Sunday, August 21, 2011

tinyKart: Round 5

tl;dr tinyKart works:

It ran outdoors for the first time at Swapfest, our de-facto vehicle debut venue. This time, tinyKart and Charles Guan's Straight RazEr were revealed to the world crazy swapfolk. It took a while to get there, as evidenced by the greater-than two week gap between tinyKart posts. But here's where I fill in that gap. When we left off the kart was mechanically complete, minus the steering wheel.

Problem solved.
The angular butterfly design courtesy of Max H. fits very nicely with the overall aesthetic of the kart, plus it's all 1/8" aluminum plate welded to thin-wall aluminum tubes, so it weighs less than a pound. For various reasons I can't quite say how this nice thing came into existence. But did you know that Amy Q. is awesome? Just sayin'.

The steering wheel is very important on tinyKart. It holds all of the driver inputs; there are no pedals. The two front disk brakes are controlled by cables which go to a really awesome double pull lever I found on Amazon. We had some issues stemming from unequal length brake cables (different amounts of friction). But a trip to the bike shop for two new 48" cables solved that problem. The throttle is a trigger from an RC car controller, and it really does work like an RC car. (Pull to go, push to brake, push twice to reverse.)

One interesting consequence of having no pedals is that you can put your feet wherever you want...

One other minor mechanical detail came up during the week: the drive pulleys which attach to the motor shafts had been bored out to 10mm, but that left very few threads for the set screws. As a preventative measure, I took the four prop adapters that came with the motors and pressed them into the even-more-bored-out pulleys. The Frankenpulleys are held together only by a 0.003" press fit and large quantities of retaining compound, but I still think they're better than the stock ones.

Before attaching the pulleys to the motor shafts, I filed small flats on each side for the set screws to grab. The other side of the pulley attaches to a 10mm aluminum idler shaft that runs in a bearing in the outer plate of the drive module. This way, the pulley is supported on both sides and motor shaft incurs less bending moment, all of which helps my MechE side sleep at night.

And with that, all that remained was a crapload of electronics work. The first step in that process was to mount the batteries and the motor controllers. Up until we thought about it hard enough to realize that nobody should sit in between two 3lb LiPo batteries, the plan was to put them along the sides of the kart. But it turned out that the two batteries and the controllers fit nicely in some space behind the seat. To isolated them from mechanical shock, they are attached to a separate 1/8" aluminum plate floating on left-over BWD wheel urethane:

But weren't BWD's wheels stupidly hard?
If you feel the seat warmer kick in...GTFO!
This is the second time I've used giant "long pack" LiPos on a vehicle. The first was Pneu Scooter's maiden voyage in Singapore. To be honest, they scare me...and I am not easily scared by batteries. I am used to the potential for 1,000A short-circuit currents and the precautions that go with that. I know how to monitor cell voltages and protect the pack from damage. But all I can think about with these is how far away I need to be if the battery starts spewing flame in all directions.

Tangential thought: I think I will spend some time in the near future making a 12S3P A123 pack for tinyKart. To keep the same weight while switching from LiPo to LiFe, some capacity must be sacrificed (273Wh instead of 333Wh). But I could make a couple of packs and swap them. And as much as we fret over the safety and liability of having loose cells and homemade battery packs lying around, I still feel safer that way than with these giant LiPo bombs that I can buy freely on the internet.

Finally, a good load test for eBay Hack Charger #7,523. Set at 41.0V cutoff and 5.0A, it would take almost two hours to fully charge the batteries. They arrive about half-charged, so this was a good one-hour full power (200W) test of the Vicor brick power supply I built. Seems to hold up just fine. And no fiery LiPo death yet.

Thank you,  Cell Loggers, for giving me at least some peace of mind.
With the batteries and controller mounted, the wiring could begin:

The wiring for this kart wasn't nearly as bad as Cap Kart. For one, the primary wiring could be done with double 10AWG instead of single 2AWG. So, the hammer crimper was not required for this one. Each battery gets its own 60A fuse. Then, the two positive wires come together inside a master switch, which has a precharge circuit bypassing it. Chris C-T. decided to put all of this inside the master switch, because that's just what he does.

A switch within a switch...
The master switch needs to be easily reachable by the driver, so it is attached to the right 80/20 frame rail:

The rest of the primary wiring was easy. Well, I shouldn't say that, because it turns out that the 5.5mm bullet connectors on the batteries are not that same size at the 5.5mm bullet connectors I bought... So, a couple hours of tense giant LiPo connector surgery were necessary. But other than that it was easy. The signal wiring, on the other hand, took a very long time.

Cue wiring montage...
Each Kelly controller needed a total of 12 signal connections: 5 for the Hall effect sensors and 7 for the throttle, brake, reverse, and diagnostic signals. The Hall effect sensors ride on a laser-cut acrylic mount (a la Pneu Scooter) that holds them just off the motor can, at the proper electrical angle. The cans on these motors are thin enough that the magnetic field is easily picked up from outside; no need to bury the sensors in the motor windings. The acrylic mount is tangentially adjustable so that the motor timing can be fine-tuned after finding the proper combination of wires. The process by which this happens is a matter for another entire post, but generally it's sufficient to set them for minimum current at a given throttle position.

Throttle, brake, and reverse are controlled by a sophisticated central vehicle processor Arduino that intercepts the throttle signal from the RC trigger and decides what to do with it. The Arduino reads in the trigger analog input and generates four PWMs which are low-pass filtered, buffered, and set out to the throttle and brake sensor inputs on both Kelly controllers. Ted G. and Catherine C-T. set up this fancy throttle interceptor to capture the brake/reverse function of the trigger, but also because we want to implement steering angle-based differential torque on the two motors. Future feature, once the steering potentiometer is installed. 

As it turns out, regenerative braking will also have to wait for some minor modifications. The Kelly controllers can use an analog brake signal, but apparently they still need their digital brake switch pulled low at the correct time for the braking feature to work at all. So, there will actually be 8 signal wires for each controller, one of which may not be zip-tied to the nice wiring harness that was already made.

After a week of wiring, we finally got a chance to test drive it this weekend. Here are the two videos:

8/20/11- Maiden voyage in N52.
8/21/11 - Outdoor testing at MIT Swapfest.

So, one year to the day after Cap Kart v2.0's first successful test drive, and three years after Cap Kart v1.0 first touched pavement, we made it back out to our favorite strip of trackside delivery road. This time, though,  we didn't need four people and a pick-up truck to get there. One person can easily tow tinyKart with two wheels on the ground, and two people can carry it for a long distance. It also fits in elevators and through doors.

The test drive validates some of the mechanics of tinyKart: The 17mm aluminum wheel axles seem okay, the brakes and steering work as they should, and the frame holds itself together. In the video, you can see that the chassis does flex, a lot. You can feel it in the seat. But it's not the kind of flex that will break things - more like an airplane wing flapping a bit because it's just an aluminum truss. Interestingly, it keeps all four tires on the ground.

There are definitely still some weak points, some of which we knew about and some of which were revealed in this test drive:
  • The left side motor can is loose and has a tendency to shift axially, once to the point of binding so hard that the entire rotor shifted, changing the motor timing. The only question is: repair or replace?

  • Both controllers exhibit a well-know problem of cutting out under heavy load (80A?). This seems to be common with RC outrunners running on Kelly's. I've seen it at least twice before. Since I pretty much spend all my time debugging motor controllers anyway, I can probably find a quick fix. If not, there is a higher PWM frequency option for Kelly controllers that could be worth a try. (The inductance on these motors is so low that the current ripple at 16kHz is still large.) If nothing else works, tinyKart may some day be merged with DirectDrive...

  • The weight.

The current weight is just under 55lbs. To get down to the target of 53lbs (less than one of Cap Kart's old lead acid batteries), we will have to do a bit of trimming. The current weight already represents the first major cut - the side seat mounts that Max put so much effort into had to go. We were worried about whether the rear seat mounts by themselves could handle high cornering forces, so we did a quick 1G turning test:

Removing the side seat mounts only saved 0.85lbs, though. To cut an addition 2lbs, we will have to take small bits off of several places. Some easy targets are the belt tensioners, the front of the frame, and a few of the lateral 80/20 braces which could be swapped for lighter box extrusion. After that maybe we'll drill some holes in the LiPos to reduce weight...

In the meantime, a bit more test driving may be in the near future. Also, exciting news: We will be bringing tinyKart to NY Maker Faire in mid-September. Look for it there!


  1. Could you please tell me where you got those belt pulley? McMaster I guest!



  2. They are 16 tooth aluminum HTD pulleys from SDP/SI:

  3. hallo how you make power steering where i can buy i can you tell me about motor thansk