Finishing BurnoutChibi: Transmission & Drivetrain, Controller Mounts, and Wiring

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In the previous week of work on BurnoutChibi, I’ve fully completed the vehicle but have yet to get it out to really test. This thing really is too damned fast for our indoor.. uhh, test track. A motor quality issue also prevented me from blasting it around in our usual outdoor venue (for very long, anyway). These issues have since been addressed, so it’s almost time for more test video!

As previously discussed, BurnoutChibi is a refit of the derelict Chibikart1 frame into something a little more hair-raising, as if Chibikart 1 wasn’t bad enough already. Since the last update where I had just finished reconnecting the steering, I’ve finished mounting the braking system, the transmission shifter cables and linkages, and also completed electrical hookup. At the behest of some of my students, I completed it in time for CPW last weekend, though the aforementioned motor problem meant it was not out scaring parents and wide-eyed potential freshmen.

Here’s the story in the pictorial form.

I began with a little aside in order to solve the problem of how to mount the two “Sand Castle” controllers. They have no mounting flanges and both sides are made of heat sinks, so just gluing it to a plate would make for some pretty poor thermal design. I decided to come up with a “cradle” that held the two controllers right under a fan for some forced convection  cooling. The fan I selected was out of my plentiful stock of 80mm LED case fans.

This design was an exercise in designing a snap fit for 3d printing. While I could have made the base a little wider and added some through-holes to hold the two halves together, I decided to get creative and dovetail each corner post together. The angle is extremely steep – about 85 degrees – so the whole assembly could be pulled out with force, but otherwise snaps into place cleanly.

…and it’s printed out of PLA.

Yeah, so what if it’s going to melt at about 60 celsius? It’ll just smell like delicious waffles while the ESCs burn.

I decided to try the “translucent light blue” PLA which is sold commonly, and I must say it’s my favorite PLA color so far. It’s not the vaguely jaundiced-rainwater color of natural PLA, and I also don’t like solid color PLA. A tinge of blue helps, but is not overwhelming and makes me think it’s some real plastic.

Putting together some of the electrical deck and testing the fit of the ESCs. Result: pretty perfect!

I set aside the e-deck for a while to return to the transmission and drivetrain.

First order of business is to attach the sprockets to the wheels. This basically entailed making four standoffs which acted as the lug nuts (M6 thread) on one side, and regular 1/4″-20 on the other side. The standoffs hold the sprockets a set distance from the wheel so the chain clears the tires, and also holds them concentric.

Or so I hoped.

There is practically nothing concentric or wobble-free about these shitty caster wheels. I had picked them up since they’re $10 each, but I swear not even Harbor Freight wheels are this bad. While the sprocket seemed to have minimal runout (radial misalignment), the wobble from the poorly stamped wheel rims was incredible.

I literally had to take a dial indicator to the sprocket and hammer on the wheel rims to bend them around. I got most of the axial wobble out of the sprocket this way, but this meant it all ended up in the wheels themselves, which now are a bit “googly-eyed” as a result. It will look hilarious when running.

With all wheels mounted, the frame could finally support weight. It’s definitely lost the Chibikart look a little since it’s so far off the ground (in comparison…). I have an incredible 2.5″ of ground clearance now.

The brake pedal hookup was the exact same as for DPRC. This pedal design doesn’t have a spring return on the pedal side since it is handled by the built-in spring elements in the brakes themselves.

Which, as it turned out, weren’t quite strong enough, so the pedal felt quite mushy and also did not return all the way. I added a long compression spring on each side between the cable stops and the brake lever, and this made the pedal feel much more positive. The brake cables sit in barrel adjusters so the balance could be finely tuned.

Shifting to the back again, I’ve appended the Vex sprockets to the Vex transmission’s VHex output shafts. The Vex sprockets didn’t come with any set screws or other means of axial retention, so for a quick fix, I drilled and threaded three #10-32 screws 120 degrees apart. The three set screws will offer way more retaining power than just one. I decided to forego any other spacers and shaft end-tap screws for now.

 

Here’s a view of the shifter linkage. The mechanism is a spring-balanced cable setup where I provide the pull to shift into 2nd gear, and the spring pushes the shifter back into first.

This was simple enough, but I chose springs which were way too strong initially. I figured “10 pounds of force” at max deflection was enough, but that translated through the cable into the shift lever, times two, meant it was just too hard to throw!

I went to a hardware store and bought several sizes of springs in roughly the same length that were much ‘softer’. The replacement spring is about half the spring rate, and was also too long in that it could not compress enough. The solution to that was to really quickly dremel a few loops off the spring, just  like a good ricer. The shifter now has a positive click as the ball detents lock into place.

Once that affair was taken care of, I routed the chain and moved the gearbox up to tension it (the “goalposts” having slotted mounting holes for this reason). To lock the gearbox in place, I simply tightened the…

… Oh, I can’t reach those bottom socket screws.

Must have bought those hex headed screws for a reason! I was wondering briefly where they were supposed to go on this thing. With the hex heads accessible with a regular wrench, now I could actually tighten the drive up.

With both transmissions hooked up, I spent some time getting pushed around synchronizing the cables. I put another set of barrel adjusters on the shifter cables so they could be adjusted as needed.

What I (not surprisingly) discovered during this push testing is that the brake shimmy is pretty severe. This is caused by combination of factors, two of which include my “kinematically suboptimal” rotor retention method (two screws across a diameter) as well as the complete non-concentricity of the wheels. To reduce the severity of the effect, I had to dial the cables to different tensions. The braking is still effective, but it definitely feels like it’s trying to jerk all over the place.

Ultimately, I’m likely to ditch these drums and go to a disk brake setup with its own guide bearing on the front spindles to maintain concentricity. But for now…

…back to the electronics deck. Here’s the wiring mostly in place with batteries mounted. The batteries are my old 5Ah, 10S sticks. Two of them.

The batteries are secured by Velcro ties and sandwiched between two rigid plastic panels (the baseplate on one side, a 1/4″ thick polycarbonate strip on the other). A 1/8″ silicone rubber pad sits below each battery for shock absorption and more impact protection. Combined, this ought to ensure the batteries don’t move anywhere.

The ESC power leads directly into a 150A fuse junction, and ground has its own big brass distribution block also. Overall, this is the beefiest power system I’ve built since probably LOLrioKart.

At the point, the frame was flipped over for installation of the power electronics deck. The rest of the wiring, including connections to the motors and to the main switch, happened in-place after the installation.

The long run to the power switch is doubled-up 12 gauge wire in each direction.

The only other power side wiring was to make one motor extension cable. With main power wiring completed, I quickly hooked up a HV BEC to provide 5V and a servo tester to convert the foot pedal’s analog 1 to 4 volt output to servo pulses. These two components were heat shrunk and sealed, then attached with Velcro to the top of one of the battery pack plates. The signal electronics for this thing are extremely basic – no fancy signal processing is occurring.  One thing that could happen with this system in the future is converting to electronic shifting, such as with solenoids, upon which I think a system which cuts throttle before the shift and slowly brings it back in would be helpful.

After confirming the functionality of the ESCs and calibrating the controllers, the whole rig is put together.

Here is BurnoutChibi posed next to DPRC! The wheelbases for both vehicles are the same, but BC has a slightly wider track because of the pneumatic wheels. Otherwise, they handle alike and are mututally just as difficult to sit in.

testing

The first few test runs of BurnoutChibi were done indoors, in our Conveniently Circular Building hallway. Due to the extreme acceleration ability of the vehicle, I couldn’t really test it any faster than DPRC or original Chibikart, so we decided to not take video. More testing commenced in an underground garage, then our usual spiral parking garage haunting ground. Unfortunately, I really only got a minute or two of hard driving in before the left motor threw several magnets.

The high speed of the motor caused some serious sparking as the loose magnets scraped the stator and also cut up the motor leads. Unfortunately, the only video that was taken was not focused properly…

The accomplice vehicle is the (still unnamed) tricycle.

Since that test, I’ve reglued the magnets and repaired the wiring, and BC is currently operational. I am currently waiting for a day in Boston / Cambridge when all hell is not breaking loose (in fact, as I write this) to test in the garage again. These pictures and videos will be uploaded when they are taken.

What’s Happening in 2.00gokart?

chorl

It’s been a month or so since my last post about my personal undergraduate victory garden, “2.00gokart”. At that point, nobody’s really assembled anything or completed their designs yet. That’s all changed. Here’s what’s going on now, and what will happen in the next few weeks!

Chaos and half-assembled karts is the law of the land as the “Milestone 7” checkoff and inspection draws near. This is a full “rolling frame” demonstration – brakes must work and steering must be hooked up and functional. The vehicle mechanicals do not have to be final, but to get to this point, it’s sort of implied. Hacking systems together to pass the inspection was discouraged, and nobody really tried to push anything sketchy. After MS7, the only tasks remaining should be to finish up electrical assembly.

At this point, things were being raised onto wheels and the true creativity of the students began to show.

This thing, for instance, had a custom wooden coachwork/centerpiece which was partially CNC router machined and also featured living hinges laser-cut into thin plywood.  On top of all that, the steering isn’t a normal wheel or handle, but it’s tilt based. The builders, Nelson and Carolyn, both have blogs and I swear they add content more often than I do. This creation has been officially named Zoran, which I briefly confused with Zorak.

Some of these things are conventional, others just a little off the wall, and still others… well, they have interesting operating postures. I’m kind of glad to see that nobody is really building a normal 4 wheel kart – there’s really only one. But hey, that’s not bad either – last year, the most normal and innocuous vehicle was done on time, on budget, exactly as described, and performed reliably.

Other vehicles are defined by a central feature, such as Dat Wheel.  That’s a 18″ lawn tractor tire that the team specified off Surplus Center. You’d  basically be sitting directly over it.

So that’s a sampler of the oddball creations coming out of this crew of MechE sophomores. What’s next for everyone is putting electrical systems together. I’ve been giving short “mini lectures” about places to get electrical parts such as switches and contactors, and also good wiring practices and other safety-related device (such as in the background of that picture – no male-side connectors allowed on batteries!)

In about 3.5 weeks time, everyone will once again (hopefully) fly down the service road and up the garage. This year is going to be tremendous.

On my end of things, I’ve been recently tasked with creating a relevant homework assignment for the EV students. One of the homework assignments for 2.007 proper relies significantly on you having built a competition robot. Since none of the alternate lab section students have, everyone was a little distressed. I therefore had to invent an alternative.

My alternative is a miniature (1-3 amps) current mode controller for a DC motor. That’s right, your homework is to torque control. I created the hardware hookup most of the way, requiring students to read a datasheet or two to discern what else needs to be hooked up, and also how to read the DC current sensor. The half bridge is standard fare for me – an IRS21844 gate driver hammering on some obsolete but sufficient IRF2807 N-channel FETs. The whole rig is synchronous rectified by nature. Really, this can be scaled up 100 times and be fully legitimate plus or minus some power supply changes.

Once the students finish said homework, I’ll release the working code & “class solution”, which is the most documented and commented thing I’ve ever written, as a general resource.

Here’s a picture of testing the whole setup, with a DC motor plant, ammeter to verify output, and a battery which can take regeneration current. Power supplies may be safer to hand off to a fuzzy duckling, but they cannot handle regeneration current, so a 10 amp fuse in the test battery will have to prevent things from going too awry. The controller is based off a simple integral-only (i.e. ramping) loop, which is more than suitable for driving highly inertial, dynamic loads like a vehicle.

So that’s explanation for the video last week! What really happened there was that I was pushing 15 amps (at 20 volts or so) into the field winding of the large blue motor (it’s a “separately excited” DC motor, so the mini half bridge board could crank 5 amps (briefly – I raised the limit just for the video) into it and have it just barely spin up. No, it is not creating perpetual energy. Without the torque control loop, it would have grenaded instantly.

From here out, I’m only bumping the students in the right direction to finish, and also making sure the appropriate paperwork and signoffs are in order for the final contest. In other words, “Wait, you want to do what with the garage?” “Yeah, we did it last year.”