twitter)

Things I'm not proud of:

Looking at the images above the ground planes are pretty well-contained save for a few wires that need to reach the top header. We'll see if any noise results from this, shouldn't be too bad.

There are 50 f***ing vias. On a via/area basis that isn't bad at 16.4/in^2. However, this board is being ordered 4pcb Bare Bones which allows 35/in^2 maximum, and I'm sure you could pick a given 1" x 1" where that is the case. Hopefully they don't check that so carefully but regardless, that is a shameful quantity of vias.

Switching to the mechanical realm the Robot Marketplace order is in and I suspect so is the McMaster order. In addition I have been working on the frame design. Don't believe me?


This wasn't meant to be in the "things I'm not proud of" section but now that I think of it I'm not proud of how little attention the mechanics have received. Maybe I can make it up by over-engineering servo-automated belt-tensioners...

No comments:

Post a Comment