A Hysterical Constant-Current Mode for RageBridge

chorl4 Comments

Here’s where I deviate a little from the progress updates on Null Hypothesis to focus only on RageBridge for a little while.

After detonating the first board by shorting it through a very stalled and unhappy drill motor, I decided to fill in the current sensor pads and actually put them to use: I elected to investigate if I could give current limiting to RageBridge. Yes, you heard me right.

I, Charles Z. Jesús Sergeyovich Guan Kennedy III, after years of bitching endlessly about crappy robot controllers with fancy current limiting and how they don’t work, am building a motor controller with current limiting.

The primary motivation was to not detonate my hand-built boards left and right every time a motor decides to disembowel itself such that I can better appraise the reliability of the board. It’s nice to have an ESC that can dump forever into a stalled motor without any form of protection, but such a controller would of course then be dramatically overkill for any normal loads it is expected to experience. Even a shitty drill motor can pull a hundred amps for a few seconds (before it lights on fire), and very few reasonably-sized robot controllers will happily dish out those hundred amps. They do exist – they just fit nowhere in my robots.

That aside, I also have a personal grudge to settle (okay, fine, THIS was the real motivation). Most specialized robot ESCs these days are pretty smart and do feature current limiting. The problem is, when they say current “limiting” they mean “After this number of amps, I will shut off completely, without notice, and won’t turn on again until you cycle power”. Or even if it does turn on again by itself after a few seconds, the damage is already done, so to speak, – your bot has been totally defenseless for several seconds and is now probably in more than 1 piece. Granted that these are complaints only for the combat domain, which admittedly is a small realm of robots as a whole. Generally if you have a fancy controller like a Sabertooth driving an autonomous rover or whatnot, it’s not going to be making sudden full throttle forward-reverse slams, unless it is very confused or special. In that case, it’s totally legitimate to assume that if the current goes over a certain limit, something terrible has happened.

But that kind of current limiting, more properly called current cutoff, pisses me the fuck off. It’s the reason why I keep using Victor 883s, as old school as they are, in my drives because they just don’t care. I’ve scared Banebots controllers, Sabertooths and SyRens, Robo-Claws, all purported modern intelligent motor controllers, with said shitty drill motors. Not even anything exotic or special! Cut-off is not smart – it’s stupid and easy, which is probably why it’s done.

True “current limiting” would behave much like a bench power supply. After a certain amp threshold is crossed, the output voltage will reduce to the level needed to sustain that current. Ideally it will dump x amps into a load forever, even a dead short. It would work something like:

Yeah. That. I’ve named it the “hysterical” current limiter since while it resembles a hysteresis controller I have no defined lower bound and it’s not trying to keep the current within a certain range, just giving it a ceiling.

Green represents nondimensional command (say “100%” voltage output), blue represents nondimensional voltage output, and red is nondimensional current of a load. Shown is a pure resistive load whose current just directly scales with voltage.

The idea is that with each timestep of the controller, it will increments the output voltage magnitude a small amount to follow the command. Also at each timestep, a current reading is taken, and if it is over the set limit, the limiter will take away that increment plus some more, such that the output voltage is lowered. If at the next timestep the sampled current is still higher than the limit, then the limiter will take away more output magnitude again. And so on – all this method needs is the current limiter ‘takeaway rate’ exceeding the maximum voltage ramp rate of the controller for responding to the inputs, e.g. my radio stick.

How big the current hysteresis band (the ‘triangle’)  is will depend on the ratio of the two rates and the timestep. If the current limiter takes a large amount of output magnitude away, then the input command portion will take a longer time to catch back up, the magnitude of the hysteresis band (“ripple current’) will be higher, but the response to an overcurrent condition would also be faster.  If the current limiter can’t take away as much magnitude as the input command contributes, then it doesn’t work.

Since I already have a “ramping” function in Ragebridge’s firmware, I could just inject my current limiting process directly into that. In fact, having this kind of limiting would make the command ramping I have obsolete – the maximum acceleration of the motor would only be dictated by how high I want set the current limit. A small motor which can’t pull enough amps to hit the limit will be slapped back and forth relentlessly, and in the best case, I can hitch this controller directly to twin direct drive Magmotors and have it slowly crawl along. Forever.

It’s not the same as current control which only increments/decrements as fast as needed to maintain a certain current (and your stick inputs are translated into a value representing how much current you want) – it really is current limiting. If your motor always wants more amps than the controller will output (e.g. twin direct drive Magmotors), then you will be operating always in the constant current regime – accelerating at a constant rate to your target speed.

So let’s start. First things first, I need to put together more boards:

Here we go. When I have the parts out, it’s actually more effective to just keep populating more boards since I can copy and paste my motions. The only limiting factor this time for me was the lack of MOSFETs. These fuckers are expensive and each board has 8 of them. Just these 3 boards are like $80 of MOSFETs, and by using them in my controllers I’ve pretty much sentenced them to death by burning smoky fire.

I’ve added reinforcing wire to the long return traces in the hopes of staving off another current-induced explosion. Ideally, if my hysterical current limiter works, they won’t be needed.

Three boards made, with the first board at the top background now being used as a reference  and parts junkyard. Only one of these has the current sensors built in right now – when I can verify that it works, the others will be adapted (Those current sensors are also kind of pricy – if I were to take these to production, I would try to use a resistive sense circuit instead of the hall effect sensors on there right now).

The ACS714 current sensors are bypassed externally by a 1 mohm current sense resistor, which reduces their sensitivity by about half. Otherwise, I’d only have 30 amps available to sense, whereas with 1 bypass there is “60 amps” available. Multiple current sensor bypass surgery is reasonable – current will divide itself according to the resistances of each path available, and I’ve done up to “double bypass” to get 90 amps on Tinytroller before.

The current sensors have up to 80khz bandwidth if there is no external filtering (datasheet), but the noise at those conditions is pretty frightening. Because my loop will only ever run at 500hz or 1khz (maaaaaaybe faster in the future), I can afford a little bit of external filtering to keep the peak-to-peak amps noise down. I elected to pitch 10nF filter capacitance on this board. I could go even higher if needed because the ATMega’s analog-to-digital converters take over 100us just to do their job (this is horrifically slow for modern microcontrollers), so the current rise time can be really slouchy.

This was at the stage where I was just checking the current sensor values to see which polarity they read, and how much. The huge 2.5 ohm power resistor was a decent, if a little high-resistance and induction-less load. Start gently, I suppose.

Alright, now we’re getting serious. I don’t have a real low-ohm power resistor, but did find another one of those huge green things in MITERS, so 1.25 ohms is enough for now. While I could have taken several dozen feet of fat wire and dumped through it (and it would have some meaningful inductance to boot), these things were low enough to test up to the limit of the power supply. At this point I had written a basic version of the code which only applied to 1 sensor and in 1 direction.

The next step was to start puking into an RL type load, like a motor winding. Purely resistive loads have no ability to smooth their current (since ideally it tracks the voltage in proportion directly). Inductance adds a bit of ‘mass’ to the current, making it tend to keep going. When combined with a switching output stage (like my H-bridges, or every switching power supply ever) it is one component of producing very smooth DC outputs. Conversely, an RL load will tend to ‘coast’ for a while, making it actually much easier to control.

Plus, I’m building a motor controller, so it damn better work when I run a motor with it.

The test load is a classic EV Warrior motor I’ve had sitting around for a long time. Back in the day™, if you were a newbie and were building a 60 to 120lb battlebot, EV Warriors were the shit. It’s also a good candidate motor for this test – it’s got a huge armature with lots of inertia AND inductance, and can pull a hundred amps easily. It already killed the unprotected Ragebridge once.

To up the stakes, I clamped it to the table and locked the shaft with a vise grip. Or more accurately, I locked a vise grip onto the shaft… one is more dangerous than the other. I was about to find out if my current controller worked or not – if it worked, the EV Warrior will barely nudge the vise grip against my clamp. If it doesn’t, then the EV Warrior will shoot the clamp through my stomach.

Luckily, the answer was it worked. Okay, minus an accidental sign error in the reverse direction that had the radio input fighting the current controller whenever I hit reverse, but no blood was spilled!

After verifying that the current control worked for several currents between 5 and 20 amps, it was time to get serious. I upped the limit to 30 amps (as a start) and lobbed it in Null Hypothesis. Having 2 severely overvolted drill motors per channel, it will happily suck down over a hundred amps.

Driving the robot on only 30A per side was amusing – it actually didn’t take much longer to get up to speed, because the wheelspin current of each motor was about 10 amps (typical for drillboxes – here, have a drivetrain calculator). The motors would have tried to draw a brief burst of high current to get up to speed normally, whereas now they would have been limited to 30A, so they would have had to ramp up to speed a little slower. But that transient is so short it is nearly negligible with respect to the robot’s mass accelerating with the floor’s traction limitations.

But what it couldn’t do was push anything. As soon as there was additional weight on top of the robot, the sum of the motor currents exceeded 30 amps and the motors were capped in the amount of torque they could produce. It may have handled another 30lb robot fine, but I could stop it with my foot – something that normally results in me being toppled over as I get my center of gravity displaced.

I decided to up the limit for NH to 51 amps per side (up to 25 amps per motor, shared equally, or 50A per motor if one is totally fucked or unevenly loaded) and close the bot up. The weird limit has no particular reason – I could go up to 55 or so amps, but what I don’t want to do is saturate the current sensor so it never actually recognizes that it hit the limit.

One of the perennial fixes I have to make to my controllers is the “common mode nut”. I’m not sure what it does, but on the old Melontroller it helped resolve an issue where the controller would behave oddly and occasionally reset under heavy loads.

The problem with this board is a little different. I’m still not sure on why it happens, but occasionally my 15 volt power regulator would latch up and emit an audible buzzing sound. The voltage output is typically just enough to keep the ATMega alive, but no gate drives. It’s then impossible to reset, even with a power cycle – chances of it entering a normal operating mode are rather slim. The same issue has plagued tinyTroller as well – everything I’ve made with the LM2594 has displayed this latchup problem. Melontroller had it easy because it used a linear regulator.

It’s weird because only this board does it so far – the first one I made did not have this problem at all. Even better is that the board displays no issues after I manage to successfully power it on, just that I should not try and turn it off again!

I’m fairly certain it’s inrush-current related, but it also seems to involve temperature. What fixed tinyTroller’s problem was turning the 15V regulator’s input from a diode only to a diode-resistor circuit, with a 10 ohm resistor supplying current to the input capacitors. It basically puts the LM2594 at the output of an RC filter, limiting how fast the voltage transients can be. In this case, the controller will power on fine if I

  1. Turn it on, but do not try to turn it back on from a power cycle within about 2-3 minutes
  2. Or turn it off and expect to be able to turn it on again after it has been on for more than about 30 seconds

I’ll try adding the inrush-limiting resistor to the logic regulator and see if that fixes anything. Or just put together another board and see if it’s some kind of strange assembly problem!

Hardware oddness aside, here are some pretty scope pictures.

Just a shot of gate drive voltages, measured at the gate of the FETs. Purple trace is low-side, red trace is high-side. The turn-on and -off time for the IPB034N06 FETs I’m using with the IRS21844 gate drives is only about 200ns. I’ve become used to just setting the “deadtime” on the 21844 to 1 microsecond, but that’s clearly excessive. I split that time roughly in half for this controller, and it seems to be working great. 1us isn’t terrible, but having excessive deadtime not only limits how high of a duty cycle you can achieve with purely bootstrapped gate drivers, but for very fast loads it can also cause voltage spikes to occur at the FETs if the load gets ‘cut off’ suddenly, kind of like punching your bartender in the face if he waits too long to serve you another drink.

To investigate how fast I can have my code run, I decided to time the main loop by quickly switching on a pin and then turning it off. I accomplished this using direct port manipulation – the digitalWrite procedure in Arduino adds something like 100us of overhead, which is a little distressing. A port write is done on the next machine cycle, adding negligible time if my code take several hundred microseconds to run already.

The entire control loop, from taking in the latest inputs to calculating the output magnitudes, takes only a little over 300us. This was good news – I started the code running at 500hz, and could conceivably bump it to over 2khz! The faster the loop, the tighter my ‘hysterical triangle of current’ gets.

Unfortunately, my ramping function only works with integers. I’m already only letting it ramp 2 LSBs per loop, and to take the loop much faster means I’d start having to do 0.75 or 0.5 or something, which would imply floating point math (sloooooooooooooooooooow). Maybe I could ‘prescale’ the ramping function by having it run only ever other loop or something, but that seems impractical. Given that I have the current limiter, the ramping function is almost unnecessary.

For now, I decided to just leave it in, so I had to pick a speed that could still accommodate it:

I decided on a 1khz loop. That means it is checking the current every millisecond, which ought to be fast enough save for the lowest-inductance pancake motors and raw resistors. There isn’t really harm in dumping into a non-RL type load anyway – the current ripple would just increase in magnitude.

This is a trace of the current sensor’s raw reading, showing the triangle-wave-like waveform that results from the voltage command being tugged back and forth by the limiter. It’s important to note that neutral (zero current) is at about 2.5 volts, or above the displayed waveform. Therefore, the current dips sharply down (the shorter, steeper slope) before incrementing up again (the longer slope). I can’t accurately judge the ripple current from this screenshot because the wave is barely bigger than the pixels. Maybe I’ll take a more accurate picture later.

For now, though, some video I shot during the process. The video shows me bobbing the EV Warrior around at 5 amps and 10 amps, then standing on the robot to keep it still. Even though the current sensors weren’t verified for the video, it’s important to note that the voltmeter is trying to take an average reading – the peaks might be at 5 and 10 amps, but the average is a little less because of the ripple.

 

I have yet to try the “crowbar” test where I run a motor near top speed and then suddenly short the output with a screwdriver. This would be the ultimate test of it saving itself without interfering with robot operation otherwise.

While NH may be able to make use of this for saving its drive motors, I’m more interested in this controller now for Uberclocker. Not only does it have bigger drive motors, but the lifter would certainly benefit from current limiting. I don’t have limit switches on the actuators, so the only thing saving them from fiery explosion (or unreleasable self-inflicted lock) is me not being too hard on the stick at the wrong moment. Historically, the top clamp arm has had a habit of locking itself into a position where it doesn’t have enough torque to pull back out. If I had, for instance, asymmetric current limits, then I could clamp with a lower current allowed than releasing, guaranteeing it would be able to free itself again.

I’ll need to fix that regulator inrush problem before really trusting this controller, however, but for the time being I will go ahead and populate the other two spare boards with current sensors!

Oh, yeah, before I forget: The current control firmware that is running on Ragebridge as of right now.

Null Hypothesis: Finishing Up and Testing the Ragebridge v1

chorl3 Comments

This it!

The post where my project gets stuck in motor controller hell. It happens to everything I try to build a motor controller for, so be prepared to read about exploding FETs and endless nondeterministic troubleshooting!

Well, the story is actually slightly happier than that. In the past few days,

  • The frame has been put together, all the wheels mounted, and battery pack made
  • The Ragebridge has been installed and the robot successfully tested and driven using it, seemingly exhibiting no adverse noise issues
  • The Ragebridge has been successfully detonated when a motor suddenly locked up.

Let’s start from the beginning.

With the wheels bored out, making hubs was the next step. I decided to make these more consistently on a Nice Thing with digital readouts, courtesy of the Edgerton Shop (They were also the nearest straight knurling tool, incidentally). The knurls will help the hubs not slip and strip out against the soft plastic bore of the McMasterbots wheels.

The hubs are installed with an arbor press.

Two wheels in. The hubs are very conventional “drill hubs” with a 3/8″-24 internal thread and internal shoulder that lets the reverse-threaded 5mm locking screw hold the hub in place against both threads. I’ve always found it funny how the ‘cheap drill’ converged solution ended up using an English and a Metric thread on the same part. The outer thread is not M10 or a metric thread, and the internal thread is definitely 5mm x .8 and not #10-32, which only fit down to a few threads. I’m mildly interested in how shady Chinese factories converge towards which design to copy and paste.

It’s starting to look like a robot now.

I temporarily secured the front wedge on with a few screws in preparation for a test drive.

Okay, robot’s done. Nothing more to see.

…

Using a spare 7S battery pack (from the long-retired Segfault) and the Botbuttz controllers I bought a while back, I threw together this test rig just to try out how the robot handles. Conclusion: It handles like a pushybot with a wide wheelbase and near-midpoint center of gravity! Unlike Clocker, which is back-heavy out of necessity and so oversteers very easily, this thing is extremely stable in turns. The braking effect of the BB controllers also contributes to snap turning response and smooth starting and stopping. These are all features I want to make sure Ragebridge can duplicate.

I spent a while scooting this around before moving onto the real challenge: the aforementioned Ragebridge.

I soldered up another board (the first one having been put into the Silly Media Lab Vehicle). There were no changes made to the board layout, only the modifications addressed in its first post which included moving the low-side gate drive bypass capacitors into the right place.

After verification of the hardware functionality, I spent the majority of 2 days hammering out an R/C input firmware for the board. I added features in the following order:

  • Radio signal-loss failsafes! I figured if I just wrote a dumb Servo-to-PWM program, it would stay that way forever. ESCs for robots need to have a way to shut off the motor outputs on recognition of signal loss, so the robot doesn’t just keep rolling. This part wasn’t difficult: every time a valid servo pulse is received in the pulse-reading interrupt, a flag is set to indicate the reading was good. A checking routine runs once per half-second to see if there were any channels which did not set their validity flags – if there are, then the gate drivers are shut off until all signals are good again.
  • Command ramping, meaning the ESC takes a small amount of time to slew from one direction to another. This is to prevent instantaneous “step reversing” which can cause huge current draws, up to twice the DC bus voltage to appear at your ESC’s power inputs, exploding motors, and other terrible things. I created quite an epicly named function, mapWithRampingAndDeadBand(); to perform this task.
  • Linear stick response. Directly mapping of the pulse widths to PWM output %, through the ramping limiter. This was just to get the robot driveable so I could add…
  • Invert mode, a feature which allows you to redesignate which end of the robot is forward. Useful for invertible bots like NH because otherwise your robot would drive backwards when upside down, as well as have swapped turning behavior. This is activated by the channel 5 toggle witch on my radio, and just swaps the left and right commands while adding a negative sign.
  • Exponential stick response. This makes the initial response ‘softer’ so the robot can be driven more precisely when moving slowly. It tends to reduce spin-out and oversteer tendencies. It is implemented as a simple lookup table – raw command goes in, expo command comes back.

I began to add a ‘stick calibration’ mode, but decided against it for now. Stick calibration is a luxury, because radios can generally be trimmed and have their endpoints modified to suit just about any desired pulsewidth range.

The current version of the RB firmware is here. It’s written in Arduino 1.0.1 (which finally got rid of the CTRL+SHIFT+M bug!). I generated two exponential response tables, one a little more expo than the other. I found the “more expo” one to be a little sluggish in responding without moving the sticks really far, but your experiences may vary.

Ragebridge v1 installed in the robot on its little standoffs…

After testing the controller out on a power supply first (which has nice features such as non-infinite current output in case of a fault) I put the test battery right back in because I hadn’t gotten to making the actual robot pack yet, then made some test driving videos!

The “basic test” was done right after I finished the linear stick mapping, and the more advanced testing is with exponential.

I like Ragebridge’s functionality in those tests. While open-floor driving is not as strenuous as pushing another robot (trying to push you back), high-current transients like from reversal and stopping will be what causes noise-induced failures and latchups. I didn’t observe any of those issues during that test driving, which bodes well for more sustained heavier loads. Unfortunately I neglected to put a Wattmeter on the thing, so I don’t actually have power throughput figures. Typical drill motor drivetrains in this size robot will draw about 30-40A per motor accelerating or changing directions and about 15 amps spinning out the wheels.

With at least a slim prospect of the robot working properly, I started putting together the battery pack:

The voltage of this pack will be 8S2P in lithium iron phosphate, so 25.6V nominal and 28v peak. This high voltage was just to get the speed I wanted out of the motors, but it could prove to be the proverbial Achille’s Heel… or perhaps Achille’s A123 Cell for the robot.

Those yellow things are PTC thermal fuses. I usually just lay more busbraid across the cells to put them in parallel, but the PTCs add another layer of failsafing by limiting the rate at which the cells can differentially discharge. If one cell suddenly fails (or gets punctured) and becomes a short, then the very low resistance of the bus braid will let the other healthy cell short into it, potentially causing even more damage. The PTCs will at least limit this to a reasonable current.

I chopped up some spare 4S JST balance extension cables for the cell taps. Generally I make my own using JST-XH connector shells and pins (e.g. Digikey’s 455-2217-ND), but it takes longer (and most chargers don’t have slots for 8S pack taps anyway). One JST lead is the lower half of the pack and the other is the upper half.

The balance leads are insulated by some Kapton tape layers as I soldered them in. I didn’t want them laying directly on the cell tops any more, after this happened, but on this pack there’s no space to route the wires only down the sides of the cells (it will be laying flat in the robot).

Since this pack will be more enthusiastically jostled inside the robot, I’m giving it a layer of foamy armor.

And after Giant Heatshrinking! I gave the pack its initial balance-and-charge wakeup routine with my 1010B+ charger doohickey.

While the pack was slow-roasting to perfection, I took care of the last bits of electrical work on the robot itself.

This adorable ABS-printed connector holder houses a Deans connector and an XT-60 connector. The Deans is going to be the master switch via the “Georgia Tech Key” method. In order to prevent myself from being confused at which terminal to charge the battery from versus to bridge together (hint: bridging the battery is a bad thing), I just used a totally different connector for the charge plug. The connector housing is mounted to the bottom plate.

This is the “Georgia Tech Key”, so named because I’ve only seen it on random GT creations before. Technically it’s called a removable link.

I made this loop a little too long and it sticks out the top of the robot by a little, but some tape should keep it down. In the worst case I’ll just replace it with a directly shorted Deans connector.

Alright, here it is. That’s it. The robot’s done.

Now to hand it off to other people and drive it around until it blows up! The best way for me to find the weaknesses of any of my builds is to give it to other people. I subconsciously go easy on things I know may be unreliable (the so called Creator’s Mercy), but other people have no concept of, say, the drills being severely overvolted, or for instance in the case of the Chibikarts, that flooring the throttle from a standstill may not result in satisfactory behavior.

I wasn’t able to whip out the video camera in time to capture the magic moment, but what happened was this:

And this:

What triggered the failure was a MITERS member driving (very happily and enthusiastically) NH into a solid machine frame at pretty close to top speed. The robot stopped moving, and I instructed him to give a little throttle to see if the controller was still working. It’s actually very difficult to judge what people consider ‘a little throttle’ based on experience, so I can only assume that it was near full stick or something. A small explosion was seen inside the case.

The superficial damage was the blown ground trace and one very, very unhappy MOSFET. The actual damage, though, as I found out through subsequent hours of checking and debugging, was pretty much the whole thing. The Arduino Nano was fried (noted when I powered it back on and the ATMega started drawing half an amp…), as were all 4 FETs on the exploded side (gate-to-source shorted through), both of the IRS21844 gate drivers on that side, the gate-to-source protection zeners (which I’m sure did all the protection they could before exploding), as well as half of the 74LS08 quad AND gate that is involved in piping the PWM signal to the gate drivers.

Damn.

After replacing most of the semiconductors (and the Nano) failed to make the controller work consistently again, I decided to just scrap this board. Some times, enormous transients damage components in ways that are unexpected

And that’s the motor after I took it apart.

I think a combination of being oversped (18v overvolted to 28v) and sudden impact (running into the solid object) must have blown off the commutator segment. These motors are not really what I would call “well made” since the whole drill they come in costs $20. Running 28 volts is certainly up for compromise. But, I enjoy the speed of the bot on such a voltage.

Good thing I have a small forest of these drills!

So the final version of the story is, NH was finished and blown up, and is now unfinished. Putting together a new controller (with some mods I will discuss below) and pitching the bot  back together should not take long.

ragebridge mods

Now that I’ve gotten to know Ragebridge a little better, I have in mind a list of issues and changes I’d like to make when it’s time for version 2. Since V1 actually seems to work fairly well, this is not an immediate task.

First issue to take note of is of course the misplacement of the gate drive bypass caps. This is what led to the very first board not working at all until I changed it. Clearly this is a routing matter and the second version of the board has to address it, but for now it is easy to hack by hanging the little capacitor off the pins of the 21844 chip.

Second, the row of input headers at the top of the board are not in a very useful order. Two of them were designed for “servo” style connectors which are terminated signal-power-ground, and two for ‘potentiometer’ style inputs, terminated power-signal-ground. Well, I got the servo input order wrong anyway – those two headers actually are connected signal-ground-power. So I had to make two different styles of deceptively incompatible servo cables to suit the board because I needed 3 channels of input. I’d just line them all up signal-power-ground in the future, even the “analog” inputs, for better consistency.

Next, and an issue I can hack around for now, is the way that the power traces narrow awkwardly and haphazardly:

Those yellow highlights is where the trace width drops significantly from the average. The ones at the left and right bug me in particular – I was definitely trying to keep the controller within a reasonable width, but the heavy power traces around them are easily double the width and very short too. I was intending to reinforce those with braid or wire, I think, but didn’t actually do it for this board.

The circle near left center is a really obvious design derp. Most of the power stage of the board is basically rotated 180 degrees from eachother, except that current sensor placement. The right side of the board has no corresponding narrowing – current flows down through the vias on the right center two FETs for MA2, and the right side’s current sense path is a straight shot.

Given that these traces are all made in 1 oz copper, they’re exceptionally prone to sudden current overloads, such as a near doubly-overvolted drill motor suddenly slamming to a halt. The current sense path on the left is a little more difficult to reinforce since it’s under that FET, but as long as I’m not using the current sensor on these boards yet, I could hard-jump it with a piece of wire.

Overall, I’m satisfied with how Ragebridge has been behaving (short of being inductively crowbarred by an unhappy drill motor), and I’m going to be building more of them to test on other bots soon…and of course fixing this one.