Category: Reference Posts

Posts which contain useful reference information

Revisiting the DEC Module: I should bake a cake for Maxon

chorl

At the beginning of summer, I wrote up a quick report of some experimentation with Maxon’s DEC Module. In short, they’re bite-sized little BLDC motor control boards which can run up to 50 volts and 5 continuous amps. They have real current control and also can perform closed loop speed control. Overall, I’ve found them very versatile and useful. Four DECs have now logged several whole miles!!! in the RazErBlades.

I’m always out for more amps, however. The DEC is marketed as a 5 amp controller, but it will gladly hold currents of 10 amps forever, as I found out in the ‘blades. As that post addressed, it isn’t the semiconductor which is a limit to the DEC’s output power. The FETs, while small physically, only have about 6 milliohms of resistance when switched on The gate drive itself is also fairly well matched to the FETs. It’s just Maxon’s well-known conservative rating for their parts, which probably contributes to their reliability significantly.

But what this really means is that a 10 amp Maxon controller should be capable of about 195,000 amps in reality[citation needed].

Well, 20 is close enough to 195,000. Can you spot the hack?







…





It’s the lamest thing ever. I just piled another current sense resistor in parallel with the one already on there. The DEC uses a 10 milliohm 2010 package SMT resistor. A compatible one is this, from Digikey.

What’s that cute little PCB the DEC is mounted to?

Like the RazErDEC and Double DEC’er board, it’s designed to hold a DEC module and provide some glue circuitry. Unlike those two, however, it’s a bit more “general purpose”. I lead the D2 and D1 mode pins out to their own header for setting closed loop speed. I also threw in a selectable inverting circuit which flips the state of the REVERSE pin, in case the motor is happy running backwards when you really want it to run forwards. Otherwise, it has the analog throttle and digital reverse pins brought out to another header. There’s also a supplemental bus capacitor.

The “killer app” for this board so far is the  model CityCar at the Media Lab…

Yes, those are Turnigy Short Melons attached to the Epic Robotic Steering Arms.

Moving on… The DECs are currently a stopover solution as we ditch the flakey Kelly KBS controllers and eventually move onto a fully integrated control solution involving the Hexbridge Shield.

pushing the limits

Now, realistically speaking, I don’t expect the DECs to survive at the full 50 volts and 20 amps. The FET package is simply not designed for high power. The model CityCar will only be running 12 volts, which (while admittedly a bit of a waste of the Melon’s potential – it’s being used for form factor, not power) eases the power handling of the boards significantly.

I expect if I’m actually to use the DECs at 20 amps for any length of time (e.g. for the new RazEr) I’d have to come up with a heat sinking solution. There’s many components on the power side of the board which are taller than the FETs, so it’ll involve some creative layout for certain. I’ll probably have to remove one or two of the large blocky capacitors onboard – which is fine as long as I provide it additional buscap anyway.

RazEr rEVolution: End Menial Labor

chorl

When we last left rEVolution, I was halfway through the process of turning my hands into bloody, tattered stumps. It’s a ritual that occurs with the construction of every hub motor (which means I went through it four times to make the RazErBlades).

And by all that I mean winding the motor. Here’s the end result, terminated for presentation.

I ended up keeping the “split dLRK” winding style – making 6 teeth wound AabBCc and the other 6 aABbcC.  They’re terminated in separate non-connected star points on the underside. The leads are grouped by their respective phases – motor 1 is the classic white-red-black, and motor 2 gets yellow-green-brown.

My patience deterioration is very clearly shown in the picture. I wound this motor starting from the Aa side and proceeded counterclockwise. You can see a decay in the quality of windings and the neatness of layers right after the aA phase on the left. Then by the last one, it’s all gone to hell.

To maintain engineering rigor, I took measurements of the DC resistance of each phase. This will pretty much determine how much current the motor can ever pull.

I proceeded to defenestrate engineering rigor (MITERS has very large ceiling-height windows, so this was easy) by scribbling the resistances hastily on the workbench. In dry-erase.

From the picture, you can deduce that the C phase of motor #2 is about 10% short of turns. Yeah, I think I pretty much stopped caring by that point. The other phases should have 39 to 42 TPT, but I’m guessing 35 or less made it onto the last C phase if I’m lucky.

Fit test!

I tried giving the motor a test spin using a sensorless controller (essentially by bootstrapping off Melon-scooter), but neither half-motor exited the “starting” regime. This was concerning. Did I terminate them wrong? Wind something backwards just a little bit?

A few minutes of diagnosis revealed that the motor was technically running smoothly, but the controller just didn’t increase throttle. Applying voltage to each half-motor in each combination of the 3 leads (i.e. A to B, A to C, B to C…) produced 6 identically spaced angular positions each time – which is indicative of correct winding, since that’s pretty much what your controller does anyway, just alot faster and without colliding alligator clips.

I pretty much just declared it to be a fault of shitty sensorless aircraft controllers being unable to deal with inertial loads or high friction systems.

Without a supply of hall sensors, I put the frame and motor away and started thinking about the other parts of the electrical system.

EVs generally have 3 big components – motor, controller, and battery. With the latter two, you can shoehorn anything in as a motor. With motor and battery, you can be a dumbass like me and control the vehicle with a mildly more civilized form of touching 2 wires together. But without a battery, what am I to do – run the whole thing from a bag of lemons?

Luckily for me, people have figured out how to stuff the equivalent of tanker-loads of lemons into lithium nanophosphate cells. Above is the 12S2P A123 DeWalt 36v Lithium pack during mid-assembly.

I managed to find some terrifyingly wide copper braid this time, and I think the cells just look so much better with them instead of the \m/etalpaxxx‘ quadruple braid. The massive interconnects just scream this pack will dump enough amps to destroy you and everything you ever loved.

Another serving of giga-braid later and the pack structure is complete. I decided to split the pack into 2 6S packs again for balancing purposes, but this time the pack isn’t two separate entities. Shane supplied me with Real Legit JST-XH Connectors of Battery Balancing (+14, for pin count), or otherwise I would have probably resorted to 7 pin-header again.

Bottleshrink returns once again, but this pack is so long that I needed the midsections of 3 bottles to completely enclose it. Luckily, it fit 2 liter bottles fine, and I just pulled a few out of the recycling bin.

A quick check of the cell levels revealed that these cells are in fact still quite healthy despite sitting for an unknown period of time.

Of course, that all went to hell on the first charge, and I had to spend an hour manually bringing each cell group up to full.

back to motters

Given that the motor itself seems to be correctly wound, I surmised that all my problems would go away as soon as I added the requisite hall sensors to the slots.

And here they are, temporarily CA’d in place as a soldering fixture. LRK-style motors need a physical 120 degree sensor arrangement, but I’ve always found cross-stator wiring work to be awkward. Therefore, like on the skatemotors, I take the B sensor and flip it around for a 60 degree, every-two-slots layout.

I secured a chunk of 6 pin cable to the access slot in the shaft using epoxy. At the same time, I elected to secure the phase leads to the other side of the motor. The whole epoxy-globbed mess was then introduced to my epoxy curing oven fixture implement hair dryer hanging by its power cord from the ceiling.

After the first round of epoxy set, I finished connecting the sensor wires. Unfortunately, I managed to break off the output pin of the B sensor… and so ended up having to install the opposite one anyway.

Luckily, this design left plenty of space near the edge of the stator, so running the wiring all the way around the motor wasn’t as difficult.

The sensors got their own layer of goop. I kept finding buckets of different epoxy fillers around MITERS – this is “cellulose” filler, which probably just means paper dust.

It claimed to be less cancer-inducing than either the silica or phenolic microspheres, so I gave it a shot.

After another hour or two of watching glue dry, I put the motor together. Here it is – Dual (Non-)Interleaved RazErMotor!

But does it work?

That’s a gooooooooood question.

Using the Kelly KBS which sort of miserably failed at running Melon-scooter, I was able to briefly power up and spin the DNIR to full speed. Qualitative observation tells me that at least I didn’t get it that wrong. The fact that it moved at all meant that I got the sensor arrangement correct for the “split dLRK”. Now, for this test, I just paralleled the equivalent phases of each half-motor. Once I put together the RazerDEC board, each motor will be powered by its own controller, but triggered from the same sensors.

The speed was visually in the “correct” range and it started and ran smoothly with no signs of improper winding or termination.

that is, until it started sounding like a moped.

Oscilloscope diagnosis of the sensor outputs showed that every mechanical revolution, all 3 sensors were blanking. This caused confusion for the controller for a split second until the sensors outputted a valid signal again.

Well, this is disappointing. The sensors – which, mind you, I had JUST encased permanently in a cellulose-epoxy matrix – seem to be having power issues once per mechanical revolution. I’m fairly certain it’s power, anyway, because there is no physical way for all 3 sensors to read the same field orientation the way they are set up.

This actually means it shouldn’t be hard to fix – my guess is that somewhere, a bare solder joint is nicking the metal can once per revolution and causing a short between sensor 5 volts and ground. A further test to confirm (or dispel) this would be to look for dips in the 5 volt line during each revolution.

In the mean time, there’s no pics of it running, therefore it didn’t happen.