October 26, 2017

A brief introduction to the big go kart, and machining some PolyChain sprockets

Brief Introduction:
Go read Bayley's introduction.  TL:DR:  Racing kart frame with hybrid car parts based powertrain and custom motor control.  We've done a lot of motor modeling, characterization, and control over the last year, and the go kart is getting quite performant.  But we're not quite squeezing all the possible performance out of the KIA HSGs yet.  And the kart's going to get a second motor.

Right now the go kart uses Gates Micro-V belts, because the HSG's come with those pulleys pre-attached.  However, we're driving the motors a fair bit harder than the car they came from did.  The v-belts, even with a huge amount of tension, slip at ~45 N-m of torque, and we've been able to hit nearly 60 N-m at 180 phase amps.  So we're switching the kart over to Gates PolyChain GT Carbon belts, which are basically the best synchronous belts you can get.  The inch wide Micro-V belt is getting replaced with a 12mm wide, 8mm pitch PolyChain belt, which should be good for substantially more torque.

You can download CAD models of the sprockets from Gates, but, as they warn you on the website, the tooth geometry of the sprockets is not actually accurate, since the tooth profile is proprietary.  My first plan was to try to figure out the tooth profiles from the patents (1, 2), but it turns out if you just ask nicely, Gates will send you the tooth profile drawings  drawings for the sprockets you want.

We went for a reduction of 4:1, with a 20 tooth motor sprocket and an 80 tooth sprocket on the axle.  The big sprockets were too big to fit on the MITERS CNC mill, so I did them on my lab's Haas SMM.  For fun I GoPro'd the entire machining process for one of them:

Getting a good finish on the teeth required many very shallow passes - the minimum curvature radius in the grooves is very close to matching the 3mm end mill I was using, so I had to go really slowly to avoid chatter at the high-engagement areas.

The motor-side sprocket was done on the MITERS CNC.  I still need to broach the keyway in it, and turn a clamping hub for the big sprocket.

Here's one of them partially installed on the go kart.  Still needs a hub to clamp the axle.

So low profile!

Back to the subject of squeezing all the performance out of the KIA HSG, next kart post will be about a stall test stand we through together for motor characterization.

October 4, 2017

Extremely Chinese Brushless Power System Dyno-ing

I did some data-gathering for Dane on a generic looking 48V 1800W brushless electric scooter motor widely available on ebay and aliexpress, paired with a 1500W E-bike controller. 

Check out the full data here.

The motor had basically no shaft to speak of - it has a left-hand threaded stud on the end, and 2 flats which engage with a stamped #25 chain sprocket.  Since there wasn't enough shaft to grab onto with a collet, I made a "terrorist coupling", with three steel pins that engage with the sprocket teeth:

I plugged the analog output of the talk-to-everything board  into the throttle connector so I could sweep throttle commands in an automated way.  It was a little tricky to tune the throttle ranges, because of how the e-bike controller works.  As far as I can tell, the throttle commands voltage, not current, until you hit the DC bus current limit.  That means that at low-speed, the torque vs. throttle position curve is really steep: very small changes in throttle position mean huge changes in torque, thanks to the voltage-mode control.  The motor is also capable of slightly more than the ~10 N-m the dyno is good for, so I had to carefully tune the throttle range so that the dyno could regulate speed.

Dyno-ing in progress:

Initially I put the dyno in inertia-simulation mode, and did a spin-up using a hand held twist throttle to see what the full-throttle performance was like.  Data below ~1200 RPM was cropped because it was pretty noisy.  This whole spin-up only lasted a few seconds so the plot is little rough.  The motor is legitimately good for almost 2000 mechanical watts.  Surprising given the 1500 watt rating on the motor controller.  Efficiency is rather poor, topping out at around 80%.

The high-speed end of the torque curve is a little bit strange looking, and I don't have a good explanation as to why.  Probably something to do with the internal hall sensors and commutation scheme of the controller.

After figuring out what throttle ranges worked at a range of speeds, I did a full automated sweep of speeds and throttle setpoints to get the full operating maps.  Again, the full data is in the database.  The e-bike controller wasn't set up for regen, so all the maps are 1st quadrant only.