[Part 7] Electrical Design - OSMC Motor Controllers

The Ultramobile Personal Commutator uses two OSMCs (Open Source Motor Controller) as motor drivers. Each unit is capable of operating at 12V to 60V and can output up to 160A. This is way more than what my motors are designed for. The good news is the motors will never suffer from low power situation. The bad news is that unless a proper measure is taken, these OSMCs can completely fry the motors.

I don’t use any fancy mechanism for limiting current. Instead pretty much everything is controlled by the software and it is this software that makes sure that the motor never gets to run at stalled condition (that is to say, to run at its maximum capacity).

Another good thing about these OSMCs is that they are Open Source! You can download all the little details about the circuit. You can also buy a completely assembled and tested unit but hey there is really no point of DIY if you don’t build it yourself.

Some of the features listed in the available OSMC schematics are not absolutely necessary. So I omitted some capacitors and resistors in it to cut down some costs. A commercially available blank OSMC board looks like this.

These are two identical units; the one on the right is just flipped around. The most challenging part in completing this board is probably soldering the resistors. This is not regular cylindrical resistors, but instead they come in rectangular shapes and at much smaller sizes. To show you just how small each resistor is, I took a picture of a resistor with a nickel.

It is so small that using a microtip tweezer to handle it is pretty much mandated. Otherwise I don’t see how anyone can even pick this up and hold it in place while soldering. Using regular soldering technique of “heating up the board, melt a soldering lead on it” won’t work here because the very act of melting soldering lead will push the resistor out of place or the surface tension of liquid solder pulls the resistor out of place very easily since it is basically massless. The technique I used worked splendid and it requires four instruments : Radioshack 30W Soldering Iron (or anything equivalent), tweezer, Xacto knife, solder. First, cut out solder in about a ¼ size of each resistor.

I used a tweezer to pick up and place a resistor and cut-out solder bits onto the desired soldering place.

Using the soldering iron, heat up the side where the solder bit is on, very gently.

Now there is perfectly good soldering seal on the side of resistor. This technique does not require a use of heat gun or heating pad. So it is perfect method for instrument hungry DIYers out there like me. 

The rest of elements are pretty easy to solder like this. One thing though, don’t ever try to solder the MOSFETS perfectly aligned and parallel. I tried to do this first time and ended up braking one of its leg. These guys cannot handle too much force and each leg is very fragile.

Now that the small things are soldered, moving onto the big stuffs like capacitors. One thing I noticed about this OSMC design is that some of the grounded solder points are simply just an extension of the main metal frame. Because of this very design, a lot of heat is required to heat the board up to the desired soldering temperature. I was finally forced to using two soldering irons at the same time since I was unable to heat up using only one 30W iron. If you are trying to assemble OSMCs and can’t get the ground soldered correctly, it’s not your fault. Use two irons simultaneously!

This is a finished OSMC board with 12V fan mounted on it. I didn’t bother to connect the fan wires at the time because by the OSMC design, if I do, the fan will be running regardless of whether the OSMC is active or not. I plan to control the cooling speed using the Propeller chip.

This shows the Big capacitors used for power stabilization. They bridge the power outs directly, so you don't have to connect separate capacitors for the motors. The MOSFETs around the capacitors are cooled by a fan. Considering how not powerful the CIM motors are, using any kind of active cooling mechanism is completely unnecessary. Nevertheless, there are always a room for future improvements if I over-do it; that is, just go with the fan.

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