√−1 Scooter

As part of my last free this summer, I've taken up the task of building a personal scooter to transport me around campus and to MITERS.

Design parameters:

• 5-8 mile range
• cruise speed of 20mph
• reliability

I used Charles Guan's instructable as a guide in how to succeed not to destroy everything.

Among the most useful "steps" was the motor page as, having no previous EV experience, sizing up a reasonable motor alone would lead to either utter disappointment or too much scooter for any sane person to handle.

To safely fulfill the 20 mph (8.9 m/s) cruising speed, I'd need a motor with the following output wattage:

P = Force(drag)*Distance/Time

Substituting in air's density at STP (1.1839 kg*m^-3), and my drag coefficient at 1 into the drag equation gets me 38.0N, which translates to 338W. It's recommended that this number be less than 15% of the max motor input as ideally, only half of the input wattage goes into mechanical output. 

This would mean I'd need a motor rated for ~2.3kW. 

Hot damn.

With waterjetting out of the question, I ran into the idea of constructing the entire chassis out of u-channel to avoid racking up costs (the actual theoretical budget for the project is around ~$400). 

Going for $47.06, three feet of 4"x2.25" McMaster u-channel seemed to fit the job. 

At this point, several issues arise with trying to build a suitable motor into the frame of this form factor.

• Outrunners of the correct wattage tend to be either too big for the frame, or run at voltages that would cause battery costs to be too damn high.
• Having an outrunner too long/large would mean having to machine out the appropriately sized  hole in the other side of the chassis while mounting the motor to the frame. 
• Batteries are expensive.
• Ideally, the motor should completely inside the chassis to avoid gunk acquisition that would otherwise limit its lifespan.

This meant downsizing the motor from the previously melon-esque Turnigy SK3 63mm motor to the following: http://www.hobbyking.com/hobbyking/store/__18175__Turnigy_Aerodrive_SK3_5055_280kv_Brushless_Outrunner_Motor.html

Running the system at 36V also means an overall cheaper system and a happier wallet.

This did mean, however, my safe cruising speed would take a fall to 18 mph (hardly slow).

After settling on the motor, I could finally go shopping for the other assortment of parts I'd need to make this scooter scoot. The BOM can be viewed here along with the drag equation conveniently available in table form.

More to come...

-oneTesla

Pronunciation: "ne-guh-tive wan-tess-luh"

This summer, I'm working as a slave for oneTesla, a start-up founded by three MIT students: Bayley, Heidi, and Kramnik.

The recent bout of employment happened to coincide with my recent quest to get a "singing" SSTC working, and so -oneTesla was born.

Conceived from its namesake, oneTesla, -oneTesla is the SSTC derivative of the DRSSTC. The conversion process involved, to name a few things, removing the tank cap, fiddling with the feedback, and some trace rerouting.

DISCLAIMER: I do not recommend that you undertake this path as an alternative to purchasing an actual oneTesla kit. Trust me. The amount of pure butthurt incurred in troubleshooting alone is not worth the minuscule monetary sum that you'd save.

I started out with this very humble chassis and fan assembly, orphaned from Minuet 1.x:

4020, some lasercut acrylic, and HDPE (currently smothered in electrical tape) machined on a lathe. obligatory zip-tied fan included.

The original plan involved simply etching a oneTesla. It went less than swimmingly. For the longest time, I was under the impression that the optical receiver on the board was dead. Four hours later, I tried the interrupter on a oneTesla unit and it also didn't seem to work, and it turned out that the interrupter was just running out of batteries and the LED on the output just wasn't bright enough. Oh, and this was after I accidentally shorted +12 to ground, frying a few traces.

Important lessons learned:
1. Humans are not manufacturing houses.
2. Traces are not magical.
3. Always check the battery to your opto-coupled interrupter.

Version 1.1(6/10/13)

After receiving the board file for one of the original oneTeslas (meant to be etched), some more finagling occurred, which lead to the creation of another functional board.

Results: 3" of spark run at ~8% duty cycle.

Not bad considering the low duty cycle. Too bad that the bridge fried itself a few times before I could reflash the interrupter to increase the pulse width.

To be continued...

UPDATE (7/22/13)

New and improved -oneTeslas with secondary base feedback have been sent to the fabhouse. ETA July 29th.

UPDATE (7/30/13)

ERMAHGERD ETCHED BOARDS

Minuet 1.x

Time to go solid state!

For my first solid state coil, I decided to go with something on the moderate side of complexity, basing it off of Steve Ward's mini SSTC schematic: http://www.stevehv.4hv.org/SSTC5.htm

Operating Principles

Tesla coils are resonant air-cored transformers.

A primary inductor is coupled to a secondary inductor, resulting in a voltage step-up that allows for the breakdown of air after the secondary exceeds air's dielectric voltage (~3 × 10⁶ V/m). 

By using an air cored transformer, Tesla coils avoid the pesky core saturation that would otherwise limit the magnetic field and the resulting high voltage. 

The simplest variation of a Tesla coil, a spark gap coil, consists of the following:

SGTC Schematic
Source: http://enterpriseprogrammer.files.wordpress.com/2012/07/tesla_coil_circuit_diagram.png

When the primary tank capacitor reaches the voltage neccesary to arc across the spark gap (literally two electrodes separated by air), an LC circuit is formed that exhibits a resonant frequency.

Spark gap Tesla coils (SGTCs) and Dual resonant solid state Tesla coils (DRSSTCs) use two resonant tuned circuits (The primary and secondary) to more effectively transfer energy from the primary to the secondary coils. This is achieved by the phenomenon in which at resonance, impedances and reactances effectively cancel each other out.  

Given the resistive nature of the primary and secondary, the oscillations at the resonant frequency in both sides of the transformer will undergo decay and display waveform damping. This damping effect produces distinct RF "cycles" that are offset by 90 degrees. It's during each cycle that energy is exchanged through the changing magnetic field.

Simple solid state Tesla coils only have a secondary which is coupled to a primary inductor. The primary inductor is then driven at the resonant frequency of the secondary. 

Being driven at resonance requires current to flow back and forth in the inductor at a resonance frequency (simulating AC). In a solid state coil, this is achieved through transistors assembled in either a half-bridge or a full bridge (H-bridge) with the primary inductor acting as the load. The resonant frequency of most secondaries is around a few hundred kHz and bus currents and voltages linger around in the tens of amps and 240V (bus voltages will vary from topology to topology), this puts quite a bit strain on the transistors, limiting the field of options down to two types of transistors: MOSFETs (Metal-Oxide-Semiconductor Field Effect Transistors) and IGBTs (Insulated Gate Bipolar Transistors); the transistor strain is limited by the fact that most of the oscillations are induced in pulses i.e. the coil is kept in operation in short bursts to prevent overheating. Spark production is also noticeably different when driven in an "interrupted" mode as opposed to continuous duty. 

More often, IGBTs are favored over MOSFETs because they exhibit diode-like voltage drop while MOSFETs exhibit a resistive drop that increases with current.

Dual resonant variations of solid state coils include a tank capacitor in the primary side circuit, creating, like a spark gap coil, an LC circuit while in operation. The addition of a capacitive element to the circuit also cancels out the inductive reactance of the primary coil, thus allowing for currents orders of magnitude greater than regular SSTCs to propagate within the primary.

Tuning

Spark gap coils are tuned via physical parameters: the inductance of the primary, the capacitance of the tank capacitor, etc. 

In solid state coils, the resonant frequency is maintained by some sort of feedback mechanism. SSTCs generally use antenna based feedback (literally reaching into the electric field produced by the secondary), which induces a changing voltage as the field changes, while DRSSTCs required primary feedback as the secondary is out of phase with the primary. SSTCs are unable to use primary feedback because the current isn't effectively measurable due to the lower primary currents found in SSTCs. 

Construction:

Minuet 1.x was intended to be a single-boarded audio modulated SSTC; unfortunately, it's had a hard and very confusing life.

Evolution (to be added when I bother to take some pictures)
At one point, it did produce sparks, albeit only in continuous wave operation:

6'' sparks run continuous wave

The board since exploded after the addition of a fiber-in jack to more reliably interrupt operation. Maybe duty cycle issues?

If any good came out of this project, it's the list of things you shouldn't do

The project was abandoned in lieu of -oneTesla (negative oneTesla) 5/29/13. 

Herp Derp

me:  ben, I'm in school

get me out of here

halp

halp me

 Ben:  hue hue

 Sent at 10:23 AM on Monday

 Ben:  I'm at home

 me:  T^T

Spark Gap Tesla Coil 1.0

Conceived during a summer that probably could've gone to better use (Junction, whyyyyyyy did I attend you???), SGTC 1.0 was what first got me into the whole business of shooting lightning.

Long exposure is best exposure.

Design Considerations:

Modularity

- At some point, I want this thing to shoot monster streamers, so being able to swap components would be nice.

Reliability

- Demos are far more fun when things work.

Sexiness

-Hnngggghhhh *ahem*

Construction breakdown:

Chassis

-Two sheets of HDPE held up by threaded rod columns

- Fan bolted onto a piece of plexi that was subsequently ziptied onto the threaded rod (farming hard)

Secondary

- Donated by the Daniel Kramnik

- 4" diameter, 2.5'~ tall

- 24 AWG???

- Resonant frequency ~555kHz

- Added toilet flange for increased mounting stability to chassis

Primary

- 1/4" copper refrigeration tubing wound spiral-style

- Added supports for sexiness

Topload

-  Some very sad aluminum ducting wrapped in even sadder aluminum tape. Not exactly my best handiwork.

MMC

- Scavenged Panasonic metal film caps

- Rated for ~8kV 

-Capacitance: ??? (something in the 0.005uF range)

NST

- 9kV, 30mA (problem, MMC?)

Spark Gap

- Zinc-plated hardware (a bad idea)

- Cooled via muffin fan (a yummy idea)

Here, have some pictures media!

Note: bleed resistors help
 prevent accidental electrocution

Protip: when laying the primary,
 go from the center outward 

mmmm, HDPE

Results:

14"~ streamers 

When's the best time to start a blog?

3AM on a school night, of course.