TARS Part 2

<< Part 1

Fabrication has started in earnest. I decided to go with the most complicated piece first – possibly not the best approach but I reasoned if I failed to do this, the whole project would be off.


The first task was to produce some stock from which to machine all the pieces. I made a column from sheet styrene 160mm x 30mm x 20mm – big enough to make half of one quarter of the main body – this is about as big a piece as I can work on in one go on the mill. It was long enough to also include some spare material at the end that I could use for mounting the piece securely on the workbed.

A silicone mold of the column was poured and left to cure. In this I then cast a slab of urethane resin as my workpiece.

The first job was to mount the resin on the mill and then machine one surface completely flat (known as facing). This then becomes the bottom face and all subsequent operations are then guaranteed to be perpendicular or parallel to this face. The block was 20mm thick and the final thickness needed to be 16.7mm so I took off around 2mm at this time using a 3mm end mill. This was the first of several long, noisy and dusty operations! The process took around 30 minutes, milling off the resin manually layer by layer. Holding the vacuum cleaner hose as close to the cutter as possible as it worked sucked up the vast majority of the dust and swarf, but a fair amount still escaped, covering everything in a snowy layer!

Job 2 was turn the block over and reduce the thickness to 16.7mm. This and all subsequent operations were done via CNC. The data from the Sketchup drawings was exported to a series of DXF files which were then imported into CAM software. This allows you to generate the g-code instructions for the CNC controller that tell the mill where to go and what to do.

After watching it grind away for another 20 minutes, my piece was now exactly the right size on one dimension!

Job 3 was to engrave all the detail onto the front surface – the TARS lettering, the braille underneath and the panel lines. This was quickly achieved using a 45 degree “V” engraving cutter. I figured that it was best to do this first since everything else was going to take some time and I’d hate to mess things up on the final hurdle!

Job 4 was to create the recessed strip in front of the two screen holes which will hold some tinted material, and the two screen holes themselves.

I flipped the piece over and started on job 5 – removing the bulk of the resin inside the piece to allow for the display board. This was another long operation and nearly ended in disaster. By some miscalculation on my part, it ended up going deeper than it should – the recessed strip at the front should have ended up 1mm thick, but actually ended up being 0.5mm! Luckily it’s just about salvageable.

The last big operation was to remove the channel at the bottom of the piece. this is only really to allow the wires up from the battery back and in hindsight could have been a lot smaller ( = quicker to mill!)

2 small recessed were cut which will contain the magnets used to hold the whole assembly together. A half circular one at the top which will mate with the same in the other half, and a full circular one at the bottom side.

All that remained was to mill the outside dimension of the piece to free it from the excess stock. Well not free entirely – two small holding tabs were left, one at each end to ensure everything remained steady as the rest of the machining progressed – much like plastic sprue attachment points.. Once finished, these were cut through with a micro saw and the remaining nubs sanded flush.

Overall it came out better than I hoped but not entirely perfect. There’s a small bit of scoring on the sides due to the mounting not being secure enough which caused the piece to move during machining and also I think I was cutting a bit too fast, overloading the drive mechanics a bit. Nothing that won’t polish out. But it was a good learning exercise!

There is still more work to be done on this piece on the outside-side – more panel lines and holes for magnets but these have yet to be CAD-ed up.

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TARS from Interstellar

After the success of adding a working display screen to Gerty last year I’d been thinking about doing the same to something else. Re-watching Interstellar a while ago made me realise TARS the robot would be a great choice for a scratchbuild project. TARS himself is a relatively simple design, and there were some blueprints on the Blu-ray extras that showed the overall measurements of the full sized prop. From this it was easy to extrapolate all the others.

The first task was to draw up the parts in Sketchup full size and then resize down to get some figures to work with. A 1/6th scale model seemed to be a good place to start.

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TARS has 2 portrait orientated screens behind a tinted facia. Initially I figured I could use a single small LCD in landscape mode and use a portion of the LCD for each screen. The modules have a small plastic surround that holds the glass LCD in place. It’s only a couple of mm on 3 sides but unfortunately one end is much thicker to accommodate the flexible connector to the controller electronics. This means a) with the module in landscape orientation, the active LCD area is offset from one side and b) the wide border is too wide compared to the border of the TARS screen on one side. So I have to use the display in portrait mode, which in turn means most of it will not be used.

Looking around at available LCDs, a module with a 2.8″ active area seemed to be in the right ballpark – this would just be wide enough in portrait orientation for both TARS’ screen at exactly 1/7th scale. Slightly smaller than originally planned but this will alleviate some other issues to do with making the body too.

I intend to mill out and engrave the body from solid blocks of probably just resin (or maybe PU foam) but my small CNC mill has a very limited work area. The next job was breaking TARS down into manageable pieces. The arms were easy; these can be split in 2 in the middle. Despite the inside face of the arms being slightly different (no fine detailing and pivot point locations) I can make 2 complete arms from 4 copies of one half. The body is slightly more complex – it’s basically 4 pieces, 1 for the top front (including name and holes for screens) and 3 identical ‘blanks’ for the others.

The front of the LCD needs to be as close to the inside of the bezel around the screen holes as possible, which means the top part will need to be almost hollow. A battery pack of 3 AA cells will power the module, and this will be inside the bottom half, so each of these will need to be hollowed out as well, but not to such an extent. Access to the battery pack will be necessary occasionally so my plan is to hold everything together with magnets. These will also be used to hold the arms to the body, allowing both rotational movement and changing of pivot point from top, middle or bottom to provide a number of posing opportunities.

A few more hours with Sketchup and I ended up with this:

Next up was the electronics. I drew up a design based around an Atmel ATMEGA328 microcontroller (as used in the Arduino development board). This communicates with the LCD over a simple serial interface. I decided to add in some extra components since this may end up being used in other projects – a micro-SD card reader (handy for showing images on the LCD like in the Gerty project) and an expansion connector to allow some of the other spare I/O ports of the microcontroller to be connected up. The board sits neatly on the back of the LCD module and is only a few mm thick.

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Most of the time, all you see on TARS’ screens is a continual stream of text printing out – the left one is usually white and the right one is green. The LCD resolution (128 x 160 pixels) is far too low to show real text, so this was simulated with single pixels. A loop constructs a line of random length made from words of random length and draws each line out, one letter pixel at a time. This happens simultaneously on both screens. Once a screen is full, it is cleared and the process starts from the top again.

Part 2 >>