Monday, June 26, 2017

DISASSEMBLING MOTORS, TINY BIKES, AND A SHAFT


Hello again! Project directions have shifted quite drastically since last post. While compiling the bill of materials for Tee-mobile, it became clear that this was going to be an expensive endeavor.

Cost of raw materials sans motor, wheels, controller, batteries..

Luckily, the quest for a fast thing didn't end there. Last summer, I wound up with one of these babies:


[Vrooming Intensifies]

A kid's pocketbike complete with things like brake (singular), belt drive, chassis mounts for electronics, and bodywork ripe for plastering with obnoxious stickers.

Stock, they come with a 250W brushed DC motor and controller, and some SLAs. However, this was soon going to change.


Womp @_____@

Courtesy of MITERS I was given one of these things. A copy of a Hacker motor, and mechanically very similar to Hobbyking's 150cc Rotomax offerings (same mount pattern and external dimensions), this little guy (dubbed the Tiramisu) is good for ~10kW with a nice and low kV (100RPM/V) for vehicular operation.

Running the show is a 72V 100A Kelly sourced from no less than Dane.

But Noel, the motor has the shaft on the wrong end!

Have no fear, italicized text: we thought of that already. We have the technology.

In search of the one true shaft, I began by removing the end-bell shaft. Unfortunately, these motors had gone swimming once upon a time, and the four fasteners keeping the shaft attached had suffered from the softening. 
Ripe for stripping

Initial attempts involved using the correctly sized hex heads, but that resulted in the inevitable stripping of the heads, so I resorted to gently hammering in a slightly larger torx bit.

Success!

In hindsight, I should've left the end-bell shaft on for initial disassembly, however, it led to the discovery of the bolt keeping the real shaft secured to the rotor.



After the liberal application of WD-40, the shaft bolt was removed and I undid the motor mount-side screws keeping the can attached. The rotor and stator assemblies then happily came apart after some pulling. Undocumented is the use of the mill to pull things apart: I clamped the motor mount in the vise and used the drill chuck attachment to grab hold of the reinstalled end-bell shaft.

Afterwards, I took the rotor assembly, removed the end-bell shaft and the bolt securing the one-true shaft and clamped the can in the vise and pulled yet again.


Pliers unrelated


Tape was applied to the magnets promptly after removal to prevent crap from accumulating in the air gap.

With the shaft extracted, I took some measurements and turned a new, longer one that would protrude out of the mount side. The taper was completely unnecessary and may as well just be a step to make machining easier.

Left: Original. Right: New and improved.
In reality, this process took a few tries to get right because I tried to jam things together with a hydraulic press after machining the initial shaft slightly too large and wound of destroying a bearing that had to be angle-ground off of it  I suck at turning things and was too lazy to do it right the first time.

The tolerances on the motor are such that the bearing interfaces with the shaft are slip fits, while the interface between can and shaft is a press fit.

Reassembled, things looked pretty good!


Whew

The motor mounting bracket was also made out of a piece of U-Channel left over from last season's car.

U-Channel evolved into ... angle bracket ಠ_ಠ
In the back of my mind this whole time was also the fact that I had to mount a pulley to this thing, so afterwards, I went ahead and also filed down some flats on the shaft for the pulley that would mount on the shaft. Thankfully, the bike came with a standard 5M HTD belt, so I could buy a pulley off of ebay.

Almost a thing
Also sent out was a hall sensor board to play with the Kelly. The motor is a 10 pole pair affair and using some math, I established the mechanical spacing between sensors. That is:

1 rotation = 360 mechanical degrees = (360 * N pole pairs) electrical degrees.

For this motor, we get 10 times the number of electrical cycles per mechanical cycle.

Going further, we get a conversion factor of

0.1 mechanical degrees = 1 electrical degree

So for 120 degrees of electrical offset, we need the sensors to be mechanically offset by 12 degrees.

The idea of having the hall effect sensors mounted to a board was also attractive, so I went ahead and made a board in Circuitmaker in part to get some more experience in the Altium-like environment, but also because importing funny board shapes was especially convenient.





The design is a blatant knock-off of the boards Charles has made in the past with adjustments made to fit the larger can size and pole count of the Tiramisu and the addition of a JST-XH connector on the back. You can buy Charles' stuff here.

As a Circuitmaker project, all of the files are free to access in their community vault. Just search for "Tiramisu Hall Board" in the public projects bar.

IRL



With board made, I made up a quick mount in Solidworks that I ended up printing and bolting to some holes I drilled in the motor mount.


Rawr
Note: I'll probably redo the design and add some support material between the standoffs to prevent bending and the terribleness that is encoder misalignment.
Motor on a Bench
Unfortunately no video was taken during the encoder tuning process, but it went a little something like this:

Whirrrrrrrrr

>Looks at current draw
>Adjusts Encoder

Whirrrrrrrrr

...

Once the position was settled, I filed down some flats on the motor shaft, mounted the pulley and everything was dumped into the frame.




Next up: batteries and wiring!

Monday, May 15, 2017

INTRODUCING TEE-MOBILE

Following this year's FSAE season, I had the burning desire to make something "fun" again. No more messing with the Sevcon until dawn, or trying to convince other people to show up and do some work, or getting bogged down by schoolwork (not that it really stopped me from working on car).

Of course, this resulted in taking things way too seriously. As a result of planning out the car team class' curriculum for next fall, all I could think of was the meat of the design process. Designing to spec, meeting a performance goal, taking into account your budget, getting to make something cool to get around a problem.

And so, I met myself somewhere in the middle - an electric go-kart. In partial fulfillment of my desire to whip around the new engineering building at Tufts in style, and to have a working electric vehicle again, I started on the aptly named Tee-Mobile about three days ago.


Partially Transparent-Mobile

I started with a performance goal and a budget- hitting 20 mph in around 10 seconds, but also spending less than $600. So, I whipped out a spreadsheet and used some good ol' idealized physical models to get me some enumerations.

Spreadsheet

From my mechE friends, I've been told that when you start designing a vehicle, you start with the tires, then go to the suspension and chassis, and then the powerplant you need to hit the numbers you want.

I of course, was granted the luxury that there was almost no chance that I'd be doing burnouts with the current state of cheap scooter-worthy parts, so contact points weren't an issue. (I'd need something upwards of 22kW to get a burnout going from standstill)

I had no intention of having a suspension given the terrain would be entirely smooth, and I didn't have that many restrictions on the chassis - a fairly small turn radius would be nice, however the primary concern was whether I could fit on it or not.

So, I got to skip a lot of the hard stuff and went to pick out components.

Quickly, I discovered that this would be kind of hard to pull off.

The game was essentially "find the low end torque" - not especially hard with electric motors, but kind of hard with hobby airplane parts.

Starting with the motor:

I settled on the Turnigy SK3 6374.

Has a kV of 192 RPM/V, and is rated for 2.75 HkW (Hobbyking kilowatts). Alternatives included some offerings from Alien power systems, and other wholesale RC sites, but the 6374 wasn't so fast that I couldn't gear it down sufficiently, nor was so large that it couldn't fit in my preferred u-channel vehicle construction method.

I also wouldn't feel too bad putting ~4kW through it for short stints. Cooling will definitely be negotiated, with an auxiliary 12V supply on board to power fans, but also fancy lights and the like.

Once that was settled, I could then move forward with futzing with the steering geometry. Run of the mill Ackerman was chosen because we're going to be spending most of our time at 'low speeds' and I'd like the wheels to last more than 5 turns.

Going in circles: no longer a bad thing

Other niceties include an actual chain tensioner that has a spring. Having learned my lesson from the scooter, I have also opted to include all four mounting posts for the motor.

Impeccable
You may have also noticed the lack of brakes: having gotten sick of doing mechE things, and not enough of the zappy things (although in the spirit of the sloth, I may even avoid that), I opted to go for the latest and greatest in cheap Chinese motor controller technology, which implements not only regenerative braking, but also SINE DRIVE. Let's just hope I'm not rolling down a hill when the batteries are already dead. 


Big controller or small cart?
For now, the 1.5kW, 18 FET version of the latest Chinese brushless motor controller was chosen, but we may downsize and hope that whatever FETs in the smaller, 12 FET variety are equally capable of being overdriven via forcible resizing of the current shunt.

To be continued!







Monday, January 9, 2017

AND NOW BACK TO OUR REGULARLY SCHEDULED PROGRAMMING

MOTORS

An autopsy of the YASA was performed in lieu of its failure at competition. On site, the controller was throwing encoder faults, the reason for which became incredibly apparent once we'd disassembled the drivetrain.

Motor Exposed to the Elements

Tears T____T

Due to some negligence on behalf of the mechEs, the motor regularly sat in the shop without its dust covers on, which resulted in a foreign object entering the gap between the optical encoder and the encoder grating. One of the optical encoder housings was then bent and upon bench testing, was shown to be broken.

Top Side

Extracting the encoder board from the motor revealed a fairly run of the mill quad diff amp package that amplifies the photodiode current from the encoders to give the usable UVW output used by the motor controller. The encoders themselves are shimmed with a 3D printed member that gives the appropriate curvature when seated around the motor's circumference.

Bottom Side

The JST style connectors are for thermistor breakouts that are fed through the main "DATA" connector. Further, the board appears to have a conformal coating, presumably for HV isolation (the encoder board sits in proximity to the phase leads of the motor), however, gently probing the soldered through-hole leads showed that it didn't really do much in terms of insulation (lol).,

The replacement component comes in the form of the Sharp GP1A58HRJ00F, which is their drop in replacement for the broken part (The old part has since fallen into obsolescence).

Testing of the repaired board will continue once the rest of the drivetrain is back in order.

BOARDS

Motherboard
The motherboard and steering wheel boards were respun. The motherboard features many of the same things from last year's car, just in a condensed form. Since we were moving all of the low voltage electronics into a smaller, weatherproof box, space savings was the biggest priority. Further, simplifications to the I/O were made, which meant we could use just a single AMPSEAL 35 pin connector. Note: the relays required for the FSAE hybrid latching circuit took up a considerable amount of space.

Following last year's cue sheet, the micro takes care of the start-up sequence, and also monitors system conditions (tire pressure, motor rpm, current draw, speed, etc) which it can relay to the steering wheel micro for display to the driver.

Refinements were also made to the steering wheel electronics (see mechanicals here).



The steering wheel board was separated out into three parts: a main board that has the microcontroller that drives the main display, shift registers, buffers, and level shifters, and two LED breakout boards for indicating state of charge for the high and low voltage supplies. Having the boards separate meant that we could independently mount each section to its respective location in the steering wheel assembly, making our lives a little easier.

Development for both platforms is also done on Arduino given the low complexity and performance of the required systems.

Infotainment System

The display chosen was a 128x64 pixel Adafruit OLED module; using their included GFX library, the dashboard layout was done in a single evening.

LIDAR SCANNERS AND STUFF

In an effort to enhance the engineering curriculum at Tufts, a suitemate and I got involved in making some lab material for ES2, a general coding class for engineering students.

We were tasked with making a 1D and 2D scanning system that used one of the dinky ultrasonics sensors you see floating around everywhere.
http://arduino-info.wikispaces.com/file/view/sku_133696_1.jpg/495546144/632x632/sku_133696_1.jpg

To keep costs low, we decided to go with a nested approach; that is, the 1D scanner would be a subset of the 2D scanner. Pictured below is the 2D scanner - the 1D version would simply exclude the L-shaped "arm" and secondary motor, and then mount the sensor directly to the main motor stage, giving control of only yaw.

2D Lidar Scanner: Now with 50% more floating Arduino
The base plate consists of a lasercut piece of acrylic upon which the main stepper motor, arduino, and legs are mounted. The "arm" member is attached to the main motor using a set screw and shaft that's integrated into the arm's body.

Since the arm would be 3D printed, a tradeoff was made between print-time and strength: it features an I-beam structure that decreased print time and weight, and also allowed for convenient cable routing. 

The secondary stepper controlling the pitch angle, is mounted on the arm and holds the sensor using another 3D printed member.

Cable routing is done to reduce the cable strain without resorting to drilling out motor shafts.
Beginning from the sensor side, a small cavity is included past the expanded through hole for the motor shaft that feeds into a port in the arm - this will also double as the feedthrough for the motor leads. Shielded cable would be used to minimize interference between the sensor data lines and the motor lead wires.
Cable Routing
The cables then travel along the inner wall of the arm and pop out the bottom through a port located next to the main motor. 


 Also visible are the endstops - just some brass rod that's press fit to prevent either axis from over-rotating and pulling out a cable.

Electronics to be done soon...