Monday, August 7, 2017

New Go-Kart!

Firstly, credit where credit is due. This project is a collaboration between myself, Ben Katz and Jared DiCarlo, with assorted contributions from Michael DeTienne, Nick Kirkby, and Fred Moore, and moral support from Austin Brown.

For the past year or so, a bunch of us at MITERS had been working on a new go-kart, built around a commercial racing kart chassis. The goal was to finally use hybrid car parts in a project which was stable enough to be fun to drive (the motorcycle was not very pleasant to ride, and the battlebot had rather serious technical issues). 

It all started with this and this. ORNL had done some tests on the hybrid starter-generator ('HSG') from the Hyundai Sonata/Kia Optima hybrids. The Sonata uses a unique hybrid system amounting to what is essentially a Honda IMA combined with a GM BAS, the difference being Hyundai's IMA pancake motor is much larger (30KW) and is capable of powering the car for a few miles in full electric mode. After dealing with remounting the rotors and stators from various Prius-derived hybrid transaxles, the HSG seemed like a dream come true - it had a housing, a water jacket, and even mounting feet.

Locally known as an "altermotter"

The motors are mounted via clamps to the frame and are watercooled using standard computer watercooling parts. Power transmission is done via Gates Polychain GT Carbon belts - we had originally used Gates Micro-V belts, but it was impossible to tension the very long outside belt. The Polychain Carbons are truly remarkable; they have about the same power handling capabilities as roller chain of the same width.

In the back is the big pile of electronics. Instead of an enclosure, the control boards are conformal coated, and the exposed HV busbar is covered in liquid electrical tape. This has done us remarkably well, even on grimy Cambridge roads. Logic is powered by a A123 12v7, which is good for about 4 hours of operation on a single charge (the converters inside the power module draw about an amp at 12V).

The rear disc brake is a generic 200mm Amazon moped brake. It is made of a terrible alloy that warps and discolors under heavy load, and generally doesn't seem to be suitable for anything. It stops the kart just fine though. In the future, a proper vented disc brake will probably be needed, especially at higher voltages and speeds.

The traction pack (42s LiPo) is made of 14 Admiral Pro 4000mAh in a 7S2P configuration. The Admiral Pro's have excellent performance when they work, but out of the original 18 we bought three have already failed - two had physically leaky cells and one has severely low capacity. The pack seems to work OK once the bad packs have been weeded out, but a 16% DOA rate is not really OK for what claims to be a premium battery.

The pack is mounted in the front to improve the weight balance of the kart. It compensates for the extra weight the inverter adds to the rear (~8kg); moving the traction pack from the side to the front greatly improved cornering performance, albeit at the additional risk of battery damage in case of an accident.

The front brakes are built out of moped calipers and some no-name discs we found in a drawer. Making the hubs and mounts was pretty straightforward; however, getting them plumbed in proved to be quite challenging.

Key specs are:
  • Motor: 2x Hyundai Sonata HSG, total 85 ft-lbs (115 Nm) @ 0-2,500 RPM, 40 HP (30 kW) @2,500-10,000 RPM on 160V nominal.
  • Traction pack: 42S 2P lithium polymer, total 168 V peak 8.0Ah.
  • Transmission: 4:1 (80t/20t) 15mm Gates Polychain GT Carbon.
  • Motor controller: custom field-oriented control; control stage: STM32F446RE, power stage: 2nd-gen Prius power module + capacitor.