Thursday, November 28, 2013

LED Dress

About six months ago my Sister told me that she was going to a Taylor Swift concert and that she would love me to make her an LED enhanced outfit to wear. Taylor specifically asks her fans to get out their glowwy things such as cellphones so I took this as an opportunity shine... Ahem.

A quick check of the conditions of entry didn't say anything against going to concerts covered in red LEDs.

Skip to the middle if you just want to see the final result.
What could possibly go wrong?

Having done this kinda thing in the past I was only too keen to do something similar. As she is sometimes wheelchair bound due to having dysautonomia I decided that her wheelchair had to feature in the outfit too.

She had a red cocktail dress that was to be the basis of the creation. We ordered some cheap waterproof red LED strips, and LED controllers from eBay along with a rather chunky LiFePo4 RC flight battery from Hobbyking.

Battery, Power
electronics and controller.

The battery is 19.8V and 4.5A/h and I use a switchmode buck converter to supply a constant 12V dc to the LED controller.

LED Controller: $4.80 from eBay, Bargain.

Sudden realisation as to
how much of my hair
has fallen out recently.
At the end of my most recent contract with VUW I organised to make a trip up to my sister's place and begin creating...

 I measured out the the strips to fit the bodice and the skirt. Mum then stitched the strips on with invisible thread.

Ready for wiring.
I then began the rather painstaking task of stripping and soldering some 22 joints to each of the LED strips. The strips are just paralleled LEDs, that is they all glow at the same brightness.

Due to using hot glue to reinforce all of the joints, I made doubly sure to test every joint and avoid nasty dark strip surprises later. The dress is impressively bright with all the strips running on 12V (about 25W)

With the strips all wired it was time to test...

25% power.
100% power.

It would be rude to not do any light painting with a creation capable of such ludicrous luminous flux, the results are rather breathtaking.

Battery Belt:
Sketch of the net for the pouch.
Needing a place for the rather large battery and the electronics to live I set about measuring everything up and devising a "net" or pattern for the battery pouch. The design is a basic "cereal box" with a lid that's held down by velcro. The sketch roughly shows the dimensions with a 10mm seam allowance around the edges.
Not everyone uses vernier calipers for sewing projects.

I then transferred the sketch to the fabric stiffened with iron on interfacing. Then cut out all the notches which give the corners clearance.

It's red on the other side.

After a bit of fun with the sewing machine the pouch was done. I made a velcro belt from the same fabric.

Battery pouch with electronics hidden inside.

Update Friday 29th November: I have received an email from the touring company and they are going to allow the dress at the concert! Thank you Frontier Touring!
Update Monday 2nd of December: My sister got into the concert with no trouble at all and had a great time :D

Patiently waiting for the doors to open.

Wednesday, November 13, 2013

Closed Loop CNC 4th Axis.

My friend has an Abene 3-Axis CNC machine, it's capable of lifting 1000kg on the table... Pretty grunty. I've used it for making Tesla coil parts, making custom camera accessories, and even teaching his 12 year old stepdaughter CNC programming.

Due to making such things as the centre plate for a twin plate car clutch, and because it's well cool. We decided it was time to build an addon rotary 4th Axis.
The controller, a Heidenhain TNC 155b which despite being as old as me is still capable of driving 4 servo axes with 1 micron positioning accuracy. It's even capable of 3D graphics!

 The controller outputs 0-10Vdc which is fed into a dc servo amplifier for each axis.

Left to right: XYZ

The amplifiers then output up to 100Vdc to the DC brush motors which drive the ballscrews on the axes.

The servo motors have a tachogenerator mounted to the opposite end of the shaft: This generates a DC voltage proportional to the speed of the motor. This signal is fed back to the servo amplifier for the speed control loop.

On the linear axes the position control loop is closed by use of linear glass scales which output a quadrature signal of two sinewaves back to the CNC controller.

The 4th axis is a little different. Mechanically it's a rotary table that would normally be used for manual indexing of parts on a milling machine.

The table is driven by a 1000w dc servo motor with a 3:1 belt drive reduction. When the servo is at maximum speed (3000rpm) the table rotates at around 10RPM, a little exciting when there is a large part in the chuck.

Due to the machine originally being only 3 axes with an optional 4th axis, we had to add an extra servo amplifier.

The servo we used has a higher voltage tachometer than the amplifier was designed for. Not wanting to allow many dollars worth of smoke out, I modified the "Personality Module" of the amplifier slightly to allow a higher input voltage.

If only humans came with such a module.

The mechanical loop is closed in this case by a Heidenhain rotary encoder which is connected to the input shaft of the rotary table.

The encoder is held in place by a large block of aluminium machined to suit. Inside there is a flexible shaft coupling between the encoder and the input shaft of the rotary table.

Electrically the 4th axis is pretty simple:
A small proximity switch is used for the initial homing of the axis on startup.

All the connections are made inside a tiny electrical enclosure, all the holes were CNC machined of course :)
The lid carries the encoder connections.
The base carries the motor and
tachogenerator connections

All the parts are mounted to a 12mm thick aluminium base plate that was also machined in the CNC.

Mounting bracket for the servo motor.

It took a couple of evenings working till way past midnight but the result was well worth it.

The video below shows a test piece being machined with the 4th axis mounted horizontally and then the clutch centre plate being machined vertically.

Using a touch probe to set the X zero point.
Drilling the edge of the clutch centre plate.

Monday, November 11, 2013

Opening Lego Mindstorms batteries with a CNC milling machine.

Working for Victoria University of Wellington, we use Lego Mindstorms for teaching some of our robotics papers as part of our Bachelor of Engineering. (yes we get paid to play with Lego)
The Mindstorms battery packs have been wearing out lately. In the interests of being environmentally friendly we are repacking them with cells from Battery Space.
The cases are held together with aluminium rivets which have to be drilled out to get inside.

With around 20 battery packs to open: I wrote a quick program on one of my friend's CNC machines to do the drilling for me. Each pack took less than 30 seconds to open as a result.

Sunday, August 11, 2013

Pyramider: The evolution from Spark Gap to IGBT

A number of years ago I was approached about a Tesla coil in a deceased estate of a friend of a friend you might say. Naturally this was an opportunity too good to pass up.

Pyramider circa 2006.
Upon first inspection I found out the coil had three 12kV neon sign transformers and a rotary spark gap.

I duly agreed to take responsibility for the creation and it then sat at work for a few years occasionally being tinkered with.

Around 2011 He who must not be named on the internet moved back to little ole NZ bringing back with him enthusiasm for building tesla coils and a REALLY big piece of power silicon: Thus began my renewed interest in power electronics...
2.5kv at 1800A

The idea was to build what's called an Off-Line Tesla Coil (OLTC), in that there is no supply transformer: The incoming mains supply is rectified and fed directly into the tank capacitor.

OLTCs have some interesting design criteria: With the primary voltage being very low (compared to spark gap coil) The tank capacitor has to be huge in order to supply sufficient energy to the system.
The huge tank capacitor in turn means that the primary inductance has to be tiny for the system to have a sensible resonant frequency.

The primary currents involved are pretty massive, as a result the "wiring" has to be pretty heavy duty: In this case the conductors are made from 1.6mm copper sheet, 190mm wide. That is 300 square millimetres of copper!

Initial back of a envelope calculations were based on an input power of around 5 to 10kw to make things reasonably interesting. With a reasonably safe maximum switching frequency of around 7-800Hz this meant the tank capacitor needed to store around 10 to 12 Joules. Charging the capacitor to around 1KV, (more on this later) around 20µF was necessary.

Primary Construction:
As mentioned earlier the primary conductors are made from copper strip 190mm wide, there are a couple of reasons for this besides the ludicrous discharge currents...
The tiny primary inductance: Total inductance, which includes the parasitic inductances of the conductors, transistor, ESL of the capacitors etc had to be about 0.1µH or less. This is so that the resonant frequency of the primary matched the secondary.
The wider a conductor is the lower its inductance, therefore transmission lines made of nice wide strips of copper have lower parasitic inductances.
Flat conductors are also a lot easier to mount closer together: This reduces the parasitic inductance of the system further still. (more on this later)

The "Snorkel"Primary:
Owing to the low inductance needed, the primary had to be very small in diameter (the diameter actually ended up less than the height) this resulted in much headscratching... The required diameter was less than the diameter of the secondary... Eventually the solution of internally mounting the primary was devised by use of a little trickery:

In order to minimise parasitic inductance, I had to devise a suitable feed line to connect the primary turn to the capacitor and transistor assembly. A bit of a hunch and a few hours of sketching later a plan was devised to fabricate a shape from copper strip and then roll it into a combined primary turn and feed line... Below is a sketch of the result:
Sketch of the fabricated primary.
The above shape was made by cutting strips of copper sheet that were then TIG welded together, this approach was taken as there was a shortage of material (it was rather expensive)

The welds were linished flat and all the sharp edges were polished off. I then put the whole part through a sheet metal roller to give the final shape.

The many, many bolts used to connect
the snorkel to the capacitor assembly.
Snorkel featuring polyethylene
sheet to keep the feed line separated. 

Tank Capacitor:
Capacitors in Tesla coils have a pretty hard life, they're charged to high voltages and discharged very quickly... Only to be told to do it many times per second. So naturally they have to be very low loss and rated to huge pulse currents.
Capacitor plate to the left.
Currently about the best off the shelf solution is to connect multiple smaller capacitors in series-parallel to give the appropriate capacitance, voltage and current rating. In this case depending on tuning there are about 40 capacitors in parallel. The capacitors are mounted to a parallel plate feed line (think double sided PCB) made from two sheets of 1.6mm copper spaced 5mm apart. This is an idea I borrowed from Steve Connor and Greg Leyh. The capacitors connect to the plates using threaded brass studs and copper tubing spacers.
Brass studs and copper spacers fitted.

One day I would like to build a coil using water cooled capacitors designed for induction heating from a supplier such as Celem: Megawatts anyone?

Capacitors fitted to the assembly.
Top and bottom mounting bushes.

Mechanical Assembly:
Making the components necessary to hold everything together is a little interesting:
For one tesla coils generate high voltages - Many of the components have to be very good insulators.
Ferrous components can get hot due to induction heating (Tesla coils tend to generate rather large EM fields around the primary)
The result of all this is that I tried to use plastic components as much as possible...
Bottom mounting bush and threaded
nylon rod.
The four mounting rods.
The secondary former is supported by a combination of 10mm threaded nylon rod, 3d printed bushings and PVC pipe columns.
Columns and top bushing fitted.
Secondary former sitting on the bushings.

Water Cooling System:
For a lot of the initial running, the power consumption was limited to around 5kw. This changed quite dramatically when I finished building a three phase input supply. A brainwave to do with the implementing a pulse forming in the charging circuit made things even more dramatic.

The side effect of much more power is well, much more heat... Until this point we had been using a fan cooled aluminium heatsink, this was ok but things were getting a little hot.

The shiny new waterblock.

The transistor with heat sink.

Enter water cooling: I have a friend who's a rather brilliant machinist. We spent an evening drawing up a water cooling block to suit the transistor. An hour or so on the CNC machine later and presto.

Bolted together.

Getting ready to bolt the transistor to the waterblock: Toons,
safety glasses, and two finished cups of coffee... Serious work.

Nobody breathe.
For the first test I lashed the system together using a plastic bucket and a spare PC water cooling radiator I had lying around.

Hose restraint.

The first results were more than a little exciting.... The cooling water stayed below 27 degrees celsius during many power runs like the video below... Making Toccata and Fugue even more epic.

So What's Next??
I've made a better water reservoir than a bucket, Hansen plumbing fittings are great for building crazy stuff like this.

10 Litre reservoir.

Still to be fitted is a new radiator from Koolance: As a final teaser here's the CNC machined mounting brackets for it... Next post will detail the power system.
Problem: "Radiator bracket for water cooled Tesla coil"
is not a part that exists...
Solution: CNC machine.