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6-Axis Micro Manipulator

Micron level manipulator, using printed and low cost components

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This is a Gough-Stewart platform type, 6-axis platform / manipulator. Which will be used in helping to fabricate my digital UV exposer for alternative printing chemistrys.

The overall dimensions will be a function of the available motors and required resolution for translation and rotation. The initial design work indicates this to be in the region of 40mm cube, with maximum translation of 2.8mm and rotation of 20degrees.

I'm sure there are may uses for fine positioning in 6-axis that i've not considered, but pretty much any case where the cost of high precision actuators such as piezo stacks or alternative fabrication materials (EDM steel) are too high, or where equal behaviour could be achieved with much larger apparatus, but where there is a overall access / size constraint.

Shared under the Creative Commons - Attribution - ShareAlike 3.0 license.

The aim is to use this manipulator to position an optic fiber in front of a laser diode (when the maximum output is passing down the fiber), the fiber is then glued in position. This is how pig-tailed laser diode modules are created.

Industrially the assembly of pig-tail modules uses machines that must cost tens of thousands of dollars. These use the same layout, a Gough-Stewart platform, using six actuators to give the six degrees of freedom control.

in my case the manipulator will be manually moved so that the fiber is within a millimeter or less of the correct position and then the manipulator will make the final positioning and hold the position whilst the fiber is glued down to a support structure. 

Other possible uses:

  • As a platform for supporting small samples in bio-sciences research, either inverted as a standard stage or with a modified main frame to allow the microscope to view down through the centre.

  • As a manipulator where both triangular frames are modified to allow view down through both parts, with end effector positioned into the centre of the platform.

  • With an alternate design of platform incorporating vacuum nozzle, discrete optical elements could be manipulated as required.

  • To support end of microscope fibre optic bundle to allow viewing around a specimen

Mathcad - Gough-Stewart_leg lengths.pdf

Calculations for roughing out control equations to calculate leg lengths for required rotation and translation of the platform / end effector

Adobe Portable Document Format - 332.82 kB - 07/20/2017 at 14:40

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Mathcad - Gough-Stewart_convex_hull.pdf

Investigation in to the valid translation space for specified gepmetry, leg lengths & connection positions & looking at the effect of rotation in restricting valid positions.

Adobe Portable Document Format - 2.04 MB - 07/20/2017 at 14:40

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  • rev2 actuator body

    David Brown07/14/2017 at 15:47 0 comments

    So the initial build of an actuator has allowed me to get a better understanding of the behaviour of the SLS nylon 3D prints and the overall behaviour of the mechanism.

    • The HP multi jet fusion printed parts have much better resolution than the older 'strong and flexible' prints.
    • Need to minimise the moment generated by the stepper and the actuator pivot point, the motor needs moving as close to the pivot as possible. This also allows the nut to be better captured.
    • Slight change needed to allow the control board to be easily installed. The first revision peg has a fully enclosed hole for the resistor slider, as only the lateral force is need, the recess can be open vertically to allow the slider to be dropped into the recess.
    • Need to better control the moment about the peg to the body of the actuator, in the first revision a simple drop in cylinder was used as the pivot. Because of the print tolerances and the thickness of the body, there is a degree of movement that helps to bind the mechanism. The cyclinder has been replaced by using a brass insert in the peg and having a screw act as the pivot. This should provide better control of the movement perpendicular to the pivot axis.

    I've ordered the new geometry and it will be another few weeks wait for the parts to arrive. Meanwhile I need to sort out how and what the monofilament linkages will be attached and start looking at implementing the algorithm for controling a Gough Stewart mechanism. Images below the break

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  • initial power-up

    David Brown07/12/2017 at 21:55 0 comments

    As you can see i've transfered the components to one of the HP fusion printed actuator bodies as these have better definition on the hinges, so require less force to move. The 'strong and flexible' (Shapeways SLS nylon) had too much load on the motor (hinge material printed out thicker). The video below is of the actuator moving back and forwards twice over. This required some adjustment (use of needle files), as the printed nylon has some variation from the design geometry.

    As you may be able to see the black 'peg' that couples the motor, resistor and actuator moves around a fair bit and there is obvious backlash from the motor nut to the peg. I need to investigate a new iteration for the peg geometry to take most of this out.

    Getting the motor aligned was also a tricky and required adjustment of the brass insert to get the angles close. This is fine for small numbers, but larger numbers will require a accurate jig for placement.

  • Initial assembly

    David Brown07/11/2017 at 19:37 1 comment

    So the SLS nylon parts arrived from Shapeways and i've put together one actuator. unfortunatly the brass screw inserts which were supposed to be M1.2 turned out to be M1.5 so i managed to find x3 M1.5 screws in my bag of tiny screws from dismantled stuff. The others will have to wait another few weeks until the re-ordered inserts arrive, but at least i have one to run tests on, which I'm quite pleased about. More below the break....

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  • Actuator components

    David Brown06/23/2017 at 15:20 0 comments

    So i've ordered the printed components from shapeways (SLS nylon) but the lead time is quite long, so will see how quick those parts come through.

    The linear variable resistors (ALPS RS08U11AZ001) have arrived - really small, see below;

    And i've ordered the boards that these mount to from OSH park. Kicad render below;

    This has been designed for both the LB1935FA and LB1848MC bipolar motor control ICs, as i've still got a few pieces of the now obselete LB1935FA. There is a single pin difference, so its a case of cutting one of the tracks, depending on which one is being used.

  • Fatigue design in PA-12 / Nylon

    David Brown06/21/2017 at 01:05 0 comments

    The actuator body has now been designed for fatigue and to meet the requirements for Shapeways 'strong and flexible' sintered plastic (PA / polyamide / nylon 12).

    Figure: Final design for articulator body.

    The material data for this is available by searching for PA2200 material data sheet. The stress limits come from a paper I found [ref. High cycle fatigue properties of selective laser sintered parts in polyamide 12] the key findings of which are;

    • The building orientation of the SLS specimens has no influence on the fatigue properties, due to high percentage of fusion.
    • The fatigue life is significantly influenced by cyclic softening if the sample temperature reaches the glass transition region, leading to brittle fracture.
    • Where the print machine employs contour scanning the outer surface is of higher quality, damge to the surface by machining reduces fatigue life as cracks nucleate from unmolten powder particles and these are not present on the unmachined surface.

    The design followed an itterative process from the initial design, extending the length of the thin hinge material. Initially simply, but when I found the fatigue

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  • Design rev A2

    David Brown06/20/2017 at 02:01 0 comments

    The alternative motors arrived, so i've been updating the design to use one of the types. The originally ordered ones arrived, but need a separate tooth part to run against the screw, so they are a no-go for this application.

    Figure: Updated actuator for alternative motor

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  • Ordered some alternative motors

    David Brown06/07/2017 at 15:38 0 comments

    With the first ordered motors in customs limbo I've ordered some alternative motors with brass nuts (which may be needed for higher forces). These will take a few weeks to come through, so I will see if i can put together some hand calculations for the forces and a finite element analysis of the fused nylon mechanism to see what the local stresses are and if there will be fatigue issues.

    All of the motors look like they were originally for use in camera lenses. So one of the complications is working around the mounting arrangements.

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  • initial update

    David Brown06/06/2017 at 23:18 0 comments

    I ordered the motors shown in the images a month ago and still waiting for them to exit customs, so have pushed on with design that can be updated when they finally arrive. They were less than a dollar for 10 including postage, you get what you pay for. I may order some others for design flexibility.

    The design will use conformal joints / living hinges and use 3D printed components connected using monofilament line. This ensures that there is no backlash, which is critical for this level of resolution.

    Figure - 1.8mm thick PCB provides foundation for the motor and resistor, fitted together with M1.2 screws, the micro linear resistor provides feedback to the controler. The motor driver IC will be on the board so that only lower current drive signals are in the wiring, and reduces the number of wires required.

    Figure - The end effector is designed to hold an optical fiber to allow me to position when building a pig-tailed laser diode assembly.

    It's hard to appreciate just how small this will be; being able to fit in a rough 40mm x 40mm x 40mm volume.

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Gravis wrote 07/16/2017 at 05:27 point

The actuators look they would be great for use in automated wafer alignment for semiconductor lithography. :)

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Douglas Miller wrote 07/04/2017 at 20:23 point

Very nice! I'll be following this one closely. I have a use for something like this coming up soon, and this looks really interesting. Best of luck!

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David Brown wrote 07/04/2017 at 21:39 point

Many thanks, I'll be posting as this progresses, boards are in the post and still waiting on the plastic components being produced.

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