TOBL2: First Rolls

TOBL began as an idea that popped into my head one fateful night in the Fall of 2010 - sometime around here. Two years and a few months later, TOBL the sequel, is alive. Thinking about it this way, I'm not too far off Apple's new product release pace. In fact, TOBL original was controlled by my iPhone 3GS, where as now I have an iPhone 5. Interesting. Well, without further delay here's video of the first recorded rolls:

I cannot think of a more appropriate place for TOBL2 to be initiated than the filthy floors of the hallowed MITERS shop. After all, this is where the first TOBL was forged and where I learned most of the skills needed to create TOBLs. Okay, onto the technical part.

The numbers:

0.8 lbs
22 screws and bolts
6 wheels
2 bridges
7 different belts before getting just the right size
5mm of ground clearance
And my favorite: TOBL2 has traveled to and was worked on in 3 different states

To get to this point there were just a couple agonizingly simple loose ends to address. First was the belt and tensioning issue. Two posts (and four months!) ago, there's a picture of TOBL fully assembled and seemingly a few 1s and 0s away from action. However, the belts used in that picture were so tight they stalled the motors. Thing I learned: the pitch diameters given for FingerTech belts aren't precise enough to be designed around. I designed the whole pattern on the side plates to the millimeter to get the belt tension just right, but went through seven different belts and three different pulleys before hitting a winning combination.

Two: the radios. My brand new pair of xBees which I paired and updated the firmware on weren't talking to each other, I thought. Turns out I was using an older REV of my XPWMShield that was assembled incorrectly, denying power to the xBee on-board TOBL2. After fixing this and re-flashing the xBees to a wireless programming friendly bit rate of 57600, wireless communication was restored. I also learned a neat trick for wireless programming: when uploading the new code hit the Arduino reset button just after the program size pops up. There are fancier ways to achieve this automatically if you plan ahead a little with your circuit, but this works fine so long as you can get at the reset button! Wireless programming is a stupidly awesome upgrade for the TOBL line because otherwise there's no way to access the USB programming port.

After being shelved for months that was seriously all that needed doing...took me about 6 hours yesterday. With my volatile lifestyle and imminent graduation who knows how far I'll get on this list, but here's what I would like to happen next:

1. Clean up the wiring. A healthy application of zip-ties and more flexible wiring should take care of that. Needs more wall-flips.
2. Incorporate the servos that currently serve no function except a slight amount of structural integrity. They were intended to auto-tension or de-clutch the belts. This is mostly a mechanical task.
3. Develop the controller. My Python controller is quite rudimentary at the moment. I run the program in the shell, click on a little box, and then have the following discrete abilities: forward, reverse, left, right, and hard stop. The next step is to add speed control and a little calibration to account for unequal belt tension. A GUI would also be nice to trim different parameters.
4. IMU? Well, I am taking a controls lab this semester. Maybe I can use TOBL2 as the platform for some kind of controls problem like directional stabilization? The motors probably aren't fast enough to enable Segway-mode. Shame.
5. NEW ELECTRONICS. Bet I can take all the functionality of XPWMShield and MKI and put them onto one wireless motor controller board with a smaller footprint. It would also be nice to ditch the expensive Arduino Nano and just run straight off of an AVR chip with the Arduino bootloader. Regardless of whether this new batch of proprietary electronics makes it onto TOBL I'd like to have a more polished and convenient package to work with for whatever project comes next.

Time to go back to school.

Courtesy of Sandy

As I was sitting eating some yogurt on this rainy morning, I realized how much I miss working on my small robot friend TOBL2. Same story as always: school and my innate propensity to occupy my free time with responsibilities. Quick semi-relevant aside: I am loving being a TF this semester! The class just made the transition from the instruction-based part of the semester, to the build-and-test-an-autonomous-hovercraft portion. Already my inbox is being flooded with messages about batteries selection, which sensors to use, how to wire everything up, Arduinos (I know), etc.. In a way this class has curbed my appetite for making things, which is a nice side effect.

Anyhoo, I thought I'd share a video of TOBL2 in his current state. That is, a glorified python-enabled motor controller.

Not my finest cinematography but in the video you can see the directional keys have been mapped to forward, reverse, left, and right. There's a hard-stop function tied to the space bar as well (not shown). In this configuration, there's no speed control (I know booooo) but I will add two keys to increase and decrease speed. A lot of the controller specifics will be tweaked once I get the thing moving and can see what makes the most physical sense.

Speaking of assembling TOBL2, there are two factors impeding that from happening. The first is that no combination of the belts and pulleys I bought seem to work. Even with the nice auto-tensioning, one belt is just way too big and the other stalls the motors from being so tight. I am placing an order for some new smaller pulleys and couple different belt sizes. Selecting those was more scientific that I just made it sound, so hopefully I'll be able to drive this thing finally. The other road block is the wireless issues I've been having. Probably not a big deal, but I don't have much in the way of debugging those issues unless I go to my lab. I'll look into that once I get a working drivetrain.

Welp, thanks Hurricane Sandy for the little break from classes. Time to get caught up on everything else...

TOBL2: Press Pause

This past weekend my hard drive kicked the bucket. It's kinda my own damn fault. My external hard drive died a while ago, my computer started wheezing a bit in the Spring, and I didn't do anything about it. But I guess that's what you get for moving around all the time. Hopefully it can be recovered at some point, though it will probably be expensive.

Progress on TOBL2 is at a bit of a stand-still while I resolve computer issues (currently booting Ubuntu off a flash drive w/o a hard drive), finish up my summer internship, and transition from Colorado back to Maryland. But don't fret! It's really quite excruciatingly close to being done. Check it out:

Essentially needs code (old code is now gone :/)and some slight mechanical tweaking to get the tensioning spot on. Next time I'll take it apart and show off some of the cool machining I did last week.

It also turns out Maker Faire extended the Maker application deadline (shock) to this Friday, but it doesn't look like I'll be able to take advantage of that. More likely TOBL1 will be used as a placeholder get to get in the door. Can't do much about a hard drive dying so I'm proud of how far I got. I'll press play again after relocation.    

In the meantime debating updating computers a year earlier than anticipated...
 

TOBL2: Build Week 2

Sitting down feels nice right now. This week was quite a push to keep TOBL2 on World Maker Faire schedule. If I'm honest, I don't think I'll be able to pull off a fully functioning bot for the August 17th deadline,  because I need to focus on my actually work work. A TOBL2 running the old iPhone controller or maybe a short autonomous program and no servo clutching is definitely still possible. So that's what I'll focus on. But I'm getting ahead of myself, here's where the bot's at after this week.

I tend to stay in the machine shop until the time I need to leave to make up for it coincides with the start of the afternoon mountain rain. Cold, but totally worth it. This week I machined all the mounting blocks which resolve the 90 degree angle between the drivetrain and electronics shelf. Remember the ridiculous custom stainless steel standoffs I made for TOBL1? I suppose these mounting blocks would be the machining analogue to those standoffs. Except machining really small things beats machining stainless any day. By a lot.

Here they are. The two on the left are for the battery tray and the four on the right support the electronics.

The only issue I can spot so far is the taper on the electronics plates from the water jet. The sides of these plates butt up against the motor plates and create a little gap. That's easy to fix.

The motor controller diodes also appear to be impeding one of the servos. That's another easy fix.

Oh yeah, and slightly more troubling issue: the belt appears to be undersized. I need to cut the motor shafts before attaching the outer plates and knowing for sure, but it's looking a bit tight when I just hack it together. The next size up belt has three more teeth so I'll probably invest in a set of those. Don't really need to worry about the slack thanks to the torsion spring tensioners.  

Still left to do (mechanically):

1. Add springs (coming in the mail Monday)

2. Shorten mounting blocks to make way for servo

3. Grind down electronics plates

4. Cut motor shafts, attach outer plates and see if belts too tight (if not skip 5)

5. Replace belts

6. Assemble 

7. Add servo linkage (probably won't worry about that 'til post-deadline)     

It's a bit ridiculous to go through every little mechanical task that needs doing, so ignore that if you like, it's really just a reference for me. This is definitely doable in 5 hour blocks of machining. I'd like to be done with this list by Tuesday, leaving the last three days to wire it back up, get decent code running, and capture the required media for the Maker application. This week we should finally see this thing move! Build week 3 engage.

P.S. Last weekend I moved for legitimately the 5th time this summer. So that's why there's a new backdrop for my pictures every week. I think this is my favorite so far. 

TOBL2: Build Week 1

Enough waiting around. At the beginning of this week I received some of the last bits and pieces needed to finish TOBL2. Here's what I got:

The parts from the water jet turned out really nicely! Seeing these parts in isolation makes me realize what a bizarre hobby/design sense I have. But, I suppose the endless hours of SolidWorks dimensioning has paid off. The only slight hitch is that Big Blue Saw messed with the dwg file and forgot to include all of the outer plates - those should come in Monday.

Whilst I wait I've had the privilege of full access to an impressively well-equipped machine shop at work. They used to do production in-house so now there's just an overkill shop full of mills, lathes, a wire EDM, a water jet, and so forth. A MechE candy store, if you will. And the guys there are great. Direct quote: "Don't buy any more hardware or material, okay? We have anything you need."  This past week I've been scarfing down lunch and spending the rest of the break in the shop machining.

The first order of business was to clean up those inner plates.

With all those countersinks I tested the motor mounting assembly.

No interference! While playing with this something occurred to me: why the four letter words was I going to use tension springs to tension the belt? The thrust bearings are a perfect place to wrap torsion springs; which is a much cleaner solution. I guess you can't do all your design work in front of a screen.

But before getting too excited a little bit of math is in order. The Hitec HS-45HB feather servos are rated to provide maximum 1N of force for a 1cm arm (i.e. stall torque 1kg-cm). There is an arm on the motor mount too...about 1inch or 2.5cm where the servo will catch it. The motor is about 1.25cm from the point of rotation. On McMaster, the weakest spring that fits around a 5-40 screw is 0.64 in-lbf or 0.072 N-m. Then we do a simple moment balance:

ΣM = 0 = kθ + mgl - FL

θ = (FL-mgl)/k

If we make the safety factor 2, and assume the servo can only output 0.5 N, the spring will overcome the servo at about 1.2°. For safety factor 1 we can get up to 5.2° travel. Somewhere in that range should be enough lift to disengage the belt. If not, coils can be removed to decrease k. Here's a clearer picture of the plan (I realize this is already like plan C):

The next things that need doing are:

1. Clean up the outer plates when they come in
2. Press all the bearings
3. Machine the interfaces between the PCB shelf and inner plates
4. Servo linkage
5. Torsion springs
6. Assemble!

This is the plan for next week. In the background I'll be working on the new controller. I've been learning some python specially for the occasion and am hoping to be competent enough to write a controller that will use keyboard inputs (for now). Another new goal is to have TOBL2 operable enough to get video to submit for World Maker Faire New York. The submission deadline is in two weeks, on August 17th. Presumably I could always enter TOBL1 like last year and then bring TOBL2, but let's see if we can't do this honestly.

TOBL2: Frame Design Complete!

About two minutes ago I placed my first Big Blue Saw order. This included all* of the missing pieces for TOBL2's frame which will be water jet cut from 1/8" 6061 aluminum plate. I guess one benefit to making everything so absurdly small is that I save some money on these sort of orders. Before shipping it came out to $81.40, which I hear is about the minimum you can pay. I like how Big Blue Saw gives you a preview of what your parts will look like if they were flying through a dessert.

Keeping this guy compact took a lot of changing dimensions, looking at the part in assembly, and then fine-tuning the dimensions further. The motors are mounted to a plate which can pivot, so that eats up a lot of space. The electronics stack also grew a little with the addition of the motor controller board. Altogether though, I'm very pleased with the way it turned out. The center of mass is going to be pretty atrocious, but I'm not ready to start selling TOBLs just yet, so that's alright.

I did away with the three aluminum posts mounted at each corner, and added two shelves. The top one is for the electronics stack (shown transparent) and the bottom one will hold the battery pack (ditto). PCBs should never be structural components, as I'm learning so well at work this summer. In this case, I had to be very creative, since the axles are now live and attaching posts to them won't work anymore. I prefer the old bulky aluminum posts mechanically, but I ordered an extra electronics shelf beam, so I can always put that above the servos if this doesn't cut it. As you can see, I had a little fun with the battery shelf.

Sexy battery tray

A lot of what took so long designing this frame was also anticipating what's going to go wrong. Having a drivetrain with triangular geometery, belts, springs, and moving motors, is really just asking for it - in terms of alignment issues. In an attempt to resolve all this, I put some redundant holes in to allow a little tuning upon assembly. For example, all those holes in the motor bracket are to adjust the spring tension (not easy to see in the picture below).

The drawing geometry and dimensions for these plates are disgustingly complex

 See what I mean? Gross

I also ordered some bearings on eBay, which had no picture, so that's always good. They were a dollar a piece though, so if they're the wrong size then I'm sure I'll create some crazy thing which will need them down the road. The asterisk in the second sentence is because as hard as I tried to make this the entire frame from the water jet order, I'm going to need to machine a few thicker (1/4") blocks at the interfaces of the plates and shelves. These are really straight forward, so hopefully the machinist where I work will be kind enough to let me knock those out some time.

Also, just bought a road bike...so that may have something to do with why TOBL had been moving so slowly recently...

TOBL: Timers and PWM

Due to shear stubbornness I only brought one suitcase to Colorado. This means no soldering iron or useful tools. All I can do is SolidWorks and software. However, I've been doing a lot of CAD lately so I changed gears and took some time digging deeper into the inner workings of PWM signals. This is what I found:

Hardware, or timer-generated, PWM on AVR chips can be broken into two main categories. There is fast PWM and phase correct PWM. In fast PWM mode, the timer counts up from 0-255 (on 8-bit timers) and toggles the output once the compare register, OCRnx value, is reached. The output starts HIGH and switches LOW when this value is reached, meaning the higher the OCRnx value, the higher the duty cycle or time that the output is held HIGH. The graphs below represent the input sawtooth wave with two compare registers (most timers have two), an output PWM signal running 30% duty cycle, and one at 70% duty cycle, respectively.  

I wrote a MATLAB program to generate these waveforms; it will become more apparent why when explaining phase correct PWM.  

There is however a problem with fast PWM, for certain applications. The PWM signals above emulate the two outputs from a single timer. As can be seen above, the pulses start at the same time but at different duty cycles they do not stop synchronously. This is alright for controlling a single motor or dimming an LED, but for a pair of motors, this introduces error. Say for example there were a drivetrain with two motors, one left and one right (hmm kind of like TOBL). If TOBL were to steer left by running at 50% duty cycle on the left side and 100% on the right for x time, and repeat the other way around for the same amount of time, it would not end up at exactly it's original heading. In TOBL's case, with such a wide track and short wheelbase, this error is amplified. 

Ah hah! There is a solution to this issue called phase correct PWM. In this mode the timer counts from bottom to top and then top down to bottom again, toggling from HIGH to LOW crossing of the compare value on the way up, and from LOW to HIGH on the way down. This process looks like this:

There are a couple differences in these waveforms, starting with the triangle wave input. Because the timer has to count up and down the period of each duty cycle is doubled, or the frequency is halved, making it clear why fast PWM is named as such. You might also notice that now neither of the pulse edges align. However, more importantly, the pulses are centered around the top value. Returning to the two-motor vehicle example,  now using phase correct PWM, if the signal instruction were suddenly stopped, both motors would stop the same percentage of the way into their duty cycle. While this doesn't entirely eliminate the error, it is a lot better than fast PWM.  

On most timers fast PWM is the default and to change it you need to reset the timer/counter control register value to make the switch to phase correct. For the Arduino, which TOBL is based off, this looks like this:

TCCR2A = _BV(WGM20);

...for timer2. Of course there is a lot of other cool stuff to control at this level, so while doing this it may also be of interest to fiddle with the other registers. Setting the two outputs of the timer to non-inverting, the waveform mode to phase correct, and the prescale value to 256 would be this:

TCCR2A = _BV(COM2A1) | _BV(COM2B1) | _BV(WGM20);
TCCR2B = _BV(CS22);

The prescale value is useful because it adjusts the PWM frequency (for fast 8-bit, clock speed/prescale/255 = PWM frequency). So, while I may have cut the frequency in half by selecting phase correct mode, I can do the same to the prescale value to maintain the same PWM frequency. This is less important for motor control and more important for things like servos, which usually run at about 30-50Hz.

Anyways, the point of this tangent is that PWM can be a lot more complex than analogWrite(), and can be easily modified for any application. TOBL2 software now implements phase correct PWM, though I can't do much with it until I get a rolling chassis. If you're interested in the m.file that generated the graphs above, you can get it here. It's fun to play around with and see what's going on under the hood.

TOBL2: Fire Day

Today we had what I'm going to call a fire day. It's just like a snow day, except with fire...never thought that was an option but that's what happened. Fortunately, where I work was closed due to smoke/air quality and not because there was nothing left of the place. Going outside wasn't the best option and we don't have a TV to watch Spain vs. Portugal in the EURO, so I spent the whole day playing with TOBL instead!

One thing  you don't realize when you relocate for a significant amount of time, is all the little things you're used to having access to. For example, usually when I do any design work I like having paper around to make sure what's in my head can actually be drawn and physically embodied. Well, I don't have any paper here, but we do have some old pizza boxes lying around...

Here was the original idea, of bringing the belt to the motor

And here would be some servo mechanism to tension the belt

What's this, moving the motor to the belt. Is this opposite day? No, it's fire day...

Now whenever somebody says to do envelope calculations, I'm going to do them on a pizza box, so much more room! Anyways, a few minutes into my brainstorming session I realized that my original plan of bringing the belt to a fixed motor was going to be difficult. Initially I wanted to do something like this:

The second spring isn't shown, but if it were the servo would have a neutral point where the spring displacements are equal and thus F1 = F2. This way, the servo could be floating during driving (tension) and the spring (F2) could do all the work. Then, when you need to detension, the servo arm would rotate downwards. However, this gets complicated when trying to load the pin that the tensioner bearings are riding on. This pin would require a slot on both of the plates that sandwich the wheels, and if the spring forces on either side weren't perfectly balanced the pin would tilt and not tension very well.

At this point, I thought back to my real first idea, which was to have the motor on a pivot. I threw out that idea originally because I didn't like having a motor that's not rigid with the frame (and I still don't). Practically though, this is a lot simpler because a motor itself does not rotate like a belt does -you can't bolt anything to a belt. So, that's where my third pizza box drawing comes in, which looks like this in SolidWorks form:

Most of the drivetrain complete 

Better shot of the motor mount plate

View of the drivetrain from the inside where the electronics will eventually sit

This of course had it's own challenges. The primary difficulty was making a robust mounting system for the motor that would withstand the torque from the belt tension on a single pin! To that end, the motor is centered as much as possible so that it's entire mass isn't cantilevered. The plate that carries the motor will serve as one washer of a thrust bearing, the other will be the inner plate, and a matching thrust bearing on the other side. Thrust bearings can take a lot of lateral load, these ones are rated to about 26lbs and were the only ones on McMaster small enough for the task. This picture should clarify what that actually looks like:

In the middle of the frame you can see the darker grey thrust bearings sandwiching the inner plate. To get everything to fit and not interfere (still not entirely done with that) took a lot of geometry and trial and error. Probably took more time to dimension this plate than design all of the electronics.

Not including the servo or supports to keep the inner and outer plates together. 

There are several advantages to this design. Firstly, due to the way the motors are mounted there is more room in the middle section for the electronics and battery to sit. I'm still playing with the tray that will mount all of that but I'm going to do my best to make TOBL2 narrower -this would make driving a lot more controlled. Unlike the original design, I don't have to worry as much about the springs. As you can see above on the shots of the drivetrain in its entirety, there are holes galore on the motor mounting plate, both to accept some sort of turnbuckle from the servo and a spring(s) to maintain belt tension. Having all these mounting options will be important in finding the perfect spring displacement which keeps tension in normal and 60 degree mode, but also isn't too strong for the servo to de-clutch. Next step: figure out the supports which hold the two drivetrains together and carry the electronics. Maybe tomorrow will be a fire day too?

MKI: Motor Controller

At some ill-defined point in time routing becomes art, I think. I've spent the greater part of the last week transforming that schematic from my last post into the pretty pictures you will see below. It's my first motor controller so I don't want it to suck. Call it MKI if you like (i.e. motor kontroller one or MKI like the VW GTI). The point is I've hung around Shane too long and am beginning the motor controller phase of my life; which is sure to see many iterations.


This first - shield - controller is being produced for TOBL2 and is therefore stupid compact. I know this because routing was a beach. It's contained within a 1.65" x 1.85" form and will plug directly into the headers on XPWMShield I created last year.

MKI has some things that I'm proud of:

There are isolated ground planes on the bottom layer of the board. This technique was suggested by Shane, of course. The logic pins are tied to a ground plane (SGND) which is only connected to the main power ground plane (PGND) by an 0805 resistor. This resistor is sized such that it will explode when too much current is drawn, acting like a fuse to protect the current-sensitive pins on the logic end. In this case running at 6V and fearing 2A gives a 3ohm resistor.

Power ground plane.

Signal ground plane.

Each motor has three diagnostic LEDs. One to indicate forward, one for reverse, and another for over-current. The latter I cannot tout until I write the software for the controller. This LED is not connected to the actually current sensing pin but will use feedback from it to illuminate. More on that once I have a board to test.

It's not visible now but originally several traces wove through the seven-pin header; which the DRC did not like. Moving these traces basically meant ripping up half the wires and starting fresh...#firstworldproblems. (<-- first and last time I will hash tag...yuck...Twitter)

Things I'm not proud of:

Looking at the images above the ground planes are pretty well-contained save for a few wires that need to reach the top header. We'll see if any noise results from this, shouldn't be too bad.

There are 50 f***ing vias. On a via/area basis that isn't bad at 16.4/in^2. However, this board is being ordered 4pcb Bare Bones which allows 35/in^2 maximum, and I'm sure you could pick a given 1" x 1" where that is the case. Hopefully they don't check that so carefully but regardless, that is a shameful quantity of vias.

Switching to the mechanical realm the Robot Marketplace order is in and I suspect so is the McMaster order. In addition I have been working on the frame design. Don't believe me?

Boom.

This wasn't meant to be in the "things I'm not proud of" section but now that I think of it I'm not proud of how little attention the mechanics have received. Maybe I can make it up by over-engineering servo-automated belt-tensioners...

Space Invaders and Motor Controllers

The first order of business to address in buildapalooza is TOBL2's motor controller -that way the new chassis can be built while the board and parts are being shipped! This is the first motor controller I've designed and it's based off of the L298 Dual H-Bridge, PowerSO 20 package. Essentially it's an Arduino shield...shield, because it will stack onto XPWMShield. Here's the schematic:


Actually this is the schematic, but it does kinda look like the Space Invaders alien:



It will drive TOBL2's two new FingerTech motors at 6V, which promise not to ask for any more than 2.6A combined if both motors stall. The L298 is rated to handle up to 4A, but to be safe the current sensing pins will be in effect and there will be more status LEDs than one could ever need. Oh, and automated belt tensioning/detensioning....yeah I really don't want any more slipping or breaking (which were all part of TOBL1's charm). More details on the controller once I have a physical controller in my hand to test. Just a little amuse-bouche before the feast of blogposts and building commence.

TOBL: It's Working.

It's working, it's working. It = TOBL and here are some stats:

• 6 wheels
• 18 gears (think about it...)
• 84 oz/in Torque
• iPhone-controlled via TouchOSC app interface
• OSC/Serial communication
• 8 zip-ties
• Top speed of 0.413 mph (my personal favorite)

After much adieu, TOBL has been captured on film with my new camera -a Pansonic Lumix DMC-ZS7. It's about to make this blog a whole mess snazzier, 9.1 megapixels more snazzy to be exact. Henceforth, all videos shall be shot in 720p HD, like this one (fast-forward to 2:34 for wall flips):

Kind of dark but I still recommend watching in 720p.

Now held together by 8 zip-ties, thats 4X as strong as last time!

One of the finishing touches on TOBL was trimming the servos. However, PWMShield V1.0 is not yet wirelessly-updateable and in the monotony of disconnecting and reconnecting all the controls hardware, I vaporized yet another trace. This board should be renamed "GhettoShield" after all the loose wires I've had to solder to busted traces. The circuit works fine though, and using it saves $33. After fixing the trace the servos were trimmed.

Can you spot the latest exo-trace?

I don't talk about software very often, and that's because I don't know much about it. I do like to share the code I use though, isn't that the point of the whole open-source movement? After all, most of the communication software for TOBL was "taken" from this iPhone-controlled tank and this SFE tutorial (Thanks to both!). I modified bits of this software and wrote the rest to interface this existing code with my TouchOSC layout and electronics. In order to return the favor here is the Arduino and Processing code. You'd think a wireless iPhone-controlled robot would have denser code than ardWINDo, but take a look, it's surprisingly compact.

This project was a lot of fun. It's rare that you can take an idea and bring it into fruition exactly the way you imagined it. My initial goals were to make it iPhone-controlled and able to drive after rolling over -TOBL does both of those things. It wasn't cheap, I would estimate ~$250 including redundancies, but TOBL is an awesomely fun desk toy and an even better platform for playing with software, different sensors, and whatever else.

I also learned that servos are great for being servos, but not so great at being drive motors (especially modified micro-servos). For future projects I will venture down the new and exciting road of custom motor-controllers. Time to start playing with H-bridges...

TOBL: Testing

Done with my first exam of the semester and feeling like it's time to get back to TOBL, however briefly. I still don't have a camera to record test footage but that may change soon. Until then, here is the little guy in his most primitive form a.k.a. being held together by two whole zip-ties.

Yup, two.

So far testing has been limited to some laps around the apartment, during which I have already broken yet another spline gear. Reassembling the HS-81 plastic gear servos has worked out well. One of the wheel gears was a little tight, but has since been fixed. So now the plastic gears should last longer than 10 minutes.

Since the servos were modified they don't center very well. I suspect this is because the resistors that replaced the potentiometer do not match exactly. However, I did trim them in software and even after wearing the battery down a bit the neutral point doesn't drift much. As often as I ramble about servos I've learned my lesson and my future projects will likely use motors, for drive purposes at least.

Speaking of future projects, my V1.1 XPWMShields are finally in. Somehow they were delivered twice? Go figure. I think the perfectly good V1.0 board will remain on TOBL as not to have wasted $33 and shipping. I'll be able to comment more on the functionality (namely wireless uploading!) once I get some new Nanos to test them.

On a side note, I've recently started working at the Micro Robotics Research Lab. I thought I was into making things on a small scale...not even close! This lab concentrates on MEMS, fabrication techniques, and small scale robotics in general. For example, one team is designing a similar wireless communications board to XPWMShield, except that the board is approximately 1cmx1cm! Very cool stuff. Technically I'm working there for a program I'm in, but we'll see what happens. As for TOBL I will try to acquire a camera asafp!

TOBL: Teaser

What happened to TOBL?? Well it's alive, better than alive. In an effort to actually finish the thing before returning to school I focused more on the building and less on the documenting. It was close but I had to take it back with me to school:

What's up TSA?

I used 1-1/2" Dubro tail wheels for the drivetrain, mainly because they have ridiculous grip and aluminum hubs. To my surprise, the smallest outer diameter of the hubs was ~0.401". To support the steel standoffs I bored them out to 3/8", so that's like 0.013" wall thickness or less at the very center. We're good though, the two halves didn't torque themselves apart on the lathe. The hub diameter just barely supported the gear-mounting holes as well, but they don't post these dimensions online so not my fault.

This just looks goofy.

Small snack for the mill.

After making those aluminum plates I thought lasering some out of acrylic would be cooler. It was good milling experience still, I guess? It took a few tries to adjust from the gears' theoretical to actual pitch diameter -in the end an extra 0.02" radius. The servo-gears are also not properly supported i.e.they are cantilevered off of the spline gear. So they wobble, just a little. Enough though that the servos stalled and stripped some teeth internally. This was actually a good wakeup call because the stronger HS-82MG motor wasn't designed with the HS-81 plastic gears in mind. New gears are on the way, enough to bring two HS-81s to life.

Not even including the set that actually made it to production.

Look for test footage next post! (waiting on parts in the meantime :/)

XPWMShield V1.0

After a long wait (it says 2010 on the board because it was sent out last year!) XPWMShield has arrived from AdvancedCircuits. And with a bag of microwave popcorn in the packaging? Awesome company.

XPWMShield V1.0

This revision of the board lived a short life. I only got it yesterday, soldered on all the components today, and just broke it five minutes ago. Before that though here's V1.0 in all its glory, running off a 1300mAh, 7.4V, 2-cell, LiPo battery I got off HobbyKing:

She lives.

Need a better camera.

Just as I was going to test the wireless, one of the leads from the battery touched the bottom of the board, contacted a pin/via and blew up a trace coming out of the voltage regulator. Great. But at least the $34 Arduino Nano and $24 XBee are alright.

As it turns out the diode I put in between the XBee DOUT and Arduino RX was disabling radio communication. I put it there so that the software could be updated wirelessly without having to remove the XBee. So, I removed the diode and soldered a wire in its place -the same for the busted trace.

Ghetto wiring aside I can't complain, it works really well. With some minor revisions it's time to order more for future projects!

TOBL: Quick Software Update

There was an issue controlling both servos at once. With one fader value being sent over to the Arduino one servo would work fine. With both, that same servo would tweak out. After isolating the error to the Processing side, it turns out the float defining the fader values was preventing the second fader from registering. With that cleared up the code finally works as intended*!

* The interface needs some work to be more user friendly. For example, a deadband at the neutral position so you don't have to land the fader on the exact pixel that corresponds to STOP. Also, there's a point - about halfway through the range - where there's no noticeable change in speed. Playing with the fader ranges in the touchOSC editor should fix that. My only other complaint is a little lag in feedback due to the speed of wifi. On the other hand, it can be controlled from anywhere in range of the network aka college campus, so we'll let that slide.

TOBL: HS-82MG-360

A key ingredient for this project is a solid drive system. I opted to take the servo route as opposed to straight motors. Servos come pre-geared-down, they're easy to mount, and easy to control. What's not to love? Well they're perhaps less interesting from a controls standpoint, and in certain applications they scream rookie. If you've been following this project though, you know they're not trivial to work with, especially micro servos.

I've encountered plenty of problems with these little guys. Finally though, after buying two HS-81s, two HS-82MGs, and three HS-81 gearsets, then breaking two spline gears and a potentiometer, I have created the perfect (for my purposes) servo. Behold, the HS-82MG-360:

They said it couldn't be done. Seriously:

That's right it is spinning 360 degrees proving that impossibility is no match for being both stubborn and patient. If you're really interested in building one of these miniature torque monsters then you should see this post for details. The quick summary of how to make an HS-82MG-360 is as follows:

1. Replace the spline gear with the plastic one from an HS-81 gearset.

2. Cut out the first mechanical restrictor, found in the top of the gearcase with a pocket knife.

3. Cut out the second mechanical restrictor, found in the potentiometer. This holds the gears in place so be careful not to cut too much.

4. Cut the three wires connecting the potentiometer to the control board.

5. Solder small low-watt resistors to the loose wires keeping in mind you should:

a. Pick resistors of the same value such that their sum is the value indicated on the potentiometer. In this case, the pot read 5k and used two 3kohm resistors, so it doesn't have to be exact. I'm pretty sure the control board is just looking for a value somewhere in the designated vicinity.

b. Connect them such that the two outside wires initially from the potentiometer are individually connected to the central wire. Red-Yellow and Green-Yellow in this case.

6. Cover the exposed leads of the resistors with your choice of hot glue, heat shrink, or electrical tape.

7. Reassemble and test.

Control is handled the same way as before only this time the delay corresponds to a speed instead of a an angle. With this particular servo that's 600usec to 2400usec, 1500usec is neutral. Until next time...