Archive for July, 2014

 

Chibi-Mikuvan: Detroit Recap and The Conclusion

Jul 31, 2014 in Chibi-mikuvan, Events

I have the worst go-kart hangover ever.

In the end, the reversing module, spare parts, and electrical system refactor didn’t happen, so I basically went down to the race with Chibi-Mikuvan in its late Wednesday condition. However, all the sciencing and preparation did pay off: Chibi-Mikuvan won 1st place in its 30-lap sprint race. The Endurance race the next day, however, saw my very much still beta-version non-refactored electrical system catch a case of the loose wires from the very bumpy track, an issue which couldn’t be diagnosed in time for the end of the race. The final ranking for Chibi-Mikuvan for the weekend was #6 out of a field of 19.

Also, this picture happened:

We actually didn’t travel down with Chibi-Mikuvan in the open, to avoid things like rocks and bird droppings. Instead, it was tarped over, but the tarp came loose at times and required readjustment, and ultimately had a few large holes torn in it – heavy duty it was certainly not. It was easy to see when the tarp was loose: the intrigued and possibly disgusted stares of passing motorists (and when you’re me, carrying four people and hundreds of pounds of pit equipment, everyone passes you) was the clue that we needed to check the tie-downs.

I’m also fairly sure that if we DID travel with it in the open, we would have caused at least one accident.

But wait! There’s MORE!

In the roughly 1.2 weeks before leaving, the MITERS folks who helped me test became inspired enough to cobble together their own independent entry using literally whatever was standing around in MITERS and upstairs in my shop at the time. I’m sure you’ll hear more of this from the likes of Benkatz and Dane.

Here’s the load-out shot from above as I was scanning the shop for things which I forgot to bring, showing the tarp bubble. The choice of route was straight west on I-90, and then a right turn. I wanted to go through Canada, but figured the 1.5 hours saved was not worth the potentially more than 1.5 hours being hassled by border crossings. Plus, not everyone who came had passports. Sadness – maybe a future vans-on-vans trip into Canada is warranted if they ever hold a race at the future Toronto Maker Faire or whatever. Google Maps said the trip would take 12 and a half hours, but for me it was more like 14 each way.

The Detroit track was made in the parking lot of the Henry Ford Museum using tire barriers. It’s easily the largest PRS track I’ve seen (and I think the series has set up) – it’s easily twice as large as the New York Maker Faire one. For the most part, it was smooth with the exception of three very large bumps that ran perpendicular to the track. I think these bumps were ultimately the cause of the failures I experienced during the Endurance race due to the wiring not being secured well.

One half of pit row. Pit space was tight, and we had to intermingle tools and parts with surrounding teams. This was a shot from the morning, before everyone’s pit became earthquake disaster zones.

We’re contemplating something here… Probably our newly acquired safety bracelets.

An hour or two of open practice was available, so I took the time to get a feel for the track and try out the racing line.

The reason PRS appeals to me as much as it does is how varied the level of technology in the entrants is. It reminds me very much of my other favorite sport of fighting robots – you have personal garage builds, schools, and basically professional shop spaces playing at the same level, and the professionals don’t always win. The rules are purposefully open-ended instead of heavily codified like most motorsports. It’s purposefully and unironically derpy. As I’m sure I’ve made abundantly clear, I’m a huge fanatic about learning via building derpy shit taking initiative and having fun.

Here I am running a qualifier lap. I got the 2nd and 1st best times for qualification, but the 1st place one was negated due to me not stopping in time (it has an integrated brake test). It was no fault of the brakes – my first run, I braked so hard the front discs locked up and one of the Harbor Freight tires got a huge divot eaten into it. So I took it easy – a bit too easy the second time around.

The video of that run is here:

After the qualifying round was a short break before the “Moxie round”, which is a freestyle showoff kind of affair. A few teams acted out skits, one made a simple presentation, and there were some drifting and speedruns. Then there was me:

When I go, I go hard.

In the picture of the qualifying run, notice the small antennae-like structures glued onto the helmet. They’re mounting points (such technical words) for the Miku pigtails seen above. I cut them to 2ft instead of the stock 4 feet to avoid becoming a hazard to everything.

However, all things considered, this was probably the wrong audience for such a display and most people were seemingly more confused than amused – coupled with the fact that I went last (with helping Hack Pittsburgh with their Back to the Future skit, and MITERS with their small goofy vehicle parade), means I didn’t score many moxie points.

During the intermission period, there were, of course, shenanigans. Along for the ride were eNanoHerpyBike (which later gave up a throttle to Chibi-Mikuvan), this contraption the One-Day Kart, and Bentrike.

Here’s MITERS relaxing, right before the heat races began in the afternoon. The field was split into odd and even qualifier positions, so the MITERS entry was in the first race starting near the middle of the field.

Some kind of strategy discussion or trash talking before the 2nd heat race that I was in pole position for. That, or we’re just talking about who to spin out and who to run into the tire walls.

The 30-lap sprint confirmed to me a weakness of Chibi-Mikuvan that I had predicted – that anyone with a DC motor is going to get the jump on me. The Trackstar takes a sweet second to ramp through its starting routine, during which Hack Pittsburgh (in a superbly built custom-bodied DeLorean with Back to the Future theme and everything) was able to get around me. We’re also fairly matched for speed, so it was difficult for me to catch back up unless there was traffic ahead. In the end, it dropped from the race due to a loose wire – letting me take first place. The same fate was to befall Chibi-Mikuvan in the endurace race.

The ‘press shot’ for the weekend, during the 30-lap heat.

Here’s one video of the race, shot from the finish line truss.

A fancy new hubcap!

I discovered during this race that heavy is not necessarily a bad thing. In fact, it keeps you on the ground a whole lot better. I had to slow down more for the turns than some teams to avoid rolling over – I got on two wheels two or three times. This prevented me from effectively using the turns to pass. I may have to reconsider that full-size battery pack for Chibi-Mikuvan in the future.

Both MIT vehicles in one shot during the endurance race.

The endurance race for the first 15 minutes or so was a faceoff between Chibi-Mikuvan and NIMBY‘s Ferrari. There was “some” contact made. Nearing the end, the battery began running down and I started seriously feeling the power dip, so I pulled into the pits, allow them to regain the first position for laps. I neglected to remember that the Hyperion 1420i charger has a charge capacity limit of 9.9Ah – it always stops at 9.9Ah, and I have to manually restart it. I left the battery charging on Saturday, but forgot to restart the charge on Sunday, and so with the sprint race taking out (most likely, due to our science’d data collection) more than 10Ah, it wasn’t running on a full charge.

The starting lineup for the endurance race.

 

 Chibi-Mikuvan caught in a bit of a traffic jam next to NIMBY. We traded spots a few times.

Around 20 minutes into the race, after the battery change, was when things start going wrong. At first it manifested itself as an intermittency in the throttle – it would work after one bump, but not after the next. It quickly progressed into Chibi-Mikuvan having to be carried off the track and into the pits. I did some short voltmeter investigation and discovered that the 5V throttle wire broke off at the terminal strip.

After re-stripping and pouring hot glue into the area to restrain the rest of the cables better, the next thing that happened was the throttle straight up dying on the second driver 0.9 volts all the time, no response. I harvested the throttle quickly from eNanoHerpyBike and Chibi-Mikuvan got in the running for a couple more laps, but very quickly had to return to the pits because I smelled something funny.

Thinking it was the Trackstar ESC, I shouted for a replacement ESC to be made ready, but it wasn’t that simple – the Trackstar was working great right until the end. I’m not sure what exactly happened but the Arduino Carrier board’s thick ground trace was blown apart, and 14 volts was present on all the 5V rails. The Arduino itself was nonresponsive, probably due to said 14 volts.

In a moment of either desperation or insanity, Dane chopped apart a Harbor Freight 4.5v flashlight, I rigged its battery into a servo tester knob, and then I cruised very slowly like that for a few more laps before something just completely gave out. And that was it for the Endurance race!

The picture above is of the servo tester and flashlight hack. It was taped to the seat back, so I had to drive with 1 hand behind me to gently twiddle the knob. I got used to this style and actually began to try and run the race again until the final, as of yet known failure occurred.

Chibi-Atomic-Jeep (among other names), the MITERS one-week entry, actually did finish the endurance race, but not before busting off a few Harbor Freight tires. I’ll let the others tell that story.

Chibi-Mikuvan didn’t escape the tangles with NIMBY unscathed. I clearly also scrubbed someone’s tire, and that knocked off the catface, which was never recovered. Otherwise, the front body mounts were broken off in a run-in with the tire wall, and I drilled through the body (cringe) and used large zip ties to secure it for the rest of the time. The rear body mounts were redone shortly after completion using E6000 rubber construction adhesive and never had problems again, whereas the fronts stayed epoxied on. The epoxy seems like it was just too inflexible for the task.

Obviously body repair will need to happen, but all things considered it’s in great shape. I had expected far worse. This is all stuff that can be taken care of with some Bondo. I probably won’t be able to get it back running again before Dragon*Con (there is not a race there anyway, just a short exhibition), but I and MITERS alike are making serious plans for Maker Faire New York…

conclusion and next steps

So now that the race is over, how well do I think Chibi-Mikuvan performed overall? Time for helpful infographics in my usual style. As I have said since the start, Chibi-Mikuvan is intended to push several technological and resource boundaries at once, in the interest of anyone else wanting to get into the sport of silly rideable things. Some of it went great, others not so much.

The biggest risk was probably pressing into real heavy duty racing service a R/C car controller, the Trackstar 200A. Granted, it’s for “1/5th scale” R/C cars, which I swear Chibikart is already a 1/5 scale… Ariel Atom with growth stunting problems or something. Chibi-Mikuvan is a 1/3.14 scale R/C car… that’s close enough, right? Checking out the internal construction of this thing along with watching the current draw with the digital dashboard, I’m satisfied that this controller can output its continuous rating at full throttle. In the context of PRS, you’d probably never run 200A at full throttle unless something really interesting is happening.

During testing, I did manage to kill two – one by letting the signal board shake loose (so attach it firmly!) and the other by regenerating into a disconnected battery – instant voltage-induced death. However, it did not miss a beat during the competition, in 90F+ weather, in conjunction with the current limiting algorithm. Too bad everything else did!

One of the things which continually gets me “interesting” reactions from curious builders and onlookers is the gear ratio I’m running on Chibi-Mikuvan – a healthy 20.5:1, compared with the 5:1 to 10:1 of average small vehicles. In part, that was to take advantage of this fist-sized motor in a ground vehicle application. I’ve had 1 person so far ask me if the blue thing was a supercapacitor, thinking the motor itself was under my electronics deck somewhere. Because power is composed of both speed and torque, you get “free power” to a degree by running a faster motor. That’s why R/C outrunners are able to advertise such ridiculous power ratings – in their designed application (aircraft), the motors run very close to their no-load speed so they can indeed be very “powerful”. Chibi-Mikuvan proved that you can indeed run a very small, high speed motor in conjunction with sensorless control if you gear it highly enough.

Sensorless, though, isn’t without its trappings as I mentioned previously in the face-off with Hack Pittsburgh. The sensorless starting routine can still bite you and let someone else get a jump on you.

To use this motor above 1500W or so for an extended time, a water cooling circuit is necessary.

The way I was able to design a 20.5:1 reduction in the rear left corner of the vehicle was by passing the Aquastar motor through angle grinder gearbox. The 9″ generic Chinese angle grinders seem to come with 4.1:1 right-angle spiral bevel gear drives. Now, I can’t vouch for the gear quality, but they seem to be holding up great in this application.

There’s several types – one of which Harbor Freight uses and has a smooth input shaft, and another style which has a Woodruff-keyed input shaft. I modified the Harbor Freight input shaft and the bore of the pinion into a machine taper to get better torque transmission properties, and it worked for the most part, but I had troubles with the pinion gear getting pulled off its taper by the action of the spiral bevel gears: on negative torque (regenerative braking, for instance), the thrust force tends to pull the pinion off the taper.

I ordered two “Truepower” branded ones from Amazon, and they come with Woodruff keyed input shafts. I upgraded the key one size to a 1/8″ thick, 0.5″-circle Woodruff key, and that’s the one that was used during the race with absolutely zero problems. The gearcases were packed full of grease to minimize wear – I wanted to do an oil fill with some differential gear oil, but the gearbox has no gaskets and it would have gone everywhere. I’d recommend the woodruff key version just because it is conceptually easier to interface to. You could cut off the angle grinder motor’s shaft and use the leftover 12mm stub shaft with a coupler, or machine a custom one like I did.

The Ford Fusion batteries came through with absolutely no problems! Dumping 100+ amps is probably not good for the life of the battery, but if Ford’s actually advertising 20-30kW hybrid systems in those things, they then have to be able to push that much current. I gave an in-depth dissection of a 2009-2011 (generation 1) Ford Fusion hybrid battery in its own post (BE CAREFUL! is the theme here), as well as how I packaged them for use.  You can get 28.8v and 16Ah in about 22 pounds the way I assembled them.

A recent (a few minutes ago…) scan of New England yard listings shows most of these batteries starting around $500, which at the rate I used them, is still borderline economical given how many independent packs you can get out of one.

What can I say – the Mac G5 pump and radiator (and my reservoir made of an olive jar that I ate on the spot to get) worked great! The only issue was that the Aquastar motor wasn’t well sealed for most definitions – it leaked consistenly, and by the end of the 30 lap sprint race, the cooling system had leaked dry. A coat of silicone will probably solve that problem, so that’s on deck for New York. The system was silent and effective.

My favorite part, the Hysterical Current Limiter, worked to great effect during both the race and testing. It’s best explained in its own two posts. During the race, I did blow one fuse, possibly the result of running too hard when the weather was hot (the temperatures were nearing 90 degrees Fahrenheit in Detroit that day) – quite metaphorically flying too close to the sun. But I and anyone else could slam the throttle to full without, you know, dying or something. Without the HCL, it takes a trained throttle finger to drive Chibi-Mikuvan, which is why I basically didn’t let anyone else drive it until this was completed. Functioning code for the HCL is provided on those posts/pages.

Ah, the Harbor Freight pink tire. Full disclosure: We cleaned out the nearest three Harbor Freights to MIT of these things, as well as the closest Harbor Freight to the Detroit Maker Faire. I still think they’re a good choice for low powered vehicles. They are grippy, but the rubber is sadly too thin to last long. They’re well cost-optimized for the 0.5mph handtruck or pushcart. The tread is at most 4mm thick, with the tire carcass (plies) barely below that.

The stock hub also has well known durability issues – they crack apart at the weld under sideways pressure (from turning). I made my own hubs and designed these to be swappable, so that was not the issue. In fact, they turned out better than expected on the DMF parking lot – for some reason, their asphalt is less abrasive than ours.

Sadly, I don’t think I’m returning to these – something that I made up my mind about before the race after seeing the results of the “science”. They work, but they simply don’t last long enough. I’d like to get through a race without having to change tires, and that involves moving up to something designed for mobile vehicles. Popular at the PRS race in Detroit were riding lawn mower tires and real honest-to-Go-Kart-Jesus scooter tires, both designed to move under power.

The thing that bit me in the end was my ad-hoc wiring job. Breadboards, multiple cables running into each other, and poorly strain relieved terminal strips don’t make for a high-durability, robust mobile electrical system. The parking lot ruts finished off what the science testing surely started – fatigued wires and unknown shorts and lost connections. With this system dead, I have no choice but to refactor and rebuild it from the ground up. I’m envisioning a single PCB with all the features I need on it – or at the least, prototyping board with actual solder joints!

So what’s in store for the future? Well, nothing for the time being, since Dragon*Con and other projects (not to mention, umm, my shopmaster job) are higher priority. Strictly speaking, the vehicle can run again in its current state with just an electrical system rebuild. Cosmetically it’s beat up, but nothing some Bondo and paint will fix (as well as rebonding the front body mounts properly). Perhaps the highest priority would be repairing that electrical system.  I have a few ideas for drivetrain upgrades, but they’re a long ways off – maybe for next PRS season.

This concludes the Chibi-Mikuvan action for a while. Look out soon for details on how the summer EV Design class has been going, as well as updates on the robots for Dragon*Con!

For reference convenience, I’ve copied over the “cheat sheet” from the introductory post I wrote when I got it running back in May.

Cheat Sheet

Motivation

I started this project as a museful distraction in October of last year after returning from the New York Maker Faire and mingling with the Power Racing Series folks for the third year. Having seen the league grow immensely, I decided to finally enter something while exploring new and unusual components for hobby builders (my usual MO) while also wanting to see a change away from the “model year bloat” I saw in many teams, who started using heavy forklift motors and other salvaged industrial components. Hence, the focus on R/C electronics and non-lead chemistry batteries.

Work on the project began more in earnest with this season of “2.00gokart“, since I figured I needed to have an instructor vehicle to troll my own students with.

The project was my first jump into making a composite bodied anything, motivated in part by the bodywork repair I’ve had to perform to real-Mikuvan.

Build History

In chronological order up to the previous post, here’s the process of Chibi-Mikuvan creation from conception to implementation:

Naming

The project is named Chibi-Mikuvan after its principal design predecessor, the Chibikart twins which were the “prototypes” for the design class I teach today, and my 1989 Mitsubishi Delica known familiarly as Mikuvan.

It has little to do with Chibi-Miku-san though a few large decorative decals would not look out of place on the shell. I’m an avid follower of the crowdsourced synthetic Japanese future girl-pop that you’ve never heard of world of Hatsune Miku and Vocaloid. That’s literally the most concise way to fully describe it, as I have learned over many difficult discussions about what the inglorious shit is it that’s playing all the time in my shop.

Components

Specification

  • Top Speed: 25mph (as-geared, Y-termination)
  • Acceleration: to 25mph in < 3 seconds
  • Braking distance: < 30ft from top speed
  • Skidpad: Uhhh, gimme a sec.
  • Clearance: Still not enough for the Maker Faire cable protectors
  • Drivetrain: RR layout, 1 speed, spool axle (no differential)
  • Dimensions: 50″ L, 28″ W, 24″ H
  • Weight: 113lb with battery
  • Seats: 1, though if Chibikart was any indication to go by, up to 7.

Bill of Materials

Here’s the latest iteration of the BOM (5/1/2014 version), which contains at least 95% of everything on the thing, short of the trivial like zip ties. I went into much more detail than the average PRS list; the quality is a little more closer to what I expect out of my students when it comes to found parts and used parts. Everything, to the degree possible, is given a Fair Market Value which sort of artificially inflates the cost a little. While technically over the PRS $500 statutory budget, I believe this is a more realistic representation of the cost needed to replicate this once.

The BOM has 3 cost categories. First is the actual money I spent. I had a fair amount of parts already on hand, but did have to buy things full-price like the Ford Fusion battery pack and the motor & controller. Next is the PRS rules based accounting, exempting some things like brake parts. Finally, what this vehicle would cost under my 2.007 EV Design class rules, where some raw materials are provided to the students so they only need to count materials if they need to be purchased additionally.

Chibi-Mikuvan: The Penultimate Update!

Jul 23, 2014 in Chibi-mikuvan

With the Detroit Maker Faire only 3 days away (AAAAAAAAAAAAAAAAAAAAAAAAAHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHH), and our departure only two days away, it’s time to summarize the past week’s worth of development on Chibi-Mikuvan. The short story: There basically isn’t any. I’ve been out “sciencing” basically every other night doing reliability testing, and there have been some incremental changes in the interest of reliability and maintenance, whose stories will be told here.

First off, the Northern Tool tires were definitely not what I expected. They wore to this condition in all of maybe 7 or 8 laps:

Yikes. There’s something to be said about the futility of pressing shitty handcart/garden tires into karting service. Ones the same size that are designed for scooters (and they do exist, they’re just pricier) will surely have better longevity – even the cheap 200×50 tires I get for the class don’t exhibit this sort of wear.

It might be worth removing or reworking a feature to lower the budget cost, such that I can  run more reliable tires for the New York Maker Faire. We’ll see how fiddly things are in Detroit.

Overall, we discovered that if we don’t purposefully thrash the thing and drift and slide, the tire life is actually not bad. Here’s the “notebook of sciencing” showing the results.

The wear on the outside front tire is the greatest for obvious reasons – high speed left turns will tend to wear out the right side faster, and so on. The 2.00gokart test track is heavily left turn biased – all the fastest turns are turning left. The HF pink tire in that position saw 23 laps before the tread wore through, but the rest were still going as of the next test – in total, 70-80 laps. They’re worn right down to the cords, of course, but they did survive.

One important change that will aid maintability is swapping the wheel bolts to wheel studs, like every normal vehicle. I’d not given this very much thought before since I typically use screws with everything (bolt meaning using with a nut on the other side, not threads in the material), but I found out last time it was utterly impossible to change tires and wheels quickly, since with the bolts all out, there was no means of rotationally aligning the tire and two halves of the split rims.

To resolve this, I just stuffed the bolts in from the other side and welded them in!

Now I can have lug nuts like a normal person.

With that issue resolved, I began the quest of making spare parts. First, another battery pack.

By this time I’d forgotten exactly how I arranged the cells in the last one, but luckily my sketchy notebook sketches and last few posts on this site came in handy. I clearly marked each module with which side the cell sticks had to face out of just to make sure I don’t confuse myself.

The modules were clamped together to the correct spacing before I installed the bus bars.

I’d make another battery, but I’m totally out of the 150A size Anderson powerpole connectors! I was going to pick some up at swapfest (the MIT flea market) this past weekend, but sadly the only vendor which consistently has these……. was absent. My only remaining one therefore goes to the battery charger adapter lead.

Last on the list of things to add before the race, if I can, is reversing ability. I don’t intend to use it, but clearly if this thing spins out or ends up sideways during the race, it’ll be critical to getting out of the situation. Reverse is a ‘dumb reverse’ here – just a double pole relay pack to swap two motor wires. Even though the controller has reversing ability, I’m not using it out of caution with the one-sided sprung throttle and its shared function with regenerative braking.

The relays are “80A” units with larger 3/8″ output tabs instead of the more common 1/4″ ones on 30-40A SPDT automotive relays. I spent a while searching for these again – I had one, a long time ago, plucked from some unknown refuse pile, but lost it and wanted more.

To mount the relay pack and keep the motor-gearbox unit, I made new mounting plates with an additional little tab that sticks out of it.

The race to the finish will consist primarily of working on these spare parts, as well as an “adapter socket” for Chibi-Mikuvan:

 

Chibi-Mikuvan: Getting “Tired” of All This; Making the HCL Actually Function; Plus Other Work

Jul 14, 2014 in Chibi-mikuvan

As the Detroit Maker Faire and its attendant PPPRS race draw closer, Chibi-Mikuvan testing and modifications have been intensifying still! With each silly night testing video (when the parking lot is empty) comes more rework and rethought of the original design elements. I’m actually glad it’s all being done now, and things are being broken during testing now, because there are a handful of things on the design which are turning out to be bad ideas suboptimal for reasons like ease of maintenance, parts swapping, etc.

The story continues where it last left off: with my indoor testing of the Hysterical Current Limiter. I was gradually pushing the maximum current higher and higher and driving around while staring at the scope screen to get a handle on what kind of surge currents it was seeing with the error of the controller being quite large at low speeds. With the limiting code itself working, I began to implement the ‘time decay’ where the controller would keep track of how long it’s been running over a designated continuous maximum current, and then slowly lower the maximum allowed current over a few seconds to the continuous level. The idea of this would be to get more acceleration ‘punch’ but not risk blowing the fuse.

The way this code worked was that it counted how many loops (fixed delta-T, hence how many seconds) the current stayed above the continuous limit, and obtained a maximum current based on a lookup table with the delta-t count as the index. It would reset if the current fell back under the continuous limit. Seems simple, right?

//Obtain a current limit value for this loop
    if(delta_ts_overcurrent > 512) local_max_amps = MAX_AMPS_CONT;
    else {
      //an 500+ entry table would be overkill, so here we change the max current ceiling every 8 loops (16 milliseconds)
      //This requires only 64 entries [0-63].
      local_max_amps = MAX_AMPS[(delta_ts_overcurrent / 8)];
    }

    //Hysterical current limit algorithm
    //Unlike in Ragebridge, we only care about driving current, so the case of decrementing throttle quickly is not handled.
    if((amps_to_use > (float)local_max_amps) && limited_current) { 
      //local_max_amps = Either a fixed constant or obtained from a LUT
      throut -= OC_MULTIPLIER * RAMP_US_PER_LOOP;
      if(throut <= 1500) throut = 1500; //Don't go past neutral
      delta_ts_overcurrent++;
    } else {
      delta_ts_overcurrent = 0;
    }

That’s where the logic was flawed. Recall that the current waveform is messy. It’s a noisy sawtooth, not a smooth monotonic function. As I found out during testing, the chances that the current waveform goes under the MAX_AMPS_CONT value and therefore resets the lookup table index was almost 100% – when it worked, it was a bug.

I finally pushed it a little too far when doing a 120 amp launch (which could have seen errors as high as 200 amps, knowing the low speed performance). The current trace never sank, and I managed to mentally process that this was probably bad a few milliseconds before the fuse went out.

On dumb DC systems, a fuse-out will just cause everything to stop working. But this controller has regeneration ability – and for whatever reason, the failure mode of loss of battery input is “snap to 100% regen”. If the battery were present, a large burst of negative current will flow (into the battery) and I will faceplant through the scope screen. But without the battery there, regenerating into space just causes extremely high voltage spikes. I could smell the magic smoke coming from the ammo box right after stopping.

Fortunately, only the controller itself was toast. After resetting the power, everything else turned back on and still worked. Unfortunately, digging the controller out of the box is a 15+ minute service job. In the field, this was going to be intolerable. Furthermore, I now had a serious problem. A fuse-out is guaranteed controller death, so I have to keep the overload bursts conservative with the consequence of reduced acceleration.

I first turned my attention to the code. It took me a while of thinking to recognize the problem I described with it, and then a bit more thinking about how to fix it.

//Hysterical current limit algorithm
    //Unlike in Ragebridge, we only care about driving current, so the case of decrementing throttle quickly is not handled.
    if(limited_current) {
      if((amps_to_use > (float)local_max_amps)) { 
        //local_max_amps is either a fixed constant or obtained from a LUT
        throut -= OC_MULTIPLIER * RAMP_US_PER_LOOP;

        //This causes the time-current curve to only reset if the current flow goes UNDER the continuous limit.
        if(amps_to_use > MAX_AMPS_CONT) delta_ts_overcurrent++; 
        //Don't go past neutral
        if(throut <= 1500) throut = 1500; 

      } else {
        //Even if we don't limit the throttle this go-around, only reset the time-current decay counter IF
        //the current has fallen under the prescribed limit, indicating you've let off the throttle
        //or have used the regen brake 
        if(amps_to_use < NOLOAD_AMPS) delta_ts_overcurrent = 0;
        else delta_ts_overcurrent++; 
      }
    }

Here is the new logic structure. A new variable has been added – NOLOAD_AMPS. This isn’t a “no load” current in the motor equation sense but rather a lower limit to the current limiter – at what point does it turn off and reset? Now, the delta_ts_overcurrent counter will continue incrementing as long as the current does not fall under this lower bound. I set NOLOAD_AMPS at reasonably low number to start – 20 amps. This is the correct implementation of the behavior I described firstly – that the limit will reset essentially only if I let the throttle off in the process of being about to brake, or after the vehicle’s reached cruising speed anyway.

(Notice the flag limited_current up to… You know what this means).

With the code fixed, now I actually had to fix the thing. I decided to implement a change that I had thought about since the Trackstar failed the first time due to its signal board falling off: moving it ouside, right next to the motor. This has the effect of creating a single swappable module that consisted of the gearbox, motor, and controller, so if something goes sideways in the race, it can be swapped with a spare entire unit in a minute and then someone else can take their sweet time fixing the failed unit later.

I designed a new motor mounting plate that has an added ‘tab’ where the controller sits.

…and cut it out on my mechanical waterjet. Wait, what? Yeah, I went back to using the Shopbot gantry router as a makeshift waterjet with an 1/8″ wide nozzle. There’s nothing really stopping this machine from cutting metal, but its limitations are twofold – one, it isn’t nearly as rigid as a CNC mill, and two, it doesn’t have any provisions for chip removal or coolant.

The solution to the former is to take light passes at aggressive feedrates – keep the tool cutting, but make the cutting forces tiny. I whipped out my old experimental setting from the last time I did this, and refined it a bit more. The final settings I used were 18,000 RPM, 125 inches per minute, and 0.025″ deep passes.

The solution to the latter is more complicated. Coated carbide endmills are great about not having build-up problems, but chip recutting (chips being sucked into the next pass) will easily kill the little 1/8″ endmills from past experience. I used a dual approach of slathering the area with Tap Magic Aluminum cutting fluid as well as keeping a compressor air gun handy to clear chips on every pass. I’m not as confident machining aluminum dry, because even a little bit of build-up can break the cutter – at the least, I think it needs a mist coolant system where a tiny bit of coolant is dropped into an air blast stream.

On the whole, it worked great. Here’s the tab bent into place after the holes were cleaned up. Mechanical waterjet!

And here is one of the spare Trackstars installed onto the tab. This is now the unit that I’ll make a spare(s) of and replace whole in the event of chaos. The Trackstar is also in a much better cooling position here, being in the airstream.

As I was extending the power cable coming from the contactor inside the ammo box, I had the idea of Adding More Buscap. Galactic amounts of bulk capacitance helps smooth the DC input rails by isolating peak current spikes from the battery (and hence current sensor) – this should help clean up the readings some as well as supply the controller with more stable power. I basically tripled the mount of capacitance for the controller here, with four 820uF capacitors. The 8 gauge short wire leads shouldn’t cause trouble since the ESR of the capacitors should still dominate the amount of current ripple through them, but ideally you’d want them as close to the controller as possible. This shit’s important – the inside of high powered motor controllers are basically made of capacitors.

The caps were bundled, soldered together, then the soldered legs bent over to attach to the wire. It was wrapped up in a heat shrink burrito.

With this setup, I verified that the ‘time decay’ element of the current limiter was indeed working, and we went out once more. The idea of these testing rounds was to get everyone on the team who might be driving (and we seem to have JUST enough people to have drivers, a pit money, and an event staff helper) to get some driving time in WHILE possibly breaking it so we can fix it ASAP.

The results of this test were fantastic – the current limit was still set rather conservatively, there were no fuse problems, and I think people are realizing what they got themselves into. The noise coming out of the gearbox at the end was indicative of future troubles. That test actually ended with the tapered input shaft of the gearbox pinion slipping once again – I came into the near turnaround with full regenerative braking, then suddenly the gearbox no longer output torque.

This was to be the last time the tapered shaft will bother me, because I’m done with this 1,1,1, Trichloroethane shit.

The spare angle grinders I bought (these) have input shafts with Woodruff keys instead of…. well, nothing.

I decided to prepare these for use in the vehicle and only keep the tapered-shaft gearbox around for emergency spares if all else has failed. The stock Woodruff key seemed to be a 3/8″ or 10mm one, which is a bit small for my tastes, so I bought some 1/2″ circle ones to upgrade the size.

I took a chunk of 12mm shaft and turned one end down to 10mm to mate with the bore of the gear…

I borrowed a little woodruff cutter to make the slot in the side.

The completed new input shaft assembly. This time, it even installs from the outside of the gearbox, so I no longer have to open it up to remove the input shaft. On the previous Harbor Freight gearbox, the plate holding the bearing in came with pre-stripped screws, so I couldn’t remove it. On this one, they were removable, but low quality, so I replaced them with M4 socket cap screws.

I did repair the previous tapered shaft gearbox. This time, I took care to clean up the taper where it had slipped in both gear and shaft. Then I drilled out the screws I couldn’t remove from the bearing retainer plate, freeing it up and allowing this gearbox to also be serviced from the outside. Then I put the gear and shaft on a 5 ton press and shoved until it could move no more. This gearbox will now be on reserve as the last-ditch backup.

I also finally made a lower battery bracket to prevent the half-sized battery from pivoting on the frame because it only has two coaxial fastening locations. This makes battery changes way, way easier.

With these modifications done, we had the most insane testing session so far:

The goal of this testing round was to stress the new gearbox as much as possible. I also wanted to see how adding the shell affected the handling, so at the risk of putting another dent into it, we brought it along. The good news is that it doesn’t much – since it weighs so little, driver weight differences still dominate (Time to diet!).

However, this was when the weakness of the Harbor Freight pink wheels finally showed. I’ve had this set since the thing was built, and the rears were definitely showing wear, but little did I know how thin the tread area actually was. Ben, the test driver of the last lap, pulled probably the best save I have seen yet when the rear right tire blew out on a turn.

I may have died on the spot if it planted into the curbside.

I cut open the failed tire and noted how thin the tread was. Even if the tread were new, there would only be about 3mm or so of it surrounding the tube, which seemed very low and could result in the tire being fragile. Its also inconsistently molded, as you can see from some spots being hair-thin (around the failed location) and some spots still having thick tread.

So I swapped on another set of pink wheels with the goal of driving the tires off. We would purposefully push hard, drift, and slide, just to see a worst case tire wear scenario:

Here’s that video.

One thing we began favoring was using the powerful regenerative braking on the rear axle along with weight transfer to help kick the rear end around. I figure this wouldn’t be very useful except in certain cases of executing a pass on someone’s inside, but it was one way to get some artificial wear and tear on the tires and also beat them up. Once again, Ben is the destructor of Harbor Freight tires.

This was an eye-opening test. Even though that happened at the very end, after 14 of those S-curve laps and 15 loops around the outside, it began turning me off to the Harbor Freight 8″ wheels. They seemed okay for low speeds, but the tread was too thin to be durable otherwise.

I also don’t think they’re vulcanized, or if they are, they’re not done well. This was the wear pattern on the tread of the outside front wheel after that test – the rest weren’t much better, and all showed this “feathering” wear. After this test, I was now totally dissatisfied with the Harbor Freight pink wheels – they’re nice and grippy, but the tread life on them is extremely limited due to how thin the tread is.

Other options for tires in this “handtruck” size were limited. It’s smaller than the most common small industrial tire (4.10/3.50-4) that more manufacturers make. I ordered some of Northern Tool 189399, and since it comes from an actual known tire/wheel company, they’ll hopefully be more durable.

In the mean time, I swapped to a set of Cheng Shin 2.50-4 tires that I bought from Surplus Center (but whose page is no longer up). The difference between these and even a new Harbor Freight pink tire was striking – the tread area was far more rigid to a finger push than the HF wheels which just deformed inwards. The sidewalls were also much stiffer. I only have 6 of these tires, however, so I’m countting on the Northern Tool ones being legitimate.

Finally, we went to fool around on these, but only recorded the shenanigans:

Overall, I’m much more satisfied with these – a couple laps of hard driving and they hardly looked different.

While awaiting the order from Northern Tool, I’m going to take care of one very irritating problem that made itself painfully noticeable during all this testing: Changing the tires. Right now, you have to thread a bolt through two rim halves AND have it line up with the hub’s threaded holes! I’m not sure why I didn’t go with a stud method like every single automotive hub in existence (Probably because I was like “Ooh, these wheels come with free bolts!”).

The plan to remedy this is just to thread the bolts in from the other side, tighten them, and then tack weld them to the hub, then use proper nuts on the new “studs”.

I think there will be more hard testing to come once I perform this bolt to stud conversion, and at least one “full dress rehearsal” involving a live battery and tire change. I’ve ordered a little aluminum hydraulic jack to assist in the process once I discovered how impossible it was to change the tires quickly.

Chibi-Mikuvan: Experimenting with the Trackstar 200A and the Hysterical Current Limiter

Jul 04, 2014 in Chibi-mikuvan, Motor Controllers

As usual with something I’m working on, let me open this update by yelling at Sparkfun Electronics. Remember when I said:

I couldn’t get it to change baud rates, however, despite trying that command a few times…

This was regarding the serial LCD kit I got for the Chib-Mikuvan digital dashboard. I meant that in the sense of no matter what I tried, I couldn’t get the baud rate change command (0×81) to take effect. So what the hell? It seems like it acknowledged the command, but it wouldn’t carry through a power cycle no matter what. There was nothing on the tutorial page about why this might be the case, but there were definitely a bunch of confused commentors, a few of which were having the same issue (but no answers) The firmware link didn’t go to the firmware, but to the command page. I found, along the way, a few custom user-designed firmwares for the LCD that claimed to fix all of the ills, but I was in no mood to deal with those.

As usual with the “open source” product world, it seems like forum comments and Git repositories are an acceptable form of documentation.

Finally, after some fumbling, in a product page comment post that was three years old, I found:

Tip: ensure that you are not sending any data to the backpack for the first second after powerup. If this happens, the backpack will revert to 9600 baud as a safety measure in case you don’t know what baud rate you set it to. Note that connecting this to an Arduino’s serial port WILL send data to it when the system is powered on, as the bootloader will try to connect to a host computer if present. If you connect the backpack to another set of pins using NewSoftSerial, this won’t be an issue.

Wait, could that have been the issue? Is that why everyone, no matter what, tells me to use a software serial port, without explanation as to why? Or if there is one, it’s “Well because it might interfere with the USB communication when you’re programming the Arduino”. Why the hell is this information not front and center on the front page of the product, or at least mentioned in the “datasheet” page? How is this a remotely acceptable form of product support, making your customers rile through forum posts and internet comment threads to find relevant thread that a comprehensive datasheet or product page should have in plain view? This isn’t the first time I’ve had to deal with this kind of “let the community be your tech support” from open source hardware producers either. Anyways, stay tuned for more details as to why I had to change the baud rate on this thing. It’s been working fine after I wasted about an hour trying to make up for undocumented behaviors.

In the last post, I had mentioned that the digital dash was a critical ingredient in seeing how much current this thing actually pulled during acceleration. I wanted to understand if I could run a race entirely using “tuned finger control” or not: the vast majority of R/C type motor controllers, like the Trackstar 200A unit I decided to use, don’t have current control or even current limiting, so a little twitch of the throttle at higher throttle levels can result in huge battery current spikes, which would cause trouble with the PPPRS regulation fuse. I’m glad to say that it served its purpose well:

This video featuring test pilots Dane and Mike. Unfortunately, it showed that my predictions were true: That the motor could pull spikes of 150-200 battery amps at most speeds just by blipping the throttle. This would cause some pretty instantaneous embarrassment, even though the MIDI type fuses specified are slow-blow types.

Now, the system also happily drew 200+ (pegging my current meter) amps at low speeds too, so the motor-side current must have been incredible. I tried, near the end of the video, to “current limit” by carefully ramping in and out of throttle. This only worked to a degree – any mess-ups resulted in spikes of high current. That’s what you get when your motor is more or less a dead short.

It was clear from that night of testing that, if Chibi-Mikuvan wasn’t going to consume 50A fuses every lap, I had to do something in software or hardware to limit the battery current draw. But what?

My ideal system would let the motor current be whatever it felt like – to exact maximum acceleration at low speeds – so long as the battery side current didn’t exceed 50 amps for more than a brief period. The MIDI type fuses are actually very tolerant of overloads, as their datasheet shows – the minimum opening time for a 200% overload is 3 seconds. That, for me, means hopefully enough time to push several hundred amps out to the motor as it spooled up, before falling back to a more reasonable current limit (which it seems like is 60A or so).

Since I don’t have direct PWM percentage access, and the DHAB-S/34 current sensor pulled out of the Fusion battery only has a 250Hz (???!!!) bandwidth, and I can only update an R/C controller so fast, I can only do “average” current control – there will always be spikes and transients. This wasn’t going to be some kind of kilohertz-bandwidth torque controller, but that isn’t the idea anyway.

I spent a few on and off moments thinking about how this might be feasible, including producing quite a few whiteboard sketches that ostensibly seem to be full of science, but which I assure you are mostly made of gibberish (Engineering folks, back me up here…)

What that diagram-like abhorrence says is basically the following: Don’t limit the battery current unless it exceeds a certain minimum threshold; then, manipulate the input signal such that the current draw stays under a time-dependent limit that starts high (such as 100 or 150 amps) but decays quickly to a sustainable current like 50 or 60A.

Sounds simple enough? I was mostly in battles over how to accomplish the “manipulate the input signal” part. How should it be manipulated? The downside of not having access to the PWM % of the output directly is you don’t really ever “know” how fast the motor is spinning OR how much average voltage is being put out by the controller at any point in time. Now, I did consider rigging a sensing line to the output of the motor to capture the PWM % information, but I wanted first to see if it’s possible to do with ONLY the (non contact, not soldered or hard-connected) current sensor. Sensing the PWM % would be a direction to take if I truly wanted to shave the yak and build an intricate torque control loop around an R/C controller, which seems… excessive. Fuses, being thermal systems, don’t care that you’re carefully controlling your current at thousands of Hz.

After a while, I decided to be sensible and work on this in stages. First, I can’t make the current draw stay under anything without an algorithm that first can manage it. For this, I turned back to Ragebridge and its “Hysterical current limiter“.

I designed RageBridge on the same principles – the motor current can be whatever it wants, up a point, then that is all it’s ever getting. It was done this way as a rebellion against the recent trend in “self protecting” robot controllers which shut down if you go over their current limit. As someone who drives robots like a maniac, I couldn’t stand this at all. To me, “current limiting” meant behavior like a bench supply: Once you go over a current threshold, the voltage floats to whatever is needed to maintain that current, including shorting through your stalled motor. That’s basically what Ragebridge does, usually with a +/- 5 or so ampere accuracy. Good ’nuff for the application.

The gist of how it works is: The output command is only allowed to “ramp” up or down at a certain rate, related to the controller’s loop time (delta-t) and the amount of output units it’s allowed to deviate by each time the loop runs. At each timestep, current is sampled; if the current is over the limit, then the controller will forcibly reduce (or increase) the output command at a rate much steeper than the user-controlled ramp rate. Aside from some second-order effects such as propagation delay/leg present in the output, motor inductance, etc., the current will change on the next cycle. If it wasn’t enough (still over current), the controller will modify the output command again. And so on. The result is a current waveform that looks a bit like a sawtooth.

In that screenshot from RageBridge testing, the current sensor is reading negatively i.e. the downward slopes indicate more current being drawn. As you can see, the current increases to a certain level, then is knocked back down again, then takes several cycles to build back up to that level.

RageBridge handles this bidirectionally, and it handles both regeneration and driving current, so there was a whole ton of state machine code in that thing that I don’t hope to repeat again. In Chibi-Mikuvan, I really only care about driving current, since the way the controller performs braking doesn’t seem to “regenerate” much based on what we saw in testing. That made the code easier.

However, it was still more math that had to be done in the loop. Up until this point, I had no clue 1. how long my code took to run, or 2. if the Trackstar 200A controller could even handle high-frequency inputs. If it was limited to 50 or 100hz R/C command inputs, there was no point in trying this at all – by the time the controller got around to processing your new command, your motor is already halfway back to China. From keeping tabs on the low cost multirotor & drone world, I know that the majority of R/C controllers these days handle servo pulses via interrupt and so can run right up to 499Hz (500Hz being where a 2ms-long servo pulse signal goes continuous).

So step 1 was finding that information out.

I changed the REFRESH_INTERVAL in the Servo library for Arduino to run at 490Hz. Didn’t want to get too crazy right away, in case the Trackstar needed time to think between inputs.

Notice that the drive chain has been removed – if it somehow interpreted this as “LOL OK”, I didn’t want it taking off on me.

Success! The Trackstar took the 490Hz input without issue. I decided this was close enough to start developing the rest of the code.

The next issue was seeing how much time the code, as it stood, took to run. I wanted to increase the loop frequency to 500Hz to take advantage of the fast inputs.

Well isn’t that lovely! Something like 5% processor usage at the 100Hz it was running at. I diagnosed this by inserting a fast pin turnon/turnoff at the beginning and end of the loop. I could conceivably increase the loop to 500Hz with no problems, but I did want to see if I could optimize the code a bit to lessen the run time anyway.

The extra wide band seen above is the LCD writing code. Remember my beef with Sparkfun? That whole hour was spent trying to see if I could shrink this time by increasing the baud rate. Here was the random-dude’s-firmware-free hack I had to make to get it to, you know, work as advertised.

This is a hardware “enable” line I made with a dinky little 2n3904 transistor that prevents the LCD from getting power until I explicitly activate it in the code. That way, the Arduino can shit packets everywhere when it starts, and it doesn’t turn on the LCD until after setup() is done. Oh, and then it will artificially wait a second to make sure before issuing commands!

With this, I was finally able to change the baud rate.

…but I found out it was basically incapable of talking at 57.6K baud. So not only does it have undocumented behavior, but it doesn’t even meet its advertised speeds. Well then. I settled on 38400 baud, which was good enough.

Another speed-up-the-code thing I did was to knock off a significant figure from each of the readings. The minimum resolution of the current sensor with a 10 bit ADC is something like 0.05 amps anyway, so it wasn’t worth 2 digits of decimals to tell me. Likewise, with such imprecision in the V and A readings, there was no use having 2 digits on the watts reading either.

In the end, all this effort only took the write time down 500us. It’s certainly not nearly ratiometric between 9600 and 38400 baud, so I suspect the overhead of Serial.print and Serial.write might be more at play here. Nevertheless, there’s still 1000us left to do other stuff, including a few lines of subtraction for the current control code, so I left it.

After inserting Ragebridge genes into Chibi-mikuvan’s code and seeing it seemingly, allegedly working without the chain drive hooked up, it was time for SCIENCE

This. This is actually the most terrifying thing I have ever built. Worse than LOLrioKart, Segfault, or Deathcopter. This is Sciencekart.

Well, okay, I didn’t have a DSO Nano or one of those fancy cordless oscilloscopes, but I did have a 12v lithium module, an inverter, and lots of gorilla tape.

I scoped a bunch of things while driving around the empty hallways staring intently at the oscilloscope screen, with one hand on the throttle and the other perched over the RUN/STOP button. If they say texting while driving is bad, they clearly have never tried oscilloscoping while driving.

The signals I looked at were the R/C pulsewidth output in microseconds and the raw reading from the current sensor. I basically just gunned it from a stop for a bit, let go, then gunned it again, and so on. Square waving the throttle is the biggest rookie mistake for people not used to driving R/C throttles, and this basically eliminates that problem. It doesn’t account for “startup jump” since the controller’s starting routines are often dead-reckoned, but that’s where I come in.

I’m glad to say that it works! Clearly, not with decimal amps accuracy… perhaps now I don’t even need the time decay curve – the error of the controller alone is enough -_-

This is the sawtooth waveform the Hysterical Current Limiter generates. This was captured at around 25% top speed after I punched the throttle to 100%. The current sensor’s zero point is at +5 divisons (0.5V) on the vertical scale; I’ve moved the zero voltage line waaaaaay down to capture the peaks of the waveform more effectively. At 10mv/A sensitivity, the max current, as can be seen, is actually peaking at 100a. It’s interesting to see that the minimum current in this case was more or less 50A, the programmed limit!

My guess as to why? I think this is in part because the motor is so low inductance that the current changes much faster than the controller can keep up, and there is little back-EMF from the motor to help at low speeds. The controller only gets a little time below the current limit as a result: any increase in the command results in a ton more current flowing again. I will need to find a somewhat emptier hallway to capture higher speed waveforms, but based on eyeballing, the spikes decreased as speed increased.

I’ll continue experimenting to see how to reduce the error, but right now, it works great as-is (if not a little anemic now because it can no longer pound 150 amps). If the error is large, then I won’t really be able to utilize the ‘time decay’ method as well, but it could be fine just setting the current limit to “60A, pretty-pleeeease” and keeping it there.

Here’s a test of me driving straight towards a wall:

This was one of the early tests in which I set the current to only 40A to make sure the concept even had a chance of working. From still frames, I could see numbers like 40.6, 45.0, 32.5, and the random spike to 79.6 at the very beginning when the startup loop exits. During the cruise portion, the amps ranged from 29 to 48 or so. Promising start for now!  Tuning can occur from here as I drive around more with sciencekart!

Expect some more testing video this weekend possibly with the oscilloscope still attached.