Electric Orbit Wheels: Laterally Propelled Hub-less Rollerblades

While on a college tour around California, I saw college students riding up a hill on these strange wheels. They would move their legs forward and backward, as if they were walking, but they would magically move sideways. After I saw them jump off and put the wheels in their backpack to enter a building, I decided I had to get some. 

There are two types of these "magical" wheels; Freeline Skates and Orbit Wheels. There are a few main differences between them. The Freeline Skates have two small sized skateboard wheels below the foot platform. This eliminates two degrees of freedom, yaw and pitch, which makes them easier to handle for beginners. The flat top foot platform makes Freeline Skates top choice among skaters, because they enable for more complicated tricks. Because Freeline Skates use skateboard wheels, they can be easily swapped; however they are more likely to get caught in a cracks on a sidewalk. On the other hand, the Orbit Wheels have a large hub-less wheel with foot holders on an inside flat edge. The bigger wheels lend for easier travel over cracks, but they also have more degrees of freedom. The choice of which to buy ultimately comes down to the preferences of the customer. 

I chose to learn on the Orbit Wheels, because I had recently learned how to ride a unicycle and I liked the idea of having many degrees of freedom to control to remain balanced. After about six hours of dedicated practice over a two-week-long period, I was able to wiggle my way across rooms. I soon found their limitations and starting thinking about how I could improve them. First of all, the Orbit Wheels have a narrow and pointed wheel causing the wheels to dig into soft grass. I wanted to improve on their design by making a thicker and faster wheel. I decided I would build my version of a new Orbit Wheel while at my first semester of MIT.

Upon arriving to MIT, I stumbled around and found the MIT Electronics Research Society. MITERS is a student run shop where tinkerers of all kinds gather and build cool things. Electric vehicles were one of MITERS's many fads at the time. That's where the Electric Orbit Wheels were born, my first real project at MITERS. 


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For the Epilog Challenge:

What would I do with a Epilog Zing 16 Laser? Well... I would definitely tinker and experiment with it. Maybe try to make 3 -dimensional shapes with acrylic or make book covers out of wood. I'd raster funny images on peanut, butter, and jelly sandwiches, or even on cakes! I would certainly not limit it to a prototyping tool. 

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Submitted by MITERS for the Instructables Sponsorship Program

The concept of how orbit wheels work is kind of hard to wrap your head around. This poster has a fantastic diagram explaining the physics behind how the Electric Orbit Wheels provide sideways motion. A good image to keep in mind is if the Electric Orbit Wheels were coated in a paint, their trails would be similar to sine waves or snake paths. The two waves are offset in the direction of travel, because your feet are slightly separated. 

When it comes to mechanical things, the Electric Orbit Wheels feature a water jetted frame and a custom hub-less wheel. The wheel is driven by a water jetted ring gear embedded inside a urethane tire. The 80 shore (the same durability/hardness as skateboard wheels) urethane was poured around stacked aluminum rims in a 3D printed mold. Because of the two independent wheels, the Electric Orbit Wheels have an infinite turning radius. 

Since the vehicle is so small, the battery pack is spread out around the vehicle to maximize the range. The pack is composed of A123 28650 Systems LiFePo4 Battery Cells and arranged in a 5S2P configuration for a total of 18V and 4.4Ah. "5S" corresponds to five cells in series and "2P" corresponds to two groups of cells in parallel. This results to about 35 Watt hours of energy stored in the battery pack and a 1.5 mile/ 2.4 km range. 

The Electric Orbit Wheels use a HobbyKing Brushless Car ESC (Electronic Speed Controller) rated at 60A peak and also includes reverse. This controller is more robust than most controllers used in RC Airplanes, because it is meant for use on the ground. The HobbyKing ratings are usually overestimated, thus making the controller the limiting factor of the Electric Orbit Wheels. 

The motor controller controls a Turnigy Aerodrive SK3 - 4250-350kv Brushless Outrunner. The motor can draw 1007 Watts peak. Again, that is a HobbyKing rating so it is probably much less than 1000 Watts. The motor and motor controller are controlled by a RC HobbyKing GT-2 2.4Ghz 2Ch Tx & Rx throttle. The top no load speed is around 10 mph (16 km/h). 



Bill of Materials:

Here's a Google doc spreadsheet containing the materials, cost, and website urls:
https://docs.google.com/spreadsheet/ccc?key=0ArkLjZiIqp-ZdEx5R2trMnhNc0dickFwcUhySk5PZkE&usp=sharing

I used Autodesk Inventor 2012 to design and CAD the plates on the Electric Orbit Wheels. Autodesk Inventor is a free software for students and is among my top choices of CAD programs (alongside SolidWorks). I spent a lot of time refining my design on the computer to perfect it before cutting it out on a water jet. 

I strayed from the commercial Orbit Wheel design by making a more square-like wheel frame. The square shape allowed for more battery space and for a better way of connecting them to the frame. The battery clips, similar to motor mounts, pinch around the batteries because one leg is slightly shorter than the other. I also designed for a wider wheel in order to meet one of my design specifications: an orbit wheel that can drive over grass and soft terrain without sinking in. 

The interlocking features on the foot plate and foot plate supports are a quick and easy way to connect water jet or laser cut pieces. They also also reduce the amount of fasteners on the design. The elongated holes near the motor mount region will allow for fine adjustment to meshing the spur gear and ring gear. 

To make sure my square shaped orbit wheel design would fit my croc, I made a small prototype with laser cut acrylic and t-nuts, a quick and easy 2D fastening technique. Making the laser cut acrylic prototype was crucial to my design process, because it brought my model to life and allowed for me to interact with the design in another medium. I verified that the design would work, and made some improvements after playing with it in a physical space. The laser cut prototype helped with designing where the batteries would fit, and led to the square plate design. 

Charles, a prominent MITERS tinkerer, has a fantastic instructable in which he describes how to build a miniature go kart from available online sources. That post is relevant, because the go kart is mostly made from water jet components. Water jets are big expensive machines, but online companies are now (and have been) accepting .dxf files and shipping water jet components. You can download the .dxf files I used to water jet from my website, and send them off to be cut. 

I used Autodesk Inventor, a CAD program, to design the parts to exactly the right dimension. When it comes to water jetting interlocking parts, this can be a hassle because the water jet compensates for its curf. In order to avoid filing small bits off your freshly cut water jet components, you can ask to set the water jet tool thickness to 0.011". This "tricks" the water jet into thinking it has a smaller curf that it actually does, causing it to move closer to the part and cut a little more off for you. 

A common method of assembling water jet components is to use "t-nuts". MelonScooter uses the t-nutting build style. T-nuts allow for the parts to be easily fastened together immediately after cutting them on a water jet. I decided to drill, tap, and countersink the edges of my water jet parts to make the Electric Orbit Wheel more durable. However, by not using t-nuts, it adds another "post water jet machining" step in the build process.

The main structure of the frame is composed of six steel rods with a 5/16" outer diameter. Using a hack saw, I cut the long steel stock into small 1.6" pieces and then turned them on a lathe with a 0.136" inner diameter bore. It's hard to get all of the steel rods to be exactly the same length on a lathe, because they aren't all positioned in the same exact location with respect to the lathe. Since I need them to be exactly the same length for connecting the two front and back plates together, I stuck them in an aluminum jig and faced them an end mill to exactly 1 1/2". I then tapped the bored holes for an 8-32 screw size.

Similar to machining the steel rods, I machined 15 small aluminum spacers from a 1/2" aluminum rod stock and bored them to a 0.136" inner diameter. These spacers separate the bearings, which I obtained from VXB, so they fit in their respective tracks on the rim. The spacers also keep the wheel from sliding side to side within the aluminum frame housing. 

MITERS usually has a bunch of scrap aluminum left over from previous projects. However, there weren't any aluminum scraps with 8" by 8" of clean material and excess for clamping onto the water jet. So, I bought 1/4", 1/8", and 1/32" aluminum plates at McMaster-Carr. 

The rim is composed of a combination of aluminum parts layered together. The ring gear, and bearing tracks are complete 1/4" thick 8" diameter rings. 1/32" shims are inserted with the bearing tracks to allow for the 7 mm thick bearings to ride in the groove. The rest of the wheel is composed of 1/8" aluminum rings cut into thirds. They make up the majority of the wheel's composition, because they can be tightly packed together on a 6" wide plate and cut in bulk. 

The ring gear is probably the most unique and most difficult part of this project. I used the Autodesk gear generator (in assembly mode) to design the ring gear. The important values in the gear generator are the desired gear ratio, diametral pitch, center distance, and number of teeth. I first found a spur gear from SDP-SI (Stock Drive Products/Sterling Instruments) with 16 teeth and a 24 ul/in. pitch diameter. This helped constrain the variables a 10.375 gear ratio with 166 teeth for the ring gear. Many websites like Sterling Instruments and McMaster have CAD files along with their gears and other machined parts which can be a huge help when designing your vehicle.

The bearing tracks contain elongated holes to allow for the urethane to seep in and interlock with the aluminum rim. The inner diameter is 7" and the outer is 7.8" to give 0.4" for the holes and to fit in the 8" wide aluminum stock I purchased from McMaster. When water jetting the rim pieces, especially the ring gear, use quality 5 so the teeth mesh properly. 

In order to make the wheel compact, I screwed in 1 1/4" black alloy screws from both sides of the rim. This required a complex pattern of threaded and non-threaded holes and eliminated the need for nuts on one side of the wheel. I countersunk the first and last two holes of the outer third ring pieces. Then, tapped the three remaining holes in the middle. This pattern of taps would alternate groups of three screws. Without nuts, the space between the wheel and the front and back plates is minimized which prevents rocks from jamming the wheel (this happened fairly frequently with the Orbit Wheels). 

When water jets cut any material, the resulting edge is not a perfect flat edge. There's a very slight tapper due to the diffraction of the water and abrasive after entering the material. When assembling the side plates between the two front and back plates, I put the longer tapered edge closest to the inside to avoid seeing large gaps.

Since the bearings all fit in the groove of the rim, its difficult to put the bearings in one at a time. I solved this by slowly pushing the rim over the steel rods with one layer of bearings. By repeating this process with the spacers and bearings, the rim is eventually fitted on with all the bearings in place. The rest of the assembly is screwing the 1/2", 3/4", and 1 1/4" 8-32 black alloy steel flat head screws into the pre-tapped and drilled holes. 

The 3D printed mold is a two part mold to allow for dis-assembly after the wheel has been cast. The inside edges are filleted so the wheel will have round edges without requiring a belt sander after the urethane sets. The outer diameter of the mold is approximately 9", so the 3D printer you use must have a large enough printing region. I designed rim inserts that can be press fit into the mold in order to hold the rim up. Otherwise, without the rim inserts, the rim would sink to the bottom of the mold and would have a very thin layer of urethane on one side. 

I cut a ring gear prototype on the laser cutter to test whether the ring gear and the 3D printed mold fit each other. Laser cutters and water jets are surprisingly good at cutting gears. 

The .stl files required for 3D printing are attached below. 

The trial sizes for the Econ 80 Urethane Rubber contained plenty of liquid urethane for molding the wheel. I strongly recommend you take the necessary precautions and read the safety instructions before pouring. I made a small test mold to practice before pouring the actual wheel. The two part urethane mixture and other releasing agents can be bought from Smooth-On.

Be patient while pouring a mold. Start pouring in the most shallow region to ensure the urethane completely fills the mold and doesn't leave any air bubbles. There will most likely be a few very small air bubbles when the mold is finished curing. One way to prevent this is to stick it in a vacuum and a hot environment. 

I let the urethane set and cure for 24 hours before tampering with the mold. After removing the two part mold, I used a belt sander to touch up the thin urethane regions around the edges of the wheel. There was also some urethane leak deposits into the ring bearing tracks which I cleaned out with a flat head screw driver. 

Making a battery pack with A123 28650 Systems LiFePo4 battery cells can be very very dangerous. Ben sums it up pretty well in his Electric Tricycle Instructable. The soldering iron necessary for soldering the A123 battery cells puts out a lot of heat and must be handled with care. Also, accidentally shorting a cell, let alone a whole pack, can put out a lot of heat. If you are fairly new at soldering, I would highly recommend buying a battery pack. Only attempt building your own pack if you've been taught by a very experienced individual.  

This pack is extremely unique, because it is stretched around the frame taking up the last bits of available space. Normally, packs are assembled by first gluing them together with hot glue and then soldering. However, I primarily used electrically tape and heat shrink to hold the Electric Orbit Wheel packs together and protect them from the environment. I used braided wire to connect two batteries electrically, then folded over to get two cells in series. I ended up with 10 cells soldered in two groups of five cells in series. This formed a 5S2P pack, which I then connected with bullet connectors between the two halves of the frame. The smaller wires are balanced leads that isolate each individual cell for balance charging. 

The motor controller is the most delicate part of the Electric Orbit Wheels. In order to protect it from water damage and shocks, I buried it in a small metal compartment along with the other electronics. I took the cooling fan off the motor controller to be able to fit it into a small space. To construct the electronic housing, I used the mill to machine 1" by 2" side plates, 1" by 3" bottom plate, and a 3" by 2 1/4" top plate made from 1/8 and 1/4" aluminum scraps left over in MITERS. I used the belt sander and files to sand the inside corners of the housing to allow for the battery and balance leads to connect to the controller. 

Attaching the motor and getting the teeth to mesh was simplified because of the slots I had previously designed. To prepare the motor, I milled a flat for the spur gear's set screw and bored out the gear's inner diameter to 0.185". 

To fasten the battery packs to the frame, I used a laser cutter to make some 12 mm clear acrylic battery clips. The legs of the clips are uneven to allow for the shorter end to be tightened further making a better friction hold. The machined housing wasn't big enough to contain all the wires and capacitors from the motor controller, so I cut two hollow rectangular shapes from 12 mm clear acrylic to extend the box by 24 mm (~0.9"). The files for the acrylic accessories are below.  

Finally, I soldered all the balance leads to 6 pin JST connectors (6 pin because there are 5 cells) and all of 10 Gauge (AWG) battery wires to Deans plugs. A little tip for soldering Deans plugs: plug the male and female ends together while soldering to ensure that the metal pieces don't move if the heat from the soldering iron melts the plastic. 

After having learned how to ride the commercial Orbit Wheels, the Electric Orbit Wheel wasn't hard to pick up at all. The Electric Orbit Wheels ride similar to the Orbit Wheel; however, it is a bit heavier and thus slightly more difficult to handle. However, because they provide thrust, it is much easier to "coast" because you don't have to constantly pull your legs in.

Because I used a sensor-less motor and motor controller, the Electric Orbit Wheel can't start from a complete stop. The controller controls the motor by reading its back EMF, which means the motor has to be moving in order for the controller to work properly. Thus, the Electric Orbit Wheels start off like a normal pair of orbit wheels. Once moving, then the throttle can be applied. 

I used to break on the Orbit Wheels by dragging the foot platform on the ground or by jumping off, neither of them being very effective. On the Electric Orbit Wheel, I can reverse the throttle and make it act as a break. The effects of gerbiling, rotation in the wheel when accelerating or decelerating, are small but still present. I added a standoff post to protect the battery packs from rubbing on the ground when braking. 

I usually ride at a comfortable top speed around 10 mph (going much faster than that, and scrapped knees are the least of your worries). I've been riding it around MIT's campus to and from classes and it has significantly decreased my travel time (While similarly increasing the number of strange and fascinated looks from pedestrians).