The Graphotron Mini Makerfaire Robot

This little gadget is the graphotron 6000/graphotron 8000/remote drawing robot. The makerspace I’ve been frequenting wanted a little robot thing to display at a Mini Makerfaire and they tasked me with the project about 26 hours before it was due. I enjoy these hackathon-style binges once in a while.

I started out with a basic Arduino Uno, a handful of servos, and an equal amount of potentiometers. The initial goal was to build a remote-operated robot that plays Operation, kind of like a really dumb DaVinci surgical robot. Problems arose with figuring out the end-effector so after talking it over with the client, the Duh-Vinci became the Graphotron. The makerspace specializes in art, so drawing just kind of made sense.

The arm is a (potentially) 4 DOF manipulator made of hobby servos, dowels, and 3D printed structural components. I say potentially 4 DOF because I had some weird issues with controlling the end-effector servo dynamically, so I just stuck it at 120 degrees (or something) to hold the pen steady. The three bigger servos towards the base of the arm run on 6V, so they needed some unique regulation down from the 12V I was using. Variable voltage regulators were the solution there. The little guy on the end only needed 5V, so I used a pretty generic 5V regulator for that.

The controller, as I’ve been calling it, is almost identical to the arm in terms of dimensions to make it easier to translate the controller motion to the arm. There’s a potentiometer at each joint which measures the angles for the arm to mimic. Structurally, it consists of the same dowels and 3D printed plastic as the arm.

On the software side, linking the arm to the controller was probably as easy as it could possibly be. The Arduino measured each potentiometer on the controller and moved each corresponding servo on the arm to the same angle. Basically, it’s the Servo Knob example three times.

All-in-all the Graphotron was a hit with the kiddos. Actually using the gadget is as simple as using a pen; however, getting the result you want on the arm side is not easy due to the servos being the cheapest I could find and the angles between the potentiometers and the servos not being 1:1. The adopted slogan was “It’s like drawing, but if your hand had no nerves and your muscles only cost $60”. Super catchy.

Finished Drive (for real this time) and a Lot of Other Stuff

So this is what the small car is right now. A steel frame, set of wheels, and a steering column. Pretty good progress if I do say so myself.

So I got a nice guy at a makerspace to do the welding for me because it’s impossible to get access to MIG welders where I’m living right now. He did a great job so I’m probably going back to him to see if he can finish it when I have the rest of the metal ready. The drive sprocket is now welded to the shaft and everything spins pretty smoothly. When I plugged the motor into my 36V bench power supply, everything spun up nicely.

I got the stuff that makes up the steering column nice and put together with a great 3D printed bracket that doesn’t shatter when I bump the column which is always a plus. The tie rods I originally got in the steering kit were too long by like six inches so I replaced them with some regular threaded rods cut to size.

There’s no electrical progress to speak of for this update. I’m focusing on getting everything mechanical together so the electrical and software part will be easy to develop and test. I’m in the process of getting a deal together on some batteries so hopefully by the time that’s settled I’ll have a good platform to test everything on.

Finished Drive (pretty much) and Beginning of Frame

Tadaaa! Except the sprocket isn’t attached to the driveshaft and I haven’t done any testing.

I’m planning on just welding the sprocket to the driveshaft because I don’t have the resources to use keys. You can see the driveshaft has a little groove for a retaining ring to keep it between the motor mounting plate and snugly inserted in the transaxle, I’m pretty happy with how well it all fits together. I designed the shaft to pretty much replace the scooter motor shaft and I happened to know someone who was able to machine it by hand perfectly, which was very fortunate for me. The chain is super loose on the sprockets so I 3D printed a tensioner that I’m hoping will stand up to the wear. If it doesn’t work out, it was super cheap to make anyways and I can always design a better tensioner.

This is the super simple frame that is designed around the U-shaped bracket that came with the transaxle. I made the jig out of wood that was stored in the rafters of the house my parents recently bought because free wood. Before I ended up renting a chop saw to cut the square tube I tried using an angle grinder with a cutting wheel, but that didn’t work out as all my cuts were jagged. Lesson learned. The next step is to do some welding, which I may have to outsource because it’s impossible to rent MIG welders for some reason (I recently found out it’s because renting out the gas would be a liability).

BONUS! RGB headlights! They’re really bright if you just stare into them (pretty obviously) but probably not bright enough to be effective as actual headlights. Their purpose is just to look cool anyways so I don’t care if they’re not extremely functional. Yes, I would love to have a PCB to solder everything to, but that’s an expense I’m not willing to shell out for because just soldering them all together in an exposed metal rat’s nest is way more free. I 3D printed the bezel thing and spray-painted it chrome, then added some clear-coat which pretty much just made them gray but I really don’t mind because they still look cool and being weather-proof is nice. They’re going to be connected to their own PIC and put on a CAN bus with the rest of the car’s system components which I will get to after the car has its bare functionality (self-propulsion and steering).

Drive Assembly Progress

And thus the drive assembly is almost done!

I had some pieces cut out of 5/8″ aluminum for spacing and support of the motor. As you can see in the gallery, there is a nice little groove cut around the perimeter of the round spacer piece about a millimeter deep. This puppy took about 4 hours and a file to get just right so it would fit nice and snug in the transaxle. Yes, it would have been infinitely faster and easier if I had a lathe, but I don’t, so it was grueling and took forever.

The motor just screws in nicely to the support arm. There’s a 9-tooth sprocket for #25 chain held in place on the D-shaft by some 3D printed spacers and a lock nut. The lock nut stays there because I threaded the hardened steel shaft just enough to keep the nut there. That was another process that took a very long time, mostly because I don’t think you’re supposed to be able to thread hardened steel shafts. That’s why the rotor is out of the motor in the gallery.

The body is still in the works, but its progress took a leap when I decided to just sand down the mold as use it instead of trying to pull apart the cast from the mold. I’ll outline that a bit more when its complete and I do a rundown on the process.

Party Toaster 2.0

New and improved Party Toaster! This was a nice little week long project I did for my cousin who wanted one after she saw the mk I and mk II.

Read more here

Still working on the small car little by little (heh). It’s now more the center of attention since I’m done with the toaster.

Body + Electronics Update

Squeezing in a little bit of progress here and there is all I’ve had time to do between Capstone, homework, and work which means, unfortunately, that not a lot has gotten done. Nevertheless, here is what has happened:

So this is the current state of the body. I finished up the Bondo-ing I gave it a coat of gel coat and a single layer of fiberglass. This is the first time I’ve ever fiberglassed something and I must say it is the most frustrating thing I’ve ever done. The issue may be that I don’t have the right tools or something, but fiberglassing is just an awful experience. The resin is syrup-y and sticks to anything and everything, that is until you put the fiberglass down in it in which case the resin lets the fiberglass glide across and fold up on itself. The non-stick coating I applied before the gel coat also did not do much non-sticking because pulling the body out of the new mold took forever and ended with me ripping up the plastic and shattering all the bondo to get it done. The product is actually fairly satisfying save for a single place on the “headlight” where the fiberglass seemed to bubble up and the gel coat didn’t get a good stick. I’m expecting to do a lot of sanding and filling in of holes at the very end of this and I’m totally fine with that. I just want the stupid thing to get done. I’m dreading the second fiberglassing of the cast because I won’t be able to break it away like I did for the body, but I will cross that bridge when I come to it.

As I think I mentioned before (but am too lazy to go back and check if I did), I ordered some of the major electronic components online a while ago and finally got to testing them. In this gallery is a 6.5A 36V power supply, a Turnigy Sentilon 100A ESC, a Turnigy Aquastar T20 brushless DC motor, an Arduino Micro, and various connectors and hardware. This was just a quick test to see how the ESC worked with the Arduino. There was some disagreement at the high and low end of the Arduino’s PWM output but overall, it worked.

Quick overall project recap: body almost there, electrical system looking good, frame design constantly changing, drive transmission in progress, batteries still up in the air.

End transmission.

Small Car Shell Stuff and PIC Disappointment

After spending so much time putty-ing and sanding, I finally decided that the body of the car should be done. Any tiny cracks and bubbles can be fixed on the final product with some more putty, so I’m not going to sweat the little things right now.

The most important part of this update is actually the PIC24F16KL401 I’ve been working with on the side. I’ve learned quite a bit about the timers and oscillators on microcontrollers and all the stuff that goes into bare-bones programming. The plan was to use a fleet of PICs throughout the car to control the ESC, autonomous steering and braking motors when those came into play, and sensor data collection. Having worked with PICs in the past for college projects I figured it would be a good place to go for electronic control that’s way cheaper than an Arduino, that being said, PICs can’t efficiently drive more than one hobby-grade PWM signal at a time. The issue is that the system clock settings won’t allow for a slow enough PWM period, so the solution most people use is a pretty bad “drive digital pin high for x milliseconds, then low for x milliseconds” method. That’s totally fine for driving one servo as a demonstration, but it has very little practical application.

It looks like I’m going to have to give in and use an Arduino for motor and power supply control and monitoring as it will be significantly easier and Arduinos are practically bulletproof. More to come.

Car Body Progress and Some Other Stuff

Last I updated about the car body it was in four separate pieces and sitting on my floor. Now it is more or less one piece, held together by cardboard, duck tape, and lots and lots of Bondo putty.

Making everything nice and pretty has taken much longer than I had hoped. I severely underestimated the amount of putty-ing and sanding and re-putty-ing and re-sanding that’s required so the process has been going on for quite a while. The detailing power sander has become pretty much my best friend along with second best friend belt sander. It’s super satisfying to just hold a power tool against a rough patch of Bondo and watch it turn smooth with a puff of what I assume to be carcinogen-rich smoke.

Between waiting for the putty to dry and getting fed up with the ungodly amount of time this thing is sucking from my life, I made a cool racing team name and a logo to plaster on the hood of the car. Of course it’s toast-centric. The rest of the time I’m spending on homework, school projects, and configuring the PIC microcontrollers I’m going to use for all the power management, drive control, and other electronic stuff so progress is definitely slowing. To keep things moving along somewhat steadily while I’m focusing on classes, I’ve been ordering quite a few parts online including a motor, controller, sprockets, chain, electronic components, brakes, and other stuff.

Hopefully next update I’ll have some super cool pictures of the fiberglass mold-making process and possibly even a full on fiberglass shell.

Power Wheels Dissection and Quadrisection

As mentioned before, the Mercedes as it came out of the toy factory is much too small for a grown human to fit inside and is well within the size restrictions for PRS. As such, I had to do a bit of custom modification to the body to make it big enough for a full grown Eric which required some minor disassembly as well as some major dremel-ing.


I initially cut out the base of the body to see if I could fit inside without the seat and floor, to no avail, so it turned into a full on quartering. I stitched the thing back together with some good old cardboard and duck tape so its width fits nicely around the salvaged transaxle and the wheelbase has the same proportionality to the width as a real Mercedes Benz 300SL (about 1.34:1). The next step with the “biggening” of the body is to get some bondo or modeling clay and fill out the holes until it’s once again nice and smooth. The end result will be a fiberglass mold from which I can cast infinite fiberglass bodies to paint and interchange and replace in case of damage.

An issue with the wheels I ordered is that they are all free-spinning with bearings set into the rims. This was, of course, a problem when it comes to driving them with a motor and the solution was going to be welding two of them straight to the rear axle. Luckily, the wheels of the salvaged scooter are the same size as those that I purchased online so the split rims already screwed into the transaxle can be easily re-fitted with the tires and tubes from the aforementioned internet wheels.

Doing this took way more effort than I had hoped (taking tires off of solid rims is a nightmare), but the transplant was a success and now I have a nifty assembly for the rear axle and wheels.

A new thing that I’m finally getting around to working on is the power supply for car. It should take in the 36V from the battery and distribute it as 36V, 12V, and 5V for the drive motor, autonomous control motors, and various logic components respectively. It’s secondary function is to monitor the voltage and amperage coming out of each power rail and shutdown or limit each subsystem in the case of overvoltage or overcurrent. I drew up a really rough design that gives a pretty basic idea of how it will look, it will get refined over time as I nail down each subsystem controlling and monitoring the car.

Tangible Small Car Progress and Mobility Scooter Salvage

After what seemed like forever building up a bill of materials, I finally ordered the first batch of parts. I would have spent longer designing and measuring stuff out in CAD, but BMI Karts was having a pretty irresistible sale on some stuff I would eventually need so I couldn’t help but to get out the credit card. I got a set of wheels, a steering column, steering spindles and brackets, a pair of tie rods, a steering wheel, and a pair of pedals. After receiving the parts, the steering wheel is probably too big and the pedals too heavy but I paid $4 total for both of them so I’m not crushed. The wheels are all free wheels with inset bearings but I have an idea to resolve that issue which I will divulge soon below (last paragraph, not that soon).

I finally got my Benz! I was also lucky enough to come across a mobility scooter that happened to be on craigslist in the same area so now I have a key component of the real ‘Murrican playset. Because the sellers were actually quite a ways away from where I live, some extended family living in that area were nice enough to run my errands for me and meet me halfway for a hand-off which is why I had to wait so long to actually tear into it.

Instead of pulling apart the car right away, I decided to piece out the scooter so I could figure out what parts I’ll be able to use. Going into it, I already knew I was going to try and pull out a differential or transaxle, so everything else was just gravy. I ended up finding what I was looking for, which is why I was looking for a scooter specifically instead of a chair like a hoveround which has a pair of independent motors instead of an axle driven by a single motor.

So here’s what I needed, just gotta unscrew a few bolts and clean it up, right? Well, yes. But also, no.

I got the transaxle out of its bracket thing and started taking off the wheels, one nuts at a time. Two nuts in, the outside split rim popped out with a pretty loud bang. There was still one nut pretty tight on there so the rim was actually bent out a little which you can see in the fourth picture. At this point I got some safety glasses and put one foot on the wheel as I loosened the last bolt from a distance. It was pretty suspenseful because I was pretty sure I was going to get seriously injured, but I was too impatient to take more safety precautions. On the last turn of the last bolt, the rim popped out with so much force that it actually blew my ratchet out of my hand and threw it against the wall 5′ away. The pressure in the tire was so great that I guess the glue couldn’t hold the rims together and it blew them straight apart. You can see in the last picture the tubes nearly popping out of the tire. For the second wheel I got slapped in the face with some common sense and let the air out before I took it off. That went nice and smoothly with no damage caused to me or my environment.

So here’s all the stuff I need all disassembled and wiped down with half a roll of wet paper towels. To finally complete the thought about the free-spinning wheels I had in the first paragraph: the scooter rims are actually the same size as those in the wheels I bought, so I can just take the tires and tubes from the BMI wheels and put them on these scooter rims, easy peasy (probably not). I took the huge motor off because I’m planning to use the Turnigy Aquastar 1/5th scale boat motor (taking a page from Charles Guan’s book) and its accompanying Aquastar 200A ESC because, according to Hobby King, it can handle the 36V I want to push through it. Unfortunately, this huge axle is too big for the Benz by about 8″ so I’ll probably have to find a bigger power wheel. I still appreciate the thing for what it is, so I might keep it around to do some other stuff with (it was cheap so I’m not heartbroken).