Waag Society Fablab Amsterdam

1. Introduction

At Waag Society's Fablab Amsterdam, together with Miguel Jimenez and Karim Amhali I have developed a 3d laser triangulation scanner.

A short introduction into 3d scanning can be found here,  also, the following presentation contains lots of useful information. The whole setup is built around a webcam, a rotation platform and a focussed line laser.

By taking pictures of the object from each angle while project the laser at the object under a know degree, we can determine the position of points in space by measuring the distance of the laser line from the center of the projection.

In our final design of the scanner, we decided to add a number of improvements, such as an arduino controlled rotary platform and a second laser projecting at the opposite angle, to capture more detail and fill possible gaps caused by self-casting shadows from the object.

You can find software and some more resources at the follow url: https://github.com/mvhenten/pylatscan

This document describes in somewhat more detail the construction of a body for the 3d scanner out of plywood.

2. Considerations and partlist

The current hardware design for our prototype has evolved from extensive testing with earlier setups. A laser angle of about 30 degrees seems to be the most optimal position for the laser, and moving the laser to different angles automatically might improve scan quality, but increases the complexity of the overal design. Using a single laser proved to be insufficient, as objects protubing too much in the horizontal plane tend to obscure the laser line under certain circumstances. Therefore, a secondary laser was added under the opposite angle of -30 degrees.

Fitting the lasers rigidly as to stay in place during transport is vital for the usability of the scanner, since calibrating the whole setup is crucial in order to produce good results, but can be quite cumbersome.

The lense of our camera must be centered at the exact center of the rotation, but moving the camera up and own or closer to the object is needed to scan different sized objects, and to move the focus point of the camera to the vertical center of each object as much as possible ( to produce the best results, an almost “orthogonal projection” is the best kind of input.)

Earlier designs used plexiglass, this current design, however, uses plywood as the base material. Plywood is nice to work with, since you can cut, drill and sand it into shape very easily, using both the shopbot and more conventional tools such as a chisel and a hammer. This means the overall material is more flexible and you can work around small mistakes or omissions in the design.

2.1 Platform construction - partlist

Some of the bolts and screws could be exchanged for other parts you may have lying around - it just happened to be the pieces we could get our hands on. The SVG files for the shopbot also contain holes for the bolts, so keep that in mind when taking different sizes.

The M6 pins may not have been ideal in this case, since a thicker guiding may provide more stability.

Partlist:

1. 19mm plywood. dimensions of the platform are around 40x50mm and you need two sheets
2. “Standard” 40mm “flightcase” or “tool case” suitcase
3. about 50mm of 10M steel bar
4. about 25mm of 6M screw wire
5. one long 6M bold ( I found 12 cm the longest I managed to find )
6. two long 6M pins ( The kind with a cap on the end )
7. “draai tappen” ( or wood pivot, the kind you can slam into a piece of wood )
8. various bolts, nuts in 10M/6M
9. 2mm round- head wood screws

2.2 Scanner hardware  - partlist

Unfortunately the part number of the original stepper used in our prototypes got lost, but I’ll assume the farnel part will do just fine. Also, some exact details ( such as the type of white leds ) got lost as they were not picked by me. The led mount is made of a plexiglass scrap and hot glue. At the current time, I don’t have any connection schematics but I will provide them as soon as I have constructed the final prototype.

1. 2 Green line lasers with focus ring ( LC532-5-3-F(16x65) from laserfuchs.de )
2. 1 Logitech Webcam Pro 9000
3. 1 Arduino board
4. 1 Arduino motorshield
5. 1 Stepper engine, 400 steps (for example ST2818S1006-B from Farnell)
6. 6 bright white leds
7. wires to connect everything

2.3 Schematics, blueprints

Since this project is under active developments, I cannot be bothered with providing zipfiles, but everything will be provided here.

The shopbot cutter we used was a 6 mm, which is pretty precise. The design does not include any depth-milling, but smaller is better, since more precise. The 6mm cutter just happened to be on the machine when I used it. 

3. Construction

To explore the capabilities of the shopbot, I designed a “snap fit” laser mount and platform fix that could be cut out of one piece of plywood and put together as an IKEA package.

The platform consists of three layers: top, middle and bottom. The middle layer can be used to guide cables and provides extra stabiltiy to the laser mounts,  the bottom layer is for finishing and fixing the electronics. Overall dimensions were choosen to be able to fit into a standard size suitcase, in our case the biggest affordable aluminum suitcase we could buy at a local hardware store. However, to be honest, each layer was designed along the way in an iterative process.

3.1 The Mark One

Once the cutting was done, I constructed a camera mount out of plywood and some large bolts that could be moved up and down by turning a screw. A strong motorcycle spring provides some rigidity for t his construction, and it proved to be rather stable.  I used standard bolts so I could fix the bolt heads easily in a cavity on the bottom sinde of the the second layer. All this was done by hand, improvizing along the way, but most of this work found it’s way into the second attempt. Remember to keep a small chisel at hand for this type of work! plywood is excellent to work with in this way :-)

Once this was finished, I started to fix the lasers on their mounts, this is where I realised something had gone wrong in the milling - the cavities for the lasers were too large. I finally found out that this was caused by the transition from inkscape SVG to illustrator, for some strange reason. This can be avoided by using .eps as an intermediate format.

I improvised again, by drilling 12mm holes into the other side of the mounts. ( it is very important to do a precise job here, since the laser angles must be know and exact! ). I calibrated and ensured that the lasers were at a straight 30 degree angle by switching them on. The two lines should merge at exactly the center of your rotary platform! Again, some of this was improvised in the first attempt, for example by adding a layer of black ducktape around the laser casing to allow for more grip.

After this, it was time to fix the camera onto the mount. The packaging of the 9000 pro is not the most ideal for this setup, as it is hard to center the lens and keep the whole thing straight and horizontal. One important thing to do here is to switch on the lasers and camera, and keep looking at the picture. I’ve written a small python script that shows a line trough the center of the image, and while reassuring that everything stays straight and centered by looking at the screen, I fixed both camera’s, laser and platform in one go.

3.2 Lessons learned: the Mark II

From the inital built, I learned that:

1. The shopbot is much more precise then I had anticipated. don’t hand-drill. trust the shopbot!
2. The camera mount works pretty reliable for up-and-down movement. However, moving along the Z axis is a very nice to have, since it’ll allow for scanning small objects.
3. The snap-fit approach works pretty nice, and plywood is sturdy anough for this purpose

Altough the “Mark One” can be used within the project I intended it for, I’ve started work on an improved design ( this time without the scaling bug ) that will actually fit the suitcase as intended.

I started out by making a constructive test of a Z-Y axis mount, much like a CNC milling machine provides.

This mount eventually made it into the next design, the top layer already cut.

...more to follow!