Computing stuff tied to the physical world

Wireless CupCake

In Hardware on Feb 8, 2010 at 00:01

JeeCake, the CupCake 3D printer here, is an interesting mix of machine, electronics, and software.

The RepRap Motherboard is the main on-board controller, based on an ATmega644. It drives 3 stepper motors, communicates with the Extruder board, and talks to a desktop computer via its FTDI interface and a USB cable.

On the PC side (which can be Windows, Mac, or Linux), there is a Processing-/Arduino-like package called ReplicatorG to control the machine. It takes G-code, which is not related to Google in any way, but rather an ancient CNC control language from the 60′s.

ReplicatorG then converts this to a binary RepRap 3G protocol, as documented here and then uses that to drive the CupCake. Everything from moving the axes, adjusting the nozzle temperature, controlling the extruder motor, to writing settings in the machine’s EEPROM memory.

The neat part is that the v1.2 motherboard has an SD card adapter on board, and that the CupCake can run unattended by getting its detailed build instructions from files stored on an SD card.

Except for one little detail: there are no controls or displays on the CupCake, other than a few LEDs. This is not enough to select which file to print, adjust the zero position, or start a build. It looks like a new “Generation 4″ design is on the way, including an interface board with an LCD screen and some push buttons.

Right now, you have to connect the machine via USB, start everything up via ReplicatorG, and then you can yank the USB plug and it’ll happily continue printing what it started, right to the end (well, except that on Mac OS X 10.6, yanking USB cables can lead to kernel panics – looks like a serious bug in the FTDI USB driver).

Anyway, I don’t really care for controls, or even displays on the CupCake. All I want is some way to control a unit sitting on the other side of the room, or in a nearby room. It can be fairly noisy due to some sort of occasional wood panel resonance, and having it printing right next to me is not really my idea of fun.

Which is where JeeNodes come in: wouldn’t it be nice to be able to control the CupCake via wireless? The protocol is already perfectly suited for it, since it uses packets of max 32 bytes – well within the 66-byte limit of the RF12 driver. And if the object being printed is already on the SD card, then only a few packets need to be exchanged to get going. During printing, some status info could be sent back – again very low rate stuff, easily within the JeeNode’s wireless constraints.

Here’s the idea:

DSC_1177.jpg

Hook up a JeeNode to take the place of the USB cable, and let it behave as a RecpicatorG control program.

The interface needs to swap RX and TX for this, and because the CupCake’s signal levels are at 5V, two 1 kΩ resistors need to be inserted to prevent excessive current. One more detail is that the power pin on the FTDI connector is not connected, since the motherboard is powered off its own PC supply. So a separate wire needs to be added to power the JeeNode off the ISP connector right under the FTDI connector:

DSC_1178.jpg

The one wire not shown here is the ground wire, running from top right to bottom left on the other side of this little custom interface board.

It turns out that the motherboard is actually powered from the 5V standby supply pin, so it’s always powered, even when the power supply is in standby mode.

I’ve started writing some code on the PC/Mac side to control the CupCake without using ReplicatorG. Here’s some sample code in Tcl which works when connected directly through USB:

Screen shot 2010-02-07 at 15.05.08.png

And here’s the corresponding output:

Screen shot 2010-02-07 at 15.04.35.png

As you can see, it can access all sorts of status info, read the file names on the SD card, and control the machine. This is part of a larger project, the beginnings of which are now in the subversion code repository.

It takes a lot of work to make hookups like these work, because there are so many different bits and pieces (literally) involved. The next step is to see if the JeeNode can indeed communicate with – and control – the JeeCake, and then code needs to be written to replace the current direct-USB connection by a packet-based wireless hookup through a JeeLink + JeeNode.

Oh, and then I need to create some sort of little on-screen control panel to adjust the nozzle temperature, jog the Z axis up and down, pick a file to print, and start the print job! Not to mention making it robust and secure…

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